About the Study

SCOPAC Committee

Chairperson Councillor Mrs M Penfold MBE, West Dorset District Council.

Vice-Chair Councillor Jackie Branson, Havant Borough Council.

Technical assistance provided to Councillors by Mr Lyall Cairns (Southern Coastal Group Chair) and Dr Samantha Cope (SCOPAC Research Chair).

The STS 2012 update

The 2012 update of the SCOPAC Sediment Transport Study (STS) was funded by the Environment Agency under FDGiA, grant number LDW 41230, with additional contributions from SCOPAC.  

It is referenced as: New Forest District Council (2017). 2012 Update of Carter, D., Bray, M., & Hooke, J., 2004 SCOPAC Sediment Transport Study, www.scopac.org.uk/sts.

Sediment Transport Study 2012

Lyme Regis to Portland Bill

1. Introduction

1.1 Physical Nature of the Coast

The southwest Dorset coast is protected by a discontinuous belt of shingle beaches, of contrasting character to the east and west of West Bay. Chesil Beach (Photo 1) in the east is by far the most extensive; a linear storm beach extending 28 km eastwards from West Bay to the ‘Isle’ of Portland (Chesilton) and for 13 km backed by the shallow Fleet Lagoon. By contrast, much smaller pocket beaches at Eype (Photo 2), Seatown (Photo 3) and Charmouth (Photo 4) separated by high headlands and backed by high eroding cliffs fringe the shoreline to the west (Brunsden and Goudie, 1997).

The difference in the nature of the coast east and west of West Bay Harbour reflects the degree of protection afforded by the shingle beaches. The huge volume of Chesil Beach almost completely protects the land behind, so cliffs only exist close to West Bay where the beach is lower. To the west, the pocket beaches offer only partial protection and major landslide systems have developed on the rapidly eroding soft-rock coast (Photo 5).

Distinctly differing coastal systems exist either side of West Bay. To the east, the Chesil Beach shingle barrier structure represents accumulation over at least the past 7,000 years (Carr and Blackley, 1973; 1974). Although the origin of the beach is not understood fully, current opinion is that it is a relict accumulation at a comparatively late stage of evolution (Carr, 1980a; Bray, 1996, 1997a). Investigations indicate that the beach receives no significant contemporary supply and is a closed sediment circulation system, which displays relatively stable form, although subject to occasional overtopping, overwashing and slow recession landwards (Carr and Blackley, 1974; Carr and Seaward, 1991). By contrast, the coast to the west is an actively evolving highly dynamic system characterised by continuing sediment inputs from major mass movement complexes (Brunsden and Jones, 1976, 1980; Bray, 1996; High-Point Rendel, 1997a). However, despite input, the beaches are relatively small indicating rapid sediment redistribution by littoral transport mechanisms (Bray, 1996; Bird, 1989).

In contrast to the highly varied nature of the shoreline, the offshore zone comprises the relatively shallow and featureless embayment of Lyme Bay (Darton, Dingwall and McCann, 1980). The inshore zone is more varied as revealed by sampling and geomorphological mapping studies off Lyme Regis (Badman, et al., 2000; Brunsden, 2002), Seatown (Bray, 1996), West Bay (High-Point Rendel, 1997; 2000a) and around Portland Bill (Bastos and Collins, 2002a and 2002b). The beach face of Chesil slopes steeply seaward along the eastern and central parts, but more gently westward towards West Bay (Babtie Group, 1997). West of this point, the inshore zone is relatively shallow due to intermittent development of limestone/marl ledges and shore platforms. Relict boulder aprons are present extending several kilometres seaward of Thorncombe Beacon and Golden Cap (Bray, 1996) and buried river/stream channels have been identified off Lyme Regis (Brunsden, 2002).

Human interference involving provision of coastal and harbour defences and historical beach mining is concentrated at Chiswell (Photo 6), West Bay (Photo 7) and Lyme Regis (Photo 8) with most of the intervening areas remaining in a “natural” condition. In spite of their limited extent, the structures and practices are believed to have exerted important effects upon shoreline evolution over the past 200-300 years. Harbour structures have intercepted drift at West Bay (High-Point Rendel, 1997) and Lyme Regis (West Dorset District Council, 2000a, 2000b; Brunsden, 2002) leading to beach depletion and historical beach and foreshore mining has removed significant quantities of material from Lyme, Seatown and Chesil Beaches (Brunsden, 2002; Bray, 1996; Carr, 1980a). Recent management has been more strategic in approach and has been based upon extensive preliminary studies to develop necessary understanding prior to the design and preparation of schemes. Studies include the Lyme Bay Shoreline Management Plan (Posford Duvivier, 1998a), Chesil Beach and Chiswell investigations (Babtie Group, 1997; Posford Duvivier, 1998b) and a series of studies relating to coastal defence, harbour and environmental improvements at Lyme Regis and West Bay where a Beach Management Plan has been prepared (Posford Duvivier and HR Wallingford, 2001; West Dorset District Council, 2012).

The following internet links offer further information about various aspects of this coastline:

Wessex Coast Geology by Ian West: www.soton.ac.uk/~imw/index.htm
Jurassic Coast: www.jurassiccoast.org
Dorset Coast Forum: www.dorsetforyou.gov.uk/dorset-coast-forum
The Fleet Study Group: www.fleetandchesilreserve.org/the-fleet-study-group

A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org. The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is Teignbridge District Council, with data collection, analysis and reporting led by a specialist team at Plymouth Coastal Observatory (PCO). 

In 2008, an extensive high resolution, 100% coverage swath bathymetry dataset, known as the Dorset Integrated Survey (DORIS), was collected by the Southeast Regional Coastal Monitoring program in partnership with Dorset Wildlife Trust, The Maritime and Coastguard Agency, The Royal Navy and Viridor Credits. This survey extended from the western end of the Fleet lagoon to Handfast Point and 20km offshore from MLWS. A subsequent swath bathymetry survey was collected in 2009, through the Southwest Coastal Monitoring Programme, extending offshore 1km from MLW between Petit Tor Point and Portland Bill.

1.2 Chesil Beach

Chesil Beach is an internationally renowned feature of geomorphological and biological interest (Carr, 1983a). It has been the subject of much research, discussion and conjecture and is regarded widely as one of the most important coastal geomorphological sites in Britain (May and Hansom, 2003). Lidar data is captured for the full length of the beach through the Southwest Regional Coastal Monitoring Programme, which aids further understanding of processes and morphological behaviour. It is interesting to note that despite the high level of interest, there has not been any other, more site specific detailed monitoring undertaken. Prior to discussion of its contemporary sediment dynamics within the main parts of this report, mention must be made of its known characteristics and existing ideas relating to sources of material and mode of origin.

Morphology and Sedimentology

Chesil Beach is one of three major coarse clastic (shingle/gravel) structures on the British coast and is unique in being a linear barrier beach whilst the others (Dungeness and Orford Ness) exhibit cuspate development. The piers at West Bay represent the present western terminus of Chesil Beach, but prior to change from open to closed, impermeable construction in the 1820s (Keystone Historic Buildings Consultants, 1997), Eype Beach would have been part of the same sediment transport system (Prior, 1919; Carr, 1980; Bray, 1996, 1997a). Other work is beginning to suggest that the beach could once have extended as far west as Beer Head or Otterton Ledge (Brunsden, 2002). Subsequent erosion to the west of the West Bay piers caused marked “setback” (Photo 9) and separation from the Chesil Beach system (Jolliffe, 1979; High-Point Rendel, 1997; Brunsden and Moore, 1999). In the east, the beach terminates against the cliffs of the Isle of Portland. The shingle beach is joined to the mainland at the eastern and western ends, but for 13 km is backed by the shallow, tidal Fleet Lagoon (Photo 1). At its eastern extremity the beach forms the dominant western portion of the tombolo structure that attaches the Isle of Portland to the mainland. It is backed by Portland harbour and the town of Chiswell. West of Abbotsbury extending to Cogden Beach, the shingle beach fronts a narrow low-lying hinterland that rises steeply inland (Photo 10). Between Burton Bradstock and West Bay, recession of the beach has intersected elevated topography resulting in the formation of steep cliffs fringed by a rather narrower portion of the beach (Photo 11 and Photo 12).

The visible pebble beach consists of about 98.5% chert and flint, the remainder being pink quartzite, quartz, locally derived limestone and occasional pebbles of igneous or metamorphic rock (Carr and Blackley, 1969). Major pebble sampling programmes in the 1960s (Carr, 1969) and in 1993 (Babtie Group, 1997) have demonstrated that pebble size above low water mark increases systematically from Cogden or West Bexington in the west to Chiswell in the east, but with some inconsistencies at Wyke Regis (Photo 13) and to the west of Cogden. Beach face gradient increases eastward in accordance with the increase in pebble size in this direction. It should be noted that exact comparison between the earlier and later studies has not been possible due to use of different sampling and measurement techniques, but results suggest the broad pattern of grading has been maintained during the period between the two studies.

Below low water mark, longshore size grading is less clear and shingle is coarser and less well sorted than on the intertidal beach (Neate, 1967). On the subtidal beach, coarser pebbles are generally recorded to the east of Abbotsbury with sands predominating to the west (Babtie Group, 1997). The shingle beach was shown to extend out on the seabed down to around –20mCD at Chiswell and –10mCD at West Bay although the “active “ shingle extends only to around 50% of these respective depths. On the basis of boreholes drilled through the beach down to bedrock (Carr and Blackley, 1973), beach shingle volume was estimated at between 25 and 100 million tonnes (Carr, 1980a). The range of values reflected the uneven distribution of borehole information and the uncertain shingle volume below low water mark due to distribution in limited discontinuous horizons. Researchers have identified this wide range of volumes as a major uncertainty in understanding of the beach (Bray, 1997b).

The crest of Chesil is continuous over most of its length (irregularities exist in the Burton Bradstock area where the beach is backed by sandstone cliffs – see Photo 12 and generally increases in height from west (an average of 6.0mOD) to east (an average of 14.7mOD). Comparisons of crest height and position between an early survey of 1853 and 1993 have revealed variable trends, but with net crest recession inland of 8m to 17m in eastern parts (Carr and Seaward, 1991) accompanied by lowering of 0.5m to 2m (Carr and Gleason, 1972; Carr, 1983b; Carr and Seaward, 1990, Babtie Group, 1997). By contrast, the crest to the west of Wyke Regis either has remained stable in net position or has increased in height by up to 1.5m (Babtie Group, 1997). An important conclusion drawn from the more recent studies is that the beach crest is only sensitive to changes during the most severe storms and/or swell wave events and that the beach to the east of Wyke Regis appears to be more sensitive than that to the west. Further detailed descriptions of the beach and comprehensive lists of references to previous research are provided by Carr and Blackley (1974); Jolliffe (1979); Carr (1980a); Bray (1996) Babtie Group (1997) and May and Hansom (2003).

Geomorphological Development

Beach clast lithology has led most authors to postulate a sediment source to the west, from Devon (Fitzroy, 1853, Baden-Powell, 1930). Such theories implied that transport was eastward from the source areas by littoral drift along former shorelines (de la Beche, 1830; Coode, 1853; Rennie, 1853; Pengelly, 1870 and Arkell, 1947), or offshore across Lyme Bay (Strahan, 1898). By contrast, Prestwich (1875) suggested northwestward transport from a precursor of the Portland raised beach situated south of the Isle of Portland. Prior (1919) suggested three possible sources: (i) a precursor of Chesil Beach stretching from Start Point, Devon to Portland; (ii) erosion of the east Devon coast: and (iii) river gravels deposited in Lyme Bay by a previously extensive “Fleet River”. This last possibility was supported by Bond (1951) who reconstructed an ancient Exe-Teign river flowing eastward up to 10 km offshore of the present coast with a mouth south east of Portland. Other possibilities such as gravel rafting by ice have also been suggested to account for clasts of exotic lithology (Arkell, 1947).

The most comprehensive study of Chesil pebble lithology was undertaken by Carr and Blackley (1969), who concluded that all types could be related to either local sources or to existing sites in south west England. The over-riding opinion of previous work therefore supported the idea of supply from the west and recent quantitative study by Bray (1996; 1997) has begun to place these ideas into a more definitive context. Re-evaluation of borehole data documented by Carr and Blackley (1973) and a new set of cored data from the Fleet Lagoon (Coombe, 1998) suggest that the present shingle accumulation of Chesil rests upon a predominantly sandy and shelly foundation. These developments have enabled construction of the following chronological sequence, based initially on Carr and Blackley (1973), but with modifications by Bray (1990a, 1990b, 1996, 1997a, 1997b), High-Point Rendel (1997) and Brunsden (1999):

1. The initial forerunner of Chesil probably existed as a bank well offshore of the present beach some 120,000 years before present (BP). This bank was contemporaneous with the second development of the Portland raised beach. It is uncertain whether this beach would have stretched across the full extent of Lyme Bay, because raised shorelines have been identified at Hope’s Nose and near to Start Point, but substantial associated raised gravel deposits have yet to be identified;
2. During the last glacial period (Devensian) when sea level was up to 120 m lower than at present, a series of sand and gravel deposits accumulated on what is now the floor of Lyme Bay. These probably comprised material from the Portland raised beach, solifluction deposits, river gravels and fluvio-glacial deposits laid down on the floor of Lyme Bay by meltwaters at the end of the Devensian;
3. Formation of the present Chesil Beach began at the end of the Devensian (20,000-14,000 years BP) when rapidly rising sea-level caused erosion of these deposits and wave action drove the sands and gravels onshore as a barrier beach;
4. Close to the land, the beach overrode existing sediments and the Fleet Lagoon was rapidly filled with silt, sand, peat and pebbles. Dating of peat samples retrieved from boreholes suggest that infilling began about 7,000 BP and was virtually complete by 5,000 BP. Such deposition requires shelter, so a significant barrier must have existed at this time indicating that Chesil Beach had formed at or slightly seaward of its present position by 4000-5000 BP when sea level approached its present elevation. Cores described by Coombe (1998) suggest that the initial Chesil Beach was predominantly sandy rather than gravel-rich, with layers of shells and coarser materials indicative possibly of intervals of overwashing;
5. Relict cliffs abandoned in East Devon and West Dorset by falling sea levels in the early Devensian were re-activated around 4,000-5,000 years BP by marine erosion and supplied large quantities of gravel to the shore. Material is believed to have been yielded initially from the reworking of extensive debris aprons located at the cliff toes with erosion cutting into insitu lithologies only in more recent millennia (Brunsden, 1999). Detailed budget and sedimentological analysis indicates that some 30-60 million cubic metres of gravel could have been supplied from these sources (Bray, 1996, 1997a; High-Point Rendel, 1997);  
6. Much of the cliff gravels supplied to the shore are believed to have drifted to the east via a series of pocket beaches (Charmouth, Seatown and Eype) regulated by alternate “open” and “closed” transport at headlands eventually to nourish and enlarge the prototype sandy Chesil Beach which would have acted as the sink for this material (Bray, 1996, 1997a, 1997b; Brunsden, 1999);
7. Coastal recession and human interventions over the past 500 years appear to have depleted the beaches to the west of West Bay and resulted in increasing prominence of headlands. It has reinforced the pocket beaches as a series of distinct sub-cells leading to dislocation of the gravel transport pathway towards Chesil. The beach must now be regarded as a closed shingle system of finite volume and is likely to be sensitive to future environmental changes e.g. sea-level rise.

This evolutionary sequence suggests that Chesil initially formed mainly from pre-existing predominantly sandy deposits in Lyme Bay swept onshore as the beach migrated landward and sea level rose. Although sufficient to provide shelter for formation of the depositional environment of the Fleet Lagoon, this supply must have virtually ceased 4,000-5,000 years ago when landward recession slowed. At this point, the debris aprons and cliffs to the west started to release gravels as they were eroded by rising sea-levels. Thus, it is postulated that Chesil would have experienced a rapid influx of gravels only relatively recently (less than 4,000 years BP) which would have nourished the original sand-dominated barrier leading to development of the present shingle barrier. This sequence of events has been incorporated within an animated model to demonstrate the evolution of the beach (Edmonds, et al., 2004).

1.3 Hydraulic Regime

Knowledge of wave conditions along this coastline has increased considerably over the past 10 years through a variety of regional studies (Posford Duvivier, 1998a; Halcrow, 2002; and Bastos and Collins, 2002), together with site-focused studies at Chesil (Babtie Group, 1997), Lyme Regis (HR Wallingford, 1997a; 2001) and West Bay (HR Wallingford, 1997d) that have involved collection and analyses of new field measurements as well several longer-term hindcast data sets. Wave exposure increases eastward from Lyme Regis to Portland Bill and locations east of Abbotsbury are exposed to swell waves arriving from the northeast Atlantic.

The Southwest Regional Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The buoy is deployed at West Bay in 10mCD water depth. Between 2006 and 2012 the prevailing wave direction is southwest-by-south with an average 10% significant wave height exceedance is 1.74m (CCO, 2012).

The Chesil Beach site is exposed and influenced by large waves generated in the Atlantic: West Bay from directions between 215 and 224 degrees; Abbotsbury, between 218 degrees and 232 degrees; and Chiswell between 220 and 240 degrees. Portland wind records show that these directions coincide with prevailing wind direction for winds greater than 17 knots (230-250 degrees) and indicate the directions from which the largest waves arrive. The beach is subject to two distinct types of waves: storm waves generated by local winds blowing across the English Channel and Lyme Bay and swell waves that penetrate into the eastern parts of Lyme Bay from the north-east Atlantic. Exposure to both types of waves increases from West Bay eastward to Chiswell.

Automatic wave measurements are first reported for 1970/71 at West Bexington and Wyke Regis (Hardcastle and King, 1972), a pressure-sensing instrument was operated for several months from November 1979 at Chiswell (Dobbie and Partners, 1980) and the Proudman Oceanographic Laboratory (on behalf of the Environment Agency and its predecessors) have operated a wave rider buoy off West Bexington since the early 1980s. A directional wave rider buoy was operated from September 1993 to April 1994 offshore in Lyme Bay and a pressure gauge was operated off Wyke Regis from February 1994 to May 1995 (Babtie Group, 1997). Results from all available datasets were analysed by Babtie Group (1997) who found that typical offshore peak heights in Lyme Bay were 4.0 to 4.8m for waves approaching from 195 to 225 degrees and up to 4m for waves approaching from 105 to 190 degrees. Hindcast as well as measured wave data were analysed to determine a likely offshore extreme wave height of 7.25m for a 1 in 100 year event with corresponding inshore values (1 in 100yr) of 6.5 and 6.65m at West Bexington and Wyke Regis respectively. A likely offshore extreme wave height of 4.5m for a 1 in 100 year event was calculated for swell waves. A novel parabolic numerical model developed by Babtie Group (1997) was applied to further investigate swell waves. It was found that these waves were extremely sensitive to the bathymetry of Lyme Bay where features in water depths of up to 50m could affect transformation processes as the waves travel inshore. A depression and mound located well offshore on the bed of Lyme Bay appear to focus the waves upon specific sections of the coastline at Portland Bill, Wyke Regis and Abbotsbury. Diffraction was also an important process as the waves enter through a narrow “window” from the northeast Atlantic and some are intercepted by Start Point. Results suggested that swell waves were only a significant phenomenon on Chesil to the east of West Bexington, whereas the shore to the west was sheltered from them. It should be noted that the analyses were undertaken using significant wave heights of 2m and 4m with periods of 15 to 25 seconds.

There are several records of “unusual” swell waves that have affected eastern parts of the beach. An event of this type overtopped the beach crest in February 1979. Local wave data are not available, but an offshore significant wave height of 7m and a period of 18 seconds was recorded 120 miles off the Isles of Scilly (Draper and Bownass, 1982). The crest of Chesil was overtopped and it can be postulated that this type of event may be a major factor in beach recession. A less extreme event of this type occurred on 8th March 2003 when it reprofiled the seaward crest face, exposing consolidated substrata and bedrock clay (Moxom, 2003). Frequency of major events is undoubtedly low, but difficult to determine due to scarcity of historical information and poor understanding of the formative conditions. Return periods of 1 in 50 years and 1 in 100 years, or more, were estimated by Dobbie and Partners (1980) and Draper and Bownass (1983) respectively, but there are too few records for reliable analysis. Other events affecting this coast are described by Dawson, et al., (2000) and would appear to represent tsunami generated by seabed earthquakes or submarine landslides in the Atlantic and around its shores that travel up the English Channel and impact upon Chesil. Several events are described in which high waves apparently arose out of an otherwise calm sea achieving heights of 2m to 9m with periods of up to 10 minutes. Eastern parts of Chesil directly facing the northeast Atlantic are especially exposed to such waves.

Offshore wave climates for West Bay have been determined by hindcasting based on Portland wind data covering 1974-84 (Hydraulics Research, 1985) and 1974-90 (Hydraulics Research, 1991d). These studies showed that prevailing wave direction was from the south-west, but that directional distribution was subject to significant change after 1982 with markedly fewer south-easterly storms and a higher proportion of west and south-west waves. It must be concluded that with existing information it has proved difficult to define a reliable wave climate, because of this variability and this site is thus understood to be highly sensitive to any future changes in wave direction (Brampton, 1993). Extreme wave heights of 5.4m and 6.4m were calculated for the 1 in 10 and 1 in 100 year return periods, respectively (HR Wallingford, 2000a; 2000b). Analysis of local wave heights has suggested an increase of some 4mm per year in recent decades, although there is much scatter in the data (HR Wallingford, 1998a).

The Lyme Regis wave climate is less severe than for West Bay or Chesil due to greater shelter from westerly and south-westerly waves afforded by Start Point. Wave climate studies using hindcasting techniques applied to Portland wind data by Hydraulics Research (1987), and Posford Duvivier (1990) and Portland data (1974-1992) supplemented with Met Office European Wave Model data (1992-2000) by HR Wallingford (2001) revealed that SW and WSW waves were most frequent. Inshore wave conditions were difficult to model due to diffraction, refraction and shoaling over the shallow nearshore bathymetry with numerous ledges and reefs. Maximum wave height was found to be limited to around 4.5 m by shallow water inshore (Hydrualics Research, 1987), although some studies have estimated potential nearshore maximum wave heights at points of maximum exposure of up to 6m (Posford Duvivier, 1991; HR Wallingford, 2001). The study by HR Wallingford (2001) sets out detailed wave climates together with assessments of extreme waves and storm surge levels for some 15 shoreline and nearshore points around Lyme Regis taking into account the potential depth limitations. Waves along much of the Lyme Regis to West Bay shoreline are also likely to be depth limited in this manner, for much of the shallow inshore zone is characterised by intermittent limestone ledges and boulder aprons.

Tidal currents alone are too slow to be a significant transport mechanism in the inshore area of Lyme Bay, although powerful currents are focused around Portland Bill (Photo 14), where there is a significant potential for sediment transport. Comprehensive metering and drogue float studies off Lyme Regis and Charmouth showed that surface currents only exceeded 0.36ms-¹ for 1% of the time with a maximum of 0.45ms-¹ (South-West Water, 1979; Offshore Environmental Systems Ltd., 1980). Bottom currents off Lyme Regis did not exceed 0.3 ms-¹ (Posford Duvivier, 1990). Residual tidal circulation is clockwise in Lyme Bay with inshore eastward flow parallel to the coast and net onshore and/or westward flow further offshore (Pingree and Maddock, 1977a). Application of the TELEMAC2-D numerical tidal model validated against measured tidal current profiles indicated peak currents of up to 3ms-¹ flowing towards Portland Bill and strong residual offshore flow at the headland. A dominant clockwise eddy which changed its location, size and strength during the tidal cycle was identified to the south-west of Portland Bill. These features are mapped and documented in detail by Bastos and Collins (2002). A tendency for weak onshore flow was suggested in the central part of Lyme Bay (Pingree and Maddock, 1977b, 1983).

Chesil was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point off Chiswell at –4.1mOD. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 3-7% variation in longshore energy and confirmed that the beach was significantly more sensitive to this factor than most other south coast locations, as might be expected of a swash aligned coastline. Wave energy was also found to be especially sensitive to sea-level rise. The effect is probably due to a reduction in shoaling and wave refraction within the shallow nearshore bed as water depths increase so that slightly higher waves will approach the shoreline at rather more oblique angles. These results accord with those of Brampton (1993) who noted that net drift at West Bay was extremely variable in direction and was highly sensitive to small changes in the directional wave climate. This phenomenon was studied in further detail by Halcrow Maritime, et al., (2001) who undertook modelling of likely future wind speeds for a climate change scenario representing 2080 using the Met Office Hadley Centre Regional Climate Model. The wind speeds output by the model were used to derive offshore extreme wave conditions in Lyme Bay and results demonstrated a potential for significant increases in wave energy e.g. 1 in 50 year wave height of 8.1m could increase to 11.3m by 2080s. The hindcast waves have also been used to study the potential changes in alongshore sediment transport. Results for the east end of Chesil Beach indicate a potential for a dramatic shift in the sediment transport regime due to a small two-degree shift in the mean wave approach direction. The current net westward drift potential of 900m³ per year would under this scenario alter to a net westward drift of up to 15,000m³ per year. A similar type of study was undertaken by Sutherland and Wolf (2002) who simulated drift on Chesil Beach up to the year 2075. Results suggested that net drift could in future increase by up to 30% due to the potential effects of climate change. Results of the two studies differed due to use of different climate model outputs and application of alternative hydrodynamic and sediment transport models and calibrations; however, they both suggest that (i) waves in Lyme Bay are likely to vary with future climate change and (ii) Chesil Beach is likely to be sensitive to these changes.

The shoreline under review is exposed to modest storm surges that travel up the English Channel driven by cyclonic weather systems that approach from the northeast Atlantic. Analyses of historical tide gauge records for Portland and Devonport with appropriate adjustments for local tidal levels reveal extreme 1 in 100 year sea-levels of 2.72mOD for Chesil at Chiswell (Babtie Group, 1997; Posford Duvivier, 1998) and 3.08mOD for West Bay (HR Wallingford, 2000a). A value of 2.8mOD is reported for the 1 in 25 year extreme level at the Cobb, Lyme Regis (High-Point Rendel, 1999a).

2. Sediment Inputs

2.1 Offshore to Onshore Feed

The majority of beaches in the study area are composed almost entirely of shingle (Carr and Blackley, 1969; Bird, 1989; Bray, 1996), whereas the nearshore is characterised by thin spreads of medium sand separated by exposed bedrock ledges and boulder aprons. Due to their critical role in coast protection/sea defence and a history of mining losses, many studies have been undertaken to examine the possibility of natural beach replenishment by onshore feed. Without exception, all studies failed to locate suitable source materials and onshore transport pathways. The evidence is reviewed below:

1. Underwater investigations by Coode (1853) and Nature Conservancy (Neate, 1967) indicated that the Chesil Beach shingle accumulation did not extend very far seaward of low water mark. The pebbles of the outer zone of Chesil Beach become replaced by sand and silt at an average distance of 100m seaward of LWM (Neate, 1967). This information is quite reliable because several diving surveys and re-sampling operations were undertaken over the period 1960-63 and shingle of the outer zone was consistently found to be weed/barnacle encrusted and immobile. The area was revisited by diving surveys in 1993 that broadly confirmed the earlier results and indicated that shingle extends seaward to around –10mCD (western parts) and up to –20mCD in eastern parts (Babtie Group, 1997). A distinct boundary between clean mobile pebbles and barnacle and weed encrusted immobile material was identified at –5mOD at West Bay and –10mOD at off Chiswell. Significant movement of pebbles seaward of these points some 150 to 300m seaward of the beach was not considered likely;
2. In 1970 the Nature Conservancy examined the area immediately offshore of Chesil by echo sounder and reported little unconsolidated material above the solid geology (Carr, 1980a). Hydrographic surveys undertaken in 1996 also revealed little coarse material further offshore and comparison of this survey with charts of 1808, 1903 and 1987 revealed that the bed had remained relatively stable over this period;
3. Diving investigations off Seatown Beach revealed three distinct areas within the nearshore zone. First, the permanently submerged part of the beach face which extends 21-30m offshore. Second, a flat mudstone shelf covered by occasional limestone boulders, patches of rippled sand and patches of clean gravel. Third, an area beginning 30-60m offshore and extending beyond the end of the transects, composed of large limestone boulders, together with patches of muddy gravel and beds of kelp. Gravel from the beach appears free to move within the first two areas, however, the third area probably acts as a barrier, preventing the loss of beach gravel to offshore areas and preventing gravel supply to the beach from offshore, thus Seatown Beach is isolated from the offshore zone (Bray, 1986, 1996);
4. Boreholes offshore of Portland/Chesilton by consulting engineers for the CEGB and Dredging Investigations Ltd for John Taylor and Sons, consulting engineers to Wessex Water Authority revealed strictly limited quantities of coarse sediment (Carr, 1980a);
5. Geophysical surveys in Lyme Bay revealed only very thin sandy superficial sediment cover over the sea-bed (IGS, 1974; Darton, Dingwall and McCann, 1980). Further studies undertaken for Kerr McGee Oil by Ambios Environmental Consultants (1995) identified a thin sheet of gravel and sand in water depths of greater than 10m at around 1km offshore of West Bay and Cogden beach;
6. Survey extending over 1km offshore from Lyme Regis was undertaken using echo-sounding, grab sampling, vibrocoring and sub-bottom profiling. Superficial sediments were generally sands and muds frequently overlying thin clayey gravel deposits up to 0.3m thickness. Superficial cover was generally less than 1m in total and quantities of gravel located were extremely limited (Offshore Environmental Systems Limited, 1980). Recent studies using similar methods supplemented by diver and video inspections revealed a complex bed morphology of exposed bedrock, shore platforms and two shallow channels infilled with fine sand overlying thin gravels (Badman, et al., 2000; Brunsden, 2002). Some transport of bed sands was indicated, but primarily in eastward and offshore directions;
7. Sidescan sonar and sediment sampling survey off the west coast of Portland and at several carefully selected locations in Lyme Bay failed to locate any substantial shingle deposits (Dobbie and Partners, 1981);
8. Survey extending 4km off Seatown Beach located small quantities of shingle, some obviously immobile in water > 18m depth, the remainder trapped in pockets amongst dense boulder aprons. Large areas of seafloor were sand/silt dominated and devoid of gravel (Bray, 1986; 1996);
9. Hydrographic survey undertaken off West Bay in 1999 (High-Point Rendel, 2000) primarily identified two extensive, but thin sand sheets separated by a zone of bare seabed with scattered boulders. It was concluded that very little gravel could be identified on the nearshore bed, although seasonal or storm related exchanges with the beach were not discounted.

Analysis of Coastal Monitoring Programme data provides no evidence of onshore gravel feed and therefore, the speculative 2004 arrows have been removed.

2.2 Cliff and Coastal Slope Erosion

» E1 · E2 · E3 · E4 · E5 · E6 · E7 · E8 · E9 · E10 · E11 · E12 · E13

Publications by Brunsden and Goudie (1997); Allison (1992a; 1999a); Bray (1996 and 1997) and May and Hansom (2003) provide general reviews of the morphology and recession of the cliffs of this coastline. A series of papers referred to within the succeeding titled sections provide specific details of the cliff behaviour recorded at individual sites.

Topography is dominated by four prominent flat-topped NE-SW oriented ridges, which rise to between 150 and 200m. Truncated by coastal erosion, these ridges form the high, cliff landslide complexes of Black Ven (Photo 15), Stonebarrow Hill (Photo 16), Golden Cap (Photo 17) and Thorncombe Beacon (Photo 18). The elevation of intervening areas has been reduced by fluvial incision, so that marine erosion cuts lower cliffs across a series of steep-sided narrow valleys mantled by superficial spreads of landslide debris, scree and "head" (Brunsden and Jones, 1972). The valleys of the Lym, Char, Winniford, Eype and Brit rivers descend to sea level. Three smaller valleys immediately east of Stonebarrow have hanging terminations on sea-cliffs, 20m to 30m high.

The coastal geology between West Bay and Lyme Regis comprises Liassic strata, mainly shales, marls and mudstones that weather to soft clays (House, 1993). The main hills (Black Ven, Stonebarrow and Golden Cap) are capped by poorly consolidated, sandy, Lower Cretaceous sediments. These are important sources of sediment to the coast, particularly the durable shingle-forming materials from the Chert Beds (Photo 19) and the superficial deposits which mantle the entire coastal area (Bray, 1986; 1996).

Continuous marine erosion on a soft rock coast of relatively high relief has caused much slope instability leading to landsliding. The Cretaceous strata are permeable and introduce water to the coastal slope at the line of unconformity with the impermeable Liassic clays. When wet, the Liassic clays are liable to flow or slide. These features accelerate coastal landsliding, causing rapid rates of retreat (Conway, 1974; Brunsden and Jones, 1976, 1980; Brunsden and Goudie, 1997; Bray, 1986, 1996; Allison, 1990, 1992).

Coastal landsliding results in the formation of coastal landslide complexes, in which the backscar is separated from the beach by zones of degradation and material transport that vary between location from several tens of metres to several hundred metres in width, e.g. (Photo 15 and Photo 16). These have been the subject of intensive research, notably by Brunsden (1974); Brunsden and Jones (1976, 1980); Chandler and Brunsden (1995) and Brunsden and Chandler (1996).

The backscar retreats cyclically with short episodes of rapid retreat by rotational sliding, separated by prolonged phases of slow retreat by small scale landslide and weathering processes. The material released is passed through the system (throughput) in a more steady manner so as to give a relatively regular supply to the beach. Brunsden and Jones (1976) estimate the throughput time for the Stonebarrow landslide complex to be between 100 and 150 years and Bray (1996) estimates shorter periods for some of the other complexes. Correlation of incidences of landsliding with climatic records has led Ibsen and Brunsden (1997) to postulate a general periodicity of landslide intensity of some 30-50 years arising from landslide responses to sequences of unusually wet years. Despite such irregularities, Brunsden and Jones (1976, 1980) considered that mean retreat was relatively constant over a 100-year period at Stonebarrow. This concept was broadly applied to other landslide complexes in West Dorset to establish representative long-term retreat rates (Bray, 1986; 1996). Cliff top and sea-cliff recession rates were calculated over the period 1901-1988 using Ordnance Survey large-scale map comparisons, air photos and a cliff top survey in 1985 and 1988 (Bray, 1986; 1996). Retreat rates were combined with information relating to cliff geology and cliff erosion sediment input was determined for the coast between Lyme Regis and West Bay. Details of the solid geology were obtained from existing literature (Wilson, et al., 1958; House, 1993). The gravel component of this input, critical to local beaches, was primarily from Chert Beds and superficial deposits at the cliff top. To determine accurately the nature and distribution of input, these deposits were subject to detailed field section mapping and comprehensive sampling (Bray, 1986; 1996). These techniques yielded overall supply rates for all materials (Photo 20) and detailed gravel (>2mm) supply rates for each coastal segment.

This analysis showed that large quantities of sediment are released in the western part of the study area between Lyme Regis and Charmouth, a result of rapid coastal retreat along this segment (Photo 15). Chert and flint gravel supply is small by comparison to that of all sediment types, but they comprise the dominant beach material because of their greater resistance to abrasion. Sand, silt and clay are easily eroded and transported offshore. The limestones form beach pebbles, but these are short-lived due to attrition and only form a small proportion of beach volume (Bray, 1996). Thus, not only are rates of coast erosion input highly variable but the nature of materials yielded can alter, with some materials (e.g. chert and flint) having greater beach-forming potential. Assuming that recent increases in retreat will persist or intensify due to future sea-level rise and climate change, total material yield could increase by up to 61% in the future to over 500,000m³ per year and gravel yield is likely to double to 12,000m³ per year (Bray, 1996; Bray and Hooke, 1997). The dynamics of recession and supply are therefore described in greater detail according to location in the following sections.

Similar rates of cliff input are reported by Posford Duvivier (1997; 1999) who also calculated sediment yields resulting from erosion of shore platforms and the nearshore bed. Inputs from these latter sources of 20,000m³ per year were estimated from Portland Bill to West Bay and 55,000m³ per year for West Bay to Lyme Regis, entirely comprising fine material.

E1 East Devon (see introduction to cliff and coastal slope erosion)

Large-scale and long-continued active landsliding between Axmouth and Pinhay provides a substantial supply of sediment, comprising gravel, sand and clay, to the littoral transport zone as described by Pitts (1981a and 1981b; 1983). Bray (1996) estimates that some 12,000m³ per year of gravel could be delivered to the shore from this section of coast. High-Point Rendel Geotechnics (1997) estimated total sediment inputs of 200,000m³ per year and shingle input of 7,500m³ per year. Cliff recession is not evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and 2012 aerial photography. The reliability of information supporting the revised arrow has been reduced to low compared to the medium level in 2004.

E2 Lyme Regis and The Spittles (see introduction to cliff and coastal slope erosion)

The site of Lyme Regis comprises the valley of the river Lim that has been truncated by marine erosion. Ancient landslides are evident on the inland valley slopes with more recent failures being associated with the cliffs cut into the Liassic clays, siltstones and limestones by marine erosion. Steep sea-cliffs of 5m to 25m are backed by sequences of scarps and benches, the latter being defined by the outcrops of more resistant limestone seams. The slopes have evolved by compound translational (and rotational) failures with material being shunted seaward across benches by mudslides. Marine undercutting and removal of fallen debris at the toe have maintained instability and failures have been triggered by increases in groundwater introduced from upslope and accumulating within the landslide debris. Some deeper-seated slides are also identified having basal shear surfaces that extend beneath the beach. Defences now protect the cliffs of the main urban frontage, significantly reducing the volume of cliff input to the rock platform and foreshore, but instability remains on the slopes above and renewed failures can be triggered during increasingly frequent wet winters (Lee, 1992; West Dorset District Council, 2005). Cliff recession on the undefended sections is evident from analysis of the Coastal Monitoring Programme 2007 and 2011 lidar and 2012 aerial photography data, with less than 1,000m³ per year of cliff derived input, a reduction from the 2004 estimated volume of more than 20,000m³ per year.

A comprehensive series of investigations have been undertaken as part of the Lyme Regis Environmental Improvements Scheme developed by West Dorset District Council (High-Point Rendel, 1999a; 1999b; West Dorset District Council, 2000; Brunsden, 2002). The work includes detailed mapping, ground investigations, and establishment of a monitoring and data-handling network (Brunsden, 2002; Davis, et al., 2002). Results have included preparation of a series of ground models for the main slide units identified (see Brunsden, 2002).

Sea walls have been present at Lyme Regis since before 1789 (Posford Duvivier, 1990; 1991), but protection has not been continuous and different sections were protected at different times. East Cliff and Church Cliffs (Photo 21) were subject to continuous recession at 0.45m per year over the period 1841-1903 (Geotechnical Consulting Group, 1987) and at variable rates (0.1-0.6m per year) thereafter until 1957 when a new sea wall was constructed. Brunsden quotes rates of 0.47 to 0.80m per year (past 150 years) for East Cliff and 1.3m per year (1888-1957) for Church Cliffs. Quarrying of limestone ledges on the foreshore at Church Cliffs in the 19th century reduced protection from wave attack and may have contributed to marine erosion at this site (Hutchinson, 1984). Sea wall protection is now complete to the west of East Cliff and coast erosion input is prevented except on occasions when ancient landslides are reactivated and material may surge seaward across the sea wall e.g. Cliff House slide of 1962 (Arber, 1973; Pitts, 1979; Posford Duvivier, 1990; Lee, 1992). Coastal geology at Lyme Regis is generally Liassic clays and limestones so local cliff erosion and landslides have only a low potential here to supply materials that can contribute to the beach (Bray, 1986).

The Spittles (Photo 22), immediately to the east of Lyme Regis comprises a series of ancient landslides beneath Timber Hill on the western flank of Black Ven that are interpreted by Brunsden (2002) as being remnants of once more extensive periglacial landslide slopes. A marked northwestward shift of activity is recognised, extending from Black Ven towards the A3052 in eastern Lyme Regis and upslope towards the vegetated presently inactive landslide scarp of Timber Hill. Retreat of 0.9m per year was estimated for 1841-1901 (Geotechnical Consulting Group, 1987) and subsequent map, air photo and ground survey comparisons showed retreat at up to 2.5m per year for 1901-60 accelerating to 8m per year for 1960-88 (Bray, 1996). It appears that marine erosion of the sea-cliffs is reactivating a series of ancient mudslides and translational slides, creating a scarp and bench topography that is retrogressing inland. Mapping studies by Brunsden and Chandler (1996) and Brunsden (2002) indicate that this trend is continuing such that fresh landslides could soon be triggered within the Upper Greensand strata of Timber Hill as suggested by Bray (1986). Other work by Gibson, et al., (1999) involving comparisons of photogrametrically-derived digital ground models demonstrates en-mass lowering and seaward displacement of the Charmouth Road car park from 1992 to 1996. This has followed loss of support to seaward due to encroachment of active landslides from the Spittles and those reactivating inland from East Cliff.

Significant quantities of sediment are supplied to the shore from the Spittles, as indicated by a major landslide over the sea cliffs that deposited a large debris lobe on the foreshore in 1988. However, the proportion of gravel yielded was small (370-445m³ per year for 1901-88) although it could increase significantly once the Upper Greensand scarp of Timber Hill is reactivated (Bray, 1996; 1997).

E3 Black Ven (see introduction to cliff and coastal slope erosion)

Black Ven is one of the largest and most active coastal landslides in Europe (Photo 15). The mass movement system is characterised by a steep upper backscar, and a series of terraces separated by scarps that extend down towards the foreshore. Material released from the backscar is transported over these terraces towards the sea by a series of mudslides, which form large lobes on the foreshore (Photo 23). Instability results from a combination of geological and hydrogeological factors coupled with continuous basal removal of material by marine erosion (Denness, 1971; Conway, 1974; Denness, et al., 1975; Brunsden and Goudie, 1981; Bray, 1986; 1996; Koh, 1990; 1992). Activity has been studied by map, air photo and field survey comparisons (Brunsden and Goudie, 1996; Bray, 1986, 1996; Chandler and Brunsden, 1995 and Brunsden and Chandler, 1996). The system can be divided into three areas according to the intensity of landslide activity: (i) a highly active central area characterised by rotational backscar failures that feed two major mudslides on the upper and mid benches; (ii) a west-central area where instability has migrated inland to the formerly inactive backscar and is generating renewed failures, and (iii) a zone of increasing reactivation along the upper platform towards Timber Hill and the Spittles where the backscar is becoming unloaded and renewed failures are likely in the near future.

A sequence of events has been assembled by Bray (1996) as follows:

1. In the 19th century, two roads connected Charmouth and Lyme Regis via traverses of Black Ven. An upper road ran inland of the backscar from Charmouth before crossing the western backscar and undercliff to reach Lyme Regis. A lower road ran direct across the undercliffs beneath the backscar. The upper and mid parts of the landslide complex must have been relatively stable for these roads to have been constructed and maintained;
2. Sea-cliff recession at 0.76m per year to the “east of Lyme Regis” occurred between 1803 and 1839. Parts of the lower, road to Charmouth reportedly had slipped into the sea by 1892, suggesting that instability progressed inland following erosion of the sea-cliffs;
3. A major landslide affecting the undercliffs between Black Ven and Lyme Regis is reported. Photographs (1907) show two small mudslide lobes on the foreshore beneath Black Ven. The lower undercliffs appear unvegetated and subject to recent landslide activity, but the upper undercliffs were vegetated and stable in appearance;
4. Reports that the lower Lyme-Charmouth road was closed in 1923 owing to subsidence. Thus, it appears that the road was at least partly usable for up to 30 years following the disturbances noted above, an indication of slow, but progressive increase in landslide activity within the undercliffs;
5. Aerial photographs of 1948 display the main backscar in a stable vegetated state and show the upper road intact. Mid and lower parts were clearly active and small mudslide lobes extended across the foreshore.
6. Aerial photographs of 1958 also display the upper road intact despite large-scale mudslide activity within the undercliffs. During these disturbances, two large mudslide lobes surged from central Bleck Ven out across the foreshore, the eastern in 1957 and the western in 1958. Degradation of the main backscar is seen in association with these mudslides, but the quantity of retreat was small, especially above the western (most recent), mudslide. The 1958 photos therefore depict the preparatory stages for a phase of rapid backscar retreat;
7. Major failures of the backscar followed immediately, completely cutting the upper road in 1958 and resulting in its permanent closure;  
8. From 1958 until 1988, retreat of the backscar has been rapid. Recession above the western mudslide has been continuous, whilst that above the eastern mudslide occurred primarily between 1957 and the late 1960s with a further phase of retreat by rotational sliding beginning in April 1986. The combined results of these events has been a mean retreat of the central backscar at 2.15 m per year between 1960 and 1987;
9. Since 1988 the central western backscar has continued to retreat, whereas the eastern backscar has been relatively inactive and characterised by slow slippage down the face and disintegration of the 1986 failed blocks. Activity has increased in western parts of the complex resulting in a westward extension of backscar reactivation towards Timber Hill (Photo 24).

Backscar evolution has also been studied using a rigorous analytical photogrammetric technique based on a 42 year (1946-88) sequence of aerial photographs (Chandler and Cooper, 1988; Chandler, 1989; Chandler and Brunsden, 1995; and Brunsden and Chandler, 1996). The results confirmed that the eastern and western backscars have evolved in slightly different manners although they are both linked to major mudslide systems. Recession peaked at the eastern (2.8m per year) and western (5.0m per year), backscars during the period 1958-1969. Thereafter, the eastern backscar was relatively stable until the failures of 1986. The western backscar continued to recede actively throughout the period 1958-1988. Mean retreat values of 1.33m per year (eastern) and 3.14m per year (western), were recorded over the full period 1958-1988 following the initial mudslide surges. This work also involved calculations of the volumetric changes involved in these processes as material was delivered seaward from the degrading landslide system. Results demonstrated that the intense mudslide events of 1957 and 1958 led to removal of large quantities of material from the Upper Platform at the base of the backscar. Major rotational slides that occurred almost immediately thereafter at the backscar suggest that unloading was probably a major preparatory factor in the failure mechanism. Rapid downslope movements of the units produced by these slides almost certainly further loaded the mudslides below and could have stimulated their activity through an undrained loading effect. Further mudslide surges would again have unloaded the upper slopes causing regeneration of instability. This positive feedback process continues unabated because the mudslides that unload the slopes remain highly active and their toes are removed continually by marine erosion. Chandler and Brunsden (1995) demonstrated using a comparison of slope angles that, even though individual components altered significantly as recession proceeded, the system as a whole exhibited a “dynamic equilibrium.” This work was developed further to produce an episodic landform change model (Brunsden and Chandler, 1996; Chandler, 1999).

On occasions, sediment supply at the mudslide toe exceeds marine erosion and temporary accretion occurs on the foreshore as mudslide lobes. Over the past 15 years marine erosion has exceeded the upslope supply so that mudslide lobes have been eroded back (Photo 24). Short-term variations in sediment supply to the foreshore appear therefore to result from storage within the landslide system and some material released to the landslide complex by recent rapid backscar retreat has yet to reach the beach (Bray, 1996). This is particularly relevant to gravel supply computations, because all suitable material is derived from the backscar. Gravel supply over the period 1901-88 therefore averaged 2,100m³ per year, but this is expected to increase towards a representative long-term rate of 3,500m³ per year as gravel currently in storage is transmitted to the beach. Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and 2012 aerial photography shows continued cliff recession with 3-10,000m³ per year of cliff derived input, a reduction from the 2004 estimated volume of more than 20,000m³ per year. Sediment supply to the beach is not easy to calculate at sites such as this characterised by an accelerating rate of landsliding together with intense short-term variations in rates of processes. As the western backscar at Black Ven and Timber Hill becomes reactivated, it is likely that a significantly wider outcrop of gravel bearing deposits will be exposed to introduce fresh materials. Bray (1996) estimated that future supply of gravel could increase to as much as 9,000m³ per year by 2100. It can be concluded that the Black Ven landslide system is the most important sediment source (particularly for gravel) within the study area due to high relief, rapid retreat and suitable geology. It is likely to become increasingly important in future.

E4 River Char to Westhay Water (see introduction to cliff and coastal slope erosion)

This sector includes the landslide complex of Stonebarrow Hill (elevation up to 147mOD) together with lower cliffs of 30m-70m elevation cut into the western and eastern flanks of Stonebarrow (Photo 16). The topography is cut by the steep sided valleys occupied by the River Char and the stream of Westhay Water, which has a hanging termination over sea-cliffs. Stonebarrow exhibits a morphology comprising steep sea-cliffs developed within the Belemnite Marls, above which there is a series of platforms mantled by degrading landslide debris and a steep upper backscar. Permeable Cretaceous strata capping Stonebarrow Hill act as primary reservoirs that supply groundwater to the coastal slope at the junction with clayey impermeable Liassic strata beneath (Denness, 1971). Chemical alteration (decalcification) and unloading at the base of the backscar slopes further weaken these strata. When the accumulated stresses become sufficient to exceed the material strength, failure occurs by large-scale, but often infrequent rotational slides. Failures of this type occurred in the early 1940s generating several large blocks of up to 20m in width. The blocks thus formed are then stored on the upper platforms of the landslide complex for up to 100 years (Brunsden, 1974; Brunsden and Jones, 1976, 1980), whilst they undergo gradual disruption by frequent smaller-scale landslips. Eventually, the material is incorporated into mudslides on the Lower Platform and transported over the sea-cliffs (Brunsden, 1974). The 1940s blocks were subject to a major movement seaward over the winter of 2000/01 that shunted large quantities of debris from the lower platform over the sea-cliffs and onto the foreshore. Mudsliding within the undercliffs results in unloading of the main scarp and eventually leads to new large-scale failure. The removal of material from the foot of the sea cliffs by the sea (Photo 25) prevents a stable slope from developing and ensures continuing instability (Brunsden and Jones, 1980).

Map and aerial photo-comparisons indicate that retreat of the backscar at Stonebarrow is intermittent, but repetitive with relatively brief 'active phases' of major landsliding occurring every 50-100 years (Brunsden and Jones, 1980; Bray, 1996). These 'active phases' are separated by long periods of slow retreat by small-scale processes, as the products of the preceding active phase of landsliding are evacuated from the upper platform. Brunsden and Jones (1976; 1980) suggest that over a 100-year period, the mean rate of backscar retreat is constant and equivalent to the rate of sea-cliff retreat. Bray (1996) calculates this rate to be around 0.39m per year. Over this period it is believed that there is an approximate sediment balance between release from the backscar and supply to the foreshore, providing a basis from which to calculate sediment inputs.

Significant quantities of beach forming sediments are supplied from Upper Greensand Chert Beds and overlying flinty deposits present in the Stonebarrow backscar (Bray, 1996). Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates cliff erosion is ongoing with 1-3,000m³ per year of cliff derived input, a reduction from the 2004 estimated volume of more than 20,000m³ per year. This volume of sediment to the lower foreshore forms a gently sloping terrace exposed at low spring tides extending sub-tidally, with fine grained material transported into the nearshore zone in suspension and not retained on the foreshore. Bray (1996) suggests that fine sands are released by Upper Greensand Foxmould Sands, with an estimated 1,750m³ per year of gravel may be supplied per year with over 90% arising from the Stonebarrow system. Reliability of this information as a long-term rate is high, but supply is subject to short-term variations due to seasonal factors and feedback mechanisms which has caused “waves of aggression” to pass through the mass movement system (Brunsden, 1974; Brunsden and Jones, 1976, 1980). It is likely that future gravel yields will increase due to the effects of sea-level rise and climate change that are likely to increase recession by up to 50% (Bray and Hooke, 1997).

E5 Westhay Water to St Gabriel's Water (see introduction to cliff and coastal slope erosion)

The coastal cliffs that cut into the southeast flank of Chardown Hill are composed primarily of Lias Clays, but with variable overlying cover of gravel-rich superficial deposits and relic landslides. The cliffs are of lower relief than at adjoining segments and comprise a steep sea cliff above which there is a gently sloping undercliff, or degradation zone that approaches 600m in width at Broom Cliff. Historical retreat rates determined from map and aerial photo-comparisons (1901-1987) by Bray (1996) range between 0.2 and 1.1m per year according to location, although a general range of 0.4 to 0.5m per year is considered representative.

At Broom Cliff, accelerating retreat has been recorded since 1901 suggesting that the system is undergoing a prolonged active phase. Although it is possible that the system will eventually become so choked with debris that retreat could slow down in the future, no such signs are discernible. Observations of the system between 1984 and 1989 revealed intense mudsliding and excavation of debris from within the landslide complex, thus suggesting that recent rates of backscar retreat could be maintained or even accelerate. Retreat over the period 1960-1987 (0.99m per year) is therefore taken as the most appropriate basis for estimating future behaviour.

Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates minimal cliff recession with less than 1,000m³ per yearof beach grade sediment supplied to the foreshore, due to the lack of coarser materials within the cliff geology, with fine grained material transported into the nearshore zone in suspension and not retained on the foreshore. This revised volume of cliff derived input is a reduction from the 2004 estimated volume of more than 20,000m³ per year.

E6 Golden Cap (see introduction to cliff and coastal slope erosion)

The Golden Cap headland (Photo 16) forms the highest cliff (191mOD) within the study area and exhibits a markedly steeper profile than the landslide systems to the west. This may be related to geological differences, but increased wave exposure is also cited as a possible factor (Brunsden and Goudie, 1981; 1996). Retreat is variable and some cliff sections have showed no measurable retreat in recent decades, whilst others along its western flank receded at up to 1.0m per year (Bray, 1996). Long-term rates of 0.05 - 0.30m per year are indicated by map, air photo and field survey comparisons covering the period 1901-87, but their reliability as a guide for future recession is uncertain. This is because observation of landslide morphology shows relic mudslides, rotated blocks and numerous foreshore boulder arcs (Photo 26). These features are characteristic of phases of rapid landsliding, so it is probable that retreat has been underestimated (Bray, 1996). It is concluded by Bray (1996) that the retreat recorded historically between 1901 and 1987 is therefore representative only of the quiescent phase of weathering and small-scale failures that occurs between major landslides. It is not typical of the full range of behaviour at this site and thus underestimates the long-term retreat rate. Assuming that future failures adopt a similar mode to those at Stonebarrow, up to 20m of recession might be expected to occur in a few large events. Although the timing of such events cannot be predicted, new failures appear imminent so future retreat rates are likely to be greater than those recorded historically.

Cliff geology comprises Lias clays, sands and limestones overlain by Upper Greensand Foxmould sand and thin Chert Beds. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that less than 1,000m³ per year of beach grade sediment is supplied to the foreshore, a reduction from the 2004 estimated volume of more than 20,000m³ per year. Fine grained sand material transported into the nearshore zone and not retained on the foreshore. Large limestone boulders are supplied from the Three Tiers unit of the Middle Lias, forming dense foreshore aprons, which effectively trap any gravel supplied and prevent output to adjoining beaches (Bray, 1996). The boulder aprons provide protection against wave attack because they induce offshore breaking of large waves; so dissipating some of their energy.

E7 Seatown (see introduction to cliff and coastal slope erosion)

Cliffs backing Seatown Beach differ with respect to geology and morphology either side of the River Winniford (Photo 27). To the west, cliffs are of Lower and Middle Lias clay with a wide degradation zone. Backscar retreat is rapid on the eastern flank of Golden Cap (up to 1m per year for 1960-1987) where ancient landslides are being reactivated, but much slower on average closer to Seatown (0.06m per year) (Bray, 1996). This is because erosion has mostly involved recession of the cliff toe and this had yet to affect the backscar significantly by 1987 the date of Bray’s (1996) final survey. Since 1988, observations of the sea-cliff and undercliffs suggested landward migration of instability that is now beginning to trigger backscar failures. Concerns that instability could migrate to affect properties at Seatown led to the construction of a small stabilisation scheme in 1997 covering a frontage extending some 100m to the west of the public road turning circle. It involved toe protection, drainage through the body of the landslide and also landward of it and an attempt to “pin” the backscar to prevent landward retreat (WS Atkins Ltd., 1996).

Between the River Winniford at Seatown, and Doghouse Hill (Photo 3 and Photo 28) the cliffs are formed within sandy units of the Middle and Upper Lias and the coastal landslide complex is very narrow, being some 40-70 m in width, consequently the backscar and sea-cliff systems are more intimately associated. Erosion of the sea-cliffs is therefore transmitted more rapidly through the system to result in failures of the backscar. These cliffs appear to retreat mainly by infrequent, but fairly large-scale, falls or slumps, which are separated by long periods of slow retreat, dominated by small-scale processes. The overall effect again is of a cyclic retreat. From 1901 to 1948 retreat was very slow (0.04m per year) and no large-scale failures occurred (Bray, 1996). However, aerial photographs, together with field survey and observations indicate that since 1948 there has been a marked increase in the frequency of large-scale backscar failures. During this time, at least two major falls have occurred and the mean backscar retreat rate has increased to 0.2 to 0.3m per year. These changes might be related to historical beach mining activities that removed almost 300,000 tonnes of shingle from 1940 to 1987 and must have resulted in some loss of protection from wave attack. Indeed, a field survey of the base of the sea-cliffs by Dorset County Council in 1984 revealed retreat at 0.9 m per year since 1960. Backscar failures may therefore have occurred in response to a steepening of the profile. Thus, it is possible that natural cyclic retreat has accelerated so that major events may be triggered every 20 or so years. Post-war retreat is therefore the most relevant for extrapolating into the future i.e. 0.25m per year. It is likely recession will increase by up to 40% due to the effects of sea-level rise and climate change (Bray and Hooke, 1997).

Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that less than 1,000m³ per year of beach grade sediment is supplied to the foreshore, a reduction from the 2004 estimated volume of more than 20,000m³ per year. There is a negligible supply of gravel due to cliff geology. Fine grained sand material, supplied from the Middle Lias strata, are transported into the nearshore zone in suspension and not retained on the foreshore.

E8 Thorncombe Beacon (see introduction to cliff and coastal slope erosion)

Degradation of predominantly sandy Upper and Middle Lias sediments has created high cliffs (up to 155mOD) of relatively steep profile (Photo 18). Retreat averaged 0.17m per year over the period 1901-1960, although faster rates of up to 0.5m per year operated at some locations (Bray, 1996). Post-1960 measurements of retreat are not available. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that less than 1,000m³ per year of beach grade sediment is supplied to the foreshore, a reduction from the 2004 estimated volume of more than 20,000m³ per year. There is a negligible supply of coarse-grained sediment due to extremely thin or absent gravel bearing strata. Fine grained sand material are transported into the nearshore zone in suspension and not retained on the foreshore. Cliff erosion supplies large quantities of sandstone and limestone boulders, which densely litter the foreshore in large boulder aprons and provide protection against wave attack because they induce offshore breaking of large waves, so dissipating some of their energy.

E9 Eype and West Cliff (see introduction to cliff and coastal slope erosion)

Cliffs along this segment are lower and of distinctly different geology either side of Fault Corner, which marks the plane of a major fault with marked downthrow to the south-east (Photo 2). To the west, cliffs retreat at 0.05 - 0.5m per year and supply mostly Upper Lias clays and sands, some of which contribute to the western end of Eype Beach (Bray, 1986; 1996). To the east, cliffs are composed of Middle Jurassic marls and clays with bands of shelly limestones. Retreat has been quite rapid (0.37m per year) over the period 1887-1962 (Jolliffe, 1979) and is linked with a period of beach lowering in front that is thought to have occurred in response to interception of drift caused by infiIlling of the West Bay piers in the 1920s (High-Point Rendel, 1997; Brunsden and Moore, 1999). The low-lying West Beach retreated over this period, which caused a significant “set-back” of the coast to the west of the West Bay piers – see Photo 9 (Hydraulics Research, 1979; Jolliffe, 1979; Brunsden and Moore, 1999). The esplanade was extended along the toe of West Cliff in 1968/69 and from the mid-1970s onward worsening cracking and bulging at the cliff face resulted in various remedial works from 1985 onward including re-profiling and installation of drainage (High-Point Rendel, 1997; Keystone Historic Buildings Consultants, 1997).

Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that less than 1,000m³ per year of beach grade sediment is supplied to the foreshore, a reduction from the 2004 estimated volume of more than 20,000m³ per year. There is a negligible supply of chert or flint gravel is supplied from the cliffs due to absence of suitable deposits. Fine grained sand material are transported into the nearshore zone in suspension and not retained on the foreshore.

E10 East Cliff and Burton Cliff (see introduction to cliff and coastal slope erosion)

Near vertical sandstone cliffs extend east from East Cliff, West Bay (Photo 11) to Burton Cliff (Photo 12), where thin oolithic limestones cap them. Further east towards Cogden, there are clay cliffs of Fuller’s Earth. All cliff sections are devoid of flint and chert (Bray, 1986; 1996). The sandstone cliffs are composed of Bridport Sands and possess a number of bands of harder calcareous sandstone. As the cliffs gradually retreat, the more resistant material is released as boulders or cobbles, which temporarily contribute to beach volume before wave action results in rapid abrasion and disintegration. Recession of these sandstone cliffs was investigated by map comparisons over the period 1902-62, which indicated retreat of 0.03m per year and yielded a supply rate of 1,400 tonnes  per year (630m³ per year) of calcareous sandstone (Laming, 1985). With the exception of a significant cliff fall in July 2012 at Burton Bradstock, analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that cliff recession is negligible during this period. The 2004 estimated volume suggested 3-10,000m³ per year. There is a negligible supply of gravel due to cliff geology, and fine-grained sand material is transported into the nearshore zone in suspension and not retained on the foreshore.

E11 Chesil Beach Recession (see introduction to cliff and coastal slope erosion)

Beach recession may supply sediment from underlying deposits that become exposed in the seaward beach face as the beach is pushed landward. Carr and Blackley (1984) report slabs of Forest Marble Limestone and peat being yielded and Moxom (2003) records some direct observations of this process. Observations of gravel clasts thrown over the crest by storms led several authors to postulate recession of Chesil Beach by “in-rolling” (Carr and Blackley, 1974), or “rollover” (Carter, 1988).

Analysis of beach profiles measured originally by Coode (1853), with more recently surveyed profiles revealed that only opposite Portland Harbour was the amount of crest recession (17 m) greater than the possible errors (Carr and Gleason, 1972; Carr and Seaward, 1991). Comparisons of these data with aerial photos of 1993 revealed recession of the crest by 8m towards Chiswell and recession of the MHW by 10-15m, along the eastern portion of the beach, indicating a slight steepening of the beach face (Babtie Group, 1997). Furthermore, beach profile measurements following storms revealed that short term variations resulting from “cut” and “fill” cycles were frequently greater than any long term trend (Gibbs, 1980; Babtie Group, 1997). Available evidence, therefore, suggests that eastern parts of the beach are subject to slow recession at 0.06 to 0.12m per year, but the period covered by accurate measurements remains insufficient for conclusive long-term trends to be determined. Any recession is therefore likely to be slow and may result from delayed response to sea level rise, increased storminess, changes in wave direction and diminution of beach volume (Carr and Blackley, 1974).

The sediments located beneath the beach have been investigated by several studies. Probing traverses of the Fleet revealed a 2.5m thick mud layer resting upon a gravely layer of unknown thickness (Bird, 1972). Borehole investigations indicated a deep channel (to –26mOD) between the mainland and the Isle of Portland (Carr and Blackley, 1973). This is infilled by thick sand, gravel and peat deposits, which have potential to supply Chesil Beach as it recedes. Other coring and borehole studies within the Fleet Lagoon have indicated black marine sands at depth with clays and peats above and a basal layer of cobbles (Coombe, 1998). Overall, it appears that only relatively small quantities of coarser materials could be yielded to the beach through reworking of the substratum due to the very slow rate of beach retreat, although this could alter in the future.

E12 The Fleet (see introduction to cliff and coastal slope erosion)

At several locations, erosion of the shores of the Fleet has created low cliffs and supplied sand and gravel to small local beaches (Bird, 1972). Sediment volumes are extremely small and material appears to be retained on thin local beaches on the Fleet. In places, e.g. Wyke Regis (Photo 13), cliff recession has resulted in formation of shore platforms several metres wide in limestones and siltstones of moderate resistance. These features are difficult to reconcile with the prevailing opinion of the Fleet as a low energy environment and suggest occurrences of active wave erosion in the past.

E13 West Coast of “Isle” of Portland (see introduction to cliff and coastal slope erosion)

The northwest Portland coast is subject to major landsliding where soft erodible Kimmeridge Clay is overlain by hard Portland Stone (Brunsden and Goudie, 1997). However, analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data indicates that cliff recession is negligible during this period. Fine-grained sand material are transported into the nearshore zone in suspension and not retained on the foreshore. The landslides have been carefully mapped, documented and classified (Brunsden, et al., 1996). At West Weare, (110m OD) blocks of the resistant cap rock become detached at the backscar controlled by NE-SW master joints and fissures (Photo 29). The blocks descend close to seal-level and rotate backwards, often forming sequences of multiple rotated blocks. Some blocks topple directly from the backscar. Although toe erosion is important, the occurrence of groundwater springs and the loading of blocks by quarry waste that in the past was tipped directly down the coastal slope have contributed to the instability; with the last major failures being recorded in 1858 (Brunsden, et al., 1996). The result of these processes is an undercliff strewn with displaced Portland Stone blocks and boulders that gradually narrows to the south. The Portland Stone blocks are hard and resistant and provide a protective apron at the toe so that map comparisons showed little measurable change over the period 1867-1960 (May, 1966).

Futher south, the west-facing coastline (50-70mOD) is almost linear due to the effect of the governing master joints (Photo 30). Failures occur by joint controlled linear slides, topples, sags and falls. Hard Portland Stone forms an impressive near vertical face and collapsed blocks form boulder aprons at the toe (Brunsden, et al., 1996). The site is very exposed and all softer materials are removed rapidly by high-energy waves.

Sediment supply from the Isle of Portland is nonetheless indicated by presence on Chesil Beach of pebbles of Portland Stone and a distinctive dark chert found within the Portland Stone (Carr and Blackley, 1969; Jolliffe, 1979). Overall, it is envisaged that natural supply is extremely slow, although it probably was augmented at West Weare by tipping of significant, but undocumented quantities of quarry waste down the undercliffs – see (Photo 29) (Carr and Blackley, 1969; Jolliffe, 1979).

2.3 Fluvial Input

» FL1 · FL2

No major rivers discharge along the coastline of this unit. Chesil Beach forms a barrier in eastern parts so that the small streams draining its hinterland discharge into the Fleet Lagoon rather than into the open sea. To the west the small rivers Bride (Photo 31), Britt, Winniford (Photo 27), Char and Lim discharge to the shoreline. All are narrow gravel or sand-bed rivers with small catchments, but steep channel gradients that generate a modest potential to supply sediments to the shoreline, especially during high discharge events. The Britt is regulated in its lower course and its discharge into West Bay Harbour is regulated by a sluice. It is therefore likely that the majority of sediments delivered would be deposited in its lower channel and floodplain with very little input to the shore. Gravel beaches usually impound the mouths of the Bride and Winniford. Only the Char would appear to have a significant potential to supply materials to the shore.

Rendel Geotechnics (1996) studied potential fluvial sediment yields through the south coast region and found very little direct data available from which to make assessments. Therefore they undertook empirical estimations based on available discharge data coupled with assessment of sediment availability. They concluded that the West Dorset streams collectively had the potential to supply some 2,700m³ per year of suspended sediments and 300m³ per year of coarser bedload sediments (sands and gravels).

FL1 River Char (see introduction to fluvial input)

Large quantities of angular flint and chert gravel line the bed, but it is uncertain whether or how frequently these materials might be mobilised and transported towards the shore. Observations of the river at all times of the year during the period 1984-86 revealed very little, if any bedload gravel transport (Bray, 1996). Although the river flows across the beach at Charmouth, banks of gravel deposited by marine processes always restrict its discharge. Thus, the river is usually "ponded" for up to 300m inland during the summer when it percolates through the beach. Any bedload carried by the river would be deposited in this area and could not be supplied to the foreshore without a flood event. These observations suggest that the River Char cannot normally transport gravel-sized material. Nevertheless, it is reported that the River Char does flood during exceptional circumstances and channel gravel deposits might then be supplied to the foreshore. Therefore, the speculative 2004 arrows suggesting 3-10,000m³ per year have been removed due to the lack of supporting information and quantitative evidence of ongoing fluvial inputs.

Evidence of an episodic input is provided by a fan of gravel and shingle at the mouth of the Char (Photo 32), although some of this is composed of beach shingle displaced by river flow down the beach face as the tide retreats. The only record of an exceptional discharge for the River Char is provided by an oblique aerial photograph showing extensive flooding of the river at Newlands Bridge, 0.6km from the mouth (Chaplin, 1985). Flood waters extended up to 50m either side of the channel and it was reported that several caravans were carried along by floodwaters. Discharge is not measured routinely by the Environment Agency and its predecessors, so there are no direct data. Precipitation is measured just outside the catchment at Beaminster, Bridport and Lyme Regis. Records for 1979 showed that flooding occurred following intense rainfall on 30th May 1979.

In the absence of discharge and bedload transport information, the volume of gravel supplied to the beach was estimated by Bray (1996) using details of the fan deposited on the foreshore. The volume of material contained within the fan was estimated as being between 16,000m³ and 32,000m³. However, because the flood event is infrequent, its return period is uncertain. Existing records revealed a single flood between 1881 and 1989 suggesting a minimum 100 year return period thus giving a possible mean annual supply of between 160m³ per year and 320m³ per year. Despite being very imprecise, these estimates do suggest that fluvial supply is a relatively minor component of the long-term beach shingle budget compared with input from cliff retreat.

FL2 The Fleet (see introduction to fluvial input)

The Fleet is a shallow brackish estuarine lagoon, which receives fluvial discharge and tidal flow. Probing traverses and boreholes revealed several metres thickness of mud, which accumulated over the past 6000-8000 years (Bird, 1972; Carr and Blackley, 1973). It is suggested that the Fleet is a sediment sink but it is uncertain whether it has been dominated by terrestrial supply sources. It has been suggested that terrestrial sediments derive from wave erosion of the margins and fluvial input from several small inflowing streams, chiefly the Abbotsbury Stream (Bird, 1972), but coring studies from the Fleet suggest that the majority of sediments are of marine origin (Coombe, 1998).

2.4 Beach Nourishment

» N1 · N2

The only recorded beach nourishment in the study area relates to 6,000m³ of imported gravel placed on the Marine Parade Beach at Lyme Regis, immediately North-east of the Cobb in the 1970s (Posford Duvivier, 1990). A further recharge of 2,500m³ of shingle was derived directly from Monmouth Beach in the 1990s (West Dorset District Council, 2000b). Nourishment is also a component of Beach Management Plan at West Bay (West Dorset District Council, 2012) and Environmental Improvement Schemes at Lyme Regis (West Dorset District Council, 2005/06) for coast protection/sea defence and amenity purposes.

N1 Lyme Regis (see introduction to beach nourishment)

The urban frontage of Lyme Regis has suffered declining beach levels for several decades leading to exposure of sea-walls and loss of amenity. Consequently, a number of coast protection proposals involving beach replenishment using imported sands and gravels have been examined since the late 1980s. Earlier schemes involved replenishment using 6,000m³ of imported gravel placed along the Marine Parade Beach in the 1970s (Posford Duvivier, 1990). A further recharge of 2,500m³ of shingle was derived directly from Monmouth Beach in the 1990s (West Dorset District Council, 2000b). These actions did not solve long-term beach depletion problems. Tests using a physical model (Hydraulics Research, 1989) led to development of a preferred scheme involving a major shingle nourishment, but this was not constructed. Potential problems were identified in retaining material at the shoreline so construction of three offshore breakwaters was advised. An alternative scheme proposal to reform a former littoral drift path from Monmouth Beach to Marine Parade was found not to be feasible and various proposals for construction of groynes were considered (Posford Duvivier, 1990; 1991). Since the mid-1990s a series of investigations and proposals under the programme of Lyme Regis Environmental Improvements has been developed (West Dorset District Council, 2000b; Brunsden, 2002). This has led towards the commissioning for 2005 of Phase 2 of the scheme involving major beach replenishment between Cobb Gate and the Harbour. It is planned that sand should be used to replenish the beach around the harbour, with shingle being preferred for the beach to the east (West Dorset District Council, 2004a; 2005). Two new jetties and an extension to Beacon Rocks are also planned to control the new beaches and reduce future losses of sediment.

N2 West Bay (see introduction to beach nourishment)

A series of studies including physical and numerical modelling was undertaken as part of the West Bay Coastal Defence and Harbour Improvements Scheme (e.g. Hydraulics Research, 1991e; HR Wallingford, 2000a; 2000b; 2000c). Based upon the results of these and other studies a scheme was developed involving construction in 2004 of a replacement West Pier and a range of other measures to increase beach stability and improve the coastal defences –see Photo 7 (West Dorset District Council, 2004b). A key element of the scheme due for completion in November 2004 is a replenishment of West Beach by some 18,000m³ of shingle. The East Beach is managed so as to maintain an optimum width of some 120m to 160m. The East Beach and Freshwater Beach beach management plan (CH2M, 2009) and West Beach, West Bay beach management plan (West Dorset District Council, 2012; CH2M, 2012) supercede previous plans, and have been formulated to include actions to import beach material if this width diminishes and recycle beach material to the east should it accrete beyond this width.

3. Littoral Transport

» LT1 · LT2 · LT3 · LT4 · LT5 · LT6 · LT7 · LT8 · LT9 · LT10 · LT11 · LT12 · LT13

Transport occurs along a series of pocket beaches between Lyme Regis and West Bay and along the 28km expanse of Chesil Beach between West Bay and the Isle of Portland. Headlands and artificial structures intercept drift resulting in the identification of four beach drift sub-cells as follows (Bray, 1996; High-Point Rendel, 1997):

1. Charmouth Beach (Lyme Regis to Golden Cap);
2. Seatown Beach (Golden Cap to Doghouse Hill);
3. Eype Beach (Doghouse Hill to West Bay);
4. Chesil Beach (West Bay to Isle of Portland).

It should be noted that there are several additional transport discontinuities within some of these cells as detailed within the individual sections below.

Analysis of Coastal Monitoring Programme lidar (2007 and 2011), aerial photography (2012) and baseline topographic data supports net eastward littoral drift from Lyme Regis to West Bay, although reversals are also evident between Golden Cap and West Bay. The net drift direction on Chesil Beach is more difficult to discern for it is a swash-aligned feature and net drift rates are very low compared with the quantities of eastward and westward drift occurring. Onshore and offshore sediment transport also occurs between the beach face and inter-tidal foreshore to the sub-tidal limit of the beach profile, rather than fully offshore except under extreme conditions.

Gravel accumulations occur on the western (updrift sides) of structures that intercept beach drift such as the Cobb, Lyme Regis and a rock groyne at Charmouth and headlands act as barriers to drift (Bird, 1989). Gravels increase in size from west to east on each of the pocket beaches and tracer experiments undertaken by Carr (1971) and Bray (1996) indicated that the largest pebbles moved the fastest suggesting that accumulations of these types should indicate the net direction of drift. Calibrated modelling of transport on Charmouth Beach by Bray (1996) indicated considerable drift reversals in eastward and westward directions, but with a net annual eastward drift.

LT1 Monmouth Beach (see introduction to littoral transport)

The Cobb, has existed in various forms since around the 12th century to provide shelter for Lyme Regis Harbour (Photo 33). However, since it became connected to the mainland in its present configuration in 1754, it has functioned as a large terminal groyne intercepting west to east drift promoting the up-drift accretion of Monmouth Beach. High-Point Rendel (1999c) argue that accretion against the Cobb has been due to redistribution of existing materials by eastward drift, to re-orientate the beach westward, rather than fresh inputs from the west. They conclude that Monmouth Beach as a whole has experienced loss of sediment volume over the past 250 years due principally to the substantial reduction of drift input from feeder beaches in Pinhay Bay, to the immediate west. This, in turn, is ascribed to the impedance of the drift pathway imposed by post mid-eighteenth century landslides at and close to the Humble Point (see the text covering the neighbouring unit of Beer Head to The Cobb, Lyme Regis). Thus, the continuing west to east longshore transport along the relatively sediment starved Monmouth Beach has resulted in a progressive increase in the imbalance between input from the west and storage/output adjacent to the Cobb in the east to re-orientate the beach. Posford Duvivier (1998a) calculate a long-term average rate of accretion against the Cobb of approximately 1,500m³ per year, but modelling studies suggest that potential rates of up to 10,000m³ per year may be sustained for short periods during winter storms. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data supports a net eastward drift of 1-3,000m³ per year of beach grade sediment, although the rate of drift varies according to wave height and approach direction with increased eastward drift occurring in stormier years. This volume is a reduction from the 2004 estimated volume of 3-10,000m³ per year. Investigations using the DRCALC numerical modelling package by HR Wallingford (2001) indicated that the potential shingle drift averaged 2,400m³ per year (inter-annual variation from 1, to 3,500m³ per year).  

In 2009, the Coastal Monitoring Programme completed a 100% coverage swath bathymetry survey, extending offshore 1km from MLW between Petit Tor Point and Portland Bill. Between Monmouth beach and Lyme Regis the nearshore zone is dominated by a continuous rock platform. Geological features such as ridges and folds are discernible extending from the toe of the beach through the inter-tidal and extending from MLW approximately 400m sub-tidally. A pocket beach to the west of the Cobb comprises a range of coarse-grained sediments, such as cobbles and boulders. The thickness of substrate is generally insufficient to mask the sub-tidal rock platform and outcrops further offshore, indicating variable but generally thin seabed sediment thicknesses. Where there are larger patches of sediment, such as south of the Cobb, there is a lack of discernible bedforms to confirm sediment transport pathways.

LT2 The Cobb (see introduction to littoral transport)

The Cobb extends across the foreshore well seaward of LWMST and effectively intercepts littoral drift of shingle (Photo 33). Initial structures date back to 1272-1307 and generally comprised offshore breakwaters, which only partly intercepted drift. The present form attached to the shore dates from around 1754 (Posford Duvivier, 1990) although numerous modifications and reconstructions have been undertaken subsequently. Although it would thereafter have formed a more complete barrier, it is documented that some material could pass across the wall at its root where it joined Monmouth Beach (Posford Duvivier, 1990). Transport through the Cobb Wall was also possible by sluice gates constructed specially in the mid-1800s. At present, the Cobb wall is a more complete barrier to drift due to heightening of parapets at its landward root and permanent closure of the sluice gates in 1959 (Posford Duvivier, 1990).

The Cobb is not an absolute boundary to littoral movement, as evidenced by the accumulation of gravel between two arms of Lyme Regis harbour (Photo 34) that occurs under high-energy wave conditions (Bray, 1996). It is not known how much gravel bypassing actually occurs, although it is thought to be small and infrequent. Bypassing by north-eastward transport of sand along the nearshore sea-bed is likely to occur more frequently and mobile spreads of sand have been surveyed in nearshore waters (West Dorset District Council, 2000a). In conclusion, the Cobb appears to be an effective barrier to beach drift of shingle as witnessed by long-term accretion of Monmouth Beach and severe depletion of beaches to the east (West Dorset District Council, 2000b). As part of Phase 2 of the Lyme Regis Environmental Improvements scheme (2005-2007) Beacon Rocks were extended some 110m eastwards from the eastern extremity of the Cobb, thus potentially increasing the barrier and sheltering effects of this structure.

Analysis of 2009 swash bathymetry data, offshore of Lyme Regis indicates that the variable thickness of nearshore sediments is generally sufficient to mask the underlying sub-tidal rock outcrops. Geological features such as ridges and folds are more clearly discernible approximately 400m offshore. Offshore of Lyme Regis, there is a lack of bedforms or downdrift accumulations of sediment in connection with rock ridge outcrops to confirm an eastward sediment transport pathway.

LT3 Lyme Regis (see introduction to littoral transport)

A northward change in the shoreline orientation to the east of the Cobb (Photo 8) means that there is a significant potential for eastward drift along Town Beach and Marine Parade (HR Wallingford, 2003) that results in sediment accumulations against the western sides of the various jetties and groynes along this frontage. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data supports a net eastward drift of 1-3,000m³ per year of beach grade sediment, although the rate of drift varies according to wave height and approach direction. This volume is a reduction from the 2004 estimated volume of 3-10,000m³ per year.  Small-scale replenishments have been undertaken in the mid-1990s including use of material rich in distinctive pink Budleigh Salterton quartzite pebbles. Subsequent observations clearly indicated an eastward movement of this natural tracer material.

LT4 Church Cliffs to Black Ven (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data supports a weak net northeastward drift of less than 1,000m³ per year of beach grade sediment, although the rate of drift varies according to wave height and approach direction, and supply of material. This volume is a reduction from the 2004 estimated volume of 3-10,000m³ per year. Observations of sediment distribution in groyne compartments beneath Church Cliffs indicate net north-easterly directed drift (Hutchinson, 1984; Bray, 1996); the amounts actually transported are very small for the foreshore at Church Cliffs and East Cliff is severely depleted (Photo 21). These beaches were formerly quite substantial, but lost volume over the past 150 years following modifications made to the Cobb and construction of defences at Lyme Regis that intercepted incoming drift (West Dorset District Council, 2000b).

Fresh gravel is supplied to this segment by landslides from the Spittles and the western part of Black Ven. Eastward drift is intercepted periodically by lobes of landslide debris which surge seaward from undercliff mudslides and small beaches of angular gravel accumulate to the west of such barriers (Photo 15 and Photo 24). The Black Ven mudslide lobes have functioned as major barriers, since their extension across the foreshore in 1957 and 1958 and a beach of some 5,000 to 6,000m³ of gravel accumulated immediately to their west by the mid-1980s (Bray, 1996). By 1988 mudslide extension had diminished such that marine erosion could cut back into the lobes enabling formation of a continuous beach around the toe. At this point material held up to the west of the lobes could drift eastward as a pulse to supply the western end of Charmouth Beach. The small gravel beaches beneath the Spittles are therefore dependent upon the barrier effect of the Black Ven mudslide lobes and their volume fluctuates accordingly. Their capacity to accrete is nevertheless low, due to the limited coast erosion input and the periodic rapid losses of gravel eastward. In the long term, all gravel supplied from the East Cliff and Spittles landslides drifts eastward representing a net eastward drift of around 320m³ per year (Bray, 1996). The beaches are therefore likely to remain small and offer very little protection to the cliff toe beneath the Spittles.

Analysis of 2009 swath bathymetry data, offshore of the Spittles, between Church Cliffs and Black Ven, indicates that the variable thickness of nearshore sediments is generally insufficient to mask the underlying sub-tidal rock outcrops, which are clearly discernible between MLW and approximately 400m offshore. There is a lack of bedforms or downdrift accumulations of sediment in connection with rock ridge outcrops to confirm a sub-tidal eastward sediment transport pathway.

LT5 Black Ven to the River Char (see introduction to littoral transport)

Field observations of eastward deflection of the river Char and sediment accumulation against the west side of two groynes constructed at Charmouth in 1954/56 and the 1994 rock groyne indicates an eastward transport. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) data supports a net eastward drift of 1-3,000m³ per year of beach grade sediment. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability, which fluctuates in response to variable supply at Black Ven.

Analysis of 2009 swath bathymetry data, between Black Ven and Charmouth, indicates that the variable thickness of nearshore sediments is generally insufficient to mask the underlying sub-tidal rock outcrops, which are clearly discernible between MLW and approximately 400m offshore. There is a lack of bedforms or downdrift accumulations of sediment in connection with rock ridge outcrops to confirm a sub-tidal eastward sediment transport pathway.

LT6 Charmouth Beach (see introduction to littoral transport)

This segment comprises the beach extending from the river Char to Golden Cap (Photo 4 and Photo 25), where littoral drift is intercepted by the protruding headland and its lobes of landslide debris. The beach comprises of mixed sand and gravel in the west, but becomes increasingly dominated by coarse pebbles and cobbles towards Golden Cap.

Eastward deflection of the river Char,  a gradual eastward increase in beach volumes and analysis of Coastal Monitoring Programme lidar and aerial photography data indicates a weak net eastward drift of less than 1,000m³ per year along this frontage. This volume is a reduction from the 2004 estimated volume of 3-10,000m³ per year. Analysis of 2009 swath bathymetry data, between Charmouth and Golden Cap, indicates that the variable thickness of nearshore sediments is generally sufficient to mask the underlying sub-tidal rock platform, with occasional rock outcrops interrupting a generally gently southward sloping seabed. Although there is a lack of bedforms, there are downdrift accumulations of sediment in connection with rock ridge outcrops; these provide indicative evidence of both eastward and westward sub-tidal sediment transport pathway. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability, so short-term localised reversals may occur between Charmouth and West Bay.

Beach drift has been studied by a variety of techniques including beach pebble sampling, aluminium tracer experiments and budget analysis involving consideration of sediment supply and storage (Bray, 1996). Analysis of some 6,000 pebbles from six transects located along the beach revealed longshore trends in size grading, roundness, sorting and pebble shape from which net littoral drift could be inferred. Pebbles were extremely angular at Black Ven, but become progressively better-rounded eastward towards Golden Cap, clearly demonstrating eastward drift away from the major cliff input. Pebble size grading does not follow the pattern predicted by supply (concentrations at Black Ven and Stonebarrow), so redistribution of gravels by littoral drift must be an important process. Size is greatest at the western (Black Ven) and eastern (St Gabriel’s) extremities. The large mean size at Black Ven results from the major inputs of poorly sorted gravels from coastal landsliding, whereas the large mean size and improved sorting at St Gabriel’s results from a net eastward drift of this material. The eastern end of Chesil Beach displays a similar eastward increase in pebble size and sorting (Carr, 1969). Tracer experiments (Carr, 1971) indicated that this pattern of grading at the eastern end of Chesil Beach might have developed by net eastward drift, during which the largest pebbles were transported the furthest. As the Isle of Portland prevents further eastward movement, this results in an accumulation of large, well-sorted pebbles. A similar process appears also to operate on Charmouth Beach, to result in the accumulation of large well-sorted pebbles in the St Gabriel’s-Golden Cap frontage, where further eastward movement is blocked by landslide debris, beneath Golden Cap. Pebble sphericity also increases eastwards towards St Gabriel’s, which again suggests eastward net drift, although it is uncertain whether downdrift abrasion or sorting is the dominant causal mechanism (Bray, 1996).

Littoral drift was measured directly by Bray (1996) during a series of experiments covering a nine month period and employing aluminium tracer pebbles. The tracers were efficiently recovered to a maximum depth of 0.45m beneath the beach surface. Recovery rates approaching 100% were achieved. Tests were conducted simultaneously on shingle (St Gabriel’s) and mixed sand and shingle (Charmouth) beaches during high, medium and low wave energy conditions. The data were analysed using multivariate techniques, and results indicate that shingle transport is most rapid on the upper beach near the high water mark and the larger tracers were the most rapidly transported. It was concluded that the observed increase in pebble size from Charmouth to Golden Cap could therefore have been developed by differential transport and sorting during net eastward littoral drift. Longshore transport volumes were calculated from the velocity, thickness and width of the moving shingle layer. The volumes were 2 to 22m³day-¹ during frequent periods of low energy westward drift, increasing to a maximum of 168m³day-¹ during less frequent periods of high wave energy induced eastward drift. Although a wide range of wave conditions were tested, they were not necessarily representative of long-term conditions. Estimations of transport using a calibrated wave power model (CERC equation) and a locally adjusted 10-year hindcast wave climate for West Bay (Hydraulics Research Station, 1985) indicated net eastward drift of shingle of around 2,000m³ per year.

At Charmouth, the application of a similar estimation procedure using a site specific CERC calibration derived from the tracer experiments indicated net eastward drift of around 5,000m³ per year, but 40% of the material is composed of sand and grit so that shingle drift is around 3,000m³ per year. Drift of the entire beach sediment size spectrum is therefore greatest at Charmouth, but decreases towards the east due to the progressive offshore loss of fines. Although the drift experiments themselves provided information of high reliability, the calculated long-term annual rates are only of medium reliability for two reasons:

1. Refraction and shoaling analysis was not possible for the tracer sites, so it was assumed that the inshore wave climate was similar to that of West Bay with adjustment for different shoreline orientation;
2. The 1974-84 West Bay wave climate was not necessarily representative of long-term conditions because subsequent studies have indicated a marked change in climate beginning in 1982 (Hydraulics Research, 1991d; Brampton, 1993). This has been shown to increase net eastward drift.

Bray (1996) produced local calibrations of the CERC equation for the St Gabriel’s (shingle) and Charmouth (mixed shingle and sand) sites and these are recommended for further modelling the West Dorset gravel and gravel/sand beaches.

A further check on the drift rates along this beach was undertaken by an analysis of the volumes and budgets of beach material and a comparison against the tracer/wave power derived values (Bray, 1996). Budget analysis showed that beach volume increased eastward, but gravel supply was concentrated in the west indicating that drift should increase eastward due to cumulative supply and increased gravel availability. Integration of these inputs together with probable outputs indicated net eastward drift of between 3-5,000m³ per year. These values are very similar to those predicted by tracer experiments so that the overall assessment of drift is believed to be of high reliability.

It should be noted that the operation of these drift rates would suggest that a substantial beach should have accumulated against the Golden Cap headland since 1901. However, comparisons of old maps, field reports and the contemporary volume of the beach do not support this idea. Instead, the imbalance can be explained by intermittent phases of eastward littoral drift that occur beneath Golden Cap as explained by the following section.

LT7 Golden Cap (see introduction to littoral transport)

The high landslide complex of Golden Cap forms a headland due to the occurrence of thick resistant limestone seams of the Three Tiers unit that outcrop in the sea-cliffs (Photo 17). As the cliffs retreat debris falls onto, or surges across the foreshore within mudslide lobes. These features completely cut the eastern extremity of St Gabriel’s beach separating it from the neighbouring Seatown beach. Marine erosion cuts back into the lobes removing all fine sediments, but leaving the foreshore densely covered by resistant limestone boulders that form a protective apron. Field examination revealed that pockets of poorly sorted gravel were frequently present between the boulders (Bray, 1996). Chert was the dominant lithology, with a high proportion of large, angular (poorly abraded) material. Barnacles, limpets, and algae encrusted much of the gravel suggesting that movement of gravel was impossible where boulders were densely distributed. The only areas where movement of gravel appears possible are on the beaches at the toes of any eroded mudslides and on those portions of the mudstone pavement where boulders are less densely distributed (generally adjacent to beaches).

Analysis of Coastal Monitoring Programme lidar and aerial photography data supports a net eastward drift of 3-10,000m³ per year along this frontage. Analysis of 2009 swath bathymetry data, offshore of Golden Cap, indicates that the variable thickness of nearshore sediments is insufficient to mask the underlying sub-tidal rock platform and former headland foundations. These features are clearly discernible between MLW and approximately 400m offshore, with rock outcrops further offshore. The lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability.

“Impeded” and “Unimpeded” drift conditions have been identified occurring at the headland according to the relative intensities of mudsliding and marine erosion. At present, the combined effect of active mudslide lobes and dense boulder aprons is to completely block littoral drift of gravel along beaches, adjacent parts of the shore platform and within the nearshore (“impeded” condition). However, old photographs show that landslide debris became sufficiently eroded between 1934 and 1949 (and partially from 1949-1962) to allow a continuous beach to exist from Charmouth to Seatown – the “unimpeded” condition (Brunsden, 1985; Bray, 1996). During such periods the headland operates as a one-way "valve" permitting only eastward drift. Analysis of shoreline orientation and dominant wave approach directions indicates that westward drift is minimal (Bray, 1996). A gross eastward drift of 8,600m³ per year was estimated from tracer experiments at St Gabriel's and this may be taken as being characteristic of the unimpeded condition at Golden Cap (Bray, 1996). Long-term transport around the headland therefore depends upon frequency of the unimpeded condition. Assuming that the period (1901-1987) is representative, full transport was possible for 15 years (plus 13 years of partial transport) in each 86-year period; equivalent to a long-term value of around 2,200m³ per year (Bray, 1990b). This analysis establishes the operation and direction of intermittent transport around the headland with high reliability, but quantitative information is less reliable chiefly due to assumptions relating to periodicity of the unimpeded condition.

The westernmost mudslide at Golden Cap (Photo 17) remains active and observations of similar, though larger mudslides at Black Ven suggests that at least 20 years of marine erosion is required from cessation of major activity to re-establishment of a beach (Brunsden, 1985; Bray, 1996b). The next episode of eastward transport is therefore not expected until 2020, or later.

LT8 Seatown Beach (see introduction to littoral transport)

Seatown Beach comprises a small pocket beach some 1.8km in length bounded by high headlands to the west and east (Photo 3). The beach is extremely sensitive to short-term drift reversals, which produce rapid redistributions of material and re-orientations of the beach from one end of the bay to the other. An additional arrow has been included to reflect that analysis of Coastal Monitoring Programme lidar and aerial photography data indicates a weak westward drift along this frontage. During periods of consistent wave approach from the south-west (most frequently), or the east, the updrift end of the beach can become totally stripped of gravel to expose the underlying bedrock platform. As demonstrated by Brunsden and Moore (1999), beach volume changes and distribution are therefore an unreliable guide to net littoral drift and simply demonstrate the tendency for the operation of both eastward and westward drift according to prevailing directions of wave approach (Laming, 1988; Bray, 1996).

Analysis of 2009 swath bathymetry data, between Golden Cap and Seatown, indicates that the variable thickness of nearshore sediments is sufficient to mask the underlying sub-tidal rock platform, with occasional rock outcrops interrupting a generally gently southward sloping seabed. However, the lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability.

A total of 3,000 pebbles were sampled by Bray (1996) and net eastward drift was indicated by longshore pebble grading which showed increased size, roundness, sphericity and sorting of linear dimensions to the east. The gradings were generally less well pronounced than on Charmouth Beach, which may indicate that drift is weaker or that clast sorting has been affected by historical beach mining at the site see section 4.2. Reliability is medium because sediment sampling was completed on a single day in 1985. However, numerous subsequent observations over several years suggest that the main trends identified were typical (Bray, 1990b) and studies by Laming (1988) and Bird (1989) have revealed a similar size grading.

LT9 Thorncombe Beacon (see introduction to littoral transport)

A continuous gravel beach formerly connected Eype and Seatown Beaches, but this link was broken by shingle depletion and subsequent landsliding at the Doghouse Hill (Photo 28) and Thorncombe Beacon (Photo 18) headlands (Brunsden, 1985; High-Point Rendel, 1997b). Beach depletion and recession increased the protrusion of the headlands that appears to have triggered landsliding that then deposited lobes of debris, which blocked drift. This sequence of events is thought to have been associated with renovation of West Bay Harbour (1742-46), subsequent infilling of piers protecting the entrance channel (1820s) and rapid depletion of West Beach, West Bay. Old charts reveal a near continuous beach extending around the headland toe in 1787 and depletion mostly occurred before 1948 because air photos suggest that only a patchy beach connection then existed around the headland (Brunsden, 1985; High-Point Rendel, 1997). Field inspection reveals the contemporary foreshore to be littered with limestone and sandstone boulders forming protective aprons, which are complete barriers to drift (Bray, 1996). Much well rounded shingle remains trapped in pockets between boulders and analysis reveals an identical character to that on Seatown and Eype beach, although there is no obvious local present day source from the eroding cliffs (Bray, 1996). The shingle pockets appear to be a residue from the former continuous beach prior to its depletion. This provides evidence of medium reliability supporting the idea of a former connection. Resumption of transport around the headland is possible theoretically, but would require shingle accretion sufficient to bury the boulder aprons and this is unlikely in the foreseeable future (Bray, 1996).

Analysis of 2009 swath bathymetry data, offshore of Doghouse Hill, indicates that the variable thickness of nearshore sediments is insufficient to mask the underlying sub-tidal rock platform and former headland foundation scars. These features are clearly discernible between MLW and approximately 300m offshore, with rock outcrops further offshore. There are localised patches of sediment that may appear to be constrained by the underlying geology. Further offshore sediments are of greater thickness. The lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals. Approximately 1km offshore southeast of the headland is an area of significantly large sinuous, relatively shallow bedforms on the northern margin of a rock outcrop, possibly formed due to localised variability of current directions and velocities. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability.

LT10 Eype Beach (see introduction to littoral transport)

This frontage comprises an open gravel beach from Thorncombe Beacon to West Cliff (Photo 2), where outcropping rock ledges and a rock groyne effectively separates West Beach from the main beach to the west. Analysis of Coastal Monitoring Programme lidar and aerial photography data indicates that net drift direction is not conclusive, as littoral drift is indicated to occur in both eastward and westward directions. Although the balance of evidence would appear to support a weak to moderate net component towards the east, in the order of 1-3,000m³ per year, a reduction from the estimated 2004 volume of 3-10,000m³ per year, there is also some evidence of a weak unquantified westward drift along this frontage.  

Longshore trends in beach volume, size, grading and sorting support net eastward drift on the main beach, although West Beach has become severely depleted over the past 150 years making identification of drift difficult – see Photo 35 (High-Point Rendel, 1997). Approximately 3,000 pebbles were sampled and analysis revealed that at high water mark and on the upper beach, pebble size increases eastward and sorting also improves (Bray, 1996). A subsequent pebble sampling survey undertaken by High-Point Rendel (2000) revealed generally similar results. Using the same argument as was applied to Charmouth Beach, a slight net drift to the east is suggested. No clear longshore trend in sphericity was discernable, but roundness improved westward suggesting drift in that direction. Fluorescent pebble tracer experiments undertaken over a short period also indicated a tendency for westward drift (Jolliffe - personal communication), but it is uncertain whether the time period was fully representative. Based on beach volume changes analysed using a sediment budget approach High-Point Rendel (1997; 2000a) estimate a west to east drift of 2,500m³ per year.

The long-term natural direction for drift is from west to east (High-Point Rendel, 1997), but human interference caused by the construction of the West Bay Harbour Piers, the West Bay coastal defences and the effects of historical beach mining have made it difficult to identify the drift regime that prevailed over the past 200 years.

Analysis of 2009 swath bathymetry data, between Thorncombe and West Bay, indicates that the variable thickness of nearshore sediments is sufficient to mask the underlying sub-tidal rock platform, with occasional rock outcrops interrupting a generally gently southward sloping seabed. However, the lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability.

LT11 West Bay (see introduction to littoral transport)

The piers at West Bay have effectively intercepted littoral drift since cavities in the structures were infilled in the 1820s (Jolliffe, 1979; Keystone Historic Buildings Consultants, 1997; High-Point Rendel, 1997). This has led to a trend for variable accretion/erosion and shoreline fluctuation to the east, but erosion and setback of the shoreline by up to 100m to the west (Photo 9). These effects are not simply the result of a classic terminal scour immediately downdrift of the piers because many researchers believe that drift operates for significant periods from west to east. Indeed, Rendel Geotechnics (1997) present a convincing argument that the coastal setback can be accounted for by differential sediment supply to the east (abundant supply from Chesil) and west (deficient supply from Eype Beach) of the piers.

Analysis of Coastal Monitoring Programme lidar and aerial photography data indicated a weak westward drift along this frontage. However, net drift direction is not conclusive, as littoral drift appears to occur in both eastward and westward directions. Onshore and offshore sediment transport also occurs between the beach face and inter-tidal foreshore to the sub-tidal limit of the beach profile, rather than fully offshore except under extreme conditions. Analysis of 2009 swath bathymetry data, offshore of West Bay, indicates that the variable thickness of nearshore sediments is sufficient to mask the underlying sub-tidal rock platform, with occasional rock outcrops interrupting a generally gently southward sloping seabed. However, the lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals.

The process of change either side of the piers only dates back to the mid-19th century, despite the harbour having been constructed in 1742-1746. Infilling of the piers and the consequent barrier effect of the structures is recognised as a critical factor in this coastal change (Jolliffe, 1979; Hydraulics Research, 1978; Brunsden, 1985; High-Point Rendel, 1997). The following sequence of events has been identified based upon a series of models presented by High-Point Rendel (1997):

1. Before construction of the present general harbour configuration in 1742-46 (Keystone Historic Buildings Consultants, 1997), littoral drift would have been virtually unhindered in either direction, because the relatively small channel of the river Brit would have been the only obstacle. Drift is likely to have deflected the river mouth and blocked it periodically to allow intervals of free transport. It can be envisaged that a continuous beach would have stretched from the Isle of Portland, past West Bay to Thorncombe Beacon and probably as far west as Seatown (Brunsden, 1985; High-Point Rendel, 1997);  
2. After harbour construction in 1746, channel maintenance by flushing using a sluicing system and training structures probably interfered with drift but had little net effect because no major trend for accretion or erosion was recorded and a continuous beach evenly distributed on either side of the piers was maintained (Jolliffe, 1979; High-Point Rendel, 1997; Brunsden and Moore, 1999);
3. Beginning in the late 18th century landslides along the Doghouse Hill and Thorncombe Beacon headland engulfed cliff toe beaches and severed the drift pathway from the west. This isolated Eype and West Beaches from their only significant sources of natural supply;
4. After 1820s, the infilled piers intercepted littoral drift and exchange between West Beach and East Beach was reduced. In combination with events outlined in iii. the effect was to cause depletion, erosion and set-back of the West Beach by up to 100m (Jolliffe, 1979; High-Point Rendel, 1997; Brunsden and Moore, 1999). The situation was exacerbated from the late 19th century onward when the first esplanades and seawalls were constructed, causing wave reflection and scour. A total loss of 500,000m³ of beach material is estimated since the mid-19th century. The separate identities of Eype, West and East beaches therefore date from this time.

Periodic occurrences of shingle accumulations in the harbour entrance indicate that the piers do not form a complete barrier to drift and limited bypassing is possible. This process has probably been facilitated by the documented accretion pattern to the east of the piers that built out the beach to the tip of the east pier by the early 1980s (Jolliffe, 1979; Hydraulics Research, 1978, 1985, 1991a). Fluorescent pebble tracer experiments undertaken over an 18-month period demonstrated shingle transport from the East to the West Beach, but no return movement was recorded or likely due to the large coastal setback (Jolliffe, 1979). Physical model studies also showed that shingle bypassing could operate from east to west, but not in the reverse direction (Hydraulics Research, 1991b). Based on the amounts of dredging typically required to maintain a navigable harbour entrance channel a minimum bypassing rate of 1,200m³ per year has been estimated. Due to the depleted nature of West Beach it is thought that very little material passes from East Beach across the harbour entrance.

LT12 Western Chesil Beach incl. East Beach (see introduction to littoral transport)

Chesil Beach is a swash-aligned barrier so it is to be expected that gross drift is likely to be significantly greater than the net drift typically occurring. The great length and massive volume of the beach very effectively buffer any short term changes in sediment distribution and plan-shape morphology unless trends in sediment transport are long established. Exceptions occur at each end of the beach where hard features restrict the movement of beach sediments to produce patterns of accretion and/or erosion within years to decades according to shifts in wave climates. East Beach, West Bay is perhaps the most frequently studied location and a series of studies provide the main basis from which to attempt to understand drift on the western portion of the beach.

The marked coastal setback to the west of the West Bay piers would appear at first sight to be the result of a classic terminal scour process immediately downdrift of the piers enabling confident identification of net east to west drift (Hydraulics Research, 1978; 1985; Jolliffe, 1979). Map comparisons and field survey covering 1901-91 showed that accretion was not continuous, but interspersed with significant periods of erosion e.g. 1961-64 and 1985-91 (Hydraulics Research, 1985; 1991a; Brampton, 1993). The end result has been a fluctuation of shoreline position against the East Pier at West Bay such that MHW has varied by up to 65m with little net change (High-Point Rendel, 1997; HR Wallingford, 2000c). The setback was thus produced by depletion to the west and not by net accretion to the east, making it difficult to apply as an indicator of net drift.

Analysis of Coastal Monitoring Programme lidar and aerial photography data indicated a weak but unquantified eastward drift along this frontage. However, net drift direction is not conclusive, as littoral drift appears to occur in both eastward and westward directions. Therefore, the arrows have been revised to indicate that no quantitative volumes have been determined, compared to the 2004 speculative 10-20,000m³ per year of eastward and westward movement. Onshore and offshore sediment transport also occurs between the beach face and inter-tidal foreshore to the sub-tidal limit of the beach profile, rather than fully offshore except under extreme conditions. Analysis of 2009 swath bathymetry data indicates that the thickness of nearshore sediments is sufficient to mask the underlying geology, with the exception of the extensive rock platform along the Burton Bradstock section. This platform, which extends 400m offshore is covered with a relatively thin veneer of sediment. Offshore, 900m due south of West Bay, are rock outcrops interrupting a generally gently southward sloping seabed. However, the lack of bedforms or sediment accumulations connected to the underlying geology provides no conclusive evidence of either net eastward and westward sub-tidal sediment transport or short-term, localised reversals. Littoral drift rates varies according to wave height and approach direction, and is also dependent on shingle and sand availability.

It has proven difficult to understand drift at this location due to variations in wave climate to which the rate and direction of drift are extremely sensitive. SW approaching waves generate eastward drift so that East Beach erodes, whereas SE waves generate westward drift causing the beach to accrete against the eastern pier (Photo 9). Increase of the beach width to beyond 160m extended the beach face towards the end of the pier caused bypassing of gravel and accumulation in the harbour entrance. Erosion to widths of less than 120m significantly increases the risk of overwashing and possibly breaching e.g. Photo 36 (HR Wallingford, 2000c).

Fluorescent concrete tracer experiments undertaken over an 18 month period on East Beach adjacent to the piers revealed net westward drift (Jolliffe, 1979), but it is uncertain whether the study period was typical. The authenticity of these results was also questionable on the basis of the concrete pebbles used potentially being an unrepresentative tracer material (Carr, 1980). Littoral drift between Cogden and West Bay was further studied by means of a mathematical beach plan shape model (BEACHPLAN) employing a wave climate derived from hindcasting based on Portland Wave Data covering the period 1974-84 (Hydraulics Research, 1985). This study marked the first attempt to determine a reliable long-term estimate of drift on Chesil Beach and indicated mean net westward transport at 8,000m³ per year, which was validated by the documented trend for accretion against the East Pier. The application of the analysis must be questioned because it ignored swell waves and only compiled a 10 year wave record to represent the long term wave climate.

Subsequent observations of the beach in fact indicated it to be eroding, suggesting a possible change in drift regime. Analysis of beach profiles and air photos covering the period 1977-1990 revealed that the previous widely recognised trend for accretion ceased about 1982, whereupon a convincing switch to erosion resulted in MHWM retreat of 40m by March 1990 - compare Photo 7 with Photo 9 (Hydraulics Research, 1991d). To explain this change, wave climate and littoral drift were analysed with techniques similar to those employed for the 1985 study (Hydraulics Research, 1991d). Wave climate was hindcast using Portland wind data covering the period 1974-1990. Results showed that the wave climate changed remarkably after 1982 with fewer south-easterly storm waves and increased prevalence of westerly waves. Littoral drift calculations from this data revealed highly variable gross transport, with net westward drift before 1982 and a switch to net eastward drift of up to 14,000m³ per year thereafter (Hydraulics Research, 1991d). This model was re-run subsequently to include hindcast wave data covering the period 1974-96. The results reported a net east to west drift of some 18,000m³ per year (range eastward 10,000m³ per year to westward 70,000m³ per year over this period (HR Wallingford, 2000c). This does not accord with the net pattern of accretion and contradicts previous studies (e.g. Hydraulics Research, 1991d; High-Point Rendel, 1997), so that bypassing of the East Pier at a rate of some 17,000m³ per year was proposed. However, research cannot explain where this gravel might be transported to because the West Beach has depleted and no significant quantities of gravel have been identified by nearshore surveys (High-Point Rendel, 2000). These uncertainties suggest that the model may have significantly overestimated the true long term net drift and its results would need to be interpreted with caution by coastal managers.

It appears that net littoral drift is very delicately balanced at this western end of Chesil Beach so that slight wave climate/storm frequency variations can cause significant drift reversals as clearly indicated by previous studies e.g. Brampton (1993). Available hindcast wave data still cover too short a time span to be sure that a truly “typical” wave climate has been defined making it difficult to establish a long term net drift rate and direction. It is also difficult to predict the likely future rates and directions of drift due to the possible effects of climate change as explained in the following section. Flexible and adaptive beach management underpinned by regular monitoring should provide for the best way to cope with the uncertainties.

LT13 Central and Eastern Chesil Beach (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme lidar (2007 and 2011), aerial photography (2012) and baseline topographic data indicates that the net drift direction on Chesil Beach is more difficult to discern for it is a swash-aligned feature and net drift rates are very low compared with the quantities of eastward and westward drift occurring. Onshore and offshore sediment transport also occurs between the beach face and inter-tidal foreshore to the sub-tidal limit of the beach profile, rather than fully offshore except under extreme conditions.

Chesil Beach is a swash-aligned barrier so it is to be expected that gross drift is likely to be significantly greater than the net drift typically occurring. The great length and massive volume of the beach very effectively buffer any short term changes in sediment distribution and plan-shape morphology unless trends in sediment transport become long established. Exceptions occur at each end of the beach where hard features restrict the movement of beach sediments to produce patterns of accretion and/or erosion within years to decades according to shifts in wave climates. Research into the nature of drift on the beach and the distribution and sorting of its pebbles has involved the following methods (i) a series of sedimentological and topographic studies that essentially described the features of the beach e.g. Carr and Blackley (1974); (ii) tracer experiments using a variety of materials to simulate pebble movement e.g. Carr (1971) and (iii) numerical modelling approaches that attempt to calculate rates of transport based on wave climate data e.g. Babtie Group (1997).

Much of the central part of the beach, particularly the segment between Cogden and Langton Herring has until recently received relatively little attention except for sedimentological and topographic studies (Neate, 1967; Carr, 1969; Carr and Blackley, 1969; Carr and Gleason, 1972; Carr and Seaward, 1991). These studies yielded much valuable information, but failed to provide any clear indication of the net drift direction. Most earlier research has concentrated on the eastern segment between Wyke Regis and the Isle of Portland. Early tracer experiments using brickbats (Richardson, 1902) and painted pebbles (Adlam, 1960) showed net eastward transport governed by waves, which also caused preferential movement of larger material. These experiments were over very limited time periods so their results were in no way representative of long-term transport. More extensive and detailed experiments undertaken with pebbles of a foreign or exotic lithology confirmed the more rapid eastward transport of larger material (Carr, 1971). These experiments demonstrated marked eastward drift at Wyke Regis, but more variable and random movement near Portland. It was concluded, therefore, that there was no permanent net drift near Portland, thereby accounting for the absence of a vast accumulation of pebbles at the eastern end. Whilst providing useful information relating to sorting processes (e.g. Carr, 1974), the overall patterns of drift recorded by these experiments were inconclusive because recoveries were low and unrepresentative of injected tracers and the experimental period was not fully typical of long-term conditions.

Modelling of drift using a long term hindcast wave climate for the Lyme Bay and South Devon SMP indicated that there was zero net drift, although gross drift to the east and west were significant. Chesil was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point off Chiswell at –4.1mOD. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 3-7% variation in longshore energy and confirmed that the beach was significantly more sensitive to this factor than most other south coast locations, as might be expected of a swash aligned coastline.

Future drift variability was investigated specifically by Halcrow Maritime, et al., (2001) who undertook modelling of likely future wind speeds for a climate change scenario representing 2080 using the Met Office Hadley Centre Regional Climate Model. Initially, the present day conditions were modelled at Chiswell and West Bexington prior to examination of future conditions. Wind speeds output by the Regional Climate Model were used to derive offshore extreme wave conditions in Lyme Bay and results demonstrated a potential for significant increases in wave energy e.g. 1 in 50 year wave height of 8.1m could increase to 11.3m by 2080s. The hindcast waves have also been used to study the potential changes in alongshore sediment transport. The present condition at Chiswell was established as being a net westward drift of 900m³ per year based on a typical gross drift of 39,000m³ per year to the west and 38,000m³ per year to the east. At West Bexington a net eastward drift of 1,000m³ per year was estimated based on a typical gross drift of 21,000m³ per year to the west and 22,000m³ per year to the east. Assessment of the climate change results for Chiswell indicate a potential for a dramatic shift in the sediment transport regime due to a small two-degree southerly shift in the mean wave approach direction. The current net westward drift potential of 900m³ per year would alter under this scenario to a net westward drift of up to 15,000m³ per year. At West Bexington, the current net eastward drift potential of 1,000m³ per year would reverse to a net westward drift of up to 7,800m³ per year (Halcrow Maritime, et al., 2001). If such changes were to occur depletion could occur rapidly at Chiswell generating coastal defence problems, whereas at West Bexington incoming sediment would compensate for losses so there would be no perceptible changes for many decades. In the long-term this pattern of transport alteration would cause a slight southerly re-orientation of the beach with significantly increased risk of breaching between Wyke Regis and Chiswell.

A similar type of study was undertaken by Sutherland and Wolf (2002) using an alternative climate model to generate future wave scenarios and then to simulate drift on Chesil Beach up to the year 2075. In this instance the present condition for a beach facing 225 degrees was estimated as being an eastward drift of 24,000m³ per year. Results suggested that net drift could in future increase by up to 30% due to the potential effects of climate change. The two numerical modelling studies discussed above present conflicting results with respect to establishing the present conditions on Chesil Beach. This is because they have set out to examine future scenarios rather than to focus on the present and to precisely predict the future. They have, however, generated some valuable insights suggesting that (i) waves in Lyme Bay are likely to vary with future climate change and (ii) Chesil Beach is likely to be sensitive to these changes with the potential for reversals and accelerations of drift. If such changes were to occur variations in beach morphology and volume would become most apparent at the two extremities of the beach where transport is impeded.

The unique longshore size grading of the beach is possibly related to the delicate balance of littoral drift along the coast. Most theories attribute the grading to wave action, although several in the 19th century, now largely discredited, also considered tidal mechanisms (Carr and Blackley, 1974). Differential transport has been widely advocated, with large waves from the south-west transporting all pebble sizes to the east, whilst smaller waves from the south east could only transport smaller pebbles back towards the west. The latter would almost certainly account for the considerable accumulation of fine shingle to the east of the east pier at West Bay. This pattern of movement was first advanced by de Luc (1811) and embellished by later authors to include concepts of sorting and abrasion. Preferential transport of larger pebble sizes in tracer experiments (Richardson, 1902; Jolliffe, 1964) led to the belief that pebble selection was associated with breaking wave height (Jolliffe, 1964). However, the first detailed sedimentological and tracer studies indicated that pebble thickness was the critical parameter and grading on the beach as a whole was unlikely to have a direct relation to breaking wave height (Carr, 1969; Carr, 1971). Further studies revealed that pebble sorting was also related to wave frequency, the square root of significant wave height and the angle of swell approach (Gleason and Hardcastle, 1973). A review by Bird (1996) attributed the unique longshore size grading to a complex interaction of factors arising from local sediment availability coupled with the incident wave climate, especially the increasing wave energy towards the east of the beach. Carr and Blackley (1974) also identified the fossil nature of the beach as an important factor, arguing that fresh sediment inputs would tend to diminish size grading. However, Bray (1996; 1997a) and Brunsden (1999) argued that massive recent supply of gravel from cliffs to the west was important in ensuring a wide range of clast sizes was available to feed processes of sorting and differential transport. Brunsden (1999) also advanced a novel concept longshore moving sediment “slugs” as being an important feature of the sorting process.

4. Sediment Outputs

The Isle of Portland acts a large terminal groyne that intercepts coarse beach sediments preventing them from leaving this coastal unit. There are, however, notable processes by which sediment is lost from the shoreline. Fine sediments are winnowed from beaches by waves and deposited offshore in Lyme Bay, whereas coarse sediments suffer losses from attrition and entrapment. Finally, this unit has a history of beach mining that has removed substantial quantities of shingle from the shoreline.

4.1 Onshore to Offshore Transport

Coastal landsliding transports large quantities of sediment to the beach (see section 2.2). Only the coarser materials are stable on the foreshore when exposed to wave action. The majority of sediments supplied are degraded and weathered clays, shales, mudstones and weakly cemented sandstones. These readily disintegrate under wave attack and their fine constituent grains/particles are transported offshore in suspension e.g. Photo 15. Transport pathways are uncertain, but it is likely that a significant proportion of sediment is deposited in Lyme Bay, because offshore surveys reveal that silts and fine sands of similar character are the dominant surface sediment within the bay (Institute of Geological Sciences, 1983; Dobbie and Partners, 1980; Offshore Environmental Systems Ltd., 1980; Bray, 1986). Clays are likely to be transported more widely into suspension to contribute to estuaries within Lyme Bay and possibly more widely along the English Channel. The offshore transport of fines therefore closely follows the pattern of sediment supply, with large quantities yielded from Black Ven and Stonebarrow and extremely small quantities from Chesil Beach (Bray, 1986; 1996).

Gravel and coarse sand are more stable on the foreshore and are generally the beach forming sediments (Bray 1986). Significant offshore shingle transport is unlikely due to absence of offshore banks or "sinks" revealed by the review of offshore information undertaken in section 2.1. Offshore loss of sand from the foreshore is likely, because extensive areas of the Lyme Regis sea bed are covered by this material type (Darton, et al., 1980; Institute of Geological Sciences, 1983; Bray, 1996; High-Point Rendel, 2000; West Dorset District Council, 2000a). Available evidence of offshore transport revealed the following pathways.

A variety of studies including physical modelling (Hydraulics Research, 1989; HR Wallingford, 2003), numerical modelling (HR Wallingford, 2001) and geomorphological interpretation (High-Point Rendel, 1999b) indicate formation of an offshore transport pathway from the central part of Marine Parade during eastern and south-eastern wave conditions, although it is uncertain whether material is lost permanently, or whether it can be transported back onshore during favourable conditions. It appears that an offshore-directed current can transport sands and some gravels seaward down Town Beach Channel as identified by (High-Point Rendel, 1999b). Large quantities of sand are supplied to Charmouth Beach by coastal landslides (section 2.2) and significant quantities are present on the beach face and lower foreshore on the western part of the beach – see Photo 4 (Bray, 1996). Studies revealed net eastward littoral drift, yet the proportion of sand declines in this direction and very little is present east of Westhay Water. Progressive offshore loss of sand is therefore inferred, as it is transported eastward on the beach (Bray, 1996).

Analysis of 2009 swath bathymetry data, offshore of Lyme Regis, between Charmouth and Golden Cap, and offshore of West Bay indicates a lack of bedforms or downdrift accumulations of sediment in connection with rock ridge outcrops, and provides no evidence for an offshore pathway or offshore sinks of sediment. The speculative 2004 arrows have therefore been removed. Fine grained material is transported into the nearshore zone in suspension and not retained on the foreshore. The variable thickness of nearshore sediments is generally sufficient to mask the underlying sub-tidal rock outcrops, with occasional rock outcrops interrupting a generally gently southward sloping seabed.

4.2 Beach Gravel Mining

History of Extraction

The beaches of South-West Dorset have a long history of mining, which extends back at least 700 years at West Bay (Hydraulics Research, 1978; Jolliffe, 1979; High-Point Rendel, 1997). All operations have now ceased due to concerns over probable adverse coastal defence and environmental impacts. The last remaining operations ceased in 1986. The pebbles taken have been traditionally used for construction and ornamental purposes. Additionally, durable well-rounded pebbles have been particularly valuable for water filtration and as grinding media in the ceramics industry. Rounded to sub-rounded chert and flint pebbles of 20-60mm long axis were favoured for extraction at Seatown. Larger, more spherical pebbles were usually hand-picked where available and smaller “pea” gravel has been removed from East Beach, West Bay and sites. Estimates of volumes and locations of extraction are only available for this century and are presented in Photo 37. These figures are based upon details extracted from files held by Dorset County Council and from compilations of such information (e.g. Jolliffe, 1979; Carr, 1980a; Bray, 1996).

Carr (1980a) estimated that between the mid-1930s and 1977, a minimum of 470,000 tonnes of gravel was taken from the East Beach and 370,000 tonnes from Cogden Beach. During this time, selective picking of larger more spherical pebbles was conducted in the Chesil Cove (Chesilton) area involving removal of 9,400 tonnes between 1944 and 1972. This practice could have adversely affected size grading and crest level locally and was discontinued in 1973 after a public inquiry. It is estimated (Carr, 1980a) that 1.1 million tonnes of gravel was extracted from Chesil Beach between the mid-1930s and 1977. Given a total estimated volume of pebbles and cobbles of between 25 and 100 million tonnes (Carr, 1980a) this suggests that between 1.1% and 4.4% of the beach was removed over this period. Although the most intense extraction occurred during the Second World War and immediately afterwards, large quantities were also extracted prior to the 1930s. For example, at least 50,000 tonnes of gravel were extracted between 1905 and 1907 for the foundations of the oil tank depot at Portland Naval Base (Carr, 1980a; 1983b). Extraction ceased on Cogden Beach, the last remaining active site on Chesil, in December 1986 (Photo 38).

Further to the west, the major extraction site was Seatown Beach (Photo 3). Between 1939 and 1945 large quantities of gravel were removed and eyewitness accounts suggest that the beach was almost completely stripped (Rhoden, 1974; Brundell, 1985; Laming, 1988). Assuming that the pre-1939 beach was of similar volume to the contemporary beach, this entailed removal of 240,000 - 270,000m³ of gravel (Laming, 1988; Bray, 1996). Extraction records held by Dorset County Council Planning Department revealed that between the mid-1950s and 1970, an estimated 38,000 tonnes were removed (2,500 tonnes per year) and from 1971 until extraction ceased in April 1987, removal totalled 17,000 tonnes (1,000 tonnes per year). Total beach gravel extraction since 1939 is therefore estimated at 275,000-305,000m³. There are no records of extraction from Seatown Beach prior to 1939, but photographic evidence of vehicle tracks on the East Beach (Lang, 1932) suggest that some may have occurred.

It should be appreciated that it was difficult to check on the exact quantities and qualities of gravel removed. Many of the figures quoted above are based upon planning consents and it has to be recognised that local authorities did not possess the resources to continually check that extraction remained within the approved quotas. Consequently, these figures may be underestimates.

Effect of Extraction

Chesil Beach is widely recognised as a unique scientific feature and also performs a vital coast protection/sea defence function. The beach is generally held to be a fossil barrier receiving no significant contemporary supply (Carr and Blackley, 1974; Jolliffe, 1979; Carr, 1978, 1980a, 1983b, 1883c; Bray, 1996; High-Point Rendel, 1997). Thus, it was argued successfully at a public planning enquiry in 1985 that any losses were irrecoverable and would lead to reduction of volume and detrimental effects to the scientific value and coast protection function (Carr, 1985). More specifically, it was also argued that extraction sites can become locally depleted in the short term, with consequent reduction of equilibrium profile during wave attack and possible flooding and crest recession (Brampton, 1985). Localised beach depletion may also temporarily cause exposure of soft underlying beach core sediments rendering them liable to erosion (Brampton, 1985). These effects were supported by mathematical modelling of Cogden Beach with the assumption of continued beach extraction (Hydraulics Research, 1985).

Seatown Beach was subject to major wartime extraction, which must have severely depleted the beach and may be linked with beach loss from the foreshore beneath Thorncombe Beacon that has isolated Eype and West beaches from their sources of supply and has therefore contributed to their depletion (High-Point Rendel, 1997). Transport around the Golden Cap headland also became blocked partially in the late 1940s and completely blocked by the early 1960s, so Seatown Beach has been an isolated pocket beach since this time (Brunsden, 1985; Bray, 1996). All subsequent extraction was therefore a net loss, which reduced the protection afforded to the base of the cliffs. Cliff retreat has accelerated since 1948, although the link with extraction cannot be conclusively proven (Bray, 1986). Conerns over cliff recession has led to construction of a cliff stabilization scheme at Seatown (WS Atkins Ltd., 1996). Pebbles selected for extraction were larger and more durable than the average for the beach (Laming, 1989; Bray, 1990b), so extraction also affected the sedimentological character (Bray, 1996). Sampling has shown that Seatown pebbles are very similar to those on neighbouring beaches in terms of lithology, shape and sphericity but their mean size and roundness are less (Bray, 1996). Since these beaches are considered to share a common gravel source and eastward transport pathway the differences may reflect the effect of extraction (Bray, 1996). Pebble size reduction may result in flattening of the profile and landward migration of the beach crest, while loss of durable pebbles may expose less resistant materials to erosion (Bray, 1996).

Dredging

Dredging is undertaken in the West Bay harbour entrance to maintain a navigable channel. The amount removed has averaged 1,200m³ per year since at least 1964 (Jolliffe, 1979; Hydraulics Research, 1985; High-Point Rendel, 1997; HR Wallingford, 1997b) and this therefore provides a minimum for bypassing of the piers. Annual dredging since 2007 removes approximately 5,000m³ per year of sand from the outer harbour and placed on West Beach (Robert Clarke, West Dorset Council, pers. comm., 2012).

4.3 Attrition

During transport by wave action on the beach, gravel clasts are subject to repeated impacts with other clasts and with the substratum. Attrition rate varies inversely according to the amount of attrition a clast has already undergone. Thus, fresh angular gravels from coastal landslides are most susceptible, whilst well-rounded beach pebbles only abrade very slowly (Bray, 1990b). Detailed comparative study of the morphometry of cliff gravels with beach gravels indicated that attrition output involved two elements:

1. Attrition loss of freshly supplied gravel during the first year on the beach. This is estimated at 10% of the annual supply (Bray, 1996);
2. A much slower attrition loss of gravel stored on the beach in excess of one year. This was determined indirectly by examination of improved pebble rounding downdrift and comparison of drift rates and duration (Bray, 1996). This technique indicated attrition loss equivalent to 0.2%pa of the beach store. This estimate requires validation because some pebbles may spend much time buried and hence attrition does not continually affect the relatively angular pebbles of Charmouth Beach. Losses are much less on more mature beaches so adjustment was required for Seatown and Eype Beaches (Bray, 1996). Using these techniques, the attrition outputs were calculated:

• Charmouth Beach = 1,260m³ per year
• Seatown Beach = 84m³ per year
• Eype Beach = 82m³ per year

These rates reflect rapid attrition of angular material supplied by landslides at Charmouth Beach. The technique was adapted for Chesil by High-Point Rendel (1997) to reflect its high clast roundness (pebbles resistant to attrition) and very great beach volume such that only a small proportion of clasts would be subject to attrition at any one time. They estimated that losses of around 0.01% could occur per annum within a mobile shingle layer amounting to around 2million cubic metres. Results indicated that attrition losses on Chesil could amount to around 200m³ per year. Attrition is therefore a small, but continuous output that is especially important in clise proximity to cliff erosion inputs and on beaches such as Seatown, Eype and Chesil that are isolated from sources of fresh gravel supply.

5. Transport in the Offshore Zone

» O1 · O2 · O3

Until recently, very little attention had been given to sediment transport in the offshore zone. Some broad pathways had been identified from bedforms and others were postulated according to the patterns of residual tidal currents. The Dorset Inner Shelf Seabed Mobility Study (Bastos and Collins, 2002) has made a major contribution to knowledge covering an area of around 10km to the east and west of Portland Bill and up to 5km offshore from the tip of the Bill. The study set out to investigate wave and tide driven transport at the bed, to identify transport pathways, predict seabed mobility and identify the effects of sandbanks and shoals. It involved assembly of existing information on seabed sediments and morphology, tidal currents and waves. Original field measurements included hydrographic and sidescan sonar surveys, sediment sampling, seismic boomer surveys of sediment layer thickness and 3D tidal current velocity profiling (Acoustic Doppler Current Profiler). Data collected were applied to develop numerical models of tidal flows (TELEMAC 2-D), wave refraction (RCPWAVE) and sediment transport at the bed (SEDTRANS). Results from the modelling were compared against bed morphology and bedforms to verify, where possible, processes and pathways. Outputs from the study were presented in a series of maps within an atlas of seabed mobility and transport pathways. Topics included included bathymetry and bed morphology, sediment distribution and thickness, bedforms, maximum and residual depth-averaged tidal currents seabed mobility for various grain sizes and hydrodynamic conditions, sediment transport directions and an interpretation of key pathways. The following summarise some of the key results for the area to the West of Portland Bill:

1. A symmetrical arrangement of banks was identified to either side of the Isle of Portland. To the west, the sandy/shelly West Shoal (4-8m sediment thickness) was identified in 30m water depth. Sand waves upon its surface strongly suggested east-south-eastward transport. To the southwest, the highly shelly Portland Bank was identified (8-12m sediment thickness). Large sand waves upon its surface indicated convergent, or possibly clockwise rotational transport;
2. Tidal currents increase from 0.25-0.50ms-¹ within Lyme Bay southwards with strong currents of up to 2.5ms-¹ in the vicinity of Portland Bill. During the tidal cycle tidal eddies develop on both sides of Portland Bill;
3. There is a high mobility of the generally sandy bed sediments. Fine sand was estimated to be mobile around the Bill and throughout much of eastern Lyme Bay for greater than 90% of the time. Coarse sand was estimated to be mobile for some 75-90% of the time over Portland Bank and for 50-90% over West Shoal. Its mobility was otherwise limited to areas close inshore within eastern Lyme Bay where heavy wave action can mobilise bed sediments;
4. Net sand transport operates towards Portland Bill and zones of sand convergence have developed to the east and west of the headland in association with the tidal eddies and the occurrence of sandbanks. Bastos and Collins (2002) provide two rather more detailed interpretations of the transport pattern in relation to (i) tidal eddies and (ii) peak tidal current asymmetry. The transport paths at and around the sandbanks are verified by bedforms and are believed to be of medium to high reliability;
5. Maximum transport potential occurs at the tip of Portland Bill where the bed for several kilometres around comprised scoured bedrock because the transport conditions are erosive and too energetic for any local sediment to remain.

O1 General Eastward Flow, Lyme Bay (see introduction to offshore transport)

A general eastward flow is indicated in Lyme bay for fine sands. The flow is directed towards Portland Bill where is strengthens, being sufficient to transport coarse sands. It is produced by the combined interaction of tidal and wave induced currents at the bed.

Tidal models validated by measured currents revealed a general eastward residual tidal flow within 10km of the shore in Lyme Bay and a westward flow further seaward (Bowles, et al., 1958, Pingree and Maddock, 1977a; 1977b; Bastos and Collins, 2002). The east moving stream generally flowed parallel to the coast from Lyme Regis to Chesil Beach and was then deflected south and concentrated significantly by the Isle of Portland (Pingree and Maddock, 1977a; Bastos and Collins, 2002). This information was supported by studies undertaken for the Lyme Regis and Charmouth sewage outfalls (South-West Water, 1979; Offshore Environmental Systems Ltd, 1980; Smith, 1989). These investigations included current metering, float tracking, bacterial dispersion tests and side-scan sonar. Peak surface currents of 0.45ms-¹ and bed currents of 0.30ms-¹ were measured. Bastos and Collins determined tidal depth averaged currents of 0.25-0.50ms-¹ inshore within Lyme Bay increasing southwards and eastwards with strong currents of up to 2.50ms-¹ in the vicinity of Portland Bill.

Numerical modelling studies found that the fine sands of the bed of Lyme Bay would be highly mobile for much of the time making it likely that sediments are transported east-south-eastwards towards Portland Bill (Bastos and Collins, 2002). Bedforms mapped off Lyme Regis (Offshore Environmental Systems Ltd., 1980; West Dorset District Council, 2000a; Badman, et al., 2002) and along the surface of West Shoal (Bastos and Collins, 2002) confirm this transport direction.

O2 Tidal Eddy West of Isle of Portland (see introduction to offshore transport)

A clockwise tidal eddy that alters its location, size and strength during the tidal cycle was identified to the south-west of Portland Bill by Bastos and Collins (2002). A feature of this type had also been identified earlier by (Pingree and Maddock, 1977a; 1977b; 1983) who attributed it to deflection of tidal flows by the Isle of Portland. Its significance for bed sediment transport is uncertain although it appears to result in a convergence of sand transport over Portland Bank, and is possibly the mechanism that has formed and maintained the bank (Bastos and Collins, 2002). Cause and effect is of course difficult to determine for a bank begins to alter its own hydrodynamic environment as it forms.

An area of longitudinal furrows and ridge bedforms has been recognised some 10km off Portland Bill (well seaward of the limit of the Bastos and Collins 2002 study area) with orientations parallel to offshore flow (Institute of Geological Sciences, 1983). Surface sediment is predominantly gravel in this area (IGS, 1983), so it is possible that sediments up to this size are transported.

O3 Tidal Eddy East of Isle of Portland (see introduction to offshore transport)

A strong southwestward directed residual tidal flow operates from the east of the Isle of Portland and is associated with an anticlockwise tidal eddy and convergent sand transport focused upon the Shambles Bank. It converges upon Portland Bill and contributes to a potential for net southward transport at this point. These processes occur outside of the area of the unit currently under examination and are discussed fully within the Unit text covering the Isle of Portland and Weymouth Bay.

6. Sediment Budgets

Integration of sediment transport information enabled calculation of gravel budgets for Charmouth, Seatown and Eype Beaches by Bray (1996). These budgets are briefly presented here, but the original source should be consulted for further details. A budget for Chesil Beach based on that compiled by Rendel Geotechnics (1997) is also presented.

6.1 Charmouth Beach

A detailed long-term budget of the beach and landslide system is presented in Table 2 and Photo 39 based on research by Bray (1996). The analysis revealed total inputs of gravels to the beach of some 6,400m3a- 1 (96% from coastal landsliding) and total output of 3,900m³ per year (33% attrition, 55% littoral drift at Golden Cap). Mean accretion was therefore indicated at 2,500m³ per year. This was slightly greater than recorded beach accretion, although the difference could easily be explained by (i) past unauthorised beach shingle extraction at Charmouth, (ii) some undetected nearshore storage, and (iii) a lag between backscar release (used to calculate long-term input) and actual supply to the beach at Black Ven where landsliding has accelerated at the backscar.

A gravel flow model constructed by Bray (1996) graphically demonstrates the supply and subsequent distribution of gravel on this beach (Photo 40). A key point is that the major beach accretion is towards the east, whilst the major landslide supply sites are to the west. Virtually no littoral drift is received from the west due to The Cobb and Lyme Regis defences. The western part of the beach system is therefore characterised by high sediment delivery from the cliffs and throughput on the beaches with relatively little beach storage. No material is sufficiently stable on the western part of Charmouth beach to provide long-term protection to the toe of the cliffs. Clay and sand are supplied in large quantities but are easily eroded and transported offshore. Limestone boulders, chert and flint gravel provide only temporary protection because of the low durability of the former and the potential for longshore transport of the latter. The active phase of landsliding initiated at Black Ven in the late 1950s is therefore likely to continue, or intensify because natural basal protection is unlikely to keep pace with increasing future sea-levels without increased landslide throughput. It is likely to supply increasing quantities of gravel to the beach due to accelerating recession rates and lateral extension of the active landslide complex.

In recent decades landslides at Golden Cap have intercepted beach transport so that a period of beach accretion can be anticipated. Eventually the landslide debris should suffer marine erosion sufficient to enable a surge of drift as the sediments impounded rapidly move around the headland to Seatown Beach.

6.2 Seatown Beach

Gravel budgets for several intervals as well as a mean long-term budget are presented in Table 3 based on research by Bray (1996). The budget of Seatown Beach has until recently been dominated by phases of input from eastward littoral drift beneath Golden Cap (unimpeded transport) and output by mining. Both processes have operated intermittently, so that the shingle budget has been time dependent.

The effects of these changing inputs and outputs are illustrated by the alternative budgets calculated for various time intervals and explained as follows:

1900-1949

Over this period Seatown Beach received significant gravel inputs from neighbouring Charmouth Beach during an "unimpeded" transport condition covering the period 1934-1949. However, it was also subject to large-scale mining during 1939-45 that would have more than offset the gains. An overall net loss of 32,000m³ is estimated for this period.

1949-1987

During the early part of this period the beach continued to receive additional material as shingle drifted intermittently beneath Golden Cap before landslides finally blocked that pathway in 1960. Thereafter, beach mining was the major influence, resulting in net shingle loss and beach depletion. Recent increases in the recession of both the base of the sea-cliffs and the backscar behind the East Beach may have resulted from loss of protection caused by this factor and have had to be controlled by a cliff stabilisation scheme in one instance (see E7). The effects are compounded because extraction involved preferential removal of the larger more durable pebbles.

Present Situation

At present, the shingle budget of Seatown Beach is delicately balanced with relatively small inputs and outputs resulting in slow net depletion of 39m³ per year. This situation is expected to continue for at least the next twenty years until marine erosion of mudslide lobe barriers beneath Golden Cap is sufficient to permit incoming shingle drift from Charmouth Beach.

Future Evolution

Understanding of the general principles of the Seatown Beach gravel budget allows estimation of future evolution given that (i) the rate of sea-level rise will accelerate in future and (ii) no further beach gravel extraction will be permitted.

The natural state of Seatown Beach appears one of episodic accretion by pulses of shingle supplied from Charmouth Beach during brief intervals of unimpeded transport beneath Golden Cap. Now that extraction has ceased, accretion should resume when inputs from the west again become possible, perhaps shortly after 2020. Until this time, Seatown Beach will remain in a delicate state of balance with slight net loss (39m³ per year). The recent trend for increased backscar and sea-cliff erosion is therefore expected to continue or intensify in the short term, because beach volume should diminish slightly, while the current sea-level rise is predicted to continue or accelerate. Beyond this point it is postulated that brief pulses of rapid accretion (episodic inputs from Charmouth Beach), should be separated by longer periods of gradual depletion by attrition and entrapment ("The Accretion Phase"). This, however, would represent a stage in the beach's recovery from mining and is not a "natural" long-term state. This is because accretion cannot be sustained indefinitely before again shingle "overflows" eastward and creates new beaches beneath the Doghouse Hill and Thorncombe Beacon headlands ("The Bypassing Phase"). Such beaches would eventually transport surplus shingle to Eype Beach when the drift pathway became restored. The hypothesis is that eventually Seatown Beach, without further interference, would attain an equilibrium volume, whereupon shingle inputs and outputs should balance over an appropriate timescale (100 to 200 years), so that beach volume shows no trend ("Dynamic Equilibrium Phase"). The net effect is that Seatown Beach should alternate between being a closed and an open system according to transport conditions at its two landslide controlled headlands. At some times, the beach might only be open to inputs (temporary sink), whilst at others it would only be open to outputs (temporary store).

In conclusion, before 1900, Seatown Beach was probably of significantly greater volume than at present and was maintained by a balance between inputs and outputs regulated by the two headlands. Between 1900 and 1987, rapid beach gravel extraction was sufficient to unbalance the system resulting in accelerated coastal erosion. With the cessation of mining in 1987, a series of accretionary phases should be initiated after 2020, which should eventually return the system to its pre-1900 state. The fully closed contemporary Seatown Beach is therefore not typical and in the past it is likely that the beach served as an important transport link between the dynamic Charmouth gravel supply area and the Eype/Chesil Beach shingle sink.

It should be noted that the pocket beaches of this shoreline operate as a complex system that is sensitive to artificial and natural disturbances. For example, the projected gravel budget at Seatown may be modified in the following manner by an accelerating rate of sea-level rise:

1. Although cliff erosion will probably increase, gravel inputs will remain small because only limited quantities are present within the cliff behind the beach;
2. Erosion of mudslide lobes beneath Golden Cap may initially accelerate so that unimpeded littoral drift might occur earlier. However, the unloading effect produced at the mudslide toe may trigger additional landslide activity and more frequent blocking of the foreshore. The net effect is not easily assessed;
3. Climate change and sea-level rise might significantly increase cliff erosion gravel inputs to Charmouth Beach and thus cause accelerated beach build-up against the western side of mudslide lobes beneath Golden Cap. In time, this might allow more frequent, higher magnitude and prolonged phases of unimpeded drift eastward.

6.3 Eype Beach

A long-term budget for the beach is presented in Table 4 based on research by Bray (1996) and High-Point Rendel, 1997). The beach currently appears to be a relatively enclosed pocket beach with negligible inputs, but some significant historical outputs. Its present budget regime probably dates from the 1820s, when the piers at West Bay were made continuous barriers, thereby blocking shingle exchange with Chesil, which would previously have been a major process. Since 1825, outputs by attrition and entrapment (133m³ per year) and beach mining (discontinuous but estimated at 400m³ per year) would only have had a modest effect upon the beach volume, however, the eastern part of the beach and West Beach depleted severely with loss of up to 500,000m³ (High-Point Rendel, 1997). The full sequence of events relating to the loss of West Beach has been reconstructed by High-Point Rendel (1997), although cessation of transport around the Doghouse Hill headland and construction of coastal defences at West Bay are believed to have been contributory factors.

A major uncertainty concerns the fate of the lost beach material because it is unlikely that much could have been supplied eastward to Chesil due to the coastal setback that was produced as West Beach depleted. The only feasible explanation is of a massive offshore loss of fines from the beach and its substratum. A major offshore loss of coarse gravel is considered unlikely because nearshore surveys have failed to identify any corresponding deposits on the seabed (High-Point Rendel, 2000). Understanding of this loss has been important because West Beach has now been replenished and appropriate control structures had to be designed to minimise future beach losses (HR Wallingford, 2000b; 2000c).

The present Eype Beach is therefore a relict shingle accumulation characterised by depletion or undernourishment, albeit at a slow rate. It is uncertain whether further episodes of offshore loss could occur. Although West Beach has been artificially replenished with 18,000m³ of shingle, new coastal defences have been designed to avoid such occurrences. If it is assumed that further offshore losses do not occur from Eype Beach the future evolution of the beach may involve the following:

1. Attrition of the finite shingle volume of Eype Beach will gradually result in decreased pebble size and increased roundness. In time, this may cause reduction of the beach face gradient and some shingle might become reduced to a size susceptible to offshore transport;
2. The gradual diminution of beach volume has, and will continue to reduce protection to the toe of the sea-cliffs, allowing increased marine erosion. Because the gravel content of the cliff-forming deposits is negligible, increased coastal retreat is unlikely to provide adequate beach replenishment. Although limestones from the Forest Marble and calcareous sandstones from the Bridport Sands are supplied by erosion of these cliffs, their limited quantity and low durability restricts their replenishment potential;  
3. Natural drift inputs are unlikely in the future. Beach depletion and increased coastal recession reduces the likelihood of a continuous shingle beach reforming beneath Doghouse Hill so as to link Seatown and Eype Beaches.

6.5 Chesil Beach

Compilation of the information collected by this study has enabled construction of a budget for the beach presented in Table 5. The budget has altered through time due to changes in inputs from the west and outputs by mining, although the other elements have remained constant. Three alternative states can be identified:

Late Holocene to 1820s

Prior to construction of the Cobb, Lyme Regis and the piers at West Bay drift inputs from the West Dorset and East Devon coasts could have been as great as 8, to 12,000m³ per year (Bray, 1996). These inputs would have dominated the budget so that the beach would have been in an accreting condition. Bray (1996), High-Point Rendel (1997) and Brunsden (1999) discuss the effect of this upon the evolution of the beach. The consensus is that the much of the gravel of Chesil would have been derived comparatively recently over the past 2-4,000 years from this source to the west where some 40-60 million cubic metres could have been supplied, sufficient to account for the present beach volume of 16-63 million cubic metres (see section 1.2).

1820s to 1986

From the 1820s onward drift from the west would have ceased due to infilling of the West Bay piers. The beach altered from an open to a closed system isolated from any major shingle supplies. At this point losses become important to the beach and in the 20th century some 750,000 cubic metres of gravel were mined prior to cessation of operations in 1986. The assessment in Table 5 is therefore that outputs totalling 17,400m³ per year could have occurred.

Post 1986

Following cessation of the last remaining mining operation in 1986 the beach has depleted much more slowly with net losses of 1,400m³ per year due to drift bypassing of West Bay East Pier and attrition of the gravel as it is worked by waves. Although the beach has suffered substantial depletion over the past two centuries, the effects on profile and crest height are not easily determined due to the massive volume of the beach and high potential for rapid shingle redistribution. Recent measurements show variable but widespread lowering and some recession of the Chesil Beach between Chiswell and Wyke Regis (section 1.2). Following concerns over possible increases in damaging flooding events at Chiswell a coastal defence scheme was constructed involving measures to improve drainage within the beach and also novel gabion mattresses (Photo 41) to reinforce the stability of the crest (Hook and Kemble, 1991; Heijne and West, 1991).

The depletion of the beach has generated some debate concerning its likely future evolution and its resilience to climate change and sea-level rise; a topic that has been addressed by the West Dorset Coast Research Group (Bray, 1997b) and the DEFRA FutureCoast project (Halcrow, 2002). It is thought likely that the beach will continue to retreat by a “rollover” mechanism with acceleration in eastern parts. It is possible that the beach could breach between Wyke Regis and Chiswell, although the precise location and timing are highly uncertain. This is supported by modelling of profiles by (Babtie Croup, 1997) using Powell’s (1990) model, which indicated that a 1 in 100-year wave event in combination with future sea-level rise could result in up to 31m of crest recession. This particular model is not designed for application to a barrier beach so the results should be treated with some caution, however the increased sensitivity of eastern parts of the beach is apparent. A permanent breach would create a new tidal inlet in the Fleet, changing its regime and would greatly affect the transport and distribution of the beach shingle (Halcrow, 2002). Some researchers have suggested that such events could trigger the break-up of the beach, while others feel that en masse landward recession of the beach as a continuous barrier is the more likely (Bray, 1997b).

7. Opportunities for Calculation and Testing of Littoral Drift Volumes

Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping.  Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org.

The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is Teignbridge District Council, with data collection, analysis and reporting led by a specialist team at Plymouth Coastal Observatory (PCO). Although at present a relatively short-term time series of data has been collected (~ 6 years), longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future.

The shoreline to the west of West Bay has been studied in detail by Bray (1996) who successfully combined sediment tracing, wave power transport modelling and sediment budget analysis to derive estimates of drift on each beach. Useful further work would be to monitor the volumes of each of the pocket beaches in order to test those estimates and/or detect any changes in regime. Certainly, monitoring of beach replenishments at Lyme Regis and West Bay would enable likely drift rates to be ascertained from changes in beach volume (with allowances for the effects of control structures).

There are major opportunities to study drift occurring along Chesil Beach to obtain a clearer understanding of its overall drift regime. Previous studies have measured or modelled drift at only a few sites rather than investigating the beach as a whole. A possible approach would be to set up a numerical transport model (perhaps a beach plan shape model) and to attempt to calibrate it for the whole beach. This would be a major undertaking that would involve the following key elements:

1. Production of representative inshore wave climates at several points along the beach (preferably based on 25-30 years of hindcast data to overcome the problem of wave climate variability);
2. Application of different gain sizes for modelling at different locations along the beach so as to simulate the effect of the unique size grading upon transport (transport efficiency is known to be very sensitive to grain size);
3. Attempt to calibrate the model using known morphological changes along the whole of the beach. The 1993-95 aerial photography and photogrammetrically derived profiles provide a baseline with which a contemporary survey could be compared. Ground modelling techniques could be applied to determine changes in beach volume between surveys. Assessment of the likely errors of survey and ground modelling would be required to separate clearly those volume changes indicative of drift occurring between survey intervals.

8. Knowledge Limitations and Monitoring Requirements

The Southwest Regional Coastal Monitoring Programme commenced in 2006. Although a relatively short duration of quality data collection, analysis of the data between 2006 and 2012/13, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. However, at certain sites either due to a lack of long-term data, data coverage or sedimentological information (e.g. composition and proportion of beach grade material arising from cliff erosion), quantification of sediment transport rates of gravel and sand has not been possible.

There has been an impressive increase in both the quality and quantity of knowledge and understanding of the coastal sediment transport process system on this frontage. A series of studies at both Lyme Regis and West Bay have offered deep insights into the coastal processes and longer-term geomorphology at those sites. Major studies by Bray (1996) and Babtie Group (1997) effectively compiled information and undertook new research to fill gaps relating to the pocket beaches and cliffs and Chesil Beach, respectively. Various previous studies have published lists of suggested future research including High-Point Rendel (1997); Bray (1997b); Babtie Group (1997) and Posford Duvivier (1998a).

Notwithstanding results from the Southwest Regional Coastal Monitoring Programme, and the summarised information collated in the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), recommendations for future research and monitoring that might be required to inform management include:

1. Formulation of initial cliff behaviour models for each main section of cliff line based on methods set out within the recent DEFRA Soft Cliffs Project (Lee and Clark, 2002). Existing data together with updated cliff top positions, site reconnaissance and geomorphological interpretation should be sufficient to achieve this and provide a basis for estimation of future styles and rates of landslide activity;
2. Littoral drift has been studied in West Dorset using a variety of techniques involving both field experiments and mathematical modelling, but these have yet to be effectively combined. In particular, the CERC transport equation has been calibrated using tracer experiments on West Dorset beaches by Bray (1996) and it is suggested that these should be utilised in future modelling studies to increase the realism of the prediction process. Studies of drift on Chesil covering the whole beach are also needed.
3. A public-access long-term archive of survey material relating to Chesil Beach should be established. Due to its size and capacity for short-term change, long-term trends on the beach can only be determined by comparative studies covering long time periods. Existing data sets should therefore be compiled together with detailed description of survey techniques and field conditions etc. Information should include hydraulic studies, beach morphometry, sedimentology and geology;
4. Losses from beaches occurring due to attrition of pebbles during wave action are uncertain. Attrition is a particularly important factor on South-West Dorset beaches because: (i) all are primarily composed of gravel; (ii) many beaches are now closed systems and attrition represents a significant net sediment loss; (iii) rapid coastal erosion supplies large quantities of terrestrial gravels and these are particularly susceptible to attrition; (iv) little is known of attrition on replenished beaches and (v) existing estimates of attrition are approximate and uncertain. Several approaches to this problem have been considered (Bray, 1996) and it is concluded that fundamental research will be required involving linkage of laboratory abrasion experiments to field conditions and validation by comparative studies on naturally and artificially replenished beaches;
5. Future evolution scenarios for Chesil Beach and the Fleet Lagoon could be produced in greater detail than presently available and should include an assessment of the potential consequences for habitats.