1. Introduction
This is an internationally celebrated coastline, a UNESCO World Heritage Landscape for its Jurassic/Cretaceous geology and coastal geomorphology. Textbooks on physical geography, at all levels and published in many countries, use the landform sequence between Warbarrow Tout and Bat's Head to derive an evolutionary model of coastline development. The elegance and simplicity of this approach has been its main strength. In reality, each 'stage' is a function of site-specific-interaction between pre-existing sub-aerial relief; Quaternary period environmental change, in particular former periglacial weathering and mass movement; sea-level oscillation and neotectonics (Small, 1970; Jones et al., 1984; Brunsden and Goudie, 1997; Bird, 1996; Canning and Maxsted, 1979; Mottram, 1972; May, 2003). This coastline also features a sequence of 23 valleys, most of which have been truncated by the retreating cliffs; some are now abandoned by running water, and “hang” above beach level, others discharge as waterfalls. May (2003) provides a brief resume of hypotheses advanced to explain their development and significance in the context of local coastline evolution.
The geological structure, rock sequence and range of rock lithologies are the most influential factors determining the several dramatic changes in cliff height and morphology along this 55km coastline. This relationship has been described in detail in several textbooks, monographs and papers (e.g. Arkell, 1947; Davies, 1956; Sparks, 1971; May, 1971, 2003; Bird, 1984, 1996; King, 1976; Perkins, 1977; House, 1958 and 1993, Nowell, 1997). Much of the length of this coastline is longitudinal, i.e. it trends nearly parallel to the approximately east to west axis of the denuded Weymouth-Purbeck monocline, the dominant tectonic structure of this region. Compressional structures increase in intensity between the eastern unit boundary at Durlston Head (Photo 1) and its western counterpart at White Nothe (Photo 2), resulting in progressive westwards narrowing of the outcrop areas of the Cretaceous and Upper Jurassic rock groups that occur south of the Chalk. Rock dips are nearly vertical, with some overfolding, between Bat's Head and Lulworth Cove (Photo 3), but decline to low angles east of St Aldhelm's (or St. Alban’s) Head (Photo 4). Subsidiary fold, fault and other tectonic structures are responsible for several local complexities of the outcrop pattern of older Jurassic rocks, west of Ringstead Bay. The Cenomanian overstep between Bat's Head and White Nothe where Chalk overlies softer strata is responsible for the stratigraphical succession that has promoted large-scale semi-rotational slope failure. The behaviour of the different rock types under compressional stress has created a variety of structures that contribute to differential rock resistance to marine and sub-aerial processes. Examples include the brittle fracture of the incompetent Purbeckian strata, best appreciated in the exposed section of the drag fold ("Lulworth Crumple") at Stair Hole; and several thrust planes in the Chalk producing brecciation and crushing (Nowell, 1997).
It is the view of several researchers that cliff morphology is as much the product of long-term sub-aerial denudation as marine processes, although the latter function primarily to remove fallen debris from cliff toes (e.g. Small, 1970;May, 1971; Bird, 1996; Brunsden and Goudie, 1997; Allison, 1986 and 1989; Jones, et al., 1984). This is readily demonstrated at the various sites of active mass movement. Almost the entire range of failure mechanisms operate, at widely varying frequencies and magnitudes (Brunsden, 1973; Allison, 1986; Brunsden and Goudie, 1997). The major landslip sites, e.g. White Nothe; Emmett's Hill/St Aldhelm's Head (Photo 4) and Houns-tout cliff (Photo 5) have not received the same level of documentary research and process monitoring as those further west on the Dorset coast.
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.channelcoast.org. The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See CCO Annual Survey Reports for further details).
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 modelling study, based substantially on hindcasting using regional wind records (1975-1996) concluded that there is a strong contrast in exposure to wave energy between east and west sectors (HR Wallingford, 1998). The dominant wave directions are stated to be from the south-west and south-south-west, thus the coastline west of White Nothe is relatively protected by the presence of the "Isle" of Portland and offshore banks and shoals, such as the Shambles and Adamant shoals (HR Wallingford, 1994; Bastos and Collins, 2002). These latter features, and the Lulworth Banks, also set up nearshore wave refraction. Maximum extreme wave heights are predicted to occur offshore Kimmeridge Bay, which is open to a south-west fetch of over 8,000km (extending to the Venezuelan coast of South America). The decline in incident wave energy eastwards from Kimmeridge is only slight, but is modified by changes in coastline orientation, such as the north to south trend of the west-facing shoreline of the St Aldhelm's promontory. Deep water waves are transformed as they enter the nearshore zone by offshore bathymetry. This, and the presence of inshore reefs and ledges at various locations e.g. Ringstead Bay (Photo 6) and around Kimmeridge (Photo 7), sets up complex local wave refraction. HR Wallingford (1998) state that mean significant inshore wave height in Ringstead Bay is 0.5m; this is confirmed by Bastos and Collins (2002) based on hydrodynamic modelling. HR Wallingford (1998) have modelled extreme wave heights for a range of return frequencies applying to Ringstead Bay and the entrance to Lulworth Cove. Thus, for a 1 in 1 year recurrence, heights are calculated as 3.1m and 4.2m, respectively. For 1 in 10 years, expected extreme values are 3.9m and 5.1m; and for 1 in 50 years, 4.5m and 5.6m.
The Southeast Regional Coastal Monitoring Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Weymouth in 10mCD water depth. Between 2006 and 2012, the prevailing wave direction was from the south-southeast. Average 10% significant wave height exceedance is 0.86m (CCO, 2012).
Ringstead Bay 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 Meteorological Office Wave Model and then transformed inshore to a prediction point in Ringstead Bay at -2.93mOD. This study suggested that that the dominant wave direction is from the south-south-east, with significant offshore wave heights of up to 2.75m; for waves moving shorewards directly from the south, and thus propagating over a shorter fetch, a mean significant offshore wave height of 1.25m was computed. Potential sensitivities to likely climate change scenarios were 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 3.5% to 7% variation in longshore energy and confirmed that the Bay was significantly more sensitive to this factor than many south coast locations. Wave energy at Ringstead was also found to be especially sensitive to predicted scenarios of sea-level rise and storminess. It may be that at higher water levels the dissipative function of the protective nearshore reefs becomes reduced significantly.
The tidal regime is microtidal, thus tidal currents are comparatively weak with a range that is everywhere less than 2m except at Durlston and St Aldhelm's headlands (Bruce and Watson, 1998; Mouchel, 1998) and where flows are confined between the shoreline and nearshore/offshore reefs, as at Man O’ War Cove and Ringstead Bay (HR Wallingford, 1998). Current vectors are parallel to the coast and reverse before and after high water. A slight net westwards flow, towards Weymouth Bay, operates west of Warbarrow Tout. St Aldhelm's Head, creating a tight clockwise movement in the sea area immediately west of this promontory, sets up a vortex with current speeds in excess of 5m.sec-¹ during part of the tidal cycle.
Sediment transport is therefore dominantly wave-induced. Despite active cliff erosion, there is a shortage of mobile littoral sediment on most shorelines so that actual rates of sediment moved by longshore drift are well below their theoretical potential (Heeps, 1986; HR Wallingford, 1998). The dominant net direction of littoral transport is west to east, consistent with the direction of approach of dominant wave fronts. However, there is some evidence of a drift divide in the vicinity of the headland of White Nothe. Beaches are of variable dimensions and composition, mostly conditioned by locally available inputs. Numerous "pocket" bays, coves and other re-entrants, separated by headlands and associated shore platforms, interrupt transport and cause coarser sediment particles to be trapped and graded within discrete compartments (Heeps, 1986). This, together with the presence of deep water in front of some cliffed sections, inhibits long distance longshore transport. Many pathways are also very short as a result of changes in coastline orientation. There is substantial onshore to offshore removal of fine sediment in suspension, resulting from dispersal of much clayey landslide debris and both inter-tidal and sub-tidal shoreface erosion (Posford Duvivier, 1999). Quantities are a function of the lithology of substrate materials, wave and tidal energy, shoreface width, water depth and sea bed relief.
2. Sediment Inputs
2.1 Marine Inputs
F1 Ringstead Bay
Analysis of 2008 bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Between Redcliff Point and Durdle Door the rock platform extends subtidally 200-600m; eastward from Worbarrow to Peverill Point the extent of the rock platform and nearshore geology exposed is considerable. These bedrock exposures are only covered with a variable but thin veneer of sediment. Between Durdle Door and Worbarrow the platform is less extensive, extending approximately 100m offshore. Within the centre of Ringstead Bay and extending offshore, the nearshore sediment thickness is generally sufficient to mask the underlying geology, and the relatively featureless seabed gently slopes southeastwards. HR Wallingford (1998) note that shore parallel tidal stream velocities can be considerably greater in Ringstead Bay than is usual for the Purbeck and Weymouth Bay shorelines. This reservoir of sediment may provide an intermittent or pulsed cross-shore input to the local beach. However, the lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of onshore (or offshore) sub-tidal sediment transport.
2.2 Fluvial Inputs
The only sediment inputs of any significance are: (i) Osmington stream in the western part of this sector, and (ii) the stream draining into Chapman's Pool, West of St Aldhelm's Head. Other streams, such as Winspit, drain very small catchments and contribute negligible quantities of material.
Rendel Geotechnics and the University of Portsmouth (1996) estimate an annual delivery at Osmington of 63 tonnes of bedload and 192 tonnes of suspended load. During an extreme rainfall event on 18.07.55 (the most intense rainfall ever recorded in England up to that time was at Martinstown, 8km inland) an exceptional stream discharge was produced, causing deep scouring of the bed and supply of "many tons of boulders and other debris into the sea" such that a temporary fan or delta was formed across the foreshore (Arkell, 1955). This persisted as a coastal feature for several years.
A stream draining into this embayment contributes small quantities of sand and clay. During extreme flow events it can flood its narrow valley floor and supply small quantities of gravel to the shore.
2.3 Cliff, Shore Platform and Shoreface Erosion
» E1 · E2 · E3 · E4 · E5a · E5b · E6 · E7 · E8 · E9 · E10 · E11 · E12
Structural and lithological variation occurs over small spatial scales, resulting in considerable contrasts of cliff erosion behaviour and in the relative significance of erosionally-derived input into the transport system (Bird, 1996). Hinterland topography creates a cliff height-range of some 180m, a factor in combination with recession rate that controls sediment yield. Overall sediment yields covering the full extent of the frontage of this unit have been assessed by Posford Duvivier, (1999) as being 140,000m³ per year from cliff erosion and 60,000m³ per year from erosion of the shore platform and shoreface. The majority of the material supplied comprises fine sediment that is rapidly removed seaward in suspension, although some limestones that form boulders and pebbles on the foreshore are also supplied. Cherts and flints that can form long-lived gravel beaches are supplied only in relatively small quantities at around 2,500m³ per year to 3,000m³ per year, mostly from cliff erosion. Details of cliff behaviour and recession rates (where known) are provided for the following subdivisions, which are divided by significant headlands and re-entrants.
The stratigraphical sequence of clays, sandstones and oolitic limestones is affected by local folding and faulting. Between Redcliff Point and Shortlake there is an upfaulted block, whilst other more minor faults create local aquicludes. Immediately east of Redcliff Point a narrow but persistent gravel beach offers some cliff toe protection. Active cliff instability characterises the cliffline between Black Head and Bran Point, with several mudslides, mudflows and translational failures (Bird, 1996). At Black Head, a complex of mudslides which developed on the Kimmeridge Clay between 1910 and 1914 were triggered by excess groundwater supply from infaulted permeable Greensand and Chalk behind the cliff face (Arkell, 1947). This continues to be an active area of slope instability, possibly subject to some recent intensification due to cliff toe erosion in the absence of any significant beach accumulation (Mouchel, 1998). Overall, between Black Head Osmington Mills, Hannah's Ledge and Bran Point, some 1,400m of this frontage is occupied by mudflows and slides (Richardson, 1900); indeed Black Head is reputed to have produced the biggest single mudflow event recorded in the UK during the twentieth century (Arkell, 1955; Bird, 1996) - see section FL1. Instability at this point extends at least 600m inland. Peter Brett Associates (1995) note that mudsliding at Osmington Mills may be episodic, with periods of 10 to 15 years of alternating quiesence and renewed activity. Spalling, toppling and gulleying are also active processes, though the latter is more dominant on the Oxford Clay cliffs between Redcliff Point and Black Head. Photo 8 illustrates a medium size cliff failure within Kimmeridge Clay on the eastern flank of Black Head that has generated a mudslide that surged across the foreshore.
Mass movement accounts for the detachment of oolite blocks from the upper cliffs, particularly in the vicinity of Osmington. These accumulate at the toe of the marine cliff and provide some protection against wave erosion (Peter Brett Associates, 1995). Corallian limestone ledges and shore platforms, which occupy the lower foreshore and offshore, also modify incident wave energy. There are several estimates of rates of cliff recession, ranging from 0.05 to 0.40m per year at Black Head (May, 1966) to 0.11 to 2.2m per year at Osmington Mills (Peter Brett Associates, 1995). Mouchel (1998) and Posford Duvivier (1997; 1999) calculate cliff retreat at 0.5m per year at Osmington for 1970-96 with an average rate of 0.46m per year for this shoreline as a whole. Most researchers agree that erosional loss from cliff retreat has increased since the 1930s and 1940s along the eastern part of this sector. Using both OS maps and aerial photography, Peter Brett Associates (1995) carried out a detailed study of cliff erosion for the headland in front of the settlement at Osmington Mills. This revealed a mean rate of recession of 1.1m per year, 1946-71 and 0.8m per year, 1983-95, with cliff top erosion marginally faster than cliff toe retreat. Although this may appear to contradict the observation of accelerating erosion, averaged figures conceal wide temporal and spatial variability for the various cliff profiles that were surveyed. The exceptional event of 1955 may have substantially remodelled details of cliff morphology east and west of Osmington.
Cliff recession is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material from a mostly thin mantling of superficial deposits, is less than 1,000m³ per year, with fine-grained fractions transported offshore in suspension. The 2004 arrows stated no quantitative data was available.
Ringstead Bay (Photo 6) is cut into a series of clay dominated lithological units, with cliff heights falling from 30m at Bran Point to 5m in central Ringstead Bay. Rates of cliff recession are relatively modest, although accelerating during the last four decades of the twentieth century. May (1966) calculates 0.13 to 0.64m per year (with 0.4m per year as a mean) for 1880-1963, whilst Mouchel (1998) concludes that rates have been between 0.34 and 0.6m per year since 1970. May (2003) states that at the eastern end of Ringstead Bay a section of cliffline varying between 2 and 35m in height retreated 3m between 1996 and 1998. These longer-term rates are likely to reflect the role of a series of offshore ledges composed of resistant Corallian limestones outcropping along the foreshore and nearshore seabed parallel to the shoreline, that dissipate incoming wave energy (Cole, 1995; Bird, 1996; HR Wallingford, 1998). Beach nourishment comprising 25,000 cubic metres of marine dredged gravel, undertaken in 1996, and cliff foot rock armour defences introduced in 1998, may have reduced recent and current rates of recession. To the east of Ringstead Bay is the large deep-seated multiple rotational landslip, which extends eastwards some 2km around White Nothe headland. The 150 to 160m high cliffs have a characteristic form, described in more detail in May (2003), induced by the Chalk and Upper Greensand overstep of underlying Gault and Kimmeridge Clay (Davies, 1956; House, 1958 and 1993; Bird, 1997). This is a long-established rotational failure complex, which experiences infrequent but high magnitude falls, slips and slides. There are few reliable measurements of changes in the position of the cliff toe, although toe erosion and release of landslide debris held in high angle partly vegetated screes is clearly an ongoing process (Photo 2). Arcuate boulder ramparts seawards of cliff toes are residual evidence of several previous slide events. Kimmeridge clay outcrops across eastern Ringstead Bay, but is largely concealed by landslide debris; failures in the basement clay may be partially responsible for rotational slip blocks (May, 2003).
Cliff recession is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material from a mostly thin mantling of superficial deposits, is 1-3,000m³ per year, with fine-grained fractions transported offshore in suspension. The 2004 arrows stated no quantitative data was available.
The most westerly part of this sector is a continuation of the White Nothe landslip complex, where relict scree slopes below weathered free faces are characteristic. Further east, vertical Chalk cliffs above a narrow shore platform and truncating a sequence of “hanging” dry valleys dominate. The "needle eye" arch of Bat's Head and nearby stack is evidence of long-term recession. This coastline is more exposed to high energy waves, propagated across an extensive fetch, than the cliffline westwards. May (1966) gives an average retreat rate of 0.22m per year (1890-1960), but for Middle Bottom, west of Bat's Head, May and Heeps (1985) calculate a lower rate of 0.05m per year (1882-1975). This is probably due to the presence of infrequent small rockfalls, which offer temporary toe protection prior to their removal by abrasion and solution processes.
Cliff recession is not evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data during this period. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
The vertical Chalk cliffs of this sector reach a maximum elevation of 120m at Swyre Head, and truncate three "hanging" dry valleys. As with E3, localised and short-lived rockfalls with a relatively short residence time occur as a result of block detachment from bedding, joint, shearing and other tectonically imparted planes of weakness. A recession rate of between 0.2 and 0.46m per year (1880-1975) is calculated by May (1990) and May and Heeps (1985) for the Chalk cliffs, but is likely to be faster along the slumped and gulleyed outcrop of the Wealden clays and sandstones forming the Durdle Door isthmus (Photo 9). The 'Door' itself (a world famous arch) is evidence of high wave energy, but the cliffline to the west is screened by a submarine rock ridge of Portland Limestone, small parts of which are exposed at low water. Immediately west of the Durdle promontory is a set of caves developed in the crush breccia of a major thrust structure. It is not clear if these have a marine origin, as they are inaccessible to wave action; they may be the product of groundwater seepage and/or solution weathering. Cliff top recession is faster than at the cliff base across "hanging" valleys, due to the exposure of less consolidated Coombe Rock infill (Bird, 1996) which is actively denuded by weathering and rilling. Sub-aerial slumping and gulleying of the Wealden slopes is almost continuous, particularly during the winter.
Minimal cliff recession is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
Cliff height is variable, reflecting the differential structure and strength of the rock sequence along this sector. The Chalk cliffs in the centre of St Oswald's Bay rise to 140m in elevation, whilst the Portland Limestone cliffs that form Dungy Head and the rock rampart protecting the Durdle Door promontory are some 65-80m in height. Cliffs on the intervening outcrop of Wealden sands and clays are as low as 8m. The coastal form reflects the removal of most of the very tightly folded and compressed rock sequence in front of the Chalk, between Durdle Door and Dungy Head (Nowell, 1997). The apparently "chance" survival of the Durdle Door promontory is probably a function of topography, as it marks the position of a former sub-catchment divide (Small, 1970; Bird, 1996; Brunsden and Goudie, 1997) Man 'O' War Rocks (Photo 10) are a detached reef of Portland Limestone that absorbs wave energy and has created a subsidiary headland behind. Large boulders and coarse debris stores have accumulated around Dungy Head; offshore rocks, such as Pinion Rock, may be remnants from previous large-scale cliff falls or topples. There have been several falls affecting the Chalk cliffs in recent decades, most recently one of significant magnitude in 2013, with cliff top retreat associated with active removal of periglacially weathered debris and the presence of two deep solution shafts. Mouchel (1998) estimate a retreat rate of 0.07m per year for the cliffs east of the Man O' War Rocks (1880-1990), although May (1966) gives a figure of 0.2-0.25m per year (1890-1960). Basal notching is evident at various locations (Canning and Maxted, 1979), and cliff fall debris has a short residence time (May and Heeps, 1985). Comparatively very high rates of slope denudation are taking place on the Wealden outcrop. Gully erosion and slumping are active processes on the east-facing side of Durdle Door, whilst immediately north of Dungy Head, shallow translational slides and flows are more characteristic (Jones, et al., 1984; Allison, 1986, 1989). This contrast is likely to be related to differences in exposure to precipitation, temperature changes and wave energy (Canning and Maxted, 1979; Allison, 1986).
Minimal cliff recession is evident on the remainder of this sector, from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
The rock sequence around the perimeter of the quasi-eliptical planform of Lulworth Cove (Photo 3) has been described numerous times (e.g. Davies, 1956; Arkell, 1947; House, 1958; King, 1976; Bird, 1996; Perkins, 1977, Nowell, 1997). The Chalk forms the steep backwall, whilst the narrow entrance is flanked by overhanging cliffs of Portland Limestone. Much of the lateral expansion of the cove has been confined to the intervening outcrops of Purbeckian Limestone and Wealden clays, marls and sandstones. Both are comparatively erodible, though for different physical reasons (Arkell, 1947; Allison, 1986, 1989, 1990). The geomorphological origin of Lulworth Cove has long attracted attention (e.g. Burton, 1937; Small,1970; Mottram, 1972; Komar,1976; Jones, et al., 1984; Horsfall, 1993; Bird, 1984 and 1996; Brunsden and Goudie, 1997). The classic evolutionary model, that places Lulworth Cove in a sequence of stages starting with Stair Hole and concluding with Durdle or Warbarrow Bay, has been criticised as both simplistic and idealised. The role of fluvial erosion, creating a valley form that was subsequently partly inundated during several successive stages of Pleistocene and Holocene sea-level rise is more consistent with local detail (Small, 1970; Brunsden and Goudie, 1997; May, 2003). Cliff height and morphological development is influenced by the strong contrasts in rock lithology and structure. Cliff falls and topples are characteristic of the Chalk, Purbeckian and Portlandian Limestone, whilst differential weathering has also helped to create the ribbed and recessed form of the Purbeckian Limestone outcrop. As in sector 5a, mass movement, primarily mudslides, on the Wealden beds creates headwards retreat and rapid changes in the profile of the coastal slope developed on this lithologically weak unit (Allison, 1986, 1990; Bird, 1997). There is obvious visual evidence of dynamic cliff change on the exposed west-facing slopes closer to the Cove entrance. The initiation of new mudslides is partially documented in Allison (1986), but there are no reliable quantitative measurements of rates of cliff recession; Allison (1986) does however record the loss of some 320m² at one site, 1880-1980. Partly submerged small boulder arcs on the east side of Lulworth Cove may record earlier mudslide surges.
Stair Hole, to the immediate west of Lulworth Cove, is the result of wave energy breaching the Portland Limestone barrier creating several arches and causing the lateral expansion of a small inlet along the comparatively incompetent brittle and fractured Purbeckian rocks. The north wall of this feature is developed in the Wealden beds, creating a zone of instability extending some 250m inland (Bird, 1996). Brunsden and Goudie (1997) record the pattern of slide tracks and lateral shears that connect a well-defined source area to the zone of discharge at the slope base. Here, lobes of clay and silty sand frequently move across and partly conceal the basal boulders of Purbeckian Limestone. Mudslide activity here is the product of a complex relationship between the mechanisms controlling rates of supply and removal of debris (Jones, et al., 1984; Allison, 1986).This includes the role of overland flow generated by intensive rainfall. Canning and Maxted (1979) consider that the top of the mudslide zone retreated 30-40m between approximately 1880 and 1970. Most researchers (e.g. Allison, 1986, 1990 and Brunsden and Goudie, 1997) note that instability has increased substantially since the mid-1950s. Removal of vegetation, together with trampling due to recreational pressure, may be an important cause (Lulworth Cove receives over 11/2 million visitors annually). It is possible that the rate of backwearing of the adjacent Purbeck Limestone cliffs has declined in the last 30-40 years (Canning and Maxted, 1979; Jones, et al., 1984), thus the supply of coarse debris that protects the base of the Wealden beds opposite the three points where the sea has penetrated the Portland Limestone barrier has diminished. Brunsden (1996) notes that there are several uncertainties regarding mechanisms of mudsliding and soft cliff slope failure on the Dorset coast, citing Stair Hole as an example.
Minimal cliff recession is evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
The dip of the Portlandian and Purbeckian rocks declines rapidly along this frontage, with the former present only as the complex of Mupe Rocks offshore stacks (Photo 11 and Photo 12) at its eastern extremity. Cliff height averages 30m, and cliff form is near vertical due to undercutting of the well-jointed Lower Purbeck Series at their base. May (1966) and Mouchel (1998) calculate minimal erosion for the period 1880-1990, between zero and 0.01m per year. At Cockpit Head, adjacent to Mupe Ledges, May (1966) gives a rate of cliff base retreat of 0.23m per year.
Cliff recession is not evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
Cliff height and morphology is precisely controlled by the same shore parallel sequence of strata encountered further west. Maximum height is 170m at Ring's Hill in the eastern sector of the bay, where a wide degradation zone characterises the Chalk cliffs (Nowell, 2000). The cliff line is interrupted at Arish Mell, where a fault-guided valley has almost penetrated the Chalk down to sea-level. Historical recession rates of the main backwall, 1880-1962 were between 0.11 to 0.16m per year (May 1966). Posford Duvivier (1997) calculate an average recession rate of 0.135m per year. These figures require some upwards revision given the major slide in March 2001 which released 50,000m³ of material and resulted in 3m of cliff top retreat; this failure site had not recorded any movement during the previous century, but occurred following several months of exceptional rainfall. Material resulting from falls and topples are relatively quickly redistributed (May and Heeps, 1986). Cliff top retreat at Ring's Hill has removed slightly more than one half of a late Neolithic hill fort (Flower's Barrow). On the arguable assumption that it was originally built close to the cliff edge, this would give a long-term recession rate of 0.08m per year over some three millennia. Cliff height diminishes to less than 25m on the Wealden Beds, the outcrop area of which is widest on the east side of Warbarrow Bay (Photo 13). Translational slides, flows, semi-rotational slips, gullies and linear erosion by a small stream are particularly active over the 350m length of frontage immediately north of Warbarrow Tout (Moore and Brunsden, 1996, Nowell, 2000). Allison (1986, 1990) and Allison and Brunsden (1990) report short-term cliff base recession rates of between 2.5 and 3.0m per year at this location. These high rates are the result of individual slope failures, which rapidly create, redistribute and remove basal debris stores. Mass movement on the Wealden outcrop of Mupe Bay is also active, but this sector is less exposed to wave activity. An average recession rate here is approximately 0.12m per year for the last 100 years (Mouchel, 1998), compared to twice or three times that on the opposite eastern side of the bay. Posford Duvivier (1999) calculate that 1,400m³ per year of fine material is removed from the shoreface outcrop of Wealden beds. To the east of Mupe Bay cliff toe debris fans are relatively rapidly removed by marine erosion. Posford Duvivier (1997; 1999) calculated a total sediment yield for the bay of 20,000m³ per year, mostly comprising fine material from the Chalk and Wealden Beds.
Minimal cliff recession is evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Negligible quantities of flints are yielded from the Chalk together with clasts of a variety of lithologies from beds within the Wealden strata and contribute to the narrow gravel beaches in the eastern and western extremities of Warbarrow Bay, with fine-grained fractions transported offshore in suspension.
The Chalk cliffs descend to sea-level at Arish Mell, where they truncate a large dry valley. An embayment is developing here, partly due to the exposure of weakly consolidated alluvial and colluvial infill.
This coastline is characterised by high (140m) Portland Stone and Portland Sand cliffs of Gad Cliff (Photo 14) and the 'pocket' bays of Pondfield Cove and Brandy and Hobarrow Bays. Allison (1986, 1990) and Allison and Kimber (1999) note the presence of areas of instability where the basal Kimmeridge Clay has been exposed. At Gad cliff, a landward dipping caprock of Portland Stone forms a steep upper cliff free face that fails by rockfall creating a debris accumulation slope below; the latter is currently colonised by vegetation, suggesting stability. As the toe of the slope is eroded by wave action protective aprons of hard Portland Stone boulders are left remaining at and offshore the shoreline. This, together with the landward dip of outcropping strata, helps to explain the generally slow recession of this cliff sector. May (1966, 2003) Posford Duvivier (1997) and Mouchel (1998) calculate a spatially variable rate of cliff recession of between 0.01 to 0.15m per year for the period 1880-1990 with fault structures determining specific areas of weakness (Nowell, 2000). Rockslides and falls are active on the seaward side of Warbarrow Tout headland, for which Allison (1986) calculates recent retreat of 0.11 to 0.16m per year. Posford Duvivier (1999) determine an annual yield of fine sediment of 15,000m³ from the cliff and 4,500m³, from shore platform and shoreface abrasion assuming vertical corrasion at 4mm per year.
Cliff recession is not evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
The two main characteristics of this short sector are low, 5-20m, cliffs and a wide inter-tidal shore platform developed upon thin limestone or calcareous mudstone seams within the Kimmeridge Clay (Photo 7). Shales and clays are interbedded with a series of limestone bands ("Cementstones") that are responsible for the widest and most resistant elements of the inter-tidal ledges and platforms (Arkell, 1957; Davies, 1956; Bird, 1996; Mouchel, 1998). Rates of cliffline and platform recession are high compared with much of the westward coastline. For Kimmeridge Bay, which is the result of long-term differential erosion, May (1966) gives a rate of 0.38 to 0.45m per year, approaching 0.8m per year where platform relief and width is least (Mouchel, 1998). Erosion is less between Clavel Tower and Hen Cliff (0.13 to 0.20m per year, May, 1966), although exposure to prevailing south-westerly waves is actually slightly greater. The reason for this is uncertain, but is probably related to a change in the lithology of the Kimmeridge Clay. Although not reported in the literature, the fastest rate of cliff recession would appear to be in the vicinity of Gaulter Gap, where a stream is sharply incised. A Second World War defence structure at this point is now some 12m in front of the cliffline, whilst cliff top denudation now threatens the oil well, located here in 1958. Very rapid physical weathering occurs where flaking of paper shales creates redistributed talus, e.g. near the boathouse site.
Cliff recession is not evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
The Kimmeridge Ledges between Clavell's Hard and Egmont Bight, which extend seawards at an oblique angle to the general coastline trend, promote wave energy dissipation and complex local refraction. Thus the 35-50m high cliffs of this remote sector are comparatively stable; a retreat rate of 0.09m per year (1870-1985) is proposed by Posford Duvivier (1997). However, the Ledges form part of a wide shoreface abrasion platform that is retreating at its seawards edge, thus providing evidence of long-term recession. In most respects, this sector is poorly documented.
Cliff recession is not evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
E11 Houns-tout Cliff - Chapman’s Poole - St Aldhelm’s (or St Alban’s) Head (West) (see introduction to coastal erosion)
Egmont Point forms an integral part of the large scale unstable back-tilted coastal slope complex of Houns-tout (Photo 5) (May, 2003). The latter has a well defined upper free face of Portland Limestone, but the Kimmeridge Clay at the base has promoted slope degradation through a sequence of slides, slumps and falls as well as extensive gulleying. The undercliff west of the semi-circular cove of Chapman's Pool has been substantially modified since the early 1970s by several mudslides and large falls (Bird, 1996). Boulder accumulations provide some basal armouring, but fines are rapidly removed offshore. The recess of Chapman's Pool (Photo 15) is due in part to a local increase in the height of the Kimmeridge Clay outcrop. This is due to an upfold that accounts for the abrupt change in coastline orientation to the east, thus creating the prominent St Aldhelm's (sometimes St Alban's) Head. Because of the frequency of rockfalls and slides delivering material to the toe, the shoreline beneath Houns-tout experiences some temporary advances of mean high water followed by longer term recession, thus map analysis indicates only a slow overall retreat. May (1966) suggests a maximum of 0.10m per year, 1880-1960, whilst Mouchel (1998) calculate almost no net recession, 1890-1990. In reality, actual erosion rates involved in removing basal debris supplied by cliff falls are probably at least ten times higher than this, but material eroded is replaced by renewed debris slips. A retreat rate of 0.03m per year is perhaps representative for recent decades. The cliffline around Chapman's Pool retreated between 1880 and 1960 at a higher rate, approaching 0.20m per year (May, 1966) creating, or maintaining, a small inter-tidal platform. A small sediment yield comes from fluvial erosion via the deeply incised West Hill stream, but most of this loss is the result of wave attack of the cliff base. Cliff elevation between Chapman's Pool and St Aldhelm's Head (Photo 4) is over 120m (Emmett's Hill). This west-facing slope complex is the product of deep-seated semi-rotational failure (Allison, 1989). The rock sequence of impermeable Kimmeridge Clay at the base and relatively permeable and porous Portland Sand and Portland Limestone above provides appropriate hydrogeological conditions for slope instability, creating a wide undercliff. In addition there is direct exposure to the highest wave energy received along any part of this coastline (May, 1971). A series of major failures, possibly originating in the early or mid-Holocene (Allison, 1986; Allison and Kimber, 1999) and even perhaps associated with late glacial tundra conditions have contributed to the substantial mid-slope and basal store of boulders. These originate from the uppermost free face (backscar) of Portland Limestone, but the failure surface may be seated in the underlying Portland Sands and translate upslope. It is not, however, transmitted downslope (Jones, 1980; Jones, et al., 1984). This is apparent from visual monitoring of the development of instability since 1978. Nineteenth and early twentieth century quarrying has created several debris fans, and may have been a factor, inducing slope instability.
Minimal cliff recession is evident from analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
Vertical sea cliffs (30-38m in height) of Portland Limestone that plunge directly into deep water front virtually the entire length of this coastline (Photo 16). Above, the Portland Sand supports a lower gradient slope, giving an overall “slope-over-wall” profile. Local diversity is provided by the mouths of the graded Winspit and truncated Seacombe Valleys, the latter usually dry, and elevated platforms and benches, e.g. Dancing Ledge. Cliff morphology is largely controlled by sets of rock joints orientated at high angles to the near-horizontal bedding planes. Basal undercutting and cave formation are recurrent features due to solutional weathering and bioerosion as well as wave corrasion (Bird, 1996). However, coastal quarrying and smuggling, between the mid-eighteenth and early twentieth centuries, account for enlargement of several ledges, platforms and caves (e.g. Blacker's Hole, Dancing Ledge, West Man Quarry and Tilly Whim Caves, close to Anvil Point). Rates of natural cliff line recession are 0.02m per year for the period of reliable map evidence (May, 1966; Mouchel, 1998). Where former quarrying near cliff top sites has weakened rock coherence rates might be higher. However, this may be counter-balanced by the dumping of quarrying waste, artificially creating protective talus slopes in a few locations, e.g. Seacombe Cliff. As the shoreface is not less than 20m deep, and is at or below average wave base, erosional losses are likely to be small per year. Inter- and supra-tidal solutional loss is an active weathering process, though not quantified.
Cliff recession is not evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography. Therefore, cliff input, particularly of sand or flint gravel grade material is considered negligible, with fine-grained fractions transported offshore in suspension.
2.4 Beach Replenishment
N1 Ringstead
A beach nourishment comprising 25,000m³ of marine dredged fill retained by a terminal rock groyne was undertaken in 1996 (Photo 6). The scheme was developed following beach losses and accelerating cliff recession over the 1980s and early 1990s that was beginning to threaten properties (S W and H Pattison Ltd. 1995; HR Wallingford, 1994).
3. Littoral Transport
LT1 Recliff Point to White Nothe
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data supports no net direction of drift in the self-contained pocket beach at Ringstead beach, although weak easterly drift has been observed in Ringstead Bay when waves are approaching from the south-east and south-south-west (Mouchel, 1998). Thus, there may be a transport divergence in the vicinity of White Nothe (HR Wallingford, 1998). Transport is mostly of gravel and coarse sand, with gravel more dominant in Ringstead Bay. Coarse clastic material is supplied from flint within Chalk debris detached by failures at the western end of the White Nothe landslide complex, and released by marine erosion (however, much of the potential supply is trapped behind inter-tidal boulder ramparts at the cliff foot). This source is evident from the graded composition of the beaches of this unit, dominated by rounded, oxidised flint particles, though gravel recharge at both Osmington Mills (East) and Ringstead in the mid to late 1990s has modified particle size-range distribution. Rates and volumes of transport may be substantial over short periods, particularly after beaches have experienced drawdown due to exceptionally high energy waves. This was observed at Ringstead Bay in 1989-90 (Pattison S. W. and H., 1995; Mouchel, 1998) (Photo 6). Offshore rock outcrop reefs are composed of relatively resistant Corallian strata and are only exposed at maximum low water (Jolliffe, 1976). May (2003) states that routine measurements of beach profiles in Ringstead Bay over 15 months in 1983/4 revealed substantial erosion and accretion, but with an overall balanced sediment budget; this condition of stability has apparently not prevailed since the late 1980s.)
Transport bypassing of promontories at Bran Point, Osmington, Black Head and Redcliff is not supported by analysis of Coastal Monitoring Programme data, although a weak westward drift is evident within each pocket beach. Arkell (1947) noted the absence of Corallian Limestone clasts east of central Ringstead Bay, was evidence of net east to west longshore movement. He also reports that the coarsest sediment particles accumulate on the eastern sides of headlands, with size grading occurring immediately downdrift of these partial barriers to movement. Mouchel (1998) infers that this pattern is most apparent during periods of beach accumulation under constructive wave action. Waves approaching from the south-east may be the most effective, implying (from their comparative infrequency) that significant drift movement only takes place when forcing conditions are favourable. Coastal erosion and mudflows provide supplemental sources of mostly sand sized sediment, but an unknown proportion of the gravel supply must derive from the degradation and recession of the White Nothe landslides.
Analysis of 2008 swath bathymetry data indicates that within the centre of Ringstead Bay and extending offshore the nearshore sediment thickness is generally sufficient to mask the underlying geology, and the relatively featureless seabed gently slopes southeastwards. This reservoir of sediment may provide an intermittent or pulsed cross-shore input to the local beach. However, the lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of onshore (or offshore) sub-tidal sediment transport.
LT2 White Nothe to Durdle Door
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data supports a weak eastward drift within each pocket beach, with negligible rates and volumes due to limited supply of cliff-derived beach grade material; therefore a drift divide at or slightly east of White Nothe appears to be evident. Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Between Redcliff Point and Durdle Door the rock platform extends subtidally 200-600m and only covered with a variable but thin veneer of sediment. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
A narrow steep swash-aligned quasi-dynamic gravel beach has accumulated west of the Durdle Door promontory (Photo 9), which functions as a local drift boundary. The sub-rounded form of most of the flint and chalk clasts indicates constant attrition and abrasion in this confined transport sub-cell. Although exposed to south-west waves, an offshore reef of Portland Limestone, breached at two points, absorbs some of their energy. Small parts of this latter feature are exposed during both the rising and falling tides. The well-sorted nature of this coarse clastic beach infers selective net offshore loss of finer grade material. May (2003) considers that the majority of clasts are derived from comparatively recent cliff falls in the immediate vicinity, but a small proportion originate from the landslip debris stores to the west. Posford Duvivier (1999) calculated that the erosion of Chalk cliffs along this sector yields 200-400m³ of coarse debris, and some 25,000m³ of fines, annually. Much of this material derives from rock falls, which have a short residence time due to rapid breakdown by mechanical and chemical processes. An example, in 1983, is described by May and Heeps (1985). Cross-shore transport is indicated by rapid changes in beach volume and profile, particularly associated with storm waves. Heeps (1986) reports, from a 15 month survey in the early 1980s, that the most substantial losses resulted from easterly storms, but that almost all of this material was subsequently returned. This beach therefore appears to be in a quasi-equilibrium condition.
LT3 Durdle Door to Lulworth Cove (including within the cove)
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data supports a weak eastward drift within each pocket beach, with negligible rates and volumes due to limited supply of cliff-derived beach grade material; therefore a drift divide at or slightly east of White Nothe appears to be evident. Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Between Durdle Door and Worbarrow the platform extends subtidally approximately 100m and only covered with a variable but thin veneer of sediment. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
A mixed gravel and coarse sandy beach extends from the eastern side of the Durdle promontory to Dungy Head in a closed sediment transport sub-cell. From Dungy Head to Lulworth Cove entrance there is a discontinuous accumulation of boulders; these derive from occasional rock falls from the Portland Limestone cliffs. The presence of the inshore reef of Man O' War Rocks (Photo 10) immediately east of Durdle Door dissipates much potential wave energy and creates behind its eastern end a cuspate beach modelled by waves propagated by the local fetch. Sets of dimensionally organised beach cusps are a characteristic feature here (Canning and Maxsted, 1979), as is sediment grading, with the largest particles moving towards the centre of Man O’ War Bay (May, 2003). HR Wallingford (1998) estimates a minimal rate of eastwards littoral drift of approximately 220m³ per year. This is partly due to limited supply of sediment stable on the beaches of this sector; and partly to a complex wave climate that is affected by the presence of the offshore Corallian Limestone outcrop of the Lulworth Banks (May, 1990) and the inshore reef. Occasional rock falls are detached from the high, solutionally weathered Chalk cliffs, as in March 2013, but are relatively rapidly broken down by abrasion and solution (May and Heeps, 1985). This yields a small supply of flints, whilst fine material is presumed to move offshore. A coarse boulder beach is trapped within Stair Hole, mostly the product of sub-aerial breakdown of Purbeckian Limestones.
Lulworth Cove
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates there is no net drift direction within this isolated well-developed gravel beach, which widens eastwards. Fine-grained material mostly derived from sub-aerial denudation of the Wealden clays and sands likely to be transported offshore in suspension. Negligible rates and volumes due to limited supply of cliff-derived beach grade material (Photo 3). There appear to be localised, short-term series of drift reversal. It has been described by numerous authors (e.g. Bird, 1996; Canning and Maxted, 1979), but there is no quantitative data on which to base a morphodynamic assessment. Refracted waves pass through the cove entrance, creating a swash-aligned beach; numerous student projects have measured clast size and shape distribution, suggesting that there may be a weak net easterly drift. The well rounded nature of most Chalk and limestone clasts indicates abrasion of material confined to the cove.
LT4 Lulworth Cove (eastern entrance) to Mupe Rocks
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates that with very little supply of material from the steep, plunging cliffs of this sector, littoral transport is virtually zero. The negligible volumes of fine-grained material transported offshore in suspension. Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Between Durdle Door and Worbarrow the platform extends subtidally approximately 100m, covered with a variable but thin veneer of sediment. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
LT5 Warbarrow Bay (Mupe Rocks to Warbarrow Tout)
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates littoral transport is negligible along this discontinuous series of localised pocket beaches, with fine-grained material transported offshore in suspension. Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Between Durdle Door and Worbarrow the platform extends subtidally approximately 100m and only covered with a variable but thin veneer of sediment. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
Arkell (1947) inferred, from patterns of sorting of beach particle size and composition, a complex pattern of littoral transport, with larger particles moving towards the centre of Warbarrow Bay. Smaller clast sizes were, he considered, selectively moved both east and west. However, both HR Wallingford (1998) and Mouchel (1998) observe that the characteristically multi-bermed beaches are swash aligned and adjusted to wave refraction around both confining headlands. Coarse sand and gravel beaches have accumulated in Mupe Bay and north of the headland of Warbarrow Tout reflecting local supply from the degradation of the Wealden Clay cliffs immediately behind as well as flint particles from the breakdown of Chalk debris in the centre of the bay. As gravel is dominant, there is some doubt about the effectiveness of the Wealden outcrop as a constant source of supply. The central near vertical Chalk cliff section of Warbarrow Bay is backed by Chalk debris rather than beaches at the toes of high cliffs so that any actual drift is likely to be low. A progressive increase in beach width eastwards from Arish Mell is consistent with the asymmetric planform of Warbarrow Bay and infers net littoral transport in the same direction, as coarse sediment is apparently unable to by-pass Warbarrow Tout. The potential for longshore movement driven by refracted waves approaching from the south-west is limited, and actual volumes and rates are low. There may be periodic reversals of drift direction associated with changes in incident waves (Heeps, 1986). Warbarrow Bay is therefore a self-contained transport sub-cell, although some onshore to offshore movement of fine grained sediment is likely. This is estimated at 1,500m³ per year (Posford Duvivier, 1999). Approximately 100m³ per year of flint shingle derives from falls and topples from the Chalk cliffs east and west of the truncated valley of Arish Mell (May and Heeps, 1985; Posford Duvivier, 1999; May, 2003) and an additional but small quantity of gravel is supplied by erosion of a series of thin pebble beds within the Wealden formation. Heeps (1986) describes sonargraph survey evidence of an arc-like submerged reef connecting Mupe Rocks and Warbarrow Tout, with its apex opposite Arish Mell. This feature is composed of a complex series of ridges and troughs, with flatter sea bed topography to north and south. That part of the reef developed on the outcrop of Portland Limestone is jagged and unbreached, and up to 9m in height (May, 2003). Sand and shell ripples on the floor of Warbarrow Bay indicate alongshore transport in the nearshore zone, but in which net direction is uncertain. The presence of large numbers of unbroken shells indicate a low energy environment and probably low rates of transport.
LT6 Warbarrow Tout to Egmont Point
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates a weak eastward littoral drift along this discontinuous series of beaches. Brandy, Hobarrow and Kimmeridge Bays effectively function as virtually self-contained units, with small gravel "pocket" beaches. Negligible volumes of fine-grained material transported offshore in suspension, with no evidence of headland bypassing of material. Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent, almost continuously exposed. Eastward from Worbarrow to Peverill Point the sub-tidal extent of the exposed rock platform and nearshore geology is considerable and only covered with a variable but thin veneer of sediment. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
The precise compositions of Brandy, Hobarrow and Kimmeridge Bays closely reflect the availability of local material from cliff erosion. HR Wallingford (1998) calculate gross annual drift in the nearshore zone to be in the order of 1,500m³ per year eastwards. Tidal velocities are low (< 0.5ms-¹), thus all transport is generated by breaking waves. Wave energy, however, is dissipated by the presence of wide shore platforms that are particularly well developed in Hobarrow and Kimmeridge Bays, off Broad Bench (Photo 7) and the Kimmeridge Ledges between Hen Cliff and Egmont Bight. Where the alternating clay, shale and cementstone lithology creates obliquely-trending ridges potential drift rates may be as low as 200m³ per year. A regionally high rate of shoreface erosion, in the order of 13,200-14,000m³ per year (Posford Duvivier, 1999) is due to the predominance of the clay and shale outcrop areas. Much of this material is removed offshore, in suspension, and is added to by locally high rates of cliff erosion where platforms are absent or poorly-developed (e.g. central Kimmeridge Bay). There is a slight increase in the supply of coarse material east of Rope Lake Head, and much of this appears to move eastwards into the confined pocket beach of Egmont Bight where transport is intercepted by the debris lobe of Egmont Point beneath Houns-tout cliff (Photo 5). It is uncertain if Egmont Point is a transport barrier to fine sand, which may therefore feed the next sector downdrift (Chapman's Pool). North of Clavel Tower, in eastern Kimmeridge Bay, there appears to have been some small-scale, private, beach renourishment in the past; previous shale workings have also contributed a legacy of basal debris.
LT7 Egmont Point to At Aldhelm’s Head (West)
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates there is no net drift direction within the Chapman’s Pool pocket beach. This beach exhibits periodic fluctuations in both width and sediment composition, sometimes possessing a gravel-dominated backshore as well as a more persistent sandy foreshore.
Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent. Eastward from Worbarrow to Peverill Point the sub-tidal extent of the exposed rock platform and nearshore geology is considerable and are covered with a variable but thin veneer of sediment. Negligible volumes of fine-grained material is transported offshore in suspension, with no evidence of headland bypassing of material. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
A well-defined clockwise tidal eddy, set up by the blocking effect of St Aldhelm's Head, exists immediately to the west and offshore. This may create tidal currents capable of moving sand.
Around Egmont Point beneath Houns-tout cliff (entrance to Chapman's Pool) a boulder beach, with several sand traps, represents debris from previous cliff failures (Photo 5). Aprons of Portland Stone boulders are well developed on the west-facing coast of St Aldhelm's Head, where there is little sand and gravel accumulation.
LT8 St Aldhelm’s Head (East) to Durlston Head
Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography data indicates that with very little supply of material from the steep, plunging cliffs of this sector, littoral transport is virtually zero.
Analysis of 2008 swath bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent. Eastward from Worbarrow to Peverill Point the sub-tidal extent of the exposed rock platform and nearshore geology is considerable and are covered with a variable but thin veneer of sediment. Between Durlston Head and St Aldhelm’s Head there are a few localised patches of sediment constrained between the rock formations, which contain symmetrical bedforms. There is no evidence for transport around this large headland. The dominant westerly directed tidal stream off St Aldhelm's Head does not appear to be a significant factor in either near or offshore sediment movement. The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of along-shore or cross-shore sub-tidal sediment transport.
4. Sediment Outputs
4.1 Nearshore and Offshore Transport
O1 Westward and South-westward Sand Transport on the Offshore Sea Bed
A general tendency exists for westward or south-westward transport of sand on the seabed for up to 10km seaward of the shore to the west of Warbarrow Bay (Bastos and Collins, 2002). Donovan and Stride (1961) suggest that tidal currents may have sufficient energy to entrain and "sweep" silt and sand and move it offshore. HR Wallingford (1998) has made a general inference that this material could join a southwest-directed tidal current pathway, possibly feeding a sink (The Shambles) east of Portland Bill. This is supported by Bastos and Collins (2002), who investigated seabed mobility and transport paths in the vicinity of the Shambles.
Analysis of 2008 bathymetry data indicates that along the Dorset coastline, the rock platform and nearshore geological features become more prominent. Between Redcliff Point and Durdle Door the rock platform extends subtidally 200-600m; eastward from Worbarrow to Peverill Point the extent of the exposed rock platform and nearshore geology is considerable. These bedrock exposures are only covered with a variable but thin veneer of sediment. Between Durdle Door and Worbarrow the platform is less extensive, extending approximately 100m offshore. Apart from nearshore ledges, the only significant features of the offshore seabed are the Lulworth Banks, seawards of Lulworth Cove (Donovan and Stride, 1961 a and b; May, 1990), and a distinct linear 'step' south of St Aldhelm's Head. The first is the result of the outcrop of Corallian Limestone, due to local anticlinal flexure; the second has been tentatively interpreted as an abandoned cliffline associated with a lower Quaternary sea-level (Mouchel, 1998). In the nearshore zone a complex pattern of ridges and depressions constitute a reef that runs with only occasional breaches between White Nothe and Gad Cliff.
Coastal Monitoring Programme data have informed British Geological Survey Offshore Solid Geology and Seabed Sediment maps covering this area at 1:250,000 scale, and confirm that the sediment thickness over large areas of the offshore zone, extending some 10km offshore, are generally insufficient to mask the underlying geology, with significant features exposed. There are, however, localised patches of muddy and sandy gravels, offshore Ringstead Bay, White Nothe, Lulworth Cove, the eastern part of Warbarrow Bay and immediately west of St Aldhelm's Head. A small area of thin, gravel dominated, sediment cover exists seawards of Osmington, whilst gravelly sand conceals bedrock over a small area due east of St Aldhelm's Head (Dorset Coast Forum, 1998). The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of offshore (or onshore) sub-tidal sediment transport.
The presence of large boulders scattered across several parts of the nearshore seabed is reported by Donovan and Stride (1961 a and b), but there is insufficient detail given to determine any pattern. Boulders are likely to be relict features of former large scale coastal landslips, mudflows, etc. that occurred during, and since, periods of lower sea-level. One recent Chalk rockfall event is described by May and Heeps (1985), who note the rapid removal of fine grained sediment and the isolation of individual boulders in the nearshore zone.
Analysis of Coastal Monitoring Programme data provided no evidence to support a sediment sink or offshore transport, to the west of St Aldhelm’s Head, therefore the speculative 2004 arrows have been removed.
5. Summary of Sediment Pathways and Budget
- The South Purbeck coast is subject to relatively high wave energy, which declines westwards due to protection from the "Isle" of Portland. Tidal currents are weak in the inshore region, but intensify at the major headlands.
- Much of the shoreline is composed of cliffs, which vary considerably in height and morphology. The latter is controlled by the influences of regional and local tectonic structures, rock lithology and stratigraphic successions. Clay and shale outcrops are dominant in the west and in the central area where Kimmeridge Clay outcrops. Elsewhere, Calcareous rocks are the major ground forming materials. Several deep-seated landslide complexes result from the exposure of interbedded competent and incompetent strata to basal marine erosion, as well as hydrogeological controls. Rockfalls and topples; translational and rotational slides; slumps and mudslides are active cliff-forming processes at many locations. The balance between the different cliff degragation processes is a function of the spatial variabilities of rock structure and exposure to climatic influences and wave energy. It has produced a complex series of headlands and bays along this coast. An evolutionary model based on time-space integration of the various coves and embayments between Durdle Door and Warbarrow Bay is widely quoted, but oversimplifies local topographical and morpho-structural reality.
- In spite of a large overall input of sediment from cliff and shoreface erosion (200,000m³ per year), much of the material supplied is either clays or other weakly resistant rock that is rapidly abraded by wave action and transported seaward in suspension. Solution removes a substantial proportion of material detached from Chalk and Limestone cliffs. Some harder limestones are supplied which persist as boulder accumulations and aprons upon the foreshore providing protection to cliff toes. Only small quantities of flint and chert materials can form persistent beach gravels, amounting to a small fraction of total coast erosion sediment yield.
- Littoral drift is discontinuous being intercepted by numerous headlands and embayments. A weak littoral transport divide exists in the vicinity of White Nothe, giving net directions of drift westwards towards Weymouth Bay and eastwards towards Durlston Head. The latter is a fixed and absolute transport boundary for coarse sediment. Actual rates and volumes of wave-driven littoral transport are well below potential values, due to: (i) trapping of sediment in various embayments and coves confined by hard rock headlands, nearshore reefs and shoreline debris aprons; (ii) dissipation of wave energy by offshore and nearshore reefs, ledges and platforms, and (iii) low availability of littoral sediment and/or the presence of deep water adjacent to several sectors of cliffed coastline. Except at the major headlands, tidal currents are of limited significance in the nearshore zone, but in combination with waves they may promote the movement of sand across the offshore seabed. In western parts, sand transport on the offshore bed appears to operate in a net south-westwards direction, feeding the sediment stores of the Adamant Shoal and The Shambles Bank.
- There are relatively few coastal "problems" because this is an undeveloped and partially inaccessible coastline with very limited and localised attempts at shoreline protection. Habitat and Earth Science conservation interests are exceptionally high. Partly because of the absence of population centres and intensive land uses there have been few studies of the hydraulic and morphodynamic regimes.
6. 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 Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Analysis of 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.
Very few estimates of drift are available and those that have been made are potentially unreliable due to the large differences between potential drift and the drift actually occurring that is controlled by sediment availability, frequent headland obstructions and nearshore reefs. Indeed, the discontinuous nature of the shoreline of this unit with its numerous headlands, nearshore reefs, boulder aprons and pocket beaches means that it is unsuited for definitive studies of drift due to its complexity.
There are, however opportunities to study drift occurring on the major pocket beaches e.g. Ringstead and Warbarrow Bays where each bay would appear to operate as a relatively closed system for gravel and coarse sand. A possible approach would be to compare the sediment accretion/depletion (based on profile measurements) against confining headlands within each bay with estimates of transport derived from modelling based on hindcast waves. Potentially, a beach plan shape model could be set up to simulate the beach responses to SW and SE waves that would tend to cause significant beach re-orientations within the confined bays. Major problems could be introduced by the complex nearshore reefs at these locations that would cause shoaling and refraction of waves, potentially setting zones of wave energy focusing and causing hydrodynamic discontinuities in drift.
7. Knowledge Limitation and Monitoring Requirements
The Southeast Regional Coastal Monitoring Programme commenced in 2002. 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.
Foregoing sections have revealed considerable uncertainties relating to sources and volumes of sediment input and both volumes and rates of sediment throughput via longshore transport. Outputs or losses from beaches are largely a matter of inference as none of these components of the regional sediment budget have been quantified. These deficiencies largely reflect the fact that most of this coastline is relatively remote and undeveloped such that there are few coastal defence issues requiring investigation and the practical needs for further research and monitoring might be considered to be a lower priority than for adjoining coastlines. This situation is unlikely to change in the foreseeable future.
Notwithstanding results from the Regional Coastal Monitoring Programmes, and the summarised information collated in the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), the following recommendations for future research and monitoring that might be required to inform management include:
- Sedimentology surveys, particularly at locations of dynamic change such as between Ringstead and Osmington, could assist in calculating beach volume changes enabling better understanding of beach responses such as drawdown and re-orientation induced by storm waves and changes in wave direction, respectively.
- A systematic analysis of all cartographical and air photo records of this coastline, to produce geomorphological mapping of cliff and shoreline features and for comparisons of present beach, shore and cliff feature positions with historical data determine rates of change with improved precision.
- Instrumentation of selected unstable slopes, to determine operative processes and rates of movement of ground materials.
