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

Otterton Ledge to Beer Head

1. Introduction

This coastline has a shallow arcuate zetaform plan shape between the relative "hard" points of Otterton Ledge (Photo 1) and Beer Head (Photo 2). It is mostly characterised by retreating high cliffs developed in moderately resistant bedrock. Discontinuity of cliffline development occurs at Sidmouth, where the River Sid discharges across, and through, the beach (Photo 3). Several other valleys occupied by small streams have been truncated by cliff retreat at Salcombe Mouth (Photo 4), Weston Mouth and Branscombe Mouth. Southwest of Sidmouth the coastal planform consists of a succession of headlands (Photo 5 and Photo 6), and associated shore platforms, separating a sequence of relatively shallowly defined coves and bays occupied by pocket beaches, e.g. Photo 7. To the west of Sidmouth, cliffs are steep to near vertical (Photo 5 and Photo 8) and are controlled by a spatially variable combination of: (i) basal wave erosion and/or seepage; (ii) periodic failure by rockfalls and landslips, and (iii) gulleying by overland flow. To the east of the town, the rock succession dips eastwards (Photo 9), creating long-established conditions for large-scale and complex coastal slope failure (Photo 10). Here, particularly between Beer Head and Branscombe, an undercliff of fallen blocks and other landslide debris conceals the solid geology (Photo 11).

Beaches within this unit comprise an upper berm of coarse clastic material, often exhibiting a sequence of parallel storm ridges, and a low gradient foreshore of fine sand (Photo 9 and Photo 12). The latter is relatively thin and is subject to periodic removal, exposing underlying shore platforms cut into bedrock. Low elevation platforms and more pronounced rocky reef-like forms and boulder accumulations are a feature of many of the headlands south-west of Sidmouth (Posford Duvivier, 1999). To the east of the mouth of the Sid and especially towards Beer Head, the inter-tidal beach is significantly higher and wider (Photo 11 and Photo 13) than it is to the west. The beach at Sidmouth itself has a history of fluctuation of level and volume, in part due to the presence of protection structures since the late nineteenth century. It is now subject to a beach management strategy involving offshore breakwaters (Photo 14), rock groynes (Photo 9) (Posford Duvivier, 1998a; Posford Haskoning, 2001) and routine monitoring.

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 2009, the Southwest Coastal Monitoring Programme completed a 100% coverage swath bathymetry survey, extending offshore 1km from MLW between Petit Tor Point and Portland Bill.

Net longshore transport is directed from south-west to east-north-east, although actual transport is constrained by headlands and other features. Three distinct transport zones can be identified as follows:

1. Otterton Ledge to Chit Rocks, Sidmouth (Photo 15): dominated by rocky headlands and shore platforms enclosing discrete gravel pocket beaches. The shoreline becomes increasingly rocky and beaches less frequent to the west of Ladram bay. Transport is believed to be confined primarily to each pocket beach with negligible by-passing of headlands.
2. Sidmouth frontage. Beach drift is controlled by Chit Rocks, two artificial offshore breakwaters (Photo 14), three shore-attached rock groynes (Photo 9) and the River Sid training wall (Photo 3).
3. East Sidmouth to Beer Head: A shallow embayment occupied by a gravel beach that becomes increasingly substantial towards the east (Photo 13). Relatively free transport is possible along this beach excepting occasional temporary interruptions due to cliff fall debris that can partially block the transport pathway following major failures e.g. Dunscombe Cliff.

The sediment transport system regulating coarse gravels upon the upper beaches of these three zones is believed to be relatively closed, except for cliff inputs. Significant quantities of coarse and fine sediment are introduced by cliff and platform erosion, but the latter are removed seawards, in suspension.

Hydraulics Research (1992), HR Wallingford (1993) and Posford Duvivier (1992) note that waves from the south, south-east and south-west are experienced at different frequencies: Laver (1985) undertook a daily analysis of wave approach direction at Sidmouth beach between July 1981 and June 1982. 40% were from the south, 29% from the south-west and 31% from the south-east. Inshore waves are the result of the transformation of offshore waves by locally complex refraction resulting from irregular nearshore seabed morphology. Wave shoaling can commence 200m seawards of the low tide shoreline, depending on tidal level, wave height and period. Hydraulics Research (1992) calculated that a wave height of 3.9m, off the mouth of the Sid, has a 1 in 50 year recurrence. Maximum significant nearshore wave heights at Sidmouth were calculated to be between 2.5m (south-south-west waves) and 3.5m (south-south-east waves) during the 1980s (Hydraulics Research, 1992), but these figures are derived from hindcasting based on non-local wind data. Laver (1985) reported breaking wave heights of between 20cm and 90cm during a one-year daily survey at Sidmouth in 1981/82. The accuracy of measurement, however, is in some doubt. Wave energy from the south-west and west suffers greater attenuation and decay than from the east or south-east. This is because the former suffer refraction around Start Point, Berry Head and Otterton Ledge, whereas the latter are generated over a less interrupted fetch (and normally occur during winter months).

The Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The nearest measurement stations to this cell are at Dawlish and West Bay. The buoy deployed at Dawlish in 2011 is in 11mCD water depth. The prevailing wave direction is south-by-east. Average 10% significant wave height exceedance is 1.05m. The prevailing wave direction from measurements from the buoy deployed at West Bay in 10mCD water depth, between 2006 and 2012 is southwest-by-south, with an average 10% significant wave height exceedance of 1.74m (CCO, 2012).

The tidal range at Sidmouth is 3.95m (springs) and 1.55m (neaps). Typical north-eastward flood tidal currents offshore in Lyme Bay are 0.20ms-¹ (neap tide) to 0.36ms-¹ (spring tide) and corresponding south-westward ebb velocities are 0.26ms-¹ (neap tide) to 0.50ms-¹ (spring tide) (Posford Duvivier, 1998b). There are no inshore data on residual tidal current directions and velocities (Posford Haskoning, 2001).

2. Sediment Inputs

2.1 Fluvial Input

FL1 River Sid

The River Sid discharges to the east of Sidmouth, where its mouth is constrained by a training wall (Photo 3). It is non-tidal and is regulated and channelised in its lower course through the town, with weirs upstream. It has a compact catchment with steeply sloping valley sides and tributary streams. It is estimated to deliver an annual load of approximately 400m³ of fine sediment and 100m³ of coarse material with much of this is likely to occur during high discharge events (Rendel Geotechnics and University of Portsmouth, 1996; Posford Duvivier, 1999). Longshore sediment transport of gravel, usually to the east of the Sid channel is characterised by short-term increases in both rate and volume; this blocks the river mouth, which then discharges via seepage through the temporary barrier. Laver (1981) reported that, for the period between the early 1930s and late 1960s, the average length of time during which the river mouth was blocked was 16 days. There were, however, some occasions when direct discharge was impeded for 3 to 4 months. Aerial photography suggests the presence of a small delta of sand and gravel immediately seawards of the mouth of the Sid, but there are no records of any mapping or sediment sampling from this feature. It is therefore uncertain what proportions of the coarse bedload discharged by the Sid contribute to the beach and nearshore delta.

2.2 Cliff and Shore Platform Erosion

E1 Otterton Ledge to Sidmouth

The cliffs immediately west of Sidmouth, continuing south-west to Otterton Ledge, are composed of moderately resistant to relatively unresistant Triassic Sandstones. Between Otterton Ledge and Ladram Bay they form near vertical profiles some 20m to 40m in elevation that retreat slowly by rockfall and weathering in response to persistent basal undercutting (Photo 5) Headlands coincide with relatively more resistant formations in the Otter Sandstone sequence. There is evidence from boulder aprons of former cliff falls within some of the coves, and shore platforms up to 140m in width are extending seaward from the cliff toes. May (2003) notes that the co-existence of mass movement features and well defined platforms is a relatively unusual morphological condition on eroding coasts. Stack detachment, at Ladram Bay is proof of past and continuing relatively low energy wave and sub-aerial erosion of major joints and bedding planes (Photo 6). The several stacks immediately seawards of headlands are located where more resistant strata, with a low angle of dip, outcrop at sea-level (May, 2003) thus forming supporting pedestals. Cliffs rise to 120m at High Peak, where the upper portion is formed within erodible Keuper Marls with Otter Sandstone forming the lower portion. This results in a compound stepped cliff form with the less resistant upper strata failing by slides and gullying to form a steep undercliff above the near-vertical Otter Sandstone sea cliffs. Posford Duvivier (1998), using comparisons of historical maps back to 1886, suggest a mean recession of 0.2m per year for the cliffs between High Peak and Otterton Ledge. To the east of Peak Hill, geological dip brings relatively erodible Keuper Marls to sea level and more rapid erosion rates are characteristic (Photo 12 and Photo 15).

Cliff recession is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and 2012 aerial photography, however, there is a lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes (although a range of estimates have been postulated by other studies and researchers). This means that it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material.

E2 Sidmouth to Beer Head

To the east of Sidmouth, geological dip brings relatively erodible Keuper Marls to sea level ; further east to Beer Head, the overlying Upper Greensand and Chalk rest unconformably upon the Triassic strata forming plateau and hilltop caps that increase in thickness to the east. The soft relatively impermeable Keuper series yields readily to shallow landsliding, rockfalls, mudflows and gullying. The hill capping Upper Greensand and Chalk fail by rockfall and also by high magnitude low frequency detachments of large blocks that form a narrow undercliff above the Keuper Marl sea-cliffs. Examples of this high (150m -160m) compound cliff morphology are found at Salcombe Hill, Dunscombe Cliff (Photo 4), Coxe's Cliff (Photo 10) and Branscombe Cliff. In places, the cliffs are relatively well vegetated and appear inactive, though there is much evidence of past landsliding e.g. Dunscombe Cliff and for up to 1km to the west and east of Branscombe Mouth. Elsewhere, there is much evidence of recent cliff falls and instability affecting the sea cliffs, including some falls from the upper cliffs and several locations where the whole cliff has been affected. Increasing seacliff erosion could be destabilising some of the inactive landslides above Salcomb Cliff, Dunscombe Cliff and Coxes Cliff. There are, however, locations where upper cliff failures have occurred above inactive sea cliffs, suggesting that increasing groundwater levels may also be a factor promoting instability.

Between Branscombe and Beer Head, long-term coastline recession has cut back to create a 160m high landslide complex with active marine eroded cliffs between 40 and 90m in height. This is primarily due to the presence of permeable, well-fissured and seaward dipping Upper Greensand and Chalk on top of relatively impermeable marls and clays. This has promoted multiple rotational slope failures, giving a well-defined backscar and debris slope (Photo 11). Debris release occurs as a result of basal erosion and sub-aerial processes of weathering and minor mass movement; however, most input appears to be the product of high magnitude, low frequency failure events, of which a well-documented example occurred in 1790 (Woodward and Ussher, 1911; Perkins, 1971). On this occasion, a deep, wide cliff top fissure detached a Chalk block 250m in width that moved en masse downslope and advanced the position of the cliff foot by some 200m. At the same time, the inter-tidal foreshore was elevated some 6m in 3 hours and a temporary offshore reef created. The several slender Chalk "stacks" that make up the Pinnacles are the product of this, and earlier, major landslide-induced block detachments subsequently exploited by weathering (Arber, 1940; Ager and Smith, 1965). The geomorphology of the coastal slope between Salcombe Hill and Coxe's Cliff, as well as the vicinity of Beer Head, is partly the product of Greensand quarrying between early medieval times and the nineteenth century; thus some of the material on the lower debris slope is spoil, albeit now largely overgrown. There is evidence elsewhere for high magnitude erosion events, such as the substantial reduction of the area of Chit Rocks, Sidmouth, during an exceptionally severe storm in 1824. Gallois (2005), referring specifically to the east-facing cliffs between Beer Head and Beer Roads, observes that falls have been confined to a layer of partially dissolved, or redissolved, Chalk up to 30m in thickness. The details of cliff morphology relate closely to stratigraphic variations in bulk lithology. The inherent vulnerability to failure in this sector of shoreline has produced several cliff falls in recent years, most following prolonged or intensive rainfall. One failure at Pound’s Pool involved the displacement of 50,000 tons of rock.

The long-term erosion and recession of this coastline is evident from the truncated "hanging" valleys between Salcombe Hill and Branscombe. In recent years, cliff toe recession and up-slope instability at Salcombe Hill has accelerated (Posford Duvivier, 1998b, 1999; 2000).

There are few calculations of detailed erosion rates; Posford Duvivier (1998), using historical maps back to 1886 suggest a mean of 0.3m per year for the cliffline between Sidmouth and Beer Head. For the western part of Salcombe Hill, recession rates of 1.5m per year (1980-1995) and 1.7m per year (1990-1996) are calculated by Posford Duvivier (1999, 2001.) Such recession rates immediately adjacent to the town of Sidmouth (Photo 3) have caused concerns for infrastructure and residential properties and several alternative options have been proposed for a partial stabilisation scheme (Posford Haskoning, 2002). Deliberations  on management options remain in progress due to the extreme sensitivity of the location covered by World Heritage and SAC designations for geology and ecology respectively (Posford Haskoning, 2003; see also the current Shoreline Management Plan.)

The mean recession values quoted conceal considerable spatial variation resulting from local cliff failure and the temporary presence of basal debris. There is evidence, specifically from Pennington Point, immediately east of Sidmouth (Photo 3), of a doubling of the cliff toe erosion rate, 1980-2000 (Posford Duvivier, 2001). It is not known for certain if this is characteristic of other adjacent cliffed units although a visual inspection of aerial videography reveals evidence of cliff reactivation throughout the frontage. In a few locations, cliff stabilisation has been attempted, thus reducing natural rates of sediment yield from ongoing erosion. An example is the cliffs behind Chit Rocks, protecting Connaught Gardens, Sidmouth (Photo 16), where there has been a progressive extension of protection measures since 1957 (Posford Duvivier, 1994).

For the section of cliffed coast between approximately Chapman's Rock and Weston Combe, east of Sidmouth, Posford Duvivier (1998) indicate that some net advance of the cliff base has occurred over the preceding century. This is based solely on map analysis; if accurate, it would presumably be the result of cliff falls and outward growth of the landslide debris store. Posford Duvivier (1999) discount this frontage as a significant source of beach sediment. Total volumes of cliff material may be significant. Cliff recession is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and 2012 aerial photography, however, due to a lack of detailed study information regarding cliff sediment composition and proportions of sediment grain sizes (although a range of estimates have been postulated by other studies and researchers), it has not been possible to quantify volumes of cliff input yielding shingle or sand grade beach material. Posford Duvivier (1999) estimates that at least 50% of the supply is likely to be fine material that becomes lost offshore in suspension. However, sands are likely to be supplied from the Otter Sandstone and Upper Greensand and cherts and flints would be supplied from the Upper Greensand and Chalk respectively. Studies of these strata in West Dorset found that cherts and flints could comprise up to 5%, or even 10% locally, of the total sediment input (Bray, 1996; 1997). These results therefore indicate that overall supply could be extremely significant, especially if the cliffs continue to reactivate and erosion accelerates further as might be anticipated with future climate change and sea-level rise (Halcrow et al., 2001). Further studies, involving more detailed assessment of recession rates and sampling of cliff face lithological units, are needed to determine the true significance of cliff sediment inputs.

Posford Duvivier and the British Geological Survey (1998) and Posford Duvivier (1999) calculate an annual shoreface erosion sediment yield of 16,000 to 49,000m³ per year for the entire length of this coastline. This is based on an average shoreface width of 700m and a theoretically determined rate of vertical erosion of between 1.3 and 4mm per year. The wide range of this estimate reflects uncertainty of the effectiveness of the processes of bedrock abrasional scour by wave action. Almost all of this yield is fine sand, silt and clay and is moved offshore in suspension e.g. Photo 15.

2.3 Beach Replenishment

A major scheme at Sidmouth was constructed in stages between 1994 and 1999. It involved two obliquely orientated, offset detached rock breakwaters close to Chit Rocks (Photo 14); a sequence of three rock groynes and 185,000 tonnes of gravel renourishment distributed behind the breakwaters and between the rock groynes (Andrews, 1996). The replenisment fill comprised mostly flint gravels sourced from a local inland quarry, providing material similar in size to the indigenous beach sediment. Subsequent monitoring and the behaviour of the fill is covered in Section 4.

3. Littoral Transport (Beach Drift)

» LT1 · LT2, LT3 · LT4

Most authorities consider that there is a weak net south-west to north-east (from Otterton Ledge to Big Picket Rocks) and west to east (from immediately west of Sidmouth towards Beer Head) littoral transport pathway. Due to reversals in the direction of approach of incident waves, as well as local refraction and diffraction effects, gross transport rates are much higher than the net quantities of movement eastwards. It should also be noted that headlands, defences at Sidmouth and other obstructions frequently block drift pathways and reduce the transfers that might otherwise occur at the shoreline.

LT1 Otterton Ledge to Chit Rocks (see introduction to littoral transport)

The sequence of bays and coves along this coastline traps much of the beach sediment as “pocket” beaches, so that only small quantities, if any, by-pass the intervening headlands. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) showed no discernible net transport between the independent pocket beaches. Suspended sediments in the nearshore indicates possible cross-shore exchange to the sub-tidal zone. Tindall (1929) noted that the proportion of beach-incorporated metaquartzite clasts originating from the Permo-Triassic Budleigh Salterton Pebble Beds outcrop to the west diminishes steadily north-eastwards in the small bays between Otterton Ledge and Ladram Bay. This suggests a very low net rate of south-west to north-east longshore transport, possibly also involving abrasion wear as coarse particles move from one re-entrant trap to the next. The possibility of offshore to onshore movement feeding high, wide beaches composed of well-rounded and highly sorted clasts (e.g. Ladram Bay - Photo 7) cannot be discounted, although inconclusive from Coastal Monitoring Programme data.

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. The nearshore zone between Otterton Ledge and Chit Rocks the seabed is dominated nearshore by rocky platforms interrupted by a number of small discrete gravel pocket beaches; however the rock platforms often encloses these beaches. Further offshore, the seabed is featureless, comprising homogeneous coarse sediments and gradually deepens at a shallow gradient. No bedforms are discernible and there appears to be negligible sediment exchange between the pocket beaches, nearshore or offshore. West of Chiselbury and Ladram Bays the cliff and rock platform are continuous for 4km until Otterton Ledge.

LT2, LT3 Sidmouth Beach (see introduction to littoral transport)

Net west to east drift is apparent from a variety of observations and measurements, but significant short-term reversal of movement is a characteristic of this beach (Posford Duvivier, 1994, 1998a; 1998b; Posford Haskoning, 2001). Hydraulics Research (1992) calculated a gross potential drift rate of over 52,000m³ per year, based on mathematical modelling calibrated by regional data on wave period and direction. From these gross values this study estimated a net west to east residual transport flux of 6,350m³ per year, which was adjusted to 2,120m³ per year to take account of the storage role of the then existing (1992) beach groynes.

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. The bathymetry offshore of Sidmouth shows a featureless, gently sloping homogenous seabed comprising of coarse sediments. The training bank and breakwater are obvious anthropogenic structures. No bedforms are discernible. In addition, there is no discernible evidence of sediment from the River Sid forming an offshore sink of fine grained sediments.

Although there have been detailed measurements of beach volume losses and gains following replenishment and rock groyne construction since the mid-1990s (Posford Haskoning, 2001), these figures have not been converted into a transport rate for the entire frontage. Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) indicated that Sidmouth beach is part of a virtually closed transport sub-cell, with almost no drift from the west bypassing Chit Rocks and the adjacent nearshore detached breakwaters. The downdrift (easternmost) terminal rock groyne and the mouth of the Sid inhibit eastwards movement of sediment away from the Sidmouth frontage driven by south-west or south-south-west waves. Although not supported by the Coastal Monitoring Programme data, researchers have postulated that east and east-south-east wave conditions create a short-term littoral drift reversal of sand and gravel, suggesting that both inputs and outputs are possible at this eastern margin. It is considered (Laver, 1981, Posford Duvivier, 1994, 2001) that this material is mobilised from a nearshore store that accumulates south and east of the delta-like feature where the Sid discharges across the foreshore, although not evidenced by analysis and interpretation of nearshore swath bathymetry data collected through the Coastal Monitoring Programme. The importance of this postulated source of supply, which effectively arrives in "pulses", is indicated by the composition of natural clasts on Sidmouth Beach. Most are either flint or chert, and thus must ultimately derive from cliff erosion between Salcombe Hill and Beer Head - i.e. they are introduced from sources east of the beach. Clasts with a western provenance are relatively fewer, although no quantitative analysis of the lithology of particles on Sidmouth Beach has been undertaken. Evidence for direct offshore to onshore transport is lacking, though Tindall (1929) makes an oblique reference to possible inputs from offshore by weed-rafting of gravel. Laver (1981, 1985) observes that the offshore limit for seawards transport of gravel is between 0.5 and 1km from low water, where further movement is limited by water depth. The possibility of direct onshore transport was acknowledged by Hydraulics Research (1992), but since further evidence has not been collected, the operation of this transport mechanism remains unconfirmed and speculative.

LT4 Salcombe Cliffs to Beer Head (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme lidar (2007 and 2011) and aerial photography (2012) provides evidence of the progressive eastwards increase in beach width along this sector of coastline indicating an increase in the rate of net west to east littoral drift as a function of increasing exposure to wave energy. Drift reversals however, occur during moderately frequent periods of southeasterly wave approach. At the western, Sidmouth, end short-term pulses of gravel feed associated with waves from the south-southeast or southeast can result in significant east to west movement. This would appear to be the main cause of the periodic blockage of the mouth of the Sid (Posford Duvivier, 2001). The permanent eastward deflection and damming of the Branscombe Stream, further east, is indicative of the net eastward drift at this point. The strong cliff inputs and presence of gravelly deposits among boulder aprons at low water mark indicate that there could be a significant store of gravel sized material in the nearshore zone along this sector, given that slope failure has been a long continued process. Offshore to onshore transport may therefore contribute to this substantial beach. It remains uncertain if small quantities of coarse debris are able to by-pass Beer Head, or whether losses to deeper water could occur there (Photo 2). 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. The nearshore zone between East Sidmouth and Beer Head is often separated from the beach material by an exposed rocky ledge, which extends from the toe of the beach into the sub-tidal zone. For short sections along this frontage the beach sediments overlie the platform. Direct cross-shore transport between beach and sub-tidal zone may occur; suspended sediments in the nearshore indicate possible cross-shore exchange to the sub-tidal zone. Seaward of the rock ledges the seabed comprises a continuous extent of homogenous coarse grained sediment. No bedforms are discernible. In places, the surficial sediment thickness is insufficient to mask the underlying bedrock. The seabed gradient is slightly steeper than in the west.

4. Sediment Stores and Sinks: The Beaches

4.1 Sidmouth Beach

Hydraulics Research (1992), Posford Duvivier (1992, 1998a and 2001) and Andrews (1996) provide descriptions of the history and present morphosedimentary character of this beach. An unusually detailed set of observations of its behaviour between 1922 and 1928 are provided by Tindall (1929); these are interpreted, and supplemented, by Laver (1981, 1985).

In the early nineteenth century, previous to the construction of defences, the backshore was a wide gravel bank. A catastrophic storm in 1824 severely eroded Chit Rocks, which had formerly provided significant protection from southwesterly waves as well as being the western "hard point" that stabilised the beach. Rapid loss of beach volume necessitated the building of a seawall founded on the backshore gravel beach berm to protect the settlement in 1830. This was subsequently replaced by successively more substantial structures, with foundations in the Keuper Sandstone bedrock, in response to overwashing, breaching and progressive beach drawdown. A major groyne along the eastern bank of the Sid was inserted in 1918 to stabilise the river mouth and promote updrift beach accretion. However, losses also occurred due to removal of gravel for local road construction. This amounted to approximately 2,000 tons, 1900-1908, (Tindall, 1929); indeed, some 300 tons were used to repair the seawall in 1924! This practice was finally discontinued in the late 1920s. Tindall (1929) records that, between March 1922 and June 1926, Sidmouth beach fluctuated considerably in shape and volume. There was a definite pattern of winter drawdown and summer aggradation, with several random movements imposed by storms. Tindall concluded that the major factor regulating the behaviour of Sidmouth Beach was the direction and continuity of longshore drift, itself determined by incident wave direction. Laver (1981) observed that beach levels in the 1920s were lower than in the late 1970s. He also quotes an unpublished manuscript recording weekly beach level measurements over the period 1953 to 1957, demonstrating that this beach was comparatively high and stable over this period. The low levels of the 1920s are probably attributable to two major storms in 1923 and 1924, recovery from which occupied the following 5 to 7 years.

Throughout the 1980s beach levels and volumes steadily fell, with severe beach drawdown and losses to the nearshore/offshore occurring rapidly during the storms of the winter of 1989/90. It should be noted that these storms were particularly severe and caused damaging impacts throughout the south coast of England (Maritime Engineering Board, 1990). Given the small throughput of sediment via longshore transport, it was considered that Sidmouth Beach had only a limited, and diminishing, capacity for natural recovery. In 1992, the seawall behind the beach was 10-15m seawards of the adjacent, unprotected updrift shoreline, thus it was becoming exposed as a minor salient upon which wave energy could become focused. The system of closely-spaced groynes operating at that time (installed between 1953 and 1957) were intercepting longshore transport and thus inhibiting natural recovery from the impact of both short-term and prolonged periods of beach drawdown.

To address these problems, a comprehensive coast protection scheme was initiated; involving detailed physical, hydrodynamic and mathematical modelling to determine an optimum solution for future beach stability (Hydraulics Research, 1992). The critical requirement was to reduce levels of wave energy incident on the beach face and minimise reflective scour from the seawall. The final scheme, constructed in stages between 1994 and 1999, involved two obliquely orientated, offset detached rock breakwaters close to Chit Rocks; a sequence of rock groynes and 185,000 tonnes of gravel renourishment (Andrews, 1996). The latter was sourced from a local inland quarry, providing material similar in size and general type to the indigenous beach sediment. The breakwaters were designed to introduce local shelter and wave diffraction effects that slow down the rate of eastward longshore drift to produce protective tombolos of accreted sediment (Photo 14). The breakwaters and the rock groynes, were thus designed to control the replenishment fill and promote natural beach recharge when higher energy waves from the south-east or east operated. This is likely to include material that has previously by-passed the mouth of the Sid under net eastwards littoral transport driven by waves from the south or south-southwest. Posford Haskoning (2001) report on monitoring surveys of Sidmouth Beach, October 1995-June 2000, to evaluate the performance of this scheme and inform a beach management plan (up to 2006). Substantial inter-groyne movement of sediment occurred following re-nourishment in 1995. Reconstruction of the Bedford Steps rock groyne involved the redistribution of 18,000m³ of nourished material from its western to its eastern sides, and an additional 6,000m³ was placed to the east of York Steps groyne. All of these measures were undertaken to maintain original beach design parameters. Two additional surveys, in April 2000 and February 2001, revealed a net annual loss of 4,000m³ from the gravel beach and a pattern of accretion/depletion within groyne compartments that indicated net east to west longshore drift (Posford Haskoning, 2001). It is uncertain whether this reflects a change in the drift regime or is a result of the improved sediment availability and free transport possible from the east but not the west due to the obstructing Chit Rocks and the breakwaters. During the monitoring period, the beach crest revealed some narrowing, due in part to the transfer of sediment from the backshore to the foreshore, and in part to the washing out of fine material. Sandy foreshore accretion has occurred behind each of the detached breakwaters to create tombolos (Photo 14). Unusually, the plan shape of the gravel beach does not appear to have been affected in this manner. It is thus apparent that, as a result of human modification over nearly two centuries, Sidmouth beach has been transformed from a natural to a managed system requiring continued monitoring and beach management to maintain shoreline stability. Although a sound qualitative knowledge of its morphodynamic condition now exists, future management would benefit from improved quantitative understanding of nearshore and offshore wave climate and the overall sediment budget so as to assess the long-term implications and sustainability of maintaining Sidmouth as a "hard point" on a retreating coastline (Section 6).

4.2 Other Beaches

Little knowledge exists of the morphodynamic attributes of beaches to the east and west of Sidmouth. To the west, multibermed gravel backshore ridges behind narrow sandy foreshores characterise all of the larger bays e.g. Photo 12. They appear to have little mutual dependence via the longshore transport system, and their origins and behaviour are a matter of speculation. Whilst local cliff erosion provides a sufficient source of supply in the case of the smaller "pocket" beaches, the lithological composition and shape grading character of beaches in the larger embayments suggests past, if not contemporary, offshore sediment supply. West of Green Point, boulder aprons are major features of smaller pocket beaches, and derive from cliff falls or slides. The upper beach in the bay between Chit and Tortoiseshell Rocks has some of the diagnostic features of a former barrier structure that would have moved onshore. However, there is insufficient field data to argue this idea further.

East of Sidmouth, beach behaviour has been occasionally monitored over short periods for a few specific locations. The beach at the foot of Salcombe Hill has suffered depletion during at least the last 20 years, possibly due to the impedance of longshore transport by the Sidmouth defences and River Sid training wall (Posford Duvivier, 1998b, 2001). Rather like Sidmouth beach, this beach is subject to relatively rapid losses during storms followed by and protracted intervals of slow recovery, though rapid build-up can occasionally occur under a period of sustained east or east-south-east waves. The wide gravel beaches along the far eastern sector of this coastline are reported to have been stable over a long period (Posford Duvivier, 1998a, 1998b), but also experience rapid short-term changes of profile form and volume during storm events. This normally involves foreshore erosion and, less frequently, the creation of a high backshore storm berm. The composition, size and shape characteristics of the majority of beach gravel clasts indicates that chert and flint released from adjacent Lower Greensand and Chalk landslips have been a long-term source of supply. This may arrive via both longshore and possibly offshore to onshore pathways. The beaches tend to become wider and higher eastward and an eastward coarsening of their pebbles has been noted (Bird 1989).

5. 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 discontinuous nature of the shoreline of this unit with its headlands and pocket beaches means that it is unsuited generally for definitive studies of drift. There are, however, opportunities to study drift occurring between Sidmouth and Beer Head to obtain a clearer understanding of its overall regime. An initial approach would be to revisit the HR Wallingford (1992) physical model study to identify data from which to develop a numerical model of littoral drift potential at a series of points along the full beach length based on an analysis of a long-term (greater than 20 years) hindcast wave climate. Uncertainties encountered in applying numerical model studies would include:

1. Imprecision in the selection of synthetic wave climates in the absence of field validation of inshore waves. Shelter provided by the Otterton Ledge headland and the tendency for significant wave energy to approach from south west and south east direction sectors introduces complexity in the waves actually experienced at the shore;  
2. The problem of selecting a representative sediment gain size on the mixed gravel beaches (sediment mobility is highly sensitive to grain size);  
3. The large differences likely between potential drift and the drift actually occurring that is controlled by sediment availability, landslide talus obstructions, foreshore boulder aprons and nearshore reefs.

The resulting potential littoral drift volumes could then be tested by means of a thorough examination of the budget of beach sediments, especially those which accumulate immediately to the west of the Pinnacles landslide and Beer Head. This method would assume that long-term transport can be inferred from changes in beach volume and would offer an independent check on modelling results. For this to be feasible, it is important that beach volumes should be monitored and historical beach volumes and cliff erosion sediment inputs are reconstructed (e.g. using map comparision, historical documentary evidence, perhaps supplemented by photogrammetrically derived data from historical air photos dating back to the 1940s).

6. Research 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.

Only one small sector of this coastline, Sidmouth Beach, has been the subject of significant research. As much of the rest is undeveloped, there are few economic incentives to promote original research, although effective management of Sidmouth Beach ideally requires a wider understanding of the subcell within which it operates i.e. High Peak to Beer Head. Notwithstanding results from the Southwest Regional Coastal Monitoring Programme, and the summarised information collated in the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), future shoreline management would therefore benefit from:

1. Quantitative assessment of the wave climate at two inshore points along the unit such as Sidmouth and Branscombe Mouth. An initial examination of data collected by HR Wallingford (1992), Posford Duvivier (1998a and 1998b) and Posford Haskoning (2001) should attempt to identify the extent to which suitable data already exists, together with any additional studies needed to fill gaps. It ideally requires a representative long-term hindcast offshore wave climate based on some 20-30 years of wind data, together with shoaling and refraction analysis to derive inshore climates for the points selected. Temporary inshore field measurements of waves would be beneficial to validate model studies of the effects of refraction and diffraction on waves approaching from different directions. A magnitude-frequency analysis should also be linked to a quantitative study of the recurrence probabilities of extreme water levels. This is considered important for it is storm waves and storm tidal surges in combination that appear to cause erosion events and beach drawdown at Sidmouth and on neighbouring beaches.
2. To understand beach profile changes it is important to have knowledge of the beach sedimentology (gain size and sorting). Sediment size and sorting can alter significantly along this frontage due to crossshore and longshore transport, cliff sediment inputs and could also be affected by beach management. Ideally, a one-off field-sampling programme is required to provide baseline quantitative information together with a provision for a more limited periodic re-sampling to determine longer-term variability. Such data would also be of great value for future modelling of sediment transport, because uncertainty relating to grain size is often a key constraint in undertaking modelling. It would also provide insights into potential transport paths (via grain size and sorting analyses) and likely sediment provenance.
3. Detailed historical analysis of rates and patterns of coastline recession throughout the unit. This could involve using Tithe Map and Ordnance Survey large scale map comparisions, perhaps supplemented by photogrammetrically derived data from historical air photos dating back to the 1940s. Mapping should involve plotting the positions of the cliff top, cliff toe, MHW and MLW. Work should also involve an assessment of: (i) the degree of present cliff activity and (ii) the potential for reactivation of relic landslides, an important consideration in anticipation of future climate change as recommended by (Halcrow et al., 2001).

Details are needed of sedimentological composition of the cliffs in order to compute definitive estimates of cliff erosion sediment yield. This would require section mapping of the main lithological units of the cliffs together with some sampling of their sedimentological compositions, especially the beach forming chert and flint gravels. Once cross section areas and compositions are known then sediment yields can be computed by applying the relevant recession rates. This would offer a robust method for estimation of future changes in cliff supply as revised recession rates could be inserted as cliff sections reactivate, or otherwise vary in their behaviour.