Lyme Regis to Portland Bill

1. Introduction - References Map

1.1 Physical Nature of the Coast

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

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

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

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

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

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

Wessex Coast Geology by Ian West: http://www.soton.ac.uk/~imw/index.htm
Jurassic Coast: http://www.jurassiccoast.com/
Jurassic Coast Education pages: http://www.swgfl.org.uk/jurassic/
West Dorset District Council http://www.westdorset-dc.gov.uk/index.jsp?articleid=394
Dorset Coast Forum: http://www.dorsetcoast.com/
Fleet Lagoon Study Group: http://www.swan.ac.uk/biodiv/fsg/index.htm
West Dorset Coast Research Group: http://www.envf.port.ac.uk/geo/research/environment/wdcrg/

1.2 Chesil Beach

Chesil Beach is an internationally renowned feature of geomorphological and biological interest (Carr 1983a). It has been the subject of much research, discussion and conjecture and is regarded widely as one of the most important coastal geomorphological sites in Britain (May and Hansom, 2003). Prior to discussion of its contemporary sediment dynamics within the main parts of this report, mention must be made of its known characteristics and existing ideas relating to sources of material and mode of origin.

Morphology and Sedimentology

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

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

Geomorphological Development

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

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

1.3 Hydraulic Regime

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

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

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

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

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

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

Chesil was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point off Chiswell at –4.1m O.D. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 3-7% variation in longshore energy and confirmed that the beach was significantly more sensitive to this factor than most other south coast locations, as might be expected of a swash aligned coastline. Wave energy was also found to be especially sensitive to sea-level rise. The effect is probably due to a reduction in shoaling and wave refraction within the shallow nearshore bed as water depths increase so that slightly higher waves will approach the shoreline at rather more oblique angles. These results accord with those of Brampton (1993) who noted that net drift at West Bay was extremely variable in direction and was highly sensitive to small changes in the directional wave climate. This phenomenon was studied in further detail by Halcrow Maritime et al (2001) who undertook modelling of likely future wind speeds for a climate change scenario representing 2080 using the Met Office Hadley Centre Regional Climate Model. The wind speeds output by the model were used to derive offshore extreme wave conditions in Lyme Bay and results demonstrated a potential for significant increases in wave energy e.g. 1 in 50 year wave height of 8.1m could increase to 11.3m by 2080s. The hindcast waves have also been used to study the potential changes in alongshore sediment transport. Results for the east end of Chesil Beach indicate a potential for a dramatic shift in the sediment transport regime due to a small two-degree shift in the mean wave approach direction. The current net westward drift potential of 900m³a^-1 would under this scenario alter to a net westward drift of up to 15,000m³a^-1. A similar type of study was undertaken by Sutherland and Wolf (2002) who simulated drift on Chesil Beach up to the year 2075. Results suggested that net drift could in future increase by up to 30% due to the potential effects of climate change. Results of the two studies differed due to use of different climate model outputs and application of alternative hydrodynamic and sediment transport models and calibrations; however, they both suggest that (i) waves in Lyme Bay are likely to vary with future climate change and (ii) Chesil Beach is likely to be sensitive to these changes.

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

2. Sediment Inputs

2.1 Offshore to Onshore Feed - F1 F2 References Map

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

F1 Weed Rafting at Chesilton (see introduction to Offshore to Onshore Feed)

F2 Diffuse Feed in Central Lyme Bay (see introduction to Offshore to Onshore Feed)

2.2 Cliff and Coastal Slope Erosion - E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12 E13 References Map

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

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

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

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

The backscar retreats cyclically with short episodes of rapid retreat by rotational sliding, separated by prolonged phases of slow retreat by small scale landslide and weathering processes. The material released is passed through the system (throughput) in a more steady manner so as to give a relatively regular supply to the beach. Brunsden and Jones (1976) estimate the throughput time for the Stonebarrow landslide complex to be between 100 and 150 years and Bray (1996) estimates shorter periods for some of the other complexes. Correlation of incidences of landsliding with climatic records has led Ibsen and Brunsden (1997) to postulate a general periodicity of landslide intensity of some 30-50 years arising from landslide responses to sequences of unusually wet years. Despite such irregularities, Brunsden and Jones (1976, 1980) considered that mean retreat was relatively constant over a 100-year period at Stonebarrow. This concept was broadly applied to other landslide complexes in West Dorset to establish representative long-term retreat rates (Bray 1986; 1996). Cliff top and sea-cliff recession rates were calculated over the period 1901-1988 using Ordnance Survey large-scale map comparisons, air photos and a cliff top survey in 1985 and 1988 (Bray 1986, 1996). Retreat rates were combined with information relating to cliff geology and cliff erosion sediment input was determined for the coast between Lyme Regis and West Bay. Details of the solid geology were obtained from existing literature (Wilson et al 1958; House, 1993). The gravel component of this input, critical to local beaches, was primarily from Chert Beds and superficial deposits at the cliff top. To determine accurately the nature and distribution of input, these deposits were subject to detailed field section mapping and comprehensive sampling (Bray 1986; 1996). These techniques yielded overall supply rates for all materials (Table 1 and Photo 20) and detailed gravel (>2mm) supply rates for each coastal segment.

This analysis showed that large quantities of sediment are released in the western part of the study area between Lyme Regis and Charmouth, a result of rapid coastal retreat along this segment (Photo 15). Chert and flint gravel supply is small by comparison to that of all sediment types, but they comprise the dominant beach material because of their greater resistance to abrasion. Sand, silt and clay are easily eroded and transported offshore. The limestones form beach pebbles, but these are short-lived due to attrition and only form a small proportion of beach volume (Bray 1996). Thus, not only are rates of coast erosion input highly variable but the nature of materials yielded can alter, with some materials (e.g. chert and flint) having greater beach-forming potential. Assuming that recent increases in retreat will persist or intensify due to future sea-level rise and climate change, total material yield could increase by up to 61% in the future to over 500,000m³a^-1 and gravel yield is likely to double to 12,000m³a^-1 (Bray, 1996; Bray and Hooke 1997). The dynamics of recession and supply are therefore described in greater detail according to location in the following sections.

E1 East Devon (see Introduction to Cliff Erosion)

E2 Lyme Regis and The Spittles (see Introduction to Cliff Erosion)

A comprehensive series of investigations have been undertaken as part of the Lyme Regis Environmental Improvements Scheme developed by West Dorset District Council (High_Point Rendel 1999a; 1999b; West Dorset District Council 2000; Brunsden 2002). The work includes detailed mapping, ground investigations, and establishment of a monitoring and data-handling network (Brunsden 2002; Davis et al 2002). Results have included preparation of a series of ground models for the main slide units identified (see Brunsden 2002). A listing of historical risks and an explanation of the scheme are provided at: http://www.swgfl.org.uk/jurassic/chiswell.htm

Sea walls have been present at Lyme Regis since before 1789 (Posford Duvivier, 1990; 1991), but protection has not been continuous and different sections were protected at different times. East Cliff and Church Cliffs (Photo 21) were subject to continuous recession at 0.45 ma^-1over the period 1841-1903 (Geotechnical Consulting Group 1987) and at variable rates (0.1-0.6 ma-1) thereafter until 1957 when a new sea wall was constructed. Brunsden quotes rates of 0.47 to 0.80ma^-1(past 150 years) for East Cliff and 1.3ma^-1(1888-1957) for Church Cliffs. Quarrying of limestone ledges on the foreshore at Church Cliffs in the 19th century reduced protection from wave attack and may have contributed to marine erosion at this site (Hutchinson 1984). Sea wall protection is now complete to the west of East Cliff and coast erosion input is prevented except on occasions when ancient landslides are reactivated and material may surge seaward across the sea wall e.g. Cliff House slide of 1962 (Arber 1973, Pitts 1979, Posford Duvivier 1990; Lee 1992). Coastal geology at Lyme Regis is generally Liassic clays and limestones so local cliff erosion and landslides have only a low potential here to supply materials that can contribute to the beach (Bray 1986).

The Spittles (Photo 22), immediately to the east of Lyme Regis comprises a series of ancient landslides beneath Timber Hill on the western flank of Black Ven that are interpreted by Brunsden (2002) as being remnants of once more extensive periglacial landslide slopes. A marked northwestward shift of activity is recognised, extending from Black Ven towards the A3052 in eastern Lyme Regis and upslope towards the vegetated presently inactive landslide scarp of Timber Hill. Retreat of 0.9 ma^-1was estimated for 1841-1901 (Geotechnical Consulting Group 1987) and subsequent map, air photo and ground survey comparisons showed retreat at up to 2.5 ma^-1for 1901-60 accelerating to 8 ma^-1for 1960-88 (Bray 1996). It appears that marine erosion of the sea-cliffs is reactivating a series of ancient mudslides and translational slides, creating a scarp and bench topography that is retrogressing inland. Mapping studies by Brunsden and Chandler (1996) and Brunsden (2002) indicate that this trend is continuing such that fresh landslides could soon be triggered within the Upper Greensand strata of Timber Hill as suggested by Bray (1986). Other work by Gibson et al (1999) involving comparisons of photogrametrically-derived digital ground models demonstrates en-mass lowering and seaward displacement of the Charmouth Road car park from 1992 to 1996. This has followed loss of support to seaward due to encroachment of active landslides from the Spittles and those reactivating inland from East Cliff.

Significant quantities of sediment are supplied to the shore from the Spittles, as indicated by a major landslide over the sea cliffs that deposited a large debris lobe on the foreshore in 1988. However, the proportion of gravel yielded was small (370-445 m³a^-1 for 1901-88) although it could increase significantly once the Upper Greensand scarp of Timber Hill is reactivated (Bray 1996; 1997).

E3 Black Ven (see Introduction to Cliff Erosion)

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

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

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

Backscar evolution has also been studied using a rigorous analytical photogrammetric technique based on a 42 year (1946-88) sequence of aerial photographs (Chandler and Cooper, 1988; Chandler, 1989; Chandler and Brunsden 1995 and Brunsden and Chandler 1996). The results confirmed that the eastern and western backscars have evolved in slightly different manners although they are both linked to major mudslide systems. Recession peaked at the eastern (2.8 ma-1) and western (5.0 ma-1), backscars during the period 1958-1969. Thereafter, the eastern backscar was relatively stable until the failures of 1986. The western backscar continued to recede actively throughout the period 1958-1988. Mean retreat values of 1.33 ma^-1(eastern) and 3.14 ma^-1(western), were recorded over the full period 1958-1988 following the initial mudslide surges. This work also involved calculations of the volumetric changes involved in these processes as material was delivered seaward from the degrading landslide system. Results demonstrated that the intense mudslide events of 1957 and 1958 led to removal of large quantities of material from the Upper Platform at the base of the backscar. Major rotational slides that occurred almost immediately thereafter at the backscar suggest that unloading was probably a major preparatory factor in the failure mechanism. Rapid downslope movements of the units produced by these slides almost certainly further loaded the mudslides below and could have stimulated their activity through an undrained loading effect. Further mudslide surges would again have unloaded the upper slopes causing regeneration of instability. This positive feedback process continues unabated because the mudslides that unload the slopes remain highly active and their toes are removed continually by marine erosion. Chandler and Brunsden (1995) demonstrated using a comparison of slope angles that, even though individual components altered significantly as recession proceeded, the system as a whole exhibited a “dynamic equilibrium.” This work was developed further to produce an episodic landform change model (Brunsden and Chandler, 1996; Chandler 1999).

Sediment supply to the beach is not easy to calculate at sites such as this characterised by an accelerating rate of landsliding together with intense short-term variations in rates of processes. On occasions, sediment supply at the mudslide toe exceeds marine erosion and temporary accretion occurs on the foreshore as mudslide lobes. Over the past 15 years marine erosion has exceeded the upslope supply so that mudslide lobes have been eroded back (Photo 24). Short-term variations in sediment supply to the foreshore appear therefore to result from storage within the landslide system and some material released to the landslide complex by recent rapid backscar retreat has yet to reach the beach (Bray 1996). This is particularly relevant to gravel supply computations, because all suitable material is derived from the backscar. Gravel supply over the period 1901-88 therefore averaged 2,100 m³a^-1, but this is expected to increase towards a representative long-term rate of 3,500 m³a^-1 as gravel currently in storage is transmitted to the beach.

It is likely that future gravel yields will increase due to the effects of sea-level rise and climate change that are likely to increase recession by up to 22% (Bray and Hooke, 1997). Furthermore, as the western backscar at Black Ven and Timber Hill becomes reactivated, it is likely that a significantly wider outcrop of gravel bearing deposits will be exposed to introduce fresh materials. Taking these factors into account Bray (1996) has estimated that future supply of gravel could increase to as much as 9,000 m³a^-1 by 2100.

It can be concluded that the Black Ven landslide system is the most important sediment source (particularly for gravel) within the study area due to high relief, rapid retreat and suitable geology. It is likely to become increasingly important in future.

E4 River Char to Westhay Water (see Introduction to Cliff Erosion)

Significant quantities of beach forming sediments are supplied from Upper Greensand Chert Beds and overlying flinty deposits present in the Stonebarrow backscar (Bray 1996). Fine sands released by Upper Greensand Foxmould Sands contribute to the lower foreshore forming a gently sloping terrace exposed at low spring tides. Mean gravel supply for the coastal segment was calculated as being 1,750 m³a^-1 with over 90% arising from the Stonebarrow system (Bray 1996). Reliability of this information as a long-term rate is high, but supply is subject to short-term variations due to seasonal factors and feedback mechanisms which has caused “waves of aggression” to pass through the mass movement system (Brunsden 1974, Brunsden and Jones 1976, 1980). It is likely that future gravel yields will increase due to the effects of sea-level rise and climate change that are likely to increase recession by up to 50% (Bray and Hooke, 1997).

E5 Westhay Water to St Gabriels Water (see Introduction to Cliff Erosion)

The coastal cliffs cut into the SE flank of Chardown Hill are composed primarily of Lias Clays, but with variable overlying cover of gravel-rich superficial deposits and relic landslides. The cliffs are of lower relief than at adjoining segments and comprise a steep sea cliff above which there is a gently sloping undercliff, or degradation zone that approaches 600m in width at Broom Cliff. Historical retreat rates determined from map and aerial photocomparisons (1901-1987) by Bray (1996) range between 0.2 and 1.1 ma^-1according to location, although a general range of 0.4 to 0.5 ma^-1is considered representative.

At Broom Cliff, accelerating retreat, has been recorded since 1901 suggesting that the system is undergoing a prolonged active phase. Although it is possible that the system will eventually become so choked with debris that retreat could slow down in the future, no such signs are discernible. Observations of the system between 1984 and 1989 revealed intense mudsliding and excavation of debris from within the landslide complex, thus suggesting that recent rates of backscar retreat could be maintained or even accelerate. Retreat over the period 1960-1987 (0.99ma-1) is therefore taken as the most appropriate basis for estimating future behaviour.

Although much clay is supplied to the foreshore, all is eroded and lost offshore in suspension. The supply of potential beach forming materials is relatively low due to the lack of coarser materials within the cliff geology. Taking this sector as a whole a gravel supply of 390 m³a^-1 was calculated for the period 1901-88 (Bray 1996).

E6 Golden Cap (see Introduction to Cliff Erosion)

E7 Seatown (see Introduction to Cliff Erosion)

Between the River Winniford at Seatown, and Doghouse Hill ((Photo 3 and Photo 28)) the cliffs are formed within sandy units of the Middle and Upper Lias and the coastal landslide complex is very narrow, being some 40-70 m in width, consequently the backscar and sea-cliff systems are more intimately associated. Erosion of the sea-cliffs is therefore transmitted more rapidly through the system to result in failures of the backscar. These cliffs appear to retreat mainly by infrequent, but fairly large-scale, falls or slumps, which are separated by long periods of slow retreat, dominated by small-scale processes. The overall effect again is of a cyclic retreat. From 1901 to 1948 retreat was very slow (0.04 ma-1) and no large-scale failures occurred (Bray 1996). However, aerial photographs, together with field survey and observations indicate that since 1948 there has been a marked increase in the frequency of large-scale backscar failures. During this time, at least two major falls have occurred and the mean backscar retreat rate has increased to 0.2 to 0.3 ma-1. These changes might be related to historical beach mining activities that removed almost 300,000 tonnes of shingle from 1940 to 1987 and must have resulted in some loss of protection from wave attack. Indeed, a field survey of the base of the sea-cliffs by Dorset County Council in 1984 revealed retreat at 0.9 ma^-1since 1960. Backscar failures may therefore have occurred in response to a steepening of the profile. Thus, it is possible that natural cyclic retreat has accelerated so that major events may be triggered every 20 or so years. Post-war retreat is therefore the most relevant for extrapolating into the future i.e. 0.25ma-1. It is likely recession will increase by up to 40% due to the effects of sea-level rise and climate change (Bray and Hooke, 1997).

Cliff erosion gravel supply is negligible (145 m³a^-1) due to the thinness of gravel-bearing deposits in these cliffs (Bray, 1996). Large quantities of sandy sediments are supplied from the Middle Lias strata of these cliffs, but these are only stable on the westernmost part of the beach and the inshore and offshore seabed.

E8 Thorncombe Beacon (see Introduction to Cliff Erosion)

E9 Eype and West Cliff (see Introduction to Cliff Erosion)

E10 East Cliff and Burton Cliff (see Introduction to Cliff Erosion)

E11 Chesil Beach Recession (see Introduction to Cliff Erosion)

Analysis of beach profiles measured originally by Coode 1853), with more recently surveyed profiles revealed that only opposite Portland Harbour was the amount of crest recession (17 m) greater than the possible errors (Carr and Gleason 1972; Carr and Seaward 1991). Comparisons of these data with aerial photos of 1993 revealed recession of the crest by 8m towards Chiswell and recession of the MHW by 10-15m, along the eastern portion of the beach, indicating a slight steepening of the beach face (Babtie Group 1997). Furthermore, beach profile measurements following storms revealed that short term variations resulting from “cut” and “fill” cycles were frequently greater than any long term trend (Gibbs 1980; Babtie Group 1997). Available evidence, therefore, suggests that eastern parts of the beach are subject to slow recession at 0.06 to 0.12 ma-1, but the period covered by accurate measurements remains insufficient for conclusive long-term trends to be determined. Any recession is therefore likely to be slow and may result from delayed response to sea level rise, increased storminess, changes in wave direction and diminution of beach volume (Carr and Blackley 1974).

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

E12 The Fleet (see Introduction to Cliff Erosion)

E13 West Coast of “Isle” of Portland (see Introduction to Cliff Erosion)

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

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

2.3 Fluvial Input - FL1 FL2 References Map

FL1 River Char (see Introduction to Fluvial Input)

Evidence of an episodic input is provided by a fan of gravel and shingle at the mouth of the Char (Photo 32), although some of this is composed of beach shingle displaced by river flow down the beach face as the tide retreats. The only record of an exceptional discharge for the River Char is provided by an oblique aerial photograph showing extensive flooding of the river at Newlands Bridge, 0.6 km from the mouth (Chaplin, 1985). Flood waters extended up to 50 m either side of the channel and it was reported that several caravans were carried along by floodwaters. Discharge is not measured routinely by the Environment Agency and its predecessors, so there are no direct data. Precipitation is measured just outside the catchment at Beaminster, Bridport and Lyme Regis. Records for 1979 showed that flooding occurred following intense rainfall on 30.5.79.

In the absence of discharge and bedload transport information, the volume of gravel supplied to the beach was estimated by Bray (1996) using details of the fan deposited on the foreshore. The volume of material contained within the fan was estimated as being between 16,000 m3 and 32,000m3. However, because the flood event is infrequent, its return period is uncertain. Existing records revealed a single flood between 1881 and 1989 suggesting a minimum 100 year return period thus giving a possible mean annual supply of between 160 m³a^-1 and 320 m³a^-1. Despite being very imprecise, these estimates do suggest that fluvial supply is a relatively minor component of the long-term beach shingle budget compared with input from cliff retreat.

FL2 The Fleet (see Introduction to Fluvial Input)

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

2.4 Beach Nourishment - BN1 BN2 References Map

N1 Lyme Regis (see Introduction to Beach Nourishment)

N2 West Bay (see Introduction to Beach Nourishment)

3. Littoral Transport - LT1 LT2 LT3 LT4 LT5 LT6 LT7 LT8 LT9 LT10 LT11 LT12 LT13 References Map

a) Charmouth Beach (Lyme Regis to Golden Cap);
b) Seatown Beach (Golden Cap to Doghouse Hill);
c) Eype Beach (Doghouse Hill to West Bay);
d) Chesil Beach (West Bay to Isle of Portland).

Net littoral drift is generally from west to east from Lyme Regis to West Bay: based on the following evidence: (i) gravel accumulations that occur on the western (updrift sides) of structures that intercept beach drift such as the Cobb, Lyme Regis and a rock groyne at Charmouth; (ii) the gravel volumes on each of the pocket beaches which increase from west to east where it appears that headlands act as barriers to drift (Bird 1989); (iii) gravels increase in size from west to east on each of the pocket beaches and tracer experiments undertaken by Carr (1971) and Bray (1996) indicated that the largest pebbles moved the fastest suggesting that accumulations of these types should indicate the net direction of drift and (iv) calibrated modelling of transport on Charmouth Beach by Bray (1996) indicated considerable drift reversals in eastward and westward directions, but with a net annual eastward drift.

The net drift direction on Chesil Beach is more difficult to discern for it is a swash-aligned feature and net drift is very low compared with the quantities of eastward and westward drift occurring.

LT1 Monmouth Beach (see Introduction to Littoral Transport)

It should be noted that it is not simple to use the accretion rate against the Cobb as a direct indicator of likely net drift potential, because Monmouth Beach is relatively starved of sediment and the Cobb has not necessarily always acted as a complete barrier to drift.

LT2 The Cobb (see Introduction to Littoral Transport)

The Cobb is not an absolute boundary to littoral movement, as evidenced by the accumulation of gravel between two arms of Lyme Regis harbour (Photo 34) that occurs under high-energy wave conditions (Bray 1996). It is not known how much gravel bypassing actually occurs, although it is thought to be small and infrequent. Bypassing by north-eastward transport of sand along the nearshore sea-bed is likely to occur more frequently and mobile spreads of sand have been surveyed in nearshore waters (West Dorset District Council 2000a). In conclusion, the Cobb appears to be an effective barrier to beach drift of shingle as witnessed by long-term accretion of Monmouth Beach and severe depletion of beaches to the east (West Dorset District Council 2000b). As part of Phase 2 of the Lyme Regis Environmental Improvements scheme due for construction in 2005, it is planned to extend Beacon Rocks some 120m eastwards from the eastern extremity of the Cobb, thus potentially increasing the barrier and sheltering effects of this structure.

LT3 Lyme Regis (see Introduction to Littoral Transport)

High-Point Rendel (1999b) produced a conceptual model of transport along this frontage and identified three small scale sub-cells with eastward drift along the eastern half of the frontage, a zone with a tendency for offshore and onshore exchanges of sediment with the nearshore feature of Town Beach Channel and a zone of sediment accretion in the lee of the North Wall Rockery.

Other studies have investigated transport along this frontage using the DRCALC numerical modelling package to calculate drift volumes using nearshore wave climates for several points (HR Wallingford 2001). Results revealed that drift can vary greatly according to location due to variable shore orientation and exposure/shelter. Nearshore rock ledges and channels also have significant effects according to tidal level and wave height. It confirmed the presence of a relatively static drift divide in front of the shelters along Marine Parade. From this point, drift is eastward towards Cobb Gate at between 2,000 m³a^-1 to 5,000 m³a^-1 (averaging around 4,000 m³a^-1). Much less rapid drift occurs westward from the divide, with mostly material becoming deposited in the lee of the North Wall Rockery. It should be noted that potential drift rates are quoted based on the assumption that shingle is continuously available for transport and is unimpeded by control structures.

Further studies have used the BEACHPLAN numerical modelling package and physical models to simulate the effects of implementing a range of alternative schemes needed to control a future replenishment as part of Phase 2 of the Lyme Regis Environmental Improvements scheme (HR Wallingford 2001; 2003). The scheme to be constructed in 2005 includes sand (around harbour) and gravel (Marine Parade) replenishment together with two new jetties and an extension to the North Wall Rockery with the aim of controlling the drift of the imported sediments (West Dorset District Council 2004a).

LT4 Church Cliffs to Black Ven (see Introduction to Littoral Transport)

Fresh gravel is supplied to this segment by landslides from the Spittles and the western part of Black Ven. Eastward drift is intercepted periodically by lobes of landslide debris which surge seaward from undercliff mudslides and small beaches of angular gravel accumulate to the west of such barriers (Photo 15 and Photo 24). The Black Ven mudslide lobes have functioned as major barriers, since their extension across the foreshore in 1957 and 1958 and a beach of some 5,000 to 6,000m3 of gravel accumulated immediately to their west by the mid 1980s (Bray 1996). By 1988 mudslide extension had diminished such that marine erosion could cut back into the lobes enabling formation of a continuous beach around the toe. At this point material held up to the west of the lobes could drift eastward as a pulse to supply the western end of Charmouth Beach. The small gravel beaches beneath the Spittles are therefore dependent upon the barrier effect of the Black Ven mudslide lobes and their volume fluctuates accordingly. Their capacity to accrete is nevertheless low, due to the limited coast erosion input and the periodic rapid losses of gravel eastward. In the long term, all gravel supplied from the East Cliff and Spittles landslides drifts eastward representing a net eastward drift of around 320 m³a^-1 (Bray 1996). It is only a small proportion of the transport potential given earlier, indicating clearly that drift is limited by material availability. The beaches are therefore likely to remain small and offer very little protection to the cliff toe beneath the Spittles.

LT5 Black Ven to the River Char (see Introduction to Littoral Transport)

LT6 Charmouth Beach (see Introduction to Littoral Transport)

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

Littoral drift was measured directly by Bray (1996) during a series of experiments covering a nine month period and employing aluminium tracer pebbles. The tracers were efficiently recovered to a maximum depth of 0.45 m beneath the beach surface. Recovery rates approaching 100% were achieved. Tests were conducted simultaneously on shingle (St Gabriel’s) and mixed sand and shingle (Charmouth) beaches during high, medium and low wave energy conditions. The data were analysed using multivariate techniques, and results indicate that shingle transport is most rapid on the upper beach near the high water mark and the larger tracers were the most rapidly transported. It was concluded that the observed increase in pebble size from Charmouth to Golden Cap could therefore have been developed by differential transport and sorting during net eastward littoral drift. Longshore transport volumes were calculated from the velocity, thickenss and width of the moving shingle layer. The volumes were 2 m3day-1 to 22 m3day-1 during frequent periods of low energy westward drift, increasing to a maximum of 168 m3day-1 during less frequent periods of high wave energy induced eastward drift. Although a wide range of wave conditions were tested, they were not necessarily representative of long-term conditions. Estimations of transport using a calibrated wave power model (CERC equation) and a locally adjusted 10-year hindcast wave climate for West Bay (Hydraulics Research Station, 1985) indicated net eastward drift of shingle of around 2,000 m³a^-1.

At Charmouth, the application of a similar estimation procedure using a site specific CERC calibration derived from the tracer experiments indicated net eastward drift of around 5,000 m³a^-1, but 40% of the material is composed of sand and grit so that shingle drift is around 3,000 m³a^-1. Drift of the entire beach sediment size spectrum is therefore greatest at Charmouth, but decreases towards the east due to the progressive offshore loss of fines. Although the drift experiments themselves provided information of high reliability, the calculated long-term annual rates are only of medium reliability for two reasons:

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

A further check on the drift rates along this beach was undertaken by an analysis of the volumes and budgets of beach material and a comparison against the tracer/wave power derived values (Bray 1996). Budget analysis showed that beach volume increased eastward, but gravel supply was concentrated in the west indicating that drift should increase eastward due to cumulative supply and increased gravel availability. Integration of these inputs together with probable outputs indicated net eastward drift of between 3,000 and 5,000 m³a^-1. These values are very similar to those predicted by tracer experiments so that the overall assessment of drift is believed to be of high reliability.

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

LT7 Golden Cap (see Introduction to Littoral Transport)

“Impeded” and “Unimpeded” drift conditions have been identified occurring at the headland according to the relative intensities of mudsliding and marine erosion. At present, the combined effect of active mudslide lobes and dense boulder aprons is to completely block littoral drift of gravel along beaches, adjacent parts of the shore platform and within the nearshore (“impeded” condition). However, old photographs show that landslide debris became sufficiently eroded between 1934 and 1949 (and partially from 1949-1962) to allow a continuous beach to exist from Charmouth to Seatown – the “unimpeded” condition (Brunsden 1985, Bray 1996). During such periods the headland operates as a one-way "valve" permitting only eastward drift. Analysis of shoreline orientation and dominant wave approach directions indicates that westward drift is minimal (Bray 1996). A gross eastward drift of 8,600m³a^-1 was estimated from tracer experiments at St Gabriels and this may be taken as being characteristic of the unimpeded condition at Golden Cap (Bray 1996). Long-term transport around the headland therefore depends upon frequency of the unimpeded condition. Assuming that the period (1901-1987) is representative, full transport was possible for 15 years (plus 13 years of partial transport) in each 86-year period; equivalent to a long-term value of around 2,200m³a^-1 (Bray 1990b). This analysis establishes the operation and direction of intermittent transport around the headland with high reliability, but quantitative information is less reliable chiefly due to assumptions relating to periodicity of the unimpeded condition.

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

LT8 Seatown Beach (see Introduction to Littoral Transport)

LT9 Thorncombe Beacon (see Introduction to Littoral Transport)

LT10 Eype Beach (see Introduction to Littoral Transport)

LT11 West Bay (see Introduction to Littoral Transport)

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

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