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 SedimentologyChesil Beach is one of three major coarse clastic (shingle/gravel) structures on the British coast and is unique in being a linear barrier beach whilst the others (Dungeness and Orford Ness) exhibit cuspate development. The piers at West Bay represent the present western terminus of Chesil Beach, but prior to change from open to closed, impermeable construction in the 1820s (Keystone Historic Buildings Consultants 1997), Eype Beach would have been part of the same sediment transport system (Prior, 1919; Carr, 1980; Bray 1996; 1997a). Other work is beginning to suggest that the beach could once have extended as far west as Beer Head or Otterton Ledge (Brunsden, 2002). Subsequent erosion to the west of the West Bay piers caused marked “setback” (Photo 9) and separation from the Chesil Beach system (Jolliffe 1979; High-Point Rendel, 1997; Brunsden and Moore, 1999). In the east, the beach terminates against the cliffs of the Isle of Portland. The shingle beach is joined to the mainland at the eastern and western ends, but for 13 km is backed by the shallow, tidal Fleet Lagoon (Photo 1). At its eastern extremity the beach forms the dominant western portion of the tombolo structure that attaches the Isle of Portland to the mainland. It is backed by Portland harbour and the town of Chiswell. West of Abbotsbury extending to Cogden Beach, the shingle beach fronts a narrow low-lying hinterland that rises steeply inland (Photo 10). Between Burton Bradstock and West Bay, recession of the beach has intersected elevated topography resulting in the formation of steep cliffs fringed by a rather narrower portion of the beach(Photo 11 and Photo 12). The visible pebble beach consists of about 98.5% chert and flint, the remainder being pink quartzite, quartz, locally derived limestone and occasional pebbles of igneous or metamorphic rock (Carr and Blackley 1969). Major pebble sampling programmes in the 1960s (Carr 1969) and in 1993 (Babtie Group 1997) have demonstrated that pebble size above low water mark increases systematically from Cogden or West Bexington in the west to Chiswell in the east, but with some inconsistencies at Wyke Regis (Photo 13) and to the west of Cogden. Beach face gradient increases eastward in accordance with the increase in pebble size in this direction. It should be noted that exact comparison between the earlier and later studies has not been possible due to use of different sampling and measurement techniques, but results suggest the broad pattern of grading has been maintained during the period between the two studies.
Below low water mark, longshore size grading is less clear and shingle is coarser and less well sorted than on the intertidal beach (Neate 1967). On the subtidal beach, coarser pebbles are generally recorded to the east of Abbotsbury with sands predominating to the west (Babtie Group 1997). The shingle beach was shown to extend out on the seabed down to around –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 DevelopmentBeach clast lithology has led most authors to postulate a sediment source to the west, from Devon (Fitzroy 1853, Baden-Powell 1930). Such theories implied that transport was eastward from the source areas by littoral drift along former shorelines (de la Beche 1830, Coode 1853, Rennie 1853, Pengelly 1870 and Arkell 1947), or offshore across Lyme Bay (Strahan 1898). By contrast, Prestwich (1875) suggested northwestward transport from a precursor of the Portland raised beach situated south of the Isle of Portland. Prior (1919) suggested three possible sources: (i) a precursor of Chesil Beach stretching from Start Point, Devon to Portland; (ii) erosion of the east Devon coast: and (iii) river gravels deposited in Lyme Bay by a previously extensive “Fleet River”. This last possibility was supported by Bond (1951) who reconstructed an ancient Exe-Teign river flowing eastward up to 10 km offshore of the present coast with a mouth south east of Portland. Other possibilities such as gravel rafting by ice have also been suggested to account for clasts of exotic lithology (Arkell 1947).
The most comprehensive study of Chesil pebble lithology was undertaken by Carr and Blackley (1969), who concluded that all types could be related to either local sources or to existing sites in south west England. The over-riding opinion of previous work therefore supported the idea of supply from the west and recent quantitative study by Bray (1996; 1997) has begun to place these ideas into a more definitive context. Re-evaluation of borehole data documented by Carr and Blackley (1973) and a new set of cored data from the Fleet Lagoon (Coombe, 1998) suggest that the present shingle accumulation of Chesil rests upon a predominantly sandy and shelly foundation. These developments have enabled construction of the following chronological sequence, based initially on Carr and Blackley (1973), but with modifications by Bray (1990a, 1990b, 1996, 1997a, 1997b), High-Point Rendel (1997) and Brunsden (1999):
1.3 Hydraulic RegimeKnowledge of wave conditions along this coastline has increased considerably over the past 10 years through a variety of regional studies (Posford Duvivier, 1998a; Halcrow 2002; and Bastos and Collins, 2002), together with site-focused studies at Chesil (Babtie Group 1997), Lyme Regis (HR Wallingford, 1997a; 2001) and West Bay (HR Wallingford, 1997d) that have involved collection and analyses of new field measurements as well a several longer-term hindcast data sets. Wave exposure increases eastward from Lyme Regis to Portland Bill and locations east of Abbotsbury are exposed to swell waves arriving from the northeast Atlantic.
The 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 MapThe majority of beaches in the study area are composed almost entirely of shingle (Carr and Blackley 1969; Bird 1989; Bray 1996), whereas the nearshore is characterised by thin spreads of medium sand separated by exposed bedrock ledges and boulder aprons. Due to their critical role in coast protection/sea defence and a history of mining losses, many studies have been undertaken to examine the possibility of natural beach replenishment by onshore feed. Without exception, all studies failed to locate suitable source materials and onshore transport pathways. The evidence is reviewed below:
F1 Weed Rafting at Chesilton (see introduction to Offshore to Onshore Feed)Kelp beds exist offshore of Chesilton and occasional rafted pebbles are recognised on Chesil Beach. Thus an extremely small and very local feed is possible by this mechanism (Jolliffe 1979). Reliability is low and volume of feed is probably insignificant.
F2 Diffuse Feed in Central Lyme Bay (see introduction to Offshore to Onshore Feed)Numerical tidal modelling validated by some current metering suggested weak onshore residual flow in central Lyme Bay (Pingree and Maddock 1983). Short wavelength asymmetric megaripples identified within bed sediments by Ambios Environmental Consultants (1995) are indicative of onshore sand transport to within one km of West Bay and Burton to Abbotsbury. However, work by Bastos and Collins (2002) fails to shed further light on this potential pathway because their study area extended only 15km to the west of the Isle of Portland. Within their area they primarily identified potential sand transport pathways operating parallel to the shore rather than towards it.
2.2 Cliff and Coastal Slope Erosion - E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12 E13 References MapPublications by Brunsden and Goudie (1997); Allison (1992a; 1999a); Bray (1996 and 1997) and May and Hansom (2003) provide general reviews of the morphology and recession of the cliffs of this coastline. A series of papers referred to within the succeeding titled sections provide specific details of the cliff behaviour recorded at individual sites.
Topography is dominated by four prominent flat-topped NE-SW oriented ridges, which rise to between 150 and 200m. Truncated by coastal erosion, these ridges form the high, cliff landslide complexes of Black Ven (Photo 15), Stonebarrow Hill (Photo 16), Golden Cap (Photo 17) and Thorncombe Beacon (Photo 18). The elevation of intervening areas has been reduced by fluvial incision, so that marine erosion cuts lower cliffs across a series of steep-sided narrow valleys mantled by superficial spreads of landslide debris, scree and "head" (Brunsden and Jones 1972). The valleys of the Lym, Char, Winniford, Eype and Brit rivers descend to sea level. Three smaller valleys immediately east of Stonebarrow have hanging terminations on sea-cliffs, 20m to 30m high.
The coastal geology between West Bay and Lyme Regis comprises Liassic strata, mainly shales, marls and mudstones that weather to soft clays (House 1993). The main hills (Black Ven, Stonebarrow and Golden Cap) are capped by poorly consolidated, sandy, Lower Cretaceous sediments. These are important sources of sediment to the coast, particularly the durable shingle-forming materials from the Chert Beds (Photo 19) and the superficial deposits which mantle the entire coastal area (Bray 1986; 1996).
Continuous marine erosion on a soft rock coast of relatively high relief has caused much slope instability leading to landsliding. The Cretaceous strata are permeable and introduce water to the coastal slope at the line of unconformity with the impermeable Liassic clays. When wet, the Liassic clays are liable to flow or slide. These features accelerate coastal landsliding, causing rapid rates of retreat (Conway 1974; Brunsden and Jones 1976, 1980; Brunsden and Goudie 1997; Bray 1986, 1996; Allison 1990; 1992).
Coastal landsliding results in the formation of coastal landslide complexes, in which the backscar is separated from the beach by zones of degradation and material transport that vary between location from several tens of metres to several hundred metres in width, e.g. (Photo 15 and Photo 16). These have been the subject of intensive research, notably by Brunsden (1974); Brunsden and Jones (1976, 1980); Chandler and Brunsden (1995) and Brunsden and Chandler (1996).
The backscar retreats cyclically with short episodes of rapid retreat by rotational sliding, separated by prolonged phases of slow retreat by small scale landslide and weathering processes. The material released is passed through the system (throughput) in a more steady manner so as to give a relatively regular supply to the beach. Brunsden and Jones (1976) estimate the throughput time for the Stonebarrow landslide complex to be between 100 and 150 years and Bray (1996) estimates shorter periods for some of the other complexes. Correlation of incidences of landsliding with climatic records has led Ibsen and Brunsden (1997) to postulate a general periodicity of landslide intensity of some 30-50 years arising from landslide responses to sequences of unusually wet years. Despite such irregularities, Brunsden and Jones (1976, 1980) considered that mean retreat was relatively constant over a 100-year period at Stonebarrow. This concept was broadly applied to other landslide complexes in West Dorset to establish representative long-term retreat rates (Bray 1986; 1996). Cliff top and sea-cliff recession rates were calculated over the period 1901-1988 using Ordnance Survey large-scale map comparisons, air photos and a cliff top survey in 1985 and 1988 (Bray 1986, 1996). Retreat rates were combined with information relating to cliff geology and cliff erosion sediment input was determined for the coast between Lyme Regis and West Bay. Details of the solid geology were obtained from existing literature (Wilson et al 1958; House, 1993). The gravel component of this input, critical to local beaches, was primarily from Chert Beds and superficial deposits at the cliff top. To determine accurately the nature and distribution of input, these deposits were subject to detailed field section mapping and comprehensive sampling (Bray 1986; 1996). These techniques yielded overall supply rates for all materials (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.Similar rates of cliff input are reported by Posford Duvivier (1997; 1999) who also calculated sediment yields resulting from erosion of shore platforms and the nearshore bed. Inputs from these latter sources of 20,000 m3a-1 were estimated from Portland Bill to West Bay and 55,000 m3a-1 for West Bay to Lyme Regis, entirely comprising fine material.
E1 East Devon (see Introduction to Cliff Erosion)Large-scale and long-continued active landsliding between Axmouth and Pinhay provides a substantial supply of gravel, sand and clay to the littoral transport zone as described by Pitts (1981a and 1981b; 1983). Bray (1996) estimates that some 12,000 m3a-1 of gravel could be delivered to the shore from this section of coast. High-Point Rendel Geotechnics (1997) estimated total sediment inputs of 200,000 m3a-1 and shingle input of 7,500 m3a-1. Although a potential for eastward movement of materials at the shore would appear to exist, recent and relict boulder aprons and mudslide lobes cut the across the foreshore at numerous locations to introduce transport discontinuities that are believed to isolate the East Devon coarse sediment inputs from the shoreline to the east. Periodicities of movement along the net easterly pathway, are discussed further in the text covering the neighbouring unit of Beer Head to The Cobb, Lyme Regis.
E2 Lyme Regis and The Spittles (see Introduction to Cliff Erosion)The site of Lyme Regis comprises the valley of the river Lim that has been truncated by marine erosion. Ancient landslides are evident on the inland valley slopes with more recent failures being associated with the cliffs cut into the Liassic clays, siltstones and limestones by marine erosion. Steep sea-cliffs of 5m to 25m are backed by sequences of scarps and benches, the latter being defined by the outcrops of more resistant limestone seams. The slopes have evolved by compound translational (and rotational) failures with material being shunted seaward across benches by mudslides. Marine undercutting and removal of fallen debris at the toe have maintained instability and failures have been triggered by increases in groundwater introduced from upslope and accumulating within the landslide debris. Some deeper-seated slides are also identified having basal shear surfaces that extend beneath the beach. Defences now protect the cliffs of the main urban frontage, but significant instability remains on the slopes above and renewed failures can be triggered during increasingly frequent wet winters and also by inappropriate construction and drainage works within the town (Lee, 1992).
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:
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)This sector includes the landslide complex of Stonebarrow Hill (elevation up to 147m OD) together with lower cliffs of 30m-70m elevation cut into the western and eastern flanks of Stonebarrow (Photo 16). The topography is cut by the steep sided valleys occupied by the River Char and the stream of Westhay Water, which has a hanging termination over sea-cliffs. Stonebarrow exhibits a morphology comprising steep sea-cliffs developed within the Belemnite Marls, above which there is a series of platforms mantled by degrading landslide debris and a steep upper backscar. Permeable Cretaceous strata capping Stonebarrow Hill act as primary reservoirs that supply groundwater to the coastal slope at the junction with clayey impermeable Liassic strata beneath (Denness, 1971). Chemical alteration (decalcification) and unloading at the base of the backscar slopes further weaken these strata. When the accumulated stresses become sufficient to exceed the material strength, failure occurs by large-scale, but often infrequent rotational slides. Failures of this type occurred in the early 1940s generating several large blocks of up to 20m in width. The blocks thus formed are then stored on the upper platforms of the landslide complex for up to 100 years (Brunsden 1974; Brunsden and Jones, 1976, 1980), whilst they undergo gradual disruption by frequent smaller-scale landslips. Eventually, the material is incorporated into mudslides on the Lower Platform and transported over the sea-cliffs (Brunsden, 1974). The 1940s blocks were subject to a major movement seaward over the winter of 2000/01 that shunted large quantities of debris from the lower platform over the sea-cliffs and onto the foreshore. Mudsliding within the undercliffs results in unloading of the main scarp and eventually leads to new large-scale failure. The removal of material from the foot of the sea cliffs by the sea (Photo 25) prevents a stable slope from developing and ensures continuing instability (Brunsden and Jones, 1980). Map and aerial photocomparisons indicate that retreat of the backscar at Stonebarrow is intermittent, but repetitive with relatively brief 'active phases' of major landsliding occurring every 50-100 years (Brunsden and Jones 1980; Bray 1996). These 'active phases' are separated by long periods of slow retreat by small-scale processes, as the products of the preceding active phase of landsliding are evacuated from the upper platform. Brunsden and Jones (1976; 1980) suggest that over a 100-year period, the mean rate of backscar retreat is constant and equivalent to the rate of sea-cliff retreat. Bray (1996) calculates this rate to be around 0.39ma-1. Over this period it is believed that there is an approximate sediment balance between release from the backscar and supply to the foreshore, providing a basis from which to calculate sediment inputs.
Significant quantities of beach forming sediments are supplied from Upper Greensand Chert Beds and overlying flinty deposits present in the Stonebarrow backscar (Bray 1996). 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)The Golden Cap headland (Photo 16) forms the highest cliff (191m OD) within the study area and exhibits a markedly steeper profile than the landslide systems to the west. This may be related to geological differences, but increased wave exposure is also cited as a possible factor (Brunsden and Goudie, 1981; 1996). Retreat is variable and some cliff sections have showed no measurable retreat in recent decades, whilst others along its western flank receded at up to 1.0 ma-1(Bray 1996). Long-term rates of 0.05 - 0.30 ma-1are indicated by map, air photo and field survey comparisons covering the period 1901-87, but their reliability as a guide for future recession is uncertain. This is because observation of landslide morphology shows relic mudslides, rotated blocks and numerous foreshore boulder arcs (Photo 26). These features are characteristic of phases of rapid landsliding, so it is probable that retreat has been underestimated (Bray 1996). It is concluded by Bray (1996) that the retreat recorded historically between 1901 and 1987 is therefore representative only of the quiescent phase of weathering and small-scale failures that occurs between major landslides. It is not typical of the full range of behaviour at this site and thus underestimates the long-term retreat rate. Assuming that future failures adopt a similar mode to those at Stonebarrow, up to 20m of recession might be expected to occur in a few large events. Although the timing of such events cannot be predicted, new failures appear imminent so future retreat rates are likely to be greater than those recorded historically. Cliff geology comprises Lias clays, sands and limestones overlain by Upper Greensand Foxmould sand and thin Chert Beds. Only relatively small quantities of gravel are supplied (225 m3a-1 for 1901-1988), although significant quantities of sand and limestone are also yielded (Bray 1996). Sand is unstable on the foreshore and transported offshore, or into Seatown Bay, but large limestone boulders are supplied from the Three Tiers unit of the Middle Lias. These boulders persist on the foreshore as dense aprons, which effectively trap any gravel supplied and prevent output to adjoining beaches (Bray 1996). The boulder aprons provide protection against wave attack because they induce offshore breaking of large waves; so dissipating some of their energy. It must be concluded that supply information for this segment is only of medium reliability, because it is possible that recorded retreat was uncharacteristically slow during the study period. This will have limited impact on overall gravel supply, because cliff-top deposits are thin and boulder aprons trap materials supplied to the foreshore.
E7 Seatown (see Introduction to Cliff Erosion)Cliffs backing Seatown Beach differ with respect to geology and morphology either side of the River Winniford (Photo 27). To the west, cliffs are of Lower and Middle Lias clay with a wide degradation zone. Backscar retreat is rapid on the eastern flank of Golden Cap (up to1 ma-1for 1960-1987) where ancient landslides are being reactivated, but much slower on average closer to Seatown (0.06 ma-1)(Bray 1996). This is because erosion has mostly involved recession of the cliff toe and this had yet to affect the backscar significantly by 1987 the date of Bray’s (1996) final survey. Since 1988, observations of the sea-cliff and undercliffs suggested landward migration of instability that is now beginning to trigger backscar failures. Concerns that instability could migrate to affect properties at Seatown led to the construction of a small stabilisation scheme in 1997 covering a frontage extending some 100m to the west of the public road turning circle. It involved toe protection, drainage through the body of the landslide and also landward of it and an attempt to “pin” the backscar to prevent landward retreat (WSAtkins, 1996). The scheme is described and illustrated at: http//www.swgfl.org.uk/jurassic/opencost6.htm.
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)Degradation of predominantly sandy Upper and Middle Lias sediments has created high cliffs (up to 155m OD) of relatively steep profile (Photo 18). Retreat averaged 0.17 ma-1over the period 1901-1960, although faster rates of up to 0.5 ma-1operated at some locations (Bray 1996). Post-1960 measurements of retreat are not available. Negligible gravel is supplied because gravel-bearing deposits are extremely thin, or absent. Erosion supplies large quantities of sandstone and limestone boulders, which densely litter the foreshore in large boulder aprons.
E9 Eype and West Cliff (see Introduction to Cliff Erosion)Cliffs along this segment are lower and of distinctly different geology either side of Fault Corner, which marks the plane of a major fault with marked downthrow to the south-east (Photo 2). To the west, cliffs retreat at 0.05 - 0.5 ma-1and supply mostly Upper Lias clays and sands, some of which contribute to the western end of Eype Beach (Bray 1986; 1996). To the east, cliffs are composed of Middle Jurassic marls and clays with bands of shelly limestones. Retreat has been quite rapid (0.37 ma-1) over the period 1887-1962 (Jolliffe 1979) and is linked with a period of beach lowering in front that is thought to have occurred in response to interception of drift caused by infiIlling of the West Bay piers in the 1920s (High-Point Rendel 1997; Brunsden and Moore 1999). The low-lying West Beach retreated over this period, which caused a significant “set-back” of the coast to the west of the West Bay piers – see Photo 9 (Hydraulics Research 1979; Jolliffe 1979; Brunsden and Moore 1999). The esplanade was extended along the toe of West Cliff in 1968/69 and from the mid 1970s onward worsening cracking and bulging at the cliff face resulted in various remedial works from 1985 onward including re-profiling and installation of drainage (High-Point Rendel 1997; Keystone Historic Buildings Consultants 1997). Virtually no chert and flint gravel is supplied from the cliffs of this segment due to absence of suitable deposits (Bray 1986; 1996).
E10 East Cliff and Burton Cliff (see Introduction to Cliff Erosion)Near vertical sandstone cliffs extend east from East Cliff, West Bay (Photo 11) to Burton Cliff (Photo 12), where thin oolithic limestones cap them. Further east towards Cogden, there are clay cliffs of Fuller’s Earth. All cliff sections are devoid of flint and chert (Bray 1986; 1996). The sandstone cliffs are composed of Bridport Sands and possess a number of bands of harder calcareous sandstone. As the cliffs gradually retreat, the more resistant material is released as boulders or cobbles, which temporarily contribute to beach volume before wave action results in rapid abrasion and disintegration. Recession of these sandstone cliffs was investigated by map comparisons over the period 1902-62, which indicated retreat of 0.03 ma-1and yielded a supply rate of 1,400 tonnes a-1 (630 m3a-1) of calcareous sandstone (Laming 1985). The limited durability and the small quantities involved mean that this material is not an important source of supply.
E11 Chesil Beach Recession (see Introduction to Cliff Erosion)Beach recession may supply sediment from underlying deposits that become exposed in the seaward beach face as the beach is pushed landward. Carr and Blackley (1984) report slabs of Forest Marble Limestone and peat being yielded and Moxom (2003) records some direct observations of this process. Observations of gravel clasts thrown over the crest by storms led several authors to postulate recession of Chesil Beach by “in-rolling” (Carr and Blackley 1974), or “rollover” (Carter 1988).
Analysis of beach profiles measured originally by Coode 1853), with more recently surveyed profiles revealed that only opposite Portland Harbour was the amount of crest recession (17 m) greater than the possible errors (Carr and Gleason 1972; Carr and Seaward 1991). Comparisons of these data with aerial photos of 1993 revealed recession of the crest by 8m towards Chiswell and recession of the MHW by 10-15m, along the eastern portion of the beach, indicating a slight steepening of the beach face (Babtie Group 1997). Furthermore, beach profile measurements following storms revealed that short term variations resulting from “cut” and “fill” cycles were frequently greater than any long term trend (Gibbs 1980; Babtie Group 1997). Available evidence, therefore, suggests that eastern parts of the beach are subject to slow recession at 0.06 to 0.12 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)At several locations, erosion of the shores of the Fleet has created low cliffs and supplied sand and gravel to small local beaches (Bird 1972). Sediment volumes are extremely small and material appears to be retained on thin local beaches on the Fleet. In places, e.g. Wyke Regis (Photo 13), cliff recession has resulted in formation of shore platforms several metres wide in limestones and siltstones of moderate resistance. These features are difficult to reconcile with the prevailing opinion of the Fleet as a low energy environment and suggest occurrences of active wave erosion in the past.
E13 West Coast of “Isle” of Portland (see Introduction to Cliff Erosion)The North-West Portland coast is subject to major landsliding where soft erodible Kimmeridge Clay is overlain by hard Portland Stone (Brunsden and Goudie 1997). The landslides have been carefully mapped, documented and classified by (Brunsden et al 1996) At West Weare, (110m OD) blocks of the resistant cap rock become detached at the backscar controlled by NE-SW master joints and fissures (Photo 29). The blocks descend close to seal-level and rotate backwards, often forming sequences of multiple rotated blocks. Some blocks topple directly from the backscar. Although toe erosion is important, the occurrence of groundwater springs and the loading of blocks by quarry waste that in the past was tipped directly down the coastal slope have contributed to the instability; with the last major failures being recorded in 1858 (Brunsden et al 1996). The result of these processes is an undercliff strewn with displaced Portland Stone blocks and boulders that gradually narrows to the south. The Portland Stone blocks are hard and resistant and provide a protective apron at the toe so that map comparisons showed little measurable change over the period 1867-1960 (May 1966).
Futher south, the west-facing coastline (50-70m 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 MapNo major rivers discharge along the coastline of this unit. Chesil Beach forms a barrier in eastern parts so that the small streams draining its hinterland discharge into the Fleet Lagoon rather than into the open sea. To the west the small rivers Bride (Photo 31), Britt, Winniford (Photo 27), Char and Lim discharge to the shoreline. All are narrow gravel or sand-bed rivers with small catchments, but steep channel gradients that generate a modest potential to supply sediments to the shoreline, especially during high discharge events. The Britt is regulated in its lower course and its discharge into West Bay Harbour is regulated by a sluice. It is therefore likely that the majority of sediments delivered would be deposited in its lower channel and floodplain with very little input to the shore. Gravel beaches usually impound the mouths of the Bride and Winniford. Only the Char would appear to have a significant potential to supply materials to the shore. Rendel Geotechnics (1996) studied potential fluvial sediment yields through the south coast region and found very little direct data available from which to make assessments. Therefore they undertook empirical estimations based on available discharge data coupled with assessment of sediment availability. They concluded that the West Dorset streams collectively had the potential to supply some 2,700 m3a-1 of suspended sediments and 300 m3a-1 of coarser bedload sediments (sands and gravels).
FL1 River Char (see Introduction to Fluvial Input)Large quantities of angular flint and chert gravel line the bed, but it is uncertain whether, or how frequently these materials might be mobilised and transported towards the shore. Observations of the river at all times of the year during the period 1984-86 revealed very little, if any bedload gravel transport (Bray 1996). Although the river flows across the beach at Charmouth, banks of gravel deposited by marine processes always restrict its discharge. Thus, the river is usually "ponded" for up to 300m inland during the summer when it percolates through the beach. Any bedload carried by the river would be deposited in this area and could not be supplied to the foreshore without a flood event. These observations suggest that the River Char cannot normally transport gravel-sized material. Nevertheless, it is reported that the River Char does flood during exceptional circumstances and channel gravel deposits might then be supplied to the foreshore. Indeed, the Char catchment exhibits several flood enhancing qualities including its dendritic pattern of tributaries, impermeable Lias clay geology and steep channel gradients.
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 MapThe only recorded beach nourishment in the study area relates to 6,000 m3 of imported gravel placed on the Marine Parade Beach at Lyme Regis, immediately North-east of the Cobb in the 1970s (Posford Duvivier 1990). A further recharge of 2,500 m3 of shingle was derived directly from Monmouth Beach in the 1990s (West Dorset District Council 2000b) Nourishment was proposed at Chesil Cove (Wraxall 1972) and Seatown Beach (Laming 1988) to compensate for beach mining, however operations on these beaches ceased soon thereafter and no gravel was supplied. Nourishment is also proposed at Lyme Regis and West Bay for coast protection/sea defence and amenity purposes:
N1 Lyme Regis (see Introduction to Beach Nourishment)The urban frontage of Lyme Regis has suffered declining beach levels for several decades leading to exposure of sea-walls and loss of amenity. Consequently, a number of coast protection proposals involving beach replenishment using imported sands and gravels have been examined since the late 1980s. Earlier schemes involved replenishment using 6,000 m3 of imported gravel placed along the Marine Parade Beach in the 1970s (Posford Duvivier 1990). A further recharge of 2,500 m3 of shingle was derived directly from Monmouth Beach in the 1990s (West Dorset District Council 2000b). These actions did not solve long-term beach depletion problems. Tests using a physical model (Hydraulics Research 1989) led to development of a preferred scheme involving a major shingle nourishment, but this was not constructed. Potential problems were identified in retaining material at the shoreline so construction of three offshore breakwaters was advised. An alternative scheme proposal to reform a former littoral drift path from Monmouth Beach to Marine Parade was found not to be feasible and various proposals for construction of groynes were considered (Posford Duvivier 1990; 1991). Since the mid 1990s a series of investigations and proposals under the programme of Lyme Regis Environmental Improvements has been developed (West Dorset District Council 2000b; Brunsden 2002). This has led towards the commissioning for 2005 of Phase 2 of the scheme involving major beach replenishment between Cobb Gate and the Harbour. It is planned that sand should be used to replenish the beach around the harbour, with shingle being preferred for the beach to the east (West Dorset District Council 2004a). Two new jetties and an extension to Beacon Rocks are also planned to control the new beaches and reduce future losses of sediment.
N2 West Bay (see Introduction to Beach Nourishment)A series of studies including physical and numerical modelling was undertaken as part of the West Bay Coastal Defence and Harbour Improvements Scheme (e.g. Hydraulics Research 1991e; HR Wallingford 2000a; 2000b; 2000c). Based upon the results of these and other studies a scheme was developed involving construction in 2004 of a replacement West Pier and a range of other measures to increase beach stability and improve the coastal defences –see Photo 7 (West Dorset District Council 2004b). A key element of the scheme due for completion in November 2004 is a replenishment of West Beach by some 18,000m3 of shingle. The East Beach is managed so as to maintain an optimum width of some 120m to 160m. A beach management plan has been formulated to include actions to import beach material if this width diminishes and recycle beach material to the east should it accrete beyond this width (Posford Duvivier and HR Wallingford 2001). The scheme and its background is explained at http://www.swgfl.org.uk/jurassic/wbindex.htm.
3. Littoral Transport - LT1 LT2 LT3 LT4 LT5 LT6 LT7 LT8 LT9 LT10 LT11 LT12 LT13 References MapTransport occurs along a series of pocket beaches between Lyme Regis and West Bay and along the 28km expanse of Chesil Beach between West Bay and the Isle of Portland. Headlands and artificial structures intercept drift resulting in the identification of four beach drift sub-cells as follows (Bray 1996; High-Point Rendel 1997):
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)The Cobb, has existed in various forms since around the 12th century to provide shelter for Lyme Regis Harbour (Photo 33). However, since it became connected to the mainland in its present configuration in 1754, it has functioned as a large terminal groyne intercepting west to east drift promoting the up-drift accretion of Monmouth Beach. High-Point Rendel (1999c) argue that accretion against the Cobb has been due to redistribution of existing materials by eastward drift, to re-orientate the beach westward, rather than fresh inputs from the west. They conclude that Monmouth Beach as a whole has experienced loss of sediment volume over the past 250 years due principally to the substantial reduction of drift input from feeder beaches in Pinhay Bay, to the immediate west. This, in turn, is ascribed to the impedance of the drift pathway imposed by post mid-eighteenth century landslides at and close to the Humble Point (see the text covering the neighbouring unit of Beer Head to The Cobb, Lyme Regis). Thus, the continuing west to east longshore transport along Monmouth Beach has resulted in a progressive increase in the imbalance between input from the west and storage/output adjacent to the Cobb in the east to re-orientate the beach. Posford Duvivier (1998a) calculate a long-term average rate of accretion against the Cobb of approximately 1,500m3a-1, but modelling studies suggest that potential rates of up to 10,000m3a-1 may be sustained for short periods during winter storms. Investigations using the DRCALC numerical modelling package by HR Wallingford 2001) indicated that the potential shingle drift averaged 2,400 m3a-1 (inter-annual variation from 1,000 m3a-1 to 3,500 m3a-1). The actual rate of drift varies according to wave height and approach direction with increased eastward drift occurring in stormier years.
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 extends across the foreshore well seaward of LWMST and effectively intercepts littoral drift of shingle (Photo 33). Initial structures date back to 1272-1307 and generally comprised offshore breakwaters, which only partly intercepted drift. The present form attached to the shore dates from around 1754 (Posford Duvivier 1990) although numerous modifications and reconstructions have been undertaken subsequently. Although it would thereafter have formed a more complete barrier, it is documented that some material could pass across the wall at its root where it joined Monmouth Beach (Posford Duvivier 1990). Transport through the Cobb Wall was also possible by sluice gates constructed specially in the mid 1800s. At present, the Cobb wall is a more complete barrier to drift due to heightening of parapets at its landward root and permanent closure of the sluice gates in 1959 (Posford Duvivier 1990).
The Cobb is not an absolute boundary to littoral movement, as evidenced by the accumulation of gravel between two arms of Lyme Regis harbour (Photo 34) that occurs under high-energy wave conditions (Bray 1996). It is not known how much gravel bypassing actually occurs, although it is thought to be small and infrequent. Bypassing by north-eastward transport of sand along the nearshore sea-bed is likely to occur more frequently and mobile spreads of sand have been surveyed in nearshore waters (West Dorset District Council 2000a). In conclusion, the Cobb appears to be an effective barrier to beach drift of shingle as witnessed by long-term accretion of Monmouth Beach and severe depletion of beaches to the east (West Dorset District Council 2000b). As part of Phase 2 of the Lyme Regis Environmental Improvements scheme 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)A northward change in the shoreline orientation to the east of the Cobb means that there is a significant potential for eastward drift along Town Beach and Marine Parade (HR Wallingford 2003) that results in sediment accumulations against the western sides of the various jetties and groynes along this frontage. Small-scale replenishments have been undertaken in the mid 1990s including use of material rich in distinctive pink Budleigh Salterton quartzite pebbles. Subsequent observations clearly indicated an eastward movement of this natural tracer material. However, the amount of transport actually occurring must be small because of severe sediment depletion and construction of intercepting groynes and jetties (West Dorset District Council 2000b). Transport potential was evaluated using an energy flux technique (Posford Duvivier 1990) that revealed a minor littoral drift divide in the lee of the north wall of the Cobb (Photo 8). Concentrating upon gravel alone a westward drift towards the Cobb at 500 m3a-1 and an eastward drift along Marine Parade at 800 m3a-1 was identified. Further analysis undertaken using an improved wave climate (Posford Duvivier 1991) indicated eastward shingle drift potential of 3,000-6,000 m3a-1 at Cobb Gate Jetty. The analysis was extremely sensitive to variable wave direction, hence accuracy was estimated to be within the range 10,000 m3a-1 to the west to 20,000 m3 to the east. The drift rate was thought to decline westward towards the Cobb due to increasing shelter. These drift rates differ from the existing regime because they did not consider sand transport and it was assumed that shingle was freely available and transport was unimpeded by groynes, jetties and outfalls. The work suggested strongly that drift would accelerate if the frontage were to be replenished and that carefully designed control structures would be required to minimise such losses.
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)Observations of sediment distribution in groyne compartments beneath Church Cliffs indicate net north-easterly directed drift (Hutchinson 1984, Bray 1996), however, the amounts actually transported are very small for the foreshore at Church Cliffs and East Cliff is severely depleted (Photo 21). These beaches were formerly quite substantial, but lost volume over the past 150 years following modifications made to the Cobb and construction of defences at Lyme Regis that intercepted incoming drift (West Dorset District Council 2000b). Extensive quarrying of the limestone fore ledges during the 19th Century would also have adversely affected the local beach. Studies using the DRCALC numerical modelling package to calculate drift volumes along the East Cliff frontage using a nearshore wave climate (HR Wallingford 2001) revealed an net eastward shingle potential of some 10,000 m3a-1, although annual variability is thought to be high.
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)Field observations of eastward deflection of the river Char and sediment accumulation against the west side of two groynes constructed at Charmouth in 1954/56 and the 1994 rock groyne indicate net eastward drift (Bray 1996). Large quantities of sand and gravel are supplied to the foreshore at Black Ven, but beach volume is only a fraction of supply since 1901 (Bray 1996). Thus, it is concluded that littoral drift transports virtually all freshly supplied gravel eastward; indeed, studies of the progressive eastward abrasion and rounding of the initially angular fresh material clearly demonstrate this process (Bray 1996; Parnell 1998). Littoral drift is therefore dependent on shingle availability and fluctuates in response to variable supply at Black Ven. Mean long-term drift is therefore estimated at 3,300 m3a-1, although more rapid rates may have occurred since 1957/58 when mudslide surges caused increased supply from Black Ven (Bray 1996). The lower foreshore is sandy and no transport calculations have determined drift rates for this material. Sand has been found to be up to 20 times more mobile than gravel elsewhere on the south coast of England (Wright 1982), so rapid eastward transport is likely along this lower foreshore.
LT6 Charmouth Beach (see Introduction to Littoral Transport)This segment comprises the beach extending from the river Char to Golden Cap (Photo 4 and Photo 25), where littoral drift is intercepted by the protruding headland and its lobes of landslide debris. The beach comprises of mixed sand and gravel in the west, but becomes increasingly dominated by coarse pebbles and cobbles towards Golden Cap. Eastward deflection of the river Char and a gradual eastward increase in beach volume, both indicate net eastward drift along this sector. Beach drift has been studied by a variety of techniques including beach pebble sampling, aluminium tracer experiments and budget analysis involving consideration of sediment supply and storage (Bray 1996).
Analysis of some 6,000 pebbles from six transects located along the beach revealed longshore trends in size grading, roundness, sorting and pebble shape from which net littoral drift could be inferred. Pebbles were extremely angular at Black Ven, but become progressively better-rounded eastward towards Golden Cap, clearly demonstrating eastward drift away from the major cliff input. Pebble size grading does not follow the pattern predicted by supply (concentrations at Black Ven and Stonebarrow), so redistribution of gravels by littoral drift must be an important process. Size is greatest at the western (Black Ven) and eastern (St Gabriel’s) extremities. The large mean size at Black Ven results from the major inputs of poorly sorted gravels from coastal landsliding, whereas the large mean size and improved sorting at St Gabriel’s results from a net eastward drift of this material. The eastern end of Chesil Beach displays a similar eastward increase in pebble size and sorting (Carr, 1969). Tracer experiments (Carr, 1971) indicated that this pattern of grading at the eastern end of Chesil Beach might have developed by net eastward drift, during which the largest pebbles were transported the furthest. As the Isle of Portland prevents further eastward movement, this results in an accumulation of large, well-sorted pebbles. A similar process appears also to operate on Charmouth Beach, to result in the accumulation of large well-sorted pebbles in the St Gabriel’s-Golden Cap frontage, where further eastward movement is blocked by landslide debris, beneath Golden Cap. Pebble sphericity also increases eastwards towards St Gabriel’s, which again suggests eastward net drift, although it is uncertain whether downdrift abrasion or sorting is the dominant causal mechanism (Bray 1996).
Littoral drift was measured directly by Bray (1996) during a series of experiments covering a nine month period and employing aluminium tracer pebbles. The tracers were efficiently recovered to a maximum depth of 0.45 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:
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)The high landslide complex of Golden Cap forms a headland due to the occurrence of thick resistant limestone seams of the Three Tiers unit that outcrop in the sea-cliffs (Photo 17). As the cliffs retreat debris falls onto, or surges across the foreshore within mudslide lobes. These features completely cut the eastern extremity of St Gabriels beach separating it from the neighbouring Seatown beach. Marine erosion cuts back into the lobes removing all fine sediments, but leaving the foreshore densely covered by resistant limestone boulders that form a protective apron. Field examination revealed that pockets of poorly sorted gravel were frequently present between the boulders (Bray 1996). Chert was the dominant lithology, with a high proportion of large, angular (poorly abraded) material. Barnacles, limpets, and algae encrusted much of the gravel suggesting that movement of gravel was impossible where boulders were densely distributed. The only areas where movement of gravel appears possible are on the beaches at the toes of any eroded mudslides and on those portions of the mudstone pavement where boulders are less densely distributed (generally adjacent to beaches).
“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)Seatown Beach comprises a small pocket beach some 1.8km in length bounded by high headlands to the west and east (Photo 3). The beach is therefore extremely sensitive to short-term drift reversals, which produce rapid redistributions of material and re-orientations of the beach from one end of the bay to the other. During periods of consistent wave approach from the south-west (most frequently), or the east, the updrift end of the beach can become totally stripped of gravel to expose the underlying bedrock platform. as demonstrated by Brunsden and Moore (1999). Beach volume changes and distribution are therefore an unreliable guide to net littoral drift and simply demonstrate the tendency for the operation of both eastward and westward drift according to prevailing directions of wave approach (Laming 1988, Bray 1996). In this situation, beach sedimentology can be studied to determine net drift direction. A total of 3,000 pebbles were sampled by Bray (1996) and net eastward drift was indicated by longshore pebble grading which showed increased size, roundness, sphericity and sorting of linear dimensions to the east. The gradings were generally less well pronounced than on Charmouth Beach, which may indicate that drift is weaker or that clast sorting has been affected by historical beach mining at the site see Section 4.2. Reliability is medium because sediment sampling was completed on a single day in 1985. However, numerous subsequent observations over several years suggest that the main trends identified were typical (Bray 1990b) and studies by Laming (1988) and Bird (1989) have revealed a similar size grading.
LT9 Thorncombe Beacon (see Introduction to Littoral Transport)A continuous gravel beach formerly connected Eype and Seatown Beaches, but this link was broken by shingle depletion and subsequent landsliding at the Doghouse Hill (Photo 28) and Thorncombe Beacon (Photo 18) headlands (Brunsden 1985; High_Point Rendel 1997b). Beach depletion and recession increased the protrusion of the headlands that appears to have triggered landsliding that then deposited lobes of debris, which blocked drift. This sequence of events is thought to have been associated with renovation of West Bay Harbour (1742-46), subsequent infilling of piers protecting the entrance channel (1820s) and rapid depletion of West Beach, West Bay. Old charts reveal a near continuous beach extending around the headland toe in 1787 and depletion mostly occurred before 1948 because air photos suggest that only a patchy beach connection then existed around the headland (Brunsden 1985; High-Point Rendel 1997). Field inspection reveals the contemporary foreshore to be littered with limestone and sandstone boulders forming protective aprons, which are complete barriers to drift (Bray 1996). Much well rounded shingle remains trapped in pockets between boulders and analysis reveals an identical character to that on E.Seatown and Eype beach, although there is no obvious local present day source from the eroding cliffs (Bray 1996). The shingle pockets appear to be a residue from the former continuous beach prior to its depletion. This provides evidence of medium reliability supporting the idea of a former connection. Resumption of transport around the headland is possible theoretically, but would require shingle accretion sufficient to bury the boulder aprons and this is unlikely in the foreseeable future (Bray 1996).
LT10 Eype Beach (see Introduction to Littoral Transport)This frontage comprises an open gravel beach from Thorncombe Beacon to West Cliff (Photo 2), where outcropping rock ledges and a rock groyne effectively separates West Beach from the main beach to the west. Net drift direction remains uncertain on this beach due to conflicting evidence. Longshore trends in beach volume, size, grading and sorting support net eastward drift on the main beach, although West Beach has become severely depleted over the past 150 years making identification of drift difficult – see Photo 35 (High-Point Rendel 1997). Approximately 3,000 pebbles were sampled and analysis revealed that at high water mark and on the upper beach, pebble size increases eastward and sorting also improves (Bray 1996). A subsequent pebble sampling survey undertaken by High-Point Rendel (2000) revealed generally similar results. Using the same argument as was applied to Charmouth Beach, a slight net drift to the east is suggested. No clear longshore trend in sphericity was discernable, but roundness improved westward suggesting drift in that direction. Fluorescent pebble tracer experiments undertaken over a short period also indicated a tendency for westward drift (Jolliffe - personal communication), but it is uncertain whether the time period was fully representative. Based on beach volume changes analysed using a sediment budget approach High-Point Rendel (1997; 2000a) estimate a west to east drift of 2,500 m3a-1. In conclusion, littoral drift is indicated to occur in both eastward and westward directions, although the balance of evidence would appear to support a weak to moderate net component towards the east. There is rather better consensus that the long-term “natural direction for drift is from west to east (High Point Rendel 1997), but human interference caused by the construction of the West Bay Harbour Piers, the West Bay coastal defences and the effects of historical beach mining have made it difficult to identify the drift regime that prevailed over the past 200 years.
LT11 West Bay (see Introduction to Littoral Transport)The piers at West Bay have effectively intercepted littoral drift since cavities in the structures were infilled in the 1820s (Jolliffe 1979; Keystone Historic Buildings Consultants 1997; High-Point Rendel 1997). This has led to a trend for variable accretion/erosion and shoreline fluctuation to the east, but erosion and setback of the shoreline by up to 100m to the west (Photo 9). These effects are not simply the result of a classic terminal scour immediately downdrift of the piers because many researchers believe that drift operates for significant periods from west to east. Indeed, Rendel Geotechnics (1997) present a convincing argument that the coastal setback can be accounted for by differential sediment supply to the east (abundant supply from Chesil) and west (deficient supply from Eype Beach) of the piers.
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):
LT12 Western Chesil Beach incl. East Beach (see Introduction to Littoral Transport)Chesil Beach is a swash-aligned barrier so it is to be expected that gross drift is likely to be significantly greater than the net drift typically occurring. The great length and massive volume of the beach very effectively buffer any short term changes in sediment distribution and plan-shape morphology unless trends in sediment transport are long established. Exceptions occur at each end of the beach where hard features restrict the movement of beach sediments to produce patterns of accretion and/or erosion within years to decades according to shifts in wave climates. East Beach, West Bay is perhaps the most frequently studied location and a series of studies provide the main basis from which to attempt to understand drift on the western portion of the beach.
The marked coastal setback to the west of the West Bay piers would appear at first sight to be the result of a classic terminal scour process immediately downdrift of the piers enabling confident identification of net east to west drift (Hydraulics Research 1978, 1985, Jolliffe 1979). Map comparisons and field survey covering 1901-91 showed that accretion was not continuous, but interspersed with significant periods of erosion e.g. 1961-64 and 1985-91 (Hydraulics Research 1985; 1991a; Brampton 1993). The end result has been a fluctuation of shoreline position against the East Pier at West Bay such that MHW has varied by up to 65m with little net change (High-Point Rendel 1997; HR Wallingford 2000c). The setback was thus produced by depletion to the west and not by net accretion to the east, making it difficult to apply as an indicator of net drift.
It has proven difficult to understand drift at this location due to variations in wave climate to which the rate and direction of drift are extremely sensitive. SW approaching waves generate eastward drift so that East Beach erodes, whereas SE waves generate westward drift causing the beach to accrete against the eastern pier (Photo 9). Increase of the beach width to beyond 160m extended the beach face towards the end of the pier caused bypassing of gravel and accumulation in the harbour entrance. Erosion to widths of less than 120m significantly increases the risk of overwashing and possibly breaching e.g. Photo 36 (HR Wallingford 2000c). It should be noted that a new pier arrangement is being constructed in 2004 (West Dorset District Council 2004b) so that accretion/erosion patterns are likely to alter in response.
Fluorescent concrete tracer experiments undertaken over an 18 month period on East beach adjacent to the piers revealed net westward drift (Jolliffe 1979), but it is uncertain whether the study period was typical. The authenticity of these results was also questionable on the basis of the concrete pebbles used potentially being an unrepresentative tracer material (Carr 1980). Littoral drift between Cogden and West Bay was further studied by means of a mathematical beach plan shape model (BEACHPLAN) employing a wave climate derived from hindcasting based on Portland Wave Data covering the period 1974-84 (Hydraulics Research 1985). This study marked the first attempt to determine a reliable long-term estimate of drift on Chesil Beach and indicated mean net westward transport at 8,000m3a-1, which was validated by the documented trend for accretion against the East Pier. The application of the analysis must be questioned because it ignored swell waves and only compiled a 10 year wave record to represent the long term wave climate.
Subsequent observations of the beach in fact indicated it to be eroding, suggesting a possible change in drift regime. Analysis of beach profiles and air photos covering the period 1977-1990 revealed that the previous widely recognised trend for accretion ceased about 1982, whereupon a convincing switch to erosion resulted in MHWM retreat of 40m by March 1990 –compare Photo 7 with Photo 9 (Hydraulics Research 1991d). To explain this change, wave climate and littoral drift were analysed with techniques similar to those employed for the 1985 study (Hydraulics Research 1991d). Wave climate was hindcast using Portland wind data covering the period 1974-1990. Results showed that the wave climate changed remarkably after 1982 with fewer south-easterly storm waves and increased prevalence of westerly waves. Littoral drift calculations from this data revealed highly variable gross transport, with net westward drift before 1982 and a switch to net eastward drift of up to 14,000m3a-1 thereafter (Hydraulics Research 1991d). This model was re-run subsequently to include hindcast wave data covering the period 1974-96. The results reported a net east to west drift of some 18,000 m3a-1 (range eastward 10,000m3a-1 to westward 70,000m3a-1 over this period (HR Wallingford 2000c). This does not accord with the net pattern of accretion and contradicts previous studies (e.g. Hydraulics Research 1991d; High-Point Rendel 1997), so that bypassing of the East Pier at a rate of some 17,000m3a-1 was proposed. However, research cannot explain where this gravel might be transported to because the West Beach has depleted and no significant quantities of gravel have been identified by nearshore surveys (High Point Rendel 2000). These uncertainties suggest that the model may have significantly overestimated the true long term net drift and its results would need to be interpreted with caution by coastal managers.
It appears that net littoral drift is very delicately balanced at this western end of Chesil Beach so that slight wave climate/storm frequency variations can cause significant drift reversals as clearly indicated by previous studies e.g. Brampton (1993). Available hindcast wave data still cover too short a time span to be sure that a truly “typical” wave climate has been defined making it difficult to establish a long term net drift rate and direction. It is also difficult to predict the likely future rates and directions of drift due to the possible effects of climate change as explained in the following section. Flexible and adaptive beach management underpinned by regular monitoring should provide for the best way to cope with the uncertainties.
LT13 Central and Eastern Chesil Beach (see Introduction to Littoral Transport)Chesil Beach is a swash-aligned barrier so it is to be expected that gross drift is likely to be significantly greater than the net drift typically occurring. The great length and massive volume of the beach very effectively buffer any short term changes in sediment distribution and plan-shape morphology unless trends in sediment transport become long established. Exceptions occur at each end of the beach where hard features restrict the movement of beach sediments to produce patterns of accretion and/or erosion within years to decades according to shifts in wave climates. Research into the nature of drift on the beach and the distribution and sorting of its pebbles has involved the following methods (i) a series of sedimentological and topographic studies that essentially described the features of the beach e.g. Carr and Blackley (1974); (ii) tracer experiments using a variety of materials to simulate pebble movement e.g. Carr (1971) and (iii) numerical modelling approaches that attempt to calculate rates of transport based on wave climate data e.g. Babtie Group (1997).
Much of the central part of the beach, particularly the segment between Cogden and Langton Herring has until recently received relatively little attention except for sedimentological and topographic studies (Neate 1967, Carr 1969; Carr and Blackley 1969; Carr and Gleason 1972; Carr and Seaward 1991). These studies yielded much valuable information, but failed to provide any clear indication of the net drift direction. Most earlier research has concentrated on the eastern segment between Wyke Regis and the Isle of Portland. Early tracer experiments using brickbats (Richardson 1902) and painted pebbles (Adlam 1960) showed net eastward transport governed by waves, which also caused preferential movement of larger material. These experiments were over very limited time periods so their results were in no way representative of long-term transport. More extensive and detailed experiments undertaken with pebbles of a foreign or exotic lithology confirmed the more rapid eastward transport of larger material (Carr 1971). These experiments demonstrated marked eastward drift at Wyke Regis, but more variable and random movement near Portland. It was concluded, therefore, that there was no permanent net drift near Portland, thereby accounting for the absence of a vast accumulation of pebbles at the eastern end. Whilst providing useful information relating to sorting processes (e.g. Carr 1974), the overall patterns of drift recorded by these experiments were inconclusive because recoveries were low and unrepresentative of injected tracers and the experimental period was not fully typical of long-term conditions.
A major study of Chesil undertaken by Babtie Group (1997) over the period 1993-97 failed to significantly improve the understanding of transport along the beach although it did collect many valuable wave, beach profile and sediment data sets was well as providing some key insights into profile responses and swell wave exposure. It undertook numerical modelling of sediment transport using a COSMOS 2-D energetics based model with a sediment size of 0.75mm to 2mm. The model and the sediment size selected are not applicable to a shingle beach such as Chesil so the exercise produced few meaningful results except to indicate that transport was highly sensitive to wave height and period. Interestingly, the study did include the first aerial photogrammetric mapping of the whole beach in October 1993, 1994 and 1995 to generate some 166 profiles at 50 to 200m intervals. Comparisons revealed mixed erosion and accretion trends due to the short period covered, but a basis has been established for volumetric monitoring of the beach that might in future be used to detect long-term drift from the distribution of volume changes.
Modelling of drift using a long term hindcast wave climate for the Lyme Bay and South Devon SMP indicated that there was zero net drift, although gross drift to the east and west were significant. Chesil was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point off Chiswell at –4.1m 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.
Future drift variability was investigated specifically by Halcrow Maritime et al (2001) who undertook modelling of likely future wind speeds for a climate change scenario representing 2080 using the Met Office Hadley Centre Regional Climate Model. Initially, the present day conditions were modelled at Chiswell and West Bexington prior to examination of future conditions. Wind speeds output by the Regional Climate Model were used to derive offshore extreme wave conditions in Lyme Bay and results demonstrated a potential for significant increases in wave energy e.g. 1 in 50 year wave height of 8.1m could increase to 11.3m by 2080s. The hindcast waves have also been used to study the potential changes in alongshore sediment transport. The present condition at Chiswell was established as being a net westward drift of 900m3a-1 based on a typical gross drift of 39,000m3a-1 to the west and 38,000m3a-1 to the east. At West Bexington a net eastward drift of 1,000m3a-1 was estimated based on a typical gross drift of 21,000m3a-1 to the west and 22,000m3a-1 to the east. Assessment of the climate change results for Chiswell indicate a potential for a dramatic shift in the sediment transport regime due to a small two-degree southerly shift in the mean wave approach direction. The current net westward drift potential of 900m3a-1 would alter under this scenario to a net westward drift of up to 15,000m3a-1. At West Bexington, the current net eastward drift potential of 1,000m3a-1 would reverse to a net westward drift of up to 7,800m3a-1 (Halcrow Maritime et al. 2001). If such changes were to occur depletion could occur rapidly at Chiswell generating coastal defence problems, whereas at West Bexington incoming sediment would compensate for losses so there would be no perceptible changes for many decades. In the long-term this pattern of transport alteration would cause a slight southerly re-orientation of the beach with significantly increased risk of breaching between Wyke Regis and Chiswell.
A similar type of study was undertaken by Sutherland and Wolf (2002) using an alternative climate model to generate future wave scenarios and then to simulate drift on Chesil Beach up to the year 2075. In this instance the present condition for a beach facing 225 degrees was estimated as being an eastward drift of 24,000m3a-1. Results suggested that net drift could in future increase by up to 30% due to the potential effects of climate change. The two numerical modelling studies discussed above present conflicting results with respect to establishing the present conditions on Chesil Beach. This is because they have set out to examine future scenarios rather than to focus on the present and to precisely predict the future. They have, however, generated some valuable insights suggesting that (i) waves in Lyme Bay are likely to vary with future climate change and (ii) Chesil Beach is likely to be sensitive to these changes with the potential for reversals and accelerations of drift. If such changes were to occur variations in beach morphology and volume would become most apparent at the two extremities of the beach where transport is impeded.
The unique longshore size grading of the beach is possibly related to the delicate balance of littoral drift along the coast. Most theories attribute the grading to wave action, although several in the 19th century, now largely discredited, also considered tidal mechanisms (Carr and Blackley 1974). Differential transport has been widely advocated, with large waves from the south-west transporting all pebble sizes to the east, whilst smaller waves from the south east could only transport smaller pebbles back towards the west. The latter would almost certainly account for the considerable accumulation of fine shingle to the east of the east pier at West Bay. This pattern of movement was first advanced by de Luc (1811) and embellished by later authors to include concepts of sorting and abrasion. Preferential transport of larger pebble sizes in tracer experiments (Richardson 1902; Jolliffe 1964) led to the belief that pebble selection was associated with breaking wave height (Jolliffe 1964). However, the first detailed sedimentological and tracer studies indicated that pebble thickness was the critical parameter and grading on the beach as a whole was unlikely to have a direct relation to breaking wave height (Carr 1969, Carr 1971). Further studies revealed that pebble sorting was also related to wave frequency, the square root of significant wave height and the angle of swell approach (Gleason and Hardcastle 1973). A review by Bird (1996) attributed the unique longshore size grading to a complex interaction of factors arising from local sediment availability coupled with the incident wave climate, especially the increasing wave energy towards the east of the beach. Carr and Blackley (1974) also identified the fossil nature of the beach as an important factor, arguing that fresh sediment inputs would tend to diminish size grading. However, Bray (1996; 1997a) and Brunsden (1999) argued that massive recent supply of gravel from cliffs to the west was important in ensuring a wide range of clast sizes was available to feed processes of sorting and differential transport. Brunsden (1999) also advanced a novel concept longshore moving sediment “slugs” as being an important feature of the sorting process.
4. Sediment OutputsThe Isle of Portland acts a large terminal groyne that intercepts coarse beach sediments preventing them from leaving this coastal unit, which has therefore become a sink for coarse sediment. There are, however, notable processes by which sediment is lost from the shoreline. Fine sediments are winnowed from beaches by waves and deposited offshore in Lyme Bay, whereas coarse sediments suffer losses from attrition and entrapment. Finally, this unit has a history of beach mining that has removed substantial quantities of shingle from the shoreline.
4.1 Onshore to Offshore Transport - WO1 WO2 WO3 References MapCoastal landsliding transports large quantities of sediment to the beach (see Section 2.2). Only the coarser materials are stable on the foreshore when exposed to wave action. The majority of sediments supplied are degraded and weathered clays, shales, mudstones and weakly cemented sandstones. These readily disintegrate under wave attack and their fine constituent grains/particles are transported offshore in suspension e.g. Photo 15. Transport pathways are uncertain, but it is likely that a significant proportion of sediment is deposited in Lyme Bay, because offshore surveys reveal that silts and fine sands of similar character are the dominant surface sediment within the bay (Institute of Geological Sciences 1983, Dobbie and Partners 1980, Offshore Environmental Systems Ltd 1980, Bray 1986). Clays are likely to be transported more widely into suspension to contribute to estuaries within Lyme Bay and possibly more widely along the English Channel. The offshore transport of fines therefore closely follows the pattern of sediment supply, with large quantities yielded from Black Ven and Stonebarrow and extremely small quantities from Chesil Beach (Bray 1986; 1996).
Gravel and coarse sand are more stable on the foreshore and are generally the beach forming sediments (Bray 1986). Significant offshore shingle transport is unlikely due to absence of offshore banks or "sinks" revealed by the review of offshore information undertaken in Section 2.1. Offshore loss of sand from the foreshore is likely, because extensive areas of the Lyme Regis sea bed are covered by this material type (Darton et al 1980; Institute of Geological Sciences 1983; Bray 1996; High-Point Rendel 2000; West Dorset District Council 2000a). Available evidence of offshore transport revealed the following pathways.
WO1 Lyme Regis (see Introduction to Onshore to Offshore Transport)A variety of studies including physical modelling (Hydraulics Research 1989; HR Wallingford 2003), numerical modelling (HR Wallingford 2001) and geomorphological interpretation (High-Point Rendel 1999b) indicate formation of an offshore transport pathway from the central part of Marine Parade during eastern and south-eastern wave conditions. It appears that an offshore-directed current can transport sands and some gravels seaward down Town Beach Channel as identified by (High-Point Rendel 1999b). This same location corresponds with a littoral drift divide on the shingle beach - see LT 3. It is uncertain whether material is lost permanently, or whether it can be transported back onshore during favourable conditions.
W02 Charmouth Beach (see Introduction to Onshore to Offshore Transport)Large quantities of sand are supplied to Charmouth Beach by coastal landslides (Section 2.2) and significant quantities are present on the beach face and lower foreshore on the western part of the beach – see Photo 4 (Bray 1996). Studies revealed net eastward littoral drift, yet the proportion of sand declines in this direction and very little is present east of Westhay Water. Progressive offshore loss of sand is therefore inferred, as it is transported eastward on the beach (Bray 1996).
W03 West Bay (see Introduction to Onshore to Offshore Transport)Onshore-offshore exchange of fine gravel has been postulated to explain beach profile changes on East and West Beaches (Hydraulics Research 1978; Jolliffe 1979; HR Wallingford 2000). Furthermore, it is estimated that West Beach has lost some 500,000m3a-1 of sediment and transport offshore is the only feasible explanation (High-Point Rendel 1997) Nearshore hydrographic and sediment sampling surveys, however, have failed to locate any gravel deposits that could be the sinks for such losses. For example, High-Point Rendel (2000), 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. It was concluded that banks fed by long-term sediment loss were not present, although temporary accumulations could form very close inshore related to profile variations arising from storms.
4.2 Beach Gravel Mining
History of ExtractionThe beaches of South-West Dorset have a long history of mining, which extends back at least 700 years at West Bay (Hydraulics Research 1978; Jolliffe 1979; High-Point Rendel 1997). All operations have now ceased due to concerns over probable adverse coastal defence and environmental impacts. The last remaining operations ceased in 1986. The pebbles taken have been traditionally used for construction and ornamental purposes. Additionally, durable well-rounded pebbles have been particularly valuable for water filtration and as grinding media in the ceramics industry. Rounded to sub-rounded chert and flint pebbles of 20-60mm long axis were favoured for extraction at Seatown. Larger, more spherical pebbles were usually hand picked where available and smaller “pea” gravel has been removed from East Beach, West Bay and sites. Estimates of volumes and locations of extraction are only available for this century and are presented in Photo 37. These figures are based upon details extracted from files held by Dorset County Council and from compilations of such information (e.g. Jolliffe 1979, Carr 1980a, Bray 1996).
Carr (1980a) estimated that between the mid-1930s and 1977, a minimum of 470,000 tonnes of gravel was taken from the East Beach and 370,000 tonnes from Cogden Beach. During this time, selective picking of larger more spherical pebbles was conducted in the Chesil Cove (Chesilton) area involving removal of 9,400 tonnes between 1944 and 1972. This practice could have adversely affected size grading and crest level locally and was discontinued in 1973 after a public inquiry. It is estimated (Carr 1980a) that 1.1 million tonnes of gravel was extracted from Chesil Beach between the mid-1930s and 1977. Given a total estimated volume of pebbles and cobbles of between 25 and 100 million tonnes (Carr 1980a) this suggests that between 1.1% and 4.4% of the beach was removed over this period. Although the most intense extraction occurred during the Second World War and immediately afterwards, large quantities were also extracted prior to the 1930s. For example, at least 50,000 tonnes of gravel were extracted between 1905 and 1907 for the foundations of the oil tank depot at Portland Naval Base (Carr 1980a, 1983b). Extraction ceased on Cogden Beach, the last remaining active site on Chesil, in December 1986 (Photo 38).
Further to the west, the major extraction site was Seatown Beach (Photo 3). Between 1939 and 1945 large quantities of gravel were removed and eyewitness accounts suggest that the beach was almost completely stripped (Rhoden 1974, Brundell 1985, Laming 1988). Assuming that the pre-1939 beach was of similar volume to the contemporary beach, this entailed removal of 240,000 - 270,000m3 of gravel (Laming 1988, Bray 1996). Extraction records held by Dorset County Council Planning Department revealed that between the mid 1950s and 1970, an estimated 38,000 tonnes were removed (2,500 tonnes a-1.) and from 1971 until extraction ceased in April 1987, removal totalled 17,000 tonnes (1,000 tonnes a-1). Total beach gravel extraction since 1939 is therefore estimated at 275,000-305,000m3. There are no records of extraction from Seatown Beach prior to 1939, but photographic evidence of vehicle tracks on the East Beach (Lang 1932) suggest that some may have occurred.
It should be appreciated that it was difficult to check on the exact quantities and qualities of gravel removed. Many of the figures quoted above are based upon planning consents and it has to be recognised that local authorities did not possess the resources to continually check that extraction remained within the approved quotas. Consequently, these figures may be underestimates.
Effect of ExtractionChesil Beach is widely recognised as a unique scientific feature and also performs a vital coast protection/sea defence function. The beach is generally held to be a fossil barrier receiving no significant contemporary supply (Carr and Blackley 1974, Jolliffe 1979, Carr 1978, 1980a, 1983b,1883c; Bray 1996; High-Point Rendel 1997). Thus, it was argued successfully at a public planning enquiry in 1985 that any losses were irrecoverable and would lead to reduction of volume and detrimental effects to the scientific value and coast protection function (Carr 1985). More specifically, it was also argued that extraction sites can become locally depleted in the short term, with consequent reduction of equilibrium profile during wave attack and possible flooding and crest recession (Brampton 1985). Localised beach depletion may also temporarily cause exposure of soft underlying beach core sediments rendering them liable to erosion (Brampton 1985). These effects were supported by mathematical modelling of Cogden Beach with the assumption of continued beach extraction (Hydraulics Research 1985). Seatown Beach was subject to major wartime extraction, which must have severely depleted the beach and may be linked with beach loss from the foreshore beneath Thorncombe Beacon that has isolated Eype and West beaches from their sources of supply and has therefore contributed to their depletion (High-Point Rendel 1997). Transport around the Golden Cap headland also became blocked partially in the late 1940s and completely blocked by the early 1960s, so Seatown Beach has been an isolated pocket beach since this time (Brunsden 1985, Bray 1996). All subsequent extraction was therefore a net loss, which reduced the protection afforded to the base of the cliffs. Cliff retreat has accelerated since 1948, although the link with extraction cannot be conclusively proven (Bray 1986). Conerns over cliff recession has led to construction of a cliff stabilization scheme at Seatown (WS Atkins 1996). Pebbles selected for extraction were larger and more durable than the average for the beach (Laming 1989, Bray 1990b), so extraction also affected the sedimentological character (Bray 1996). Sampling has shown that Seatown pebbles are very similar to those on neighbouring beaches in terms of lithology, shape and sphericity but their mean size and roundness are less (Bray 1996). Since these beaches are considred to share a common gravel source and eastward transport pathway the differences may reflect the effect of extraction (Bray 1996). Pebble size reduction may result in flattening of the profile and landward migration of the beach crest, while loss of durable pebbles may expose less resistant materials to erosion (Bray 1996).
DredgingDredging is undertaken in the West Bay harbour entrance to maintain a navigable channel. The amount removed has averaged 1200m3a-1 since at least 1964 (Jolliffe, 1979, Hydraulics Research 1985; High-Point Rendel 1997, HR Wallingford 1997b) and this therefore provides a minimum for bypassing of the piers.
4.3 EntrapmentThis refers to the process whereby beach and landslide derived gravel becomes trapped by boulder aprons on the seabed as the coastline retreats landward. Large boulder aprons are present on the foreshore and extend up to 3-4km offshore of the headlands of Doghouse Hill/ Thorncombe Beach, Golden Cap and also to a much lesser extent at Black Ven. Seaward of MLW, the beaches at these locations are also fringed by boulder accumulations (Bray 1986, 1996). Numerous pockets of chert and flint gravel were observed trapped between these boulders within the intertidal zone and highly angular gravels were sampled from the nearshore extensions of these aprons (Bray 1986; 1996). The deposits are believed to represent ancient eroded mudslide lobes stranded offshore as a consequence of coastal recession and slowly rising sea-level (Bray 1986; 1996). The trapped gravel is probably derived from three sources:
EN1 Black Ven = 75 m3a-1 (see Introduction to Entrapment)
EN2 Charmouth Beach = 75 m3a-1
EN3 Golden Cap = 38 m3a-1
EN4 Seatown Beach = 45 m3a-1
EN5 Thorncombe Beacon = 36 m3a-1
EN6 Eype Beach = 51 m3a-1Total entrapment output therefore amounts to 320m3a-1 between Lyme Regis and West Bay, a small proportion of overall input. Losses due to this process need to be added to those occurring by attrition that are described below.
4.4 AttritionDuring transport by wave action on the beach, gravel clasts are subject to repeated impacts with other clasts and with the substratum. Attrition rate varies inversely according to the amount of attrition a clast has already undergone. Thus, fresh angular gravels from coastal landslides are most susceptible, whilst well-rounded beach pebbles only abrade very slowly (Bray 1990b). Detailed comparative study of the morphometry of cliff gravels with beach gravels indicated that attrition output involved two elements:
5. Transport in the Offshore Zone - O1 O2 O3 References MapUntil recently, very little attention had been given to sediment transport in the offshore zone. Some broad pathways had been identified from bedforms and others were postulated according to the patterns of residual tidal currents. The Dorset Inner Shelf Seabed Mobility Study (Bastos and Collins 2002) has made a major contribution to knowledge covering an area of around 10km to the east and west of Portland Bill and up to 5km offshore from the tip of the Bill. The study set out to investigate wave and tide driven transport at the bed, to identify transport pathways, predict seabed mobility and identify the effects of sandbanks and shoals. It involved assembly of existing information on seabed sediments and morphology, tidal currents and waves. Original field measurements included hydrographic and sidescan sonar surveys, sediment sampling, seismic boomer surveys of sediment layer thickness and 3d tidal current velocity profiling (Acoustic Doppler Current Profiler). Data collected were applied to develop numerical models of tidal flows (TELEMAC 2-D), wave refraction (RCPWAVE) and sediment transport at the bed (SEDTRANS). Results from the modelling were compared against bed morphology and bedforms to verify, where possible, processes and pathways. Outputs from the study were presented in a series of maps within an atlas of seabed mobility and transport pathways. Topics included included bathymetry and bed morphology, sediment distribution and thickness, bedforms, maximum and residual depth-averaged tidal currents seabed mobility for various grain sizes and hydrodynamic conditions, sediment transport directions and an interpretation of key pathways. The following summarise some of the key results for the area to the West of Portland Bill:
01 General Eastward Flow, Lyme Bay (see Introduction to Offshore Transport)A general eastward flow is indicated in Lyme bay for fine sands. The flow is directed towards Portland Bill where is strengthens, being sufficient to transport coarse sands. It is produced by the combined interaction of tidal and wave induced currents at the bed.
Tidal models validated by measured currents revealed a general eastward residual tidal flow within 10km of the shore in Lyme Bay and a westward flow further seaward (Bowles et al 1958, Pingree and Maddock 1977a; 1977b; Bastos and Collins 2002). The east moving stream generally flowed parallel to the coast from Lyme Regis to Chesil Beach and was then deflected south and concentrated significantly by the Isle of Portland (Pingree and Maddock 1977a; Bastos and Collins 2002). This information was supported by studies undertaken for the Lyme Regis and Charmouth sewage outfalls (South-West Water 1979, Offshore Environmental Systems Limited 1980, Smith 1989) These investigations included current metering, float tracking, bacterial dispersion tests and side-scan sonar. Peak surface currents of 0.45ms-1 and bed currents of 0.30ms-1 were measured. Bastos and Collins determined tidal depth averaged currents of 0.25-0.50ms-1 inshore within Lyme Bay increasing southwards and eastwards with strong currents of up to 2.50ms-1 in the vicinity of Portland Bill.
Numerical modelling studies found that the fine sands of the bed of Lyme Bay would be highly mobile for much of the time making it likely that sediments are transported east-south-eastwards towards Portland Bill (Bastos and Collins 2002). Bedforms mapped off Lyme Regis (Offshore Environmental Systems Limited 1980; West Dorset District Council 2000a; Badman et al 2002) and along the surface of West Shoal (Bastos and Collins 2002) confirm this transport direction.
02 Tidal Eddy West of Isle of Portland (see Introduction to Offshore Transport)A clockwise tidal eddy that alters its location, size and strength during the tidal cycle was identified to the south-west of Portland Bill by Bastos and Collins (2002). A feature of this type had also been identified earlier by (Pingree and Maddock 1977a; 1977b;1983) who attributed it to deflection of tidal flows by the Isle of Portland. Its significance for bed sediment transport is uncertain although it appears to result in a convergence of sand transport over Portland Bank, and is possibly the mechanism that has formed and maintained the bank (Bastos and Collins 2002). Cause and effect is of course difficult to determine for a bank begins to alter its own hydrodynamic environment as it forms. An area of longitudinal furrows and ridge bedforms has been recognised some 10km off Portland Bill (well seaward of the limit of the Bastos and Collins 2002 study area) with orientations parallel to offshore flow (Institute of Geological Sciences 1983). Surface sediment is predominantly gravel in this area (IGS 1983), so it is possible that sediments up to this size are transported.
03 Tidal Eddy East of Isle of Portland (see Introduction to Offshore Transport)A strong southwestward directed residual tidal flow operates from the east of the Isle of Portland and is associated with an anticlockwise tidal eddy and convergent sand transport focused upon the Shambles Bank. It converges upon Portland Bill and contributes to a potential for net southward transport at this point. These processes occur outside of the area of the unit currently under examination and are discussed fully within the Unit text covering the Isle of Portland and Weymouth Bay.
6. Sediment Budgets - References MapIntegration of sediment transport information enabled calculation of gravel budgets for Charmouth, Seatown and Eype Beaches by Bray (1996). These budgets are briefly presented here, but the original source should be consulted for further details. A budget for Chesil Beach based on that compiled by Rendel Geotechnics (1997) is also presented.
6.1 Charmouth BeachA detailed long-term budget of the beach and landslide system is presented in Table 2 and Photo 39 based on research by Bray (1996). The analysis revealed total inputs of gravels to the beach of some 6,400m3a- 1 (96% from coastal landsliding) and total output of 3,900m3a-1 (33% attrition, 55% littoral drift at Golden Cap). Mean accretion was therefore indicated at 2,500m3a-1. This was slightly greater than recorded beach accretion, although the difference could easily be explained by (i) past unauthorised beach shingle extraction at Charmouth (ii) some undetected nearshore storage and (iii) a lag between backscar release (used to calculate long-term input) and actual supply to the beach at Black Ven where landsliding has accelerated at the backscar.
A gravel flow model constructed by Bray (1996) graphically demonstrates the supply and subsequent distribution of gravel on this beach (Photo 40). A key point is that the major beach accretion is towards the east, whilst the major landslide supply sites are to the west. Virtually no littoral drift is received from the west due to The Cobb and Lyme Regis defences. The western part of the beach system is therefore characterised by high sediment delivery from the cliffs and throughput on the beaches with relatively little beach storage. No material is sufficiently stable on the western part of Charmouth beach to provide long-term protection to the toe of the cliffs. Clay and sand are supplied in large quantities but are easily eroded and transported offshore. Limestone boulders, chert and flint gravel provide only temporary protection because of the low durability of the former and the potential for longshore transport of the latter. The active phase of landsliding initiated at Black Ven in the late 1950s is therefore likely to continue, or intensify because natural basal protection is unlikely to keep pace with increasing future sea-levels without increased landslide throughput. It is likely to supply increasing quantities of gravel to the beach due to accelerating recession rates and lateral extension of the active landslide complex.
In recent decades landslides at Golden Cap have intercepted beach transport so that a period of beach accretion can be anticipated. Eventually the landslide debris should suffer marine erosion sufficient to enable a surge of drift as the sediments impounded rapidly move around the headland to Seatown Beach.
6.2 Seatown BeachGravel budgets for several intervals as well as a mean long-term budget are presented in Table 3 based on research by Bray (1996). The budget of Seatown Beach has until recently been dominated by phases of input from eastward littoral drift beneath Golden Cap (unimpeded transport) and output by mining. Both processes have operated intermittently, so that the shingle budget has been time dependent. The effects of these changing inputs and outputs are illustrated by the alternative budgets calculated for various time intervals and explained as follows:
Over this period Seatown Beach received significant gravel inputs from neighbouring Charmouth Beach during an "unimpeded" transport condition covering the period 1934-1949. However, it was also subject to large-scale mining during 1939-45 that would have more than offset the gains. An overall net loss of 32,000m3 is estimated for this period.
1949-1987During the early part of this period the beach continued to receive additional material as shingle drifted intermittently beneath Golden Cap before landslides finally blocked that pathway in 1960. Thereafter, beach mining was the major influence, resulting in net shingle loss and beach depletion. Recent increases in the recession of both the base of the sea-cliffs and the backscar behind the East Beach may have resulted from loss of protection caused by this factor and have had to be controlled by a cliff stabilisation scheme in one instance (see E7). The effects are compounded because extraction involved preferential removal of the larger more durable pebbles.
Present SituationAt present, the shingle budget of Seatown Beach is delicately balanced with relatively small inputs and outputs resulting in slow net depletion of 39m3a-1. This situation is expected to continue for at least the next twenty years until marine erosion of mudslide lobe barriers beneath Golden Cap is sufficient to permit incoming shingle drift from Charmouth Beach.
Future EvolutionUnderstanding of the general principles of the Seatown Beach gravel budget allows estimation of future evolution given that (i) the rate of sea-level rise will accelerate in future and (ii) no further beach gravel extraction will be permitted.
The natural state of Seatown Beach appears one of episodic accretion by pulses of shingle supplied from Charmouth Beach during brief intervals of unimpeded transport beneath Golden Cap. Now that extraction has ceased, accretion should resume when inputs from the west again become possible, perhaps shortly after 2020. Until this time, Seatown Beach will remain in a delicate state of balance with slight net loss (39 m3a-1). The recent trend for increased backscar and sea-cliff erosion is therefore expected to continue or intensify in the short term, because beach volume should diminish slightly, while the current sea-level rise is predicted to continue or accelerate. Beyond this point it is postulated that brief pulses of rapid accretion (episodic inputs from Charmouth Beach), should be separated by longer periods of gradual depletion by attrition and entrapment ("The Accretion Phase"). This, however, would represent a stage in the beach's recovery from mining and is not a "natural" long-term state. This is because accretion cannot be sustained indefinitely before again shingle "overflows" eastward and creates new beaches beneath the Doghouse Hill and Thorncombe Beacon headlands ("The Bypassing Phase"). Such beaches would eventually transport surplus shingle to Eype Beach when the drift pathway became restored. The hypothesis is that eventually Seatown Beach, without further interference, would attain an equilibrium volume, whereupon shingle inputs and outputs should balance over an appropriate timescale (100 to 200 years), so that beach volume shows no trend ("Dynamic Equilibrium Phase"). The net effect is that Seatown Beach should alternate between being a closed and an open system according to transport conditions at its two landslide controlled headlands. At some times, the beach might only be open to inputs (temporary sink), whilst at others it would only be open to outputs (temporary store).
In conclusion, before 1900, Seatown Beach was probably of significantly greater volume than at present and was maintained by a balance between inputs and outputs regulated by the two headlands. Between 1900 and 1987, rapid beach gravel extraction was sufficient to unbalance the system resulting in accelerated coastal erosion. With the cessation of mining in 1987, a series of accretionary phases should be initiated after 2020, which should eventually return the system to its pre-1900 state. The fully closed contemporary Seatown Beach is therefore not typical and in the past it is likely that the beach served as an important transport link between the dynamic Charmouth gravel supply area and the Eype/Chesil Beach shingle sink.
It should be noted that the pocket beaches of this shoreline operate as a complex system that is sensitive to artificial and natural disturbances. For example, the projected gravel budget at Seatown may be modified in the following manner by an accelerating rate of sea-level rise:
6.3 Eype BeachA long-term budget for the beach is presented in Table 4 based on research by Bray (1996) and High-Point Rendel 1997). The beach currently appears to be a relatively enclosed pocket beach with negligible inputs, but some significant historical outputs. Its present budget regime probably dates from the 1820s, when the piers at West Bay were made continuous barriers, thereby blocking shingle exchange with Chesil, which would previously have been a major process. Since 1825, outputs by attrition and entrapment (133m3a-1) and beach mining (discontinuous but estimated at 400m3a-1) would only have had a modest effect upon the beach volume, however, the eastern part of the beach and West Beach depleted severely with loss of up to 500,000m3 (High-Point Rendel 1997). The full sequence of events relating to the loss of West Beach has been reconstructed by High-Point Rendel (1997), although cessation of transport around the Doghouse Hill headland and construction of coastal defences at West Bay are believed to have been contributory factors. A major uncertainty concerns the fate of the lost beach material because it is unlikely that much could have been supplied eastward to Chesil due to the coastal setback that was produced as West Beach depleted. The only feasible explanation is of a massive offshore loss of fines from the beach and its substratum. A major offshore loss of coarse gravel is considered unlikely because nearshore surveys have failed to identify any corresponding deposits on the seabed (High-Point Rendel 2000). Understanding of this loss has been important because West Beach has now been replenished and appropriate control structures had to be designed to minimize future beach losses (HR Wallingford 2000b; 2000c).
The present Eype Beach is therefore a relict shingle accumulation characterised by depletion or undernourishment, albeit at a slow rate. It is uncertain whether further episodes of offshore loss could occur. Although West Beach has been artificially replenished with 18,000m3 of shingle, new coastal defences have been designed to avoid such occurrences. If it is assumed that further offshore losses do not occur from Eype Beach the future evolution of the beach may involve the following:
6.4 Chesil BeachCompilation of the information collected by this study has enabled construction of a budget for the beach presented in Table 5. The budget has altered through time due to changes in inputs from the west and outputs by mining, although the other elements have remained constant. Three alternative states can be identified:
Late Holocene to 1820sPrior to construction of the Cobb, Lyme Regis and the piers at West Bay drift inputs from the West Dorset and East Devon coasts could have been as great as 8,000 to 12,000 m3a-1 (Bray 1996). These inputs would have dominated the budget so that the beach would have been in an accreting condition. Bray (1996), High-Point Rendel (1997) and Brunsden (1999) discuss the effect of this upon the evolution of the beach. The consensus is that the much of the gravel of Chesil would have been derived comparatively recently over the past 2,000-4,000 years from this source to the west where some 40-60 million cubic metres could have been supplied, sufficient to account for the present beach volume of 16-63 million cubic metres (see Section 1.2).
1820s to 1986From the 1820s onward drift from the west would have ceased due to infilling of the West Bay piers. The beach altered from an open to a closed system isolated from any major shingle supplies. At this point losses become important to the beach and in the 20th century some 750,000 cubic metres of gravel were mined prior to cessation of operations in 1986. The assessment in Table 5 is therefore that outputs totalling 17,400 m3a-1 could have occurred.
Post 1986Following cessation of the last remaining mining operation in 1986 the beach has depleted much more slowly with net losses of 1,400 m3a-1 due to drift bypassing of West Bay East Pier and attrition of the gravel as it is worked by waves. Although the beach has suffered substantial depletion over the past two centuries, the effects on profile and crest height are not easily determined due to the massive volume of the beach and high potential for rapid shingle redistribution. Recent measurements show variable but widespread lowering and some recession of the Chesil Beach between Chiswell and Wyke Regis (Section 1.2). Following concerns over possible increases in damaging flooding events at Chiswell a coastal defence scheme was constructed involving measures to improve drainage within the beach and also novel gabion mattresses (Photo 41) to reinforce the stability of the crest (Hook and Kemble 1991; Heijne and West 1991). The scheme is explained in detail at: http//www.swgfl.org.uk/jurassic/chiswell.htm The depletion of the beach has generated some debate concerning its likely future evolution and its resilience to climate change and sea-level rise; a topic that has been addressed by the West Dorset Coast Research Group (Bray 1997b) and the DEFRA FutureCoast project (Halcrow 2002). It is thought likely that the beach will continue to retreat by a “rollover” mechanism with acceleration in eastern parts. It is possible that the beach could breach between Wyke Regis and Chiswell, although the precise location and timing are highly uncertain. This is supported by modelling of profiles by (Babtie Croup 1997) using Powell’s (1990) model, which indicated that a 1 in 100-year wave event in combination with future sea-level rise could result in up to 31m of crest recession. This particular model is not designed for application to a barrier beach so the results should be treated with some caution, however the increased sensitivity of eastern parts of the beach is apparent. A permanent breach would create a new tidal inlet in the Fleet, changing its regime and would greatly affect the transport and distribution of the beach shingle (Halcrow 2002). Some researchers have suggested that such events could trigger the break-up of the beach, while others feel that en masse landward recession of the beach as a continuous barrier is the more likely (Bray 1997b).
7. Coastal Defence and Habitats Issues - References MapThis coastline is of prime international, as well as national, importance for its geological and geomorphological features and was granted UNESCO World Landscape Heritage status in 2001. The World Heritage Site is promoted and managed by the Jurassic Coast Project that maintains an informative website at: http//www.swgfl.org.uk/jurassic/
Chesil Beach and Golden Cap to Lyme Regis are also Geological Conservation Review sites for coastal geomorphology and their qualities are discussed further by May and Hansom (2003).
A consequence of these widely recognised qualities and international designations is a potential for significant conflicts between habitat, or earth science conservation and shoreline management, wherever the latter could affect the morphology and exposure of the cliffs. As part of its overall management plan for the World Heritage site (Jurassic Coast, 2003) the Jurassic Coast Project is promoting a mechanism for consultations between coastal engineers and the earth science community. It has set up a consultative scientific network so that potential conflicts and issues can be addressed (http//www.swgfl.org.uk/jurassic/consult.htm).
The main habitats of interest comprise the coastal vegetated shingle of Chesil Beach and the saline lagoon of the Fleet. The beach is one of the most important shingle sites in Britain and the lagoon is the largest and most important site of its type in Britain. Collectively, these features are designated as a European Special Area of Conservation and a Special Protection Area. Other interests relate to vegetated soft rock sea cliffs for which the coastline from: (i) Sidmouth to West Bay and (ii) Isle of Portland to Studland Cliffs are also designated as European Special Areas of Conservation.
In managing Chesil Beach and the Fleet Lagoon it is important that understanding is gained of the likely future evolution of these features and of the likely consequences for their dependent habitats. It would be valuable to establish a baseline of the extent and quality of the present European Designated Habitats, their likely future changes to 2100 and the opportunities available for habitat management. Results could be needed as part of the preparation for the forthcoming SMP revision in order to ensure that the future provision of defences does not adversely affect European designated habitats. In the Solent, a specific Coastal Habitat Management Habitat Plan (CHaMP) has been prepared (Bray and Cottle, 2003). Aspects of the style and content of this Plan together with guidelines that have been prepared by the Living with the Sea Project (English Nature et al., 2003) should provide guidance on the type of approach that would prove beneficial. To assist in the management of the Chesil vegetated shingle, a detailed survey to create a baseline inventory for the resource could be useful. An example of a successful programme elsewhere on the south coast is provided by the West Sussex Vegetated Shingle Project (2003). The project has mapped the resource, sought to increase general awareness, and has provided guidance for contractors working on vegetated shingle. Details of appropriate management and habitat creation techniques for this resource have been set out by Doody and Randall (2003).
8. Opportunities for Calculation and Testing of Littoral Drift Volumes - References MapThe shoreline to the west of West Bay has been studied in detail by Bray (1996) who successfully combined sediment tracing, wave power transport modelling and sediment budget analysis to derive estimates of drift on each beach. Useful further work would be to monitor the volumes of each of the pocket beaches in order to test those estimates and/or detect any changes in regime. Certainly, monitoring of beach replenishments at Lyme Regis and West Bay would enable likely drift rates to be ascertained from changes in beach volume (with allowances for the effects of control structures). There are major opportunities to study drift occurring along Chesil Beach to obtain a clearer understanding of its overall drift regime. Previous studies have measured or modelled drift at only a few sites rather than investigating the beach as a whole. A possible approach would be to set up a numerical transport model (perhaps a beach plan shape model) and to attempt to calibrate it for the whole beach. This would be a major undertaking that would involve the following key elements:
9. Knowledge Limitations and Monitoring Requirements - References MapThere has been an impressive increase in both the quality and quantity of knowledge and understanding of the coastal sediment transport process system on this frontage over the past ten years. A series of studies at both Lyme Regis and West Bay have offered deep insights into the coastal processes and longer-term geomorphology at those sites. Major studies by Bray (1996) and Babtie Group (1997) effectively compiled information and undertook new research to fill gaps relating to the pocket beaches and cliffs and Chesil Beach, respectively. Various previous studies have published lists of suggested future research including High-Point Rendel (1997); Bray (1997b); Babtie Group (1997) and Posford Duvivier (1998a). The West Dorset Coast Research Group produced a specific response on information gaps and recommended future monitoring and research for the Shoreline Management Plan (Bray 1998). The recommendations for future research and monitoring here attempt to emphasise issues specific to the reviews undertaken for this Sediment Transport Study and do not attempt to cover the full range of coastal monitoring and further research that might be required to inform management as follows:
10. References - Map
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MMIV © SCOPAC Sediment Transport Study - Lyme Regis to Portland Bill