South and East Isle of Wight
1. INTRODUCTION - References MapAlmost the entire length of this coastline is characterised by active cliff development, with adjoining beaches and shore platforms of variable length, height and width e.g. Photo 1. Contrasts in cliff height and morphology are the product of the outcrop pattern of Cretaceous rocks of varied lithology, and of controls imposed by geological structure. The western and eastern coastlines truncate the axis of the Isle of Wight monoclinal fold, such that the same sequence of outcrops is encountered; detailed variations in rock dip introduce spatial variations in outcrop width (Osborne White, 1921). The southern coastline is distinctive in that it is developed in debris that is the product of long-established slope instability - (Photo 2) (Preece, 1980; 1987; Chandler and Hutchinson, 1984; Hutchinson, Brunsden and Lee, 1991; Hutchinson, 1991). The landslides of this sector of the island's coast owe their fundamental character and impressive scale to rock lithology and succession, hydrogeological controls both above and below mean sea-level, structural form, and wave climate. (Halcrow, 1997; Hutchinson, 1991; Rendel Geotechnics, 1995).
1.1 Coastal Evolution
Variations in coastal orientation, wave exposure, relief and geological outcrops have controlled behaviour enabling sub-division into three distinct behavioural units summarised as follows based upon evaluations undertaken for Halcrow (2002):
South West Coast
1.2 Wave conditions
Whilst geological factors control variations in the scale, erosional resistance and detailed morphological character of the cliffed coastline, changes in coastal orientation introduce contrasts in exposure to wave energy. The west-facing coastline is open to high-energy Atlantic swell waves that can propagate across a fetch distance in excess of 4,000km. The well-documented history of shipwrecks along this largely unprotected rugged coast is a testimony to this fact. HR Wallingford (1999), calculated, using numerical modelling of synthetic data for wave climate that the range of maximum wave height, for a 1 in 1 year recurrence is up to 5m for the coastline between Freshwater Bay and the Needles. For Compton Bay, it extends up to 4.26m. Estimations for longer recurrence intervals are also given. Variation is due to the range of different wave types and approaches. The south-facing coastline has a maximum fetch of 150km, determined by the opposing Channel coast of France, but is also affected by refracted ocean swell from the west and southwest. Offshore wave heights are depth-limited by extensive submerged boulder aprons, resulting in maximum heights of 2.6 m at a 1 in 5 years recurrence (Rendel, Palmer and Tritton, 1993). By contrast, the east-facing coast is relatively protected from waves generated by dominant westerly winds, although subject to the residual energy of swell waves refracted by a combination of offshore seabed topography and the acute change in coastal plan at Dunnose. However, the east coast is fully exposed to a 170km fetch extending east and east-south-east; this can propagate waves in excess of 3.2 m height in association with infrequent easterly gale-force winds (Hydraulics Research, 1977b; 1984; 1991; HR Wallingford, 1992). Thus, no part of this coastline is immune to the effects of storm waves of exceptional energy with a high potential for effecting substantial erosion of both cliffs and beaches.
Freshwater and Sandown Bays were two of the locations for which wave modelling excercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). Offshore wave climates were synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to prediction point in Freshwater Bay at -3.83m O.D. and Sandown Bay at -4.44m O.D. At Freshwater, results indicated that the major approach direction for waves was between 180 degrees and 210 degrees with the highest waves typically up to 3m and a significant swell wave component (periods greater than 8 seconds). At Sandown, results indicated that the major approach direction for waves was between 150 degrees and 180 degrees with the highest waves typically up to 2.5m and little swell wave exposure. The lesser impact of the latter would be attributable to the shelter provided by Dunnose so that waves arriving from west of 180 degrees would be refracted and diffracted around the headland. 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 would have relatively little effect at either site. Wave energy was, however, found to be sensitive to sea-level rise with up to a 20% increase estimated for a high scenario of 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. Freshwater Bay was also found to be sensitive to an increase in Atlantic storminess with up to a 5% increase in energy likely according to the scenario tested. Sandown, by virtue of its more sheltered position, was not considered sensitive.
1.3 Human Influences
The character of this coastline also reflects the history of defence and protection, which in turn relates to land and resource development over the last 150. The west coast, with the exception of Freshwater Bay is, and always has been, largely devoid of protection structures. Thus cliff development has been unrestrained, and the beaches here are in adjustment to natural sediment supply from available sources e.g. (Photo 1. The western and central sectors of the south, or 'Undercliff', coast have also experienced little modification by human activity (Photo 2). This is equally the case for the southeastern coastline between Dunnose and Horse Ledge (Photo 3) and the Chalk promontory of Culver Cliff (Photo 4). By contrast, the remaining coastal frontage, especially between Ventnor (Photo 5) and Monk's Bay (Photo 6) and the east coast between Shanklin Chine (Photo 7) and Yaverland (Photo 8), is fully protected by a variety of structures. These include sea walls (Photo 9; Photo 10 and Photo 11), revetments and several groyne fields (Photo 7 and Photo 12) that have been subject to both renewal and extension for more than a century. The groyne system between Shanklin and Sandown (Photo 12 and Photo 13) has succeeded in retaining substantial quantities of sand, transported from south to north by the net direction of littoral drift. Along the undefended natural coastline, beach character exhibits rapid spatial changes, from sand through various grades of gravel to boulders.
There would appear to be a direct relationship between sediment released from sites of active cliff recession and landslide reactivation of the coastal slope and the character of beaches short distances downdrift. There are, however, uncertainties over the sources of supply to the 'pocket' beaches located in isolated bays and coves along the southern and south-eastern coastlines,e.g. (Photo 14 and a general lack of reliable information on offshore to nearshore and inshore sediment supply.
Much of the literature on the details of coastal processes is in the form of unpublished consultants' reports. There remain several sectors of the shoreline for which process information is scarce, largely because lack of commercial, residential and infrastructure development has not hitherto required formal shoreline management. (Halcrow, 1997; Brampton, et. al, 1998). The policies of the Isle of Wight Council are in favour of maintaining the natural environmental attributes of this coastline (McInnes et al, 1998).
2.1 Marine Input - F1 References Map
F1 Onshore Feed
The offshore to onshore supply of sediment by wave-induced or tidal currents may account for a proportion of beach stores at certain locations. However, knowledge of nearshore sediments and possible pathways of transfer to littoral transport is very limited and is largely a matter of conjecture (Brampton et al, 1998). It is known that parts of the shoreface between The Needles and St Catherine's Point are current-swept bedrock surfaces (Posford Duvivier and British Geological Survey, 1999), thus implying limited supply potential. Tidal currents achieve relatively high velocities of 1.5 to 2.1 ms-1, and flow sub-parallel to the coastline. They may effect scour around large boulder accumulations and gravel patches. Sand and sandy gravels occur as large lobate accumulations seawards of the inshore rock platform and reefs, especially south of Freshwater Bay and between Atherfield and Walpen Chine. This may represent a sediment sink that could supply some net onshore feed (Brampton et.al, 1998). However, echo-sounder survey data, commissioned by English Nature (1995, unpublished) did not reveal evidence of sediment mobility in these areas.
The descriptions of littoral drift by several authors (Posford Duvivier 1989a, 1990a and b; Barrett 1985; Kay 1969) also give indirect implications of onshore feeds of sand and gravel from offshore or nearshore stores. This remains highly speculative (Rendel, Palmer and Tritton, 1993, Halcrow, 1997; Brampton, et.al, 1998). Halcrow (1997) have postulated a possible offshore to onshore feed of medium sand to fine gravel, derived in part from relict fluvial sediment deposited by the now largely destroyed channel network of the West Yar. This might operate to supply beaches, but there is no substantive evidence to confirm this pathway. Brampton et.al (1998) also postulate net onshore movement of gravel south of Brook Bay, on the argument that cliff and shoreface erosion south of the Chalk outcrop provides insufficient coarse clastic debris for beach building. On the accompanying map offshore to onshore feed is provisionally shown, but awaits confirmation.
2.2 Fluvial Input FL1 References Map
It is possible that sediment is discharged as traction or bedload by the several chines that interrupt the continuity of the cliffline of the western and southeastern shorelines. Flint (1982) suggests that most of the debris transported by the chine streams of the west coast are in the fine sand and silt fractions, though her conclusion is not based on sampling. The bedload of some of the more deeply incised valleys is coarse, and derives from large ironstone doggers exposed in valley-side slopes. Whale and Brook Chines have boulder chokes at their mouths, indicating occasional high discharge and bedload-transport competence. However, the petrographic character of this material is an unlikely source for durable beach gravel. Nonetheless, local beaches may retain a small proportion of the sediment discharged by the chine streams. Human interference with discharge: load ratios may have affected this contribution in the last century or more, as Flint (1982) notes that Shepard's Chine, north of Atherfield Point, has cut down at least 15m since 1820 as a result of local diversion of drainage. Rendel Geotechnics and University of Portsmouth (1996) calculate that the West coast chines collectively transport 803 tonnes a-1 of suspended load and 259 tonnes a-1 of bedload. Of these quantities, only 82 tonnes a-1, is delivered to the coastal transport system; the remainder is diverted to various forms of channel storage, both naturally and artificially induced.
2.3 Coast Erosion E8 E9 E10 E11 E12 References Map
The entire length of this coastline is subject to active shoreface and inter-tidal marine erosion and cliff development, with the exception of sites such as Shanklin and Ventnor where sea walls and promenades have removed the former cliffline from the direct influence of wave-induced attack. Vertical shoreface erosion rates of between 3 and 10 mma-1 for the Undercliff, and 1.2 to 4 mma-1 along the coastline north of Shanklin Chine are estimated by Posford Duvivier and British Geological Survey (1999). Cliff morphology is spatially varied, and reflects differing combinations of factors, the main ones being:
Extrapolation of measurements of coastal recession for the past 150 years (e.g. Posford Duvivier, 1989a, 1999; Halcrow, 1997; Tomalin, 1977) supports the conclusion that there has been up to 6km of retreat of the western coast since the start of Holocene sea level recovery between 12,000 and 11,000 years BP This estimate can be applied with most confidence to those sectors where there are outcrops of comparatively weak, erodible sandstones, clays, marls and interbedded limestones. Along the Undercliff, erosion of the landslide toes has released residual aprons of basal boulder-sized material and created a series of small bays protected by the aprons, so that there has been some localised enhancement of cliff resistance to wave energy. This may also be the case for (i) the coastline from Luccombe south to Monk's Bay, where cliff erosion releases large inter-joint blocks that litter the inter-tidal zone (Photo 16); and (ii) the Chalk coast in the extreme eastern and western sectors. The cumulative effect of successive major and minor landslides in parts of the Undercliff has been to temporarily advance the position of parts of this coastline, but the distance of erosional retreat over the past 5 or 6 millennia is probably in the order of 2to 3kms. Since the mid Holocene wave erosion has been concentrated within a relatively narrow height-range, which has promoted the expansion of shoreline platforms at suitable locations. Their effect may have been to dissipate the potential assailing force of breaking waves and thus slowly decelerate coastline recession rates. This, and other, factors have not been quantified for this coastline, but they serve to qualify calculations of land loss based solely on contemporary rates of shoreline retreat (Halcrow, 1997).
Whatever the longer-term variations in rates of erosion may have been, substantial quantities of sediment have been released from cliff erosion. This must be a primary source of the sediment, mobilised principally by waves, which has contributed to beach stores. An additional input, from offshore, is also a possibility (Brampton et.al, 1998)
The Undercliff Coast: Chale to Dunnose
The entire Undercliff coast is fronted by landslip debris, mostly comprising massive multiple blocks of Chalk and Upper Greensand underlain by Gault clay and Lower Greensand Photo 2). The geological and geotechnical properties and morphological character of the area have been described in detail (Hutchinson, 1965; Hutchinson and Bromhead, 2002; Hutchinson, Chandler and Bromhead, 1981; Hutchinson, 1983, 1987; Lee, Moore and McInnes, 1998; Geomorphological Services Ltd, 1991; Hutchinson, Brunsden and Lee, 1991; 1994a; McInnes and Jakeways 2000; Rendel Geotechnics, 1994, 1995a, 1995b, 1996, 1997; Rust, 2002; Posford Duvivier, 1991, 1993c, 1994a), as part of a comprehensive study of the magnitude, frequency, socio-economic impacts and future management of the main components of this extensive landslide system (McInnes, 2000). The principal processes that have contributed, in varying relative proportions, to spatial variation in the scale and types of slope instability and coastal erosion are "run out" and rotational slips (slope and base failures); translational slides; rockfalls; mudflows and seepage erosion. Erosional cycles of unloading, steepening and debris accumulation, in the order of 100 to 150 years duration, affect both basal and higher slopes ( Rendel Geotechnics, 1995; Ibsen and Brunden, 1996; Bray and Hooke, 1997; Hutchinson and Bromhead, 2002). The contribution of direct wave energy impact at the cliff base to the initiation and acceleration of slope failures has yet to be precisely determined, but this factor undoubtedly accounts for some of the contrasts in the longer-term stability of different units of the Undercliff coast as a whole. Debris storage is influenced by the relatively narrow shoreface, with the 10m isobath approaching to within 200 m of the coastline in places. St Catherine's Deep, exceeding 60m in depth, is less than 2km offshore, and is bounded by landslip derived boulder-covered slopes from which fine material may have been removed by tidal currents. This 35 km long depression is controlled by geological structure, and may be eroded into a Gault Clay Outcrop. It may be, in part, an original palaeo valley partly deepened by tidal scour.
The "classic" component of the Undercliff coast, between St Catherine's Point and western Ventnor, is developed in the full succession of Lower Greensand, Gault Clay, Upper Greensand and Chalk. The regional dip is 1.5-2.5o to the south and south-south-east. A strongly defined 60-80m free face in the Chalk and Upper Greensand provides a backscar to the landslip complex, which is made up of a series of terrace-like features. These are irregular in width and relative height (70-110m) although most are elongate in plan. Each terrace is a large rock mass that has moved seawards across a deep-seated semi-rotational failure plane. Multiple rotational slides dominate the upper of the Undercliff, with several back-tilted blocks of Upper Greensand in positions of longer term but temporary stability. A zone of similar width occupies the lower Undercliff, consisting of a sequence of compound slides; some of these give rise to ridge-like forms defining small grabens (areas of relative subsidence). The failure planes are located within both the Gault Clay and clay-rich horizons in the underlying Sandrock, and have different configurations for various parts of the Undercliff (Geomorphological Services Ltd, 1991; Rendel Geotechnics, 1995a, b). Geotechnical explanation of failure principals involves the contrasting hydrogeology of the Upper Greensand-Gault Clay-Carstone-Sandrock sequence (Hutchinson, 1991; Rust, 2002).
A relatively continuous steep scarp slope, up to 20m in height, defines the Undercliff terrace west of Ventnor. It is underlain by Gault Clay and is the site of occasional mudslides. Degraded mudslides, also in Gault Clay, are located immediately above cliffs cut into the Lower Greensand, e.g. at Binnel Point. The Gault Clay scarp is regarded as a major influence on the mechanisms of slope failure within the Undercliff as a whole. Its intermittent unloading, and retreat, has triggered rotational failures that, together with slope movements on the lower Undercliff, have contributed large aprons of landlside debris. During pre-Holocene times, these debris stores would have provided basal loading, and in the early stages of Holocene sea-level rise, natural protection against erosion. Progressive removal of this apron of material resulted in slope instability (Photo 24 and Photo 25), either creating new failure planes or re-activating old ones. Curved backscars in the lower landslide units appear to have been a direct response, thus isolating a series of broad, roughly triangular, spurs. These have also failed as a result of unloading on either side (Rendel Geotechnics, 1995a). An inverse relationship may exist between the degree of preservation of debris aprons and coastal slope sensitivity to shoreline conditions.
In general terms, the types and scales of past and current slope instability suggest a two tier cascading system involving a combination of different failure mechanisms at different elevations. The lower elevation ("Zone 1") failures are principally translational, and are controlled by clay horizons in the Sandrock (Lower Greensand). Above these, "Zone II" failures are of the multiple rotational type, related to slip surfaces in the Gault Clay. Characteristic morphology in Zone I is of one or more sub-parallel linear ridges, whilst relatively narrow terrace-like flats separated by well defined scarps are diagnostic of Zone II. In places, such as the vicinity of St Lawrence, debris aprons partly overlie ridges, both of which consist of landslide debris in slow downslope transit. At the extreme eastern end of the complex, mudslides and recent landslides replace the Zone I landform assemblage as a consequence of recent, and continuing, active slope movements.
The evolution of this spatially variable geomorphic pattern is related to the inter-relations between progressive removal of formerly larger and more extensive landslide debris stores as sea-level rose and the retreat of the coastline. Deep-seated failure planes developed in Zone I in response, creating several isolated triangular shaped spurs. Their defining slopes subsequently failed, due to erosional unloading, thus initiating the Zone II backscars. For all parts of the unprotected Undercliff frontage, marine erosion is therefore a critical control of both slope instability and sediment supply to the littoral transport system.
E8 Rocken End to Castle Cove (see introduction to coastal erosion)
The material composing the sea cliffs is made up largely of landslide debris, dominated by boulder-sized blocks of Upper Greensand (Photo 14 and Photo 24). The debris apron is at its widest either side of St. Catherine's Point, where borehole data indicates that its seaward portion abuts against a probable buried marine cliff at -7m OD (Hutchinson, 1987; Hutchinson, Bromhead and Chandler, 1991; 2002) which has been tentatively dated at between 7 and 9,000 years BP. After the creation of this cliff line, there was a period of active debris sliding, followed by an interlude of stability represented by tufaceous deposits, and renewed slope wash and intermittent sliding between approximately 5,000-4,500 years BP (Hutchinson, 1987). The series of thick, confluent and possibly superimposed debris aprons provide toe weighting that accounts, in part, for the stability of parts of the Undercliff complex eastwards of Reeth Bay. The seawards extension of these debris aprons is unknown, but there are narrow shore platforms cut across landslide debris at headlands such as Binnel and Woody Points that demonstrate marine abrasion and semi-continuous cliff recession in the period since 4,500 years BP. A recession rate of 0.4ma-1 has been estimated for the Gault clay cliffs at Castle Cove (Barrett, 1985); this is also quoted for the cliffed coast between St Catherine's Point and Steephill Cove, with a total sediment yield of 115,000 m3a-1 mostly sand and clay, but with some sandstone boulders, chert, and flints that contribute to the local beaches (Posford Duvivier, 1997). Some of the beach material in the small pocket bays trapped between large blockslides does not appear to be of immediately local origin.
Comprehensive coastal protection schemes at critical locations have reduced the potential for toe erosion and slope failure reactivation. For example, the Castle Cove scheme (Photo 29), completed in 1996, has involved slope regrading, provision of a debris accumulation area, drainage of mudslide sources, a toe loading and wave energy dampening rock armoured revetment and terminal rock breakwaters (HR Wallingford, 1996; Rendel Geotechnics, 1996; 1997; Lee, et al, 1998). Western cliffs at Ventnor have been protected by a rock revetment constructed at the cliff toe in 1992 (Photo 30). The A3055 between St Lawrence and Niton has been realigned over a 300m section following a major landslide re-activation during the winter of 2000/01. The reactivation followed mudslides and failures lower in the undercliffs (Photo 31) and resulted in a period of road closure (Photo 32). Work began in Autumn upon a stabilisation scheme at Castlehaven, Niton involving application of novel ground drainage measures. The problem involved a sharp acceleration, since January 2004, in the rate of landward reactivation of relic landslides, but remedial work also needed to consider sensitive environmental qualities in developing an appropriate scheme. This particular site, together with many of the others identified above are explained in greater detail by McInnes and Jakeways (2000).
E9 Ventnor to Monks Bay (see introduction to coastal erosion)
This coastline is developed in a complex of multiple semi-rotational slips, (Geomorphological Services Ltd, 1991; Rendel Geotechnics, 1995; Hutchinson, 1987; 1991; Hutchinson, et al, 1991. Because of development since the early Victorian period along and immediately inland of much of this frontage, there have been successively more robust and comprehensive schemes to protect the cliff toe (Photo 9 and Photo 10) as documented by McInnes and Jakeways (2000). Between 1896 and 1974 there was approximately 12.2m of shoreline retreat, releasing somewhat over 500,000 metric tonnes of sediment into the nearshore transport zone (South Wight Borough Council, 1985). Protection measures introduced between 1992 and 2000 have effectively reduced toe recession rates to zero e.g. Photo 6, though cliff-face weathering and shoreface abrasion continue (Posford Duvivier, 1999). Shallow translational slides have been recorded for a number of locations either without protection or where defences have become dilapidated. The role of beach accumulation as a natural defence measure has been debated, and it remains uncertain what proportion derives from local cliff erosion. The renewal of slope instability at the site of Collins Point following beach and debris removal to supply material for the construction of a harbour in 1863 indicates interdependence (Royal Commission on Coast Erosion, 1911). Previously Collins Point had retained a beach of fine shingle supplied by littoral drift, but erosion of the Gault Clay outcrop across the foreshore by wave action resulted in undermining of the newly built seawall. The result of an equiry was the building of a rock masonry groyne to substitute the former role of Collins Point headland (Posford Duvivier, 1994b). This case example would seem to endorse the view that local beaches, in spite of their relatively small width and volume, are important in regulating rates of coastal slope erosion, as well as adding to the loading of cliff toes.
Another environmental factor that may play a role has been discussed by Hutchinson (1983) and Hutchinson, et al (1991) with specific reference to the Mirables Undercliff, east of St Catherine's Point. Referring to the zone dominated by mudslides derived from the Gault Clay outcrop, they point to the fact that the critical Gault/Carstone contact is located 1m to 2m above OD, with mean high water spring tides at +1.5m OD and mean low water springs at -1.6m OD. Thus clay outcrops exposed above normal beach levels lie within this tidal range and are subject to the direct mechanical and hydraulic effects of breaking waves. A wave height of 15m once in 50 years has been predicted for this area by numerical modelling (Hydraulics Research, 1990, 1991). Toe erosion may consequently stimulate mudslide activity, the rate of which would be primarily dependent on the efficiency of debris removal. Hutchinson (1983, 1987 and 1991) developed this explanation through comparative analysis of several landslide sites and it may have a relevance to other locations. A factor not taken into account, and which also relates to other sectors of the Undercliff coastline, is the presence of numerous boulders and inter-joint blocks of Upper Greensand scattered across the inter-tidal platform. Derived from previous, more extensive but, now eroded, debris aprons - or possibly from single run-out falls or slides - they constitute a type of nearshore reef that must absorb some incident wave energy. Chandler and Hutchinson (1984) report, using borehole log evidence, that the base of slide-generated aprons extends to approximately -20m OD and rest on an erosional surface cut into the Sandrock. The substantial recession of this cliffline during Holocene times is thus implied, with several alternating cycles of stability and active landsliding. Chandler and Hutchinson (1984) use a radiocarbon date from preserved organic sediments in west Ventnor to indicate general stability between 4,000 and 4,500 BP, with renewed activity between 4,000 and 3,500 BP. A similar chronology is proposed by Preece et al (1995) with reference to colluvial infill in a dry valley near Ventnor.
The rates of erosion reported for this coastline all refer to the situation predating the installation of localised seawall, groyne and breakwater protection systems. They vary from 0.15ma-1 for the Bonchurch frontage, 0.3ma-1 between Steephill and St. Catherine's to less than 0.05ma-1 in the Upper Greensand boulder-dominated debris aprons immediately west of the Western Esplanade, Ventnor (Posford Duvivier, 1991, 1993c: McInnes, 1994). Rendel Geotechnics (1993) suggest 0.1ma-1 for Woody Bay but with considerable temporal variation affecting both the cliff base and cliff top. A cyclical behaviour of alternating periods of rapid and comparatively slow erosion is tentatively proposed. In the early 1990s a massive, stepped sea wall was built along the length of Wheeler's Bay to replace pre-existing sloping timber defences (Photo 9 and Photo 10). The latter, completed in 1973, proved ineffective in trapping sufficient beach shingle and was breached on several occasions by wave action. The seawall was strengthened in 2000 by the addition of a seaward buttress, soil nailing, drainage and grass matting of the rear slopes (Posford Duvivier and Malcolm Woodruff, 1998; McInnes and Jakeways, 2000). Other sea walls, at Bonchurch Eastern and Western Parades, have recently been rebuilt, reinforced and made more resistant to wave assailing forces, so that release of sediment by erosion is now restricted to wave scour of the foreshore platform (Posford Duvivier, 1999). The toe loading effect of these structures should also enhance the stability of the former sea cliffs. Monk's Bay at Bonchurch was formerly protected by a partial sea wall and concrete groynes; Posford Duvivier (1990b) calculated an erosion rate here of 0.2 to 0.3ma-1, with up to 15m of cliff recession since 1970 where the sea wall had virtually collapsed. During the winter of 1989/90, a semi-rotational landslip released 100,000m3 of material, but slope stability has been improved since the completion in 1992 of a comprehensive protection scheme incorporating offshore rock breakwaters, 30,000 tonnes of shingle beach replenishment, slope reprofiling and drainage (Posford Duvivier, 1992). Consequently, sediment input from this source is now likely to have been substantially reduced (Photo 6).
E10 Dunnose to Shanklin Chine (see introduction to coastal erosion)
At Dunnose, there is a sharp change in coastal orientation, but cliff height and form is initially similar to the immediate west of Ventnor. The 40m high cliffs in the south part of this sector are cut into landslide debris, but more into the Gault Clay and Lower Greensand further north. South of Luccombe Chine there is evidence for marine erosion of the cliff base, although translational slides and mudflows are frequent and often temporarily conceal bedrock. Terrace recession has exposed mudslide and landslide material within the Gault Clay, resulting in some reactivation of cliff top retreat. Mudslides move across successively lower benches, where they are contained as temporary stores. Cliff profiles assume a more degraded form where there are substantial accumulations of boulders across the foreshore. These derive from fresh falls, slides and toppling failures and the removal of less resistant clays and sands within landslip debris aprons created by previous major landslips. The cliffs north of Luccombe Chine assume a more complex composite form with one or more distinct benches. The area directly inland of Luccombe Chine has a well-documented history of part translational and part rotational slope failure (Geomorphological Services, Ltd., 1989). The re-exposure of failure planes in both the Gault Clay and consolidated landslip material by toe erosion may have been a trigger to major landslide events in 1810, 1820, 1988 and 1995 although groundwater conditions - notably critical pore water pressures - are important. The latter event displaced approximately 800,000m2 of rock debris (Rendel Geotechnics, 1995b). Despite a wide inter-tidal sandy beach (over 100m at maximum spring tides) at the mouth of Luccombe Chine, basal cliff trimming and notching by waves is an active process. Debris loading of the benches by landslip debris from above is associated with groundwater seepage at the junctions between interbedded sandstones and clays. Geomorphological Services, Ltd., (1989), calculates that breaking waves of a probable maximum height of 3m could be propogated across an easterly fetch of up to 170km, and might generate rip currents of sufficient velocity to rapidly remove the fine-grained fraction of cliff fall debris and beach sediments.
There are no estimates of rates of erosion in this coastal sector based on experimental data; the few figures quoted in the literature rely upon analysis of the position of the cliff top on successive editions of Ordnance Survey maps, the accuracy of some being in doubt. Barrett (1985) and Halcrow (1997) quote a rate of recession of 0.2 to 0.3ma-1 at Dunnose, Posford Duvivier (1981) calculated a retreat rate of 0.3ma-1 and Posford Duvivier (1987) suggest to 0.5ma-1 for this length of shoreline. This may not take full account of the loss of temporary stores of landslip material, and it undoubtedly generalises significant spatial variation, e.g. a rate of 1.0 to 2.5ma-1 in the vicinity of Borderwood Lodge (Halcrow, 1997). A mean recession rate of 0.4ma-1 gives a total potential sediment yield of 75,000m3a-1 (Posford Duvivier, 1999). At Luccombe, this latter figure may be a close approximation to the recession rate maintained over the past 140-150 years (Posford Duvivier, 1990b), but is probably exceeded along the shoreline between Yellow and Horse Ledges. McInnes (1994) proposes cliff retreat at a rate of 0.2 to 0.3ma-1 between Appley Steps and Steed Bay. North of Horse Ledge, cliff foot erosion is inhibited to some extent by partly redundant groynes, and the scree that has accumulated in front of Knock Cliff and Apsley Steps may indicate that small scale but frequent rockfalls and toppling failures, due to weathering and stress relief, are now more significant than basal notching by waves. The general morphology of the near-vertical cliff line between Yellow Ledge and Shanklin Chine indicates the longer term dominance of block failure and bench formation associated with aquicludes. Here, and also at Luccombe (Geomorphological Services, Ltd., 1989; Moore, et al, 1991), the rate of cliff top recession, at 0.3ma-1, appears to have accelerated over the past century, inducing failure reactivation on several occasions (Posford Duvivier, 1990).
E11 Shanklin to Yaverland (see introduction to coastal erosion)
Almost the entire frontage is made up of substantial sea walls and groynes, so that the former cliffs are no longer subject to marine erosion and cannot make a contribution to sediment supply. The history of protection can be traced back to the 1830s and is fully documented (Lewis and Duvivier, 1974; Posford Duvivier, 1981, 1989a, 1993a). All but one of the sea wall sections were finally joined together in 1934. Barrett (1985) quotes a figure of 0.2 to 0.4ma-1 retreat for the Littlestairs cliffs prior to their protection in the mid-1970s. Assuming that there has been little subsequent change in cliff height, this rate would yield a former supply of about 3,000m3 of sand per annum. The effect of this loss of sediment feed could be reflected in the estimate of foreshore recession of 0.3ma-1 between 1960 and 1970 (Barrett, 1985). This figure appears to have been derived from surveys of the Lower Greensand shore platform exposed seawards of Lake at low spring tides. It is part of a feature that is now fully exposed in front of the sandy beach around much of the perimeter of Sandown Bay, possibly pointing to long term reduction of beach sediment storage. Although isolated from wave activity, the former 40m. high sea cliffs remain geomorphologically active, due to sub-aerial weathering and mass movement (Rendel, Palmer and Tritton, 1988). Barton (1985, 1991) estimates 5m of slope crest recession between 1907 and 1980 (0.06ma-1), for part of the cliffline between Shanklin Chine and Hope Road, a figure derived from measurement of the dimensions of basal scree deposits. Various protection techniques including cliff-top regrading, drainage, timber shuttering, geofabric/grass matting, netting, rock bolting and talus reprofiling and removal have been implemented to manage this problem (Clark et al, 1993; Rendel Geotechnics, 1991b; 1992; McInnes, 2000) over a 3.5km length at Shanklin. This has not been entirely successful with several small free face detachments and a major talus slope failure in March 2001.
E12 Yaverland to Culver Cliff (see introduction to coastal erosion)
Immediately north-east of Yaverland the seawall terminates and there is no northwards protection against marine erosion. The outcropping strata are lithologically varied but collectively unresistant, excepting the Chalk. Shallow translational slides and mudflows are characteristic of the Wealden shales and clays, and give an irregular low, cliff profile that frequently exhibits basal notching. The Ferruginous Sandstone of Red Cliff is comparatively more coherent and supports a near vertical lower cliff face. Multiple translational sliding and mudslide surging in the Atherfield Clay has created a 130m. wide degradation zone that has been semi-continuously active since at least 1910 (Hutchinson 1965) whilst both deeper seated and superficial mass movement affect the Gault Clay outcrop, giving a retreat rate of 0.5ma-1 (Halcrow, 1997). The rate of erosion in the Wealden Marls at Yaverland was calculated to be 60m between 1910 and 1945 (Lewis and Duvivier, 1973), with annual rates varying between 0.5 and 2ma-1, depending on beach width. Foreshore recession of 0.3ma-1, 1896-1969 is indicated from Ordnance Survey maps (Posford Duvivier, 1981). A figure of 0.35ma-1, for the coastline south of the Chalk outcrop, is contained in an analysis of all historical sources (Posford Duvivier, 1997; Posford Duvivier and British Geological Survey, 1999). Lithological changes impose spatial variation of recession rates of change between 0.2 and 0.8 ma-1. The foundations of early nineteenth century buildings at Yaverland Fort, now exposed on the foreshore, indicate 0.5km of cliffline retreat, and on the Gault Clay outcrop, between 1870 and 1980 (Barrett, 1985). Repeated semi-rotational slides, and their rapid removal by wave action, have resulted in as much as 20m of cliff top retreat in less than one year at specific sites (Barrett, 1985) with instability evident up to 70m inland. Hutchinson (1965) reported that the remains of a seawall built in 1924 to protect Yaverland Castle was 80m seawards of the cliffline in 1964. There are no reliable records of shoreline change along the south facing Chalk cliffline seawards to Culver Cliff, but basal accumulation of boulders, shallow slides, rockfalls and talus cones, indicates the contemporary, as well as long-term, effectiveness of both marine and sub-aerial denudation. Posford Duvivier (1999) propose an overall recession rate of 0.23ma-1, but it is not clear how this figure was calculated. McInnes (1994) calculates retreat at about 0.1ma-1. Active erosion is evidenced by caves, incipient stack formation and a well-defined shore platform at Whitecliff Ledges northwards to the blunt headland defining Whitecliff Bay. Taking average cliff height and the above quoted erosion rate, Posford Duvivier (1997) suggest that the Chalk Cliffs yield some 30,000m3a-1, of which about 2% (<500 m3 )consists of flint.
In addition to sediment losses from cliff erosion, abrasion and scour of the intertidal shoreface also contributes input to the littoral transport system. A range of figures are presented in Posford Duvivier (1999), all of them derived from the application of a basic methodology to local conditions of shoreface width, water depth and rock erodibility. Rates vary from 2 to 35mma-1 of vertical corrasion, yielding from less than 500 to over 24,000m3a-1 of sediment. Quantities are largest where sandy or clayey rocks form the shoreface substrate on the West and South coasts. It is presumed that almost all of this sediment is fine grained and this therefore not retained local beaches.
3. LITTORAL TRANSPORT LT3 LT4 LT5 References Map
The literature covering beach sediment characteristics and littoral transport consists largely of consultants' reports on conditions observed and/or inferred during brief field visits. For small sectors there have been irregular programmes of beach profile surveys, but overall there is relatively little data of a systematic and quantitative nature on which to draw. It is assumed that sediment transport is principally wave-driven, as tidal current velocities are everywhere low (0.3-0.5ms-1). A large proportion of the fine sediment released by erosion of sandstone, clay and marl rock units is not retained by local beaches, and ultimately moves offshore, in suspension (Posford Duvivier 1999; Brampton, et al, 1998). As a general trend, beaches consist of a gravel backshore and sandy foreshore, and progressively steepen between Freshwater Bay and Rocken End. The gravel component becomes more dominant in this direction, although the median grain size of coarse clastic material gets smaller in a south-eastwards direction
LT3 Rocken End to Dunnose (see introduction to littoral transport)
Rocken End, made up of landslip debris, retains a beach composed of fine gravel described as "pea" gravel (Barrett, 1985; Posford Duvivier, 1981, Rendel, Palmer and Tritton, 1993; Halcrow, 1997). The latter is characterised by well sortedwell-sorted, sub-angular to sub-rounded flint clasts of a mean diameter of 10mm (range of 6 to 15mm). It is usually highly polished, indicating abrasion. The transition from coarse to fine gravel between Walpen Chine and Rocken End is striking, but defies ready explanation. Narrow beach widths may be due to the updrift interception of littoral drift. Between St. Catherine's Point and Ventnor there are several well-defined pocket beaches of similar "pea" gravel (Photo 5, Photo 14 and Photo 24) but there is no convincing evidence that they are supplied by material by-passing St. Catherine's Point. Barrett (1985) has observed that these beaches are adjusted to incident wave approach and exhibit weak west to east littoral drift. Brampton et al (1998) suggest that some 45% of nearshore waves approach from the west or south-west, and 28% from east or south-east. There appears to be little exchange between adjacent bays, but by-passing may take place when there are oblique south-westerly long period waves. Some beaches, particularly at the eastern end of this coastline, have been subject to draw down, indicating that potential rates of transport exceed available supply. Tidal currents may play a minor role in moving finer grained material, particularly in the vicinity of St. Catherine's Point where velocities are at a maximum. Posford Duvivier (1989a) record that attempts in the 1970s to accrete beach material along Ventnor's Eastern Esplanade (Photo 5), using groynes, were unsuccessful. In this case, however, wave abrasion of the concrete seawall was as important a cause of this failure as the difficulty of retaining a permanent beach of sufficient height and volume to absorb wave energy. Groynes and breakwaters elsewhere, e.g. Swale and Linnington groynes at the Western Esplanade, indicate net west-to-east littoral transport, but the ultimate source of the "pea" gravel that they retain is uncertain. Posford Duvivier (1981, 1989a) infer that it is derived from Chale Bay, but in view of the evidence of negligible lateral transfer from one bay, or cove, to the next (ie. a sequence of small independent sub-cells) a pathway of shore parallel movement that bypasses Rocken End and then moves onshore further east is conceivable (F1) (Rendel, Tritton and Palmer, 1993; Brampton et al, 1998). Nearshore sediment sampling has revealed the presence of sandwaves, between 400 and 900m. offshore; sand patches and ribbons are also present, suggesting inshore movement of sand between more stable gravel deposits. Net transport of sand offshore appears to be west to east (Rendel, Palmer and Tritton, 1993). The few beaches that have been monitored show cyclical "cut" and "fill" during winter and summer months respectively, with some ambiguous evidence at Western Esplanade for longer term stability of form over the past 50 years (Posford Duvivier, 1989a; Posford Duvivier Enviroment, 2002). The role of St. Catherine's Deep (over 60m deep in places), less than 2km offshore, as a sediment sink for material moving towards St. Catherine's Point has not been investigated. The origin of this striking submarine depression is enigmatic.
In some of the bays, e.g. Reeth Bay (Photo 25), sand is co-dominant with gravel, and elsewhere coarse sand patches alternate with fine gravel under varying wave conditions. It is therefore probable that sand provides a foundation for a comparatively thin veneer of gravel, which is apparently quite mobile (Barrett 1985). Norman (1887) described a beach "near Ventnor" as "covered with ... sand ... twenty years ago", and implied that there was no gravel along the entire southern coastline before the 1840s. Boulder- sized Upper Greensand blocks constitute the beach frontage wherever sand or gravel is absent, having been released by the weathering and erosion of landslide debris. The sand fraction derives from the same source. Littoral sediment volumes are temporarily augmented by sudden inputs from slides or slumps, although the only documented example is the rapid break-up and disruption of a rockfall or slide cone at Rocken End in 1825 (Barrett, 1985). Immediately east of Ventnor, particularly between Bonchurch and Dunnose, beach sediments wholly derive from landslip debris and include coarse gravel from the flint and chert content of fallen blocks of Chalk and Upper Greensand respectively. Coastal protection schemes throughout this century have progressively reduced inputs from cliff erosion, so that offshore to onshore transport may be an intermittent source of supply to beaches. Nourishment has also been undertaken since the mid-1970's e.g. Monk's Bay (Photo 6). Between Western Esplanade and Wheelers Bay the completion of massive seawall protection in 1988 (Photo 9 and Photo 10) now ensures no contemporary contribution from basal cliff erosion. Severe downdrift reduction in sediment supply (as evidenced by the undermining of groynes) may have been a cause of accelerated erosion in Monk's Bay between 1950 and 1990 (Posford Duvivier, 1981). The lack of any eastwards transport of "pea" gravel beyond Ventnor Bay has attracted some comment (e.g. Posford Duvivier, 1981), but the reasons for this are obscure. It does, however, suggest that the littoral transport system is compartmentalised, perhaps due to complex patterns of near and offshore submarine relief related to submerged landslide debris.
LT4 Dunnose to Shanklin (see introduction to littoral transport)
There are few defences along this coastline, and a very limited literature on which to draw. On the assumption that active cliff erosion is the chief source of sand contributing to the beaches in Sandown Bay, net littoral drift is presumed to be from south to north. Direct field evidence to corroborate this deduction is lacking, except at three degraded groynes to the north of Luccombe Bay. Much of the backshore and inter-tidal zone, which includes rock-cut platforms, is littered with large Upper Greensand and Lower Greensand sandstone boulders. The main exception is Luccombe Bay, where a wide, fine-grained sandy foreshore has accumulated. There are, however, distinct pockets of coarse, angular chert and flint particles at the cliff base between Dunnose and Luccombe confined between salients of both bedrock and residual landslide debris lobes (Photo 3 and Photo 16). Between Yellow Lodge and Shanklin Chine this coarse material becomes a more defined backshore berm. Both Yellow and Horse Ledges intercept the littoral transport of sand, but are comparatively easily by-passed if the rapid extension in the width of sandy foreshores immediately north of both of these features are reliable indicators. The principal features of the beach and foreshore of this sector have not changed appreciably in recent decades if the description by Colenutt (1938) is to be relied upon.
LT5 Shanklin Chine to Culver Cliff (see introduction to littoral transport)
With the exception of the zone immediately east (downdrift) of Yaverland, this beach changes eastwards from patchy gravel across a sandy foundation to homogeneous gravel. There are clearly defined offsets in beach width associated with the numerous groynes, which indicate that the dominant longshore transport is from south to north (Photo 7, Photo 12 and Photo 13). With the change in coastline orientation east of Yaverland, and the effect of Culver Cliff on wave refraction, an east to west counter-drift may operate at times when waves from an easterly or south-easterly fetch prevail. The gravel beach between Red Cliff and Culver (Photo 37 and Photo 39) is field evidence in support of this, although an offshore supply source, or a possible barrier origin, cannot be discounted. Occasional severe draw down of the sandy beaches at Shanklin and Sandown has revealed a shingle basement (Lewis and Duvivier, 1973), most probably derived from transport from the south.
The whole of this frontage has been subject to intensive protection, with successively more comprehensive measures incorporating seawalls and groynes installed since the 1860s. Unusually complete and detailed records of shoreline management have survived, and are summarised in several consultants' reports (Lewis and Duvivier, 1973, 1981; Posford Duvivier, 1989a, 1990b; 1993b;1995, Halcrow, 1997). These documents record the length, height and date of construction of all the major groynes and provide some measurements of volumes of sand retained updrift. As an example, Herne Hill groyne at Sandown, established in 1860 and rebuilt in 1893, accumulated sufficient sand to promote a 12m seaward advance of the mean High Water Mark and provide the foundation for the forward building of the Esplanade between the 1890s and 1930s (Photo 40). Small Hope groyne, at Shanklin, built in 1860 and rebuilt in 1901, created a downdrift offset of 60m and a "step" of 15m (Photo 7). South of Shanklin Chine there has been some 12m of seaward migration of Mean High Water since 1896. There have been many instances where inter-groyne sectors of the beach have suffered depletion e.g. Photo 34 and where Mean High Water has advanced landwards and the beach has steepened in response (Photo 8, Photo 41 and Photo 42); in one case, in northern Sandown, the rate of movement was approximately 30m in as many years in the period 1920-1960 (Posford Duvivier, 1989a). The negative downdrift effects of groyne construction between 1890 and 1950 is a classic example of an effect experienced in numerous other locations on the south coast of England (Barrett, 1985, Halcrow, 1997, McInnes, 1994). Because of the arcuate shape of Sandown Bay, the rate of littoral transport diminishes northwards in response to a reduction in the obliquity of angle of wave front approach. Volumes of littoral drift also diminish along this unit, partly or substantially because of increasing distance from supply sources. The long-term problems of retaining a wide and stable beach have therefore been greater in the northern part of this sector. It is probable that volumes of sediment moving downdrift also decrease in the same direction as a consequence of storage in groyne bays. Supply deficit is also a consequence of the removal of sediment supply from cliff erosion as a direct result of seawall/esplanade construction. With the exception of the Littlestairs section (up to 1974), all of thisthis entire coastline has been "walled up" since the late 1940s. The only sources of natural replenishment now available are from the erosion of the sandstone cliffs south of Appley Steps, and - possibly - from offshore. The latter has not been investigated in any quantitative sense, but one unpublished survey (quoted by Lewis and Duvivier, 1973) describes the seabed below the 30m isobath as "mostly shingle", with sand dominant across the seabed to an approximate depth, at low water, of "7 to 8 fathoms". It is arguable whether refracted swell waves would disturb this material, and tidal velocities are inadequate to account for significant offshore to onshore transport. If there is a tidal component in the sediment budget of Sandown Bay, it is probably represented by net offshore movement of suspended sediment (Dyer, 1972, 1980, 1985; Halcrow, 1997; Posford Duvivier, 1999).
Surveys and observations carried out in the 1980s (Posford Duvivier 1989a) appear to indicate that some of the inter-groyne beaches have stabilised and achieved an equilibrium condition adjusted to the current volumes of input and output. In others, the absence of a permanent backshore shingle berm has promoted wave reflection from seawalls, and therefore beach drawdown (Photo 11, Photo 41 and Photo 42). Some complications to sediment grading resulted from beach cleansing operations during the 1980s and 1990s, but the practice of removing coarse material has now been discontinued.
Littoral drift at the northern end of this sector may have contributed to the transport pathway north of Culver Cliff, before its emergence during the Holocene transgression as a barrier to movement. Dyer (1985) notes that the mineralogy of Ryde Sands bears some comparison to that of the Lower Greensand in Sandown Bay. However, erosional scour of rock outcrops over outer areas of the bay may be a supply source, and this may continue to operate.
4. SEDIMENT OUTPUTS
4.1 Offshore Transport O2 References Map
02 Sandown Bay
A net offshore transfer of suspended sediment has been postulated, but not demonstrated (Posford Duvivier, 1999; Dyer, 1980). Water depths in Sandown Bay do not exceed 20m, with the 10m isobath between 1 and 2 km seaward of the position of Mean Low Water. The possibility that it represents a local sink for fine to medium sand moving both off and alongshore (from the south) remains a possibility, but is very uncertain.
4.2 Beach Mining Freshwater Bay Collins Point References Map
There are three sites for which there are historical records of the removal of beach sediments for construction materials, as follows:
Colenutt (1904) provides a brief account of an apparently long history of unregulated removal of gravel and states that: "many hundreds of tons of shingle have from time to time been carried away." It was used for concrete in the building of local forts and batteries and the seawall/esplanade in the late nineteenth century. It was the view of Colenutt (and others) that this activity was the principal cause of accelerated beach and cliff erosion in the first decade of this century; he records 0.8m of cliff recession in the western part of the bay during the winter of 1903/4 and the destruction of the esplanade by "damaging gales" over several successive winters. It is, of course, very possible that the construction of the esplanade promoted onshore to offshore drawdown of the beach due to reflective waves. Colenutt also reports an absence of sediment accumulation "below high water" in 1902. There is no record of when this activity ceased, but Colenutt's paper proved to be an was an effective articulation of local concern and it is probable that gravel removal had ceased by 1910 or thereabouts.
The removal of all beach material for concrete aggregate used to construct harbour breakwaters occurred in the mid 1860s. The acceleration of cliff instability was an almost immediate consequence and caused considerable public concern and resulting official remedial action in the mid 1870s.
Sandown and Shanklin
Lewis and Duvivier (1974) refer to selective "pebble" removal from Sandown and Shanklin beaches for amenity purposes. It is uncertain when this commenced but it presumably refers to the removal of gravel patches overlying sand. It is, however, known to have been discontinued in the mid 1990s. Patches of gravels, spread as a thin veneer, do occasionally accumulate on the mid-beach area of Sandown Bay after winter storms but appear to be fairly rapidly removed either offshore or alongshore.
4.3 Dredging Pot Bank South-East and South of Dunnose References Map
As elsewhere along the south coast of England, there has been speculation that offshore aggregate dredging might be an independent cause of beach depletion. Reliable data on offshore to onshore transport is too limited to either confirm or deny this possibility, despite some strong assertions. Marine aggregate removal has taken place for more than 30 years in a number of licensed blocks off the western, south-western, south-eastern and eastern coastlines. Combined extraction rates increased from 1,143,750m3 in 1982 to 1,500,000m3 in 1987. An unusual feature is that the Isle of Wight Council has licensing rights purchased from the Crown Estate Commissioners in 1949 by a predecessor authority. The consensus view (Hydraulics Research, 1984, HR Wallingford, 1992) is that coarse sediment in water depths exceeding 15m is effectively immobile, and thus does not contribute to the littoral sediment budget.
The permitted dredged area is slightly more than 754,000m2, and is centred about 1100m due south of the western extremity of the Bridge, a submarine ledge extending westwards of the Needles. Dredging commenced in the late 1940s and between 1950 and 1980 a total of approximately 5,062,500m3 of gravel was removed. During the 1980s and early 1990s quantities have been much smaller, amounting to less than 315,000m3 between 1982 and 1987. Licenced extraction was suspended in 1994. A detailed analysis of seabed configuration between 1950 and 1970 using hydrographic chart data (Geodata Unit, 1989) revealed fairly modest seabed changes and concluded that some gravel may be moving into the area to partially replenish that lost to dredging. As Pot Bank is 7km from Freshwater Bay, this is an unlikely of supply pathway and its small size and limited height even before dredging would have a limited impact on wave refraction patterns. Moreover seabed changes during the period of most active dredging were no greater than those recorded on pre-dredging hydrographic charts between 1937 and 1951. Dyer (1985) indicates, without direct reference to the supporting evidence, an offshore movement of sand and shingle from south-east to north-west some several kilometres seaward of the west Wight coastline. This may be a source of partial replenishment of Pot Bank. The general conclusion that may be drawn is that Pot Bank has not been a source of supply to shingle beaches along the coast between Freshwater and Brook Bay. Thus, the impact of dredging may be provisionally discounted on the basis of circumstantial data from this (and adjacent) areas. This is supported by a closely reasoned case using data relating to tidal streams, wave refraction patterns and seabed levels (Geodata Unit, 1989). The same conclusion, based on work by Hydraulics Research (1977) on dredging of the Shingles Bank and Dolphin Bank, as well as Pot Bank, is reported in the section on Christchurch Bay.
South-East and South of Dunnose
Hydraulics Research (1977b, 1984, 1987) investigated the effects of proposed dredging for gravel at a site approximately 6 kms due east of Dunnose, at a mean water depth of 20m. It was concluded that gravel up to a median diameter of 25mm may, exceptionally, be mobilised in water depths of up to 22m, but is normally immobile below a depth of 15m. The prevailing wave climate would be incapable of moving gravel from that distance and at that depth onto the beaches of Sandown Bay. Based on wind fetch values, it was calculated that maximum wave heights vary from 4.63m (90 degrees ) to 7.41m (240 degrees).
5. SUMMARY - References Map
The entire southern coast of the Isle of Wight is subject to erosion, although its intensity varies due to wave exposure and its cliff morphology and behaviour varies according to the nature of the sequences of predominantly soft rock ground forming materials. Wave exposure is greatest along the southwest coast where there is a relatively narrow southwest facing fetch extending into the northeast Atlantic that coincides with the prevailing wave approach direction. By contrast, Sandown Bay in the east is sheltered from southwest approaching waves and is exposed primarily to moderate energy waves generated locally in the central and eastern English Channel. Sandown Bay and the South West Coast are composed of soft Wealden and Lower Cretaceous sediments, forming cliffs of moderate height, whereas the high "Undercliff" coast is characterised by sequences of soft Lower Cretaceous Sediments capped by permeable Greensand and Chalk lithologies within classic landslide generating sequences. The coastlines can thus be sub-divided into three distinct behavioural units summarised as follows:
South West Coast
6. KEY COASTAL DEFENCE AND HABITAT ISSUES - References Map
The main habitat of interest comprises vegetated soft rock sea cliffs that vary in terms of lithological substrates (clay, sands and Chalk) and their degree of activity from highly active erosion to inactive e.g. many parts of the Undercliff.
With the exception of the fully defended shoreline frontages between Shanklin Chine and Yaverland, and Western Ventnor to Monks Bay, defence and protection measures are limited, site-specific responses to local threats to property and infrastructure. The absence of baseline environmental assessment surveys prior to the early 1990s make it difficult to know if earlier cliff stabilisation measures, sea wall construction and groyne building have impoverished cliff face and inter-tidal ecology. Environmental evaluation of the Castlehaven protection scheme (Rendel Geotechnics, 1996; 1997; McInnes and Jakeways, 2001) revealed no habitats of special regional significance, although the adopted scheme makes provision for the recolonisation of regraded slopes. Where and when existing defences are upgraded, or protection measures are introduced where they have not previously existed, it will be necessary to ensure that detailed local ecosystem inventories and potential impact studies are completed. This is especially important in the southern Undercliff coast, where topography and microclimate combine to create special floristic assemblages of national significance. For much of this coastline, habitat character is a direct response to ground instability. It is assumed that where a strategic policy option of non-intervention is in force (which currently covers over 80% of this shoreline), natural processes will operate, and thus at least maintain ecological diversity. The wealth of local, national and international (E.U.) conservation designations should ensure this. Principles and practical advice established by Rendel Geotechnics (1998), Lee et al, (2001) and Lee and Clark (2002) provide guidance for the maintenance of environmental qualities on soft rock cliffs.
The Western Undercliff and southwestern coast of the Island is of special geological and geomorphological value and a long coastal segment was recently designated as a SSSI for these purposes. A consequence of these qualities and 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. It may be that some lessons learned in the management of the Dorset and Southeast Devon World Heritage site could be applied to the Island's environmentally sensitive coastline. For example, 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 to address potential conflicts and issues (http://www.swgfl.org.uk/jurassic/consult.htm) allowing opportunities to identify and resolve issues at the earliest possible stage.
This coastline does offer one theoretical opportunity for managed realignment where the truncated alluvial valley of the East Yar is confined by a seawall, at Yaverland (Photo 43). It is not under serious consideration for it would involve severing of the A3395 although it does offer potential sites for the extensive establishment of both brackish and freshwater wetland. As it is contiguous with the downstream extension of the East Yar valley to Bembridge Harbour, any change of strategic defence option cannot be undertaken in isolation and any realignment could be more effectively progressed from that direction. Whilst the lower East Yar valley is a possible site for accommodating substitution for wetland losses from elsewhere in the region, the socio-economic constraints are strong. This issue is currently in the remit of the Coastal Defence Strategy Studies for Sandown Bay and North-East Isle of Wight (Posford Duvivier Isle of Wight Council and University of Portsmouth, in preparation).
7. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map
The discontinuous nature of the shoreline of this unit with its headlands and pocket beaches in means that it is unsuited generally for definitive studies of drift. There are, however, opportunities to study drift occurring within Sandown Bay. An initial approach would be to develop a numerical model of littoral drift potential at a series of points along the full beach length based on an analysis of a long-term (greater than 20 years) hindcast wave climate. Uncertainties encountered in applying numerical model studies would include:
8. RESEARCH AND MONITORING REQUIREMENTS - References Map
The SMP (Halcrow, 1997) and Sandown Bay Coastal Defence Strategy Plan (Posford Duvivier, Isle of Wight Council and University of Portsmouth, in preparation) have reviewed much of the available information and made recommendations for monitoring and research. Some recommendations are in the process of implementation by the Strategic Regional Coastal Monitoring Programme, a consortium of coastal groups working together to improve the breadth, quality and consistency of coastal monitoring in South and South East England (Bradbury, 2001). A Channel Coastal Observatory has been established at the Southampton Oceanography Centre to serve as the regional co-ordination and data management centre. Its website at www.channelcoast.org provides details of project progress (via monthly newsletters), descriptions of the monitoring being undertaken and the arrangements made for archiving and dissemination of data. Monitoring includes wave and tidal recording, provision of quality survey ground control and baseline beach profiles, high resolution aerial photography and production of orthophotos, LIDAR imagery and nearshore hydrographic survey. Not all of these actions are presently planned for this unit. Data is archived within the Halcrow SANDS database system and the aim is to make data freely available via the website.
The recommendations for future research and monitoring here therefore 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:
9. REFERENCES - Map
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CHANDLER, M.J. and HUTCHINSON, J.N. (1984) Assessment of Relative Slide Hazard Within a Large, Pre-existing Coastal Landslide at Ventnor, Isle of Wight, Proc. 4th Int. Symp. on Landslides (Toronto), Vol.2, 517-522.
CLARK, A.R. et al. (1993) The Management and Stabilisation of Weak Sandstone Cliffs at Shanklin, Isle of Wight, in: J.C. Cripps and C.F. Moon (Eds) Engineering Geology of Weak Rocks. Proceedings of 26th Conference of the Engineering Geology Group of the Geological Society, London: Geological Society, 392-410.
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MMIV © SCOPAC Sediment Transport Study - South & East Isle of Wight