West & South 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 E1 E2 E3 E4 E5 E6 E7 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.
E1 Needles to Freshwater Bay (see introduction to coastal erosion)
This frontage is dominated by high, near vertical Chalk cliffs with a few small coves and slight indentations that accumulate coarse angular gravel (Photo 15 and Photo 17). Of these, Scratchell's Bay is the largest, but is totally inaccessible. Barrett (1985) has made the general observation that cliff recession rates appear to be least where there is little, or no, beach accumulation. This would appear to imply that wave-assisted clast abrasion rather than direct cliff toe erosion along this exposed coastline is a significant factor. However, gravel accumulation could be a product of higher than average recession, rather than its cause. Beach material is presumed to derive directly from the release of flint nodules from the steeply-dipping bedding planes of the Upper Chalk at a rate of 1500 m3a-1 (Posford Duvivier, 1997, 1999) from a total yield of 15,000 m3a-1 of chalk debris. Recession of this cliff line is relatively slow, with intermittent rockfalls. A rate of shoreline recession of 0.14ma-1 is suggested by Posford Duvivier (1991) and 0.15ma-1 was calculated by Halcrow,(1997) covering the period 1866-1995. The shoreface is relatively steep, with the 10m isobath between 200 and 300m from the shoreface. This limits capacity for debris storage.
E2 Freshwater Bay (see introduction to coastal erosion)
Field and documentary evidence suggest that the prevailing rate of retreat of the Chalk cliffs is slow due to the high degree of protection afforded by the beach - approximately 0.08ma-1 (May and Heaps, 1985; Barton and McInnes, 1988). Very localised and episodic retreat of parts of the cliff-top free face occurs at rates of up to 0.3ma-1 (McInnes, 1994). Prior to the provision of coastal defences within the bay, recession occurred at a mean rate of around 0.5ma-1 for the period 1866-1909 (Halcrow 1997). Occasional rockfalls yield a small quantity of clastic material, but in the western part of the bay the cliffs support an overburden of loosely consolidated chalk and flint fragments (Coombe Rock) that is subject to mass wasting and gullying. Posford Duvivier (1999) calculate an erosion loss of approximately 2000m3a-1 of mixed sediment sizes, of which less than 100m3a-1 is flint gravel that is retained on the local beach. Cliff morphology suggests that mechanical failure due to sub-aerial denudation, toe erosion, and block fall are in phase (Geodata Institute, 1989). The presence of the Stag and Arched Rocks is proof of longer term erosion of the more exposed cliffline; a third stack was added as a result of a large block fall in 1971.
E3 Freshwater Bay to Compton Down (see introduction to coastal erosion)
Between Freshwater Bay and Compton Down, Chalk strata dipping at 45 to 80 degrees form cliffs in excess of 75m height. Material at the cliff foot suggests that gravity-assisted falls and block failure are dominant here. May (1966) calculated a rate of shoreline recession of 0.01ma-1, which is significantly less than for Chalk cliffs of similar exposure and dimensions at other south coast locations. Posford Duvivier (1981, 1989a, 1997, 1999) propose an average long-term rate of 0.15-0.6ma-1, yielding some 15,000m3a-1 of Chalk and 500 m3a-1 of flint gravel. Haclrow (1997) calculated a long-term recession of around 0.1ma-1 from 1886 to 1975, but with an increase to 0.42ma-1 for 1975-1995 attributable to failures in superficial deposits at the cliff top. Recession of the cliff top at Afton Down has posed a particular threat to the A3055 since 1981. The cliff slope hereabouts is partly defined by east-to-west trending joints, but is mantled by slip debris derived from Chalky Head materials above. Detailed geotechnical surveys and sophisticated monitoring have been undertaken because of the threat to public safety (McInnes and Jakeways, 2000). Barton and McInnes (1988) calculate a recession for the cliffs as a whole of 0.05ma-1 with up to 18m of retreat of the cliff top between 1935 and 1980 (McInnes, 1983; 1994). They suggest that the cracks or "vents" that have appeared are indicative of incipient toppling failure, ultimately related to toe erosion. Movement has been especially noticeable during winter months, related to either the swelling of superficial drift, "ponding" of perched groundwater aquicludes or pressure-release due to previous failures (Barton, 1987; Geodata Unit, 1989).
However, the overall erosion rate of about 0.10ma-1 along this sector of the coastline is comparatively slow, and may be partially explained by the exceptional width of the offshore zone. The 10m depth contour in the north of Compton Bay indicates a 1300m wide offshore platform so that incident wave energy is strongly dissipated. However, the Chalk yields very little sediment suitable for beach building, so that protection against breaking waves is slight.
E4 Compton Down to Hanover Point (see introduction to coastal erosion)
To the south of Compton Down, the lithology of the outcropping Wealden rocks includes clays, shales, marls and sandstones. The offshore platform is wide, but modified wave erosion is partly offset by the highly erodible nature of the cliff materials. Overland flow, gullying and shallow sliding contribute to the constantly changing morphology of cliff faces, and may be at least as important as basal marine erosion in regulating retreat rates. Hanover Point owes its existence to an outcrop of comparatively more resistant bedrock.
Posford Duvivier (1981, 1989a, 1999; McInnes, 1994) report a contemporary rate of retreat of 0.2-0.5ma-1, with 24m of recession at Shippard's Chine, 1877-1955 (Isle of Wight County Surveyor, 1957). For the complete section between Compton Chine and Hanover Point, Halcrow (1997) calculated recession averaging some 0.48ma-1 for the period 1866-1995, although this value includes considerable local variation. A rate of 0.23ma-1 at Compton Chine is suggested by Barton (1987). McInnes (1983) has proposed a figure of 4ma-1 in "recent" years at one particular site where there has been particularly active mass wasting and possible seepage erosion. Barton (1987) has calculated 23m of land loss, 1861-1981 for the narrow Gault Clay outcrop. There is a well defined shoreline platform between Shippard's Chine and south of Hanover Point cut across both weak and more resistant bedrocks, providing clear proof of the long-term efficacy of linear erosion at an estimated rate of 0.55ma-1 (Posford Duvivier, 1999). For the sector between Afton Down and Shippard's Chine, Posford Duvivier (1997) propose a total sediment yield of 35,000m3a-1 dominated by sand, silt and clay. Between Shippard's and Grange Chines, an erosion yield of 45,000m3a-1 of fine sands and clay and 5,000m3a-1 of gravel-sized sediment is calculated.
E5 Hanover Point to Atherfield Point (see introduction to coastal erosion)
With the exception of the weakly arcuate plan of the inset of Brook Bay, this comparatively straight coastline is dominated by cliffs of modest (20-30m) height (Photo 18), rising to 53m at Barnes High (Photo 19 and Photo 20) and is interrupted by several chines. Geological composition is of weak materials, similar to the sector to the northwest but with significantly higher rates of localised cliff top recession due to several semi-rotational slides, block failures and mudflows. Over the past 10 years, the highest rates of retreat have been along 1km frontage south of Brookgreen. A shore platform extends to just south of Chilton Chine (Photo 1), but is frequently concealed by both beach sediments and rafts of seaweed. A detailed study of shoreline movement in the immediate vicinity of Grange Chine gave a figure of 130m of recession, 1794 to 1977 (1.6 ma-1). (Tomalin, 1977) with possibly up to 700m of retreat "since Roman times". McInnes (1983) derives a figure of 1ma-1, 1935-1982 for the low sandstone and clay cliffs in Brook Bay. Colenutt (1938) wrote that "extensive erosion has occurred north of Brook", presumably referring to immediately preceding years; he infers that this was due to storm waves gaining access to the cliff foot, which had previously been protected by a wide, stable sandy beach. May (1966) calculated 49m of retreat within Brightstone Bay between 1870 and 1963, giving a mean of 0.52ma-1. The County Surveyor (1957) derived a similar figure, 0.55ma-1, for Brook Bay, whilst Barrett (1985) proposed an average recession rate of 0.4ma-1 since 1800 and Posford Duvivier (1997) suggest 0.48ma-1. For the sector between between Hanover Point and Sudmoor Point, Halcrow (1997) calculated recession averaging some 0.40ma-1 for the period 1866-1995. They also calculated recession averaging 0.51ma-1 for the sector between between Sudmoor Point and Grange Chine and 0.47ma-1 between Grange Chine and Sheperd's Chine. This range of values may reflect genuine spatial variations in wave energy induced by platform relief (eg Brook Ledges), cliff height and rock erodibility, but may also be due to temporal variations in the magnitude of coastal storms. These rates translate into a supply of 105,000m3a-1 of mostly fine sediment between Shippard's and Shepard's Chines (Posford Duvivier, 1997); of this, less than 5,000 m3a-1 is coarse material. That some of these storms, over the past four or five centuries, were high-energy events is documented by numerous accounts of shipwrecks. The Isle of Wight County Archaeology Unit's Marine Sites and Monuments Register (MSMR), could provide an original database for the estimation of cliff recession along this and adjacent parts of the coastline, although only for wrecks known to have foundered on specific, named beaches or prominent offshore ledges on dates that can be independently corroborated.
E6 Atherfield Point to Walpen Chine; Chale Bay (see introduction to coastal erosion)
The position of the 10m isobath along this straight, actively eroding cliffed coastline approaches to within 500m of mean low spring tides, and the effect of Atlantic swell waves is greater here than on adjacent parts of the west Wight shoreline. Atherfield Point, and its offshore ledges, results from an outcrop of a more resistant unit within the Lower Greensand (Photo 21 and Photo 22). The cliffs are formed primarily within the Ferruginous Sands and Sandrock and maintain near-vertical profiles with occasional narrow ledges (Photo 21, Photo 22 and Photo 23). Long term recession rates of 0.6 ma-1 (Posford Duvivier, 1997) and 0.66 ma-1 for 1866-1995 (Halcrow, 1997)for this unit, with spatial variability due to subtle changes in rock resistance and exposure to wave energy, are characteristic. An erosion yield of 45,000m3a-1 between Shepard's and Whale Chines is derived from the above recession rate. (Posford Duvivier 1997).
E7 Chale to Rocken End (see introduction to coastal erosion)
Cliff height increases from 70m at Chale (Photo 23) to over 180m at Gore Cliff (Photo 26), where the eastwards dip of the rock succession causes a west to east change in the lithological units outcropping at the cliff base. The Ferruginous Sandstone between Walpen Chine and north of Rocken End, but particularly in the vicinity of Chale, contains bands of interbedded clays that promote shearing and seepage erosion of the overlying sandstones. Associated aquicludes promote seepage erosion and are responsible for the bench-like form of the cliff profile. Study of Ordnance Survey map editions between 1861 to 1960 give an overall recession rate of approximately 0.5ma-1. Kay (1969) noted that the true Undercliff, defined by slip and flow debris beneath an upper scarp and bench, was widest between Cliff Terrace and Blackgang (Photo 27). He reports 25.3m of upper scarp retreat between 10 December 1965 and 19 February 1966 and comments that the building of the properties of Cliff Terrace in the 1860s might suggest a period previous to that time of apparent stability. Hutchinson (1987) reports a significant acceleration of coastline retreat at this site over the past 130 years, from 0.16ma-1 between 1861 and 1907, to 0.57ma-1 in the period 1907 to 1980, giving a mean rate of 0.43ma-1. Posford Duvivier (1997) suggest a spatially averaged figure of 1.2ma-1, (0.6-2.0) giving the substantial yield of 440,000m3a-1of sand and clay for the sector between Whale Chine and Rocken End where active cliff height is at a maximum for this entire coastline. Rendel Geotechnics (1995b) calculate a rate of cliff retreat of 1.57ma-1, 1980-1994. Cliff top retreat throughout the 1980s and 1990s has been greater than that of marine erosion at the toe, with debris supplied by several failure reactivation events temporarily stored in the Undercliff. (Note that the coastal backscar developed in Lower Greensand materials has retreated relatively slowly, whereas the previously inactive landslides in front have reactivated towards the backscar toe. If this continues then renewed first time failures of the backscar can be anticipated in future (Halcrow, 2002) Material is removed by mudslides that are frequently released onto the beach at the cliff base, which has an ill-defined form compared to the steep cliffs to the immediate north.The main morphological components are: (i) The Walpen (Chale) Undercliff, developed in the Atherfield Clay, Sandrock and Ferruginous Sandstone lithological units. A sequence of near-horizontal terraces and free face segments coincide with clays and sandstones, respectively. Cliff recession takes place through spatially and temporally variable combinations of falls, mudslides and erosion by groundwater seepage. Halcrow (1997) calculated long-term recession averaging some 0.62ma-1 for the period 1866-1995, although recession has increased in recent decades with typical rates of 0.8-0.9ma-1. Historical rates of cliff top recession have exceeded those at the cliff base, but the latter provides the ultimate control. Recession of the cliff base has significantly narrowed the lower Undercliff benches; mudflows are frequently released onto the beach, but are rapidly dispersed. (ii) The Blackgang to St Catherine’s Undercliff (Photo 26 and Photo 27), comprising a linked series of landslides activated by semi-rotational failure planes. Broadly, there are multiple rotational failure units in the Gault clay, succeeded downslope by mudslides developed across a mid-slope scarp and compound block failures developed on clay-rich horizons in the basal Sandrock. Steep cliffs separate each of these units (Rendel Geotechnics, 1995a), which are functionally inter-related. The unit as a whole has a history of large scale and frequent landslide reactivation, related to complex feedbacks between rates of basal cliff erosion and the unloading of debris stores. Groundwater also plays a significant role, with pore water pressures at and above critical lithological boundaries possibly determining the magnitude of the larger failure events (Bray, 1994). Their precise timing may be linked to exceptionally high levels of antecedent rainfall, a relationship that was apparent for the 1978 and 1994 landslips at Blackgang. The latter event involved deep-seated rotational movements shunting a large volume of ancient landslide and rockfall debris over the Gault clay scarp. Simultaneously, several mudslides transported this material onto the basal cliff slope and beach. It illustrated clearly the integration and inter-dependence of the main morphological units, which have been analysed in terms of an evolutionary model by Rendel Geotechnics (1995a). This argued that retreat of the sea cliff reduces the lower tier block failure unit, which in turn triggers upslope landsliding through reactivation of ancient failure planes; the Gault Clay scarp is unloaded by the landward migration of compound block failures, thus promoting mudsliding. The critical control is exerted by marine erosion of the cliff foot, together with removal or reduction of the debris stores mantling the foreshore. Numerous small-scale failure events in the basal section of this system will promote episodic larger-scale shearing failures in the upper, landward zone. The precise relationship between the rate of recession of the Gault Clay scarp and upslope failure events is, however, a complex one, involving time lags that are not yet fully understood. Despite the intermittent delivery of large quantities of landslide debris to the foreshore, some of it of boulder size, beach formation appears inhibited. The cliff toe therefore remains exposed to marine erosion. Rates of cliffline recession have been calculated by several researchers, largely with reference to the retreat of the marine cliff. The doubtful survey accuracy of cliff features on this coastline on successive Ordnance Survey maps renders this data potentially unreliable although it is to be presumed that the cliff top has been mapped accurately. Cliff retreat varies within and between each of the major morphological subdivisions; Rendel Geotechnics (1995a) quote a rate of approximately 2.5ma-1, 1980-1984 for the Gault Clay scarp. This is probably representative of a phase of accelerated movement, as earlier measurements covering the period 1861-1980 (Hutchinson, et al, 1981) indicate a lower long-term mean of 0.41ma-1. Halcrow (1997) calculated recession averaging 0.14ma-1 for the period 1866-1909, although recession increased thereafter with typical rates of 2.0ma-1. It was noted that the recession process was linked to major reactivations of ancient landslides in 1928, 1935, 1952, 1978 and 1994. The episodic nature of recession at this site means that as much as 50m of retreat is possible in a single event (Bray, 1994). Minor ground movements involving tension cracks and pressure ridges extend inland. Mean recession rates therefore need to be considered in combination with the maximum recession likely in a single event. Sea-level rise over the next 50 years and beyond may increase access of wave erosion to the cliff base and thus sustain higher rates of recession. If so, the evolutionary model of cliff behaviour points to increased frequency of landslide events, although predictions of their locations, magnitudes and timings are not yet possible. It should be noted that this active unit provides the most appropriate analogue for the behaviour of the Undercliff should its toe protection and debris aprons be removed. The main Undercliff beach descends from approximately 55m OD at Walpen Chine to beach level at Blackgang Chine, with some subsidiary minor benches to the immediate south south east (Photo 23, Photo 26 and Photo 27). These were apparently better defined, and by implication were more stable, throughout the nineteenth century. Over a small area immediately southeast of Blackgang Chine, the cliffs are in a relatively stable condition; Hutchinson, et al (1981) suggests that seepage erosion is suppressed here because sub-surface drainage is directed towards the Chine. Elsewhere, groundwater-fed stream flow and debris transport, shallow mudflows, rotational slides and rear scarp weathering and mass wastage are the dominant erosion processes. The coastal cliff complex from a point seaward of Gore Cliff (Photo 26) has a long history of instability, with major rockfalls and landslides occurring at regular intervals (Hutchinson, 1987; Bromhead, Chandler and Hutchinson, 1991; Hutchinston, Bromhead and Chandler, 2002; McInnes and Jakeways, 2000; Rust, 2002; Rendel Geotechnics, 1994 and 1995a). Nearly 300m of cliff retreat has taken place since approximately 1880 (2.5 ma-1). At Gore Cliff itself there is strong toe erosion in the Ferruginous Sands and Sandrock, with a recorded mean rate of shoreline recession of 0.6ma-1 between 1862 and 1980 (Hutchinson, et al 1981). Preece (1980, 1987) records mollusca and Romano-British artefacts in a chalky hill wash deposit, which he interprets as having been derived by mass movement acting on a slope with a northeasterly aspect. This implies a substantial recession and probable geomorphological re-modelling of the Undercliff in this vicinity over at least two millennia. The direct contribution of marine erosion to the initiation of slope instability remains uncertain, as various other hydrogeological and geotechnical attributes are of importance (Hutchinson, 1965, 1987; Bromhead, et al 1991; Rendel Geotechnics, 1994). However, the consensus view (Rendel Geotechnics, 1994; McInnes, 2000) is that unloading and oversteepening of the cliff base provides the mechanism that prepares for slope failure events that are triggered by periods of intensive or prolonged rainfall adding critically to pore water pressures determined by local groundwater reservoirs (Bromhead et al, 1991; Hutchinson et al, 1991; 2002; Lee et al, 1998).
3. LITTORAL TRANSPORT LT1 LT2 LT3 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
LT1 Needles to Freshwater Bay (see introduction to littoral transport)
The presence of a fringe of coarse gravel and boulders derived from cliff falls has been discussed previously. The largest accumulation is at Freshwater Bay, where the beach is characterised by multiple berms and a steep backshore storm ridge, banked up against the cliffs throughout most of the year, particularly in the eastern sector. The height and continuity of this feature led Barrett (1985) to suggest that this beach is strongly reflective. The material is predominantly large, rounded flint pebbles and cobbles, suggesting that abrasion by swash and backwash reworks sediment trapped in this well-defined coastal inset. The lack of in situ flints in the Chalk cliffs in the eastern part of the Bay suggests their movement by littoral transport from the west, but there may be an input from the mass wasting and marine erosion of the soliflucted chalky-flint deposits infilling the truncated valley profile of the Yar. This would have been more significant before the completion of the first generation of sea defences in the late nineteenth century. Severe damage sustained by the sea wall esplanade and groynes, necessitating extensive repairs and reconstruction in the 1900s, 1953 and 1966, indicate the effectiveness of both abrasion and scour (Posford Duvivier, 1989a). The consensus view is that the small tidal range helps to construct and conserve an essentially stable beach, which now offers adequate protection to the foundations of the seawall, except perhaps for the salient around the Albion Hotel (Posford Duvivier, 1989b). The esplanade sea wall built in the late 1890s was constructed on the foundations of a backshore berm, thus immobilising a previously dynamic sediment store. Colenutt (1904) reported that there was little evidence for net longshore transport within the bay, but that short-term and short distance east to west movement had been observed following south-easterly winds. The beach to the immediate west of the Albion Hotel was depleted in 1902-4, possibly due to gravel trapping by groynes or deliberate shingle removal for construction materials. All subsequent descriptions of this beach fail to mention any patterns of sediment transport reversal or net accumulation/depletion. The groynes, present since the 1930s, have tended to minimise changes in beach form and orientation. The cessation of beach quarrying circa. 1910 may have been significant in restoring dynamic equilibrium after an unknown period of selective removal of clasts.
LT2 Freshwater Bay to Rocken End (see introduction to littoral transport)
This long sector of coastline has a north-west to south-east trend, except for the stretch between Freshwater Bay and Compton Down where the Chalk cliffs and boulder beach have a similar character as the coastline to the west, although the beach is marginally wider. The change in orientation at Compton Chine reflects the influence of rock type and geological structure and coincides with a marked change in beach character. As a generalisation, shingle forms the upper backshore, with fine to medium sand constituting the mid and foreshore areas. Under high energy wave conditions, this sorting pattern is often lost, and beach composition is mixed. The sand derives mostly from the long term erosion of the cliffline and of substrate silts, clays and shales exposed across the foreshore. Fine-grained sediments are transported offshore by suspension (Posford Duvivier, 1999a; Brampton et.al, 1998). The beaches between Compton Bay and Atherfield Point are of moderate gradient, and the inter-tidal zone is wide. Nearshore significant wave heights vary from 1.6 to 7.5m, the latter with an approximate 1 in 100 year return frequency (HR Wallingford, 1999). In Chale Bay, the dominantly gravel beach is substantially higher, steeper and comparatively narrow, about 30m at mid-tide in the vicinity of Whale Chine compared to 70m at Compton Bay (Barrett, 1985). Rock ledges and platforms are a further feature of the inter-tidal zone between Brookgreen, Hanover Point and Atherfield Point which help to dissipate wave energy and conserve beach form. Beach profiles and volumes are subject to substantial fluctuation, given the potential range of wave heights and periods (HR Wallingford, 1992). Large rafts of seaweed are frequently superimposed on the mid- to back-shore beach, which act as a buffer against wave scour, particularly during late spring and early summer. Norman (1887) described the southerly beaches attaining "great depth", and composed of medium to fine gravel. It was his unsubstantiated view that the material derived from offshore deposits, reworked by wave action.
Colenutt (1938) reports that the normally coarse sandy beach at Brookgreen had been "occasionally" replaced by well graded, polished (abraided?) gravel, but there are no other accounts of short term changes in beach sedimentology except for a tentative inference by Kay (1969) that the pebbles composing Chale beach had recently changed from large to smaller size. The same author notes that this beach may have been steeper in the late nineteenth century, or that a permanent storm ridge might have existed at this time. It would appear to be rare for a true storm berm to form at most points along this coastline. If it does, it only persists for very short periods.
All authors are agreed that the dominant littoral transport direction is from north-west to south-east (Colenutt, 1938; Kay, 1969; Posford Duvivier, 1981, 1989a, 1999; Barrett, 1985; Brampton et al, 1998). In view of the lack of groynes, direct evidence for this is slight, although both Atherfield and Hanover Points - whilst allowing some sediment by-passing - promote some updrift beach accretion. The reduction in the mean size of gravel from north-west to south-east is also consistent with progressive abrasion along this pathway (Barrett, 1985), although in itself it is not a reliable guide to dominant drift direction. Halcrow (1997) suggest that the coarse fraction, whilst retained on the inter-tidal slope, moves downdrift rapidly. Beach material therefore has little long-term stability, and may therefore fail to provide effective cliff toe protection. The relatively limited development of beach width in Chale Bay has not been satisfactorily explained, (Kay, 1969). Barrett (1985) has proposed that the headlands confine the littoral transport of gravel, but as they are relatively minor salients this is a debateable proposition. There is no direct evidence of offshore losses, nor of net offshore to onshore feeds of gravel (Posford Duvivier and British Geological Survey, 1999). It is unlikely that more than a fraction of the gravel constituting the beach in Chale Bay is derived from the erosion of the Chalk cliffs of Freshwater Bay, via longshore drift. Although there are cappings of valley gravels at the cliff top between Shippard's Chine and south of Brookgreen, they are thin and patchy and even long-term cliff erosion would not generate a sufficient quantity of beach sediment from this source. An off to onshore wave-transported input of gravel is therefore a reasonable inference (Brampton et al 1998). The dynamics of longshore and nearshore sediment transport for much of the western Wight coastline are therefore not understood. What little evidence that is available is based on chance observations. The important interacting variables controlling transport are offshore bathymetry, wave climate and the nature of the shallow seabed sediments potentially available for onshore transport (Brampton et al, 1998). The only characteristic that is not in doubt is the change from dissipative (sand) to reflective (gravel) beach from north-west to south-eastward and the progressive reduction in the mean size of the gravel fraction. Roundness may also increase in this 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.
4. SEDIMENT OUTPUTS
4.1 Offshore Transport O1 References Map
01 South of the Needles to Brook Bay
A map of postulated sand transport pathways around the Isle of Wight (Dyer, 1985), indicates a net onshore to offshore movement away from the approximate location of Hanover Point and possibly seawards of Freshwater Bay. The rationale for either pathway is not stated; it may be based on unpublished survey data.
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
ALGAN, O., CLAYTON, T., TRANTER, M. and COLLINS, M.B. 1994. Estuarine Mixing of Clay Minerals in the Solent Region, Southern England, Sedimentary Geology, 92, 241-255. ANDREWS, J. and POWELL, K. (1993) Monk's Bay, Isle of Wight, Papers and Proceedings, 28th MAFF Conference of River and Coastal Engineers (Loughborough), 2.1.1 to 2.1.15.
BARRETT, M.G. (1985) Isle of Wight - Shoreline Erosion and Protection, Paper to Conference on Problems Associated with the Coastline, Newport (Isle of Wight), 8pp.
BARTON, M.E. (1985) Report on the Cliff and Scree Slope Movements near Shanklin Spa Car Park, Report to South Wight Borough Council, 11pp.
BARTON, M.E. (1991) The Natural Evolution of Soft Cliffs at Shanklin, Isle of Wight, and its Planning and Engineering Implications, in: R.J. Chandler (Ed) Slope Stability: Engineering Developments and Applications, London: Thomas Telford, 181-188.
BARTON, M.E. and McINNES, R. (1988) Experience with a Tiltmeter-based Early Warning System on the Isle of Wight, Proc. 5th Int. Symp. on Landslides (Lausanne), 379-382.
BIRD, E. (1997) The Shaping of the Isle of Wight, Bradford on Avon: Ex Libris Press, 176pp.
BRAMPTON, A.H.; EVANS, C.D.R. and VELEGRAKIS, A.F. (1998) Seabed Sediment Mobility West of the Isle of Wight. London: CIRIA. Project Report 65, 171pp and 8 Appendices.
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MMIV © SCOPAC Sediment Transport Study - South & West Isle of Wight