Palaeo-Environmental Study Area P5 Sandown Bay and Brading Haven, Isle of Wight, UK
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Palaeo-environmental Study Area P5 Sandown Bay and Brading Haven, Isle of Wight, UK PALAEO-ENVIRONMENTAL STUDY AREA P5 SANDOWN BAY AND BRADING HAVEN, ISLE OF WIGHT, UK AN ASSESSMENT OF ARCHAEOLOGICAL AND PALAEO-ENVIRONMENTAL SITES ON THE CHANGING COASTLINE OF SANDOWN BAY, ISLE OF WIGHT, UK. Plate P5 Bembridge Harbour, part of a low-lying belt of land linked to Sandown Bay on the south-east coast of the Isle of Wight, UK 1. LOCATION Sandown Bay lies on the south east shore of the Isle of Wight on the English Channel coast of southern England. The shore of the bay accommodates the seaside resort of Sandown. This town lies 13km south of the major seaport of Portsmouth. 2. MODERN GEOGRAPHY The sandy beach-head of Sandown Bay is some 6.5km long. On the central section of this shoreline lies Sandown, a seaside resort which was established in the mid 19th century when the sands and the sheltered aspect of the bay were an attraction to Victorian visitors and developers. This town occupies a relatively low position and it is partially bounded by drained marshlands which border the most northerly section of the sandy shore. The attractions of the sandy shoreline were also responsible for the development of the neighbouring town of Shanklin. This town occupies the southern end of the bay and it is mostly built on the top of a 1 Palaeo-environmental Study Area P5 Sandown Bay and Brading Haven, Isle of Wight, UK sandstone cliff. The centres of the twin towns of Sandown and Shanklin lie some two kilometres apart but residential development during the 20th century has visually unified the two into a single landscape unit. A railway connecting both towns to a mainland ferry service at Ryde was responsible for the rise of a successful tourism trade in the 19th century and this still remains the key economic base to both towns. In the latter part of the 20th century an increased demand for retirement homes has also boosted the population. The total resident population figure for the two towns is now some 18,000 but in summer this is increased by approximately 20,000. 3. THE CONTEMPORARY COASTAL PROBLEM Much of the coastland at Sandown is low-lying, the topography having been determined by the course of an earlier Holocene river system which has since been truncated by the encroachment of the sea into the bay. In the northern section of the bay the persistent advance of the sea has been well documented since a Stuart fort was lost to coastal erosion in the mid 17th century (Jones 1979). A measure of the the historic advance of the sea since the c.AD 1800 has also been conveyed in the modified view of Sandham Castle presented in part 1 of this LIFE report. The shoreline management plan for the bay has divided this coastline into eight units. At the northern end of the bay units SAN 1 and SAN 2 embrace a chalk and greensand cliff-line where a policy of do nothing has been set. Where the shoreline cuts across the old palaeovalley of the Eastern Yar a policy of hold the line begins and this is sustained throughout all of the remaining management units within the bay. Along this latter sector of the coast a sand beach is headed by variable bodies of storm beach shingle set at the foot of a well- maintained concrete seawall. Longshore drift is resisted by regular series of wooden groynes. Two principal problems can be perceived on this shoreline. The indented configuration of the bay demonstrates a long history of encroachment by the sea and this is confirmed by historic observations. In the northern sector of the bay, in SAN 3 and SAN 4, the presence of the Western Yar palaeochannel signifies that the coastline is particularly vulnerable. Here there are a number of buildings sited on and within the vicinity of the drained marshlands behind the seawall. This includes properties on the eastern boundary of Sandown town. At Shanklin, development in the 1860's spawned hotels and houses at the foot of the sea cliff in a location which is currently protected by a promenade and a sea-wall. There are economic needs to hold the line along the whole of the defended coast but with sea-level rise and predictions of increased storminess two practical issues must be assessed. These are the need to avert overtopping of the sea defences and the need to predict the future behaviour of the natural buffer of sand and shingle which currently protects this coast. 4. KNOWN HISTORY AND PALAEO-ENVIRONMENTAL SETTING Other than historic evidence covering the past three centuries little is known of the behavioural history of natural coastal processes in Sandown Bay. Maps of the late Tudor period are too imprecise to show significant difference from today’s coastline even though some notable advances coastal erosion have been noted in the 17th century. An illustration of AD 1545 suggests that the Yar estuary map may have formerly opened into the sea at Sandown Bay but this is unconfirmed. There are, however, earlier historical accounts of seasonal difficulties in crossing the valley floor between the medieval settlements of Brading and Yaverland and these seem to favour a past history of periodic flooding. 5. ARCHAEOLOGICAL AND PALAEO-ENVIRONMENTAL STUDIES AND POTENTIAL Three principal sources of palaeo-environmental information can be identified in Sandown Bay. In the off-shore zone the sediment archive in the inundated catchment of the Eastern Yar offers evidence of the local environment prior to earlier Holocene sea-level rise and coastal change. On-shore, in the drained marshland, offers palaeo-environmental evidence of the nature of the coastline after the sea has truncated the accrued sediments off-shore. Finally, where development has proceeded at the foot of Shanklin cliffs, there remains a need to retrieve 2 Palaeo-environmental Study Area P5 Sandown Bay and Brading Haven, Isle of Wight, UK evidence which might establish when this location was last subject to wave attack. This may be viewed in relation to a possible return period. See Figures P5.1 and P5.2. 5.1 Geophysical prospection on the floor of the bay During the closing stages of the LIFE project commercial side-scan and sub-bottom surveys were carried in Sandown Bay prior to the planning of a new off-shore waste-water out-fall. This work, commissioned by Southern Water, was followed by the sinking of geo-technic bore-holes. The geophysical prospection identified the course of a significant palaeo-channel on the floor of the bay and this was considered to be a former element of the Eastern Yar river system prior to Holocene inundation. This palaeo-channel was traced to a depth of c.-20m OD. A preliminary assessment of the core samples by Dr Scaife suggests that a fill of valley peat was accruing in this palaeo-channel in pre-Boreal and possibly Boreal times. At present it is uncertain whether further analytic work will proceed on these cores. 5.2 Palaeo-environmental investigation of coastline history from on-shore sediment archives at Yaverland Stratigraphy of the Holocene sediments at Sandown waste treatment works Trenching activity for the laying of pipelines on Sandown Marshes exposed a sequence of peats and sediments to a depth of some 3-4 metres. It is suggested that the sequence has accumulated primarily in response to rising sea-level during the late Prehistoric period (positive eustatic change and subsequent marine retrogression. Monolith profiles were taken which span this waterlogged sequence from upper salt marsh deposits down into possible basal palaeosol and basal sands. These monoliths have subsequently been sampled for pollen assessment and described in the laboratory. The Stratigraphy The profile was described as follows in Table 1 using Munsell colour charts for colour description. 3 Palaeo-environmental Study Area P5 Sandown Bay and Brading Haven, Isle of Wight, UK Depth in cm. Colour in core 100-107 Grey (10YR 5/1) fine silty clay with brown vertical organic rootlets penetrating from above. 107-111 A darker grey, more organic horizon (10YR 4/1 or 0YR 3/1). A possible stabilisation horizon 111-185 Grey silt/clay (10YR 5/1)rapidly oxidizing to 10YR 4/2.Homogeneous but containing vertical rootlets and marine shells (cf. Scrobicularia). 185-195 Transition between overlying grey silt/clay and lower organic fen peat 195-239 Brown (2.5Y 3/2 or 10YR 3/2). Stratified monocot. peat with Phragmites or Typha reed. Some silt, vertical rootlets and occasional wood fragments. 239-249 Peat with medium/coarse sand (10YR 3/1 249-257 Grey organic silt and sand with some orange/yellow mottled sand-transition 257-268 Yellow sand with some grey mottling (10YR 4/1 or 10YR 3/1). 268-291 Predominantly yellow sand with occasional grey mottles. Table 1 Stratigraphy at Sandown Bay, Isle of Wight, UK Interpretation of the sediment archive Depth of trenching and safety did not allow solid bedrock to be reached (i.e. Lower Greensand). However, the basal sediment sampled appears to represent a waterlogged, disturbed basal sand deposit. Whilst there is a possibility that this is a beach or marine deposit, the overall character appears to be that of disturbed upper material of the bedrock Greensand. A degree of grey mottling increases upwards into more organic sands which may be a palaeosol developed in the basal sands and sealed by subsequent peat formation. A well developed highly, humified, fibrous monocotyledonous peat overlies the possible basal palaeosol at 239cm. Although this peat has a quite high detritus content, there is some evidence of a stratified structure.