Durlston Head to Handfast Point
Introduction - Map
The headland and bay sequence of this east-facing coastline reflects the alternating sequence of east to west striking sedimentary strata truncated by a north to south trending coastline. The relative resistance compared to intervening strata of the Chalk, Upper Purbeck Limestone and Portland Limestone have created the three morphologically distinctive headlands of Handfast Point (Photo 1) to Ballard Point; Peveril Point and Durlston Head (Photo 2) respectively. Swanage Bay is developed in the Wealden series, which are comparatively weak clays and sandstones, whilst Durlston Bay has been excavated from the brittle jointed and fractured sequence of limestones and "dirt bands" of the Lower and Middle Purbeckian series (Bird, 1996). The planform of Swanage Bay is evolving towards a log spiral zetaform, with equilibrium less fully achieved in the northern sector (Halcrow, 2002). Although geologically and geomorphologically this area is usually regarded as part of the "Isle" of Purbeck, there is no evidence for any marine process connection between the south-facing coast of Purbeck and this sector. Durlston Head is therefore accepted as a fixed boundary to sediment transport, with Handfast Point to the north providing a well-defined northern limit. Limited measurement and analysis of wave data for Swanage Bay (Halcrow, 1999a and b, 2002; Hydraulics Research, 1987a and b) reveal effective protection from refracted long period swell waves moving from the west and south-west through the English Channel. Occasional storm waves generated over the fetch areas to the south-east, east and north-east, can create significant levels of wave energy, sediment movement and beach drawdown. Waves approaching from the east and south-east are therefore the dominant component of the wave climate of Swanage Bay. The majority of waves generated by the east and south-east fetch do not exceed a height of 1.0m. Extreme wave heights of 3m and 4m may occur with return intervals of 5 and 50 years, respectively (Hydraulics Research, 1987b). The platform and reef offshore Peveril Point is a critical feature that provides additional protection to southern Swanage Bay by promoting rapid shoaling of waves approaching from the south-east. Highest wave energies are experienced in Durlston Bay and along the headland between Ballard and Handfast Points. Tidal currents are considered ineffective as a process of nearshore sediment entrainment (Posford Duvivier, 1998; Halcrow, 1999b), and do not exceed 0.2 to 0.3m.sec-1 in Swanage Bay. However, it is possible that tide and wave induced currents operating together in the nearshore and offshore zones entrain and move sand (Halcrow, 2002)
2. SEDIMENT INPUTS - FL1 References Map
2.1 Fluvial Input
FL1 Rivers Swan and Ulwell
There is an input of fine sediment from the discharge of the Rivers Swans and Ulwell in central Swanage Bay. There is no data on quantities, but it is presumed to be very small.
2.2 Coast Erosion - E1 E2 E3 References Map
E1 Durlston Bay
Rapid cliff recession at Durlston Bay is well documented through its impact on clifftop properties and the subsequent problems of providing protection (Purbeck District Council, 1996; High-Point Rendel 2002). Notably, a block of flats built 10m from the cliff edge in 1976 became threatened by cliff retreat, requiring construction of a protection scheme in 1988-89 (Photo 3). The cliffs are composed of closely interbedded and jointed limestones, with intervening "dirt bands," mudstones and marls of the Purbeckian series, but three major compound faults and thrust planes complicate the structural situation. These faults are an important component of slope instability and define the site of an active slide to the west of Durlston Flats, which, at times, becomes a mudflow. Cliff top recession was calculated as 0.44ma-1 over the period 1950-1980 (Trevor Crocker and Partners, 1986), although movement was not continuous. Marine erosion at the base of the cliffs and cliff face seepage quickly remove fine material, leaving large inter-joint blocks and boulder arcs marking the position of former debris slides, falls, topples and mudflows. Small talus stores do not appear to be persistent features. In the northern part of Durlston Bay, Trevor Crocker and Partners (1986) estimated that 8,000m3 of material had fallen seawards in the previous ten years, although Posford Duvivier (1999) quote an input of 1,000m3a-1of limestone boulders. In a subsequent report (Crocker, 1988), it was calculated that up to 12m of cliff top recession had taken place at certain critical locations since 1968, and that erosion rates appeared to be accelerating.
High-Point Rendel (2002) produced a detailed assessment of cliff recession processes for ten distinct sub-units that they identified within the bay. They identified a general tendency for behaviour to be controlled by marine toe erosion in the north of the bay with an increase in hillslope processes (landsliding and scarp retreat) towards the south. Particular attention was drawn to instability of the cliffs adjacent to Purbeck Heights apartments where cliff top retreat of 35m from 1955 to 2000 occurred (0.4ma-1), having previously been much slower. A further 10m of recession occurred over the exceptionally wet winter of 2000/01 such that the cliff edge migrated to within 25m of the buildings. A small stream identified as being contributory to the landslide activity was diverted in 2002. A range of options for remedial works to reduce instability by various combinations of drainage, re-profiling and shear piles have been proposed (High-Point Rendel, 2002).
A basal cliff recession rate of 0.13ma-1 for the area between Durlston Flats and Peveril Point is quoted by Halcrow (1999b). Major falls are attributed to the combination of toe erosion by waves, sub-aerial weathering of all lithologies and groundwater seepage at limestone/marl junctions. Fallen joint blocks remaining at the base of the cliff can mitigate the hydraulic effects of breaking waves, but their longevity is unknown. The chert bands release a small hard gravel component to the steep gradient fringing beach, which offers some protection to the cliff base.
In the south of the bay a landslide complex involving many types of unstable landform mechanisms is associated with both hard and soft rocks (Denness, 1970), but has not generated any ongoing slope instability in recent years.
For this entire length of shoreline (Handfast Point to Durlston Head), substantial quantities of fine sand, silt and clay supplied by cliff erosion are removed as suspended load by waves. It can be inferred that much of this is transported offshore, but its fate is unknown.
E2 Swanage Bay
The rock lithology and stratigraphy of northern Swanage Bay directly affects cliff mass movements. Toe erosion and steepening of cliffs formed in soft Wealden Beds, comprising sandstones, grits, marls and clays generates repeated shallow mass movements, particularly north of the groyne field. Instability is enhanced by groundwater seepage and mudflows, and debris fans extend periodically onto the beach. This material is soon removed offshore, as suspended load, by wave action at higher tides. Canning and Maxtead (1979) suggest there had been decreases in coastal recession at Punfield Cove due to protection by storm beach growth, but stability was only temporary. A major failure affected a 250m length of this cliffline in December 2000 and January 2001 following a period of exceptionally persistent heavy rainfall (Photo 4). The failure occurred primarily within talus on the mid to lower cliff, but the unloading of the cliff face appears to have resulted in renewed rockfall failures of insitu Chalk material extending up to the cliff top. It would appear to be an example of the type of cliff reactivation that could become more common in the future with climate change as explained in generic terms by Halcrow Maritime (2001).
Gullying, translational failures and other mass movements are active across the coastal slope further south, behind a promenade and sea wall protecting the base of the cliffs (Photo 5). Slips and slides almost annually obscure the promenade. Several attempts have been made to stabilise these cliffs using toe protection, but high groundwater levels have remained problem and in many places the upper cliff has continued to degrade. Up to 8m of sub-aerial cliff recession must have taken place since the promenade was built in the early 1920s, as shown by attempts to build a masonry wall over part of the cliff face and the isolation of former cliff steps further north. Cliff top recession is now operating faster than cliff toe movement as a result of the headward expansion of failure scars. An average recession rate of 0.5ma-1 is suggested (Halcrow, 1999b; 2002), possibly exceeding 1.0ma-1 in the sector between Shep's Hollow and Ballard Down. Spatial variability may be partially controlled by vegetation colonisation and basal beach height and width. In places, there is reed growth around cliff foot seepage points. Halcrow (2002) have proposed that, based on a spatially variable recession rate of 0.5 to 1.0 ma-1, between 100 and 1,000 cubic metres are eroded annually. It is suggested that 50% of this quantity is sufficiently coarse to be stable on the adjacent beach. This figure is of medium to low reliability as accurate estimation requires a detailed analysis of the lithology of the cliff material.
The shoreface platform of this frontage extending to a maximum distance of 750m offshore, may yield 3-4,000m3a-1 of fine material from horizontal downwearing by wave scour of 1mma-1 (Posford Duvivier, 1998).
E3 Handfast Point to Ballard Down
There is obvious and dramatic evidence of active coastal erosion of the Chalk outcrop for the area from Handfast Point to Ballard Point where there are clean near vertical cliffs of up to 50m in height. May (1971) has shown that conjugate joint set and fault/shear planes control both marine and sub-aerial erosion. May (1966, 1971, 1977) mapped coastal retreat from OS map editions between 1882 and 1962, suggesting a mean retreat rate of 0.23ma-1. Halcrow (1999b) estimate a rate of 0.25 to 0.35ma-1 for Ballard Point to Handfast Point, using both OS maps and air photo cover. Cliff erosion is substantiated by comparison of the present state of the stacks at Handfast Point (Photo 1) with 19th Century descriptions and early photographs, and by documentation of collapse of a rock arch in 1920-21; a stack ('Old Harry's Wife') in 1899, and a cave in October 1976. The Chalk cliffs to the west of Ballard Point appear less active, having larger paretly vegetated talus accumulations. However at their western extremity a major slide within accumulated tatus provides possible evidence of increasing activity (see E2)
Posford Duvivier (1997; 1999) propose a potential sediment supply of 30,000 m3a-1 from the length of shoreline from Handfast Point to Swanage, consisting of 10,000m3 of Chalk, 500m3 of flints (released from the Chalk) and 20,000m3 of sand and clay. The latter figure is likely to be an exaggeration, for such material can only be supplied from the Wealden Beds and a significant proportion of the cliffs formed in these materials are protected within Swanage Bay. Basal undercutting of the vertical Chalk cliffs is active, but only temporary accumulation of material from falls and topples occur at their base. Chalk blocks are rapidly broken down, and it is only their flint content which makes a contribution to local 'pocket' beaches and the sub-cell sediment system (May and Heeps, 1985). At Ballard Point waves are reflected at High Water, with a narrow gravel beach exposed at Low Water. In 1969, a fall at Ballard Cliff produced 500m3 of material (May and Heeps, 1985). 20m3 is calculated to be added annually by several isolated small falls. Volume loss was estimated at 50 m3a-1 because by 1984 all the basal debris had been removed and, in addition, 35m3 of bedrock had been eroded following partial excavation of earlier, consolidated scree deposits. The nearshore and offshore platform between Handfast Point and east of Punfield Cove reaches a maximum width of approximately 450m extending out to the 10m isobath. Posford Duvivier (1999) derive the theoretical calculation of a yield of 100m3a-1 of flint gravel resulting from wave scour of this surface.
3. LITTORAL TRANSPORT (BEACH DRIFT) - References Map
At the northern end of Swanage Bay there is a steeply sloping shingle beach subject to seasonal change, and a semi-permanent storm berm in Punfield Cove immediately south of the set back between the Wealden and Gault/Upper Greensand, and Chalk cliffs. The source of the coarse clastic material is uncertain, but is presumed to be flints derived from the adjacent Chalk, including both cliff face and submergent platform. Its presence suggests (probably episodic) offshore to onshore bedload transport from the Chalk shore platform by waves that approach this sector of coast across an easterly fetch. The lithological composition and morphology of this beach suggest it could be a minor store.
Evidence based on beach sediment grading as well as the persistent pattern of accumulation against groynes and the Ulwell Stream outfall culvert suggests a net northward littoral drift under the influence of prevailing waves. This is also confirmed by modelling studies (Halcrow, 1999b; 2002). The presence of sub-rounded clasts of Purbeckian limestones, presumed to have been detached from the Peveril headland, in central Swanage Bay confirm this. However, chalk and flint pebbles occur well south of the Chalk cliffs in Swanage Bay, so periodic reversals of this pathway must be inferred. These reflect the influence of north-easterly and easterly wind waves, primarily in the winter months and is also demonstrated by the transport model applied by Halcrow (2002).
LT1 Durlston Bay (see introduction to littoral transport)
In Durlston Bay, there is little definitive evidence of littoral drift, though it is assumed that any occurring would follow the same northward direction as in Swanage Bay since both have comparable orientations. Wave energy is, however, marginally higher (Hydraulics Research, 1987a). Only a limited beach occurs, dominated by large fallen joint blocks derived from the eroding cliffs above, resting upon a narrow, stepped platform. This factor ensures that littoral drift potentials cannot be due to the lack of mobile sediment and the impediment to drift posed by the boulders. Marine erosion occurs directly at the base of the active cliff sections and quickly removes fine material offshore in suspension. Abrasion and attrition slowly reduce beach clast sizes, but there is no clear evidence that there is any drift bypassing of Peveril Point and its reef-like extension seawards. The latter may function as a subsidiary fixed transport boundary, thereby isolating the littoral sediment budget of Durlston Bay. Purbeck Limestone clasts on the shingle berm of central Swanage Bay probably derive from quarry spoil introduced in the eighteenth and nineteenth centuries, rather than indicating natural supply from outcropping strata in Durlston Bay.
LT2 Swanage Bay (see introduction to littoral transport)
In Swanage Bay, beach material becomes finer in a southwards direction, with a relatively abrupt decrease in the proportion of upper beach gravel south of the most northerly section of seawall/promenade. Northern parts therefore have a gravel upper berm and a low gradient sandy foreshore, whereas southern parts are almost completely sand (Photo 6). The southern shore against Pevril Point is depleted of sediment. A general south to north drift operates within the bay, although reversals can occur as outlined below.
Halcrow (2002) state that there was a small, net accretion of beach sediment following the construction of the first seawall in 1905/06, but that erosion was initiated following the seawards extension, and upgrading, of this defence structure in the 1920s. Groynes were introduced in the 1930s to address this problem, but their success in the south was accompanied by beach depletion north of the Ulwell Stream outfall (Trevor Crocker, 1987). A further set of groynes was therefore installed, north to Shep's Hollow, in the late 1950s to early 1960s. Hydraulics Research (1987a) noted loss of beach volume over the previous ten years, which was ascribed to wave reflection off the seawall (Photo 5). Halcrow (1999b and 2002) calculate that since the early 1980s, the entire beach - between The Mowlem and Ballard Down - has been eroding at an average rate of 5,000 m3a-1. Comparison of beach volumes between May 1998 and April 2002 revealed this to be an accelerating trend with a cumulative loss of 34,000 m3 (8,500 m3a-1) over this period. The main cause was considered to be the structural deterioration of the groyne field, causing sediment that had previously been trapped to move downdrift and offshore. A subsidiary factor affecting the beach between the Mowlem and the Ulwell outfall (but also further downdrift) is the construction of the Swan river culvert and jetty (Photo 6) in 1991 (Hydraulics Research, 1991). Although this initially caused an obstruction to northwards longshore drift (Halcrow, 1999b), the beach width to the south (i,e. updrift) has adjusted to its presence, and sand by-passing is now occurring (Halcrow, 2002). Only some 20% of beach erosion since approximately 2001 can be ascribed to this factor. Ongoing beach erosion may now be irreversible without management intervention, which is likely to involve sand replenishment and groyne field re-design/re-construction.
Beach drawdown induced by storm waves approaching from the east/south-east is a frequent occurrence, and often removes all but the upper backshore accumulation of gravel. During these periods, which normally occur in winter, the clay/sandstone substrate is exposed. Its lowering by wave abrasion, and perhaps by clay liquefaction (Halcrow, 2002) is a probable additional cause of overall, longer-term, beach lowering. However, in the extreme southern part of Swanage Bay, storm waves are more likely to move sediment onto the beach. Model simulation of higher energy waves (Halcrow, 2002) demonstrated that this would give a net addition of 1,000 m3a-1, with a recurrence of storm waves of between 6 and 10 times a year. Material thus provided would then tend to drift northward rather than accumulate.
The gravel storm beach at Punfield Cove, in the northern 'corner' of Swanage Bay between the Chalk and Wealden clifflines is more difficult to explain. Given net south to north longshore transport, it might be interpreted as an accumulation form. However, as it is composed mostly of flint rather than sand, and an onshore supply pathway is implied. Halcrow (1999b; 2002) have not established any clear gravel feed moving westwards along the base of the cliffs of Ballard Down. On the contrary, computer modelling of various combinations of tidal and wave currents indicates a net eastwards movement of fine sediment (< 10 mm diameter). Whilst this would be very modest - in the order of 100 m3a-1 under "average" incident waves, it could amount to between 2,000 and 20,000m3a-1 under storm waves. It is presumed that this material moves offshore when it reaches Ballard Point (or possibly en route), eventually joining the approximately north to south tidally driven pathway in outer Swanage Bay (see Section 4). As this longshore travel pathway is selective of grain size, the Punfield Cove beach could be a 'lag' deposit.
Most research previous to Halcrow (1999b; 2002) gave qualitative desriptions of net drift directions. Hydraulics Research (1986; 1987 and b) and HR Wallingford (1991) concluded that the dominant drift direction was northwards. Mathematical models used were based on limited data on wave approach direction frequencies and wave heights, so net drift rates could not be calculated with reliability. It was, however, stated that observational evidence indicates that short-term drift reversal takes place, although often this only occurs over a limited sector of the beach system. Trevor Crocker (1987) considered drift reversal (ie north to south movement) to be confined to short periods during the winter, and that it did not significantly affect the integrity of the groyne field extant at that time. Webber (1987) accepted the probability of net southerly drift over a short length of beach between The Mowlem and the pier. However, both he and Hydraulics Research (1987a) were unable to cite convincing evidence for any long-term accretion in this extreme southern "corner" of Swanage Bay.
Halcrow (2002) provide a more definitive, quantified, assessment of both pathways and rates of longshore transport based on an improved wave climate. Their assessments are derived from computer modelling of tidal and wave-induced currents, the latter based on a 10 year, 3-hourly time series of hindcast wave data. This record covers a representative range of wave energy conditions and is adjusted for local refraction and diffraction of SW waves around Durlston Head and Peveril Points effects. Drift rates are computed as potential, rather than actual, values. For the sector north to the Mowlem, there is a balance between northwards and southwards potential transport, giving a net movement close to zero. Between The Mowlem and the Ulwell stream outfall, there is a northwards drift of 30,000 to 40,000m3a-1 of sand with no indication of any reversal. Between the Ulwell outfall and Punfield Cove, potential gross rates of sand transport commence at 20,000 m3a-1 and increase progressively northwards to 80,000 m3a-1. Reversed drift (southwards) varies between 10,000 and 15,000 m3a-1 (northern Swanage Bay) and 2,000-15,000 m3a-1 Shep's Hollow to Ulwell); prevailing rates are a function of incident wave conditions. As stated previously, these are all potential rates. Actual rates are much lower, and are determined by sediment availability, presence of gravels (less easily transported), beach volume fluctuations and groyne interception of longshore transport.
LT3 Ballard Point to Handfast Point (see introduction to littoral transport)
Pocket beaches occur around the Chalk headland; these are highly compartmentalised and contain mainly abraided flint gravels. Shore platforms extending seaward from the cliff toes show signs of surface scour, and are patchily covered by loose flints, mainly derived from erosion of Chalk (Fitzpatrick, 1987). It appears that removal of finer material from the base of the cliffs takes place without significant lateral movement. May (1977) estimates that Chalk pebbles can be moved longshore 1.5km before destruction; this accounts for the distinct trend of progressively smaller and less numerous clasts of this lithology towards the centre of Swanage Bay, moved by infrequent counter-drift.
4. SEDIMENT OUTPUTS
4.1 Transport in the Offshore Zone - O1 O2 References Map
The tidal circulation in Swanage Bay has a net anti-clockwise flow, with movement in that direction being much longer sustained than clockwise flow in each tidal cycle. Hydraulics Research (1987a and b) describe this as a large eddy that operates during most of the flood tide, disappearing at high water. Flow is directed southwards 3 hours after Low Water. Strong tidal currents occur around Handfast Point where the seabed is a shallow rock-cut platform, with a marked outer edge. Poorly-sorted gravels occur on this platform where tidal currents may effect a winnowing action through the selective transport of sand. Tidal currents increase in speed towards and offshore of Peveril Point, but within Swanage Bay are low, attaining maximum velocities of 0.2-0.3ms-1 (Hydraulics Research, 1987a; Webber, 1987; Wessex Water, 1992). Gravels are scattered thinly across the bed of Swanage Bay in the area above the 18m depth contour, but farther south they are better sorted and clean washed, with a high content of shelly material (maerl) (Fitzpatrick, 1987). Seabed topography has not been mapped in detail, but there are few indications of bedforms that might indicate either sediment mobility or directions of transport (Halcrow, 1999a). The average bedslope is less than 1:100.
O1 Southward Transport offshore of Swanage and Durslton Bays (see introduction to sediment outputs)
The net direction of sediment movement some 0.5km to 1km seawards of the coastline is southwards. Fitzpatrick (1987) states that the heavy mineral assemblage indicates a general provenance from Eocene (Tertiary) sediments from the sea floor of Poole Bay, but no indication of volumes of transport is given. In deeper water, offshore sediments become progressively finer eastwards, with an area of rippled sand about 4km east of inner Swanage Bay. Movement there appears to be in a predominantly south-west direction out towards the English Channel (Fitzpatrick, 1987).
From what little is known of the composition and mineralogy of seabed sediments it appears that little or no material is moving onshore from offshore although there are conflicting statements as to whether onshore movement occurs by weed rafting of gravels. There may be some net onshore transfer of sand towards the Molwm (Halcrow, 2002) and of gravel to the extreme north of Swanage Bay, but the latter is based on inference from beach composition. Diver inspections report that seabed gravels and sandy gravels are colonised by weed and are therefore presumed to be immobile.
O2 Handfast Point (see introduction to sediment outputs)
It appears that some coarse material may move from onshore to offshore, particularly in the vicinity of steep cliffs and especially off Handfast Point. Simulation of removal of effluent from an outfall in Swanage Bay indicates a potential for net offshore transport (Hydraulics Research, 1987b). This is the likely pathway for significant quantities of fine calibre sediments moved as suspension load by waves and - possibly - tidal currents.
5. SUMMARY OF SEDIMENT PATHWAYS - References Map
1. The headland and bay sequence of this east-facing coastline reflects the alternating east to west striking sedimentary strata of variable erosion resistance truncated by a north to south trending coastline. Durlston and Swanage Bays have eroded into less resistant sediments and are backed by active cliffs. The planform of Swanage Bay appears to be evolving towards a log spiral zetaform, with equilibrium less fully achieved in the northern sector where significant recession continues to occur. Wave exposure is controlled by the sheltering influences of Durlston Head and Peveril Point so that south-easterly and easterly waves are dominant.
2. Swanage Bay is thought to operate as weak sediment sink accumulating sediments primarily from local cliff erosion and possibly from offshore sources. Cliff erosion supplies sands and some flint gravels to Swanage Bay, whereas Durlston Bay receives clays and limestone boulders. Most inputs, however, are fine materials that become transported offshore in suspension so that only thin, narrow beaches occupy the bays.
. Recent modelling studies have identified a strong potential for net drift towards the north within Swanage Bay, but this is not reflected by the distribution of beach sediments. Indeed, all the beaches appear to be depleted of sediment with only the gravel component increasing northward. Studies have revealed that beach losses have been occurring over at least the past two decades. It could be that defences in Swanage Bay control the natural recession process and have critically reduced inputs of sediment from the Wealden cliffs in central parts of the bay. Alternatively, it may be that sand is progressively winnowed offshore from beaches as it drifts northward.
4. There are indications that cliff recession may be accelerating and some previously relic cliffs appear to be reactivating e.g. Punfield Cove. Continuation of such trends may be anticipated in the future as part of the likely coastal response to climate change as envisioned by Halcrow Maritime (2001). It could pose problems for the stability of some of the partly stabilised soft cliffs within Swanage Bay, although it should lead to increased sediment inputs to the shore.
6. KEY COASTAL DEFENCE AND HABITATS ISSUES - References Map
This coastline is nationally and internationally celebrated for its geological exposures and geomorphological features. In common with the rest of the Purbeck shoreline, it is an SSSI, with four key GCR (Geological Conservation Review) sites. The vegetated cliffs in Durlston Bay, northern Swanage Bay and Ballard Down have also been designated ecological SSSIs. In addition, it forms part of the recently notified UNESCO World Heritage Site for Jurassic Geology. 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 so that potential conflicts and issues on this highly sensitive coast can be addressed (http://www.swgfl.org.uk/jurassic/consult.htm). The coast of this unit is also included in a provisional SAC and is a component element of the Poole Bay and Isle of Purbeck SMA. Planning designations include the Dorset AONB and Purbeck Heritage Coast, both of which recognise the exceptionally high quality landscape character of this coastline and argue for minimal modification of its natural environment.
Given this strength of conservation policy, it is crucial that future coastal defence strategies do not compromise the variety and viability of its earth science recources and habitats. This is fully recognised in the recommendations for shoreline management contained in the Poole and Christchurch Bays Shoreline Management Plan (Halcrow, 1999a) and Swanage Bay Beach Management Plan (Halcrow, 1999b). Formal defences are currently restricted to central and southern Swanage Bay and to a site below Durlston Flats, in Durlston Bay. At the first site, some upgrading of the standards of current defences are proposed where cliff top and bayside properties and infrastructure are at potential risk over a timescale of not less than 10 years. If there is periodic beach feeding, informed by the results of routine monitoring of beach levels, as recommended in Halcrow (1999b; 2002), partial restoration of beach stability in the northern part of Swanage Bay may occur. This could provide some opportunities for vegetated shingle habitat creation
The armoured revetment, rock fill and cliff drainage scheme immediately below the site of Durlston Flats (Durlston Bay) and completed in 1989 (Photo 3), will continue to inhibit some yield of material from formerly unrestrained cliff top weathering and mass movement. Given an assumed lifetime of 40 to 50 years, its toe may act as a secondary salient during this interval, restraining littoral transport. It will not eliminate future movement of the upper free face and, over time, its role will diminish. Options have been proposed for further cliff stabilisation in front of the Purbeck Heights apartments (High-Point Rendel, 2002) that are immediately to the south of Durlston Flats.
There is little precise information on the littoral and sub-littoral habitats in the nearshore and offshore areas. As future coastal defence strategy is likely to be a matter of maintaining approximately the same length of protected frontage as has been the case for the previous century, no significant impacts are anticipated (Halcrow, 1999b; 2002). Undoubtedly, considerable local habitat modifications occurred when Swanage was an active stone exporting port, but this ceased in the late 1920s.
7. LOCATIONS FOR FUTURE RESEARCH TO TEST NUMERICAL MODEL ESTIMATIONS OF LITTORAL DRIFT - References Map
The bays of this frontage are pocket beaches with unique east-facing orientations so that any research conducted would be highly site specific rather than generic in application. In its recommendations for process monitoring in Poole Bay (with Swanage Bay as a component part) no sites in this sub-cell were identified as high priority in the regional SMP (Halcrow, 1999a). This is a realistic position based on the relative significance of risk and cost of providing effective strategic defences.
However, in the longer term it would be advantageous to obtain appropriate field data for central and southern Swanage Bay, especially if future coastal defence were to become based upon the maintenance of a replenished beach. The prime requirements are for regular beach monitoring to record patterns of change in beach volume and for some local wave recording to calibrate the Halcrow 2002 hindcast wave climate and enable wave transformations inshore to provide input data for transport modelling. When a reasonable beach monitoring data series has been established it should be feasible to set up a beach plan shape model. Uncertainties requiring attention would include possible offshore-onshore exchanges of sand, representation of an appropriate groyne efficiency and representation of an appropriate grain size on the sand and gravel beach.
8. RESEARCH AND MONITORING REQUIREMENTS - References Map
A full review of the monitoring requirements for Swanage Bay is set out in the Swanage Bay Beach Management Plan (Halcrow, 1999b). They are placed in the context of the sediment transport cell of Poole Bay (Halcrow, 1999a), where they are prioritised in relation to the full spectrum of regional process monitoring requirements. The conclusions of these two, complementary studies are that there is a serious deficiency of primary data relating to: (i) water levels; (ii) bathymetry; (iii) wave climate; (iv) beach morphodynamics and (v) marine cliff and coastal slope stability. The installation of a permanent tide gauge and the establishment of a routine programme of beach profiling are probably the most urgently needed initiatives to address these shortcomings. The latter, in particular, would provide much improved, if indirect, insight into the rates and volumes of littoral sediment transport. Observations and measurements of drift directions should be integral to repetitive beach re-profiling.
A start has been made by the survey data collected for the Swanage Bay Beach Monitoring Study (Halcrow, 1999b) and other 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 directional wave recording, provision of quality survey ground control and baseline beach profiles, high resolution aerial photography and production of orthophotos, provision of new ground survey control and some baseline beach profiles, 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 following suggestions are also made:
1. Data appropriate to the calculation of extreme water levels and inshore wave climate are further requirements. In the first case, a lengthy period of data collection is necessary for reliable analysis. Regarding the latter a temporary deployment of a wave recorder could suffice to validate/calibrate the Halcrow (2002) wave climate.
2. Current knowledge of cliff erosion rates is limited to restricted sectors of Swanage Bay and is based on inherently uncertain comparisons of the positions of the cliff foot and cliff toe on successive Ordnance Survey maps and plans. Detailed field, map and photogrammetric analysis of the recession of both the upper and lower segments of selected cliff profiles would provide improved data on the erosion yield to the sediment budgets of Durlston and Swanage Bays. The installation of instruments (e.g. inclinometers) to record the detail of cliff slope displacement would provide more precise knowledge of mechanisms of mass movement; this, in turn, might help to refine estimations of future yield, and give more insight into the magnitude and frequency relationships of characteristic types of cliff instability. A critical location is the cliffline in front of New Swanage, between the northern end of the promenade and Shep's Hollow. If it could be economically justified, the additional deployment of piezometers to monitor groundwater flow in the Wealden series cliffs would be highly beneficial.
3. The pathways and rates of sediment transport in the nearshore and offshore zones are very uncertain, and it should be an intermediate timescale priority to undertake a primary survey of seabed bedforms, preferably repeated throughout at least one year, to gain some inference of sediment transfer paths and directions. A grab sample survey of Swanage Bay would provide at least a provisional answer to the question of the fate of fine texture sediment. It would also be worthwhile to investigate if any sediment grades are able to by-pass Durlston Head, thereby confirming whether, or not, it is a fixed and absolute boundary dividing the south and east Purbeck transport sub-cells.
4. Given the relatively short length of the shoreline of this sub-cell; its largely self-contained low flux transport system; and the readily determined provenance of coarse clastic sediments involved in littoral transport, the availability of additional data on inputs and throughputs should make the task of calculating a sediment budget feasible. The crucial question of whether or not Swanage Bay represents a sediment sink, analogous to Weymouth Bay, might thereby be resolved.
9. REFERENCES - Map
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MMIV © SCOPAC Sediment Transport Study - Durlston Head to Handfast Point