1. Introduction
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 11) 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; Royal Haskoning, 2010). Though 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 but probably partial northern limit to sediment exchange.
Analysis of wave data for Swanage Bay (Halcrow, 1999a and b, 2002; Hydraulics Research, 1987a and b) reveal 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 Southeast Regional Coastal Monitoring Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Boscombe in 10mCD water depth. Between 2003 and 2012, the prevailing wave direction was south-by-west. Average 10% significant wave height exceedance is 1.03m (CCO, 2012).
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 (where refractive effects are less than in Swanage 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 0.3ms-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 fine to medium size sand both onshore and towards the south-south-west (Halcrow, 2002). This is enhanced where ebb flow rips operate around the major headlands.
A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org. In 2008, an extensive high resolution, 100% coverage swath bathymetry dataset, known as the Dorset Integrated Survey (DORIS), was collected by the Southeast Regional Coastal Monitoring program in partnership with Dorset Wildlife Trust, The Maritime and Coastguard Agency, The Royal Navy and Virido Credits. This survey extended from the western end of the Fleet lagoon to Handfast Point and 20km offshore from MLWS. A subsequent swath bathymetry survey was collected in Poole Bay, through the Maritime and Coastguard Agency’s Civil Hydrography Programme; this survey abutted with the DORIS survey area, although the shoreward limit was the 2mCD contour. The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See CCO Annual Survey Reports for further details).
2. Sediment Inputs
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
Analysis of the Coastal Monitoring Programme data between 2006 and 2012/13, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. However, at certain sites either due to a lack of long-term data, data coverage or sedimentological information (e.g. composition and proportion of beach grade material arising from cliff erosion), quantification of sediment transport rates of gravel and sand has not been possible.
E1 Durlston Bay
Rapid cliff recession at Durlston Bay is well documented through its impact on clifftop properties in the 1980s and the subsequent problems of providing slope stabilisation (Purbeck District Council, 1996; High-Point Rendel 2002). 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 12). The cliffs, 15m in height at Peveril Point, rising to 40m at Durlston Head, are composed of closely interbedded and jointed limestones, with intervening "dirt bands," mudstones and marls of the Purbeckian series; 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 south of Durlston Flats, which, at times, becomes a mudflow. Cliff top recession was calculated as 0.44m per year 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 1,000m³ per year of 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 either side of the extant protection works.
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 on Belle View Road where cliff top retreat of 35m between1955 and 2000 occurred (0.4m per year), 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, slope re-profiling, rock-fill loading, basal boulder revetment construction and shear piling have been implemented in stages.
A basal cliff recession rate of 0.13m per year 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. The cliffs forming Peveril Point are fronted by two incurving sets of intertidal reefs, formed in response to the synclinal structure of this salient; these provide some reduction of the assailing power of breaking waves at the cliff foot during high water.
In the south of the bay, including Durlston Head, a landslide complex involving many types of unstable landform mechanisms is associated with both hard and soft rocks (Denness, 1970).A significant landslide occurred in 2007 and was monitored through the Coastal Monitoring Programme from 2008-2012, which showed that following the fall, the coastal slope had stabilised. For this cliffed section, analysis of Coastal Monitoring Programme (2003-12) indicated minimal cliff erosion, less than 1,000m³ per year. The 2004 arrows stated no quantitative data was available. Although substantial quantities of fine sand, silt and clay are supplied by cliff erosion, which are removed as suspended load by waves, there is a lack of detailed study information regarding proportions of cliff input yielding shingle or sand grade beach material.
E2 Swanage Bay
The rock lithology and stratigraphy of northern Swanage Bay directly affects cliff mass movements. Toe erosion and steepening of cliffs averaging 30m in height formed in soft Wealden Beds, comprising sandstones, grits, marls and clays generates repeated shallow mass movements, particularly north of the beach groyne field. Instability is enhanced by groundwater seepage and mudflows, and debris fans extend periodically onto the beach (May, 2003). Analysis of Coastal Monitoring Programme (2003-12) indicated minimal cliff erosion, less than 1,000m³ per year. The 2004 arrows stated no quantitative data was available. Although substantial quantities of sediment are supplied by cliff erosion, there is a lack of detailed study information regarding proportions of cliff input yielding shingle or sand grade beach material. The eroded material is soon removed offshore, as suspended load, by wave action at higher tides.
Gullying, translational failures and other mass movements are active across the coastal slope here and further south, this latter area behind a promenade and sea wall protecting the base of the cliffs (Photo 14). Slips and slides almost annually obstruct sections of the promenade. Several attempts have been made to stabilise these cliffs, but high groundwater levels have remained a 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 the building of a masonry wall over short length of the cliff face, other largely ineffective private defences and the isolation of the base 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 0.5m per year is suggested (Halcrow, 1999b; 2002), possibly exceeding 1.0m per year 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 m per year, 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 3-4,000m³ per year of fine material from horizontal downwearing by wave scour of 1mm per year (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 115m in height at Ballard Point, falling to just over 20m at Handfast Point. May (1971, 2003) has shown that conjugate joint sets and fault/shear planes control both marine and sub-aerial erosion. These structures have strongly influenced the creation of a set of caves, arches and stacks frequently referred to in textbooks as exemplifying a model of sequential, evolutionary development (May and Heeps, 1985; May, 2003). May (1966, 1971, 1977) mapped coastal retreat from OS map editions between 1882 and 1962, suggesting a mean retreat rate of 0.23m per year. Halcrow (1999b) estimate a rate of 0.25 to 0.35m per year 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 as they are partly protected by a basal gravel beach as well as by substantial, patchily vegetated, confluent talus slopes. However at their western extremity a major 250m wide detachment and slide within consolidated talus provides evidence of more recent instability (see E2) This event, initiated in 2001, gave rise to unloading that later resulted in renewed failure of the upslope Chalk free face.
Posford Duvivier (1997; 1999) propose a potential sediment supply of 30,000m³ per year 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. Analysis of Coastal Monitoring Programme data (2003-12) indicated minimal cliff erosion. Although substantial quantities of fine sand, silt and clay are supplied by cliff erosion, which are removed as suspended load by waves. There is a lack of detailed study information regarding proportions of cliff input yielding shingle or sand grade beach material. This means it has not been possible to quantify inputs from the cliffs.
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 impersistant local 'pocket' beaches and the sub-cell sediment system (May and Heeps, 1985; May, 2003). In 1969, a fall at Ballard Cliff produced 500m³ of material (May and Heeps, 1985). 20m³ is calculated to have been added by several isolated small falls. Volume loss was estimated at 50 m³ per year 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. Between Ballard Point and the Pinnacles stacks there is virtually no development of an intertidal shore platform. Posford Duvivier (1999) derive the theoretical calculation of a yield of 100m³ per year of flint gravel resulting from wave scour of submerged platform surfaces fronting the full length of Chalk cliffs in this sector.
N1 Swanage Bay
- Swanage Bay was renourished with imported material in 2005/2006. Studies revealed that beach losses had been occurring during the thirty years prior to the recharge. Further information on scheme details can be found at www.poolebay.net/PhaseI/swanage.htm
3. Littoral Transport (Beach Drift)
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 of the shoreline at the junction between the Wealden and Gault/Upper Greensand/ 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 refracted waves that approach this sector of coast across an easterly fetch. Analysis of Coastal Monitoring Programme data and the lithological composition and morphology of this beach material suggest it could be a minor store with very low rates of exchange.
Analysis of topographic (2003-12) Coastal Monitoring Programme data, beach sediment grading as well as the persistent pattern of accumulation against groynes and the Ulwell Stream outfall culvert provides evidence for the 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, some of which are presumed to have been detached from Peveril headland, in central and northern Swanage Bay tend to confirm this (Calkin, 1968). A significant proportion of the latter may represent residual quarry spoil introduced in the nineteenth century when Swanage Pier was the main location for loading Purbeck building stone. As 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 generated by Halcrow (2002).
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 potential cannot be achieved 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.
Extensive rock formations extend offshore from the cliffs between Peveril Point and Durlston Head. A thick patch of sediment covers the formations in the centre of Durlston Bay, no bedforms are present. Maybe caused by a tidal eddy formed by Durlston Head.
In Swanage Bay, the beach material is composed of shingle covered by a (recharged) layer of sand (Photo 15). The rocky southern shore between Peveril Point and Swanage Pier is depleted of sediment, but with an inferred, if small, east to west movement (Royal Haskoning, 2010). A general south to north drift operates within the rest of the bay, as confirmed through analysis of topographic (2003-12) Coastal Monitoring Programme data, with transport rates in the order of 3-10,000m³ per year. The reduction in volume compared to the 2004 proposed rate of more than 20,000m³ per year, has resulted in changes to the arrows within Swanage Bay. Observations from Mike Goater (Purbeck District Council, pers. comm., 2016) indicate that since 2006 there has been significant nearshore movement of sediment within Swanage Bay. The offshore movement of material results in the formation of a bar at the northern end of the bay, followed by onshore movement of material onto the beach. These movements are essentially nearshore movement and not considered here as offshore transport or wave-driven onshore transport. 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 14). Halcrow (1999b and 2002) calculate that between 1980 and 1997 the entire beach - between The Mowlem and Ballard Down - eroded at an average rate of 5,000m³ per year. Comparison of beach volumes between May 1998 and April 2002 revealed this to be an accelerating trend with a cumulative loss of 34,000m³ (8,500m³ per year) over this period. The main cause was considered to be the structural deterioration of the groyne field, originally built in 1925, 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 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 could be ascribed to this factor. Recognition and acceptance that erosion would be an ongoing reality led to the comprehensive recharge in 2005/06 of central Swanage beach with 130,000m³ of sand derived from the dredging of the entrance channel of Poole Harbour. This sediment input has subsequently been subject to some loss due to northwards and onshore to offshore transport, but retention has been facilitated by the insertion of eighteen timber groynes.
Beach drawdown induced by storm waves approaching from the east/south-east is a frequent occurrence, and can remove all but the upper backshore accumulation of gravel. During these periods, at least prior to recharge, which normally occur in winter, the clay/sandstone substrate and outcropping rocky ledges are 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 1,000m³ per year, 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 inset of Swanage Bay between the Cretaceous 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, a supply pathway from the east is implied, perhaps also including some onshore directed feed. Halcrow (1999b; 2002) and Royal Haskoning (2010) have not established any 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 100m³ per year under "average" incident waves, it could amount to between 2,000 and 20,000m³ per year 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, Punfield Cove gravel beach could be a type of 'lag' deposit.
Most research previous to Halcrow (1999b; 2002) and Royal Haskoning (2010) gave qualitative descriptions of net drift directions. Hydraulics Research (1986; 1987a 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 (i.e. north to south movement) to be confined to short periods during the winter, and that it did not significantly affect the functionality 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 previous long-term accretion in the extreme southern re-entrant of Swanage Bay. Furthermore, a relatively short period of monitoring, analysis of 2003-12 topographic and lidar data collected through the Coastal Monitoring Programme, provided no evidence of drift reversals, therefore the 2004 reversal arrows have been removed.
Halcrow (2002) provide a more definitive, quantified, assessment of both pathways and rates of longshore transport based on an improved wave climate model. 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 south-westerly waves effects around Durlston Head and Peveril Points. 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 40,000m³ per year of sand with no indication of any reversal. Between the Ulwell outfall and Punfield Cove, potential gross rates of sand transport northwards commence at 20,000m³ per year and increase progressively in this direction to 80,000m³ per year. Reversed drift (southwards) varies between 10,000 and 15,000m³ per year (northern Swanage Bay) and 2,000-15,000m³ per year (Shep's Hollow to Ulwell); variable prevailing rates are a function of changes in incident wave conditions. Actual rates are much lower than the calculations of potential rates, and are determined by sediment availability, presence of gravels (which are less easily transported), beach volume fluctuations (particularly the effects of antecedent beach morphology during periods of sustained transport) and groyne interception of longshore transport. It is probable that the finer grades of sediment transferred to the north of Swanage Bay are moved offshore, but there are no calculations of the quantity of this inferred loss, nor of how much might be returned - probably only under high wave energy conditions - and where (Royal Haskoning,2010). Simulation of removal of effluent from an outfall some 1km from the shoreline of Swanage beach indicated the possibility of net offshore removal of fine calibre sediments as suspension load by waves and tidal currents in combination (Hydraulics Research, 1987b)
The majority of the Swanage Bay seabed has only a thin veneer of sediment overlying west-east oriented bedrock ridges except for a sand bank running north-north-east to south-south-west in the centre of the bay. This sand bank rises up to 2m above the seabed and the eastern flank exhibits bedforms that indicate a southwards flow of transport (O1).
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.
The northern section of the Swanage Bay seabed has a thin veneer of sediment overlying west-east oriented bedrock outcrops and ridges that extend offshore. A rock platform extends from the cliff toe between Handfast Point and Ballard Point, and is discernible approximately 600m sub-tidally before the sediment thickness is sufficient to mask the underlying bedrock.
4. Sediment Outputs
4.1 Transport in the Offshore Zone
The tidal circulation in Swanage Bay, where the nearshore seabed falls away quite steeply to water depths in excess of 15m, 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). There are few indications of bedforms that might indicate either sediment mobility or sustained directions of transport (Halcrow, 1999a). The average offshore bedslope is less than 1:100.
The northern section of the Swanage Bay seabed has a thin veneer of sediment overlying west-east oriented bedrock outcrops and ridges that extend offshore. The volume of eastward transport from the northern end of the bay is unquantified, compared to the speculated 2004 rate of 10-20,000m³ per year. A rock platform extends from the cliff toe between Handfast Point and Ballard Point, and is discernible approximately 600m sub-tidally before the sediment thickness is sufficient to mask the underlying bedrock. Bedforms were evident on the eastern flank of the sand bank in the central sector of Swanage Bay, elsewhere bedforms were not discernible from the recent bathymetry surveys to verify that the net direction of fine-grained 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, including possible weed rafting of gravels (Royal Haskoning, 2010). There may be some occasional net onshore transfer of sand towards the Mowlem (Halcrow, 2002; Royal Haskoning, 2010) and of gravel in the extreme north of Swanage Bay, but the latter suggestion is based on inference from beach composition and would only be possible when storm waves are operative. Diver inspections report that seabed gravels and sandy gravels are colonised by weed and are therefore presumed to be immobile. There may be an offshore north-east to south-west directed pathway of fine sediment transport moving from outer Swanage Bay to Durlston Bay.
A rock platform extends from the cliff toe between Handfast Point and Ballard Point, and is discernible approximately 600m sub-tidally before the sediment thickness is sufficient to mask the underlying bedrock. It appears that some coarse material, deriving from cliff erosion, may move from onshore to offshore, particularly in the vicinity of steep cliffs and especially off Handfast Point, although no accumulations of such material have been identifie; hence offshore transport is considered to be of low confidence.
5. Summary of Sediment Transport Pathways
- 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 waves travelling from south-easterly and easterly directions are more energetic than refracted Atlantic swell waves.
- Swanage Bay is thought to operate as a 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, but with some possibility that a partial return feed occurs under favourable hydrodynamic conditions. Further information may be found at http://www.scopac.org.uk/sediment-sinks.html.
- Analysis of Coastal Monitoring Programme data and modelling studies have identified a net northward drift within Swanage Bay, but this is not precisely reflected by the distribution of beach sediments. Indeed, all the beaches appear to have been progressively depleted of sediment prior to renourishment in 2005/6, with only the gravel component increasing northward. Studies revealed that beach losses had been occurring during the thirty years before it was recharged with imported material. 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. Losses have been at least temporarily reversed by deliberate recharge, but this will need to be repeated at future intervals on the assumption that beach depletion and drawdown are fundamental, underlying features of this frontage.
- There are indications that cliff recession may be accelerating and some previously relic cliffs appear to be reactivating. 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. Locations for Future Research to Test Numerical Model Estimations of Littoral Drift
Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org.
The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Although at present a 12 year time series of data has been collected, longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future.
The bays of this frontage are pocket beaches with 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 requirement is 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.
7. Research and Monitoring Requirements
The Southeast Regional Coastal Monitoring Programme commenced in 2002. Analysis of the data between 2006 and 2012/13, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. However, at certain sites either due to a lack of long-term data, data coverage or sedimentological information (e.g. composition and proportion of beach grade material arising from cliff erosion), quantification of sediment transport rates of gravel and sand has not been possible.
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 were recommended. These conditions were implemented and met by the Coastal Monitoring Programme.
Notwithstanding results from the Southeast Regional Coastal Monitoring Programme, and the summarised information collated in the Hurst Spit to Durlston Head SMP2 (Royal Haskoning, 2010b), the following suggestions are also made:
- 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.
- 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.
- 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.
