Handfast Point to South Haven Point (Studland Bay)
1. Introduction - References
Although a short (8km) section of coastline, the South Haven Peninsula is defined by two transport boundaries to coarse sediment, comprising: the promontory of Handfast Point (Photo 1) in the south and South Haven Point flanking the tidal inlet of Poole Harbour mouth in the north (Photo 2). It is thus a distinct subcell, within the Poole Bay transport system, although onshore and offshore sediment exchanges between Studland Bay and the shoreline are complex, and have not been researched in sufficient detail to clarify the local sediment budget (Lacey, 1985; May, 1997; Brampton, et al, 1998). Most authorities, however, regard Studland Bay as a net sediment sink within the wider framework of sedimentation in Poole Bay.
Cliff erosion in the south is replaced, by long term accretion north of Redend Point,that has formed the largest development of sand dunes in central-southern England, comprising the major part of the South Haven Peninsula. These have accumulated in stages during the recent historical period, implying episodic or cyclic sediment supply from offshore stores. Both Studland Bay and dunes therefore represent a regional sediment sink. Although being a much visited coastline, its geomorphological evolution presents several interpretational problems (Bird, 1996).
Studland Bay is sheltered from prevailing southwest approaching waves by the Isle of Purbeck. Refracted swell waves approaching originally from the southwest are diffracted around Handfast Point, thus creating a low wave energy environment along most of the length of this shoreline. It does, however experience intervals of exposure to easterly and south-easterly waves (from 90 to 150 degrees) generated within the English Channel which can exert a significant influence. The planform of Studland Bay is not yet fully adjusted to wave energy distribution (Halcrow, 1999). Relative exposure increases northwards from Knoll Beach, although significant waves in excess of 1m are only associated with waves from east and south-east fetches (Halcrow, 1999).
Studland Bay was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point in the bay at -3.4m O.D. 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 the bay was insensitive to one to two degree variation in wave climate direction (due to sheltering from prevailing SW approaching waves). However, a significant potential sensitivity to sea-level rise was identified. The rises associated with 2002 "medium and "high" estimates from UKCIP could result in a 12% to 22% variation in net longshore energy and a 220-480% increase in total longshore energy, suggesting that the Bay could be significantly more sensitive to this factor than many south locations. The effect is probably due to a reduction in wave refraction within the shallow bay as water depths increase so that slightly higher haves will approach the shoreline at rather more oblique angles.
Tidal currents are weak (less than 0.3msec-1) along most of this frontage, the exception being the area immediately adjacent to Poole Harbour inlet and its approach (the Swash Channel), where locally strong currents are generated by the exchange of tidal waters between the bay and the harbour. With the exception of the dredging of the Swash Channel, and the construction of a 1,500m long training wall to help maintain its stability, human modification of coastal processes is limited. Short lengths of both 'hard' and 'soft' defences have been built to prevent or restrain erosion losses, particularly between Redend Point and Knoll Park. Studland Dunes, however, have experienced some degradation resulting from previous military use and contemporary recreation pressure. A management scheme is in place to reinforce geomorphological and ecological conservation (The National Trust, 2001; May, 1997).
Historically (at least since the mid-nineteenth century), the cliffs and dunes of the southern and south-central sectors of this shoreline have recorded net erosion and retreat. The dunes of the central and northern sections have built forward, whilst accretion and recession have alternated in Shell Bay. Net erosion has been the dominant trend here since the 1960s (Carr, 1971; Halcrow, 1999). This spatially variable pattern is the outcome of a complex inter-relationship between sediment supply to the south-north littoral drift pathway, offshore to onshore sediment input from Studland Bay and longshore changes in wave climate. Bray et al. (1992) used an application of the Brunn Rule to study the effects of predicted sea-level rise and concluded that accretion rates in northern Studland Bay would reduce, whilst erosion rates would increase rapidly in the southern sector. The position in Shell Bay would probably be a modest increase in prevailing rates of erosion over the next 30-50 years. These are precisely the trends experienced since 1993, but at rates greatly in excess of those forecast. May (1997) notes that his earlier (unpublished, 1991) estimates of erosion, based on analysis of coastline exposure to refracted and diffracted waves have been exceeded almost by an order of magnitude (Photo 3). He ascribes this to a major modification of local wave climate due to several prolonged periods of winter easterly winds. The extent to which this coastline can recover from this impact, and thus reveal the role of sea-level rise alone, cannot be stated with confidence at present. It is anticipated that a sustained continuation of the recent recession rates would eventually result in a permanent change in coastal orientation, with a clockwise rotation from N-S to NE-SW.
2. SEDIMENT INPUTS
2.1 Marine Inputs and Offshore Transport System - F1 F2 References Map
Studland Bay experiences low wave energy, but theoretical modelling of wave refraction using hindcasting techniques indicates that there may be some wave convergence, or focussing of wave energy along the central sector of Studland Beach (May, 1997; Brampton, et al, 1998; Halcrow, 1999). Waves propagated over the south-east fetch are less affected by refraction, as they are characteristically short period. They therefore have higher inshore energy than the fully refracted south-westerly (swell) waves that enter Poole Bay. South-easterly waves are therefore more likely to cause beach drawdown, and thus the transfer of sand offshore. Under prolonged periods of westerly or south-westerly winds (blowing offshore), sand bars have been observed to develop and thereafter migrate onshore, thus expanding the width of the inter-tidal beach (May, 1997; Brampton, et al, 1998). Two surveys of the bathymetry of Studland Bay, in 1990 and 1991, revealed little change in seabed relief, despite the loss of 480,000m3 of sand from the inter-tidal beach as a result of two periods of sustained south-easterly waves in the intervening period (BP Exploration, 1992). This investigation demonstrated that, under these wave conditions, sand storage in Studland Bay did not appear to increase, nor was there any construction of offshore landforms. Hydraulics Research (1986, 1988, 1991), using a numerical modelling approach, indicated significant transport convergence towards Poole Bar, particularly westwards movement, during spring tidal cycles,. This would promote net accretion in Studland Bay, with some interception and re-direction of southward moving sand in the Swash Channel likely (Halcrow, 1999).
F1 Onshore Sand Transport in Studland Bay (see introduction to marine inputs)
Circumstantial evidence therefore suggests that, except under prolonged easterly or south-easterly winds and waves, Studland Bay is a sediment sink partly fed by an offshore pathway that converges on Poole Bar from the Swash Channel and Hook Sand. The lack of evidence for long-term accretion of Poole Bar (Hydraulics Research, 1988) indicates a probable transfer to Studland Bay. Furthermore, the history of dune growth and net sediment accretion along the central and northern dunes and beach of the bay suggests that material is fed ashore from the nearshore bed of the bay. Inputs may be linked to the growth and breakdown of bar topography, but the reasons for apparent periodicities of supply from offshore are not clear (May, 1997). The feed is undoubtedly wave-driven (possibly during SE storms) and sand that accumulates on the wide foreshore becomes entrained by E and SE winds and is blown landward and deposited on the fringes of the expanding dune system (Photo 4) A full discussion of sediment transport and sedimentation at, and seawards of, the entrance to Poole Harbour is provided in the section on Poole Bay (Sandbanks to Hengistbury Head).
F2 Wave Driven Transport to Poole Harbour Entrance (see introduction to marine inputs)
Wave action, especially during storms drives predominantly sandy sediments from Hook Sands into the north parts of Poole Harbour entrance. See Poole Bay unit for further details.
2.2 Coast Erosion - E1 E2 E3 E4 References Map
E1 The Foreland (Handfast Point) to the Warren
This sector comprises north facing Chalk cliffs, 18m to 26m in height (Photo 5) and the celebrated stacks and arches of Old Harry Rocks at Handfast Point (Photo 1). The latter are more properly a part of the 30-100m high, vertical to overhanging east facing Chalk cliffs extending southwards to Ballard Point. Wave-induced basal erosion, creating rock falls and topples, is strongly influenced by joint sets and other parting planes (May, 1971). The collapse of a large stack - "Old Harry's Wife" - in 1899 is evidence of exposure to relatively high energy waves and physio-chemical weathering. There are other stacks ("The Pinnacles") and fragile salients that indicate ongoing recession, estimated by May (1966) and May and Heeps (1985) to be between 0.23 and 0.46ma-1 for the period 1882 to 1975. Cliffline retreat has created a shore platform in the vicinity of Old Harry Rocks, but this feature is less apparent further south. There are, however, some isolated coarse shingle pocket beaches of gravel trapped in small joint and fault-guided embayments at the base of the stretch of cliffs named 'Old Nick's Ground' (Bird, 1996).
The north-facing Chalk cliffs (Photo5) are low in elevation partly due to their protection from high energy (refracted swell) waves; and partly because the block of Chalk north of the Purbeck thrust plane has been relatively downfaulted (Bird, 1996), giving low dip angles. This cliff face is partially obscured in places by vegetation and weathered debris, but the relatively clean cliff toes with basal notches indicate wave trimming. Minor headlands and bays cut into the cliffs and a narrow shore platform are further evidence of long-term recession, despite low wave energy. There are no published estimates of historical rates of shore platform and cliff erosion. Flints released from the breakdown of Chalk boulders provide a small supply of gravel to thin pocket beaches at the cliff toes and northwards towards Redend Point (May and Heeps, 1985).
E2 The Warren to Redend Point (South Beach)
The near right-angle change in coastline orientation at The Warren Wood is the result of the outcrop of lithologically less resistant Tertiary (Eocene) sandstones and clays (Photo 6). Recession of this low (8-20m elevation) cliffed coastline is the result of shallow landslides and small-scale slumps and mudflows, particularly in the Reading Beds, London Clay and where argillaceous and lignitic horizons occur in the Bagshot beds (Poole Formation) sequence directly north of Redend Point. The latter is a small headland composed of relatively more resistant sandstone (Photo 7). It exhibits several small caves, which follow joint planes, and is fronted by a shore platform. The latter is partly in response to the progressive northwards increase in exposure to wave erosion; however, physico-chemical weathering may account for its microrelief. The breaching and hollowing-out of case-hardened ferruginous nodules create unusual opportunities for the formation of pot holes (Canning and Maxted, 1979).
May (1997) and others have noted that the presence of mature trees in front of the steep, but partially vegetated, coastal slope immediately south of Redend Point indicate absence of marine cliffing during the twentieth century. However, the comparatively weak, friable Bracklesham Beds at this point are affected by occasional slumps, as well as by biological weathering. Direct evidence of basal undercutting is only apparent at the far southern end of this unit where the cliff faces are exposed fully. The spatial pattern of cliff morphology is evidently more a function of the lithology of ground-forming materials than of incident waves. There are no published calculations of retreat rates or volumetric losses, but it is logical to assume that historically the mediun to coarse sand component of the sediment yield (estimated at 20% of total input) remains on the beach and is available for drift northward. Much of the clay, silt and fine sand fractions are likely to move offshore as suspended load, but some may be incorporated into sandbanks and bars.
E3 Redend Point to Knoll Beach
Analysis of historical maps and charts indicate that this dune-fronted shoreline has been subject to periodic erosion since the early eighteenth century (Diver, 1933; Baden-Powell, 1942; May, 1997). Carr (1971) calculated a recession rate of 80m from 1890 to 1970, based on serial map and air photo interpretation. Erosion rates very rapidly increased in the 1990s, to an average of 3ma-1. May, (1999) reported on projections of the likely erosion for the period 1994-2020 indicating that the entire projected recession was achieved in 1997. May (1997) estimated a mean erosion rate of between 0.47 and 0.81ma-1, for 1970 to 1995, but recorded 4m of recession in February and March 1996 alone under conditions of waves generated by persistent strong easterly winds. Between 1990 and 1992, MLW opposite Knoll Beach Car Park retreated 7m; and along the southern Dunes Beach almost 10m over the same period. Beach lowering between 0.3 and 0.7m occurred simultaneously. Despite some subsequent recovery, this erosion trend has persisted, resulting in substantial losses and threats to infrastructure (The National Trust, 2001; May, 1966, 1997, 1999). Gabion defences were installed along parts of the frontage in the late 1990s, notably to protect a car park and café immediately north of Redend Point (Photo 8) and beach huts further to the north (Photo 9)
Between 1994 and 1998 a 3m high eroding cliff formed at the dune edge (Photo 3), with relatively little opportunity for progradation because of the narrow inter-tidal beach along this sector. A management policy of managed retreat has been formally adopted by the National Trust (2001) in response to this acceleration of historical trends. The option of beach foreshore replenishment, using sand gained by net accretion in the northern segment of Studland Bay, has been rejected as it might threaten the future integrity of the dunes in this latter area.
E4 Knoll Beach to Shell Bay
Although this sector has recorded net accretion since the early twentieth century, at a mean rate of 2.15 to 4.3ma-1 between 1936 and 1970 (Carr, 1971), some periods of erosion have alternated with phases of embryo dune growth (Photo 10) and expansion of the width of the inter-tidal beach. This continues a pattern observed in the past, particularly 1850 to 1880 (Diver, 1933; Brampton, et al, 1998). In Shell Bay, erosion of up to 3.3ma-1, 1880-1930, was reported by Diver (1933), which apparently continued at the reduced rate of 0.5ma-1, 1933-1970 (Carr, 1971). May (1997) notes that annual losses of up to 5m occurred in the 1980s and 1990s, but with significant short-term alternating accretion phases. Erosional losses along the fore-dune frontage south of Shell Bay are less frequent, and the shoreline has maintained a net accretion rate between 2.43 and 1.12ma-1, 1963-1993 (May, 1997). However, between 1994 and 1997, recession was dominant along the dunes immediately north of Knoll Beach, resulting in a distinct dune cliff. This was a consequence of several periods of erosive wave action linked to strong easterly winds, which also induced some limited inland dune migration. Since 1998 there has been some recovery, due largely to the substantial reservoir of sand provided by the wide inter-tidal beach that characterises this sector.
The apparent fluctuations between net erosion and accretion in Shell Bay have been ascribed by some authorities (e.g. Glover, 1972) to the construction of the training bank defining the southern boundary of the entrance channel to Poole Harbour (Swash Channel). This was built in 1860, and was extended to 1,300m in 1876 and 1,500m in 1927. By altering the configuration of this channel, and intercepting northward littoral drift, from Studland Bay, scour in Shell Bay may have been induced under specific combinations of winds, waves and tides. However, it may also promote net offshore sediment transport, thus increasing potential sediment supply to the beach system via Hook Sand and storage in Studland Bay (Hydraulics Research, 1986, 1988, 1991). This effect may therefore offset losses arising from the intercepting effect of the training wall on littoral sediment transport supply from the south (Brampton et al, 1998). However, there is no quantitative data in support of this contention, and it must be regarded as speculative. Inter-tidal shoreface erosion should be significant process in the western parts of Poole Bay, particularly as dominant south-westerly winds generate off-shore currents. It has not been quantifiedspecifically for Studland Bay, but Posford Duvivier (1999) calculate a yield of approximately 20,000m3a-1 for the shoreline between Handfast Point and Durley Chine (Bournemouth). An unknown proportion of this quantity is likely to be retained in the sediment store of Studland Bay. Indeed, it could be argued that the accretionary regime could inhibit shoreface erosion within the bay itself.
3. LITTORAL TRANSPORT - LT1 LT2 References Map
LT1 The Foreland to Redend Point
Flint and Chalk clasts, deriving from the north-facing cliffs occur on the Studland village beach and north to beyond Redend Point, where they are often trapped in small potholes on the shoreline platform. This is an indication of net east to west littoral movement along the line of outcrop of the Chalk; and net northwards movement from Warren Point. Yield of fine sand from erosion of the Eocene cliffs has created a wide sandy intertidal foreshore. A small berm of coarse sediment occurs immediately south of Redend Point, where it has been colonised by trees. Littoral drift is likely to be partially intercepted by Redend Point, as the rocky shore platform that cuts the beach is only patchily veneered by mobile sediment (Photo 7) and the inter-tidal beach to the north exhibits a marked reduction in width. There are no estimates of drift rates, but low wave energy along this sector reduces its potential to low values.
LT2 Studland Bay: the Dunes
A weak to moderate northwards littoral drift may be indirectly inferred from net coastline recession in the south, and accretion in the centre and north, of this sector. However, direct transfer of sediment from the zone of loss to that of gain has not been experimentally proven, and accretion may be the result of landward directed transport from offshore (see F1). Robinson (1955) stated that cross-shore transport accounts for beach profile variation, but Halcrow (1999) demonstrate a small residual northwards obliquity (that would power drift) to breaking wave fronts, surviving the diffracting effects of both The Foreland and offshore banks and bars in Studland Bay (BP Exploration, 1992). Accretion on the southern side of the inner training wall is indirect evidence in favour of net northwards longshore transport (Hydraulics Research, 1988). Lacey (1985) reports a northward increase in grain size of the sand fraction, from a sampling survey using a set of 10 beach cross-sections spaced 100m apart. This possibly suggests progressive winnowing of the fine-grained fraction by wave-induced littoral transport, possibly augmented by peak ebb tidal currents exiting Poole Harbour. No net drift is likely when waves approach directly from the east, and there may be short-term drift reversal (i.e. north to south) on the relatively infrequent occasions when waves are generated across the north-easterly fetch (May, 1997). Most of these statements are either theoretically based or derive from very short-term observations and measurements. The reliability of knowledge of this transport pathway is therefore low. Most observations have been conducted during the past 10-15 years, during which the frequency of higher energy waves approaching from the east/south-east has been higher than in previous decades of the twentieth century.
LT3 Shell Bay to South Haven Point
Lacey (1985) was able to infer net north-westwards wards littoral drift along this sector based on a small increase in mean grain size of beach samples. Hydraulics Research (1991) confirmed this pathway for nearshore transport, using wave refraction estimates based on hindcasting derived from wind speed/direction and bathymetric data. A north-west directed potential net drift of 95,000m3a-1 was calculated, but this may be an overestimation because it did not factor in the effects of tidal currents. Actual net rates are considered to be a fraction of this amount, because of low energy sheltered conditions, the shore-normal approach of most waves and periodic reversals in drift direction (Halcrow, 1999). Sand flux studies based on tidal flow in the Swash Channel revealed a marked potential for south-east (offshore) directed transport (Hydraulics Research, 1986, 1988, 1991), where current velocities are substantially higher than in Studland Bay. It could be that material drifting along the shoreface is susceptible to entrainment and loss to this SE directed tital transport pathway (see EO1). North-westwards beach drift at South Haven Point is apparent from the groyning effect of the ferry slipway, which creates an offset in beach width (Photo2).
4. SEDIMENT OUTPUTS - References Map
4.1 Estuarine Outputs
EO1 Swash Channel
The ebb-dominant tidal regime results in a net SE directed transport of sand delivered to the Swash Channel (Hydraulics Research, 1986, 1988, 1991). It is thought that it delivers considerable quantities of sand to the offshore bed of Studland Bay where it would be available to feed wave driven onshore transport pathways. See Poole Bay unit for further details.
4.2 Wave Driven Offshore Loss
WO1 Sandbanks to Hook Sand and Swash Channel
It is thought that easterly and south-easterly waves can transport sediment seaward from Sandbanks beach to Hook Sand where it may become entrained by tidal currents operating within the Swash Channel. See Poole Bay unit for further details.
4.3 Aeolian Transport - References Map
A1 Studland Dunes
Sand that accumulates on the wide foreshore in central and northern parts of Studland Bay becomes entrained by E and SE winds and is blown landward and deposited on the fringes of the expanding dune system in the form of embryo dunes (Photo 10). Strandline vegetation, or litter may induce initial deposition. As sand continues to accumulate Sea Lyme grass and then Marram Grass colonise, thus further increasing the "roughness" of the ground surface and encouraging further aeolian deposition of sand. The dunes form a succession with vegetation becoming more mature and continuous inland. Aeolian transport reduces as areas of bare sand diminish and higher vegetation intercepts airflows. Some areas suffer temporary loss of ground cover due to trampling of the vegetation by the large number of recreational summer visitors to the dunes. At these sites eroding basins or "blow outs" may form as bare sand once again becomes entrained by winds. These can expand causing loss of habitat and are often managed (fencing windbreaks, controlling access etc.) by English Nature and National Trust to reduce their incidence. Further details of the growth and development of the dune system are given below.
5. SEDIMENT STORES AND SINKS (BEACH MORPHODYNAMICS)
5.1 Studland Dunes Store - References Map
Detailed analysis of estate plans, charts, topographic maps and vertical aerial photography by several authors (e.g. Diver, 1933; Robinson, 1955; May, 1971; Carr, 1971; May, 1997; Brampton, et al, 1998) indicates that the present day dune complex started to accumulate against an original narrow spine of Tertiary sands no later than the 1570s. An estate plan of 1585 clearly shows the present day freshwater lagoon of the Little Sea to have a connection with the open sea. The first of a succession of approximately north-to-south trending dune ridges had accumulated by 1720 in a shallow embayment defined by low cliffs cut into Bagshot (Poole Series) sandstones. Though degraded by subsequent sub-aerial denudation, this cliff line remains visible in the modern landscape. Three distinct parallel ridges, and intervening slacks, were built at approximately 100-year intervals in the northern part of the dunes (Diver, 1933). A fourth was added in the twentieth century as a result of continued dune progradation see Photo 4 (Carr, 1971). Accretion, overall has widened the northern peninsula by up to 900m. A more complex, and compressed, sequence of ridges and slacks was created in the southern area, between approximately 1720 and 1850, with the Little Sea fully enclosed by the late nineteenth century.
Dune growth occurs as a result of sand feed from the adjacent offshore zone and inter-tidal foreshore, but vertical growth is inhibited by the removal of sand from dune crests by dominant south-west winds (i.e. blowing offshore) and the fact that dune-forming winds from the east and south-east only prevail intermittently. The very rapid and widespread colonisation of older dune ridges by Calluna heath vegetation (Good, 1935; Wilson 1960) and slacks by trees (Bray 1982) has also suppressed sand erosion. Thus, the normal sequence of embryo to foredune is not encountered here, though most other environmental gradients associated with increasing dune maturity inland occur (Wilson, 1960). Robinson (1955) and Halcrow (1999) consider that each dune chain was supplied with sand via nearshore and foreshore ridges built by onshore sediment transport. This implies periodic "pulses" of sediment supply associated with onshore sand bar migration. Brampton et al (1998) used hydrographic charts from the mid nineteenth century to demonstrate that a wide sandy foreshore, with oblique bars superimposed, preceded a phase of rapid onshore movement and new dune ridge creation. Wave modelling suggests that sediment accretes in inner Studland Bay, having been moved westwards from the Swash Channel and Poole Bar. The alternative possibility of lateral northward growth of spit platforms is excluded by the small supply of sediment via littoral transport. Between 1800 and 2000, some 4.5km2 of sand dunes were thus created, with a strong tendency towards net erosion in the south and net accretion in the north after the mid-nineteenth century. Despite some exposure of sand at various inland locations, mostly due to either human activities or rabbit grazing, the dune ridge pattern has been remarkably stable. There is no evidence of any tendency for the dunes (or their component parts) to migrate or to experience any significant in situ sand re-cycling.
The precise source of supply for each phase of dune building is uncertain. However, given their very low calcium carbonate content, it must derive from the large store of sand on Hook Sand, Milkmaid Bank and on the seabed of Studland and Poole Bays (May, 1997; Brampton, et al, 1998). Periods of expansion in the width of the inter-tidal zone are implied, possibly linked to growth of offshore sandbanks and bars close to the entrance to Poole Harbour (Brampton, et al, 1998; Halcrow, 1999). On each occasion of sustained dune growth, offshore stores grew to a threshold capacity and then relatively rapidly migrated onshore. This is borne out by examination of hydrographic charts, covering the period 1785 to 1849 (Halcrow, 1999). This revealed foreshore narrowing and an increase in nearshore water depths immediately following a major phase of dune accumulation (Brampton, et al, 1998; Halcrow, 1999). At present, there is active exchange of sand between the Lyme and Marram grass dominated embryo dunes and the inter-tidal beach; during several periods in the 1990s, the dunes in the southern sector have been eroded and cliffed and have increased in height (May, 1997, 1999). Halcrow (1999) state that there is no distinction between the mean particle size, and particle size distribution of beach and dune sands. This has been in response to steep, moderately or non-refracted, waves generated over east or south-east fetches by strong easterly winds. Details of erosion and accretion rates, since approximately 1850, are given in Section 2.2. See also the Unit covering Poole Bay (Sandbanks to Hengistbury Head), which examines the morphodynamics of the entrance channel, and adjacent near and offshore area, of Poole Harbour.
6. SUMMARY - References Map
1. Studland Bay occupies a site sheltered by the mass of the Isle of Purbeck and subject to accretion of sand forming a wide beach and dune system that appears to have originated only over the past 500 years.
2. It is understood that the major sand input must have come from onshore transport from the bed of Studland Bay and it is believed that it occurred as intermittent "pulses." The quantities involved are large for the volume accreted could be of the order of 25 to 50 million cubic metres (4.5 square km and an assumed thickness of 5 to 10m) giving mean supply required of 50,000 to 100, 000m3a-1. It is thought that the material could have been derived from SE directed tidal transport from the ebb-dominant Poole Harbour entrance channel that extends into the northern part of Studland Bay. The sediments could originally have been derived from the erosion of Tertiary sands of the cliffs of Poole Bay and transported SW possibly via Hook Sand to the Poole Harbour entrance channel. There is a considerable uncertainty attached to this interpretation, but there are no credible alternative explanations for accretion of this magnitude.
3. Net accretion appears to have continued at least up to the late 1990s, although it is uncertain whether the mean rate has altered, or whether the source could in future become exhausted.
4. Redistributions of material occur at the shoreline such that episodes of erosion have occurred in southern parts of Studland Bay and in Shell Bay. Sand appears to be transported northward by net drift within the bay, which may explain why the southern parts have tended to erode. Exposure to easterly and south-easterly waves is an important factor and major erosion over the past seven years is linked to an increased occurrence of storm waves from these directions. It is uncertain whether this trend might continue, or even be exacerbated by future climate change. Assessments of potential future sensitivities to climate change have been undertaken by Bray et al (1992), May (1997 and 1999) and Halcrow Maritime et al (2001), but there are considerable uncertainties and behaviour needs to be monitored.
5. The area is of high earth science and habitat value and management of the present shoreline is based on a philosophy of limited intervention or non-intervention so that natural processes and dynamic changes should be able to operate relatively freely in future e.g. (Halcrow 2002).
7. KEY COASTAL DEFENCE AND HABITATS ISSUES - References Map
The cliffs, dunes and inter-tidal beach are subject to a large number of national and international conservation designations. The dunes and co-adjacent heaths and wetlands include a mosaic of rare and fragile habitats that have justified the declaration of this area as a Biogenetic Reserve (only one of five in the UK) and part of a UNESCO World Landscape Heritage site. The pre-eminent importance of habitat issues is fundamental to current management approaches (e.g. The National Trust, 2001). It is therefore anticipated that there will be no extension or reinforcement of existing defences. For the eroding frontage between Knoll Beach and Redend Point, the National Trust have adopted a managed retreat option; other alternatives, in particular beach nourishment, have been rejected. The superficially attractive approach of transferring sediment accretion gains along the northern dunes sector to South and Shell Beaches - both areas of deficit - will not be pursued. This is because there is no long-term guarantee that the former area would continue to show a local budget surplus, and sand removal could threaten dune stability and ecological diversity. Management policy fully accepts that the dynamic exchange of sediment between beach and dunes should not be inhibited.
A more difficult problem is that of visitor recreation pressure over the entire dune complex, particularly vegetation loss from trampling and fire. A wide variety of approaches to control these impacts have been practised for several decades, and it is anticipated that they will continue into the future (The National Trust, 2001). However, the long-term integrity of the dunes might be most effectively assured if spatial or seasonal access zoning is introduced. This is counter to prevailing management philosophy.
The freshwater lagoon of the Little Sea is, fortunately, free of any intensive use. However, the extensive regionally important eel grass beds in the littoral zone of Studland Bay require a continuous supply of sediment to ensure their survival. Any extension of the training wall could pose a threat. Increasing use of personalised water craft and other aquatic recreational activities could affect subtidal habitats close inshore in southern parts.
8. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map
The unique east facing orientation, irregular nearshore bathymetry and the relative absence of features that can intercept and trap drift mean that it is difficult to undertake studies of drift on this frontage. Important first steps would be to establish a local wave climate, conduct hydrographic surveys of the bathymetry of the bay and to continue recent beach monitoring initiatives. Major problems would involve field monitoring of wave conditions to validate the unique wave climate and also to derive a means to validate transport estimations at sites where change in beach volume could result from onshore rather than longshore transport. It has been suggested that transport along the bay is extremely complex with the development of sand circulation sub-cells possibly related to the changing nearshore bathymetry.
9. RESEARCH AND MONITORING REQUIREMENTS - References Map
There has been a modest increase in knowledge and understanding of the coastal sediment transport process system on this frontage over the past 10 years, although it has mostly been derived or inferred from wider studies of Poole Bay, or Poole Harbour approach channel rather than work that has focused directly upon Studland Bay and its shoreline. Consequently, the following important questions remain unanswered or uncertain:
1) What is the source(s) of the sand that has accreted so rapidly in Studland Bay?
2) Is accretion likely to continue in the future, or could it be affected by: (i) future climate change and sea-level rise and (ii) management of the Swash Channel?
3) What are the typical rates of drift within Studland Bay? Can circulation sub-cells be identified?
4) Are recent trends for erosion in the south and accretion along the north of the bay shoreline likely to continue?
The SMP (Halcrow Maritime, 1999) and an in-progress coastal defence strategy plan have reviewed, synthesised and contributed to this much of the available information and have made recommendations for targeted monitoring and research. Furthermore, many of their 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, 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:
1. The wave climate of Studland Bay is unique due to its eastward exposure, the sheltering effects of the Isle of Purbeck and shoaling and refraction of approaching waves over Hook Sands and Poole Harbour Swash Channel. Although local wave climates have been prepared by several previous studies using available data, it is important that they should be validated/calibrated by some local wave measured records. The existing network of wave recorders operated/managed by the Channel Coastal Observatory extends only to a buoy at Boscombe in the east of Poole Bay so it is recommended that a temporary period of recording is required within Studland Bay.
2. The effective application of numerical modelling studies of wave transformation, sediment transport and beach behaviour requires the input of high quality nearshore bathymetric survey data. This is especially important for those sectors of the near and offshore environments with complex landform and sediment associations such as Hook Sands, the Poole Harbour Swash Channel and the nearshore bed of Studland Bay. Surveys should be completed with reasonable frequency and perhaps integrated with routine bathymetric surveys of the harbour entrance and approaches undertaken by the Poole Harbour Commissioners. Ideally, they should be combined with some sea bed sediment sampling. The latter would provide valuable information on the potential for onshore sediment transport through the compilation of large-scale maps of sediment distribution, and analyses of particle size and sorting to derive bed transport vectors as was undertaken for Chichester Harbour entrance by Geosea Consulting Ltd (1999). It might, in particular, throw light on the important question of whether offshore to onshore sand supply is a sustainable process under the contemporary hydrodynamic regime, or whether its sources could suffer interception (possibly by Swash Channel management) and/or exhaustion. A second application of the bathymetry would be to enable reliable wave transformations and determination of breaking wave climates along the shoreline. These, in turn, should enable improved modelling of sediment transport.
3. A network of beach profiles needs to be established together with arrangements made for regular re-surveys. It would also be advantageous to undertake post-storm profile surveys at critical locations e.g. around Knoll car park where severe dune cliffing has occurred. Profiles would need to be extended into the dunes since sand can transfer from the beach to the dunes, potentially depleting the former. Periodic bathymetric surveys are also required to extend profiles seawards to water depths where there are limited bed level changes. Such profiles would be especially valuable in order to attempt to identify evidence of migrating bars or other features associated with the significant offshore to onshore transport within the bay. Overall, the data would enable quantitative analyses, especially of volume changes (which have hitherto been lacking) and would provide valuable insights into the rates of operation of littoral transport, onshore feed and the effectiveness of any beach and dune management. It is understood that the Strategic Regional Coastal Monitoring Programme (www.channelcoast.org) aim to provide vertical aerial photo coverage, together with a series of survey control points and some measured baseline profiles.
4. Once a programme of profile measurement has been established, consideration is needed of how the profiles should best be analysed. It will be important to identify indicators of beach health such as sediment volume, crest height and position and possibly also some dune parameters (note that sediment transfers may occur from the beach to the dunes potentially involving significant losses from the beach). An error analysis should be undertaken to identify the minimum volumetric change that can be resolved with the techniques. Trends in these indicator parameters (annual and seasonal) need to be established as data accumulates and a system of routine analysis instituted that would provide early warning of "unusual" trends. It may be that local managers can identify critical thresholds, or minimum values of these parameters that could be applied to trigger specific warnings. To effectively interpret the trends recorded, it will also be vitally important to maintain good records of all beach and dune management activities undertaken.
5. To understand beach profile changes it is important to have knowledge of the beach sedimentology (gain size and sorting). Ideally, a one-off field-sampling programme covering the seaward (mobile) dunes as well as the beach is required to provide baseline quantitative information along this shoreline together with a provision for a more limited periodic re-sampling to determine longer-term variability. Results could also be compared with those of some previous sampling by Lacey, (1985). Grain size data would also be of great value for future modelling of sediment transport, for uncertainty relating to grain size is often a key constraint in undertaking modelling.
10. REFERENCES - Map
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MMIV © SCOPAC Sediment Transport Study - Handfast Point to South Haven Point