1. Introduction
Although a short (8km) section of coastline, the South Haven Peninsula is defined by two transport boundaries to coarse grained 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, with complex onshore and offshore sediment exchanges between Poole Harbour ebb tidal delta, Studland Bay and the shoreline (Lacey, 1985; May, 1997; Brampton et al., 1998; Cook, 2007). Most authorities regard Studland Bay as a net sediment sink within the wider framework of sediment circulation in western and central Poole Bay transport cell, but with a distinct but as yet unquantified budget that involves sediment transfers from Hook Sand, Milkmaid Bank and Bar Sand.
Cliff erosion and relatively narrow beach development in the south is replaced by long term accretion north of Knoll Beach, that has formed the largest development of sand dunes in central-southern England, comprising the seawards 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 sub-regional sediment sink. Although a much visited coastline, its geomorphological evolution presents several interpretational problems (Bird, 1996; May, 2003; Cook, 2007) which have yet to be resolved. Cook (2007) provides a detailed review of historical evolution of the Studland beach system, and offers explanatory models based specifically on volumetric and planimetric changes affecting nearshore and foreshore sub-systems since the early 1990s.
Studland Bay is sheltered from prevailing southwest approaching waves by the “Isle” of Purbeck. Refracted and diffracted 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 on beach morphodynamics. The planform of Studland Bay is not yet fully adjusted to wave energy distribution (Halcrow, 1999). Relative exposure increases northwards from Knoll Beach, which over the past two decades has been a pivotal point between net erosion to the south and accretion to the north.
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.channelcoast.org. In 2012, an extensive high resolution, 100% coverage swath bathymetry dataset was collected in Poole Bay, through the Maritime and Coastguard Agency’s Civil Hydrography Programme. 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).
The Southeast Regional Coastal Monitoring Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Boscombe in 10mCD water depth. From 2003 to 2012 the prevailing wave direction was south-by-west. Average 10% significant wave height exceedance is 1.03m (CCO, 2012). Significant waves in excess of 1m in height 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 -3.4mOD. 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 coast locations. The effect is probably due to a reduction in wave refraction within the currently shallow bay as water depths increase so that slightly higher waves approach the shoreline at rather more oblique angles.
Tidal currents are weak (less than 0.3ms-¹), 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 periodic capital and maintenance 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 have experienced some degradation resulting from previous military use and both past and contemporary recreation pressure. A management scheme is in place to reinforce geomorphological and ecological conservation (The National Trust, 2001, 2008; May, 1997), which includes managed realignment at Knoll Beach.
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 at this location since the 1960s (Carr, 1971; Halcrow, 1999), especially at its northern and southern limits. 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. Variability may also be ascribed to complex alongshore linkages between the south, central and north sectors of Studland Bay, including the migration of rhythmic topography, such as embayments with wavelengths up to 200m (Cook, 2007). Seasonal and longer-term fluctuations in beach morphology in each of these three main sectors, with attempted explanations of the relative roles of controlling variables, are detailed in Cook (2007). 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 experience 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 approximately north to south to a north-west to south-east trend.
2. Sediment Inputs
Analysis of Coastal Monitoring Programme 2005 and 2012 lidar and aerial photography datasets and 2003 and 2012 topographic baseline survey data, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified.
2.1 Marine Inputs and Offshore Transport System
Studland Bay experiences a relatively low wave energy climate in comparison to much of Poole Bay, 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-southern 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), shore-parallel 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; Cook, 2007). 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)
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 (the ebb tidal delta) from the Swash Channel and Hook Sand. The lack of evidence for long-term accretion of Poole Bar (Hydraulics Research, 1988) indicates 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 onshore from the nearshore seabed. Fresh inputs may be linked to the growth 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 south-easterly storms) and sand that accumulates on the wide foreshore becomes entrained by east and south-east winds and is blown landward and deposited on the seaward margin of the expanding dune system (Photo 4). Significantly, there is a positive correlation between characteristic beach width and foredune height. 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 (Poole Harbour entrance to Hengistbury Head). This includes the effects of dredging and deepening of Poole Harbour approach channel - most recently in 2010- which has introduced localised and relatively minor modifications to tidal current velocities and directions, and wave energy (Royal Haskoning, 2010a).
In 2012, an extensive high resolution, 100% coverage swath bathymetry dataset was collected in Poole Bay, through the Maritime and Coastguard Agency’s Civil Hydrography Programme. However the shoreward limit was defined by the 2mCD contour so coverage is limited within Studland Bay. Bedforms at the southern end of the Swash Channel confirm southward movement of sediment. In the southern section of the bay, between Studland village and Handfast Point, the chalk rock platform extends sub-tidally offshore from the toe of the cliffs. The remainder of Studland Bay is relatively flat and featureless with a sufficient thickness of surficial sediment to mask the underlying bedrock.
Wave action, especially during storms, drives predominantly sandy sediments from Hook Sands into the north parts of Poole Harbour entrance. Refer to the unit on Poole Bay unit for further details.
2.2 Coast Erosion
E4 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 high vertical to overhanging east facing Chalk cliffs extending southwards to Ballard Point, where they reach 117m in height. Wave-induced basal erosion, creating occasional rock falls and topples, is strongly influenced by joint sets and other parting planes (May, 1971; May and Heeps, 1985). The collapse of a large stack - "Old Harry's Wife" - in 1899 is evidence of exposure to relatively high energy waves and physico-chemical weathering. There are other stacks ("The Pinnacles") and attenuated salients that indicate ongoing recession, estimated by May (1966) and May and Heeps (1985) to be between 0.23 and 0.46m per year 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; May, 2003). (Refer to section E3 in the unit covering Durlston Head to Handfast Point for further detail.)
Moving west from Handfast Point, the north-facing Chalk cliffs (Photo 5) 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 (May, 2003). This cliff face has a faceted morphology which is the product of combined mass movement processes and wave action (May and Heeps, 1985). Mid-slopes are partially obscured in places by vegetation and weathered debris, but the relatively clean cliff toes with basal notches indicate wave trimming. Minor headlands and small re-entrants cut into the cliffs and a narrow but well defined 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. Flint clasts released from the breakdown of Chalk boulders provide a small supply of gravel to restricted pocket beaches at the cliff toes and thence northwards towards Redend Point (May and Heeps, 1985). They diminish in size and relative frequency in this same direction. Chalk particles also move in the same direction, but are rapidly removed from transport pathways by abrasion, attrition and solution.
For this cliffed section, analysis of Coastal Monitoring Programme 2005 and 2012 lidar and aerial photography datasets indicated minimal cliff erosion. 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. No significant landslides or cliff erosion events have been measured since the Southeast Programme commenced in 2002.
E5 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). In the most northerly sector of this unit the cliff line has been protected by rock-filled gabion baskets since 1983.
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 that 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. Halcrow (2004) calculate a retreat rate of 0.03 per yearm per year for the period 1951 to 2001. It is logical to assume that historically the medium 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, where some may be incorporated into nearshore sandbanks and bars.
Analysis of Coastal Monitoring Programme 2005 and 2012 lidar and aerial photography datasets indicated minimal cliff erosion for this cliffed section. 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.
E6 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 (0.7m per year) from 1890 to 1970, based on serial map and air photo interpretation. Erosion rates very rapidly increased in the 1990s, to an average of 3m per year. 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 0.81m per year, 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 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, 2008; May, 1966, 1997, 1999).
Between 2003 and 2008 retreat averaging 3.0m per year was recorded, but with spatial variability (Royal Haskoning, 2010b). Gabion defences were installed along parts of the frontage in the late 1990s, to protect a car park and café north of Redend Point (Photo 8) and beach huts further to the north (Photo 9). These were removed in 2003 as they were being undermined, thus demonstrating ongoing recession and consequent beach profile narrowing and steepening.
Between 1994 and 1998 a 3m high eroding cliff formed at the dune edge (Photo 9), with relatively little opportunity for progradation because of the inter-tidal beach, subject to cross-shore transport. Analysis of 2003 and 2012 topographic baselines, and 2005 and 2012 lidar and aerial photography datasets collected by the Southeast Regional Coastal Monitoring Programme indicated a continuing trend of beach and foreshore erosion.
E7 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 4.3m per year 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- some 20m in Shell Bay between 1951 and 2001 (Halcrow, 2004; Cook, 2007). 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 3.3m per year, 1880-1930, was reported by Diver (1933), which apparently continued at the reduced rate of 0.5m per year, 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 foredune frontage south of Shell Bay have been relatively infrequent, and the shoreline maintained a net accretion rate between 2.43 and 0.8m per year, 1963-1993 (May, 1997; Halcrow, 2004). Fresh embryo dunes developed in 2004, probably related to a pulse of an above average quantity of sediment supply. The measured rate of beach progradation was 1.0m per year between 2003 and 2008 (McInnes et al., 2011) with no net accretion in the succeeding three years. However, between 1994 and 1997, recession was dominant along the foredunes immediately north of Knoll Beach, resulting in a distinct backshore 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 recovery, due largely to the substantial reservoir of sand provided by the wide multi-bermed inter-tidal beach and adjacent embryo dunes that characterises this sector. Halcrow (2004) suggest that between 1951 and 2001 net accretion of sand within the sector between Knoll Beach and Pilot Point was approximately 120,000m³, two thirds of which was contributed by longshore transport and the remaining one third derived from offshore to onshore movement. However, progressive drawdown of the beach is perhaps the likely longer-term trend given scenarios of future climate change and sea-level rise.
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 with the partial intention of preventing accretion in the entrance channel. By altering the configuration of this channel, increasing the velocity of ebb tidal currents 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; Halcrow, 2004). 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 a significant process in the western parts of Poole Bay, particularly as dominant south-westerly winds generate off-shore currents. It has not been quantified specifically for Studland Bay, but Posford Duvivier (1999) calculated a yield of approximately 20,000m³ per year 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 this accretionary regime could inhibit the contribution of shoreface erosion within the bay itself.
Analysis of Coastal Monitoring Programme 2003 and 2012 topographic baselines, and 2005 and 2012 lidar and aerial photography datasets provides no evidence of continued slope or dune erosion on this frontage. Therefore the 2004 arrows have been removed.
3. Littoral Transport
LT4 Handfast Point to Redend Point
Westwards from the absolute drift boundary at Handfast 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. Analysis of Coastal Monitoring Programme data confirms low rates, less than 1,000m³ per year, of net east to west littoral movement along the line of outcrop of the Chalk; and net northwards movement from Warren Point. The 2004 arrows stated no quantitative data was available.
Yield of fine sand from erosion of the Eocene cliffs has created a sandy relatively narrow inter-tidal foreshore. Royal Haskoning (2010) postulated that the foreshore is supplied by an onshore feed estimated to be between 100 and 200m³ per year. However Coastal Monitoring Programme data provides no quantifiable evidence for this source. A small berm of coarser sediment occurs immediately south of Redend Point, where it has been partly colonised by trees. Littoral drift at an approximate rate of between 100 and 200m³ per year 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. Given the low rate of longshore drift, beach morphodynamics are dominated by cross-shore transport, with drawdown in the winter by higher energy (and very occasional) storm waves.
LT5 Studland Bay: the Dunes
Analysis of Coastal Monitoring Programme data (2005-12) confirms weak to moderate northwards littoral drift, in the order of less than 1,000m³ per year, from net coastline recession in the south, and accretion in the centre and north, of this frontage. The 2004 arrows stated no quantitative data was available. Over this period, between Warren and 1km north of Knoll beach car park, the MLW has retreated in the order of 25m, whereas further north the MLW has migrated seaward approximately 30m, which correlates with dune accretion. Royal Haskoning (2010) measured a mean recession or 27m between 1960 and 2010 i.e. a rate of 0.45 m per year but almost 50m at the critical location of Knoll Beach immediately south of the National Trust Visitor Centre. 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 suggests progressive winnowing of the fine-grained fraction by wave-induced littoral transport, estimated to be between 200 and 10,000m³ per year per year (Royal Haskoning, 2010), varying from the lower value at Redend Point, up to 5,000m³ per year per year at Knoll Beach to the higher value at the northern extremity of this sector. Wave powered sediment movement is augmented by peak ebb tidal currents exiting Poole Harbour. No net drift is likely when waves approach normal to the shoreline 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 and measurements have been conducted during the past 25 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 (Cook, 2007).
LT6 Shell Bay to South Haven Point
Lacey (1985) was able to infer net north-westwards littoral drift along this sector based on a small increase in this direction of mean grain size of beach sediment 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. Analysis of Coastal Monitoring Programme data also confirms this pathway and accretionary trend, with quantified rates in the order of 1-3,000m³ per year, which reflect the prevailing low energy sheltered conditions. Previous authors have suggested periodic reversals in drift direction may also occur, although no reversals were evident in the Coastal Monitoring Programme data between 2006 and 2012. 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 south-east directed tidal 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 (Photo 2). From the mid-1990s through to 2005 Shell Bay beach accumulated sediment, except at its extremities (Cook, 2007).
4. Sediment Outputs
4.1 Estuarine Outputs
EO1 Swash Channel
The ebb-dominant tidal regime results in a net south-east directed transport of sand delivered to the tidal delta and 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 Harbour Entrance to Hengistbury Head (Poole Bay) unit for further details.
Analysis of Coastal Monitoring Programme data provided no evidence to support wave driven offshore transport, therefore the speculative 2004 WO1 arrow has been removed.
4.2 Aeolian Transport and Deposition
A1 Knoll Beach to Shell Bay
Analysis of Coastal Monitoring Programme 2003 and 2012 topographic baselines, and 2005 and 2012 lidar and aerial photography datasets indicated a trend of dune accretion. Sand that accumulates on the wide foreshore in central and northern parts of Studland Bay becomes entrained by easterly and south-easterly winds and is blown landward and deposited at the leading seawards margin of the dune system in the form of embryo dunes (Photo 10). The southern and central section of dunes are accreting at less than 1,000m³ per year, whereas the northern section of dunes, towards Pilot Point, are accreting at 1-3,000m³ per year. Strandline vegetation, and/or litter and organic debris 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 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.
Although this sector has recorded net accretion since the early twentieth century, at a mean rate of 2.15 to 4.3m per year 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 - some 20m in Shell Bay between 1951 and 2001 (Halcrow, 2004; Cook, 2007). 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 3.3m per year, 1880-1930, was reported by Diver (1933), which apparently continued at the reduced rate of 0.5m per year, 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 foredune frontage south of Shell Bay have been relatively infrequent, and the shoreline maintained a net accretion rate between 2.43 and 0.8 m per year, 1963-1993 (May, 1997; Halcrow, 2004). Fresh embryo dunes developed in 2004, probably related to a pulse of an above average quantity of sediment supply. The measured rate of beach progradation was 1.0m. per year between 2003 and 2008 (McInnes et al., 2011) with no net accretion in the succeeding three years. However, between 1994 and 1997, recession was dominant along the foredunes immediately north of Knoll Beach, resulting in a distinct backshore 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 recovery, due largely to the substantial reservoir of sand provided by the wide multi-bermed inter-tidal beach and adjacent embryo dunes that characterises this sector. Halcrow (2004) suggest that between 1951 and 2001 net accretion of sand within the sector between Knoll Beach and Pilot Point was approximately 120,000m³, two thirds of which was contributed by longshore transport and the remaining one third derived from offshore to onshore movement. However, progressive drawdown of the beach is perhaps the likely longer-term trend given scenarios of future climate change and sea-level rise.
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 with the partial intention of preventing accretion in the entrance channel. By altering the configuration of this channel, increasing the velocity of ebb tidal currents 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; Halcrow, 2004). 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 a significant process in the western parts of Poole Bay, particularly as dominant south-westerly winds generate off-shore currents. It has not been quantified specifically for Studland Bay, but Posford Duvivier (1999) calculated a yield of approximately 20,000m³ per year 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 this accretionary regime could inhibit the contribution of shoreface erosion within the bay itself.
5. Sediment Stores and Sinks (beach morphodynamics)
5.1 Studland Dunes Store
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, 2003; Brampton, et al., 1998; Cook, 2007) indicates that the present day dune complex started to accumulate against an original narrow spine of Tertiary sands no later than the 1570s and perhaps some two centuries earlier. 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, accumulated 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 seawards 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 now freshwater Little Sea, formerly a saline lagoon, 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 model sequence of embryo to and then primary and secondary dune ridges 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 addition of embryo dunes was supplied with sand via nearshore and foreshore ridges built by “pulses” of onshore sediment transport. This implies periodic, possibly cyclic, sediment supply associated with onshore sand bar migration and welding to the inter-tidal foreshore. Brampton, et al. (1998) and Cook (2007) used hydrographic charts from the mid nineteenth century to demonstrate that an expanded wide sandy foreshore, the product of oblique nearshore bars welded and perhaps superimposed, preceded a phase of new foredune creation. Wave modelling suggests that sediment accretes in inner Studland Bay, having been moved south- 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 inland 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, Bar Sand 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, 2003). Halcrow (1999) state that there is no distinction between the mean particle size, and particle size distribution of beach and dune sands. Erosion 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 (Poole Harbour entrance to Hengistbury Head), which examines the morphodynamics of the entrance channel, and adjacent near and offshore area, of Poole Harbour. Further information may be found at http://www.scopac.org.uk/sediment-sinks.html
6. Summary
- Studland Bay occupies a site sheltered by 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 450 to 600 years.
- It is presumed 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 a mean supply of 50,000 to 100, 000m³ per year. It is thought that the material could have been at least partially derived from south-east 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 west-south-westwards 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.
- Net accretion appears to have continued at least up to the late 1980s, although it is uncertain whether the mean rate has altered, and whether the source could in future become exhausted.
- Redistributions of material occur at the shoreline such that extended episodes of beach 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 has been an important factor since the early 1990s and major erosion 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), Halcrow Maritime et al. (2001) Halcrow (2004) and Royal Haskoning (2010), but there are considerable uncertainties
- 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.
7. Opportunities for Calculation and Testing of Littoral Drift Volumes
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 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.
8. 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.
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:
- What is the source(s) of the sand that has accreted so rapidly in Studland Bay?
- 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?
- What are the typical rates of drift within Studland Bay? Can circulation sub-cells be identified?
- Are recent trends for erosion in the south and accretion along the north of the bay shoreline likely to continue?
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), recommendations for future research and monitoring that might be required to inform management include:
- 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.
- 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.
- 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.
- 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.
