Poole Harbour Entrance to Hengistbury Head (Poole Bay)
1. INTRODUCTION - References MapPoole Bay is a relatively shallow embayment delimited by Poole Harbour tidal inlet (Photo 1) to the southwest and Hengistbury Head/Christchurch Ledge (Photo 2) to the east. It has a very gentle log spiral form, curving slightly more in the west than to the east. Much of the coastline features formerly rapidly eroding soft cliffs that are now fronted by a substantial seawall/promenade built progressively eastwards, in stages, between 1878 and 1973 (Photo 3 and Photo 4). Unprotected cliffs occur along the easternmost 2 km at Hengistbury Head (Photo 5). Beaches are predominantly sandy, though they coarsen eastwards in the same net direction as longshore drift along this coastline. Intensive beach management has been practiced for over a century (Lelliott, 1989, Harlow, 2001). Up until the 1970s this was achieved through the building and maintenance of a groyne system to offset the effects of out flanking of the progressively move eastward terminal position of the seawall. In 1938, construction of the Long Groyne at Hengistbury Head, attempted to intercept drift and retain sediments within Poole Bay. It resulted in beach accretion updrift providing some protection to the toes of cliffs at the headland. During the last 30 years, more reliance has been placed on periodic recharge using imported sand and gravel, though groynes remain critical as means of controlling drift rates and beach levels. (Harlow, 2001; May, 1990; Cooper, 1997, Cooper, et al, 2001).
The solid geology of the cliffs, and the seabed beneath Poole Bay, is composed of rocks of the Tertiary Bracklesham Group, consisting of a sequence of fine, medium and coarse sands (Bristow et al, 1991). At Hengistbury Head there are younger rocks of the Bartonian group, forming an outlier (Photo 5). The Barton Clay here is made up of a series of sands and interbedded clays, with four distinct bands of ironstone nodules. These formations dip eastwards and are cut out by a north-west to south-east trending fault. This defines the eastwards boundary of Christchurch Ledge, which is a seawards continuation of the resistant ironstone strata exposed in Hengistbury Head.
The narrow entrance to Poole Harbour is acts a boundary to beach drift, but there is thought to be some bypassing of littoral sediment via Hook Sand and into Studland Bay. For this reason, Studland Bay and the South Haven peninsular can be considered as a potential sink for sediments circulating within western parts of Poole Bay. This is fully covered in this section, involving overlap and some repetition with sections covering the coast between Handfast Point and South Haven Point to ensure a holistic view of sediment movement, and processes and patterns of sedimentation.
A substantial increase of both qualitative and quantitative knowledge of most aspects of the hydrodynamics and geomorphology of Poole Bay has been achieved in the past 30 years. This is due to the promotion of internal and commissioned research by Bournemouth and Poole Borough Councils and Poole Harbour Commissioners; strategic co-operation between them; central government support of measurement and monitoring programmes both before and after the renourishments of Bournemouth beach; several academic research investigations, and the interest of commercial organisations. Of the latter, the most important - in terms of generating original data and further analysis of hydraulic conditions - derived from a proposal in the late 1980s/early 1990s to construct an artificial island, offshore of the mouth of Poole Harbour, as part of the expansion of the Purbeck oil field. Several additional studies are currently in progress (2002) as part of the Poole Bay Coastal Defence Strategy Study (Halcrow, on behalf of Bournemouth and Poole Borough Councils and Purbeck District Council). Preliminary results from some of these have been incorporated into this text and the final publication of the strategy study, will contain new analyses that should help to clarify some of the remaining questions and uncertainties.
1.1 Coastal Evolution
Pleistocene evolution was dominated by the Solent River which flowed across the floors of Poole and Christchurch Bays and hence through the Solent and Spithead (Everard 1954, Bray, 2000; Velegrakis et al, 1999). During the fluctuating sea levels of this period the Solent River and its tributaries deposited a descending sequence of gravel filled channels and terraces which mantle the predominantly sandy Eocene bedrock (Nicholls 1987; Bristow et al, 1991; Allen and Gibbard, 1993). Remnants of these fluviatile deposits occur, up to 3 m in thickness, at the summit of the modern cliffline; they have also been mapped in the offshore areas of Christchurch and Poole Bays (Fitzpatrick 1987; Velegrakis, 1994). A critical factor in the creation of Poole Bay was the breaching of the Chalk ridge, which previously extended between the Needles and Handfast Point. Opinion is divided as to the date of this event. Until recently, the prevailing view favoured breaching in the early Holocene (Everard 1954, Keen 1975, Jones 1981). Interpretation of a buried channel in Poole Harbour (Devoy 1972), studies of Chalk erosion rates, offshore buried channels and the depth, relief and inclination of the plantation surface which truncates the Purbeck-Wight ridge now indicate an early to mid-Devensian breach (Wright 1982, Lacey 1985, Nicholls 1987; Velegrakis et al, 1999; Tyhurst and Hinton, 2000; Nowell, 2000). It is thought that much of Poole Bay was opened up at this time, causing the upper Solent River (proto-Frome) river to change direction and flow south across the present area of Poole Bay (including the Chalk outcrop) as sea-levels fell during the Devensian (Wright 1982; Velegrakis et al, 1999).
Rising sea-levels of the Holocene transgression caused rapid erosion of the soft Teritiaryand much sediment redistribution over the floor of Poole Bay, as it assumed its present planform (Devoy, 1982). Velgrakis (1994) and Velegrakis et al (1999) provide a detailed stratigraphy of the sediment infill of three buried channels that are incised into the Chalk outcrop and extend further seawards. This consists of basal fluvial material, passing upwards into estuarine sediments. This sequence is a clear record of early Holocene marine invasion, creating an enclosed bay head. The simultaneous erosion of Hengistbury Head/Christchurch Ledge initiated the equally rapid, but later erosion of Christchurch Bay (Wright 1982; Nicholls and Webber 1987a; Velegrakis, 1994; Bray, 2000; Velegrakis, et al, 1999). Further details are given in the section on Quaternary Evolution of the Solent.
Both Poole (partially) and Christchurch (fully) bays have a log spiral/zeta form planshape, but concepts of crenulate bay evolution need to recognise that Poole Bay pre-dates Christchurch Bay. Poole Bay has a quasi-stable condition fixed by the "anchoring" effect of Hengistbury Head. Accelerated erosion of this headland would induce major instability and a return to an earlier unstable configuration characterised by rapid coast erosion.
Marine erosion of Poole Bay released large quantities of sediment, some of which must now be represented by thick sediment accumulations in the western part of the bay, such as Hook Sand and in the offshore area of Studland Bay. Eastward transport into Christchurch Bay may also have occurred as the precursor to Hengistbury Head would have retreated and diminished (Nicholls 1985, Nicholls and Webber 1987; Halcrow, 1999). Significant quantities of sediment may also have been lost from the bay in the early stages of its erosion, although the present whereabouts of such material is conjectural.
1.2 Hydrodynamic Regime
Wave and tidal currents are the dominant sediment transport mechanisms. The wave climate varies spatially due to the sheltering effect of Handfast Point and the "Isle" of Purbeck. Halcrow (1999) assert that the plan shape of Poole Bay is adjusted to the directions of approach of offshore swell waves. Prevailing wave direction offshore is from the south-west which also coincides with the longest fetch. Waves from this sector cannot directly enter Poole Bay, but are refracted and diffracted so as to approach from the south and south-east. The degree of shelter afforded to such waves therefore increases westwards in the lee of Handfast Point. Nearshore wave climate is characterised by transformed swell waves and waves generated by local fetches to the south and south east.
Several detailed wave climate studies have been undertaken in Poole Bay. A wave rider buoy was operated off Southbourne between 1974 and 1978 and recorded a maximum wave height of 8.8m, with a computed 1 in 10 year return period. The mean significant wave height (Hs) of 2m was exceeded 2% of the time, with a maximum (predicted) significant wave height of 3.9 m recorded twice during this period (Henderson and Webber, 1977; 1978; Henderson 1979, 1980; Halcrow 1980). In another study, using Portland wind data (1974-1984) wave hindcasting was employed to determine an offshore wave climate for Solent Beach; this indicated that Hs of 2m was exceeded 12% of the time, and Hs of 1m 39% of the time (Hydraulics Research 1986). An appropriate wave refraction and shoaling model was then used to transform offshore conditions onshore, with accuracy improved by calibration against the measured wave buoy data. The inshore wave climate was used to determine extreme wave heights using the Weibull distribution which revealed a significant wave height of 3.9m with a one year, and 5.3m for a 20 year, return period (Hydraulics Research 1986). A similar approach, also based on Portland wind data (1974-1990) was used to determine the offshore wave climate in the western part of Poole Bay (Hydraulics Research 1991b). Attention was focused on an inshore point to the east of Hook Sand and it was found that waves from the 90E-180EN sector dominated (Hydraulics Research 1991b). Approximately 4% of waves exceeded 2m and 1% exceeded 3m in height. Extreme conditions were calculated for the 120E-180EN sector which produced Hs values of 3.5-3.8m with a 1 year return, and 5.4-6.0m with a 50 year recurrence.
HR Wallingford (1995a) undertook a numerical modelling study of the wave climate of western Poole Bay. Swell waves were not included, but it was determined that 44% of waves generated over local fetches approach from the south-west, and 28% from the east/south-east. Hs values were rarely above 1.0m. A simplistic wave refraction analysis indicated wave energy convergence at (i) Southbourne; (ii) the central Bournemouth frontage, and (iii) Hook Sand, and other smaller sandbanks north and south of the Swash Channel. Earlier work by Henderson (1979) developed a series of computer-generated wave refraction diagrams for outer Poole Bay which concluded that longer period waves were most significantly modified by refraction, set up by Christchurch Ledge, Beerpan Rocks, Dolphin Bank and Dolphin Sand. Maximum wave focusing in this analysis was found to occur offshore Hengistbury Head (Henderson and Webber, 1979; 1980).
Brampton, et al (1998) employed the UK Meteorological Office (UKMO) fine mesh wave model, using data for January 1991 to December 1996, to create an estimation of the wave climate for both the offshore and inshore zones of Poole Bay. In this case, swell waves were included, and it was determined that in the offshore area where water depths exceed 6m, swell waves with a one year return period have characteristic Hs values of 2.5 to 3.0m. The effect of shoaling and refraction reduced these to 1.0 to 2.0m in the inshore zone, with maximum energy focusing along the frontage of central Bournemouth.
A similar approach was used by Halcrow (1999), also based on the UKMO model but employing a 15 year continuous record of wind speed and direction for validation . Wave height and approach were determined for an offshore "node" in Poole Bay, and it was concluded that refracted waves approaching from the west and south-west affected the inshore environment of Poole Bay for 85% of the time. Although this is the prevalent wave type, dominant waves from the east and south-east affect all sectors of this coastline. For the offshore area, maximum Hs values were calculated to vary between 5.5m (1 year return period) and 7.4m (50 year recurrence) for a location off Southbourne. Mean inshore Hs values were also determined for unbroken waves, approaching from all directions. These are greatest at Southbourne, declining rapidly westwards towards Sandbanks, and reducing slightly at the Long Groyne. Halcrow are currently engaged in further analysis of the regional wave climate as part of the Poole Bay Coastal Defence Strategy Study.
HR Wallingford (1999) report the results of an independent analysis of the offshore wave climate using the HINDWAVE numerical prediction and TELURAY wave refraction modules. Mean Hs values are computed for several approach directions and return frequencies. For locations offshore Poole Head, Bournemouth Pier and Southbourne, mean Hs for annual recurrence varies between 2.16m (155°) and 1.01m (245°); 2.46m (155°) and 2.80m (245°), and 2.26 (155°) and 3.61 (245°) respectively. Similar orders of difference are computed for a 1 in 10 year recurrence, with the highest value being 4.88m for waves approaching Southbourne from 245°.
These studies demonstrate the effect of increasing shelter in the west of Poole Bay as waves with a dominant south to south-west approach in this area are less severe. By contrast, conditions along the eastern sector are more energetic, with waves having a dominant south, south-east and south-south-east approach. These differences affect the direction and rate of longshore sediment transport and the nature of beach material distribution (Sections 3 and 5).
Tidal range is low, approximately 2.0m during spring cycles and 1.0m during neaps. A weak "double high water" component occurs, mean range increasing westwards. These characteristics concentrate wave action into a relatively narrow height range. Bed stresses created by tidal currents alone are not usually sufficient to entrain sand, but they act in conjunction with waves to transport material both onshore and offshore. The most rapid currents are in (i) the extreme western part of the bay, where the peak ebb flow at Poole Harbour mouth approaches 2.5ms-1, and (ii) offshore the Long Groyne at Hengistbury Head and above Christchurch Ledge. Tidal flow in the west is deflected south by prevailing south-easterly currents and generates currents up to 1.0ms-1 off Handfast Point (BP 1991; Riley, et al, 1994). Tidal flow has been studied in the western part of the bay by mathematical models (Hydraulics Research 1986, 1988, 1991b; HR Wallingford, 1995a). These have been validated against data obtained by Ocean Surface Current Radar (OSCR) and direct depth-averaged tidal current measurements from three locations (BP 1991, Hydraulics Research 1991b). Peak near and offshore velocities are highest in deeper water, but are generally less than 0.6ms-1, with values of 0.15-0.25ms-1 in the vicinity of Bournemouth and Boscombe piers (Riley, et al, 1994; HR Wallingford, 1995a). These figures relate to spring tides. Ebb flow is predominantly westward off Hengistbury Head and south or south westward further west. Flood flow is north or north-east in the west, becoming eastward in direction towards Hengistbury Head. Southward and south-westward residual flow occurs in the west out of Poole Bay (Hydraulics Research 1991b; Brampton, et al, 1998). There is also an eastwards residual flow towards Christchurch Ledge, with velocities progressively higher with distance offshore (Brampton, et al, 1998; Osborne, 1991). Current metering of surface and sub-surface residual currents in the offshore zone has indicated that speeds decrease from the surface towards the seabed, with an imposed anticlockwise rotation (Osborne, 1991).
Brampton, et al (1998) used the TELEMAC two-dimensional numerical model to simulate tidal current vectors and velocities. They demonstrated pathways of sand transport, for median grain sizes of 200şm and 100şm, consistent with direction of residual tidal currents in the offshore zone. For inner Poole Bay, it was concluded that tidal current velocities alone are below sediment entrainment thresholds, and that stresses imparted by shoaling and breaking waves are a necessary auxiliary to tidal currents to effect significant sand transport. Waves become the dominant force in the inshore zone, as demonstrated by the SANDFLOW numerical model. Halcrow (1999) confirmed this general pattern except in the entrance channel to Poole Harbour, where ebb tidal current velocities in the Swash Channel - particularly along its eastern boundary - are higher than elsewhere in western Poole Bay. However, even here, it is wave and tide generated generated stresses working together that account for sand mobility (Hydraulics Research, 1986; Brampton, et al, 1998). Transport rates have been observed to increase substantially under the action of storm waves.
2. SEDIMENT INPUTS - F1 F2 F3 References Map
2.1 Marine (Off to Onshore) Input
Two categories of marine input are recognised, comprising: (i) input of fresh sediment to the Poole Bay system; and (ii) sediment feed to beaches and Poole Harbour entrance from existing nearshore/offshore stores within the Poole Bay system.
F1 Westward Transport from Christchurch Bay (see introduction to marine inputs)
An offshore survey employing echo-sounding, side-scan sonar and sediment sampling revealed sand-wave bedforms which indicated net westward transport of increasingly fine sands from the Needles Channel to Dolphin Sand at the eastern margin of Poole Bay (Dyer 1970). Additional offshore surveys using similar techniques, including direct observations by divers, revealed this area to be covered by southwestward migrating sand waves, thus suggesting sediment input from Dolphin Bank (Fitzpatrick 1987). Both surveys were of limited duration but indicated similar transport pathways. Heavy mineral assemblages of sediments sampled from migratory sand waves also support the conclusion that there is sediment feed from outer Christchurch Bay (Fitzpatrick 1987).
A variety of other, indirect, evidence is also available, both supporting and opposing the assertion of input from Christchurch Bay. Comparison of Admiralty hydrographic charts covering the period 1849-1977 revealed net accretion of 727,000m3a-1, mostly in the central part of Poole Bay (Lacey 1985). Comprehensive analysis of available documents revealed no other possible sediment input that could account for this scale of accretion. Cliff erosion input was greatly reduced by coast protection along much of the Bournemouth frontage between the first and third decades of the twentieth century (Lacey 1985). Input from the English Channel is unlikely because offshore transport pathway studies reveal movement is towards the south or south-west (Hydraulics Research 1986, 1988, 1991b, Fitzpatrick 1987). Furthermore, chart comparisons revealed net erosion of the seabed in Christchurch Bay by 505,000m3a-1 for the period 1849-1977 (Lacey 1985). Westward transport from here to Poole Bay is therefore a feasible explanation for this pattern of differential erosion and accretion.
This postulated seabed transport pathway is opposed by the findings of drifter experiments (Watson 1987, Tyhurst 1976, Turner 1990). These studies show a net eastward transport potential over all areas of the sea bed east of a line drawn south of Bournemouth pier. The reliability of these studies is questionable because it is uncertain what drifters actually measure and it has been shown that they are not necessarily reliable indicators of residual seabed currents and seabed mobility (Collins and Barrie 1979). Additionally, recoveries were between 38 and 75% and there was uncertainty as to the accuracy of some information returned and the exact routes followed by drifters. Despite these problems, this technique has been shown to reproduce accurately general transport trends in other areas. This postulated pathway cannot be verified until the results of drifter studies are more thoroughly analysed, or further data becomes available.
F2 Bournemouth to Southbourne Beaches: Offshore to Onshore Transport (see introduction to marine inputs)
Major beach nourishment was undertaken between Bournemouth and Southbourne over the periods 1974-75 and 1988-89 (Photo 6). The first scheme involved import and dumping of 1.4m million m3 dredged sand at sites over 400m offshore the position of mean low water. Approximately 650,000m3 of sand was then pumped ashore and reprofiled (Newman 1978, Halcrow 1980, Wilmington 1982). The beach was intensively monitored thereafter by beach profiling, which extended up to 450m offshore. Surveys were undertaken at frequent intervals along 38 survey lines between Alum Chine and Hengistbury Head. Comparison immediately before and after nourishment revealed that the intertidal zone had gained 725,000m3 of sediment compared to the 650,000m3 pumped ashore, thus suggesting onshore transport of 75,000m3 of sand from dump sites during the operation (Lacey, 1985; Harlow and Cooper, 1994; 1996). Thereafter, the intertidal zone lost material, offshore and by littoral drift, but the nearshore and offshore zones continued to accrete.
After 1979, all zones lost material and by 1982, the intertidal zone in many areas had returned to its prenourishment volume, so onshore feed had only a brief residence time (Lacey, 1985; Harlow, 2000; 2001; Harlow and Cooper, 1994). Detailed analysis of beach profiles measured between the conclusion of nourishment and late 1979 revealed net accretion of 270,000m3 on the beach and nearshore zone extending up to 300m offshore (Hodder 1986). Assessment of littoral drift using a wave energy flux technique based on refraction studies (Henderson, 1979; Henderson and Webber, 1979) indicated littoral drift output of 340,000m3 over the same period (Hodder 1986). Total sediment input was thus estimated at 610,000m3. After examination of possible sediment sources, it was concluded that the material was derived from onshore movement of imported sand from the offshore dump sites. Sediment sampling revealed that coarser grades were preferentially moved onshore, whilst fine sand remained at the dump-sites (Hodder 1986). Beach volume analysis indicated that after 1979 this onshore feed ceased; cessation of supply was attributed to exhaustion of the dump site sediment stores, with offshore transport due to net eastward transport of sand to higher energy beaches where sediment was no longer stable in the nearshore zone (Hodder, 1986; Harlow and Cooper, 1994, 1996; Cooper, 1997). The pattern of events recorded indicates a potential for onshore transport, dependent on sediment availability within the nearshore zone. No permanent net onshore feed is likely for Bournemouth beaches because it does not appear to be maintained by supply from further offshore (Lacey 1985, Hodder, 1986; Harlow and Cooper, 1994). Detailed studies in the Solent Beach area confirmed this, because beach profiling, offshore tracer studies and mathematical modelling all failed to indicate any sustained onshore feed (Wright 1976, Webber 1980, Halcrow 1980, Hydraulics Research 1986). If onshore transport at Bournemouth Beach occurs, it is likely to be a summer seasonal effect associated with relatively calm swell conditions and simply constitutes a redistribution of existing beach material. Classic storm (intertidal erosion) and swell (intertidal accretion) profile variations were recognised from beach profiling (Lacey, 1985; Hodder, 1986; Gao and Collins, 1994), and onshore feed for a limited period after a storm was identified from beach profiles and a fluorescent tracer experiment (Kent 1988).
F3 Sandbanks (see introduction to marine inputs)
Examination of the pattern of sediment accretion against groynes first constructed at Sandbanks between 1896 and 1906 revealed a possible onshore feed from Hook Sand (Robinson, 1955). Evidence of this process is provided by examination of historic maps and charts covering the period 1785-1953. These showed a tendency for offshore bars to form between Poole Head and Sandbanks and it is envisaged that their onshore migration could supply sand to the beach (Poole Harbour Commissioners, 1995-6). Littoral drift rates were calculated for five points between Branksome Chine and Sandbanks using an energy flux technique based on wave modelling (Hydraulics Research 1991b). Longshore integration revealed mismatch of volumes between adjoining points (i.e. too much eastward drift to be sustained by the limited beach material available), which could be due to difficulties in precise modelling of drift, or perhaps linked with fluctuations of onshore-offshore transport. Historical beach volume measurements were not available as a check, so distinction between onshore transport and other possible causes were not possible. Nevertheless, this analysis indicated a tendency for either beach erosion, or onshore transport, for three of the four segments considered. This investigation was conducted as part of a programme of research into the hydraulic effects of siting an artificial island just over 1km offshore Poole Head. It did not consider the possibility of an onshore pathway of sand transport, although rigorous mathematical modelling suggested increased sedimentation would occur in the lee of the hypothetical island (Hydraulics Research, 1991b).
Interestingly, post-1991 beach erosion has tended to support the earlier HR Wallingford (1991b) analysis that indicated an excess of transport potential along the Sandbanks peninsula. Subsequent studies by HR Wallingford (1994; 1995a and b and 2000) confirmed the occurrence of net eastward drift over recent decades that might have been expected to have completely depleted the beaches and partly eroded the peninsula. That such severe erosion did not occur led HR Wallingford (2000) to infer that the beaches must have been at least partly sustained by onshore transport of sand from the direction of Hooke Sand. Direct evidence was not provided and the process was not studies further although detailed recommendations for monitoring were made. Based upon the results of these studies, rock groynes were constructed in 1995 (Photo 1) and 2001 (Photo 7) to control the beach.
A much earlier tracer experiment employing radioactive sand injected 900m offshore of Poole Head did reveal shoreward movement of the tracer centroid over a four month period (Hydraulics Research Station, 1957). However actual, supply to the beach was not proven and the experiment was of too short a duration to be fully representative of long-term trends. It must be concluded that the potential mechanism of supply is incompletely understood and onshore feed has yet to be verified beyond doubt.
2.2 Fluvial Input
No rivers of any significance discharge directly into Poole Bay from this sector of coastline. The river Bourne has discharged via a culvert since the late nineteenth century. The several deeply-dissected chines that interrupt the cliffline west of Bournemouth Pier do occasionally discharge run-off and sediment following prolonged or intense rainfall. Harlow (2001) quotes the example of Middle Chine, which discharged approximately 500m3 of sediment onto the beach in 1991 following 75mm of rainfall in 3 hours. Boscombe Chine has also discharged several inputs in excess of 3-500m3 over recent decades.
2.3 Coast Erosion - E1 References MapThe coast between Poole Harbour entrance and Hengistbury Head was previously subject to continuous erosion throughout the Holocene, resulting in development of steep cliffs between 4m and 36m in height and thus supply of sand and gravel to the beach. This situation was altered from the 1890s onwards by construction of coast protection structures west of Canford Cliffs Chine. Schemes involving protection by seawalls and groynes followed in 1907-11 (Bournemouth-Boscombe), 1927-35 (Boscombe-Southbourne) and 1955-75 (Southbourne). By 1975, virtually the whole frontage from Poole Head to Solent Road was protected (Lacey 1985, Lelliott 1989). These measures have involved protecting the toe (Photo 3) local cutback, slope grading and stabilisation (Photo 4), drainage (Photo 8) and planting, thus supply of sediment to adjacent beaches was progressively reduced as protection spread eastward along the frontage (Wigmore, 1951; Bournemouth Borough Council, 1991c). Map comparisons over the period 1867-1933 indicated cliff retreat at 0.15-0.40ma-1 between Poole Head and Bournemouth Pier, and 0.20-0.50ma-1 between Bournemouth and Southbourne (Lacey 1985).
Cliff retreat rates were combined with details of cliff height and sediment composition to determine past rates of cliff sediment supply. The analysis for 1867-1933 calculated a total supply of 115,000m3a-1, of which 91,000m3a-1 was sufficiently coarse to remain on the beach. Similar analysis for 1933-1967, taking into account the extension of the protected frontage during this period revealed quantities of 77,000m3a-1 and 66,000m3a-1 respectively (Lacey 1985). Gao and Collins (1994) undertook an independent analysis of cliff erosion yield prior to protection, and concluded that yield was 136,000m3a-1, of which 46% (approximately 100,000m3) was stable on adjacent beaches. Contemporary supply is restricted to the 2.5km length of eroding cliffs between Solent Road and Hengistbury Head and is estimated at 4,000m3a-1 (Lacey, 1985) of which 1,400m3a-1 is gravel (Bray, 1993; Harlow, 2001). Coast protection has thus had a progressive and dramatic effect in reducing sediment supply from coast erosion and this is widely cited as a major cause of the marked reduction in both drift volumes and beach levels recorded over the past 80 years (Lelliott, 1989; Harlow and Cooper, 1994; Posford Duvivier, 1998; 1999; Harlow, 2001).
Despite cliff stabilisation by regarding and planting of grasses, shrubs, etc., sub-aerial processes of weathering and mass movement continue to operate (Bournemouth Borough Council, 1991a, b and c). These include wind erosion, surface wash and gulleying, the latter promoted by groundwater seepage where there are clay and silt horizons between sandstones. Harlow (2001) estimates an ongoing rate of 1cma-1 of cliff top retreat for the frontage between Branksome Chine and Southbourne, giving a yield of 15m3a-1. All of this accumulates at the junction between the cliff foot and the promenade, and is periodically removed. The cliffs immediately east and west of Alum Chine have not been re-modelled, and are subject to periodic small scale failures induced by critical pore pressures in gravels and sands overlying clays (Halcrow, 1999). Another site of past, and potential, failure occurs east of the Toft 'Zig-Zag' path.
Detachment of sand grains by wind erosion is a minor process, but makes a contribution to the accumulation of cliff foot debris. As this material is cohesionless, it is restrained by sediment traps installed to minimise transfer to the public promenade.
E1 Solent Road to Hengistbury Head (see introduction to coastal erosion)
Two morphological units are recognised along this coast. Low cliffs composed of Valley Gravels with overlying blown sand rise eastward from Solent Road to attain 4.5m O.D. height at Double Dykes (Photo 9). Predominantly sandy cliffs at Hengistbury Head are composed of Barton Clay, Hengistbury Beds and overlying Boscombe Sands capped by Plateau Gravels (Photo 5). These cliffs rise to 36m O.D. and contain ironstone nodules and thin pebble bands (May, 1971; Lacey, 1985; Bray, 1993; Halcrow, 1999; Daley and Balson, 2000). The upper cliff (75 degrees) is distinct from the lower cliff facet (45-55 degrees), and both decline in gradient towards the headland tip. Recession has been studied using a variety of sources and timescales including O.S. maps (1840-1960) and Bournemouth Borough Council surveys (1923-1986). These surveys reveal cliff-top erosion at the following rates for the period 1840-1986: 0.75 to 1ma-1 at Solent Road; 1.12 to 1.5ma-1 at Double Dykes and between 0.95 and 3.5ma-1 at Hengistbury Head (Wigmore, 1951; May, 1966 and 1971; Hydraulics Research 1986; Parker and Thompson, 1988; Turner, 1998).
A major contributory factor to rapid erosion at Hengistbury Head was the foreshore and nearshore mining of ironstone boulders over the period 1848-1870 (Paris, 1954; Tyhurst, 1985a, Hydraulics Research, 1986; Bray, 1993; Turner, 1998). This involved direct removal of material giving cliff toe protection, and also facilitated an acceleration of littoral drift; both factors causing accelerated beach depletion. Since about 1880, rates of cliff toe erosion have declined at Hengistbury due to cessation of ironstone mining, construction of the Long Groyne in 1938 (Photo 10) and World War II anti-invasion works. These measures all promoted beach accretion and stability at the tip of the headland (Photo 2), which has continued up to the present (Hydraulics Research, 1986; Bray, 1993; Turner, 1998). However, the beneficial effects of the Long Groyne have not extended as far west as Double Dykes or Solent Beach, and the low cliffs here suffered continuing erosion at up to 1ma-1 over the period 1933-67 (Lacey 1985) and 0.4-1.1ma-1 over the period 1976-1986 (Hydraulics Research 1986). This situation is directly related to beach sediment starvation resulting from progressive eastward construction of groynes and seawalls along the updrift coastline. These measures both prevent sediment input from cliff erosion and intercept some eastward drift of sediment.
Contemporary cliff top recession at Hengistbury Head is measured at 0.2ma-1 and mostly comprises losses from gulleying, small mudflows, rockfalls and physical weathering (Photo 5). Parker and Thompson (1988) estimate that sub-aerial weathering removes some 780m3a-1 from the headland cliffline, west to Double Dykes. This is recognised as a reliable and representative long-term rate with the Long Groyne in its present configuration (Hydraulics Research 1986). Erosion reduced to 0.2 to 1.0ma-1 at Solent Beach in the vicinity of permeable groynes after they were installed in 1976-77 (Hydraulics Research, 1986; Posford Duvivier, 1997; 1999). This was due to basal undercutting during storms, cliff top runoff creating small gulleys, wind abrasion and physical weathering.
Rate of cliff erosion, used in conjunction with cliff height and lithology along the unprotected cliffline (1360m in length) enabled calculation of cliff sediment input (Lacey 1985). This analysis indicated an input (>0.08mm diameter) of 26,000m3a-1 for 1867-1933, 14,000m3a-1 for 1933-67 and 4,000m3a-1 for 1984. Supply has declined due to slower erosion of Hengistbury Head due to increasing toe protection afforded by the beach accreting against the Long Groyne, protection of Solent Beach by groynes and a beach replenishment (see Section 2.4) and declining relief landward as the cliffs at Double Dykes have retreated. Cliff foot surveys indicated erosion of 0.9ma-1 over the period 1974-82 at Solent Beach/Double Dykes, sufficient to supply 750m3a-1 of gravel (Lacey, 1985). Posford Duvivier (1997) calculate a total yield of 8,000m3a-1, 300m3 of which is gravel and the rest is sand and clay, apart from 300m3 of ironstone nodules. This is based on a long-term mean erosion rate of 0.4m.a-1. These calculations support the assertion of Wright (1986, 1982) that gravel on the upper beach between Solent Beach and the Long Groyne was largely derived from local cliff erosion.
Bray (1993) undertook a detailed analysis of gross material yield from the eroding cliffline between Southbourne and Hengistbury Head based on estimation of the lithological composition of solid strata and overlying drift sediments. This gave an annual input of 12,200m3, with some 8,100m3 contributed by the sector between Double Dykes and the Long Groyne and 4,100m3 contributed by the Southbourne to Double Dykes sector (this latter value may be an overestimate of the present situation for beach management over the past 15 years has significantly reduced recession). The mean annual yields of the different material types were as follows: sand 5,000m3; clay 5,000m3; gravel 980m3; and ironstone 235m3. The yield of gravel is comparable to the volume that accretes annually against the updrift side of the landward end of the Long Groyne (Lacey, 1985). Some of this material is retained as a talus store overlying the upper beach, until it is re-distributed by high-energy waves that reach the backshore. Naturally occurring and managed dunes close to the Long Groyne, which form an element of local cliff conservation management (Turner, 1998; Bray and Hooke, 1998), partially obscure coarse sediment forming the upper beach.
Posford Duvivier and the British Geological Survey (1998) attempted to estimate erosion loss from the 500m wide shoreface between Hengistbury Head and Bournemouth Pier. Assuming an average depth of 10m, yields are 6,000m3a-1 for the sector between the Long Groyne and Double Dykes, and 4,500m3a-1 for the sector further west. Both estimates derive from questionable assumptions about net vertical erosion rates.
2.4 Beach Replenishment, Bournemouth - N1 N2 References Map
N1 Alum Chine to Southbourne
Three phases of nourishment have been undertaken by Bournemouth Borough Council, between Alum Chine and Southbourne as follows:
The 1974/75 Nourishment: post-completion beach behaviour
Intensive monitoring of the Alum Chine - Hengistbury Head shoreline has been undertaken since July 1974, using topographically surveyed beach profiles, hydrographic survey out to 450m seawards of mean low water and systematic sediment sampling. Detailed evaluations of the accuracy, limitations and possible margins of error of the techniques used are given by Hodder (1986), Cooper (1997) and Harlow (2001). The first post-nourishment survey in 1976 revealed an intertidal gain of 745,000m3, although only 650,000m3 was actually pumped onto the beach. This gain was due to sand transported onshore by wave action during or shortly after the nourishment operation (Webber, 1980; Lacey, 1985; Harlow and Cooper, 1996; Harlow, 2001). Subsequent surveys showed substantial volume loss from the intertidal zone, which was attributed to rapid eastward littoral drift (the initial nourished beach overtopped pre-existing groynes), transport of finer materials to the nearshore zone and profile adjustment. It has been suggested that the 1 in 10 slope of the face of the nourished beach was not compatible with the wave climate in the eastern part of Poole Bay (Newman, 1978; Lacey, 1985), although the slightly smaller size of imported (compared to indigenous) sediment may also have been a factor (Hodder 1986). Intertidal losses were 287,000m3a-1 in 1976 and 1977, but diminished within two years of nourishment to a mean of 26,100m3a-1 over the next 9 years (95,000m3a-1 between 1979 and 1981) suggesting eventual attainment of an equilibrium between beach profile, sediment size and wave climate (Lacey,1985; Harlow and Cooper, 1996; Cooper, 1997). A factor, which could also have contributed to volume loss, is compaction of the beach subsequent to nourishment. This does not involve actual loss of material but is impossible to calculate retrospectively without packing density measurements from the freshly nourished beach (Harlow, 2001; Cooper, 1997).
A different view of post-nourishment beach behaviour was obtained by detailed volumetric calculations for the sediment prism extending between mean low water and a distance 300m offshore (Hodder 1986). This analysis revealed that total volume was 1 million m3 greater in October 1978 than in July 1974, and 350,000m3 greater than in July 1975. It is therefore inferred that substantial offshore and longshore losses of 270,000m3a-1 subsequent to nourishment were offset by wave driven onshore feed of 620,000m3 from the nearshore dump-sites. Losses from the intertidal zone were therefore not necessarily losses to the beach system, for much sediment was retained in the nearshore zone, contributing up to a 1m rise in seabed levels and thus facilitating dissipation of wave energy (Hodder, 1986; Gao and Collins, 1994; Harlow, 2001). Spatial analysis of these volumetric trends indicated that nourishment was most effective between Bournemouth and Boscombe Piers where imported sediment was of a slightly larger grain size, and a high proportion remained within the intertidal and nearshore zones. Indigenous beach sediment size increases eastward (Lacey 1985) and thus imported sediment becomes progressively smaller than the "background" in this direction. Losses from the nourished frontage were shown to benefit downdrift beaches, as the Solent Road - Hengistbury Head segment had accreted to 100,000m3 above its July 1974 volume by 1979 (Hodder 1986).
Overall beach volumes along the nourished segment remained at 550,000m3 above prenourishment values by November 1982 (0-300m offshore), with an annual loss of 95,000m3 in 1979, 1980 and 1981 following exhaustion of inputs from nearshore supply sites. Intertidal volumes had returned to near 1974 values at most locations by the mid 1980s (Hodder, 1986; May, 1990).
The reliability of the volumetric analysis was evaluated by Hodder (1986) and it was found that some inaccuracy resulted from survey error, spacing error (failure of profile interval to adequately represent intervening beach segments) and seasonal profile variations. Overall confidence of volumetric information for the 0-300m zone was estimated at + 131,000m3 for the nourished beach, as a whole. Volumetric accuracy could be increased by undertaking representative measurements of packing density immediately after nourishment so as to calibrate volumetric analysis for subsequent compaction effects.
The 1988/89 Scheme: post-completion beach behaviour
Routine monitoring involved continued measurement of the same 50 cross-sectional profiles for beach and offshore survey and sediment sampling as for the earlier nourishment. Analysis of beach volume change has employed a "rolling average" of five successive surveys to minimise the impact of survey errors (Harlow and Cooper, 1996; Harlow, 2001; Cooper, 1997). Results revealed that 400,000m3 was lost between 1990 and 1993 (Harlow and Cooper, 1994). In 1994, loss of recharge material was 92,000m3 , and by 2000 very little of the material added 12 years previously was still retained (Harlow, 2001). The overall behaviour of the replenished beach has been comparable to that following the earlier recharge. Initial rapid loss was due in part to over-filling of groyne bays, which therefore under-performed immediately following re-nourishment. Cooper (1997) states that the highest rates of post-nourishment decline were on the frontage between Boscombe Pier and the end of the promenade at Southbourne. This coincides with a larger groyne spacing to length ratio in comparison to updrift beaches, but also correlates with the steepest beach profiles and coarser background sediments. This is understood as a response to the progressive west to east increase in exposure to wave energy that causes an acceleration of drift and possibly offshore winnowing of the finer grainsizes. Steepening also appeared to be positively linked to grain size. Harlow (2001) notes that monitoring revealed gradual change of grain size distribution over time, with the pattern of eastwards coarsening becoming more marked. He states that a grain size of 1.8 phi is critical, with smaller particles more likely to move seawards to form nearshore bars.
Cooper (1997, 1998); Harlow and Cooper (1994; 1996); Harlow (2000; 2001), Cooper and Harlow (1998) have undertaken critical comparative analysis of both sand recharge schemes, and have observed the following shared behaviour trends:
This work thus indicates that each of the two major sand replenishments has had an effective 'life' of 12-14 years. "Borrow" material accounted for approximately 30% of net beach volume in 1997 (Cooper, 1997) A well designed and efficient groyne field has proved essential to retaining recharge materials (Cooper, 1997; Harlow, 2001), thus demonstrating that nourishment alone is not sufficient when there is a tendency for rapid littoral drift. Harlow and Cooper (1994; 1996) have asserted that differences in the particle size distribution of the borrow material used in each replenishment have not proved critical to beach stability, although comments by Hodder (1986) relating to the effects of compaction on initial volume losses are valid.
Further details on beach morphodynamics are given in Section 5. Analysis of profile and volume changes post 1974 indicates the strong influence of beach renourishment.
N2 The 1988/9 Nourishment of Solent Beach
A gravel nourishment scheme involving 143,000m3 of dredged material was undertaken on Solent Beach in 1988/9 (Photo 6) followed by construction of three rock groynes at Double Dykes (Harlow, 1989; Cooper, 1997; Cooper and Harlow 1994; 1998; Harlow and Cooper 1996). A major reason for maintaining beach levels here is the potential threat of a breach at Double Dykes under storm surge conditions that could establish a new tidal inlet to Christchurch Harbour (Bray, 1993; Hydraulics Research, 1986; Parker and Thompson, 1988; H R Wallingford, 1993; Turner 1998). Concern was expressed at the time of recharge that beach retention may be low east of Double Dykes where the beach remains ungroyned (Lelliott 1989). This was confirmed by 1995, when analyses of beach volumes indicated that most of the material added via recharge had been lost (Bournemouth Borough Council, 1995; Harlow, 2001). Proposals to build a sequence of five rock groynes between Double Dykes and a location 400m west of the Long Groyne (HR Wallingford, 1993; Bray, 1993), as a measure to conserve beach sediment, had not been implemented by 2003. However, this segment can be regarded as a confined transport system (Hydraulics Research 1986) as eastward drifting course material is at least partly intercepted by the Long Groyne. Offshore loss of fines is a characteristic response of newly nourished beaches and is probably a factor promoting loss of volume at this location.
3. LITTORAL DRIFT - LT1 LT2 LT3 LT4 LT5 LT6 References Map
Net littoral drift appears to operate from west to east within the bay, although reversals are an important feature especially in western parts. The sheltering effects of the Isle of Purbeck and Hook Sand, together with strong tidal currents generated at Poole Harbour entrance have resulted in a complex transport regime in the west of the bay that is not understood fully. Numerous researchers have attempted to model and predict drift within the bay and a wide range of estimates is available for different locations (see LT 1 to LT 6 below).
To better understand processes occurring throughout the whole bay, a systematic measurement programme of beach profiles (from 1974) and of beach levels against groynes (from 1993) was undertaken by Bournemouth Borough Council. The datasets generated were analysed to determine major trends and patterns in drift and beach accretion/erosion occurring between Poole Head and Solent Beach, as follows (Harlow 1995, 2001):
LT1 Haven Hotel to Sandbanks Car Park (see introduction to littoral drift)
The morphological form of the Sandbanks peninsula would appear to be indicative of east to west littoral drift of sand, although studies over the past decade suggest that a complex transport situation exists. In particular there is some uncertainty over the net drift direction and a tendency for drift reversals is identified.
Littoral drift modelling revealed a drift reversal west of Sandbanks car park, with net westward drift continuing at 20,000m3a-1 towards the Haven Hotel (Hydraulics Research 1991b). Southwestward drift fails to accumulate in the vicinity of the Haven Hotel so it was envisaged that tidal currents at Poole Harbour Entrance entrain sediment arriving and transport it into the main channel. The flood dominant East Looe channel flows across the subtidal beach toe so that could contribute significantly to transport and untimately control beach development at this location. Indeed, when littoral drift was modelled at different levels across the beach profile results revealed greatest transport below mean low water where tidal currents have higher velocities (Hydraulics Research 1991b).
Subsequent studies concluded that drift was variable with intervals of reversal, but with an overall tendency in recent decades for net northeastward drift. HR Wallingford (1994, 1995b, 2000) concluded, from analysis of beach profiles adjacent to groynes along the Sandbanks frontage that littoral drift has a net eastwards component. This is set up by refraction affecting waves approaching from the southwest and south southwest due to the presence of Hook Sand. HR Wallingford (2000) identified several distinct phases between 1886 and 1994 during which drift alternated between net easterly and westerly directions. West to east net movement has prevailed since 1952, though with several short-term periods of reversal. This variability is considered to result primarily from changes in incident wave climate. During periods of uninterrupted unidirectional drift, throughput rates may be up to 100,000m3a-1. During phases of frequent reversal, net rates are lower than 5,000m 3a-1, in either west or east directions. The potential for rapid transport is also indicated by rapid beach losses that occurred following groyne removal in 1990/91 and significant accretion that occurred within the embayments of new rock groynes constructed in 1994/95 (Photo 1) and 2001 (Photo 7) (HR Wallingford, 2000).
Both HR Wallingford (2000) and Brampton, et. al (1998) observe that potential drift west of the westernmost groyne (Photo 1) is towards Poole Harbour entrance, but that little material is actually retained on the beach at this location.
LT2 Sandbanks Car Park to Branksome Chine (see introduction to littoral drift)
Drift is also rather uncertain in net direction with a tendency for reversals along this frontage also. Longshore wave energy flux based on wave refraction analysis indicated a net westward drift along this segment (Henderson 1979). The following rates have been estimated according to the sand transport equations and calibrations employed: 178,000m3a-1 (Henderson 1979), 282,000m3a-1 (Lacey 1985) and 127,000m3a-1 (Hodder 1986). However, observation of sediment distribution against groynes at Sandbanks indicated eastward drift (Robinson 1955, Hydraulics Research 1991b). All other evidence, including the orientation of the Sandbanks spit, clearly favoured westward drift so it is hypothesised that eastward drift is restricted to the upper beach so that net drift across the whole profile is westward (Hydraulics Research 1991b). Calculation of littoral drift in the nearshore zone using refraction and shoaling analysis based upon a 10 year hindcast wave climate yielded a net south-west drift pathway, at a rate of 25,000m3a-1 for a point off Poole Head (Hydraulics Research 1986). Accuracy was limited due to failure to model adequately complex tidal and wave induced currents and a range of 8,000-75,000m3a-1 was given for the variability of this estimate.
More detailed littoral drift studies were undertaken between Branksome Chine and Sandbanks, to examine the hydraulic and shoreline effects of siting a proposed artificial island just over 1km off Poole Head (Hydraulics Research 1991b). The existing littoral drift regime was determined initially before modelling the effects of the island. A 15 year hindcast wave climate was used to provide input to an improved wave refraction model, from which longshore wave energy flux was determined for five nearshore points. Net drift was identified as being southwestward and increased from 20,000m3a-1 at Branksome Chine to 34,000m3a-1 at Flag Head Chine and 70,000m3a-1 just west of Poole Head. Drift at Sandbanks car park was found to be subject to reversal, and amounted to a net movement of 5,000m3a-1 to the northeast. If this analysis is correct, then the accelerating southwest drift from Branksome Chine (20,000m3a-1) to Poole Head (70,000m3a-1) must be fed either by beach erosion, or onshore feed of approximately 50,000m.a-1. There is no evidence for either process operating at such magnitude, indeed field observations over the past decade have suggested that net drift was on average eastward.
The argument for the existence of net eastward drift is advanced by Harlow (1995 and 2001), based on a detailed analysis of measurements of beach levels adjacent to groynes. This forms part of a comprehensive programme investigating drift directions for the entire Poole Bay frontage, which commenced in 1993. Surveys have been carried out twice weekly during a part of this period. Harlow (2001) concludes that there is no evidence for the operation of a drift divide close to Durley Chine, where net easterly drift has occurred during 70% of the survey period up to 2000. However, it is clear that net westerly drift does operate under favourable hydrodynamic conditions. Gao and Collins (1994) asserted that both net eastwards and westwards longshore drift pathways operate along the Sandbanks peninsula shoreline, but that the dominant transport direction was towards the east/north-east. Halcrow (1999) identify a possible zone of littoral drift convergence between Sandbanks Car Park and Poole Head, with a calculated rate of 7,500m3a-1 of beach accretion in the 1990s.
One difficulty that would appear to arise here is an explanation for the growth and development of the Sandbanks peninsula. It is mostly readily understood as a conventional spit that has extended southwestwards in the direction of dominant littoral drift, with its proximal end near Poole Head. In doing so, it has, together with the northwards growth of South Haven spit, narrowed the entrance channel to Poole Harbour. There may, however, be an alternative explanation. This would regard the rounded form of the distal end of the supposed Sandbanks spit to be an original island composed of Eocene bedrock that has been subsequently become attached to the mainland by the landward migration of a sand barrier beach. This would place emphasis on cross-shore, rather than longshore, sediment supply. Although this hypothesis requires further research, circumstantial evidence in its favour includes: (a) the evidence of the periodic development of a multi-barred foreshore in front of Sandbanks; (b) the past accumulation of blown sand and dunes behind this beach, presumably deriving from a formerly wider inter-tidal zone; (c) sediment supply from Hook Sand, and (d) borehole evidence from the southern Sandbanks peninsula relating to substrate geology.
LT3 Branksome Chine to Durley Chine (see introduction to littoral drift)
Uncertainties relating to net drift direction exist with respect to this frontage also. Modelling studies based upon hindcast wave climates have indicated that net drift at locations west of Durley Chine should be to the southwest, with a drift divide occurring at Durley Chine. By contrast, observations of beach morphology over the past decade have suggested that net drift was on average eastward and have failed to identify evidence of a divide.
Wave refraction and shoaling was analysed in Poole Bay, based on a hindcast offshore wave climate derived from wind data for 1977. Inshore wave conditions were calibrated against those recorded at the Southbourne wave rider site and corrected inshore wave climates were employed to calculate longshore wave energy flux (Henderson, 1979; Henderson and Webber, 1979). These calculations revealed a potential for the divergence of littoral drift pathways in the vicinity of Durley Chine. Observations of the distribution of sediments against groynes provided some confirmation of this (Hodder 1986, Hydraulics Research 1991b). Westwards of the divide net littoral drift was determined to be westward and several quantitative estimates have been derived from the longshore wave energy flux using different sediment transport calibrations. These calibrations are based upon the rate of loss of sediment from the 1974/75 nourishment and vary due to the timescale employed (recharged beach volume declined with time), adjustment for onshore-offshore transport and beach sedimentology. Northwestward drift of 71,000m3a-1 was estimated by Henderson (1979), but this may be an overestimation because calibration was based on the initially high loss rates from the nourished beach. Northwestward drift of 113,000m3a-1 was estimated by Lacey (1985), which is certainly an overestimate because in this case calibration was also based on losses from the intertidal zone immediately after renourishment. Calibration by Hodder (1986) derived westwards transport of 51,000m3a-1; this was over an 8 year period of measurement and included adjustment for nearshore sediment storage of recharge sediment, and is thus regarded as being more representative. It can be argued that any calibration based on monitoring of a nourished beach is likely to give unreliable results if applied to a natural beach because the former tend to be initially unstable and subject to artificially rapid drift rates. It can also be argued that a wave refraction study based on a single year of wave data may not be representative of long-term trends. The role of groynes in impeding littoral drift also needs to be calculated and detailed observations of accretion against groynes failed to identify evidence of a drift divide (Harlow, 2001).
LT4 Durley Chine to Southbourne (see introduction to littoral drift)
Observations of sand distribution within groyne compartments has indicated that net beach drift operates from west to east throughout this frontage - see Photo 4 (Harlow 2001).
Rates of drift rates along this frontage have been estimated from sediment loss measured after the 1974/75 beach nourishment scheme. Different authors have determined different rates of loss according to the timescale of analysis and various assumptions on transport conditions. Loss of 65,000m3a-1 was calculated by Newman (1978), 91,000m3a-1 by Lacey (1985) and 40,000m3a-1, over 9 years, by Hodder (1986). These values were then used to calibrate sediment transport equations and littoral drift was hindcast based on the wave refraction analysis of Henderson (1979). The estimates of Hodder (1986) are regarded as the most reliable, for reasons given in the preceding section. Cooper, Hooke and Bray (2001) quote a mean drift rate of 35,000m3a-1 following the effect of the second replenishment programme. All authors agree that net drift is eastward and that rates increase from Bournemouth Pier to Southbourne due to increasing wave energy in this direction (Henderson 1979). Harlow (2001) observes, from a detailed programme of drift monitoring, that Bournemouth and Boscombe piers locally reduce longshore transport, and promote some immediate updrift accretion. Local variations in longshore transport rates are induced by subtle variations in seawall alignment.
Fluorescent sand tracer experiments were undertaken over a limited period between Bournemouth and Boscombe piers (Kent 1988, Whitehouse, et. al, 1997). Wave and longshore current measurements were employed for empirical derivation of littoral drift rates and results compared favourably with Henderson (1979) and Lacey (1985). Calculations of drift rates based on tracer results were significantly lower by comparison; this was attributed to failure to adequately model the effect of tidal range and onshore-offshore transport, as well as a low level of tracer recovery. It was concluded that longshore transport could not be reliably predicted using existing transport equations because these fail to account properly for all factors influencing movement. Another short-term study measured suspended sediment concentration in breaking waves at a site west of Boscombe Pier (Taylor 1990). Although of too limited scope for conclusive results, the study showed a relationship between sediment concentration with elevation above sea-bed and breaker type. Measurements of this type are ultimately necessary to develop more refined sediment transport models which overcome the problems of excessive simplification noted by Kent (1988).
The effectiveness of groynes in restricting drift on this sandy beach is limited due to a marked propensity for drift in the nearshore zone seaward of the toe of the longest groynes.
LT5 Southbourne to Hengistbury Head (see introduction to littoral drift)
For the calculation of drift rates along this sector, it is particularly important to correctly calibrate sediment transport equations because the beach face is composed of a high proportion of flint gravel, which exhibits different hydraulic behaviour to sand and is less readily transported. Henderson (1979); Lacey (1985) and Hodder (1986) derived gross drift rates within the range 53,000m - 70,000m3a-1 at Southbourne, and indicated a slightly increased drift rate of gravel and sand between Double Dykes and Hengistbury Head of 63,000 to 90,000m3a-1 Hydraulics Research (1986) suggest a lower rate, at 45,000m3a-1, based on a modelling approach. Most of the transport estimated involves movement of sand rather than gravel in the nearshore zone. Differences in the estimates are attributed to use of different wave climate data and differences in calibration of sediment transport equations to represent the mixed grain sizes of the beach.
Valuable information was derived from three separate tracer experiments employing aluminium pebbles similar to indigenous clasts in terms of size-range, distribution and shape (Wright, Cross and Webber 1978 and 1980; Wright 1982). Tracer recoveries were quite high (28-61%), thus enabling determination of the width, thickness and longshore transport of coarse beach sediment. The maximum distance of alongshore displacement was greatest to the west (167m), but the average results indicated 58m movement to the west and 87m to the east. Drift volumes calculated from this information were related to three sets of incident wave conditions (each with characteristic breaking wave heights) to calibrate transport equations, and these were then used to estimate annual drift rates. Gravel transport rates of 11,000m3a-1 and 28,000m3a-1 were derived from the two main experiments, but transport was only accurately monitored over short periods that were not fully representative of annual conditions. The drift estimates of Henderson (1979), Lacey (1985) and Hodder (1986) are also likely to be excessively high because the presence of gravel on a sandy beach causes decreased sand transport (Wright 1982) and presence of sand on a dominantly gravel beach causes enhanced gravel transport. This suggests that it may be necessary to calculate gravel and sand transport separately and employ appropriate correction factors depending on beach sediment size distributions. A further problem with overall drift estimates derived for this segment is that the calibration of sand transport was derived using a groyned nourished beach, whilst that of gravel was from a natural beach. This is a cause of inaccurate prediction of total sediment transport, because sand and gravel calibrations were combined arithmetically and errors with one element affect the whole equation.
The limitations apparent in existing transport predictions, led HR Wallingford (1986) to undertake further studies based on mathematical modelling. A hindcast offshore wave climate based on Portland wind data (1974-84) was validated against inshore wave measurements from the Southbourne wave rider. Refraction and shoaling analysis was employed to determine shoreline longshore wave energy flux, from which an eastward drift of 45,000m3a-1 was calculated. This estimate however was based on the assumption that the Long Groyne was not present. As this major structure causes progressive eastward change in beach orientation, it is possible that actual drift rates would decrease in this direction due to the more stable beach plan shape produced by accretion against the groyne. Analysis of accretion rates immediately updrift of the Long Groyne reveal accumulation equivalent to eastward drift of 600-900m3a-1 (Wright 1982). However, this is an underestimate because the groyne is only a partial barrier to transport and subject to overtopping (Photo 10) and outflanking, especially by sand (Hydraulics Research 1986, Harlow, 2001). Nonetheless, the net eastwards drift of 45,000m3a-1 is more representative of conditions at Double Dykes where the Long Groyne has no updrift effect. Construction of rock groynes on Solent Beach intercepted drift and a multi-line beach plan model was employed to investigate these changes (Hydraulics Research 1986). The model calculated the distribution of littoral drift across the profile and showed that drift on the upper gravel beach was small (less than 9,000m3a-1) compared to that further down profile and in the nearshore zone. The groynes therefore intercepted only a small proportion of total drift, mostly the gravel fraction. By analogy this situation may also exist at the Long Groyne. Cooper (1997) concluded, from analyses of beach behaviour either side of the Long Groyne, between 1987 and 1993, that sand from the 1988 replenishment bypassed within 3 to 4 years of its emplacement updrift.
LT6 Hengistbury Long Groyne (see introduction to littoral drift)
Sand, and some gravel, accretion against the south-facing sides of a set of rock groynes on Mudeford Spit indicates northward transport (Tyhurst 1985). In the absence of knowledge of the specific source, it is generally held (but unproven) that sediments accumulating on the spit are derived from bypassing of the Long Groyne. Visual observations certainly suggest that overtopping of the structure by sediment is possible (Photo 10). Physical modelling of the Long Groyne and its surrounding area (Hydraulics Research 1986) indicated that coarse sediment could overtop the groyne during storms and sand could outflank it. Although subject to scaling effects, the physical modelling results corroborate findings of earlier drifter research, which indicated a strong trend for eastward transport around the Long Groyne (Watson 1975, Tyhurst 1976, Turner 1990). Thus much of the predicted 45,000m3a-1 arriving at this point (Hydraulics Research, 1990) may outflank it.
4. SEDIMENT OUTPUTS - O1 O2 References Map
4.1 Offshore Transport
Thin sheets of homogenous fine to medium sand, muddy sands and gravel, with several areas of exposed bedrock, mostly occupy the relatively shallow offshore seabed of Poole Bay. The largest rock outcrop is the Beerpan Rocks and Christchurch Ledge feature, which varies in width between 400 and 1200m and extends for some 6 km southeast of Hengistbury Hard. Its prominence as a submarine ridge or reef is due to the presence of several resistant bands of ironstone concretions; however, its inshore area, at a depth of -3.5m O.D. was affected by opencast mining during the mid nineteenth century (Velegrakis, 1994; Brampton, et al, 1998).
Sediment provenance is uncertain, but the occurrence of coarse materials seawards of the modern shoreface is probably mostly relict, derived from both terrestrial and marine deposits dating from the mid or late Quaternary (Brampton, et al, 1998). In the nearshore zone, and seawards to the central areas of Poole Bay, predominantly sandy sediments are likely to mostly derive from past cliff and shoreface erosion. Seismic studies (Velegrakis, 1994; Brampton, et al, 1998) have revealed that seabed sediments rest on an erosion surface cut across previously varied relief. This is the result of valley incision during the Devensian stage of low sea-level, when the present day area of Poole Bay was exposed to sub-aerial denudation. This is indicated by buried palaeochannels, infilled with a transgressive suite of Holocene fluvial, estuarine and marine sediments (Velegrakis, et al, 1999).
Submarine sandbanks have developed at Hook Sand seaward of the mouth of Poole Harbour and at Dolphin Sands. The latter may function as a sink for both Christchurch and Poole Bays, although some of the sand arriving there is in transit, ultimately moving into the deeper water of the central English Channel. All other sandbanks are stores, as revealed by their dynamic behaviour. Brampton, et al, (1998) report that Dolphin Sand is up to 8m above the level of the surrounding seafloor, and is an average of 6.5 km in length and between 1.0 and 2.0km in width. This represents an accumulation of over 25 million m3, with clear evidence of a basic stability of form from the end of the nineteenth century. Given these dimensions, it is difficult to argue that all of this material is mobile, even though hydrographic chart comparison for 1980 to 1990 indicate that the northern flank of Dolphin Sands eroded at much the same rate as its southern flank gained material.
O1 Bournemouth to Hengistbury Head (see introduction to offshore transport)
Sea-bed drifter experiments indicated marked eastward transport in areas seaward of the -10m OD contour and east of Bournemouth (Watson 1975, Tyhurst 1976, Turner 1990). These studies have shown a tendency for movement from Poole to Christchurch Bay within the offshore zone, but are contrary to transport directions indicated by bedforms (Dyer 1970, Fitzpatrick 1987) and modelling (Hydraulics Research, 1991; 1990a; Brampton, et al, 1998). However, they are similar to those determined for surface currents by OSCR (BP 1991; Osborne, 1991). It is therefore possible that drifter results are an unreliable guide to bedload sediment transport. Confusion may derive from profile asymmetry of smaller bedforms, subject to reversal by tidal currents. It should be emphasised that almost all survey data has been obtained under calm, summer conditions, and therefore is not representative of potentially very complex transport conditions under high energy waves (Hydraulics Research, 1990a; Brampton, et al, 1998).
O2 Central Poole Bay (see introduction to offshore transport)
Offshore surveys by sidescan sonar, echo sounding and sediment sampling revealed gravel waves, sand waves and sand ribbons (Fitzpatrick, 1987; Hydraulics Research, 1990a; Brampton, et al, 1998). Analysis of the profile asymmetry of these bedforms indicated transport away from Dolphin Sand to the southwest, with transport veering more directly southward further offshore. Currents were locally intensified by irregular bathymetry in the vicinity of the eroded Needles-Purbeck Chalk ridge and significant gravel transport was indicated in this area (Velegrakis, 1994). Sediment type throughout much of this area is moderately well sorted medium/fine sand, which forms waves moving across a stable gravel basement. Sand megaripples often have a finer composition than "host" bedforms, with silt infilling troughs. Flattened crests might indicate erosion following formation but this cannot be ascribed with confidence to tidal current reversal (Brampton, et al, 1998). A feed from Dolphin Sand was indicated by bedforms (Dyer 1970) and a heavy mineral assemblage rich in haematite and glauconite (Fitzpatrick 1987). Typical Eocene substrate-derived heavy minerals are present in the north of the area, eg. epidote, glauconite, apatite and also in smaller quantities south of the eroded Purbeck-Needles ridge, despite their relative absence from the insitu sea-bed outcrops. This implies a southward supply from Poole Bay, but as no evidence of corresponding northward return feed is available, it is concluded that this pathway comprises a net output from the Poole Bay system. Much material may have been supplied from Christchurch Bay via Dolphin Sand and possibly only passes through Poole Bay en route to a final sediment sink in the central English Channel (Halcrow, 1999; Brampton, et al, 1998; Cooper, Hooke and Bray, 2001).
HR Wallingford (2000) have suggested that some material from this pathway may eventually feed south-eastern parts of Hook Sand some 4km seaward of Poole Harbour entrance, although direct evidence is not available.
4.2 Estuarine Sediment Transport
EO1 Poole Harbour Entrance the Swash Channel and Hook Sand
Ebb tidal currents are shorter-lived, but more rapid (up to 2.5ms-1) than corresponding flood currents at Poole Harbour entrance (BP 1991; Hydraulics Research, 1986; 1988; 1990b; 1991), thus the potential exists for seaward transport along the Swash Channel (Appleton, 1994, Halcrow, 1999). This feature is 3.2km in length, and 150m in width. Its maximum depth is at to the harbour mouth, due to navigational dredging and channel maintenance by natural tidal flushing. Tidal modelling based on bathymetry recorded on a 400m resolution grid was used to determine sand transport using empirical calculations (Hydraulics Research, 1986; BP Exploration, 1991). These studies showed increasing transport potential southward down the Swash Channel and towards Handfast Point. Analysis also showed that the localised ebb tidal "jet" from Poole Harbour entrance was deflected southward by interaction with dominant south-westerly tidal flow within Poole Bay, further confirmed by large asymmetric bedforms (Brampton, et al, 1998). The zone of maximum interaction was at Poole Bar, where transport of 20,000m3a-1 across the bar was predicted. This conclusion that net transport is seawards of the harbour entrance was supported by limited sediment sampling which showed grain size reduction down the pathway, from coarse gravel at the harbour entrance to fine sand at Poole Bar (Hydraulics Research, 1986; 1990a). Progressive deepening of the Swash Channel, 1970-1995 due to dredging may have increased the transport rate (Halcrow, 1999).
A more detailed mathematical tidal current model was developed on a 44m grid, validated against float tracking, and coupled with an empirical sandflow model (Hydraulics Research, 1986). To provide a more realistic transport prediction, wave effects were also included. This latter information was obtained from a hindcast offshore wave climate (Portland wind data 1974-84) adjusted for refraction and shoaling effects due to Hook Sand. Bed conditions were specified according to available information and three contrasting wave/tide conditions were tested. The Swash Channel itself was not examined, as little material was present on the bed, but its margins were modelled in detail. During calm conditions (70% of the time), the mean transport rate was limited, and was from north-east to south-west across the bar at an average of 15,000m3a-1. Under the influence of "typical" waves (30% of the time) the gross transport rate increased four-fold and net annual movement of 29,000m3 was estimated. Storm waves (once per year) caused a ten-fold increase in transport, but net movement was only 2,000m3a-1 due to the low frequency of this condition. Net north-east to south-west residual transport of 46,000m3a-1 of suspended sediment was estimated from Hook Sand southward to Poole Bar. This was most intense under the combination of high energy wave action and spring tides. These calculations are of medium reliability because site conditions are complex and modelling necessitated simplification. For example, wave-current interactions are not fully understood and bedload transport was not adequately modelled. In addition, long-term trends were not established because only a limited number of conditions could be tested. The basic observations of this study were confirmed by Brampton, et al (1998) using the SANDFLOW 2-D sand transport model, calibrated for tidal current velocities. This study concluded that only waves and tidal currents operating together can account for the volumes of sediment moved.
A comprehensive sediment transport study of the area immediately seawards of the harbour entrance was undertaken to determine the effects of constructing an artificial island north-east of Hook Sand (Hydraulics Research, 1991b; BP et al, 1991). This involved modelling the existing regime, against which to compare predicted changes caused by the island. Sand transport was determined from mathematical wave and tide models validated against appropriate field data. Sensitivity tests were conducted to evaluate the relative importance of key variables, eg. wave approach direction, and a series of base conditions established. Transport was studied during calm, "typical" and storm conditions and it was confirmed that most rapid transport occurred with stronger wave conditions and spring tides working together. Potential deposition zones were identified in the harbour entrance, in the Swash Channel, near the crest of Hook Sand and on Poole Bar. Erosion was predicted on both the seaward face and landward flank of Hook Sand. In places, it is naturally armoured by coarse sand of lower mobility, which could protect the bank and inhibit erosion. Effects of factors such as these are difficult to model and can cause over-estimation of transport rates. Detailed analysis of the conditions in the Swash Channel revealed that transport was strongly dependent upon combined wave and tidal current action with 50-100 times more movement under typical waves and 500-1000 times more during the operation of storm waves. Deposition/erosion profiles along the Swash Channel revealed accretion on the northern margin of the bar during calm conditions, accretion further seaward with "typical" waves, and erosion of the seaward face of the bar under storm wave action.
Due to short-term variability indicated by the sensitivity tests, these results are for a limited number of conditions. They cannot be extrapolated as long-term trends without more detailed knowledge of the interactions of variables such as wave direction, wave breaking and tidal phase. Accurate long-term study is therefore required to examine small systematic short-term changes which might be cumulative and cause major long-term trends.
This feature is included here, as it can be interpreted as a part of the ebb tidal delta of Poole Harbour entrance composed of sediments from Poole Bay. Part of the crest of Hook Sand lies above -1m OD causing refracted waves to break. It is suggested that this causes: (i) sand to be driven onshore from the crest (HR Wallingford, 2000) and (ii) some sediment to move offshore in the shallows of Poole Head and then southwards along the east side of the bank (Hydraulics Research 1986; 1991a). Refraction and shoaling models based on a 10-year hindcast offshore wave climate indicated a potential for net southward transport of 25,000m3a-1. Sand supplied by this pathway may periodically partially infill the Swash Channel or be transported further south to Poole Bar, although there is no clear evidence for this (Brampton, et al, 1998). Chart comparisons covering the period 1785-1990 revealed that the Swash Channel and Hook Sand were subject to some fluctuation, but were relatively stable in position and planform. This implies a long-term equilibrium between sediment supply and loss (Velegrakis, 1994; Hydraulics Research 1986; 1991c H R Wallingford, 1994; Halcrow, 1999) despite considerable sediment throughput, thus suggesting that Hook Sand is not the ultimate sediment sink for this transport pathway. Reliability of transport-direction is high due to corroboration by model studies (Hydraulics Research 1988, 1991b), but quantitative information is of low to medium reliability because the contribution of tidal currents remains very uncertain.
Gravel and sand waves on the seabed to the east of Hook Sand (Hydraulics Research, 1991b) indicate that it is a dynamic form receiving sediment. Seismic reflection data reveals abundant evidence for alternating phases of erosion and accretion, as well as marginal scour (Brampton, et al 1998).
Overall reliability of the operation of this pathway of transport is high, but quantitative information is of uncertain quality because of the complex interaction of variables and lack of long-term modelling. The nature and importance of these variables has been established, but their long-term contributions to the overall nearshore sediment transport regime have yet to be quantitatively assessed.
4.3 Wave Driven Offshore Loss
Two types of output are recognised:
WO1 Solent Beach - Double Dykes
Mathematical modelling of littoral drift was undertaken using a single line beach plan shape model - see LT5 (Hydraulics Research, 1986; 1990). (Section 3) Littoral drift was determined from analysis of longshore wave energy flux derived from a 10-year hindcast offshore wave climate adjusted for the effects of refraction. The model was calibrated to reproduce accurately the historical accretion pattern at the Long Groyne. Results predicted slow beach accretion at Double Dykes and Solent Beach, but measured profiles actually revealed beach erosion. Offshore transport of 25,000m3a-1 was therefore estimated from the differences between predicted and observed changes. The trend for steady onshore to offshore movement was noted previously by Webber (1980) and Halcrow (1980). Sea-bed drifter experiments (Watson 1975, Tyhurst 1976, Turner 1990) also noted a major net offshore tendency for sediment transport between Solent Beach and Hengistbury Head, although there are some doubts as to the reliability of this information. Aluminium tracer experiments could not directly show offshore gravel movement because tracer recovery was not undertaken underwater (Wright, et al, 1978; 1980; Wright, 1982). Nevertheless, declining tracer recovery in this direction suggested that offshore loss was possible. Overall corroboration of available information indicates that this pathway - especially for sand - has been established with medium reliability.
WO2 Diffuse Offshore Transport: Bournemouth to Southbourne
Two phases of offshore transport from the 1974/75 nourished beach were recognised.
5. SEDIMENT STORES AND SINKS; BEACH MORPHODYNAMICS - References Map
The frontage is divided into the following three zones representing differences in transport regime and style of behaviour:
North Haven Point, Sandbanks, to Branksome Chine
HR Wallingford (2000) presents a detailed summary of the major phases of beach behaviour from the late nineteenth century through to 1994. Between 1886 and 1900, accretion and erosion were experienced at different locations, in the general absence of any defence or control structures. Groynes were introduced in 1896 in response to local beach sediment losses but were not comprehensive until 1925. Over this period, there was steady accretion in groyne bays over the central section, but net erosion at either flank. For the next 30 years, accretion was confined to the groynes in the western sector, with modest erosion affecting the eastern/north-eastern part of the beach. This suggested either net east to west drift, or a tendency for sand to move from offshore and built up the beach behind Hook Sand. After the mid 1950s, beach stability prevailed over the eastern sector but there was some erosion to the west, indicating a prevailing west to east littoral transport pathway. Between approximately 1910 and 1980, some 60 m of seaward advance occurred over central and eastern beaches. Starting in the late 1980s and accelerating over the next 10 years, erosion became dominant over the entire beach frontage; some 40 m of retreat (1.8ma-1) was experienced between 1991 and 1998 on central Sandbanks beach.
This abrupt change from accretion/stability to erosion affecting the central and eastern sectors of this beach, and a significantly increased erosion rate west to Haven Hotel, has been the subject of research and remedial action (HR Wallingford, 1994; 1995a, 2000). Accretion during much of the twentieth century has been ascribed to a non-uniform net onshore feed of sand from Hook Sand (Robinson, 1955). Under higher energy conditions, refracted (and refracting) waves entrain sand from the crest of this offshore bank. HR Wallingford (1994) Brampton et al (1998) and Halcrow (1999) report some landward movement of Hook Sand between 1990 and 1996, which tends to confirm this process. However, direct onshore transport is modified by the presence of the flood dominant East Looe channel, between Hook Sand and the beaches of the Sandbanks Peninsula. Its origin is not fully understood, but it is likely that tidal currents, flowing towards the Poole Harbour entrance, transport fine sand. Some of this is likely to be supplied by wave erosion of Hook Sand, and it is probable that waves also contribute to this shore-parallel pathway (HR Wallingford, 2000). Further complications are introduced by waves that move across East Looe Channel during high water, and by wave modification by both flood and ebb currents moving into and out of the harbour mouth. All of these mechanisms could create a net offshore feed to Hook Sand, which therefore functions as a dynamic store. Hydrographic chart analysis, from 1800, reveals that both Hook Sand and East Looe Channel (Poole Harbour Commissioners, 1995-6)) experience constant changes in the relative positions of their boundaries, as elaborated in Section 4.1 (Gao and Collins, 1994; HR Wallingford, 1994 and 1995a and b; Halcrow, 1999). Periods of beach foreshore erosion and steepening correlate closely - though not exactly - with the inshore migration of East Looe Channel.
Although rapid beach drawdown along the Sandbanks shoreline can be correlated with the incidence of a series of southeasterly storm waves between 1991 and 1994, further explanation may be linked to beach management practices. The previous phases of accretion and stability had encouraged the removal of groynes, first installed in the early twentieth century, as they deteriorated and appeared to be redundant. Rock armouring of the seawall was carried out, to reduce wave reflection and enhanced offshore transport, together with a limited recharge of 40,000m3, in 1991/2. Rock groynes were built along the southern sector of the coastline in 1995, which were successful in retaining sand (Photo1). Simultaneously, small dunes accumulated at the backshore, which were subsequently stabilised by vegetation planting. However, these groynes appear to have promoted scour to the northeast, with a loss of beach width of 0.8ma-1, 1995-1999 (HR Wallingford, 2000). This northern sector of the Sandbanks beach system did not benefit from the additional effect of the new groynes in "deflecting" tidal currents away from the foreshore. (HR Wallingford, 1995b, 2000). The effects experienced indicate that dominant littoral drift was eastwards, although short periods of reversal occurred (Harlow, 2001). Beach accretion between Midway Path and the Haven Hotel has been assisted by the placement (during winter periods) of water porous nylon sheets (Berosins) between rock groynes. However, an experiment at beach dewatering, as a further measure of sand retention, was discontinued after two years. In 2001, an additional four rock groynes were constructed along the northern, eroding section between Sandbanks car park and Poole Head (Photo7).
Bournemouth Beach (Canford Cliffs to Southbourne)
Since 1974, Bournemouth Borough Council has undertaken biannual surveys of a set of 38 beach profiles, at roughly 200 m intervals. This has been achieved through topographic survey to mean low water, and hydrographic survey to a distance from origin of 450 m or where water depth increases to more than 10 m. Although there have been some changes of the position of these profile lines with respect to groynes, and some reservations concerning hydrographic survey accuracy have been expressed, this is nonetheless a high quality monitoring record. Surveys have been conducted during spring and autumn of each year, in an effort to "even out" seasonal fluctuations of profile form, area and volume. A comprehensive graphical and statistical analysis is given by Harlow (2001), with further analysis of parts of this data set by Gao and Collins (1994) and Cooper (1997). Summaries are also given in Brampton, et al (1998) and Halcrow (1999). The following paragraphs draw heavily upon data synthesis and critical interpretation by Harlow (1994; 2001).
Net beach area is a function of exposure to wave energy, with a steady reduction from west to east. Greatest variability, as revealed by a measure of standard deviation, occurs at Southbourne; it is least where the beach is relatively more sheltered from incident waves, notably between the two main piers and in the west of this frontage. Beach gradients are characteristically low, but again increase eastwards. This is in response not only to wave energy, but also to a downdrift (eastwards) coarsening of mean particle size. Where there are local variations in beach orientation resulting from the influence of past or present defence structures, beach volumes also fluctuate. A combination of limited natural sediment supply and the presence of a steep, backing seawall results in "squeezing" of the intertidal beach and a consequent steepening of gradient. Gao and Collins (1994) give consideration to beach stability or equilibrium conditions along this frontage, concluding that recovery from "perturbations" is relatively rapid.
Although there is abundant evidence for seasonal variations in beach height, area and profile form, the timing of surveys - together with analytical methods designed to "smooth" fluctuations - does not clearly reveal this. However, maximum area is normally characteristic of late summer and early autumn. High-energy storms can result in as much as 1 m of beach lowering in a few hours (Henderson and Webber, 1977) when it is thought that sands are transported seawards to form nearshore bars. Typically, this material would be returned to the beach during calmer intervals with an associated decay of nearshore bars. These processes of short term "cut" and "fill" have not been studied directly and are not understood fully in Poole Bay although they have been documented on other sandy beaches throughout the world.
Overall convexity of profile form out to 450 m is apparent from much of the survey record (Gao and Collins, 1994). A major reason for this is the presence of near shore sand bars, below low water, that have characteristic heights of 1.5 m and form parallel to the shoreline. These features are present in water depths of up to 10 m and are best developed during winter, especially following storms. This type of inshore morphology is relatively unusual on the south coast of England, as it only occurs where tidal range is low. Gao and Collins (1994) propose that the bars are related to breaking waves (ie "break-point" bars), but the fact that they occur in progressively greater water depths, and are more persistent in an eastwards direction throws doubt on this conclusion. An alternative explanation is that they are the result of the reflection of long period swell waves from the beach face (Harlow, 2001). No clear associations between changes in bar and trough morphology on the one hand, and inter-tidal beach erosion and accretion on the other, has been determined.
Beach elevation has not been specifically analysed, but forms an inherent element of volume change, discussed below. It is clear that both past and present groynes have had a critical effect in this respect (Harlow, 1995; 2001).
Analysis of beach and inshore sediment sampling, carried out in conjunction with many of the beach surveys, indicate a high degree of both longshore and cross-shore sorting. Coarsening towards the east and around Mean High Water is evident. A value of 1.8 phi is a critical size, with finer material likely to move offshore; some of this is deposited on shore-parallel bars. The beach replenishment of 1974/5 did not significantly alter overall particle size distribution, as 'borrow' material was close to that of the indigenous beach. However, in the five or six years following the recharge scheme of 1988/9, the less well-sorted sands obtained from dredging the Swash channel approach to Poole Harbour resulted in an overall finer distribution (Cooper, 1997).
There has been considerable analysis of net beach volume changes, 1974 to 1997 (Harlow, 2001; Cooper, 1997 and summarised in Harlow and Cooper, 1996). This indicates up to 500,000m3 of fluctuation between successive surveys, but the use of a 5 year "rolling average" (to smooth out shorter-term irregularities) indicates variation between 6 million m3 in 1974 and 9 million m3 in 1990. There were fluctuations in between due to the effects of sand nourishment in 1975 and 1988/9 and subsequent decay due to selective longshore and offshore transport. Cooper (1997) and Harlow (2001) have undertaken detailed analysis of the volumetric changes induced by replenishment for different sectors of the Bournemouth beach system. The major variables are location with respect to dump sites; antecedent profile form and beach area; beach orientation; exposure to wave energy; the nature of local control structures and particle size distribution of introduced material. As with many renourishment projects, rapid losses were experienced within two years of emplacements, but thereafter stabilised at levels characteristic of "background" erosion. For example, volume losses resulting from the 1988/9 recharge were 255,000m3a-1 over the succeeding 3 years, but thereafter reduced to 44,000m3a-1. Groyne design and spacing was often critical to beach sediment retention, demonstrating its importance to the viability of management by renourishment. The return to pre-nourishment volumes was fastest along the eastward part of this frontage, between Boscombe Pier and the end of the continuous seawall (Cooper, 1997). Gao and Collins (1994) estimate that the annual loss of beach sediment for the entire Bournemouth beach system, 1970-1990, was close to 100,000m3.
Further evidence of the negative budget prevailing over recent decades, and probably since at least the early twentieth century following the progressive extension eastwards of defences (Bray, et al, 1996; Cooper, Hooke and Bray, 2002), is given by Harlow (2001). He uses the Bournemouth Borough Council profile database to calculate recession of both High and Low Water Marks between 1974 and 1997. Between 1974 and 1987, MHW receded by 1.36 m a-1 over successive autumn surveys, and 0.3 ma-1 between 1991 and 1997. The reduced rate during the later period reflects the short period elapsing since re-nourishment in 1988/89.
Analysis of the outer part of the beach between 100 m and 450 m of origin showed some volumetric increase following renourishment in 1975 and 1988 even though this part of the profile was not directly recharged. However, a major loss of volume in 1975, and a sustained gain between 1992 and 1995 that could not be linked to renourishment were also revealed. One implication is that large scale but short-term losses and gains in the nearshore area may occur independent of inter-tidal beach volume changes. Significant net onshore and offshore, but unsteady, transport pathways therefore would appear to operate.
Solent Beach to the Long Groyne, Hengistbury Head
This length of beach is characterised by a narrow upper gravel backshore and wide sandy foreshore with only relatively small-scale seasonal fluctuations of profile. Littoral drift is predominantly west to east, evident from updrift accumulation against the Long Groyne. Between 1938, its year of construction, and 1980, Lacey (1985) calculated that the position of Mean High Water advanced seawards by over 70 m (1.8 m a-1). For Mean Low Water, advance over the same period was close to 100 m. Sand dune accumulation occurs across the backshore close to the Long Groyne, assisted since the late 1980s by the trapping of saltating sand by gabion cages and fencing. This is due to the drying out of beach sand as the tide falls. Harlow (2001) estimates net dune sand accumulation of approximately 1,000 m3a-1.
The morphodynamics of Solent Beach, and the co-adjacent beach between Double Dykes and the Long Groyne has been complicated since the mid 1970s by three artificial influences. The first is the effect of updrift sand replenishments in 1974/5 and 1988/9, which were transferred to this beach, after a time lag of two to three years, by eastwards littoral drift (Cooper, 1997). The second is a direct gravel recharge of Solent Beach in 1988, involving close to 150,000 m3. The last is the construction of three rock groynes at Double Dykes in 1987/8, with additional armouring in 1991 (Photo 9). These have been relatively successful in accumulating sediment, especially sand and fine gravel (HR Wallingford, 1993; Halcrow, 1999). Inter-groyne beach area has increased as a result.
In comparison to updrift beaches, width (except adjacent to the Long Groyne) is relatively narrow due to greater exposure to wave energy and a net offshore sediment transport pathway estimated at 25,000 m3a-1 (Hydraulics Research, 1986). This is the result of deflection of littoral drift by Christchurch Ledge and Beerpan Rocks. Harlow (2001) calculates beach volume, 1974 to 1998, to be a mean of 1,578,000 m3, but with peaks in 1977 and 1992 resulting from earlier updrift replenishment schemes. The natural tendency is for erosion, calculated by Gao and Collins (1994) to be 250 m3 per metre length for the period 1974 to 1987; Harlow (2001) calculates that a loss of 200 m3 per metre prevailed between 1979 and 1984 across a regularly re-surveyed profile approximately 800 m west of the Long Groyne. It was assumed that this period post-dated any benefit from the updrift beach nourishment in 1974/5. Cooper (1997) noted that net erosion east of Double Dykes was apparent by 1993, giving only some 6 years of temporary accretion following the second nourishment scheme in 1988/89. Throughout the period 1974 to 1989 (Hydraulics Research, 1986; Cooper, 1997; Harlow, 2001), the beach 100-150 m west of the Long Groyne steadily accreted but erosion at an average rate of 100 m3 per metre set in after 1993.
Since the early 1990s, there have been proposals to extend the sequence of rock groynes at Double Dykes further downdrift (eastwards) to help conserve beach sediment (HR Wallingford 1993; Bray, 1993). These have yet to be implemented, although current strategic management policy (Halcrow, 1999) is to maintain this beach at a critical width that will allow only modest basal cliff erosion.
6. SUMMARY OF SEDIMENT PATHWAYS - References Map
7. COASTAL DEFENCE AND HABITAT ISSUES - References Map
Both the SMP (Halcrow, 1999) and the forthcoming Poole Bay Strategy Study (Halcrow, 2003) examine in detail two main issues as follows:
Double Dykes BreachA permanent breach at the low cliffs at Double Dykes, could open up an additional entrance to Christchurch Harbour rendering Hengistbury Head an island. Given rising sea-level and the increased frequency of storm waves, this scenario will become increasingly probable should there be any relaxation of current beach management practices. Thus, to avoid the impact of a second tidal pass on the ecological balance of Christchurch Harbour, it will be essential to enhance the defence standard offered by the present mix of rock groynes, gabions and periodic renourishment. Solent Beach was last recharged with gravel in 1988/9, with most of this material having moved either alongshore or offshore within six years of its emplacement. To maintain the integrity of Solent Beach, it may in future be necessary to undertake recharge, with mixed sand and gravel, at intervals of less than 10 years (depending on quantities introduced; particle size distribution of borrow material, whether introduced onto both the intertidal beach and nearshore zone, and any reprofiling, crest elevation, etc.). There may also be an increasingly more compelling case for innovative management measures, such as beach dewatering or offshore breakwaters. However, all proposals will require very careful numerical modelling based on higher quality data for nearshore wave climate and sediment transport than is currently available.
A breach at Double Dykes also has far-reaching socio-economic and environmental implications (see, for example, Parker and Thompson, 1988; Turner, 1998; Bray and Hooke, 1998a). Most cost benefit calculations support the case for investing in defences, thus maintaining the existing mix of freshwater and saline habitats in Christchurch Harbour.
Cliff HabitatsBoth the protected and unprotected cliffline has a valuable role in providing natural and artificially created habitats. All of the cliffline east of Poole Head, as far as Southbourne, has been protected from basal marine erosion by a now continuous esplanade. Significant lengths of cliff have been regarded to stable angles of between 25 and 35 degrees and planted with a variety of hardy shrubs and grasses (Photo 4). This practice dates from the early 1930s, so that today most parts of this cliffline carry a continuous vegetation sward (Walls, 1993). English Nature has declared Poole Bay cliffs to be a SSSI, and have recorded the presence of over 300 plant species. There are particularly well-developed heather and dwarf gorse communities. Poole and Bournemouth Borough Councils work in co-operation with English Nature to achieve the longer-term objective of enhancing ecological integrity and habitat stability. Particular emphasis is currently being placed on removing, or at least controlling, invasive scrub and alien exotic species, including garden escapees, such as Holm Oak and Hottentot Fig, and numerous weeds imported with the initial topsoil. This will provide more opportunity for the expansion of native or semi-natural grass and ericaceous communities, and more habitat for species such as sand lizards. The latter requires that areas of sandstone are kept free of vegetation to allow weathering to create patches of bare, loose sand.
Unvegetated slopes, particularly where they have not been regarded, as at Canford Cliffs (Photo 3), are subject to gulleying, physical weathering and - to a small extent - attack by aeolian processes. This has crated cliff foot debris stores of fine sand, which have been partially reworked into small dunes. Blown sand from the foreshore is also trapped, a process that has contributed a cliff top deposit between Southbourne and Solent Beach. Some dune vegetation has colonised both types of site, creating a small-scale habitat worthy of conservation. There is, of course, the amenity problem of drifting sand - perhaps a deliberate programme of planting dune vegetation might be a sustainable option. This has been successfully implemented along the Sandbanks coastline, where backshore dunes have been created as a component of comprehensive beach management by Poole Borough Council.
Between Double Dykes and the Long Groyne the actively eroding cliffs are not deliberately managed, so that habitat interests are not compromised. Immediately updrift and downdrift from the Long Groyne, dune creation and planting has occurred across the backshore, thus enlarging a habitat that dates from the rapid growth in the width of the foreshore following the completion of the Long Groyne in 1938. Here, and to the immediate east, Bournemouth Borough Council and English Nature have attempted to resolve a long-running conflict of interest between habitat/geological conservation, on the one hand, and coastal protection, on the other (Turner, 1998; Bray and Hooke, 1998a and b; Bray, 1993). Although current practice of trying to reduce but not halt retreat involves an element of compromise, it is difficult to identify a more appropriate alternative.
8. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMESDetailed monitoring of sediment losses from past episodes of beach replenishment have been used by a variety of researchers as a basis to determine rates of drift. However definitive results could not be produced due to the non-typical initial behaviour immediately following placement of fill and an inability to reliably separate losses occurring due to offshore transport from those resulting from drift.
A variety of modelling studies have also been undertaken, but these have tended to produce a range of transport estimates and on occasion predicted drift direction did not correspond with observations of sediment accretion within groyne embayments. Problems facing modelling involve selection of appropriate wave climates, the complex conditions occurring in the western parts of the bay, the extensive groyne fields that intercept drift and a tendency for cross shore and transport to dominate over longshore.
Two main opportunities for future can be identified:
9. KNOWLEDGE LIMITATION AND MONITORING REQUIREMENTS - References MapMost of the shoreline of this sector has benefited from what is probably the longest established and most comprehensive and detailed beach monitoring programme carried out in the United Kingdom. Starting in 1974, Bournemouth Borough Council have undertaken biannual (spring and autumn) beach profile surveys for over 30 profiles. Topographical survey has been employed for the intertidal beach down to Mean Low Water with hydrographical survey used to extend this to 450m seawards. In addition, particle size analysis has been routinely undertaken for samples from 8 sectional profiles. Since 1993, detailed monitoring of beach levels adjacent to groynes, and computation of drift volumes, has also been regularly recorded along the Bournemouth frontage. Full details of this work, together with analytical conclusions, are given in Harlow (2001).
Other regular monitoring activities includes (i) frequent bathymetric surveys of the Swash channel and Poole Harbour entrance channel by Poole Harbour commissioners, and (ii) regular intertidal beach profile resurveys of the Sandbanks peninsula, by Poole Borough Council. Other continuous data sets include tidal levels, at Bournemouth Pier, and wind speed and direction, together with a directional wave rider buoy offshore of Boscombe.
The regional Shoreline Management Plan (Halcrow, 1999) suggests several ways whereby repetitive monitoring surveys would add to existing knowledge. However, it emphasises that the work already achieved deserves to be continued indefinitely, thus providing a long, and therefore representative, record of the morphological and hydrodynamic conditions operating in Poole Bay. A specific series of recommendations for monitoring have been made by HR Wallingford (2000) to improve knowledge of the complex transport regime operating along the Sandbanks Peninsula. Further research has been undertaken for the Poole Bay Strategy Study (Halcrow, in preparation). This has involved the creation of a wave climate model, using UK Meteorological Office synthetic data; from this, vectors of nearshore and offshore sediment transport and a beach behaviour model.
Some of the 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, review and continuation of Environment Agency ABMS to incorporate new ground control, LIDAR imagery and nearshore hydrographic survey. 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 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:
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MMIV © SCOPAC Sediment Transport Study - Poole Bay