Isle of Portland and Weymouth Bay
(Portland Bill to Redcliff Point)
1. Introduction - References Map
Weymouth Bay is one of only three open coast frontages in the SCOPAC region with an east-facing orientation. It cuts across a succession of Jurassic strata related to the disposition of the Weymouth anticline, the only surviving part of the southern limb being represented by the "Isle" of Portland extending southward to form the major headland of Portland Bill - see Photo 1 (Bird, 1995; Brunsden and Goudie, 1997). The Bay itself comprises the inundated and partly eroded remains of the Wey river valley occupied by a barrier beach of gravel and sand which was formed by the landward migration of sediments that were combed up during Holocene sea level rise. Cliff erosion occurring between Ringstead Bay and Redcliff Point provided an additional local source of material for the barrier, with material being transported westwards along a locally reversed drift pathway prior to the emergence of Redcliff Point (Photo 2) as a transport boundary. The barrier enclosed a small estuary and drift deflected its entrance westward against the hard Nothe headland (Photo 3). The estuary is now largely infilled by sedimentation and is occupied by the Yacht basin and Radipole Lake. Major complex landslides occur in areas of high topography on the north of the Isle of Portland where resistant and/or porous cap rocks overlie soft impermeable clays (Photo 4).
The resort town of Weymouth has been built along the western part of Weymouth Bay and a major road (the A353) constructed behind the whole of the barrier beach (Photo 5). The town and road have been protected by a sea-wall and esplanade since the late 19th Century. Weymouth port was developed within the outer and mid Wey estuary. The eastern margin of the entrance has been stabilised, whereas the resistant limestone of the Nothe point forms the western margin (Photo 3). Some modest reclamations have been undertaken along the estuary margins. Portland Harbour was enclosed by a series of concrete breakwaters constructed in the mid 19th Century.
Parts of this shoreline have been investigated in some detail because of flooding and erosion problems, which have affected properties, amenity beaches and communications. Various defence and protection measures, and construction projects, have entailed assessment of their possible impacts and therefore required research into sediment transport processes and patterns. Some complex hydraulic and geomorphological changes have taken place, mainly resulting from the creation and subsequent development of Portland Harbour after the mid-nineteenth century. These include gradual shoreline realignment, some local modifications of pathways of littoral drift and stabilisation of the barrier (tombolo) connecting the 'Isle' of Portland with the mainland coast. The building of the Portland Harbour breakwaters has removed any dynamic adjustment between this coastline and Chesil Beach, although the tidal lagoon of The Fleet drains via a regulated outlet into the harbour. Thus, military, recreational and urban pressures have significantly altered the natural hydrodynamic and sediment transport regimes.
2. SEDIMENT INPUTS
2.1 Marine Inputs: Offshore to Onshore Sediment Transport - F1 F2 References Map
F1 Weymouth Bay: Bowleaze- Lodmoor
Jolliffe (1976) considers that in Weymouth Bay locally significant amounts of flint gravel from both nearshore and offshore sources, possibly originating from relatively deep water, are supplied to beaches by weed-rafting (kelp becomes attached to sea-bed gravels is transported by wave induced and/or tidal currents acting upon the kelp frond and dragging the pebble along the seabed). Bastos and Collins (2002) have confirmed the presence of accumulations of gravel within a much more extensive area of finer to medium sand. The total amount involved in this probably discontinuously operative pathway has not been quantified and more substantial and frequent movement of sand on to southern beaches may mask some incursions. The significance of weed-rafting is difficult to assess because of the lack of any systematic observations, but Jolliffe (1976) presents some direct evidence of the process. Pulses of supply provided by kelp rafting are probably associated with waves of greater than average energy (Brampton, 1996). Kemble (1984) has also concluded that the only substantial contemporary supply of coarse clastic materials to the Weymouth-Bowleaze sector must be from offshore. The sub-angular nature of the majority of clasts, and features such as attached marine encrustations, indicate that the material was previously on the seabed beneath relatively deep water. This is a low energy environment, but onshore movement may be induced by occasional high-energy waves. Discrete "pulses" of supply may be inferred, but definitive evidence is lacking. Bastos and Collins (2002), however, consider that only sediment up to the grade of course sand can be moved by storm waves entering Weymouth Bay, although their calculations were for clean rather than kelp attached gravels. Evidence of micro jointing defining the shapes of some particles may imply that they have been derived directly from erosion of rock outcrops on the seabed (Posford Duvivier, 1999a). This would also suggest the occasional operation of high-energy waves.
F2 Weymouth Bay and Portland Harbour
The volume of sand stored in the intertidal area of Weymouth Bay beach has increased over recent decades apparently because of natural feed of sandy material from offshore. Two independent research investigations, very different in their methodologies, come to similar conclusions. Jolliffe (1976) stated: ".... a reasonable body of factual data support the view that sand has been moved southwards across the inner bay, mainly on account of tidal flow, and has tended to collect in the southern parts of the bay. A substantial amount of this material has moved shorewards, mainly due to wave action, building up the southern foreshore and the small submarine slope." Bastos and Collins (2002) demonstrate, using hydrodynamic and transport numerical modelling, that waves and tidal currents, in combination, can mobilise fine to medium sand. The latter is only likely to move as bedload under higher energy waves approaching from the east-south-east. Once mobilised, net direction of movement is shorewards, promoting accretion.
Some problems of siltation in the outer part of Weymouth Harbour have necessitated maintenance dredging. Evidence from the median grain size of sampled material - fine sand - is that it originally came from adjacent beaches, or offshore. Calculations based on amounts of material removed by dredging (ABP, 1984a and b) indicate that average annual depth of accretion over the Harbour area is only 0.085m. Some of this material must be part of the net onshore movement of fine sand, noted above. Once inside the Harbour training walls, it is potentially available for settlement. The turbulence generated from vessel movements probably redistribute some material within the Harbour.
2.2 Fluvial Inputs - FL1 FL2 References Map
FL1 River Jordan
The River Jordan, debouching at Bowleaze Cove, drains a small catchment of highly erodible sandstones, clays and limestones, but there are no measured data on sediment delivery (Posford Duvivier, 1999). Rendel Geotechnics (1996) give an estimate of 72 tonnes a-1 bedload, 220 tonnes a-1 suspended load.
FL2 River Wey
Fine sediment in the inner part of Weymouth Harbour is considered to be derived from erosion of the Wey catchment. Measurements of suspended sediment concentrations in the River Wey indicate that supply from that source is very small (Rendel Geotechnics, 1996; ABP 1984b), probably not exceeding between 100 and 500m3a-1 of fine sand or silt delivered to the coast (Posford Duvivier, 1999; Rendel Geotechnics, 1996). If Weymouth Harbour owes its configuration to former southwards spit, bar or barrier growth, most fluvial sediment will have been trapped and stored in Radipole Lake during recent centuries. The low yield is due in part to barriers to river transport, notably the mill at Upwey. Without these impediments, suspended load would be in the order of 1,400 to 1,500 tonnes a-1 (Rendel Geotechnics, 1996).
2.3 Coast Erosion E1 E2 E3 E4 References Map
The nature and rates of coast erosion are determined strongly by the varied lithologies of the geolocical strata exposed. The geology is controlled by the east-west trending Weymouth anticline, eroded to leave a low valley developed in soft Oxford clay at its core with harder Corralian limestones outcropping along its SSW and NNE flanks forming low hills and cliffs at Redliff and Nothe points respectively. The southern limb of the anticline is now largely eroded except for the gentle southward dipping Upper Jurassic strata of the Isle of Portland. Consequently Weymouth bay is backed by low-lying estuarine deposits and flanked to the East (Redcliff Point) and West (the Nothe) by cliffs composed of alternating clays and limestones. On the north and north-east coast of the Isle of Portland, the cliffs are higher and the Portland Stone and Purbeck beds rest upon soft Kimmeridge clay in a sequence that has resulted in large deep-seated landslides (Photo 4). Southward dip of the strata on the Isle of Portland causes a reduction in land elevation in this direction and brings the clays down to below sea-level to the south of Church Ope Cove so that the tendency for landsliding is reduced and the limestone cliffs retreat extremely slowly (Photo 1). There is a long history of quarrying of building stone on the Isle of Portland and much quarry waste has been tipped around the island forming protective boulder aprons at the cliff toes.
E1 Furzey cliff to Redcliff Point (see introduction to Coast Erosion)
Between Bowleaze Cove and Redcliff Point cliffs 18-30m high are developed in Corallian clays interbedded with numerous limestone seams and are subject to landslipping by rotational movement and block slides. The landsliding delivers clays and limestone blocks to the shore; the former being removed seaward in suspension, whereas the latter remain to form boulder aprons at the toe that intercept beach drift at the headland (Photo 2). May (1966) has calculated an erosion rate of between 0.3 and 0.6 ma-1 for Redcliff Point, confirmed by Mouchel (1998).
Furzy Cliff (Photo 6) is composed of Oxford Clay with a thin capping of Corallian Beds. It degrades readily by rotational landslides at the cliff top with mudsliding occurring within a narrow undercliff to deliver primarily clayey material to the toe. The recent history of cliff recession has been fully documented (Jolliffe, 1976; Scott, Wilson, Kirkpatrick, 1994b; Holliday, 1997). Between 1850 and 1973 the average recession of the cliff top was calculated as 0.31ma-1 but rates varied through time and according to location. In the same period HWMMT receded 6-12m and between 1947 and 1967 the cliff toe and top both receded an average of 5m (Brunsden and Goudie, 1981). Jolliffe (1976) considered that marine erosion at the base of the cliff had increased during the previous 70 years, evidenced by an almost 50% reduction in the inter-tidal width of the beach, from 24m in 1902 to 12m in 1973. May (1966) mentions a large rotational slip in 1964 that produced localised cliff-top retreat of 27.4m, with a loss of 1486m2 of land. Cliff profiles for 1963, 1964 and 1969 show an apparently accelerating rate of recession (May 1977). A sea wall constructed in 1984 along part of the cliff-base has subsequently reduced sediment input from this source, but the reactivation of mass movement in the early 1990s may have restored delivery rates to their longer-term average (Scott, Wilson, Kirkpatrick, 1994b; Mouchel, 1998).
Offshore of Furzy Cliff there is an uneven low platform surface with transient deposits of sand and fine gravel, and intermittent exposure of the Oxford Clay substrate. Jolliffe (1976) considers that net erosion by abrasion is taking place in this zone, as indicated by short-term monitoring of brass rods inserted into bedrock surfaces. Erosion of the 4km shoreface between Redcliff Point and Portland Harbour is estimated to yield some 7,500m3a-1 of fine sediments Posford Duvivier (1999a and b). Cliff inputs for this frontage are not provided by those studies.
E2 The Nothe (see introduction to Coast Erosion)
Cliff instability has been a recurrent feature of the exposed coastline at The Nothe and Nothe Point, where small coves have been eroded. Regrading and toe armouring, carried out in 1988, currently exclude any significant sediment yield (Photo 3).
Erosion of the 4km shoreface between Redcliff Point and Portland Harbour is estimated to yield some 7,500m3a-1 of fine sediments Posford Duvivier (1999a and b). Cliff inputs for this frontage are not provided by those studies.
E3 Portland Harbour (see introduction to Coast Erosion)
Within Portland Harbour the derelict condition of Sandsfoot Castle, built in 1539, provides evidence of the long term progress of cliff erosion prior to the development of the harbour. At Western Ledges, Corallian Beds overlie clays prone to slipping, but erosion was substantially reduced following construction of the Portland breakwaters and shoreline stabilisation between 1846 and 1850. Cliff instability is partly due to toe loading, but the low erosion rate is principally because there is only a small fetch for wave generation and propagation within the confines of the harbour (Brampton, 1996). The narrow beach of sand and shingle between Smallmouth and the northern harbour breakwater derives from continuing small-scale cliff erosion, but is unlikely to make a significant contribution to any littoral transport.
E4 Isle of Portland (see introduction to Coast Erosion)
The beaches the south of Portland Harbour are composed of subangular and angular gravel of Portland and Purbeck limestones and cherts which can only have come from erosion of the northern and north-eastern coastal slopes of Portland prior to the construction of the harbour. Past and currently active landslips are recorded on the eastern and northern slopes of the "island" (Allison and Kimber, 1999; Brunsden and Goudie, 1981; Brunsden, et al., 1996; Kimber, 1999; Petley and Allison, 1999), giving rise to distinctive, structurally determined, cliff and costal slope morphology (Photo 4). Some cliff foot talus on the east coast of the Isle of Portland derives from dumping of Portland Stone quarry spoil since the 16th Century (Brunsden, et al., 1996; McLaren, 1990). The mechanics of landslipping affecting the northeast and east shorelines of the Isle of Portland are not fully understood (Brunsden, 1996, 1999),
Erosion of the 6 km shoreface between Portland Harbour and Portland Bill is estimated to yield some 7,500m3a-1 of fine sediments Posford Duvivier (1999a and b) that would be rapidly lost offshore to deep water. Cliff inputs for this frontage are not provided by those studies.
3. LITTORAL TRANSPORT - LT1 LT2 LT3 and LT4 LT5 LT6 References MapThe wave climate of Weymouth Bay is determined by the depth of its indentation and protection from Atlantic swell provided by Chesil Beach, the `Isle' of Portland and the several shoals and banks east and west of Portland Bill. The latter induces substantial refraction, although residual long period waves do penetrate under storm conditions. The most important fetch direction for higher energy waves is to the south-south-east, but waves of up to 1m in height can be generated by south-westerly winds blowing across the 5km fetch of the Bay itself (Hydraulics Research, 1986; HR Wallingford, 1998). Mean significant height of south-westerly approaching waves is, however, only 0.2m. For waves moving into the bay from the east or south east, which operate for approximately 15% of the time during an average year, mean significant wave height is 0.65 to 0.7m (HR Wallingford, 1998; Bastos and Collins, 2002). Storm waves do not normally exceed 2.0m in height, though model simulation (HR Wallingford, 1998) produced a 1 in 100 year recurrence of waves between 6 and 7m in height.
Ringstead Bay some 6km to the east of Weymouth Bay was one of the locations for which wave modelling exercises were undertaken as part of the DEFRA Futurecoast Project (Halcrow, 2002). An offshore wave climate was synthesised based on 1991-2000 data from the Met Office Wave Model and then transformed inshore to a prediction point in Ringstead Bay at -2.93m O.D. Potential sensitivities to likely climate change scenarios were then tested by examining the extent to which the total and net longshore energy for each scenario varied with respect to the present situation. Results suggested that a one to two degree variation in wave climate direction could result in a 3.5% to 7% variation in longshore energy and confirmed that the Bay was significantly more sensitive to this factor than many south locations. Wave energy at Ringstead was also found to be especially sensitive to sea-level rise and storminess. It is likely that similar sensitivities could hold true for Weymouth Bay also.
Considerable change of hydraulic regime has taken place over the last 140 years in Weymouth Bay following the construction of Portland Harbour and its confining breakwaters. These have had the effect of further reducing the impacts of refracted waves from south-west and westerly directions and giving enhanced influence to waves generated over easterly and south-easterly fetches. One notable change has been the realignment of the coastal planform in adjustment to this more dominant easterly direction of wave approach (Jolliffe, 1976). Waves produced by south-east gales now impact further north, as shown by progressively more northerly breaches in the sea wall at Weymouth and the accelerated erosion of Preston Beach from the late 19th Century. From about 1910 onwards concern grew about shingle being brought on to Weymouth's sand beach from sources both seawards and further north (Overcombe and Lodmoor). The hydraulic regime has also been affected by the building and widening of the esplanade, construction of a sea wall behind Preston Beach and the introduction of the Ferry terminal.
LT1 Furzey Cliff to Redcliff Point (see introduction to Littoral Transport)
In the Redcliff-Bowleaze sector it is considered that the micro-relief of the foreshore prevents any significant littoral drift supply of clastic sediments to the Furzy Cliff segment, where beach shingle increases in mean size from north to south. As the cliff line has naturally receded it has become set back from the protected line of Preston Road Beach since the early twentieth century; it is considered that there is now compartmentalisation, with little or no exchange between these units (Scott, Wilson, Kirkpatrick, 1994). Significant wave heights can exceed 1m, with wave approach ensuring a swash-aligned beach. Littoral drift, if any, is thought to be in a northerly direction. The beach is gently sloping with a discontinuous cover of fine shingle and coarse sand overlying a basement of Oxford Clay.
LT2 Preston Beach (see introduction to Littoral Transport)
The beach forming the main frontage of Weymouth Bay is widest in the south, narrowing in intertidal width and increasing in mean grain size northwards. It diminished significantly over the 20th century and was replenished artificially in 1996. In its natural state it was composed of fine to medium sand with a high proportion of organic matter and isolated patches of gravel and cobbles. Following replenishment, Preston Beach is now a wide, artificially graded, gravel beach, partly confined by a terminal rock groyne at its southern end (Photo 5). It was renourished with 2.2 million cubic metres of coarse material, imported from a source area offshore of the coast of the north-west Isle of Wight in 1996 as part of a comprehensive sea defence project (Rigby and Quarrier, 1996; Scott, Wilson, Kirkpatrick, 1994a).
In the past, gravel appears to have increased slightly in mean size from north to south as littoral drift transferred finer grained material from Preston Beach to Weymouth Beach (HR Wallingford, 1993). Based on results from a short-term tracer experiment, Jolliffe (1976) concluded that the quantity of littoral transport was very small. The evidence therefore suggests that prior to renourishment, net drift had been southwards, with evidence of brief periods of reversal associated with the highest energy incident waves. A potential net drift rate of 2,900m3a-1 is suggested by HR Wallingford (1998). South of Lodmoor, and outside of the replenished area beach width diminishes over some 300m partly due to the effect of the seawall and retaining rock groyne (Photo 7).
Lodmoor Marsh (Photo 8) is a former lagoon confined by a barrier beach (Preston Beach) that has migrated offshore to onshore (Kerr, 1989). Natural drainage was by percolation though the shingle before modification and sea wall construction in the early 20th Century. Beach recession occurred at Preston Beach after the construction of the Portland Harbour breakwaters, resulting in the road having to be realigned by 18m by 1885 and subsequently maintained at regular intervals (HR Wallingford, 1993). The causal relationship between these events has not been clarified, but must involve some modification of northward moving onshore transport.
Gravel entering the littoral system by weed-rafting north of the promenade clock tower should be transported northwards by littoral drift. Sand, however, will move southwards. South of the clock tower swash-aligned beach-nearshore exchange appears to be dominant (Jolliffe, 1976). Gravel is retained on the backshore beach, but becomes increasingly smaller in size, and impersistent, southwards.
To conclude, it would appear that drift is rather poorly understood within Weymouth Bay and some apparently contradictory statements have appeared within the literature. The uncertainties are probably a function of the following: (i) a fine balance that exists between northward and southward drift so that net drift varies in direction from year to year (ii) sand and gravel may undergo net transport in different directions and (iii) the regime may have been affected by the major replenishment such that net drift directions could have reversed since 1996. The present information points to a possible drift divergence at Lodmoor although further studies are required for confirmation.
LT3 and LT4 Weymouth Esplanade (see introduction to Littoral Transport)
Hydraulics Research (1989), operating a physical model using a hindcast wave climate calculated from Portland wind data, found a drift divergence zone in the vicinity of the old Bandstand Pier. This was considered to be an area relatively starved of material from which sand moves southwards. However, Jolliffe (1976) found that tracers simulating coarse material moved predominantly northwards. This is therefore an unstable, transient drift divergence, which may only function under specific hydrodynamic conditions for different grain sizes (Scott, Wilson, Kirkpatrick, 1994b).
Jolliffe (1976) considers that sand is moved shorewards in the vicinity of Weymouth Harbour, building up the southern foreshore just north of the Harbour and shallowing the submarine slope. The lengthening of Weymouth Harbour breakwaters over recent decades has increased this trap zone.
Hydraulics Research (1986), modelling the possible impacts of a proposed marina in Weymouth Harbour, considered that fine sand siltation in the harbour approaches derived from a beach and/or sea bed source further north, which was transported southwards by littoral drift. The barrier effects of the Weymouth ferry terminal and harbour entrance training jetties preclude any further shoreline drift south towards Portland Harbour.
LT5 Portland Harbour (see introduction to Littoral Transport)
A weak northerly-directed sediment transport operates around the western shore of Portland Harbour which appears to operate as a closed system (Brampton, 1996) where transport potential is inhibited by limited fetch available for wave generation. Previous to construction of the harbour breakwaters a moderate drift is likely to have operated to have formed a small gravel barrier beach composed of limestones and cherts from the north eastern coast of the Isle of Portland. The former sandy beach at Smallmouth is now smothered with silt (Hydraulics Research, 1986; HR Wallingford, 1996). There is likely to be some inputs and outputs of coarser suspended material through the breakwater passes, and the latter are known to induce some tidal scouring (Mouchel, 1998). There is hydrodynamic continuity with the Fleet, enclosed by Chesil Beach, but sediment delivery is negligible. Tidal flows within the harbour are weak and the tidal range is between 1.5 and 2.5m. This, together with the 4 hour stand at low water, ensures that fine grained sediment is trapped. Portland Harbour therefore functions as a partial sink, where net accretion has occurred (Hydraulics Research, 1996).
LT6 Isle of Portland (see introduction to Littoral Transport)
A potential for net northerly longshore drift exists between Portland Bill and the Portland Harbour breakwaters (Mouchel, 1998). The potential net transport rate at Church Ope Cove is assessed at 5,200m3a-1 (HR Wallingford, 1998), but lack beach deposits and the presence of a steep focky shoreface occupied in places by aprons of limestone boulders inhibits any significant drift from actually occurring (Photo 1).
4. SEDIMENT OUTPUTS - References MapThe overall tidal circulation is anti-clockwise to the east of the Isle of Portland. Within Weymouth Bay a double low-water stand occurs, thus tidal currents are of low velocity, not exceeding 0.41ms-1. Hydraulics Research (1984) used float tracking at Weymouth Harbour entrance and concluded that wave-induced currents induce sediment movement. Southward and eastward of Weymouth Bay tidal velocities are higher, reaching maximum values in excess of 2ms-1 in the zone of strong tidal eddies east and west of Portland Bill. This is generated by the meeting of tidal current systems moving eastwards and westwards in response to the configuration of the English Channel (Pingree, 1977; Pingree and Maddock, 1979).
4.1 Offshore Transport - O1 O2 O3 References Map
As Weymouth Bay is a low energy environment, particle attrition rates are low and there is no net offshore loss of beach material. This conclusion was substantiated by Hydraulics Research (1989) in a physical model experiment of the possible effects of sand beach nourishment. This indicated that beach material deposited above the breaker zone remained stable. Similarly Pingree and Maddock (1977) concluded that fine material transported that is discharged from sewers into Weymouth Bay would not be swept into the anticlockwise tidal eddy and transported seaward of Portland Bill. The suggestion (Jolliffe, 1976) that here has been a gradual lowering of the seabed surface of Weymouth Bay has not been substantiated by subsequent research (Brampton, 1996).
01 Redcliff Point - Furzy Cliff (see introduction to Sediment Outputs)
Tidal flows, operating in combination with wave disturbance of the sea floor may carry significant amounts of sand and large clasts (some of it weed rafted) from the Redcliff Point coastline southwards across Weymouth Bay for much of the tidal cycle (Jolliffe, 1976).
Offshore of Furzy Cliff there is an uneven low platform surface with transient deposits of sand and fine gravel, and intermittent exposure of the Oxford Clay substrate. Jolliffe (1976) considers that net erosion by abrasion is taking place in this zone, as indicated by short-term monitoring of brass rods inserted into bedrock surfaces.
02 Weymouth Bay (see introduction to Sediment Outputs)
The nearshore environment of Weymouth Bay, and especially the southern half, is mainly sandy (Bastos and Collins, 2002). Net deposition and shallowing has apparently taken place in the southern zone over the past 100 years, giving an intertidal zone over 1,000m in width, and a total shoreface approaching 1,400m wide. Posford Duvivier and British Geological Survey (1999) propose that abrasional scour and wave entrainment produce a total yield of 7,500ma-1 of fine sediments; its fate is currently unknown.
Locally strong tidal currents flow through the breakwater gaps of Portland Harbour leading to scouring, but the harbour bed is composed mainly of mud and silt deposits that are not disturbed by either tidal currents or waves (HR Wallingford, 1996; Brampton, 1996).
O3 Isle of Portland (see introduction to Sediment Outputs)
Further offshore the main features are the accumulations of dominantly coarse sandy sediment in the Shambles Bank and Adamant Shoal. Sonar surveys and sampling indicate a high content of shell but also superficial sand with small and large ripples (Mouchel, 1998; Bastos and Collins, 2002). Elsewhere the sea floor is fairly featureless, with occasional rock outcrops. Field measurements have shown that residual tidal flows are southwards on both sides of Portland Bill (Pingree and Maddock, 1979; HR Wallingford, 1998; Bastos and Collins, 2002). The Shambles is within the area of an anticlockwise tidal eddy east of the headland. Flows have been mathematically modelled by Pingree (1977) indicating high bottom stress during maximum tidal streaming which leads to scouring and the prevention of accumulation at Portland Bill. Material is thought to spiral into the accumulation zone of the Shambles, in the centre of the eddy circulation, which thus represents a significant offshore sink. However, no direct transport connectivity between this feature, and the Weymouth Bay sink, has been proven.
Bastos and Collins (2002) investigated the morphology, sedimentology, seabed mobility and sediment transport pathways south and east of Portland Bill the following account is based on their study. Original data was obtained using side-scan sonar, high resolution seismic profiling, and depth-averaged tidal current measurement. Grab samples of seabed sediments were also obtained.
The seabed is characterised by extensive areas of both stable and mobile sand, a discontinuous veneer of coarse sediment (mostly gravel) and bedrock outcrops. Away from Portland Bill, there is a sequential arrangement of sandbanks (The Shambles and Portland Bank); sand and gravely sand flats; sand shoals (Adamant and West Shoals), and rippled sand sheets. Each has a distinctive sedimentary facies.
The two main sandbanks are composed of very coarse sand that has accumulated to thicknesses of up to 22m (Shambles) and 19m (Portland). Sandwaves of up to 7m in height and 300m wavelength are well developed. The Shambles has an asymmetric cross-profile shape, with its steeper slope facing towards the coastline. Portland Bank is morphologically irregular, though it is more controlled by seabed bathymetry.
Seabed mobility and net bedload transport pathways were assessed using numerical (hydrodynamic and transport) models, with a range of sand grain sizes for tidal currents alone, wave actions only, and tidal and wave induced currents in combination. Sediments immediately offshore of Portland Bill were found to be very mobile to such an extent that a zone of erosion characterised by bedrock exposures was identified extending for several km southwards of the Bill. For both tidal currents alone and tidal currents and wave-induced currents acting together, shoal sediments were mobile for more than 50% of the time, whilst sandbank sediments were in motion for over 75% of the time. The action of tidal and wave-generated currents, working together, become more important as water depth increases. It was determined that only storm waves alone can occasionally disturb seabed sediments in water depths greater than 35m.
Net Bedload Transport pathways, or vectors, for sand moved by tidal currents are convergent towards Portland Bill. Maximum shear stresses are immediately offshore. This pattern is imposed by tidal eddy systems, which approximately coincide in location with the two major sandbanks. Wave action enhances transport rates, but does not determine the vectors of movement. Transport rates were derived theoretically, varying between 10-25kg/m/tidal cycle immediately south and south west of Portland Bill; and 0.25-1.0kg/m/tidal cycle further offshore. This pattern creates two distinct, `inner' and `outer' zones of sands transport. The first, characterised by relatively high rates, is towards Portland Bill; the second, with much lower rates, is directed away from the headland.
Both Portland and Shambles sandbanks are areas of convergent transport which were surveyed in detail. The Shambles Bank is composed of coarse bioclastic sand and has high bedform (mostly sandwave) mobility. An overall anticlockwise circulation appears to operate around the bank. The presence of this bank refracts waves approaching the inshore zone and shoreline. It would also appear to exert some control over the direction of movement of tidal currents during each tidal cycle. However, the exact causal relations between eddy-induced current flow and sandbank morphology were not determined. On theoretical grounds, it would appear that the headland-generated tidal eddies are primary controls of sandbanks shape, but may not be their fundamental cause.
Seismic reflection surveys revealed several apparent erosional surfaces within the Shambles sand sink. Their significance is uncertain, but their inclination (dip) is suggestive of long term shorewards migration.
4.2 Other outputs
Material periodically dredged from Weymouth Harbour is dumped south of Swanage but amounts involved are small.
Sand moved to the beach backshore in the southern part of Weymouth Bay is prone to aeolian transport, generally moving landwards and giving rise to a "minor" amenity problem (Jolliffe, 1976). No estimates are available of the amounts involved but they are probably negligible.
5. SUMMARY OF SEDIMENT PATHWAYS - References Map1. Weymouth Bay is a headland-controlled embayment occupied by a barrier beach and a partly infilled estuary. Behaviour is controlled by the sheltering influence of Chesil Beach, Isle of Portland and to a lesser extent by the Shambles Bank and Adamant Shoal.
2. Weymouth Bay and Portland Harbour operate as weak sediment sinks accumulating sediments primarily from offshore sources. Movement of gravels inshore from relic deposits by a kelp rafting mechanism is thought to be a means by which small quantities of fresh gravels may been supplied to the shore. Few littoral sediments are supplied from cliff erosion as the Redcliff Point headland inhibits the westward drift of flint gravels from Osmington and local cliffs around Bowleaze Cove primarily supply clays to the shore with only limited quantities of more durable limestones. Products of erosion of the NE Isle of Portland coast are retained in small beaches along the flanks of Portland Harbour. Many of the existing sediments are likely to be relic having been supplied during periods of rising sea-level in the late-Holocene at which time the bay may have been a much stronger sink.
3. There remains considerable uncertainty about the relative rates, quantities and directions of littoral drift occurring the between Redcliff Point and Weymouth Harbour. This appears to result from the fine balance maintained between westward and eastward drift and the possible occurrence of divergence zones along this frontage. It may also be that the sand and gravel drift pathways operate in different net directions. In general terms, the Weymouth shoreline operates as a low flux system with a weak tendency for sand accumulation in southern parts, and a potential for erosion and landwards transgression (if unrestrained) in northern parts (Halcrow 2002).
4. The regional wave climate and tidal regime, result in westward net sand movement along the inshore bed into this large embayment, from source areas to the east (HR Wallingford, 1996, 1998; Bastos and Collins, 2002). Sand transport is then deflected southward offshore of the eastern flank of the Isle of Portland and towards a zone of convergence south of Portland Bill. Some sand becomes entrained within an anticlockwise tidal eddie to the east of Portland Bill and circulates as part of the Shambles sandbank.
5. Intensive management involving the holding of a largely fixed line of coastal defence along the Weymouth frontage for much of the past 100-150 years has inhibited the natural tendency for landward migration of the shoreline and appears to have led to the depletion of Preston Beach. This frontage is now defended by a major beach replenishment that is likely to have altered the nature of beach drift within its immediate vicinity.
6. KEY COASTAL DEFENCE AND HABITAT ISSUES - References MapImportant estuarine lagoon and reedbed habitats occur at Radipole Lake and Lodmoor (Photo 8). The former site is threatened by siltation, a consequence of its water level regulation and limited tidal flushing of the partly reclaimed estuary. Lodmoor, was previously at risk from the overtopping and potential breaching of Preston Beach, but has now been protected by the major beach replenishment of its fronting beach and seawall renewal. It is, however, potentially vulnerable to urban encroachment, although that would be inadvisable within this flood risk area.
The wide backshore of the replenished Preston Beach (Photo 5) could provide opportunities for development of shingle flora although it would involve minimising disturbances by beach management within selected areas. Details of appropriate management and habitat creation techniques have been set out by Doody and Randall (2003) A shingle flora also exists along the gravel beaches of the western shoreline of Portland Harbour, but is poorly documented with most attention being directed at nearby Chesil Beach.
The remaining undefended cliffs of this frontage make a very small contribution to the overall sediment budget of this sub-cell. Sediment yield from these sources is mostly removed in suspension to central Weymouth Bay or offshore of Portland Bill. Existing conservation designations will ensure that these sources continue to function. The input of weed-rafted shingle from offshore sediment stores in Weymouth Bay identifies a need for improved understanding of the sub-littoral habitats in this zone. Research should focus on the inter-relations between substrate and ecological dynamics, with particular attention given to the mobilisation of gravels by seaweed colonisation and wave disturbance.
Portland Harbour accommodates a range of relatively stable inter-tidal habitats. They are threatened long-term by any deterioration in the effectiveness of the existing breakwaters. This issue is fully covered in the report by HR Wallingford (1996), which provides essential baseline data for future management strategies.
7. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References MapAlthough a general understanding of the pathways of net sediment transport in the littoral zone of Weymouth Bay has been obtained, there is insufficient data on: (i) prevailing directions, rates and volumes of beach drift and (ii) the relative proportions of different grain sizes involved. Arguably, the two most critical locations where reliable quantitative data are required for beach management are, firstly, the replenished Preston Beach and secondly, the more sandy southern part of Weymouth beach. A vital preliminary piece of research is to create a comprehensive model of the hydraulic regime of Weymouth Bay. This would generate a basis for numerical modelling of drift and plan shape modelling of the beaches, tasks that have yet to be undertaken. Quality monitoring of beach volumes would also be valuable so as to provide calibrations for more effective applications of transport models.
The data from tracer studies of beach drift, reported in Jolliffe (1976), provided some useful information relating to short-term measurements for "average" wave and current conditions. However, Preston Beach is now totally transformed by the 1996 replenishment so it is uncertain whether the pre-replenishment research results would remain applicable. To resolve uncertainties it would be valuable to add to this via a systematic programme that integrated the use of a range of modern tracers under a representative range of hydrodynamic conditions.
8. KNOWLEDGE LIMITATION AND MONITORING REQUIREMENTS - References MapThe basis of recommended future monitoring of coastal processes is outlined in Mouchel (1998). 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.
On this basis, 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: 1. A critical priority is to gain quantitative understanding of the offshore wave climate. Ideally, additional measured data are required on inshore wave characteristics, so that more precise local wave climates might be determined. The existing network of wave recorders operated/managed by the Channel Coastal Observatory presently has a major gap within this sector with the nearest recorder being at Boscombe in the east, whilst other wave recorders are located well to the west of Portland Bill. Such locations are not useful for Weymouth Bay because they do not experience the unique local wave climate created by the sheltering effect of Chesil Beach and the "Isle" of Portland on incident waves from the west and south-west. Wave monitoring and preparation of local wave climates is an essential basis for progressing further research on sediment transport rates and pathways, outlined in the previous section.
2. The effective application of numerical modelling studies of beach behaviour and sediment transport processes requires the input of high quality nearshore bathymetric survey data. This is especially important for the frontage between Redcliff Point and Portland Harbour. Surveys should be completed with reasonable frequency and ideally be combined with some seabed sediment sampling. This would enhance understanding of inshore sediment distribution and mobility and reduce uncertainty over links between offshore, nearshore and littoral transport systems.
3. A network of beach profiles needs to be established together with arrangements made for regular re-surveys. It would also be advantageous to undertake post-storm profile surveys at critical locations e.g. Preston Beach, where there could be a tendency for significant cross-shore exchanges of beach material. Periodic bathymetric surveys to extend profiles seawards to water depths where there are limited bed level changes. The approach of the authorities in Poole and Christchurch Bays provides a model. It could also be integrated with routine bathymetric surveys of the harbour entrance and approaches undertaken by the Weymouth port authority. Overall, such data would enable quantitative analyses, especially of volume changes (which have hitherto been lacking) and would provide valuable insights into the rates of operation of littoral transport and the effectiveness of beach management. It is understood that the Strategic Regional Coastal Monitoring Programme (www.channelcoast.org) aim to provide vertical aerial photo coverage, a together with a series of survey control points and some measured baseline profiles.
4. Once a programme of profile measurement has been established, consideration is needed of how the profiles should best be analysed. It will be important to identify indicators of beach health such as sediment volume, crest height and crest position. An error analysis should be undertaken so as to identify the minimum volumetric change that can be resolved with the techniques. Trends in these indicator parameters (annual and seasonal) need to be established as data accumulates and a system of routine analysis instituted that would provide early warning of "unusual" trends. It may be that local engineers can identify critical thresholds, or minimum values of these parameters that could be applied to trigger specific warnings. To effectively interpret the trends recorded, it will also be vitally important to maintain good records of all beach management activities undertaken.
5. To understand beach profile changes it is important to have knowledge of the beach sedimentology (gain size and sorting). Sediment size and sorting can alter significantly between Redcliff Point and Weymouth Harbour due to sorting processes and possible "leakages" of recharge material from Preston Beach. Ideally, a one-off field-sampling programme is required to provide baseline quantitative information along this shoreline together with a provision for a more limited periodic re-sampling to determine longer-term variability. Such data would also be of great value for future modelling of sediment transport, for uncertainty relating to grain size is often a key constraint in undertaking modelling.
6. By collecting data on beach volumes, cliff erosion yields littoral drift rates, it would in future be feasible to produce initial sediment budget estimates for discrete sectors of shoreline. Key trends occurring could then be identified with confidence and major uncertainties requiring specific research could then be highlighted and prioritised. This work has significant long to medium-term implications, for the effective management of sediment for coastal defence and deserves critical evaluation.
7. Sediment transport from the nearshore to local beaches is poorly understood. It is not known, for example, the extent to which kelp rafting contributes gravel and/or wave action moves sand onshore. Although difficult to justify on economic grounds, every opportunity should be taken to make visual observations of beaches for evidence of pebbles cast ashore with kelp holdfasts attached following storms.
8. A detailed retrospective study of historical coastal changes and their management, particularly since the mid-nineteenth century, would help to elucidate the cause and effect relationships arising from human modification of the geomorphological and hydraulic regime. Existing research on this topic has provided a range of essentially qualitative insights that would benefit from re-examination using improved methodologies.
9. REFERENCES - Map
ALLISON, R. and KIMBER, O. (1999) Distinct Element Computer Modelling of Portland Limestone Coastal Cliffs, in: R.J. Allison (Ed) Dorset Revisited: Position Papers and Research Statements, West Dorset Coastal Research Group, 53-62.
ASSOCIATED BRITISH PORTS (1984a) Weymouth Harbour. Bed Investigation. Report No. R619, 5pp.
ASSOCIATED BRITISH PORTS (1984b) Weymouth Harbour Accretion Study. Report No. R311, 12pp.
BASTOS, A. AND COLLINS, M. (2002) Seabed Mobility and Sediment Transport Pathways Along the Dorset Inner Continental Shelf (Southern UK). Report to SCOPAC, School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre. Vol. 1, 93pp; Atlas, 21 plates.
BIRD, E. (1995) Geology and Scenery of Dorset, Bradford-on-Avon: Ex Libris Press, 178pp.
BRADBURY, A.P. (2001) Strategic monitoring of the coastal zone: towards a regional approach. Report to SCOPAC, South Downs Coastal Group, South East Coastal Group and Environment Agency, 91p.
BRAMPTON, A.H. (1996) Portland Harbour Study, in: Papers and Proceedings, SCOPAC Seminar on the Development of Shoreline Management Plans: Experiences and Initiatives, SCOPAC, 13-22.
BRUNSDEN, D. (1996) Landslides of the Dorset Coast: Some Unresolved Questions, Proceedings of Ussher Society, 9, 1-11.
BRUNSDEN, D. (1999) The Isle of Portland Landslips - The Case of the Missing Ductile Layer, in: R.J. Allison (Ed) Dorset Revisited: Position Papers and Research Statements, West Dorset Coastal Research Group, 39-42.
BRUNSDEN, D. and GOUDIE, A. (1981) "Landform Guides, No. 1. Classic Coastal Landforms of Dorset", First Edition The Geographical Association. 39p.
BRUNSDEN, D. and GOUDIE, A. (1997) "Landform Guides, No. 1. Classic Coastal Landforms of Dorset", Second Edition The Geographical Association.
BRUNSDEN, D. et al. (1996) The Structural Geomorphology of the Isle of Portland, Southern England, Proceedings Geologists' Association, 107, 209-230.
DOODY P and RANDALL R (2003) Guidance for the Management of Coastal Vegetated Shingle. Report No MAR 05-03-02 to English Nature 130p. electronic version available at: http://www.english-nature.org.uk/livingwiththesea/project_details/good_practice_guide/ shingleCRR/shingleguide/home.htm
HALCROW, (2002) Futurecoast: research project to improve the understanding of coastal evolution over the next century for the open coastline of England and Wales. Report and CD-ROM produced by Halcrow-led consortiun for DEFRA.
HOLLIDAY, A. (1997) Managing the Wessex Coast: the impact of mass movement on the Furzy Cliff coastline, Geography Review, 10(5), 32-35.
HR WALLINGFORD (1993) Preston Beach Road Sea Defence. Hydraulic Model Studies: Final Report. Report EX2914. Report to Scott, Wilson, Kirkpatrick, 15pp.
HR WALLINGFORD (1996) Effect of the Deterioration of Breakwaters, Portland Harbour, 2 Volumes. Report to Weymouth and Portland Borough Council.
HR WALLINGFORD (1998) Durlston Head to Portland Bill Shoreline Management Plan. Report EX3824. Report to Mouchel Consulting Ltd, 10pp. and 4 Appendices.
HYDRAULICS RESEARCH (1986) Portland Harbour: A Wave Refraction Study, Report EX1373.
HYDRAULICS RESEARCH (1989) Extension of Ferry Terminal, North Pier, Weymouth, Report EX1944, 51pp.
JOLLIFFE, I.P. (1976) Weymouth Bay, Dorset. Coastal Regimen Conditions and Resource Use, Report to Weymouth and Portland Borough Council and Dorset County Council, 73pp.
KERR, P.E. (1989) A Palaeoenvironmental Study into the Origins of a Coastal Marsh, using Molluscan and Foraminiferal Evidence. Lodmoor, Weymouth, Dorset, unpublished BSc Geographical Science Dissertation, Portsmouth Polytechnic, 59pp.
KIMBER, O. (1999) Isle of Portland Slope Instability and Computer Modelling, in: R.J. Allison (Ed) Dorset Revisited: Position Papers and Research Statements, West Dorset Coastal Research Group, 63-66.
MAY, V.J. (1966) A Preliminary Study of Recent Coastal Changes and Sea Defences in South-East England, Southampton Research Series in Geography, 3, 2-24.
MAY, V.J. (1977) Earth Cliffs, in: R.S.K. Barnes (Ed) The Coastline, Chichester: John Wiley, 215-235.
McLAREN, J.N. (1990) Global Monitoring of Unstable Slopes in the North-East of Portland, Dorset, in: Proceedings Nottingham Conference on Geotechnical Instrumentation in Practice, 145-164.
MOUCHEL CONSULTING LTD (1998) Portland Bill to Durlston Head Shoreline Management Plan. Report to Portland to Durlston Coastal Group (Lead Authority: West Dorset District Council), 2 Volumes.
PETLEY, D. and ALLISON, R. (1999) The Mechanics of Deep-Seated Landslides: Some Implications for the Isle of Portland, in: R.J. Allison (Ed) Dorset Revisited: Position Papers and Research Statements, West Dorset Coastal Research Group, 43-50.
PINGREE, R.D. (1977) The Formation of the Shambles and other Banks by Tidal Stirring of the Seas, Journal Marine Biological Association UK, 58, 211-226.
PINGREE, R.D. and MADDOCK, L. (1979) The Tidal Physics of Headland Flows and Offshore Tidal Bank Formation, Marine Geology, 32(304), 269-289.
POSFORD DUVIVIER (1999a) SCOPAC Research Project: Sediment Inputs to the Coastal System. Summary Document. Report to SCOPAC.
POSFORD DUVIVIER and BRITISH GEOLOGICAL SURVEY (1999b) SCOPAC Research Project. Sediment Inputs to the Coastal System. Phase 3. Inputs from the Erosion of Coastal Platforms and Long Term Sedimentary Deposits. Report to SCOPAC.
RENDEL GEOTECHNICS and UNIVERSITY OF PORTSMOUTH (1996) Sediment Inputs into the Coastal Zone: Fluvial Flows, Report to SCOPAC, 52pp. and 2 Appendices.
RIGBY, P.J. and QUARRIER, G. (1996) The Preston Beach Road Sea Defence Scheme, in: Papers and Proceedings, SCOPAC Seminar on the Development of Shoreline Management Plans: Experiences and Initiatives, SCOPAC, 65-70.
SCOTT, WILSON, KIRKPATRICK (1994a) Preston Beach Road Sea Defence Scheme. Engineer's Report, 2 Volumes. Report to Weymouth and Portland Borough Council, Dorset County Council and the National Rivers Authority.
SCOTT, WILSON, KIRKPATRICK (1994b) Impact of Preston Beach Road Sea Defence Scheme on the Stability of Furzy Cliff, Report to National Rivers Authority.
MMIV © SCOPAC Sediment Transport Study - Portland Bill to Redcliff Point