About the Study

SCOPAC Committee

Chairperson Councillor Mrs M Penfold MBE, West Dorset District Council.

Vice-Chair Councillor Jackie Branson, Havant Borough Council.

Technical assistance provided to Councillors by Mr Lyall Cairns (Southern Coastal Group Chair) and Dr Samantha Cope (SCOPAC Research Chair).

The STS 2012 update

The 2012 update of the SCOPAC Sediment Transport Study (STS) was funded by the Environment Agency under FDGiA, grant number LDW 41230, with additional contributions from SCOPAC.  

It is referenced as: New Forest District Council (2017). 2012 Update of Carter, D., Bray, M., & Hooke, J., 2004 SCOPAC Sediment Transport Study, www.scopac.org.uk/sts.

Sediment Transport Study 2012

Isle of Portland and Weymouth Bay

1. Introduction

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 during Holocene sea level rise. Lodmoor wetland was previously a lagoon thus confined by this transgressive barrier, though there is an absence of sufficient data on the timescale of this extended period of shoreline emergence. 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 southwards against the hard Nothe Point 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 highest topography on the north of the Isle of Portland where resistant and/or porous cap rocks overlie soft impermeable clays (Photo 4). Smallmouth is located at the outlet of the saline Fleet lagoon created by the Chesil barrier to the west; the subsidiary tombolo spit (now built over) that connects Portland to the mainland is thought to be contemporary with Chesil (See the unit on Lyme Regis to Portland Bill for further detail.)

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 18th Century, gradually extending northwards from the mouoth or the River Wey and harbour as the town expanded; the promenade was destroyed by the great gale of 1824 (Dave Picksley, Environment Agency, pers. comm., 2016). Weymouth port developed from medieval times within the outer and mid Wey estuary. The northern margin of the entrance has been stabilised, whereas the resistant limestone of Nothe Point forms the southern margin (Photo 3). Some modest reclamations have been undertaken within parts of the estuary. Portland Harbour was artificially created by the construction of a series of rock breakwaters in the mid to late-19th Century; this has modified the process regime operating along what was formerly an open coastline.

Parts of this shoreline have been investigated in some detail because of flooding and erosion problems, including those predicted in the context of climate change and sea-level rise (Pirazzoli, et al., 2006; Pirazzoli and Tomasin, 2008) 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.

A major new source of coastal data is from the Defra-funded National Network of Regional Coastal Monitoring Programmes. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org. The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See CCO Annual Survey Reports for further details). 

In 2008, an extensive high resolution, 100% coverage swath bathymetry dataset, known as the Dorset Integrated Survey (DORIS), was collected by the Southeast Regional Coastal Monitoring program in partnership with Dorset Wildlife Trust, The Maritime and Coastguard Agency, The Royal Navy and Viridor Credits. This survey extended from the western end of the Fleet lagoon to Handfast Point and 20km offshore from MLWS.

2. Sediment Inputs

2.1 Marine Inputs: Offshore to Onshore Sediment Transport

F1 Weymouth Bay: Bowleaze to Lodmoor

Analysis of 2008 swath bathymetry data indicates that the largely featureless seabed topography within Weymouth Bay gently slopes southeastwards and the thickness of nearshore sediments is sufficient to mask the underlying geology, apart from former headland scars and rock outcrops south of Redcliffe Point. Further east along the Dorset coastline, the rock platform and nearshore geological features become more prominent, typically extending sub-tidally 200-400m, and are covered with a relatively thin veneer of sediment.  The lack of bedforms or sediment accumulations connected to the underlying geology or outcrops provides no evidence of onshore (or offshore) sub-tidal sediment transport to support Kemble’s (1984) conclusion 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 relatively low energy environment, but onshore movement may be induced by occasional high-energy waves. Bastos and Collins (2002), however, consider that only sediment up to the grade of coarse sand can be moved by modified storm waves entering Weymouth Bay. 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

Analysis of Coastal Monitoring Programme lidar, aerial photography and baseline topographic data indicates a southward drift of sand on the southern sector of Weymouth Bay, with possible sub-tidal exchange of fine-grained material, as opposed to a significant offshore input of sediment. This supports Jollife (1976) that sand has been moved southwards across the inner bay, mainly on account of tidal flow and wave action, and has tended to collect in the southern parts of the bay. Bastos and Collins (2002) demonstrated, 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, potentially promoting accretion.

Analysis of 2008 swath bathymetry data indicates that the largely featureless seabed topography within Weymouth Bay gently slopes southeastwards and the thickness of nearshore sediments is sufficient to mask the underlying geology. The lack of bedforms or sediment accumulations connected to structures such as harbour walls, the underlying geology or outcrops provides no evidence of onshore (or indeed offshore) sub-tidal sediment transport.

Some problems of siltation in the outer part of Weymouth Harbour necessitated maintenance dredging, although no significant dredging has been undertaken since the 1990s (Robert Clarke, West Dorset Council, pers. comm., 2016). Evidence from the median grain size of sampled material - fine sand - is that it originally came either from adjacent beaches or offshore. Analysis of Coastal Monitoring Programme lidar, aerial photography and baseline topographic data indicates a low southward drift (LT4) with possible subtidal exchange of fine-grained material, as opposed to a significant offshore input of sediment. The speculative 2004 arrow indicating onshore transport into Portland Harbour from Weymouth Bay has been removed as there is no supporting information or evidence from Coastal Monitoring programme data.

2.2 Fluvial Inputs

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 per year bedload, 220 tonnes per year suspended load.

FL2 The Fleet

The Fleet is a shallow brackish estuarine lagoon, which receives fluvial discharge and tidal flow. Probing traverses and boreholes revealed several metres thickness of mud, which accumulated over the past 6000-8000 years (Bird, 1972; Carr and Blackley, 1973). It is suggested that the Fleet is a sediment sink but it is uncertain whether it has been dominated by terrestrial supply sources. It has been suggested that terrestrial sediments derive from wave erosion of the margins and fluvial input from several small inflowing streams, chiefly the Abbotsbury Stream (Bird, 1972), but coring studies from the Fleet suggest that the majority of sediments are of marine origin (Coombe, 1998).

FL3 River Wey

Varoius reclamations associated with expansion of the town and harbour over the past centuries have involved dredging silt from the harbour and sand from the sand bar as fill material for waterfront reclamations (Alan Frampton, CH2M, pers. comm., 2016). 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 500m³ per year 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 per year (Rendel Geotechnics, 1996).

2.2 Coast Erosion Inputs

» E1 · E2 · E3

The nature and rates of coast erosion are determined strongly by the varied lithologies of the 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 Corrallian 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 north (Redcliff Point) and south (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 relatively non-resistant 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 onto the island shorelines forming protective boulder aprons at several of the cliff toes.

E1 Furzy 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 0.6m per year for Redcliff Point, confirmed by Mouchel (1998).

Furzy Cliff (Photo 6) to the west of Bowleaze Cove 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 0.31m per year but rates have varied through time and location. In the same period HWMST 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).

Cliff recession of Furzy or Bowleaze Cliifs is evident through analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography, although cliff input, particularly of sand or gravel grade material, is negligible.

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. Slope regrading and toe armouring, carried out in 1988, and completion of the Newtons Cove seawall in 2003 exclude any further sediment yield (Photo 3). The 2004 arrow depicting erosion at this location has been removed.

E2 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. A significant section of the surviving structure was lost to cliff retreat in the 1950s.  At Western Ledges, Corallian Beds consisting of sandstones and grits are subject to block detachment; they overlie clays prone to shallow mudsliding and slipping. Erosion was substantially reduced following construction of the Portland breakwaters and shoreline stabilisation between 1849 and 1872, but there are records of failure events subsequent to these interventions. Cliff instability is partly due to toe loading and groundwater seepage at lithostratigraphical junctions, 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). HR Wallingford (1996) calculate that mean significant wave height is now approximately 2m lower than would have characterised waves incident on this shoreline before the breakwaters were built.  Private and public defences constructed and maintained to variable standards since the early part of the twentieth century (some of which are currently in a poor condition) have also inhibited rates of erosion. The most substantial of these is a rock revetment-whose initial construction is of uncertain age- protecting the toe of the Kimmeridge Clay cliffs along the southern sector of this frontage. Halcrow (2012) propose an erosion rate of 0.14m per year for the undefended or inadequately protected cliffs, 1866 to 1988; however the toe lobes of the run-out landslip of 2001 were trimmed back at a rate of 1.57m per year over the succeeding ten years (Halcrow, 2012)  The 2001 event was the first of a number of cliff failures that occurred during the next eight years, notably those close to Old Castle and Belle Vue Roads, that caused several meters of cliff-top recession (Halcrow, 2012). The latter study provides a detailed classification of cliff morphodynamics for this entire frontage. 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. Prior to the completion of Portland Harbour breakwaters inter-tidal flats were present at Smallmouth; their absence today is suggestive of erosion, but the main cause is probably the lack of sediment supply following the virtual cessation of cliff degradation immediately to the north due to the insertion of defences and the diminution of the energy of breaking waves within the harbour.

E3 Isle of Portland (see introduction to coast erosion)

The beaches of 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 coastal 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), but over an extended period have generated several run-out landslides that temporarily advanced the MHWST position of the north and north-east sections of this shoreline before its stabilisation in the late nineteenth century.    

Erosion of the 6 km length of shoreface between Portland Harbour and Portland Bill is estimated to yield some 7,500m³ per year of fine sediments Posford Duvivier (1999a and b) that would be rapidly lost offshore to deep water. Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography, indicates cliff input, particularly of sand or gravel grade material is negligible.

3. Littoral Transport

» LT1 · LT2 · LT3, LT4 · LT5

The 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, 1994; 1996; 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, 1996; 1998; Bastos and Collins, 2002). For the northern sector of Weymouth Bay (Preston Beach) HR Wallingford (1994) calculated that the peak wave approach direction is in the sector 105 to 195 degrees, whereas modelling undertaken by Babtie, Brown and Root (2002) consider it to be between 135 to 165 degrees. The latter research estimated extreme wave heights to be 3.0m for a 1:1 year occurrence, 3.7m for 1:10 years and 4.4m for 1: 100 years.  Between the northern breakwater and Sandsfoot Castle HR Wallingford (1996) estimated an annual extreme wave height of 0.75m, and 0.62m for waves opposite the mouth of the Fleet; in both cases these values were for waves approaching from the south-east. 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.

The Southeast Regional Coastal Monitoring Programme measured nearshore waves using a Datawell Directional Waverider buoy deployed at Weymouth in 10mCD water depth. From 2006 to 2012 the prevailing wave direction was from south-southeast, with an average 10% significant wave height exceedance of 0.86m (CCO, 2012).

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 transported on to Weymouth's sand beach from sources both seawards and further north. 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 Bowleaze Cove to Redcliff Point (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme 2007 and 2011 lidar and aerial photography indicates that this gently sloping beach, with a patchy cover of fine shingle and coarse sand overlying a basement of Oxford Clay receives less than 1,000m³ per year, although over this period there is no conclusive evidence for either net northeastward drift from the managed Preston Beach or southwestward drift. This volume is a reduction from the speculative 2004 volume of 3-10,000m³ per year. Over the period analysed, no accumulation or removal of material within this compartmentalised beach was evident, which may have been expected resulting from beach recycling operations updrift along Preston beach. However, since the replenishment of Preston Beach in 1996, a weak or episodic northeastward drift may be inferred from periodic recycling of coarse clastic sediment (by the Environment Agency) that accumulated on the beach fronting Furzy Cliff, although volumes not known.

LT2 Preston Beach to Furzy Cliff (see introduction to littoral transport)

This beach, some 1.4km in length and exposed to a maximum fetch of 240km from the south-east, forms the main northern frontage of Weymouth Bay. It 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 with 214,000m³ of coarse sediment dredged from offshore the north-west coast of the Isle of Wight (Scott, Wilson Kirkpatrick, 1994a; Babtie, Brown and Root, 2002.) 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 an artificially graded mixed sand and gravel beach, partly confined by a terminal rock groyne at its southern end (Photo 5) but with no restraint at its northern limit. Replenishment was undertaken in response to progressive drawdown during the previous 40 years, which resulted in overtopping and flooding of the immediate hinterland. The original “design” beach had a 25m crest width at 3.5mOD, with a graded slope of 1:7.5 and was considered to be an effective defence against a 1 in 50 year storm event. In the subsequent fifteen years there has been substantial loss of volume, which has necessitated periodic recycling. This material has been sourced from the accumulating 800m long beach fronting Furzy Cliff to the north-east, thus establishing a recycling routine to counter the net longshore transport of coarse sediment from south-west to north-east (Babtie, Brown and Root, 2002; Environment Agency, 2009). Erosion has tended to focus on a “hot spot” close to the central sector of the beach. In late 2000 the beach levels were reduced by a series of storm events to such low levels as to expose the underlying rock revetment layer along parts of the frontage before recycling works could restore the beach. This occurred again in March 2008 (and also in January/February 2014) due to a series of large storm events.

The estimated rate of littoral drift along the northern most section of Preston Beach may be as high as 15,000m³ per year, although the actual gain of material at Furzy Beach was in the order of 3,000m³ per year between 2001 and 2008 (Environment Agency, 2009). The difference between these two figures is accounted for by abstraction at Furzy Beach to recharge losses updrift. Most of the losses of volume have been the result of the impact of storm wave events, which have also flattened the cross-profile and severely cut back the crest width. Bradbury, et al., (2009) provide an analysis of the effects of two storms in 2007 and 2008. The first occurred on a neap tide, with a wave period of 8 seconds and maximum wave height of 2.5m, but had minimal impact on beach morphology. The second storm was characterised by waves of the same height but a 20 second period during a spring tide, and caused recession of the crest to within 1m of the backing seawall. The overall outcome of the behaviour of Preston Beach over the last decade and a half has been to significantly reduce its effectiveness as a defence structure, although most of the changes following replenishment in 1996 can be ascribed to its attempt to acquire a new equilibrium form.

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, 1994). Based on results from a short-term tracer experiment, Jolliffe (1976) concluded that the quantity and rate of littoral transport was small. This is as would be expected given that waves characteristically break at the shoreline at a near normal alignment, and are depth-limited due to the low gradient of the nearshore seabed. The evidence 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 2,900m³ per year was suggested by HR Wallingford (1998). South of Lodmoor, and outside of the replenished area, beach width diminishes over a distance of some 300m partly due to the effect of the seawall and retaining rock groyne (Photo 7). Monitoring and observation since 1996 has revealed that there is a transient drift divide located approximately at its central point; the net transport pathway is southwards to the south and more evidently northwards to the north of this boundary zone (Environment Agency, 2009). However, as previously (i.e. before replenishment) short-term reversals of these directions of longshore transport have been recorded under specific incident wave conditions. Estimated drift rates, quoted above, are relatively modest- cross-shore transport is a more significant influence on beach morphology than longshore. During storms, some sediment is moved offshore, a proportion of which may not be returned later (Babtie, Brown and Root, 2002.)       

Lodmoor Marsh (Photo 8) is a former lagoon confined by a barrier beach (Preston Beach) that has migrated onshore (Kerr, 1989). Natural drainage was by percolation through the shingle before sea wall construction in the early 20th Century; now drainage is more regulated through the beach via culverts and outfalls. 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 (HR Wallingford, 1993).

Analysis of Coastal Monitoring Programme 2007 and 2011 lidar, aerial photography and baseline topographic data for the managed Preston beach, which the Environment Agency maintain through beach recycling operations, indicates a weak net northeastward drift direction. However, the downdrift compartmentalised Bowleaze Cove experiences very little or no accumulation or removal of material over this period. The location of a drift divergence zone in the vicinity of the southern boundary of Preston beach has not changed, although it appears to be weak and may not operate continuously. This suggests that (i) a fine balance exists between northeastward and southwestward drift so that net drift varies in direction from year to year according to prevalent and dominant winds and waves (as indicated by a reversal arrow of low reliability; (ii) sand and gravel may undergo net transport in different directions, and (iii) the regime may have been affected by the replenishment of Preston Beach such that net drift directions could have altered since 1996. The revised arrows east of the divergence indicate eastward drift of less than 1,000m³ per year, which is a change from the speculative westward drift of 3-10,000m³ per year in 2004.

LT3 and LT4 Weymouth Esplanade (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme 2007 and 2011 lidar, aerial photography and baseline topographic data for the southern sector of Weymouth Bay, indicates a net southwestward drift direction of sand, with possible sub-tidal exchange of fine-grained material, as opposed to a significant offshore input of sediment. This supports Jollife (1976) that sand has been moved southwards across the inner bay, mainly on account of tidal flow and wave action, and has tended to collect in the southern parts of the bay. A drift divergence at the southern boundary of Preston beach appears to be weak and may not operate continuously. There is no evidence of a drift convergence in the eastern sector of Weymouth Beach as indicated in the 2004 review. West of the divergence, analysis of Coastal Monitoring Programme data indicates a westward drift of less than 1,000m³ per year compared to the unquantified speculative 2004 eastward drift.

Hydraulics Research (1989), operating a physical model using a hindcast wave climate calculated from Portland wind data, found a relatively weak 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).

Gravel entering the littoral system by weed-rafting north of the promenade clock tower should be transported northwards, but sand is more likely to move to the south. South of the tower swash aligned beach-nearshore exchange appears to be the dominant process (Joliffe, 1976). Some gravel is retained on the backshore beach, becoming increasingly smaller in mean clast size, as well as impersistent, southwards.

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. Prior 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 clasts from the north eastern coast of the Isle of Portland.

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).

Isle of Portland (see introduction to littoral transport)

Analysis of Coastal Monitoring Programme data provides no evidence to support the 2004 speculative northward drift along the eastern shore of the Isle of Portland. These arrows have therefore been removed.

4. Sediment Outputs

The 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 some 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 Redcliff Point to Furzey Cliff (see introduction to sediment outputs)

Analysis of 2008 swath bathymetry data indicates that the largely featureless seabed topography within Weymouth Bay gently slopes southeastwards and the thickness of nearshore mainly sandy sediments is sufficient to mask the underlying geology. The lack of bedforms or sediment accumulations connected to structures such as harbour walls, the underlying geology or outcrops provides no evidence of offshore (or indeed onshore) sub-tidal sediment transport or loss of beach material.

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).

O2 Weymouth Bay (see introduction to sediment outputs)

Analysis of 2008 swath bathymetry data indicates that the largely featureless seabed topography within Weymouth Bay gently slopes southeastwards and the thickness of nearshore mainly sandy sediments is sufficient to mask the underlying geology. The lack of bedforms or sediment accumulations connected to structures such as harbour walls, the underlying geology or outcrops provides no evidence of offshore (or indeed onshore) sub-tidal sediment transport or loss of beach material. 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 produces a total yield of 7,500m per year 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)

Analysis of 2008 swath bathymetry data indicates the seabed east of Portland Bill is dominated by exposed and scoured bedrock, ridges, faults, palaeo-channel and canyons and a fossilised ox-bow lake (approximately 11km east of Portland Bill). Southwest of Portland Bill is a north-south oriented 80m deep channel, the eastern slope of which rises steeply, before flattening to form an extensive plateau of exposed bedrock, that extends some 1.8km south of the Bill. Where seabed sediments occur, they are characterised by extensive areas of both stable and mobile sand, a discontinuous veneer of coarse sediment (mostly gravel) and bedrock outcrops.

The northeast-southwest oriented Shambles Bank is a large accumulation of very coarse sandy sediment, approximately 5.5km long, 1km wide and 20m high. Sonar surveys and sampling on the Shambles Bank and Adamant Shoal indicate a high content of shell but also superficial sand with small and large ripples (Mouchel, 1998; Bastos and Collins, 2002). This feature has an asymmetric cross-profile, with a steeper slope facing towards the coastline. Parasitic small-scale sand waves are evident along the length of the bank atop larger sand waves/ripples, with wavelengths up to 300m. Bedform analysis supports an anticlockwise circulation of sediments around the bank, with a dominant southwestward direction of transport across the bulk of the bank. Bedforms along the northern flank of the bank are symmetrical which maybe caused by a weaker currents travelling from east to west. There is no conclusive evidence of transport connectivity between this bank and Weymouth bay.

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.

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.

Analysis of Coastal Monitoring Programme swath bathymetry data shows that offshore of Grove and northwards, the thickness of sediments increases and is sufficient to mask the underlying geology, with occasional rock outcrops and ridges interrupting the relatively featureless sediment substrate. Further east the underlying bedrock features are masked beneath the east-west oriented Adamant Shoal, another very coarse sandy sediment set of banks and bars, approximately 3km long and 1km wide. Large and small sand waves run along its length. Analysis of the asymmetry of the bedforms suggest transport is from east to west on the northern flanks and west to east on the southern flank, suggesting anticlockwise tidal eddy circulation. And additional arrow has been included to indicate the anti-clockwise circulation associated with the Adamant Shoal.

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). 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 action 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, becomes 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 sand transport. The first, characterised by relatively high rates, is towards Portland Bill; the second, with much lower rates, is directed away from the headland.

4.2 Other outputs

Material periodically dredged from Weymouth Harbour is dumped offshore 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 and Budget

1. Weymouth Bay is a headland-controlled embayment occupied by a now stabilised barrier beach and a partly infilled estuary. Behaviour is controlled by the sheltering influences 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 relict deposits by a kelp rafting mechanism is thought to be a means by which small quantities of fresh gravels may be 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 from the north-east Isle of Portland coast are retained in small beaches along the flanks of Portland Harbour. Many of the existing sediments are likely to be relict having been supplied during periods of rising sea-level in the late Holocene, at which time the bay may have been a much more effective sink.
3. There remains considerable uncertainty about the relative rates, quantities and directions of littoral drift occurring between Redcliff Point and Weymouth Harbour. This appears to result from the fine balance maintained between net northwards and southwards 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.
4. The regional wave climate and tidal regime result in westward net sand movement across the inshore bed into this large embayment from source areas to the east. 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 eddy 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 150 years has inhibited the natural tendency for landward migration of the shoreline and appears to have led to the depletion of Preston Beach in the northern part of the bay. This frontage is now defended by a major beach replenishment that has required careful management as it has failed to perform to design specifications and predictions.

6. Opportunities for Calculation and Testing of Littoral Drift Volumes

Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change.  The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping.  Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.coastalmonitoring.org.

The Southwest Regional Coastal Monitoring Programme commenced in 2006. The Lead Authority is Teignbridge District Council, with data collection, analysis and reporting led by a specialist team at Plymouth Coastal Observatory (PCO). The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Analysis of longer term Coastal Monitoring Programme data, when combined with other data sets, academic research and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future.

Although 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.

7. Knowledge Limitation and Monitoring Requirements

The Southeast Regional Coastal Monitoring Programme commenced in 2002. Analysis of the data between 2006 and 2012/13, combined with other datasets, academic research and historical studies has enabled sediment budgets, transport rates and directions to be identified or verified. However, at certain sites either due to a lack of long-term data, data coverage or sedimentological information (e.g. composition and proportion of beach grade material arising from cliff erosion), quantification of sediment transport rates of gravel and sand has not been possible.

Notwithstanding results from the Regional Coastal Monitoring Programmes, and the summarised information collated in the Durlston Head to Rame Head SMP2 (SDDCAG, 2011), the following recommendations for future research and monitoring that might be required to inform management include: e.g.

1. 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.
2. 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.
3. 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.
4. 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.