Pagham Beach Estate to R. Adur, Shoreham-by-sea
1. Introduction - References Map
Coastal morphology and evolution
This sector of shoreline has a slightly arcuate planform, with a near west to east alignment. Its continuity is interrupted by the mouths of the Rivers Arun (Photo 1), at Littlehampton and Adur, at Shoreham-by-Sea (Photo 2). The latter is an artificially maintained absolute boundary to drifting of coarse sediments and is adopted as the eastern limit to this sector. The western boundary is the tidal inlet of Pagham Harbour, which is a transport discontinuity, but, only a partial barrier to transfer under most wave and tidal conditions. The coastal hinterland is very low-lying forming the West Sussex Coastal Plain with typical land levels being less than 10m O.D. for several kilometres inland. There is also a wide nearshore shelf, with the -20m seabed contour occurring between 15 and 20 km seaward. The plain is backed by the Chalk hills of the South Downs that meet the coast and form cliffs well to the east between Brighton and Beachy Head. The southward flowing rivers Arun and Adur, the latter reaching the coast at Shoreham, dissect the South Downs and the coastal plain. These rivers have cut deep channels into the Chalk bedrock that have subsequently been filled with alluvium. Bellamy (1995) has described the sediment infill of the buried channel of the former extension of the Arun, which was a tributary of the Northern Palaeovalley of the English Channel during the Devensian glacial period.
The West Sussex Coastal Plain narrows progressively eastwards and consists of low gradient bevelled erosion surfaces resulting from successive Middle and Late Pleistocene sea-level transgressions and regressions (Hodgson, 1964; Bates, et al., 1998; Roberts, 1998). Bedrock is largely concealed by a sequence of soft easily eroded mid to late Quaternary sand and gravel drift deposits. However, Eocene and Chalk basement rocks are exposed at various locations along the lower foreshore and in the offshore and nearshore zones. Relatively more resistant Eocene calcareous sandstone units create inter-tidal and offshore rock outcrops and reefs, as at Bognor Regis and between Felpham and Elmer. These result from subsequent differential sub-aerial and later marine erosion of the previously bevelled surface of the west-north-west/east-south-east striking Littlehampton anticline and Chichester-Worthing syncline, which reveals a lithologically contrasting set of offshore outcrops (Young and Lake, 1988; Castleden, 1996).
A well-defined inter-tidal beach, with a predominantly steep and often narrow gravel backshore and low gradient wide sandy foreshore, dominates much of this coastline. It is a product of inundation and erosion of the coastal plain by rising sea-levels of the Holocene transgression over the past 15,000 years. As sea-levels rose, quantities of sand and especially gravel were combed up and driven landwards as a series of ancestral shingle barrier-type beaches located seaward of the present shoreline. It is thought that the beaches were driven progressively (on occasions, catastrophically) landward by continuing sea-level rise and storm activity, and perhaps also by relative sediment shortages. The present beaches remain barrier-type features with a continuing tendency to migrate landwards. The conjectured barrier coastline was indented by several estuary re-entrants, which progressively infilled with marine, brackish and terrestrial sediments after sea-level rise stabilised at approximately 3,000 to 3,300 years before present. The rivers Arun and Adur once formed estuaries extending several kilometres inland, but have infilled by natural sedimentation and have been embanked and reclaimed over the past 500 years (Wallace, 1990a; 1996). Aldingbourne Rife, Bognor Regis is another example of a former estuarine inlet whose infilling has resulted in the smooth planform of the modern coastline. The deep infill of sediment in the former estuary of the lower Arun Valley is an indication of the rapidity of mid to late Holocene sedimentation. Low bluffs, cut into both drift and substrate materials, are temporary local features of the upper backshore, usually appearing after storm action. Submerged low cliffs ledges and reefs also occur in the offshore zone, e.g. between Pagham and Bognor Regis.
Waves and tidal conditions - Map
Most of this coastline is open to a moderate to high-energy wave climate comprising English Channel wind generated waves from the south-east, south and south-west as well as swell waves propagating up the Channel from the west that become diffracted around the Isle of Wight. Shoreline wave exposure increases gradually eastward from Pagham to Shoreham, which is least affected by the sheltering effects of the Selsey Bill (and Isle of Wight) and Beachy Head headlands. Hydraulics Research (1989), Robert West and Partners (1991) HR Wallingford (1994) and Gifford Associated Consultants (1997) report that prevailing waves are from the west-south-west and south west, with maximum significant wave heights of 5.0m generated over a fetch distance of 320km. For waves from the south-east, maximum fetch is 160km and maximum wave height is 4.7m (figures relate to offshore of Bognor). Jelliman, et al. (1991) examined wave conditions at Littlehampton as part of a national study attempting to predict future wave climates in the context of relative sea-level rise and climate change. Using a hindcasting approach based on wind data, they determined that 40% of waves (1974-1990) came from the west or south-west, with a mean significant wave height (Hs) of 1.8m. Analysis revealed a slight increase over this period in incident waves propagating from the east and south-east, thus slightly reducing the proportion of maximum wave height values. However, for waves approaching from the south-west, Hs increased by 1.3cma-1 for heights above 3.5m, and 0.5cma>-1 for heights between 2.1 and 3.5m. For Hs values below 2.1m there was a 0.2cma>-1decrease over the time period studied. Halcrow, et al. (2001), using wind data for Shoreham observe similar trends, with a small increase in Hs values during the autumn period. Draper and Shallard (1971) analysed the wave climate at Owers Light Vessel, 11km south-east of Selsey Bill. A wave rider buoy was deployed for one year, providing 2,917 significant wave height values. A maximum of 7.6m was recorded, with Hs of greater than 1.25m occurring 54% of the recording period; and Hs between 0.6 to 1.25m for 28% of this time. Maximum offshore wave height has been estimated to be as high as 15m to 20m for a 1 in 50 year frequency, but for inshore waters, the equivalent value is approximately 6.90m (Hydraulics Research, 1989). Immediately offshore Pagham Harbour entrance, maximum significant waves of less than 1m height occur for 35% of the year; and greater than 4.5m for 10% of an average year. The above data refer to offshore waves that have not been modified by inshore conditions of reduced water depth and complex bathymetry.
Shoreham and Middleton (east Bognor) are locations for which wave modelling excercises 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 prediction points at -5.3m and -5.1m O.D. Potential sensitivities to likely climate change scenarios were then tested by examining the extents to which the total and net longshore energy for each scenario varied with respect to the present situation.
Although the tidal range increases from west to east, the inshore surface tidal current velocities decrease eastwards. Typical spring tidal range is 5.3m at Pagham increasing to 6.5m at Shoreham, with a neap range of 3.0m. Locally strong ebb and flood tidal currents are generated by the exchange of tidal waters at the Pagham, Arun and Adur inlets. At Pagham Harbour entrance, velocities range from 1.4m.sec-1 (springs) to 0.7m.sec-1 (neaps), whilst at Shoreham the corresponding speeds are 0.8m.sec-1 and 0.4m.sec-1.
Dominant shoreline processes and management - Map
Net beach drift and nearshore longshore transport is from west to east. Evidence of this drift direction is provided by the development of Shoreham spit over several centuries; the probable formerly eastwards deflection of the mouth of the Arun and the modern pattern of sediment retention in the near continuous present day groyne series (Robinson and Williams, 1993). A sequence of transport sub-cells has been identified, using spatial variations in littoral drift rates in combination with various natural and artificial boundary conditions associated with the R. Arun and R. Adur inlets (Gifford Associated Consultants, 1997). Coastline recession has historically been rapid (e.g. Harper, 1985) though some temporary progradation may have occurred in response to landward barrier translation in earlier centuries (Smail, 1969). Since the early nineteenth century, coastline retreat has been inhibited by management interventions to "hold the line".
Previous to shoreline management, beaches were subject to the impacts of high magnitude (storm) events. Examples include a 600m breach at Widewater Lagoon in 1908 and 20m of recession in less than 7 hours, at Lancing, in 1877. Although this type of catastrophic response has been constrained by defences since the early decades of the twentieth century, several locations have continued to experience overtopping during surge events (Halcrow Maritime, et al., 2001). It provides modern evidence of a continuing tendency for landward barrier migration.
Following increasing ubanisation of this coastline from the 19th Century onward the beaches have become heavily managed. The shoreline has been stabilised extensively by seawalls, revetments and groynes built on, or behind the backshore gravel beach ridges over the past 100 to 150 years such that the entire frontage between Selsey and Brighton is almost completely defended (Photo 3 and Photo 4) . The Arun and Adur river channels have been embanked and their estuaries have been almost completely reclaimed, thereby reducing their tidal prisms and inlet flushing capacity. To maintain fixed inlets the tidal inlets at Pagham, Littlehampton (R. Arun) and Shoreham (R. Adur) have been stabilised by breakwaters and/or training structures. Together, these hard defences and interventions have had the effect of significantly reducing the net drift on the shingle upper beach, but have only partially affected transport of sands on the lower foreshore. Over the 20th Century these defences (groynes and inlet training structures) induced downdrift shingle deficits, natural sources of shingle supply reduced and many of the beaches diminished, requiring increasing structural control to maintain existing lines of coastal defence (Gifford Associated Consultants, 1997). These problems have led in more recent decades to intensive beach management involving traditional methods accompanied by increasing replacement of timber groynes by rock groynes, offshore breakwaters at Elmer, use of beach recharge, shingle recycling (Littlehampton, West beach) and bypassing (Shoreham). Seawalls, revetments, rock armour and bunds built on, or behind backshore gravel beach ridges provide protection against wave action. Some immediate hinterland areas, however, are below maximum high water (e.g. Lancing and Goring-on-Sea) and have been subject to repeated flooding resulting from the natural tendency for landward migration and overtopping of beach crest levels during storms.
2. SEDIMENT INPUTS - Map
Three naturally occurring potential sources of sediment are identified for this coastline comprising offshore to onshore transport, fluvial discharge and shore erosion. These have been supplemented in recent decades by beach replenishment at several sites.
2.1 Offshore to onshore sediment transport - F1 F2 References Map
F1 Diffuse Gravel Feed by Kelp-Rafted Shingle Between Pagham Harbour Entrance and Shoreham (see introduction to sediment inputs)
Jolliffe and Wallace (1973) and Jolliffe (1978) reported that kelp could become attached to sea-bed gravels and be transported by currents acting upon the kelp frond (process of kelp-rafting). They postulated that in areas of weak tidal currents effective movement should involve a net shoreward component induced by wave motion. This process, which may occur in a zone up to 5km offshore (according to water depth), was confirmed by diving observations of kelp-attached shingle in the process of shoreward movement. The spatial and temporal representativeness of the observations is poor so it is difficult to evaluate the significance of this process as a discrete and sustained gravel feed. Large casts of weed are often deposited on beaches on this coast (e.g. at Worthing and Felpham) in summer and after storms. These casts of weed frequently have stones attached to holdfasts, providing direct evidence of this process. Measurements of the total weight of a seaweed cast at Worthing were made by Binnie and Partners (1987, 1988), who estimated the proportion of attached shingle and calculated that each year some 1,200-3,000 tonnes, equivalent to 100 to 250 m3 per km length of shore, could be supplied by each cast event. The mean number of casts per annum and the representativeness of the measured casts were unknown, so that overall reliability of this volumetric calculation is uncertain. This study did however imply that weed rafting may constitute a small but regular and significant gravel supply in areas subject to large casts. It is estimated that there is 30-40km2 of offshore weed bed between Littlehampton and Shoreham, thus such potential supply is a particularly important source of natural gravel replenishment on groyned coasts where beach drift is inhibited.
Septarian nodules and reworked flint pebbles are released from nearshore outcrops of the London Clay (Aldwick Beds) between Bognor and Felpham, but this is probably a very small input into local beaches.
Jolliffe and Wallace (1973) and Jolliffe (1978) also report that gravel attached to kelp may be transported landwards from water depths of at least 30m, below which there is lower productivity of suitable weed in the English Channel. Kelp attachments therefore act to magnify the influence of offshore waves and currents on the transport of gravel, as this material moves at depths greatly in excess of those possible for unattached pebbles. Submerged and relict barrier beaches (Wallace, 1990b; 1996) may provide a shingle source, an example of which occurs approximately 1.5km south of Middleton-on-Sea at approximately -5mOD resting directly on an abraded Chalk platform. Such features could provide additional small sources of "fresh" flints, together with some input from the eroding Eocene sandstones. However, the largest source of gravel is likely to derive from the former seaward extension of Quaternary raised beach and periglacially weathered and fluvially transported Coombe Rock (Bellamy, 1995).
F2 Wave-Powered Onshore Shingle "Creep" (see introduction to sediment inputs)Experiments conducted between 4km and 10km offshore of Worthing and Shoreham by Crickmore et al. (1972) indicated a landward drift of gravel, which was small in volume and decreased inshore. At the 9m water depth onshore feed was measured at 1,000-1,500m3a-1 km-1 whilst at 12m depth it reduced to < 500m3a-1 km-1. No transport was recorded in excess of 18m depth. If these studies are representative of the 48km of coast between Brighton and Selsey then this feed might total a potential of 48,000-72,000m3a-1 over this whole frontage, but this rate of feed will only occur where potentially mobile shingle exists on the seabed. Furthermore, it would be expected that the operation of the process over time could have already depleted potentially mobile inshore gravel reserves since re-supply would be unlikely from further offshore due to lack of gravel mobility in greater water depths. The knowledge of these processes was derived from experiments with radioactive tracers carried out over 2 years and correlated with wave data. As the study period was representative of storm conditions, the results are therefore reliable. HR Wallingford (1993) subsequently confirmed patchy distribution of inshore gravels together with the lack of shingle mobility beneath water depths greater than 15m to18m.
Jolliffe (1978) conducted painted pebble experiments 8-9km offshore of Shoreham to assess shingle transport, and also noted net onshore movement. The smaller, more angular and discoidal pebbles moved the most rapidly, so any onshore input is likely to consist preferentially of these types. Transport was also found to be more rapid over areas of seafloor composed of exposed bedrock or gravel banks, (e.g. Kingston Rocks and offshore Ferring), suggesting that onshore feed might be spatially variable according to seabed roughness. No correlation of pathways of movement with tidal current vectors was apparent, and it was thus presumed that movement was entirely wave-propagated. Rate of supply was calculated to be between 750 and 1,5000m3a-1 km-1. Few experimental details are given, so the representativeness of this estimate is difficult to assess.
2.2 Fluvial Inputs - FL1 FL2 References Map
Two regionally significant rivers, the Arun and the Adur, discharge at Littlehampton and Shoreham-by-Sea respectively. Rendel Geotechnics and the University of Portsmouth (1996) have assessed the potential inputs of mostly suspended sediments from the sandstones, clays and flints from the Chalk, that each of these catchments provide as follows:
FL 1 River Arun (see introduction to fluvial inputs)
Estimated potential input of 17,000 tonnes a-1 of suspended load, but actual delivery is reduced by the presence of various barriers to transport and by flow diversion at times of high discharge in the lower flood plains. Actual quantities of fine sediment delivered at the coastline are estimated as being approximately 11,000 to 12,000 tonnes a-1 The river is considered unable to contribute a significant gravel (bedload) input, due to the restricted source area and upstream and in-channel storgae of any gravels entering the system. See Photo 1 of river mouth.
FL 2 River Adur (see introduction to fluvial inputs)
Estimated potential input of 20,000 to 26,000 tonnes a-1 of suspended load, but actual delivery is reduced by the presence of various barriers to transport and by flow diversion at times of high discharge in the lower flood plains. Actual quantities of fine sediment delivered at the coastline are estimated as being approximately 2,600 tonnes a-1. The river is considered unable to contribute a significant gravel (bedload) input, due to the restricted source area and upstream and in-channel storgae of any gravels entering the system. See Photo 2 of river mouth.
2.3 Coastal Erosion/Shoreline Recession - Map
Although significant lengths of this shoreline are currently either stable or accreting, the long-term trend has been that of erosion and recession. Shoreline behaviour has been substantially modified by the insertion of defence structures over the past 100-150 years. Extensive groyne systems, in particular, have been designed to maintain or increase beach volumes. In recent decades, these structures have been supplemented by practices of gravel recharge and recycling. Full details of historical beach behaviour and estimated rates of retreat and advance of the position of mean high and low water are given in Gifford Associated Consultants (1997), Scott Wilson Kirkpatrick (2000b, 2000c) and Halcrow (2002). In summary, the late Holocene history of this coastline would suggest an earlier stage (probably several discrete stages) of barrier beach emplacement, followed by erosion and depletion in historical times up to the introduction of formal shoreline management. Indirect evidence favouring the concept of a sequence of barrier beaches includes the apparent blockage of the former mouth of the Aldingbourne Rife sometime after the twelfth century and the presence of former barrier lagoons at Bognor, Brooklands (Worthing) and Lancing. Widewater persists as a brackish lagoon (Photo 5), but partly occupies a probable former channel of the River Adur. Wallace (1994) has noted the extensive offshore presence of a Chalk platform, patchily overlain by cemented beach cobbles, offshore Felpham. He interprets the latter as the residue of one or more former barrier beaches that were submerged by an acceleration of sea-level rise after approximately 2,500 years before present. Some of the material contained in this ancestral barrier may have been incorporated into the modern beach, although it does not appear to be mobile under contemporary wave action.
Analysis of the relative movements of the positions of mean high and low water on successive Ordnance Survey maps from 1875 to 1979, reveals long-term coastline recession, intertidal narrowing and beach steepening for the majority of this coastline. However, since the mid to late 1960s, the overall trend has been one of overall equilibrium or net accretion, largely due to shoreline management practices (see Photo1); and West Beach, Shoreham-by-Sea (Photo 2), although in this case there has been regular removal of sediment excess to compensate for net downdrift losses resulting from the presence of the Shoreham Harbour breakwater. Several locations reveal fluctuating trends, notably immediately to the east of the entrance to Pagham Harbour where significant earlier recession has been replaced by steady accretion since the middle of the twentieth century.
For perhaps the majority of regional beaches, annual variations of both planform and volume are greater than long-term mean fluctuations, with a high potential for substantial very short-term losses during major storm events, followed by slower recovery.
Posford Duvivier and British Geological Survey (1999) have calculated that vertical wave erosion of the shoreface zone between Bognor and Rottingdean yields between 1,900 and 4,000m3a-1 of coarse material and 2,000 to 7,000m3a -1 of fine material. Most of the latter is presumed to be lost to the coastal transport system as suspended load. An unknown proportion of the former feeds net onshore wave-driven and kelp-rafted shingle movement (see F1 and F2 in Section 2.1).
2.4 Beach Nourishment and Recycling
Several significant gravel beach nourishments have been completed by coastal defence authorities over the past 20 years with numerous local, small-scale beach feeding episodes (Photo 6) that date back to the late nineteenth century at a few locations, e.g. Brooklands (Lancing). The table below, based substantially on Gifford Associated Consultants (1997) provides data on approximate quantities of renourishment, by site. It omits several small scale "top ups" and may, therefore, underestimate this source of input to the sediment transport system.
|Arun DC||Aldwick; Bognor
Replenishments are regarded generally as having been been successful, although rapid initial diminution of beach fill volume by 20-40% has been recorded at several locations. This probably results from net offshore transport of fines, abrasion of less resistant constituents and profile adjustment. Material is lost even with well-designed and well-maintained groyne systems, thus frequent renourishments to offset losses are now common practice. Source areas for primary renourishments are normally from offshore gravel deposits that are not contributing to the contemporary littoral sediment budget (Emu Environmental Ltd, 2000). Sites of periodic removal of accumulated beach sediments occur at West Beach, Littlehampton and West Beach, Shoreham, where materials retained by river entrance breakwaters are taken for periodic recycling updrift (Climping) and bypassing downdrift (Shoreham). At Shoreham, extraction is confined to the 100m length of beach adjacent to the western breakwater owned by Shoreham Port Authority (Photo 2 and Photo 7). Based on a calculation of net input of 10-15,000m3a-1 due to eastwards longshore transport (Halcrow, 1990), an average quantity of 8,500m3a-1 has been removed between 1993 and 2000 (Vaughan, 2001). It is used to replenish the 3km length of beaches, immediately downdrift of the harbour entrance that are the management responsibility of the Port Authority. Monitoring of West Beach for the period 1993-2000 has demonstrated no loss of volume and is thus regarded as a viable source for continued bypassing operations (Vaughan, 2001).
3. LITTORAL TRANSPORT (BEACH DRIFT) - LT1 LT2 LT3 References Map
Net longshore transport along the West Sussex coast from Pagham Harbour to Shoreham-by-Sea is eastwards, a process clearly evident from historical and contemporary observations of the eastward deflection of the mouth of the River Adur by the western spit at Shoreham (West Beach) and the consistent pattern of sediment accumulation in inter-groyne compartments throughout the frontage (Ballard, 1910; Brookfield, 1952; Smail, 1969, and Castleden (1996). Movement is substantially wave-induced, with tidal currents being insufficiently strong to move coarser sands and gravels independently, except at estuary mouths. Transport rates are spatially variable, and reflect not only the energy available, but also barriers to movement (i.e. the effectiveness of by-passing mechanisms) and sediment availability. However, most studies either assume, or have measured, higher rates over the west sector of this shoreline. This reflects, above all, incident wave approach and energy; waves become progressively more shore-parallel from west to east. Predicted longshore transport rates, based on theoretical calculations, are everywhere significantly higher than actual observed or measured rates. Most authors fail to distinguish between sand and shingle, and between upper and lower shore movements. Rates of transport across the lower foreshore are likely to be higher because of lack of interruption by groynes and the finer grades of the sediments (greater mobility). Smail (1969) and Robinson and Williams (1983) describe apparent eastward deflection of the exit of the Adur from medieval times, so that net eastward drift would appear to have persisted over at least the past several centuries.
Many accounts are general and fragmentary and offer only limited quantitative evidence. Much more substantial are the results of several site specific numerical modelling, documentary and field-based investigations completed in recent years, with the whole frontage being covered by Gifford Associated Consultants (1997). The latter study details the results of a numerical model (LITPACK), which calculated net sediment transport across a series of shore-normal profiles, adjusted to take account of probable groyne-induced reduction of transport rates. This model was operated using local data on nearshore bathymetry, grain-size parameters, sediment sorting patterns and wave climate. Changes in profile, 1972-1992, for 30 beach sections, were used to calculate annual changes between adjacent profiles, and thus deduce erosion and accretion trends that could be applied to provide some validation of the modelling. The combination of LITPACK data with a conceptual model was used to derive net sediment transport volumes for a series of consecutive sections of shoreline. These are given below. It should be noted that all values are an exaggeration of actual rates, because shortages of sediment often result in failure to achieve the predicted drift potential.
LT1 Pagham Beach to R. Arun (see introduction to littoral transport)
A general west to east net drift operates along this frontage. Actual drift decreases eastward from Pagham as groynes at Aldwick intercept material and allow only intermittent transport. Lewis and Duvivier (1969; 1973) concluded that net drift was eastward between Pagham and Felpham, based on examination of sediment accumulation in groyne compartments. A local reversal of drift occurs at the western boundary of this frontage for the net direction of drift immediately east of Pagham Harbour inlet is considered to be westwards over a 500-700m long frontage at an approximate rate of 5,000m3a-1. The Pagham Harbour ebb tidal delta and wide, accreting foreshore sets up complex local wave refraction and provides protection against the dominant south-westerly waves enabling a very local dominance of south-easterly waves (Jolliffe, 1978; Barcock and Collins, 1991; Gifford Associated Consultants, 1997; Posford Duvivier, 2001). Wallace (1990b) suggests that this movement principally involves coarse to fine sand on the beach foreshore, although some losses of gravel also appear to have been involved. A drift divide marked by a zone of persistent beach erosion (Photo 8) is therefore identified in the vicinity of Pagham Beach estate (Wallace, 1990b; Posford Duvivier, 2001). Its operation over the period 1973 to 1992 is estimated by HR Wallingford (1995) to have removed 90,000m3 of beach material. Collins, Fitzpatrick and Gao (1995) undertook a tracer experiment, profile measurement and sediment sampling at two sites either side of this drift divide. The study concluded that the north-eastwards transport rate was an order of magnitude greater than that towards the south-west. All movement involved gravel in the wave breaker zone. Jolliffe (1978) used the Pagham Estate beach to experimentally demonstrate that, for shingle, the largest and more disc shaped clasts tend to move further, per unit time, compared to other shapes and sizes. Rock groynes (installed in the early 1990s to replace earlier timber structures), demonstrably inhibit potential drift rates, and the local transport system is further complicated by artificial beach crest elevation and some minor replenishment in the area of persistent erosion (Photo 8).
Periodic onshore gravel migration from Pagham tidal delta, together with a small kelp rafted supply, augments wave powered longshore drift. Some of this supply would originally have been derived from the Inner Owners and Kirk Arrow shingle store, offshore of Selsey Bill, which is subsequently moved north-eastwards by littoral drift. A proportion by-passes Pagham Harbour inlet, involving temporary storage on the tidal delta. Thereafter, a proportion is driven shoreward where it feeds the beach drift system that delivers material to Aldwick. Updrift of Aldwick, beach retreat indicates that longshore transport is faster than onshore shingle supply (Wallace, 1990b). Between Pagham Beach Estate and West Bognor, Gifford Associated Consultants (1997) estimated a potential drift rate (all sediment grades) of 60,000m3a-1, which reduces eastwards to 47,000m3a-1 along the main Bognor frontage.
The construction of eight shore-parallel, inter-tidal detached rock breakwaters at Elmer Beach, Middleton-on-Sea (Photo 9 and Photo 10), completed in August 1993, has generated several physical and numerical modelling studies, and an ongoing programme of monitoring. King, et al. (1996) report that beach planform monitoring, carried out over the 32 months following scheme completion, revealed that stability was achieved after rapid initial adjustments. Individually numbered aluminium pebbles and fluorescent coated indigenous gravel sized particles were used to identify sediment transport pathways and rates of movement. Cooper, et al. (1996) and Cooper (1997) state that these tracer materials moved more rapidly as breaking wave energy increased, with the coarsest particles moving furthest. The tracer studies clearly identified a potential for sediments to drift through (along) the scheme, but they could not reliably estimate typical annual volumes that might be involved. It should be noted that these experiments were conducted on a recently renourished beach that had only undergone partial re-working and natural shaping of its configuration. It is apparent that attempts at the simulation of both cross and longshore transport need to trace all size ranges of sediment. This is due primarily to the fact that grading is initially absent on artificial beaches and initial behaviour may therefore depart from that which might be anticipated as the beach approaches a more mature sedimentological and morphological configuration. Furthermore, Loveless and MacLeod (1999) point out, in conclusion to their research at Elmer examining the hydrodynamic and morphodynamic effects of building submerged rubble mound breakwaters, that characteristic "set-up" currents generated behind breakwaters (when water and crest levels are approximately coincident) have only partially understood impacts on beach form.
Over 200,000m3 of mixed sand and gravel was used to replenish Elmer Beach, with the main function of the breakwaters being to retain this by reducing nearshore wave energy. The gaps between breakwaters are designed to effect a longshore transport system that minimises the downdrift impact of the terminal (attached) rock groyne on beach volumes. King, et al. (2000) calculate, using 10 crosshore beach profiles resurveyed monthly between April 1994 and April 1996; together with analyses of Photogrammetric measurements and sediment tracing, that some 5,300m3 of material accreted on the immediate west of this frontage whilst a loss of 14,250m3 occurred to the east. The latter volume would have been greater without some interim renourishment. Well-developed shingle salients formed in the lee of each breakwater and extended seawards thereafter towards each breakwater as sand tombolos. Tracer experiments indicated gravel movement around these salients, but with no apparent net onshore to offshore losses. Monitoring of rates of sediment motion within one salient-defined inter-tidal embayment indicated a maximum drift of 57m3 per tidal cycle during a moderate storm; most of the other study intervals indicated transport of at least an order of magnitude lower than this. King, et al. (2000) thus conclude that, at present, longshore transport takes place via the salients, and does not occur in the immediate lee of the breakwaters. The presence of the latter has reduced longshore transport rates by a factor of at least 2, compared to the shingle beaches immediately up and down drift. Compared with the original "ideal" design criteria, the embayments are wider and the salients narrower than originally anticipated, thereby giving Elmer Beach a markedly sinuous beach planform. Between 1993 and late 1997, depletion of shingle immediately downdrift (eastwards) of this scheme (Photo 11) amounted to a potential figure of 90,000m3 suggesting that effective interception and retention by the scheme was not allowing sufficient throughput to feed the drift potential at Poole Place (Climping). This shortfall has been offset since 1993 by biannual recycling of material taken from Littlehampton West Beach where it accumulates against the R Arun training works.
Littoral drift of gravel was calculated to be between approximately 60,000m3a-1 (Mouchel, 1997a, b, c) and 50,000m3a-1 (Gifford Associated Consultants, 1997) at Felpham. At West Beach, Littlehampton, Hydraulic Research (1987) using a calibrated wave power model based on a 15-year hindcast wave climate estimated net eastwards beach drift of coarse sand and gravel either side of the harbour mouth to be 65,000m3a-1 (13,000m3 of which is gravel) to the east. The transport equations used in this latter study to derive sediment transport from wave power were theoretical, but the general order of magnitude is considered to be representative although an earlier study (Posford Duvivier, 1987) proposed a marginally lower volume. The LITPACK figure is similar, at just under 60,000m3a-1 (Gifford Associated Consultants, 1997). Scott Wilson Kirkpatrick (2000a, b, c) derive the lower figure of 50,000m3a-1 for West Beach that includes some consideration of the downdrift effexts of the Elmer breakwaters and terminal groyne. It should be noted that transport uninterrupted by groynes is possible along a 1200m frontage of West Beach.
River training works and piers at the mouth of the River Arun have intercepted beach drift since the earliest constructions in 1736 and 1793 (Harris, 2003). They caused accretion to the west resulting in increasing stability of what had previously been reported as a low and marshy river delta subject to frequent inundations. By 1830, the accretion to the west had produced a distinct offset in the coastal planform and sand dunes (Photo 1) began to form behind the wide accreting foreshore (Harris, 2003). Not all drift is intercepted for a proportion of the sand and fine gravel moves around, or over the western training wall and thus bypasses the Arun inlet. Much of this is likely to be sand (Hydraulics Research, 1987). West Pier preferentially intercepts drift of medium and coarse gravel, of which approximately 24,000m3 was removed and recycled annually between 1994 and 1998 for renourishment of beaches at Poole Place and Climping. The progressive accretion of Littlehampton West Beach, to the immediate west of West Pier, indicates that historically there has been greater input of littoral sediment than natural losses. However, detailed study of beach profiles measured over the period 1973 to 2002 identified a trend for accretion at West Beach from 1973 to 1993, but with a tendency for erosion thereafter, especially within a zone extending 300m to the west of West Pier (Harris, 2003). It can be postulated that less fresh sediment is entering the system due to retention by the Elmer breakwaters and that the recycling operations at West Beach are possibly removing material at a more rapid rate than it can be replaced naturally by eastward drift.
Throughout this sector, it is probable that drift rates are substantially higher in winter than during other periods of the year (Hydraulics Research, 1989). Annual variations in transport rates are greater than long-term (20 years) mean variance (Gifford Associated Consultants, 1997). It should also be noted that Jelliman et al. (1991) have drawn attention to the sensitivity of beach drift along this frontage to relatively small alterations in wave climate direction. Such variations could occur in future due to climate change and isolated groyne-free segments such as West Beach could be especially sensitive.
LT2 R. Arun to Lancing (see introduction to littoral transport)
The mouth of the River Arun at Littlehampton appears to form a partial barrier to bedload movement of shingle due to strong tidal flushing and the intercepting effect of river training structures. Posford Duvivier (1987) report some overflow of fine shingle into the channel of the Arun and Hydraulics Research (1987) and Environment Assessment Services (1997) report significant sand transport across the Littlehampton Bar further offshore. Therefore, it is likely that the River Arun inlet is a more effective barrier to shingle than sand. Indeed, if bypassing of sand did not occur, recession of Littlehampton East Beach would have been significantly greater than it has been in recent decades. Jezard (2003) has analysed the historical evolution of the mouth of the Arun since the late 16th century. The first piers date back to the early 18th century and were associated with river straightening operations to improve navigation. The first reliable map to show the offset alignment of the west and east beaches dates to 1830, although there are some documentary records of west pier extension in 1793 and 1825, possibly arising due to accretion to the west. Over 100m of retreat of the position of low water along the East Beach (Photo 12) occurred between 1875 and 1979 (Gifford Associated Consultants, 1997), but most of this adjustment to the effects of the piers and training works upon sediment bypassing had occurred by 1900. Indeed, some advance of low water mark has occurred since 1930 (Jezard, 2003).
Natural potential for littoral drift on East Beach, Littlehampton is estimated to be between 20,000 and 65,000m3a-1 eastward (Hydraulics Research, 1987; Posford Duvivier, 1987; Scott Wilson Kirkpatrick, 2000c), but this higher figure is probably not achieved due to shortage of sediment supply and interception and storage by groynes. The LITPACK figure (Gifford Associated Consultants, 1997) for East Beach is 37,500m3a-1, which makes allowance for the effect of groynes on temporary sediment storage.
Drift peaks to over 70,000m3a-1 along the East Preston to Ferring Rife sector. Scott Wilson Kirkpatrick (2000a, b, c) calculate potential drift for this entire frontage to be as high as 120,000m3a-1, but they estimate actual transport at 16,000m3a-1 between East Beach and Worthing.
For the Worthing frontage, the LITPACK figure of 38,500m3a-1 is based on observed and estimated storage in the numerous inter-groyne compartments (Photo 4). This reduces slightly, to 35,000m3a-1, at Lancing. Jelliman, et al. (1991) calculated changes in offshore wave climate at Littlehampton between 1974 and 1990. Based on evidence of a shift in the wave climate direction of around 6 degrees, a potential increase in littoral drift rate of 1,720m3a-1 was estimated to have occurred over the period 1974-1988. Holmes and Beverstock (1996) have also concluded that longshore transport rates between Worthing and Shoreham vary with short-term changes in wave climate. Nevertheless, the most significant factor that controls drift rates is the presence of groynes. Their role, for the most part, is to reduce natural throughput within the longshore transport system. The highest rates of interception and accretion take place along those sectors of coastline whose orientation is closer to being parallel to the predominant direction of wave approach.
A major rock groyne and gravel replenishment scheme has been completed at Lancing over the period 1997 to 1998 (Photo 13 and Photo 14). Studies of the initial behaviour suggested that the new groynes generally intercepted drift and retained beach material effectively, but that they were partly permeable allowing some throughput of gravel to Shoreham West Beach (Coates, et al., 1999). It should be noted that the studies were undertaken over a three month autumn season shortly following completion of one section of the scheme. Beach behaviour and groyne permeability may have altered over the following years as the sedimentology and morphological configuration had time to develop.
LT3 Shoreham West Beach (see introduction to littoral transport)
West Beach comprises the western portion of a shore-parallel barrier type shingle spit fed by net west to east shingle drift that has deflected the mouth of River Adur eastwards as far as Portslade in 1810. Up to the early 19th Century there was complex history of natural and artificial breaching to reinstate the mouth of the Adur opposite Shoreham, typically followed by renewed eastward extension of the spit (Brookfield, 1949 and 1952; Castleden, 1996; Robinson and Williams, 1983). The patterns of spit growth and river deflection therefore suggest that a net eastwards littoral transport pathway has prevailed for several centuries. The beach drift and spit behaviour has been modified since 1821 by various harbour training works designed to stabilise the inlet at approximately its present position (Gifford Associated Consultants, 1997). These structures interfered with the dominant west to east drift leading to accretion of shingle and growth of the West Beach. The present configuration of harbour breakwaters (Photo 2) dates from 1955-58 when various modifications were undertaken to improve navigation in connection with construction of the power station on the east spit (Ridehalgh, 1958). Beach evolution over the past Century has been studied by Baily (2001) based on historical maps and aerial Photos. The western breakwater was significantly upgraded and lengthened to around 250m of which over 150m extended seaward of the mean low water mark on the beach. It has functioned to intercept all shingle drifting along the shoreline, although it only partly intercepts sand transport in the nearshore.
All of the shoreline westwards to Worthing and on to Littlehampton is controlled by extensive arrays of timber and rock groynes, wooden revetments, and sea walls. These defences prevent free drift of shingle and control inputs of shingle to the West Beach. Modelling using the LITPACK package by Gifford Associated Consultants (1997) suggested a potential for net eastward drift of all sediment grades of 35,000m3a-1 at Lancing and 26,000m3a-1 at West Beach Shoreham, although these rates cannot be achieved due to the impeding effects of groynes along the former frontage. However, for a distance of 1.5km westward of the western breakwater, upon the West Beach itself transport is uninterrupted. The dominant westerly wave climate causes net shingle drift on the beach from west to the east so that shingle accumulates against the breakwater. Since the western breakwater appears to act as a total barrier to gravel drift, the accretion occurring has provided a basis for making estimates of the drift occurring on the beach. Several estimates of accretion have been produced based upon data from the Environment Agency Annual Beach Monitoring Program (Riddell and Ishaq, 1994), including 15,500m3a-1 by Chadwick, (1987; 1988a, b, c, and d; 1989a and 1989b and 1990) for the period 1975-84 and 470,000m3, or 19,000m3a-1 from 1974 to 2000 (Scott Wilson Kirkpatrick, 2000a, b and c). It would appear that net drift varies annually within the general range of 15,000-20,000m3a-1 as suggested by Scott Wilson Kirkpatrick (1995). Longshore transport rates along this sector have also been shown to vary with short-term changes in wave climate (Holmes and Beverstock, 1996). The Shoreham Harbour Authority have bypassed shingle (Photo 7) eastward around the harbour entrance from the West Beach to the East Beach at a mean rate of around 8,500m3a-1 from 1993 to 2000 (Vaughan, 2001). It is uncertain whether this has been accounted for in the more recent drift estimates based on accretion. Some of the differences between authorities on net transport rates may be due to different weighting of, or allowance for, gravel removal for downdrift recycling (see Section 2.4), although differences can also be attributed to the different time intervals studied.
As part of the Shingle Beach Transport Project (funded by MAFF and the EA) undertaken between 1996 and 1999 (Coates, et al., 1999), the deployment of six different tracer types over a 700m frontage indicated a net west to east drift of between 15,000 and 20,000m3-13. Van Wellen, et al. (1997, 1999) and Lee, et al. (2000) report that there was preferential transport of larger particles, suggesting the operation of sorting process that could result in beach grading. Most transport was confined to the beach face, with negligible exchange with the nearshore zone. Transport rates were closely related to wave energy flux, with "bursts" of more rapid drift. (During the experimental period 48% of waves approached between west and south, with a maximum offshore wave height of 4m). Tidal currents are weak (0.1 to 0.7m.sec-1) and of no significance in effecting sediment transport. It was additionally concluded that vertical mixing depths have a linear relationship to significant wave height (Lee, et al., 2000). Beach elevations were found to oscillate by as much as 1m during single tidal cycles. Sediment exchange rates of 10m3 per tide for low energy waves, and 150m3 per tide for medium to high energy waves were recorded for specific experimental areas. Especially large high energy events were found to resulted in fluxes of up to 3,000m/tide (Bray et al., 1996; Stapleton, et al., 1999). Taking a longer perspective, Mason, et al. (1999) found reasonable agreement for both short-term and tidally-averaged longshore transport rates, based on the application of an energetics model for shingle transport. Van Wellen, et al., (1999) however found differences between field measured (short timescale) transport and the longer term accretion based evidence. Scott Wilson Kirkpatrick (2000a, c) calculate the current eastward drift to be 16,000m3a-1, with most of the gravel fraction retained by the substantial breakwater protecting Shoreham Harbour. This study concluded that West Beach accumulated by 98,000m3, between 1993 and 2000.
Beach Plan Shape Models (Halcrow, 1990; Chadwick, (1988a, b, c and d, 1989a and b, 1990; Baily, 2001; Duane 1998) have been tested at Shoreham by comparing their predictions with observed beach accretion. Models were calibrated so that littoral drift volume and direction, together with its effect on beach plan/volume, could be determined from suitable wave climate data. Despite this, wave refraction and reflection from the artificial breakwaters protecting the harbour mouth were found to cause anomalous effects on the drift pattern and beach volumes nearby, which could not be consistently predicted. Additionally, sediment transport was measured in the field using shingle traps and related to wave power to create calibrated transport equations (Bray et al., 1996). Halcrow (1990) reported that wave reflection from the breakwater caused a progressive eastwards reduction in the rate of longshore transport. This was also apparent over the frontage west to Lancing, possibly because of a slight change in shoreline orientation.
4. SEDIMENT OUTPUTS - O1 O2 O3 References Map
4.1 Transport in the Offshore Zone
01 Eastward Moving Pathway of Kelp-Rafted Shingle (see introduction to sediment outputs)
Direct observations during diving surveys indicated an eastward moving path for kelp-rafted gravel up to 5km offshore from east of Selsey Bill to Brighton (Jolliffe and Wallace, 1973). The reliability of this information is low because it is uncertain whether the observations were representative of long-term average conditions. These observations raise a series of questions. For example, does this process occur over the whole area? What volumes of shingle might be involved? Is it a consequence of both wave and tidal current powered transport? Jolliffe and Farnham (1986) presented proposals to study this phenomenon further which have yet to be followed up.
02 Sand Transport between Pagham and Worthing (see introduction to sediment outputs)
There is relatively little detail on the nature and pattern of offshore sediments, except for a few site-specific surveys, and considerable uncertainty over rates, volumes and net directions of transport. Gifford Associated Consultants (1997) state that much of the seabed is veneered by sandy gravel and fine to coarse gravel, with seaweed colonisation over extensive shallow water areas. There are, however, some bedrock outcrops which are presumed to be source areas for weed-rafted shingle. HR Wallingford (1993) suggests that these areas may be scoured by tidal currents provides a provisional map of large and small sandwaves, and patterns of megaripples further offshore. All possess a distinctive cross-sectional asymmetry, indicating a net eastwards to north-eastwards transport pathway. The quantities of sand transport occurring are estimated by HR Wallingford (1993) and Scott, Wilson Kirkpatrick (2000b) using modelling approaches and hindcasting of forcing conditions. Results indicate a progressive reduction in transport rates for a mean grain size of 0.2mm as water depths and distance from the shoreline increase. However, movement is likely at least 20km offshore. In the western part of the offshore sector, net transport of 100,000m3a-1/km is estimated. Eastwards, this reduces to 50,000m3a-1/km in the same direction within 4km of the coastline. This presumably creates a discontinuity zone of depletion or potential erosion in between. However, there is an indication of a peak transport rate of approximately 200,000m3a-1/km offshore of Worthing, decelerating towards Littlehampton. For coarse sand (1.1mm diameter), transport rates are lower (20-30,000m3a-1/km offshore Worthing), again directed eastwards. Gravel waves also occur in deeper water, but are not considered to be mobile under prevailing hydrodynamic conditions. Gravel particles were considered to be stable on the seabed at water depths in excess of 12-15m.
Gifford Associated Consultants (1997) propose a closed anticlockwise "cycle" of sand movement offshore the entrance to Pagham Harbour, a conclusion based on conceptual analysis of the presence of the ebb tidal delta, known sediment distribution on the adjacent seabed and the propensity for swash bars to be driven ashore by wave action.
Binnie and Partners (1987 and 1988) report that the seabed 2km to 8km offshore of Worthing consists of a thin cover of sand and medium to fine gravel. Faintly defined sub-parallel ridges of very fine gravel were identified, and it was tentatively suggested that they may be the product of bedload transport by tidal currents; however, they do not clearly indicate net directions of transport. Some rock exposures up to 8km offshore may also be due to either tidal current scour.
Potential shorewards transport of sand and gravel from the Inner Owers, 15km offshore Littlehampton is regarded as very unlikely HR Wallingford (1993), and has provided some confidence that long-term aggregate extraction within this area should not have significant wider impacts. A recent environmental assessment for two proposed licenced areas (Emu Environmental, 2000) indicated possible tidal current induced sand transport pathways to the east-north-east and west-south-west. It has been suggested that dredging over the past 30 years might even have had the effect of increasing nearshore concentrations of sand through entainment of spoil materials, particularly offshore between Pagham Harbour entrance and Bognor Regis (Emu Environmental, 2000)
03 Eastward Sand Transport Off Shoreham (see introduction to sediment outputs)
A series of sand waves were mapped off Shoreham and Brighton by the British Geological Survey (1990) using information from seismic and echo sounder surveys. Sand wave morphology indicated a net eastwards sediment transport pathway. It was, however, uncertain whether these sand waves were permanent bedform features or were intermittently developed, indicating periodic transport. HR Wallingford (1993) report large areas of sand in shallower water, with dominant gravel further seawards. Much of the seabed is colonised by various flora and sedentary marine life, and sediment transport potential was thus considered to be low. Gravel was presumed to be immobile below depths of 12-18m.
In both areas, residual tidal currents provide the main transport mechanism for fine sand, although waves are a significant process in maintaining movement of suspended fine grained sediment in the nearshore zone. For coarse sand, Scott Wilson, Kirkpatrick (2000b) calculate net movement of between 5,000 to 10,000m3a-1/km within 4km of the shoreline. This may increase to approximately 50,000m3a-1/km for fine sand.
4.2 Estuarine Outputs - EO1 References Map
EO1 Littlehampton Harbour Entrance and Bar
Tidal flushing operates at the mouth of the River Arun, but as training walls (Photo 1 and Photo 12) inhibit gravel entering the inlet channel (Posford Duvivier, 1987) transport mostly involves sand. Sediment sampling by Hydraulics Research (1987) demonstrated that sandy ebb tidal delta deposits extend up to 1km seaward of the East Pier comprised of sediments that drift into the inlet channel and then become flushed seaward. A study of the estuary mouth regime (Environmental Assessment Services, 1997) estimated return shoreward transport from the delta is possible so that by-passing of sediments transported longshore occurs in both eastwards and westwards directions. The former takes place only under high wave energy conditions, as the West Pier is otherwise a major impediment. The shorter, lower level extension of East Pier (Photo 12) provides more limited impedance to littoral drift into the inlet when net westwards movement of sediment operates under waves approaching from the east or south-east. The quantity of sediment capable of moving across the harbour mouth was calculated to be a gross volume of 5,000 to 10,000m3a-1. The completion of the Elmer breakwater and recharge scheme to the west may have resulted in a reduction to this quantity. The Environment Agency has been removing gravel from Climping Beach for the purposes of updrift recycling in response to loss of volume downdrift of the terminal groyne at Elmer.
The mean tidal flow at Littlehampton Harbour mouth is 135m3sec-1, with tidal current velocities of up to 2.5m.sec-1. Tidal scour has thus maintained the inlet and the depth of clearance over the harbour bar, which consists of two parts, an inner component (West Pier Head to East Beacon, to about 150m seawards), and an outer component between 150m and approximately 500m seawards of the entrance channel. Although the inner bar is more dynamic, there has been little overall morphological change since the late nineteenth century. However, new shoals have appeared inside West Pier and on the seaward margin of the outer bar, the former tending to reform between dredging operations. A reduction of depth of clearance over the outer bar, of 0.5m between 1887 and 1993, could be due to weed growth stabilising the sandy surface of the bar, which overlies consolidated cobbles. Alternatively, this trend may indicate an increase in the volume of sediment in transit across the harbour mouth, though there are no sedimentary structures that are diagnostic of sediment mobility. No dredging over the outer bar has occurred since 1917. Using data from previous studies, (Gifford Associated Consultants, 1997; Hydraulics Research, 1987; Posford Duvivier, 1987) and the results of bathymetric analysis, Environmental Assessment Services Ltd (1997) suggest a tentative sediment budget as follows. Wave-driven longshore transport, from west to east is estimated at 60,000m3a-1, with only a small, unquantified, volume moving in the reverse direction. Much of this is retained by West Pier breakwater, but an approximate total of 5,000 to 10,000m3a-1 may be involved in inlet bypassing. Most of this is sand. An input of no more than 4,000m3a-1 is derived from the River Arun, most of which is fine grained. A further 4,000m3a-1 may be introduced by kelp rafting of gravel, although this figure would seem to be more appropriate for the whole sector of coast between Middleton-on-Sea and Kingston. Of the fine sediment that moves into the entrance channel and onto the inner and outer bars, it is currently uncertain what relative proportions are retained, transported downdrift or in the littoral and nearshore zones or moved into deeper water offshore as part of the offshore transport system (see O2).
5. SEDIMENT STORES: BEACH CHARACTERISTICS - References Map
Beaches comprise the main zones of littoral sediment storage. Ebb tidal deltas associated with the inlets of the rivers Arun and Adur are relatively poorly developed, possibly due to the loss of tidal prism resulting from infilling and reclamation of these estuaries. It can be postulated that larger tidal deltas could have been maintained in the past when the estuaries were larger, but the sediments would have been driven onshore to feed beaches following estuary infilling and reclamation.
5.1 Beach Sediments
Upper beaches are composed predominantly of coarse flint gravel and are relatively steep and flat-crested, or may have a noticeable upper storm berm. Lower beaches are usually composed of fine gravel, granules and medium to fine sand; they are significantly less steep forming a low gradient foreshore often extending for several hundred metres seaward to low water. Overall, beach profiles are convexo-concave in form.
There are sand dunes behind Climping Beach, to the immediate west of the Littlehampton Harbour that have accreted since 1830, in apparent response to the interception of drift by the River Arun training structures and the wide sandy lower foreshore that has accreted (Harris, 2003). Although stabilised in parts by vegetation planting, they remain an open, active accretion system. Former dunes to the east of the River Arun, at Littlehampton, are now covered by urban development (Jezard, 2003).
These general statements are based on analyses of beach composition at several specific locations, e.g. Shoreham (Chadwick, 1989; Coates, et al., 1999), Middleton and Felpham (Lewis and Duvivier, 1973; Mouchel, 1997a, b and c), Elmer (Cooper, 1997), Littlehampton West Beach (Harris, 2003), Worthing (Binnie and Partners, 1987) and Lancing-Shoreham (Scott, Wilson Kirkpatrick, 1995). Quantitative data on beach sediments based on significant field sampling is provided principally by Gifford Associated Partners (1997), Cooper (1997), Coates, et al., (1999), Scott Wilson Kirkpatrick (2000b, c) and Harris (2003). Gifford Associated Partners (1997) present trends of mean grain size, sorting and skewness separately for the upper and lower beaches. Although there are some site-specific deviations away from general trends, values for all three measures display expected longshore and cross-shore responses to energy gradients.
5.2 Beach Volumes
Information includes a variety of site-specific measurements and more comprehensive sets of data for the entire frontage calculated by Gifford Associated Consultants (1997), for Climping to the River Arun (Harris, 2003) the shoreline between Littlehampton East Beach and Shoreham West Beach, by Scott Wilson Kirkpatrick (2000b, c) and Jezard, (2003). The studies are primarily based on data covering 1973 to the present derived from the Environment Agency Annual Beach Monitoring programme (Riddell and Ishaq, 1994). It should be noted that there are have been some uncertainties in the past relating to the reliability of parts of the data.
Beach thickness is available for Littlehampton Harbour Mouth, where 4 boreholes were drilled (Environmental Assessment Services, 1997). On the lower foreshore (East Beach), 0.2-0.3m of sand covered a 4-5m thick sandy gravel and cobble deposit overlying in situ Chalk at 10-12m. Boreholes themselves are reliable sources of information; however, it is uncertain just how representative 4 boreholes are of local conditions at the mouth of the Arun. Gravel deposits adjacent to the mouth of the Arun are probably not representative of general beach thickness because the river has excavated a deep (up to 30m) channel, which has subsequently been infilled by gravel, sand and clay (Jones, 1981; Bellamy, 1995). Scott Wilson Kirkpatrick were unable to discern any obvious trends in beach volume changes at this location for these reasons, although there may have been a small annual loss of between 1,200 and 4,700m3 between East Beach and East Preston.
The gravel volume of Pagham Estate beach, east of the harbour entrance, is estimated at 2-3 million m3 (Wallace, 1990) based upon measurement of beach width and length and an estimated shingle thickness of 5m. No details of sediment composition changes with depth are given - the beach may have a sand, sandstone or clay base, so that gravel volume could be significantly less than the above calculation.
Scott Wilson Kirkpatrick (2000b, c) calculate a mean volume of 2.5 million m3 for central-east Worthing beach, with gains up to 1983 switching to net losses after 1988. This gives an overall annual volume addition of 7,300m3 between 1973 and 1998.
The volume of West Beach at Shoreham is approximately 2.5 million m3, calculated from analyses of beach monitoring profiles derived from Environment Agency annual aerial Photo surveys (Gifford Associated Partners, 1997). Scott Wilson Kirkpatrick (2000a, c) calculate that there has been a steady increase in volume, since 1974, in the order of 470,000m3, or 19,000m3a-1. Duane (1998) reports a net gain of 15,340m3a-1 from 1971 to 1986, but with some natural inter-annual fluctuation. Longshore feed into this beach is retained by the breakwater protecting the entrance to Shoreham Harbour, but an average of 8,500m3a-1 has been deliberately removed, and placed on East Beach, by recycling operations conducted by the Shoreham Port Authority since 1993 (Vaughan, 2001).
5.3 Recent Accretion/Depletion Trends
A comprehensive view of beach volume changes between 1973 and 2000 has been provided from analysis of the Environment Agency (and predecessor organisations) Annual Beach Monitoring Survey (ABMS) by Gifford Associated Consultants (1997) Scott Wilson Kirkpatrick (2000b, c), Jezard (2003) and Harris (2003). Analyses have sought to identify specific accretion/depletion zones and net volumetric erosion/accretion rates, an approach which is dependant on the accuracy of Photogrammetric analytical techniques and the spatial and temporal representativeness of selected beach profiles. Scott Wilson Kirkpatrick (2000b, c) used ABMS data to specifically investigate trends in beach crest height and width, and beach volumes. They considered it to be reasonably reliable as an indicator of trends over decadal periods.
The major trends between the early 1970s and mid-1990s that were identified were (i) beach depletion at Pagham Beach Estate (HR Wallingford, 1995), Middleton and Lancing and (ii) net accretion immediately west of Littlehampton (Climping), Worthing and Shoreham (West Beach). Intervening areas covered by the survey showed no clear trends. This could mean either that (a) these beaches were relatively stable; or (b) beach volume fluctuation was such that significant trends could not be established statistically (e.g. the sector between Littlehampton and Rustington showed depletion between 1983 and 1988, but slow, cumulative recovery throughout the 1990s). The above observations may have been influenced partly by beach management, especially the widespread practice of replenishment.
Harris (2003) analysed profile changes between Poole Place (Photo 11) and the River Arun and identified a trends for erosion at Poole Place and Climping and accretion at West Beach from 1973 to 1993. Erosion continued at Poole Place and Climping up to 2002, but at significantly reduced rates. At West Beach, accretion reduced from 1993 to 2002 with erosion being recorded within a zone extending 300m to the west of West Pier. It can be postulated that the Poole Place and Climping frontages suffered sediment shortfall for some time prior to construction of the Elmer breakwaters and that the recycling operations from West Beach since 1993 haveprovided some compensation for the shortfall. Erosion at West Beach since 1993 could indicate that even less fresh material has entered the system since the Elmer breakwater construction, although it could also indicate that the recycling operation may have been removing material at a more rapid rate than it can be replaced naturally by eastward drift.
Worthing Borough Council (1987) stated that beach levels along their frontage were previously appreciably lower. Thus, there has been a general trend for accretion over the past 100 years and this is attributed to the effectiveness of groynes at intercepting drift. This assessment is based on comparisons of old Photographs, plus observations of groynes periodically exposed by storms. Reliability is only low/moderate, because no quantitative evidence is presented. This report mentioned that in the mid 1980s the beach showed signs of depletion, but Binnie and Partners (1987) indicate that beach widths at Worthing are highly variable from year to year. They calculate short-term beach volume changes for this frontage of as much as 790,000m3a-1 (1982-83) with apparent long-term accretion of 100,000m3 between 1974-85. The equivalent calculation by Scott Wilson Kirkpatrick (2000b, c) for 1973-1998, is a net accretion gain of 183,000m3. Jezard (2003) also identifies net accretion along the whole frontage from Littlehampton and Worthing between 1993 and 2000, but her analysis of ABMS data from 1973 to 2000 revealed an overall reduction in volume. A switch from erosion to accretion around 1995 is implied.
A consequence of heavy shoreline management by groynes at Worthing has been progressive loss of shingle volumes and both narrowing and steepening of beach profiles between South Lancing and East Shoreham Beach (King's Walk). Dobbie and Partners (1990) and Scott, Wilson Kirkpatrick (1995) identified these trends to have been uninterrupted since at least the 1870s. Mean Low Water mark retreated 1.7ma-1 between 1875 and 1896 and continued at a mean rate of between 2 and 3ma-1 up to the early 1970s. In response to very substantial losses of beach volume in the winter of 1989/90, a recharge of 110,000m3 was carried out in 1991/2. At the same time, beach crest height and width was increased. Resulting from subsequent shoreline management strategic studies (Scott, Wilson Kirkpatrick, 1995; 2000b and c), further recharge and the construction of a series of rock groynes has been undertaken (Photo 13 and Photo 14). This, together, with future planned input, involves an additional 150,000m3 of gravel.
Analysis of beach monitoring data by Chadwick (1989) showed that the beach west of the Shoreham harbour entrance was subject to shingle accretion (1973-82) over a stretch 1.5km long. The mean rate was estimated as being 14,539m3a-1 from 1973 to 2000. A total net increase in beach volume of 166,000m3 occurred over this period, with 98,000m3 accumulating between 1992 and 1995. Sir William Halcrow and partners (1990) calculated accretion of approximately 200,000m3, 1973-1989 for the frontage 2,000m west of the breakwater. This figure, however, included an estimate of net deposition across the sandy lower foreshore. Thus an adjusted figure for accretion since 1973 is 6,600m3a-1, but some 16,000m3a-1 for the post-1992 period (Vaughan, 2001). A considerable variability therefore exists in the rate of net drift operating along the beach. It is likely that future climate change could have moderate effects upon drift on this beach. Modelling of a range of feasible future scenarios has indicated a possible tendency for increased rates of net eastward drift (Halcrow Maritime, 2001).
Baily (2001) has analysed Lancing and Shoreham beach profile data for consecutive years, and decadal periods, and has concluded that shorter-term records fail to reveal process response relations between beach morphology (including volume); transport processes and wave forcing. Antecedent beach condition was revealed as critical to beach behaviour under storm conditions. A general finding was that the upper gravel beach displayed moderate net accretion over virtually the whole coastal segment, whilst the sandy lower foreshore showed loss of volume. This would point to a tendency towards beach steepening, thus continuing a trend established over the previous 100 years. Baily (2001) has confirmed the historical pattern of steepening and narrowing, and his research reveals that recession rates, up to 6ma-1, are highest around mean low water. These losses are not necessarily sustained, and may be recovered over relatively short-term periods.
6. SUMMARY OF SEDIMENT PATHWAYS - References Map
1. This coastline is characterised by a dominant west to east directed littoral drift pathway operating along the gravel upper beaches and sandy lower foreshores. It forms the key central part of the wider circulation cell that operates between Selsey Bill and Beachy Head.
2. The drift pathway has been sustained by sediment inputs from the shore between Selsey Bill and Pagham Harbour as well as receiving modest gravel inputs from the nearshore bed. Historically rapid, coastal retreat has provided important sources of fresh sediment derived from erosion and/or transgression of the sand and gravel sediments of the West Sussex Coastal Plain.
3. Intensive management involving the holding of a largely fixed line of coastal defence for the past 100-150 years has inhibited the natural tendency for landward migration of the shoreline. It has greatly reduced the supply of fresh sediments from coastal retreat and extensive groyne fields have intercepted much of the drift of gravels and coarse sand on the upper beaches. Major cross-shore training breakwaters constructed at the inlets of the Arun and Adur rivers have intercepted drift additionally.
4. Tidal exchanges at the Arun and Adur inlets exert only a modest influence upon the sediment dynamics because both estuaries are substantially infilled and reclaimed. Prior to estuary reclamation is likely that both inlets would have generated substantial ebb tidal deltas that would have tended to dissipate wave energy in their immediate vicinity. Following the loss of tidal prism due to reclamation it is likely that the sediments of the tidal deltas were driven landward to contribute to the local beaches. These supplies are anticipated now to be largely exhausted.
5. Intensive beach management operations throughout this shoreline involving gravel recharge, re-cycling and bypassing supported by carefully designed and maintained control structures now largely control sediment transport and attempt to maintain beach stability. Given the low-lying and erodible nature of this shoreline, its limited natural sediment supplies and the potential for sea-level rise and climate change impacts there are some uncertainties relating to the sustainability of trying to hold the present defence line in the long term (Halcrow, 2002)
6. Much aggregate dredging has been undertaken in areas offshore although most studies suggest that it mined immobile deposits and was unlikely to have had significant shoreline effects. Indeed, local offshore resources the frequently been used as sources for beach re-charge operations.
7. COASTAL DEFENCE AND HABITAT INTERFACE ISSUES - References Map
The frontage is outside of the main area of the Solent ChaMP, although it is within the interactive envelope identified for potential habitat management and creation opportunities of relevance for the Solent features. It is of interest primarily for its gravel beaches and its embanked tidal rivers occupying wide low-lying flood plains that have been formed from reclaimed estuaries.
Much of this coastline has been managed using "hard" defences for some 130-150 years, extending back for over 200 years at some critical locations. This influence, together with an historical trend of natural recession, narrowing and steepening of gravel beaches, has had some negative impacts on habitat survival and development. Exceptions, however, occur where important shingle vegetation communities adapted to stable backshore gravel or gravelly sand environments have become well established at sites of accretion and beach progradation. Examples include Pagham Estate Beach (Photo 8), West Beach Littlehampton (Photo 1), West Beach, Shoreham (Photo 2) and numerous smaller sites scattered widely along the frontage. The vegetated shingle resource is potentially threatened by squeeze between fixed residential developments to landward and the natural tendency of the beaches to migrate. Intensive beach management also threatens to disturb vegetation communities. Details of appropriate management and habitat creation techniques for this resource have been set out by Doody and Randall (2003). The distribution and characteristics of vegetated shingle have been mapped by the West Sussex Vegetated Shingle Project (2003). The project has sought to increase general awareness of the local resource, it has provided guidance for contractors working on vegetated shingle with further guidance produced for residents with shingle gardens.
Several former lagoons and river mouths blocked by barrier transgression have been drained and infilled, and their characteristic habitats virtually extinguished by earlier agriculture and twentieth century expansion of urban development. Given this history of ecological losses, it is critical that future shoreline management should give priority to the conservation, and enhancement, of what survives. This may prove difficult at sites such as Widewater Lagoon, Lancing (Photo 5) that are constrained by residential development to landward such that it will be necessary to restrain landward beach recession, whilst simultaneously addressing an increasing transgressive tendency of beaches due the forcing effect of rising sea-level. The site is a valuable remnant of previously more widespread back-barrier lagoons along this shoreline and contains several valuable lagoonal specialist species. Long term maintenance would involve holding the defence line for the foreseeable future which would be counter to the latest guidance for conservation of self-sustaining functioning coastal systems (English Nature et al., 2003)
The now widespread practices of beach nourishment and sediment re-cycling may give rise to opportunities for habitat creation on the backslopes of reconstructed beach crests. Indeed, guidance for contractors involved in coastal defence operations has been -produced by West Sussex Vegetated Shingle Project (2003) Experience at Elmer indicates that offshore rock breakwaters provide additional, or new, intertidal ecological niches. The expansion of sand dune development at Climping, though modest in scale, provides scope for adding diversity to the currently limited plant and animal community.
The infilled and reclaimed estuaries of the Arun and Adur contain much low-lying land adjacent to tidal river embankments that could potentially be inundated in a controlled manner for creation of intertidal habitats. As such, these areas could potentially be considered for mitigation projects arising from the need to compensate for losses in adjoining areas such as the harbours of the eastern Solent.
The Sussex Seasearch Project (1994-1999), partly supported by SCOPAC, has undertaken valuable baseline survey work on the sub-littoral environments of this coastline. It is particularly important that future shoreline management should ensure the integrity of this habitat. It represents a condition of diversity and productivity much in contrast to the relative biological sterility of most of the adjacent beaches.
8. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References Map
Estimates of gross and net littoral drift derived from numerical modelling based upon wave hindcasting are available at numerous points along the shoreline due to previous studies in support of schemes and the decision taken within the SMP to routinely provide such information (Gifford Associated Partners, 1997). Difficulties encountered in applying these models included the problem of selecting a representative sediment gain size on the mixed sand and gravel beaches (sediment mobility is highly sensitive to grain size) and the need to estimate (or ignore) the extent to which groynes on the upper beach intercepted any potential drift. It has meant that the data generated from these studies has had to be interpreted carefully.
Shoreham West Beach provides some of the best opportunities in the region for calculation and testing of littoral drift volumes. This is due to due to its simple morphology and bathymetry, unconfined transport and abundant shingle available for transport. These qualities were recognised by a series of researchers who have attempted to measure sediment transport in the field, develop relationships with hydrodynamic forcing and undertake numerical model application, development and verification e.g. Chadwick (1989b); Morfett (1990); Bray et al. (1996); Duane (1998); Coates et al. (1999); Mason et al. (1999); Stapleton et al. (1999); Van Welen et al. (1999) and Lee et al. (2000). The presence of the breakwater was especially advantageous for much of this work because the shingle accretion that it promoted to its west offered an independent estimate of long-term drift. Results from these studies suggested that extremely large quantities of sediment could be mobilised during storm events, rapid reversals of drift could occur in response to changes in wave conditions and most gravel transport was retained on the upper beach. Difficulties were encountered in comparing short and long term transport measurements and in comparing field measurements of transport with predictions made with existing theoretical approaches. Other uncertainties involved determination of grain size on the variable mixed gravel beach together with the tendency of waves to be reflected from the breakwater and interfere with transport processes on the beach.
Other comparative work has been undertaken on beaches controlled by rock breakwaters Elmer (Cooper, 1997; King et al. 1996) and rock groynes at Lancing (Coates et al., 1999). Although it proved possible to demonstrate that transport could occur through such schemes over short intervals it proved difficult to determine generic controlling relationships that would enable scaling up of results to longer time periods. The problems encountered related to the complex configurations of these features and the tendancy for rapid initial sedimentological and morphological changes of the recently replenished beaches.
A location potentially amenable to study of drift is West Beach, Littlehampton, which possesses many of the same qualities outlined for Shoreham Beach. A key difference is that its western pier acts only as a partial barrier to drift making it more difficult to analyse the sediment accretion to its west as an independent estimate of long-term drift. Some further study of drift at this location would, however, be warranted since beach profile analyses by Harris (2003) have indicated that recent rates of Environment Agency recycling may exceed the capacity of natural drift to replenish the material.
9. KNOWLEDGE LIMITATIONS AND MONITORING REQUIREMENTS - References Map
There has been an impressive increase in both the quality and quantity of knowledge and understanding of the coastal sediment transport process system on this frontage over the most recent 10 years. The regional Shoreline Management Plan (Gifford Associated Partners, 1997) has reviewed and synthesised this much of this information (as well as contributing to it in its own right). Furthermore, many of its 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. Their rationale and approach is explained by 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:
2. Studies of beach planform and volumes, especially volume changes, provide valuable insights into the rates of operation of littoral transport and the effectiveness of beach management. These have been facilitated by routine Environment Agency ABMS aerial Photography since 1973, with subsequent Photogrammetric measurement of profiles. There are have been some uncertainties in the past relating to the reliability of parts of the profile data so that it is important both to validate the historical data and to introduce robust methods for future profile data collection. It is understood that the Environment Agency initiated such work in 2002 and intend to incorporate the new ground control provided by the Strategic Regional Coastal Monitoring Programme into their profile measurement procedures.
3. Once the quality and consistency of the ABMS profile data sets have been assured it will be important to consider 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. Volume is especially important, but can be difficult to monitor reliably using widely spaced profiles on groyned coasts. Furthermore, an error analysis should also be undertaken so as to identify the minimum volumetric change that can be resolved with the techniques. Past trends in these indicator parameters (decadal, annual and seasonal) need to be established and a system of routine analysis instituted that would provide early warning of "unusual" trends. It may be that local engineers could 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.
4. 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 along this frontage due to beach management, especially the practices of recharge and recycling. 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 was identified by Gifford Associated Partners (1997) as a key constraint in undertaking modelling in support of the SMP.
5. Combining conventional Photography with other emote sensing imaging techniques - e.g. LIDAR; CASI, infra red; thermal - should provide new insights into pathways - and perhaps approximate quantities - of sediment movements at and around major discontinuities, such as river entrance breakwaters and specific sets of groynes. It is, however, admitted that non-Photographic imaging requires simultaneous field data collection for a degree of ground truthing if it is to provide useful and accurate interpretation. The Environment Agency is understood to have made arrangements for the linking of LIDAR with some of their ABMS work.
6. Application of numerical beach plan shape models for study of coastal evolution is difficult along much of this coast due to the widespread groyne systems that interfere with drift. Ideally some research is required to determine generic principles that govern the efficiency of different groyne configurations (timber and rock). The existence of a validated dataset of ABMS beach profiles extending back to 1973 could enable study of the long term efficiency of groynes in series as well as individually.
7. Studies are needed of the losses of sediment that occur from beach replenishment schemes over the short to medium term and the relative roles of processes such as sorting, attrition and longshore leakage. Work could be extended to a involve review of re-cycling and bypassing schemes with consideration of processes occurring at both the "borrow" and "fill" sites. The validated dataset of ABMS beach profiles extending back to 1973 should enable reasonably reliable retrospective studies of the performances of schemes. Local guidelines based on historical performance could be produced for areas where cyclic or ongoing operations were required. To effectively interpret historical performance, it would also be vitally important to compile good records of all past beach management activities undertaken back to 1973.
8. More sophisticated approaches to sediment tracing and sampling need to be applied to quantifying the offshore to onshore transfer of gravel via wave-assisted kelp rafting. This may be a more significant input into the regional sediment budget than has been supposed, and a combination of direct and indirect methods of quantifying travel pathways may clarify what continues to be an enigmatic, but potentially significant, process. As seaweed deposition on beaches is an environmental and amenity problem, there are opportunities for promoting integrated applied research.
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MMIV © SCOPAC Sediment Transport Study - Pagham to R Adur