Portsmouth Harbour Entrance to Chichester Harbour Entrance
1. INTRODUCTION - References MapGravel beaches, with only a few backshore features of prominence, dominate the character of this low-lying coastline (Photo 1). Tertiary (Eocene) sands outcrop across the foreshore, and provide the foundations of the beach substrate; they are occasionally exposed during storms. Until 1985, diversity was provided by a dominantly sand beach along the eastern coastline of Hayling Island, but this has since been covered by gravel clasts and cobbles introduced as part of a large-scale beach restoration scheme.
The coastline is interrupted by the tidally dominated entrance channels of Chichester (Photo 2), Langstone (Photo 3) and Portsmouth Harbours (Photo 4), defined by strongly recurved gravel and sand spits that have developed in opposed directions. The beach at the southwestern limit of Hayling Island encloses a sequence of gravel ridges that constitute the cuspate foreland of Sinah Warren and Gunner Point (Photo 3). Offshore is the extensive area of East Winner Sand. South of Ferry Point, and at Sandy Point, limited developments of low sand dunes are present landwards of the main beach berm. Another constructional feature is the tapering form of the West Winner gravel spit west of Langstone Harbour mouth; this enigmatic feature has experienced considerable changes in size and position over the past century.
Much of this coastline is urbanised (Photo 5), except for the central and western parts of Hayling Island, and is defended by a variety of sea walls, revetments and groynes (Oranjewould, 1991). Up until the early nineteenth century, (1820-1830) much of the backshore area of Portsea Island was a swampy or marshy residue of former lagoonal conditions (The Great and Little Morass). This suggests that the tendency along this coastline since the mid-Holocene has been for the development of a shorewards migrating barrier beach, cutting off former shallow tidal embayments and creeks. (Wallace, 1988, 1990) The harbour entrance spits may post-date the breaching and breakdown of an offshore barrier, as recently as the late seventeenth century (Wallace, 1990). Chichester Harbour entrance (Photo 2), for example, is thought to have been approximately 2km in width in the early eighteenth century, with no confining spits.
Wave energy decreases progressively from east to west, largely due to the protective effect of the Isle of Wight, so that modified swell waves do not contribute to the wave climate west of Eastney. (Hydraulics Research, 1984;1992; Havant Borough Council, 1992a). Complex local refraction effects are accentuated by the East Winner and East and West Pole Sands and wind waves are also diminished by interaction with tidal streams at the entrances of each of the three harbours. The coastline between Eastney and Southsea is exposed to a larger local fetch from the southeast than it is to the west, due to a change in orientation (Photo 1), but the offshore presence of Horse and Dean Sands reduces incident wave heights along the central and eastern Portsea Island coastline. Based on hindcasting, Hydraulics Research (1984 and HR Wallingford (1995) calculate that mean wave height at Portsmouth Harbour entrance is 0.48m, diminishing to 0.25m at The Point because of local diffraction effects. A maximum significant wave height of 0.8m (increasing to 1.04m for a 1 in 200 year recurrence) is proposed by Halcrow Maritime (2000). Ship-generated waves are normally less than 0.40m height, but may be an important local component, as there are over 90,000 vessel movements (1998). Recurrent flooding of Old Portsmouth indicates a 1 in 2 year probability of a water level of +2.54m ODN, increasing to +3.03m ODN for a 1 in 200 year return period. Further east, at Southsea Castle, a significant wave height of 1.22m, with an extreme of 1.62m for a 1 in 100 year recurrence, is calculated. Corresponding wave heights for Langstone Harbour Entrance are 2.10m and 2.58m. Extreme water levels here are 2.75m ODN (1 in 2 years) and +3.14m ODN (1 in 50 years). Maximum significant wave heights for the frontage between Sandy Point and the Inn on the Sea (Hayling Island) are 2.8m (1:1 year) to 3.8m (1 in 50 years), according to analysis of various sets of wave data, back to 1968, undertaken by W.S. Atkins (1998). A wave recorder was deployed off Hayling Island during the winter of 1988/9, the results from which reveal a mean significant wave height in the nearshore zone of 0.61m. For individual storm events, this value exceeded 2.2m. Whitcombe (1995) used a 100 to 200m resolution wave refraction model that indicated that offshore waves are refracted as they enter Hayling Bay and converge on East Winner and the banks/bars at the mouth of Chichester Harbour. Most waves along the central sector of Hayling Island approach normal to the shoreline, except when propagated across south-east and east-south-east fetch directions. Wave focusing is highest at the eastern end under swell waves approaching from the south-south-west.
Two new wave recorders have been established at Chichester Bar since 2001 (see website at www.chimet.co.uk) and further seaward in Hayling Bay since early 2003 (see website at www.channelcoast.org.). Although near real time data for each is available for each via their websites, analyses of their archived data have yet to be undertaken.
Rectilinear tidal currents in the nearshore zone adjacent to the shoreline have characteristic velocities of less than 0.5m. s-1 during spring tides, with only very limited competence to entrain sediment (Hydraulics Research, 1992; HR Wallingford, 1995). Further offshore, they can attain velocities of over 1.25m s-1 in water depths exceeding 5m. This is sufficient to mobilise non-consolidated fine gravel, as well as sand. However, as most of central Hayling Bay is floored by partially consolidated, weed-encrusted and poorly sorted flint gravel clasts, it is not considered that tidal currents alone can move material (Whitcombe, 1995). However, peak tidal currents in combination with the highest energy waves incident along this coastline, may effect some net onshore transport.
Tidal current velocities at each of the estuary entrances are much higher, especially on the ebb. Their role in transporting both sand and gravel is discussed in Section 5.
Further details on pathways of tidally induced sediment transport in and seawards of the shallow - water area of Hayling Bay are given in the section of East and Central Solent. 2. SEDIMENT INPUTS - References MapMost of the Hayling (Photo 6) and Portsea (Photo 1 and Photo 5) frontages are at or below sea-level and coast erosion has been prevented by extensive coast protection structures, especially over the past 100 to 150 years. The maintenance of beaches relies upon supply from co-adjacent sediment transport systems, but significant longshore supply by littoral drift is prevented by deep water channels scoured by rapid tidal currents at Chichester (Photo 2), Langstone (Photo 3) and Portsmouth Harbour (Photo 4) entrances. Extensive control structures (including those between Selsey and West Wittering) have greatly reduced littoral drift rates. Consequently, (excepting sites of beach replenishment) the most effective potential sediment supplies to the Portsea and Hayling beaches are now via onshore feed, mostly from sediment stores associated with the harbour entrances; supply directly from the Eastern Solent is comparatively small, and may not operate under low to "average" wave conditions. Sediment supply has been historically maintained by the progressive erosion and recession of this coastline, considered by Wallace (1990) to be in the order of 2 km since the 13th century. This view is based on sub-aqua diving inspections of archaeological sites in Hayling Bay and apparent submerged relict barrier beaches. The present planform of the Hayling coastline may result, in part, from permanent inundation due to a succession of "superstorms" in the thirteenth century. The evidence for their occurrence, and some of their effects, is recorded in Thomas (1953).
2.1 Marine Inputs - F1 F2 F3 F4 F5 F6 References Map
F1 Gravel Feed from the Chichester Tidal Delta
At Chichester Harbour Entrance (Photo 2), the ebb tidal current is of shorter duration, but significantly greater velocity, than the flood current. Net transport direction of all sediment moving into the channel is therefore offshore, thereby creating a major sediment accumulation extending 3 km to 4 km offshore (Harlow, 1980; Wallace, 1988). Its sediment volume was established at 25 million cubic metres by Webber (1979) and water depths over the delta are relatively shallow, particularly the outer and inner bars (Webber, 1979; Harlow, 1980; Wallace, 1988). Significant sorting and transport of sediments occurs on the delta by combined action of waves and tidal streams. Sedimentological analysis of the delta deposits indicate that net transport of gravel is westward, resulting in accumulation offshore West Pole (Harlow, 1980, Geosea Consulting, 2000; Whitcombe, 1995). By contrast, sand is more widely distributed both eastward and westward forming the outer bar deposits (Webber, 1979; Harlow, 1980). It has been suggested by Hydraulics Research (1980) that sand could be transported onshore from this source to feed a wide frontage on Hayling Island, extending as far west as the East Winner (ABP Research and Consultancy, 2000/2001). Conclusive evidence to support this possibility, and the volume of feed, is uncertain. Further discussion is included in the unit on Portsmouth, Langstone and Chichester Harbours, including the possible impacts of dredging in Chichester Harbour entrance channel on beach morphodynamics.
Analysis of beach volumes on Hayling Island previous to 1985 revealed a transient littoral drift divergence at, or close to, the Beach Club (Harlow, 1980; Whitcombe, 1995). Despite loss of sediment by littoral drift eastward and westward, beach levels in this immediate vicinity did not fall as dramatically as might have been anticipated. Onshore gravel feed between 6 to 13,000m3 .a-1, sufficient to offset littoral drift losses, was therefore postulated by Harlow (1980). This figure probably conceals significant annual variations, for drift rates fluctuated markedly over this period. (See Section 3). Although beach levels at the Beach Club have a long history of decline, levels were particularly low over the period 1976 to 1985 and very little new material appeared to be supplied (Harlow, 1980; Hydraulics Research, 1988; Whitcombe, 1995). Thus, it appears that onshore feed declined significantly during this period (Hydraulics Research, 1988). In 1985, a 2.2km frontage between Eastoke and Sandy Point was artificially nourished with 530,000m3 of gravel (Photo 7 and Photo 8) obtained from a source 15km, offshore Littlehampton. The recharged beach showed very variable patterns of post-project accretion/depletion (Whitcombe, 1995), but with an overall trend for depletion. These changes resulted from the lack of attainment of beach equilibrium and high initial losses due to the low permeability of recharge sediments. Some 55% of material added in 1985 had been lost offshore or downdrift by 1994 (HR Wallingford, 1993a and b;1995). It is difficult to employ volumetric analysis of the nourished beach to determine the present status of any onshore gravel feed. Initial depletion of the nourished beach in the vicinity of the Beach Club indicated that onshore feed was significantly less than loss by littoral drift, but net drift rates increased after recharge was complete (Hydraulics Research, 1988); Whitcombe, (1995) attempted to calculate a provisional beach budget for the eastern and central sectors of Hayling beach. He calculated that onshore improvement of some 12,000 m3a-1 may substantially compensate for losses caused by longshore transport east and west of the drift divide.
Evidence for onshore gravel feed is indirect and relies largely upon beach volume analysis. Information is thus of medium reliability until confirmed by direct means e.g. tracer experiments. Analysis by Harlow, (1980) was for a representative time period (1842-1976) so mean rates were accurately calculated. Because the analytical technique was indirect, short-term variations in supply and the exact path of transport were uncertain. Thus, when the hydraulic regime was altered in 1985 it became difficult to calculate the volume and spatial distribution of onshore transport. The post-nourishment beach profile record is now of sufficient length to allow this to be determined, though no reported research has been located.
F2 Sand Feed from the Chichester Tidal Delta to East Head
Sand deposited on the outer bar and East Pole Sands can be transported onshore by wave action to supply East Head (Webber, 1979; Harlow, 1987; ABP Research and Consultancy Ltd, 2000; HR Wallingford, 2000). The counter-clockwise circulation pattern involving material moving out of the estuary mouth along its western side and into it on the eastern margin (Photo 2) has been quantitatively demonstrated through sediment grain size trend analysis (Geosea Consulting, 2000; ABP Research and Consultancy, 2001). However, evidence remains indirect and is mostly based upon sampling of particle size variations over the tidal delta. As with the Winner, which is a sand and shingle bank at the harbour entrance, East Pole Sands has exhibited net erosional lowering since the late 1920s (ABP Research and Consultancy Ltd, 2000). Removal of sand by dredging from the East Winner was formerly a small scale activity, but is unlikely to be an explanation of erosional loss. The proximal sector of East Head has also diminished, with thinning and recession the dominant trend since the early 1960s (Searle, 1977; ABP Research and Consultancy Ltd, 2000,2001). Further details are given in the unit on East Head to Pagham Harbour.
F3 Feed from West Pole Sands
Onshore movement of gravel to the beach immediately takes place by periodic migration of bars during storm conditions. Bar migration has been documented by Harlow (1980;1983) using air photos; this study revealed that a gravel bank/bar originating at the inner Chichester bar in 1980 was driven onshore to form an "island" on the West Pole Sands by the summer of 1983. Orientation of the feature and its north-eastward migration suggested that waves, rather than tidal currents, were the dominant formative mechanism. Onshore movement took place chiefly during southerly storms (Harlow, 1980), but the volume of gravel likely to be moved onshore from this feature was not calculated. Historic maps and charts show periodic development of similar "islands" either side of Chichester Harbour entrance, but historical records are unable to indicate the frequency of such incursions (Harlow, 1983). Hydraulics Research (1988) state that as the onshore gravel feed to the Beach Club beach frontage declined after 1976 (and probably before that) onshore transport of sand and gravel intensified over West Pole to supply the foreshore 150 m west of Eastoke Beacon. Evidence for this was recorded net accretion in the vicinity of Eastoke Beacon, but volumetric analysis was difficult for simultaneous accretion also derived from littoral drift.
F4 Diffuse Onshore Feed: Hayling Bay
A diffuse supply pathway of kelp-rafted gravel clasts has been indicated from observations of weed-attached stones on Hayling beaches. After a major incursion of kelp at Hayling, all weed-attached clasts were counted within a single groyne compartment (Harlow, 1980). The results were extrapolated to cover the entire 6km Hayling frontage and frequency of incursions was estimated at two per annum based on observations over 1977 and 1978. From this data, an input of 120m3a-1 was calculated (Harlow, 1980). Although this analysis was based on a small sample, including several assumptions and extrapolations, the magnitude of this process was thus shown to be relatively insignificant.
Diffuse wave-driven gravel feed from Hayling Bay is also regarded as insignificant on the basis of tracer experiments and sea-bed observations and sampling. Tracer data for an exposed site south-west of Selsey indicated no onshore shingle movement (Hydraulics Research, 1974). Sea-bed investigation involving divers' observations and sediment sampling revealed poorly sorted sands, gravels and boulders with abundant attached marine flora and fauna, diagnostic of stable conditions (Harlow, 1980). This evidence indicates that gravel is probably immobile on the floor of Hayling Bay and supply from this source is only possible by small quantities of both independently mobile and weed-rafted pebbles. Whitcombe (1995) has identified potential mobility of gravel in the nearshore zone of Hayling Island under high energy incident waves with a possible onshore component. On the basis of his calculations of the sediment budget for the entire post-replenishment beach system, he produces a maximum value of 12,000 m3a-1 .
F5 Feed to Eastney from Langstone Harbour entrance Tidal Delta
Map comparisons over the period 1896-1972 indicate that Eastney Beach accreted significantly during this time increasing significantly in width as new berms were deposited (Photo 9). Beach volume analysis reveals that littoral drift is weak and site observations have indicated that a transient littoral transport divide exists at Eastney. Beach accretion is therefore explained by onshore gravel feed in the vicinity of this divide, between Eastney Barracks and the eastern end of the promenade (Webber, 1979; Harlow, 1980; Halcrow Maritime, 2000). It is likely to be wave-transported, originating from the Langstone Harbour tidal delta. Webber (1979) believed that the input was sustained by littoral drift across the bar, but Harlow (1980) was uncertain whether transport occurred along this pathway, for hydrographic surveys indicated deepening of the channel across the bar and its confinement by the East and West Winner banks, thereby magnifying the tidal flow. If this is correct, supply to Eastney Beach would derive from sediment stored east of the entrance channel and could reduce as sediment stores declined. Most authors therefore believe that coarse sediment circulation at the Langstone Harbour entrance is similar to that at Chichester, where sediments entering the channel are flushed seaward by the dominant ebb current to accumulate as a delta deposit with a marked gravel dominated inner bar. Net westward littoral drift across the bar transports coarse materials west of the entrance, where they could be driven onshore by southerly waves, primarily during storms. Circumstantial evidence indicates that dredging at both sites has deepened the entrance channels and depleted the deposits of the inner bar thereby causing recent diminution of onshore feed to the shore west of each inlet (Webber, 1979; Harlow, 1980).
The volume of accretion at Eastney was calculated by Harlow (1980) to be 12,000m3a-1 over the period 1868-1967, with littoral transport away from the drift divide estimated at 3000-4000m3a-1; thus, onshore feed was approximately 15000-16000m3a-1 for the period. Independent analysis of beach profiles covering the period 1970 to 1981 concluded onshore gravel feed to be 13,000-14,000m3a-1 (Webber, 1979, 1982, 1984). All figures are estimates based on limited survey data, so it is uncertain whether they were representative long-term rates. Beach volume analysis using map comparisons over the periods 1896-1972 (Grontmij, 1973) and 1868-1967 (Harlow, 1980) suggest that supply may previously have been slightly greater, but there has also been a significant acceleration in accretion since 1980 (Halcrow Maritime, 2000).
F6 Feed to West Winner from Langstone Tidal Delta
The changing morphology and configuration of the narrow linear West Winner gravel spit was documented by Grontmij (1973) and Harlow (1980) using historic charts and Ordnance Survey plans covering the period 1786-1972. The maintenance of the spit implied a fine balance between wave-induced sediment supply and loss to tidal currents at the Langstone Harbour entrance channel. Since 1890, the spit has narrowed, elongated and moved eastward, changes attributed to possible reduction/loss of gravel supply from the Langstone bar (Harlow, 1980). Although no quantitative evidence has been presented, the development of the West Winner spit during the twentieth century suggests maintenance of supply from the Langstone bar. The information is of low reliability for the pathways, mechanisms and volumes of transport are unknown but must involve complex wave refraction and diffraction.
The presence of numerous sub-parallel, low gravel ridges that make up the cuspate foreland foreland (or ness) behind Gunner Point indicates that onshore transport also occurs from the East Winner Sands, perhaps commencing in the sixteenth century (Tubbs, 1999).
2.2 Beach Nourishment - N1 References Map
A major beach nourishment was conducted between April and December 1985 on the 2.2km Eastoke frontage at Hayling Island between Sandy Point and Rails Lane. This was in response to progressive loss of beach volume at an estimated rate of 13,000 m3a-1 over the previous 30 to 40 years (Hydraulics Research, 1980; Harlow, 1985). The seawall was built in sections between 1939 and 1954, and was a probable contributor to drawdown and increased frequency of overtopping during surge events. The scheme used marine-dredged shingle from the Owers bank off Littlehampton, supplied to a carefully designed specification (Harlow, 1985; Whitcombe, 1995). The material was deposited on the lower foreshore by shallow draught barges and redistributed by bulldozer to form the completed beach profile (Photo 7 and Photo 8). A total volume of 530,000m3 of gravel was measured at the end of the operation, with a median grain size slightly coarser than the indigenous material; the completed replenished beach was approximately 5.6m above OD, with a crest 30m in width, tapering at each end (Grant 1986). Initially the beach face was armoured with coarse gravel, but this was rapidly assimilated into the fill, and the beach formed a natural concave profile sloping at approximately 1 in 9. (HR Wallingford, 1993b). Since construction, a substantial quantity of sediment has been lost as the fill became reworked, causing notable - Photo 10 (Mc Farland et al, 1994), but this has been countered by the periodic recycling of excess material from beyond the east and west ends of the beach, rather than importing fresh material (W.S. Atkins, 1998; Havant Borough Council, 1999, 2000). Between 1986 and 1993, an annual loss of 30,000 m3a-1 was experienced, accompanied by profile flattening (Whitcombe, 1995). Between 1994 and 1998, the trend was for annual losses of approximately 25,000m3, compensated by gains from recycling of 20,000m3a-1. If this practice were to be discontinued, this negative budget would mean that the beach would return to its pre-nourishment volume around 2010. 'Top up' supplies have been derived from bulges that periodically develop the west of the area of replenishment and from sediment moved offshore at its eastern end (Whitcombe, 1995, 1996; Havant Borough Council, 1999). The beach profile has increased in concavity, due in part to (i) the expansion seawards of the foreshore (W.S. Atkins, 1998); (ii) profile lowering, and (iii) upper berm cliffing. Whitcombe (1995) and McFarland et.al (1994) ascribe this last feature to impeded sub-surface drainage due to compaction of fines filling matrix voids below the beach surface. Further details on sedimentology are given in Section 5.
At Hayling Ferry (North of Gunner Point) small quantities of gravel have been added periodically (Webber, 1974 a and b) (WS Atkins, 1998). On Southsea beach, surplus accretion of gravel on Eastney Beach is transported to the frontage between Southsea Castle and South Parade Pier, according to the scale of loss. This transport is modest, at less than 1,000 m3a-1 (Halcrow Maritime, 2000).
Coastal Erosion - E7 References Map
E7 The Harbour Entrances
Study of Langstone and Chichester Harbour entrances by Hooke and Riley (1987) revealed significant erosion at Eastney outfall (0.48 ma-1 1870-1932), together with a shortening and thickening of Eastney Spit (Photo 1) and erosion of the proximal point of Black Point, Hayling (0.32 ma-1, 1910-1968). Webber (1974) also reported erosion between Eastney outfall and Eastney Spit. These spits and associated beaches are predominantly composed of gravel so that their erosion releases material to the tidal streams in the entrance channels. Net transport at each entrance is offshore due to the greater velocity of the ebb current so erosion at the harbour entrances cannot supply sediment to the harbour except under very infrequent combinations of southerly waves and peak flood tide velocities. Further details are given in the units on (i) East Head to Pagham Harbour, and (ii) Chichester Harbour to Portsmouth Harbour Entrance "open" coastline.
3. LITTORAL TRANSPORT - LT1 LT2 LT3 LT4 LT5 LT6 LT7 LT8 LT9 LT10 LT11 References MapThe transport of sediments in the beach, breaker and nearshore zones is considered to be wholly wave induced. Tidal currents in the nearshore zone have insufficient velocities to entrain any significant quantities of material. Rectilinear tidal currents at each of the harbour inlets are much more significant (Section 4.2), but their contribution to littoral transport along the intervening sectors of shoreline is unlikely.
LT1 Westwards Drift towards Chichester Harbour Entrance
This is discussed in the unit on East Head to Pagham Harbour. It is not considered that littoral transport can directly move sediment across the harbour entrance.
LT2 Eastward Drift to Eastoke Point
The 1985 beach replenishment scheme has had significant effect on littoral drift on the southern coast of Hayling Island, so information is assessed for before and after its completion.
Pre-replenishment. Littoral drift was determined by measuring beach volume changes from Ordnance Survey 1:2500 plans covering the period 1842-1972 and thereby calculating the minimum drift rate (Harlow, 1980; 1985). This approach was effective in determining a representative long-term drift rates. Analysis indicated a transient littoral drift divide approximately 100m west of the Beach Club, confirmed by site observations of sediment distribution within groyne compartments (Harlow, 1980; Hydraulics Research, 1980). The precise location of this divide at any one time (or for short periods) is determined by variations in incident waves, with wave refraction under contrasting sets of prevailing wave directions being particularly important (W.S. Atkins, 1998; Whitcombe, 1995), and on net drift rates to east and west. Net potential eastward drift was the lesser volume, and varied between 2,000m3a-1 and 12,000m3a-1 over the study period, with a long-term mean of 5000m3a-1 (Harlow, 1980). Drift increased at Eastoke Point due to changed coastal orientation; Hydraulics Research (1980) suggested a mean rate of 10,000m3a-1 for this sector.
Post-replenishment. Littoral drift rates increased by several magnitudes after replenishment for the following reasons:
Before implementation of the scheme, both Harlow (1980) and Hydraulics Research (1980) assessed previous experience with beach replenishment and anticipated initial losses from the nourished beach would be significantly greater than long-term rates. This was confirmed by H.R Wallingford (1995) and Whitcombe (1995) who reported that slightly more than 45% of the immediate post-replenishment volume was in place in 1994, i.e. an annual loss of 30-40,000m3a-1. Data from W.S. Atkins (1998) and Havant Borough Council (1999) indicate a loss of 27,100m3a-1, 1991-1998, declining from 46,000m3a-1, 1991-94 to 25,000m3a-1, 1996-98. This is based on regular beach topographic surveys and post-1992 data on recycling inputs. The proportion of this quantity which is lost directly offshore is not known, though there is circumstantial evidence (eg. from gravel - filled gutters across the sandy foreshore) to indicate that this process operates.
The result of rapid eastward drift since replenishment has been significant accretion at Eastoke Point, particularly of sand and fine gravel. Material has been artificially recycled from this accumulation area and replaced over the replenished beach. In January 1991, a terminal rock reinforced groyne was completed at Eastoke Point, designed to restrict littoral drift into the Chichester Harbour entrance channel, thereby facilitating retention of sediment for recycling. A rock revetment was added to the frontage of Eastoke Point in 1992 to provide further beach stability.
The Eastoke Peninsula Beach Management Strategy Plan (Havant Borough Council, 1999, 2000) proposes: (i) a continuing programme of annual gravel recycling, taking accretion excesses to areas of deficit over the existing beach; and (ii) quinquennial recharge, using material obtained by dredging the approach channel to Chichester Harbour. The initial quantity will be 50,000m3, followed thereafter by inputs of approximately 25,000m3. The major advantage of this approach is that it retains material in the local sediment transport sub-cell. Without recycling, rates of loss would probably slowly but progressively increase, to at least 30,000m3a-1 by 2005 (Havant Borough Council, 1999, 2000).
LT3 Eastoke Point to Black Point (Sandy Point Spit)
Site observations indicate northward drift from Eastoke Point to Black Point (Harlow, 1980). The volume of sediment involved is probably small because wave action in the harbour entrance is relatively weak and net drift (particularly on the upper beach) is interrupted by groynes. Prior to the recharge scheme to the west, depletion at a rate of almost 8000m3a-1 was indicated by beach volume analysis (Hydraulics Research, 1980; Beard, 1984) but littoral drift was probably less rapid, for some of this material was probably diverted to the tidal channel. Studies by Harlow (1980) revealed depletion of the Black Point Spit, at 1000 to 2000m3a-1 between 1932 and 1967, which was preceded by accretion at 1000-5000m3a-1 between 1842 and 1932. Thus it appears that potential littoral drift may be between 1000m3a-1 and 8000m3a-1, although transport may be effectively prevented by coast protection structures when beaches are depleted. It is unlikely that the beach replenishment scheme has had a significant effect on this coastal segment as the major area of accretion is at Eastoke Point; further downdrift transport is more likely to enter Chichester Channel, rather than along this south to north shore (Hydraulics Research, 1988). Sediment supply has been further regulated with the completion of the terminal rock groyne and rock revetment at Eastoke Point (1990-1992). Quinquennial dredging of the approach channel to Chichester Harbour will provide a small quantity of sediment for periodic replenishment, if required (Havant Borough Council, 2000). Alternatively, any surplus will be removed to Eastoke Beach. A mix of coast protection structures between Black Point Spit and the western limit of the Sandy Point Nature Reserve currently maintain beach levels, but they remain vulnerable to extreme wave conditions. Sediment stored here might therefore rapidly increase littoral drift throughput in the event of any breakdown of stability, and increased erosion.
LT4 Westward Drift to Gunner Point
The presence of the East Winner banks, eastward of the Langstone Harbour Channel, is evidence of the long-term operation of westwards drift, particularly when waves from the east-south-east and south-east operate.
Pre-replenishment. Net westward drift from the Beach Club drift divide to Gunner Point was determined by Harlow (1979; 1980) from field observations of sediment distribution in groyne compartments, combined with map and air photo evidence. Detailed assessment of littoral transport was undertaken by volumetric beach analysis using Ordnance Survey plans. Drift rates were was calculated for nine separate periods between 1842 and 1976, which revealed significant spatial and temporal variability. Generally, the drift rate increased westward, sustained by progressive eastward beach erosion; most transport was considered to take place during high-energy waves approaching from the south-east or east-south-east. Further west towards Gunner Point, drift declined due to reduced wave energy resulting from shelter from the Isle of Wight. Maximum drift of over 80,000m3a-1 was recorded for the period 1931-1940, but rates fell subsequently due to sediment shortage as supply beaches became depleted. Immediately west of the Beach Club, a mean potential westward drift of 17,000m3a-1 was calculated. This increased to 20,000m3a-1 one kilometre to the west and declined to 19,000m3a-1 at the Beachlands surface water outfall (constructed in 1973 (Webber, 1974b)). Further west, it declined to 13,000 m3 at Beachcot (in part due to the obstructing effect of the outfall) and increased to 15,000m3 at Gunner Point (Harlow, 1980; Hydraulics Research, 1980). These figures were long-term means, the period 1842-1976 and therefore likely to be representative. Other calculations of drift over limited time periods revealed rates of 14,000m3a-1 (1967-1976) and 14,000m3a-1 (1976-1978) at Beachcot. Only the figure for Beachcot was probably unrepresentative of drift immediately before replenishment because coast protection structures at this site interrupted transport of medium to coarse gravel on the upper beach, so that drift was restricted to fine gravel and sand on the mid/lower beach (Harlow, 1980). A modest increase in drift rate probably occurred west of Beachlands after 1979, with the installation of a piped outfall. However, the protection of the 'Inn on the Beach' in 1976 added a limited impediment to westwards drift (Photo 11), necessitating the subsequent construction of a revetment (Photo 12) and groynes (Photo 13) to modify downdrift beach recession. Gunner Point has a long history of fine gravel and sand accretion extending back about 380 years; Harlow (1980) suggests that, since about 1850, the average accretion rate has been about 5,000 m3a-1. Mean High Water Mark at Gunner Point has moved 195m seawards since approximately 1880 (Havant Borough Council, 2000). HR Wallingford (1995) suggest progradation of 0.8m3a-1 since 1910 (90,000m3, 1976-1992 or 5,000 m3a-1). A total of between 10-15,000m3a-1 of sediment either passed around Gunner Point, or was transported directly into the Langstone tidal channel, between approximately 1880 and 1980. (Harlow, 1980). The north-trending, sandy gravel ridge that defines the eastern boundary of the Kench may be a remnant spit pre-dating the development of the Gunner Point foreland.
Post-replenishment. Volumetric analysis using measured beach profile data revealed that drift was 37,000m3a-1 (February 1986 to February 1987) and between 6,000 and 13,000m3a-1 (1987-97) immediately westward of the Beach Club on the replenished beach (Hydraulics Research, 1987, 1988; HR Wallingford, Whitcombe, 1995). Initially, the replenished segment was not groyned and Harlow (1985) reported significant accretion in the old groyne field and revetment to the west. Whitcombe (1995) and W.S. Atkins (1998) note this frontage subsequently steadily lost material, (see section 5.4). Hydraulics Research (1988) reported that accretion resulting from westward movement of replenished material was not apparent very far west, at that time, suggesting that drift was much less rapid in the groyned coastal segments. A peak drift rate was recorded at the western extremity of the replenished beach immediately following its construction, where the change in alignment (caused by replenishment) was greatest. Volumetric analysis derived from beach profiles for the periods 1986-1990 and 1993-97 indicated that westwards littoral drift declined to between 9,500m3a-1 (Whitcombe, 1995) and 4,800m3a-1 (W.S. Atkins, 1998). Whitcombe (1995, 1996) identified the transfer of a "wedge" of gravel west of the replenished beach. Since 1990, the beach westwards to the Fun Fair and Norfolk Crescent has shown net accretion (W.S. Atkins, 1998), thus representing a potential source for future re-cycling operations to maintain the Eastoke replenished beach. Havant Borough Council (1999) estimate that net westwards drift along this entire sector, west to the Inn by the Sea, is approximately 20,000 m3a-1
LT5 Reverse Drift at Gunner Point
Analysis of beach profiles measured from air photos for the period 1973 to 1981 revealed short-term alternations of accretion and erosion at Gunner Point, which contrasted with the well documented history of accretion by westward drift up to 1972 (Hydraulics Research, 1988). Between 1982 and 1987, significant erosion of Gunner Point coincided with net accretion further east on the beach in front of the golf course; this has been sustained up to the late 1990s (Havant Borough Council, 2000) but its causes are uncertain. By assuming that the measured changes resulted from littoral drift, a local reversal of littoral drift between Gunner Point and Beachcot is implied (Hydraulics Research, 1988; H R Wallingford 1995; W.S. Atkins, 1998). A local eastward drift is also supported by field observations of sediment distribution in groyne compartments. As it was identified for a relatively short time period (5 years), the longevity of this phenomenon is uncertain. Because the possibility of beach volume changes occurring by onshore-offshore transport could not be assessed, information specific to littoral drift was only of medium reliability. Continued monitoring over a longer time period is necessary to determine whether the reversal is simply a short-lived local variability or whether it represents a major change in the hydraulic regime (W.S. Atkins, 1998). It was suggested by Harlow (1984), Hydraulics Research (1988) and HR Wallingford (1995) that the pattern of drift may have altered as a result of dredging of the East Winner Bank and Langstone Bar. Dredging could have locally increased water depth and altered wave refraction patterns so that the directional distribution of wave energy was altered at the shoreline. A significant research effort involving hydrographic survey, wave height and frequency data and refraction analysis would be required to confirm this hypothesis.
LT6 Northward Drift from Gunner Point Along Langstone Harbour Entrance Channel
Analysis by Harlow (1980) indicated a long-term mean westward drift of 15,000m3a-1 to Gunner Point where mean accretion of 5000m3a-1, primarily gravel, was recorded. Much of the remaining 10,000m3a-1 of sediment was transported into the Langstone tidal channel and then flushed seaward to accumulate on the bar or on East Winner. However, complex but transient littoral drift reversals, sustained for periods of 10-30 years, appear to be characteristic of the western Hayling frontage (W.S. Atkins, 1998). Their causes are probably due to changes in wave climate and/or sediment supply, as well as the convex form of the Gunner foreland. Harlow (1979;1980) suggested that significant quantities of gravel are transported around Gunner Point in pulses during storms from the south-east, whereupon they form large scale sinusoidal bulges (preceded by short-term erosion phases) which migrate northward by littoral drift along the eastern side of the Langstone entrance (Photo 3). The base data for this hypothesis is of medium reliability though the observed trend is confirmed by W.S. Atkins (1998) for the 1990s. These features were traced by Harlow (1980) on maps (1842-1976) and on aerial photos (1976 and 1977); northward migration was measured at 10ma-1. No major gravel accumulations exist in the Langstone Entrance Channel so it is postulated that gravel is progressively lost to seaward transport as it moves northward towards the recurved distal point of the Hayling ferry spit. The growth of this spit between the late seventeenth and mid-nineteenth centuries, together with the more recent expansion of Gunner Point, accounts for the historical narrowing of the entrance channel to Langstone Harbour (Tubbs, 1999).
Beach profile analysis by Hydraulics Research (1988, HR Wallingford, 1995) indicated erosion of Gunner Point and also demonstrated continued gravel accumulation on the "bulges" in Langstone entrance suggesting that northward drift still prevailed. If this is correct, a littoral drift divide has also developed at Gunner Point, although its exact location has yet to be established. HR Wallingford (1995) suggest that loss of foreshore material at Gunner Point, commencing between 1985 and 1990, may be the result of dredging the approach channel to Langstone Harbour and/or removal of sediment from the west bank of the East Winner. W.S. Atkins (1998) propose that drift rates to the east of this divide, with a net eastwards component since 1982, are roughly equivalent to those with a net westwards direction, prior to that date. Actual quantities are sensitive to year to year variations in wave climate. For the period 1982-1996, net eastwards drift of gravel (only) is calculated to be in the order of 3,700m3a-1.
LT7 Eastward Drift to Langstone Channel
Harlow (1980) suggested that much of the material fed to Gunner Point was transported westward into the Langstone tidal channel, whereupon it was flushed seaward and deposited on the East Winner (primarily sand) or Langstone bar (primarily gravel). This drift potential probably averaged less than 10,000m3a-1. Circumstantial evidence of drift reversal at Gunner Point (Hydraulics Research, 1988, HR Wallingford, 1995) indicates a change in the hydraulic regime, so it is possible that transport is eastward away from the Langstone tidal channel. Despite this, northward drift on the east side of Langstone Channel appears to be fed from Gunner Point, so supply to Langstone tidal channel could be sustained by erosion of Gunner Point if the littoral drift divide is situated eastward of the Langstone entrance. Transport into the Langstone tidal channel is therefore uncertain and further research work is necessary to ascertain the present status of this pathway.
LT8 Eastward Drift at Eastney
Beach volume analysis using map comparisons over the period 1868-1967 indicated a littoral drift divide at Eastney (Harlow, 1980). Although drift was relatively weak and not closely defined, the divide appeared to migrate 500m westward between 1868 and 1960. Eastward drift to Eastney Point was calculated as being 1,000m3a-1 to 2,000m3a-1 (Harlow, 1980). Using similar techniques Webber (1982) determined eastward drift of 8000m3a-1 from the littoral transport divide. This drift reversal was attributed in part to wave refraction around the West Winner bank (Harlow, 1980). This is supported by Grontmij (1970; 1973), who calculated the wave energy flux at the Eastney shoreline and determined a capacity for weak eastward littoral drift. The analysis included detailed and reliable wave measurements but simplistic refraction and wave direction calculation. Wave energy at the shoreline was not converted to littoral drift potential using sediment transport equations, thus the information is qualitative and of medium reliability. In 1966, sewage outfalls at Eastney Point were encased by culverts and an additional outfall constructed (Webber, 1974a, 1984). These structures intercepted littoral drift, causing gravel accretion of 4,000m3a-1 (1971-1982), a volume believed to be consistent with the contemporary drift rate for the entire section (Webber, 1982). Interception of drift by these structures has continued to 2001 (Photo 3). HR Wallingford (1995) calculate a drift rate of 6,800m3a-1, with some net accretion in front of Fort Cumberland. Littoral transport is probably more rapid at Eastney Point due to the rapid change of coastline orientation and the presence of a sea wall, which causes reflection of wave energy. (Webber, 1982,1984) The outfall structures effectively prevent further littoral drift, so this transport pathway is either towards a zone of accretion west of the outfall or offshore into the Langstone Channel. Halcrow Maritime (2000) calculate an updrift input of approximately 13,000m3a-1, with the implication that a significant proportion might move into the mouth of Langstone Harbour entrance channel.
LT9 Drift from Eastney Point to Eastney Spit
Northward littoral drift from Eastney Point to the end of Eastney Spit has been indicated from site observations including sediment distribution in groyne compartments (Webber, 1974). Strong tidal currents immediately offshore provide the possibility for material on the lower foreshore to become entrained and lost to the tidal channel, so that progressive loss occurs northward; thus only a very limited supply to Eastney Spit is possible. The motive force is primarily waves from the south and south-east (particularly occasional storms), with drift fed by sediment passing around Eastney Point (Webber, 1974). Webber (1982) stated that relatively little gravel passed around/over the outfalls, so it is possible that pulses of sediment periodically overflow and bypass these obstructions. Supply around Eastney Point has declined since 1966 when the outfalls were modified, yet northward drift has continued and appears to be sustained by beach erosion in their lee (Webber, 1982).
LT10 Westward Drift, Eastney to Southsea Castle
There is some uncertainty over longshore drift rates west of the Eastney drift divide. Harlow (1980) proposed a rate of approximately 2,000m3a-1, whilst Webber suggested 6,000m3a-1 and HR Wallingford (1997) calculate an intermediate figure of 3-4,000m3a-1. All are agreed that rates decline rapidly westwards, to no more than 300m3a-1 between South Parade Pier and the coastal salient of Southsea Castle. Rates are determined by locally-generated waves, affected by refraction induced by the projecting form of the West Winner spit. Swell waves do not reach this sector of coastline, because of protection afforded by the Isle of Wight and partly and wave energy decay created by the sand and gravel banks of Horse and Dean Sands. Weak drift rates are confirmed by the long history of beach accretion between Eastney barracks and the Firing Range since at least the mid-nineteenth century. Shoreline advance has accelerated since the 1980s with 145,400m3 added to beach volumes (16,200m3a-1) between 1983 and 1992 (HR Wallingford, 1995). The source of supply of this sediment, which is dominantly gravel, is uncertain; they are most likely to drive from onshore feed from the Langstone tidal delta. There may also be some cyclical pattern of supply. Further evidence of weak longshore and cross-shore transport comes from analyses of beach clast size and shape. This reveals little discernible sorting (Grontmij, 1973; University of Portsmouth, 1990-2000).
Between the Royal Marines Museum and South Parade Pier (Photo 14), the beach width diminishes rapidly, but there has only been slight recession in the position of Mean Low Water over the past 40-50 years. The beach between South Parade Pier and Southsea Castle (Photo 1 and Photo 15), where there is effectively no beach accumulation because of the adjacent presence of deep water, has accumulated somewhat less than 600m3a-1 since the early 1980s (Halcrow Maritime, 2000).
Old maps and early editions of Ordnance Survey plans show the extent of the Great and Little Morass, former swampy areas between Old Portsmouth and eastern Southsea. Unpublished borehole data of sediment stratigraphy reveals alternating horizons of peat, fine gravel and sand, to depths of up to -2.8mOD. This is suggestive that they are former lagoons and thus the possibility that the entire length of beach along the southern Portsea Island coast has a barrier origin. The present day Canoe Lake, at Southsea, is a probable lagoon remnant. If this hypothesis has validity, a part of the backshore beach store derives from punctuated offshore to onshore barrier translation during recent centuries, probably longer (Wallace, 1990). Backshore and foreshore elements are therefore distinct in terms of their morphosedimentary properties.
LT11 North Westward Drift, Southsea Castle to Portsmouth Harbour entrance
Visual observations of gravel built up against obstructions over the past 10 to 20 years indicates very weak net northwest drift on the beach between Southsea Castle (Photo 1) and Clarence Pier (Photo 5). Documentary evidence of this process is provided by comparison of Ordnance Survey maps and charts covering the period 1868-1972 (Fishbourne, 1977; Hooke and Riley, 1987), e.g. the construction of Clarence Pier created an 18m retreat of the beach to the north west between 1868 and 1896. It is probable that the pier interrupted littoral drift to the north-west resulting in lee side scour. This evidence suggests that the historical direction of net littoral drift along the frontage is north-westward, which corresponds with present day observations (Atkinson, 2000). No quantitative information is available for this transport pathway, but Harlow (1980) suggested that littoral drift is weak because wave height is limited by fetch and nearshore diffraction/refraction; and there are also strong tidal currents in Portsmouth Harbour entrance. In view of the lack of bypassing at Southsea Castle, much of the gravel supply to this beach must derive from onshore movement. Halcrow Maritime (2000) note slight beach narrowing since the 1860s. In recent years, modest quantities (<1000m3a-1) of sediment have been periodically added to replenish winter losses. Previous periods of beach depletion have resulted in overtopping (e.g. 1818 and 1871) during exceptional storm conditions.
There is little opportunity for beach development between Clarence Pier and the Round Tower, because of several centuries of defence structures; a rapid increase in offshore water depth and very low drift rates. The exception is the small fillet of gravel, forming the "Hot Walls" beach; here there has been slow, but progressive, loss of beach volume over recent decades. Partial causes may be "drawdown" into the adjacent dredged navigation channel and wave run up patterns influenced by the numerous ship movements through this narrow entrance (Halcrow Maritime, 2000). The former has been gradually deepened, and the latter have greatly increased over recent decades.
Drift along this frontage is westwards, but at low rates and volumes as incident waves have characteristically low heights and tidal current velocities over the narrow foreshore do not have the capacity to entrain gravel. Littoral transport is probably not in excess of 300m3a-1 (HR Wallingford, 1995); this material is transferred to the harbour entrance channel and moved seawards by the ebb current. Maximum tidal current velocities are capable of moving material up to 50mm diameter (Halcrow Maritime, 1999). Thus there is no littoral drift transfer to the western (Haslar) harbour shoreline, and the entrance channel forms a well defined transport cell boundary (Bray et al., 1995).
The spit on which Old Portsmouth has been developed (Photo 4 and Photo 5) is indicative of long sustained longshore transport towards the harbour mouth. However, it may have a complex history involving barrier migration, breaching and subsequent re-orientation.
Overall, the littoral transport regime is a complex one, with several unusual features. Foremost amongst these are: (i) the interaction of waves and tidal streams at the harbour entrance, creating local patterns of wave diffraction and refraction and thus spatial variation in wave energy and sediment transport potential; (ii) pronounced seaward movement of coarse sediment, formerly building an ebb tidal delta at the harbour mouth, and thus providing an offshore store; (iii) the possible presence of a continuous, or semi-continuous, barrier beach, now partly submerged (iv) several artificial influences, particularly shoreline defence structures, aggregate extraction navigational dredging and shipping movements.
4. SEDIMENT OUTPUTS
4.1 Transport In The Offshore Zone - O1 O2 O3 References Map
01 Westward Transport to East Winner
Sand is much more easily transported than gravel, and sedimentological studies indicate that supply from the Chichester tidal delta is transported over a wide offshore area. Webber (1979) indicated that significant quantities of sand were transported eastward from Chichester tidal delta via the outer bar and Hydraulics Research (1980) stated that sand may be moved westward from the Chichester tidal delta to feed the East Winner bank. This knowledge is of low reliability, as no direct evidence of this transport pathway has been presented and existing information is based on limited interpretation of offshore sedimentology, coupled with more general knowledge of wave and tidal conditions. Comparison of sea-bed levels and sediment volumes by Foss (1978) using six hydrographic charts covering the period 1842-1976 revealed erosion of the sea bed in the east of Hayling Bay (West Pole) and deposition in the west (East Winner). Webber (1979), HR Wallingford, (1995) and ABP Research and Consultancy (2000) identify an anticlockwise circulation of sand around the East Pole, which allows some of this sediment, swept into the outer esturary channel from the East Winner, to be transported back to the West Wittering foreshore. A clockwise circulation may operate around West Pole, inferred by the periodic addition of fine sand and gravel to the beach at Eastoke Point (HR Wallingford, 1995; ABP Research and Consultancy Ltd, 2000) and the Beach Club site on Hayling Island. This pattern of erosion and accretion therefore indicates an overall sediment transport pathway from east to west in Hayling Bay.
02 Westward Sand Transport in the Solent
A series of sand waves were identified in the Eastern Solent by the British Geological Survey (1989). The information was derived primarily from surveys using sonar combined with some sediment sampling. The morphology of the sand waves indicated net westward transport. (See further discussion in unit on sediment transport in the East and Central Solent).
03 Sand Circulation on East Winner
Observations at low water combined with air photo interpretation revealed that the whole area of the East Winner bank was covered by ripples, sand waves and low dunes (Harlow, 1980). These features are characteristic of environments of high sediment mobility and the larger features suggested gyratory sand motion. Harlow (1980) suggested that sand circulated in an essentially closed system, because it was unable to move westward across the Langstone tidal channel. Thus, net transport, by tidal ebb currents, on the western flank is offshore, whilst on the eastern flank transport it is onshore, moved by wave action, and westward at the northern flank by waves and tidal currents combined. Dredging of sand from the East Winner banks has been a long-term practice, only very recently discontinued. However, there is no data on quantities removed, as all dredging records also include losses from the maintenance of the access channel across the outer bar.
4.2 Estuarine Outputs - EO1 EO2 EO3 References Map
The narrow entrances of Portsmouth, Langstone and Chichester harbours confine the tidal flow of large volumes of water into and out of the harbour, thereby setting up strong tidal currents which extend several kilometres offshore. Residual current speeds are in the order of 0.15-0.16m.s-1 Analysis of tidal currents at Portsmouth Harbour (Hydraulics Research, 1959; Harlow, 1980), Langstone Harbour (Grontmij, 1973; Portsmouth Polytechnic, 1976; Harlow, 1980) and Chichester Harbour (Webber, 1979; Harlow, 1980; Wallace, 1988; HR Wallingford, 1995) reveal that the ebb current at each entrance of shorter duration, but significantly greater velocity, than the flood current, resulting in the scour of channels to depths of up to 20m. Thus, net transport of sand and gravel entering the tidal currents is offshore and dispersal and deposition of these materials occurred as tidal currents weakened. Further details of tidal regime and current velocities are given in the separate unit covering the three harbours. Output of sediment by navigation dredging has a strong impact on the quantities moved both in and out of each entrance. This is discussed in detail in the unit on Portsmouth, Langstone and Chichester Harbour.
E01 Chichester Harbour Entrance (see Introduction to Estuarine Outputs)
Sediment supply to the main tidal channel is possible by either eastward or westward littoral drift at the harbour entrance. (See Section 3 and the unit covering East Head to Pagham Harbour). Sediment is supplied at rates up to 70,000m3a-1 from East Head via the Winner gravel bank or directly from the distal end of the spit (Webber, 1979; ABP Research and Consultancy Ltd, 2000). Supply from this source has reduced significantly with the progressive and comprehensive protection of the shoreline of Bracklesham Bay since 1874 (Harlow, 1980) and has probably now virtually ceased following extension and upgrading of the groynes at West Wittering in the mid 1980s. Eastward littoral drift at Eastoke Point supplies sediment to the tidal channel at a minimum rate of 5000m3a-1 (Harlow, 1980). Beach replenishment at Hayling Island initially enhanced eastward drift to 45-50,000m3a-1 (Hydraulics Research, 1988), later reducing to a mean of 11-15,000m3a-1 (Whitcombe, 1995; Havant Borough Council, 1999). Although some of this material accumulated at Eastoke Point, significant quantities were lost to the entrance channel and flushed offshore to be deposited in the tidal delta. A rock groyne and rock armouring, designed to retain sediment on the updrift beach, were completed at Eastoke Point in 1991/2. Compared with the other harbours, the entrance channel to Chichester Harbour is deeper and has higher ebb and flood tidal current velocities. Sediments entering the tidal channel are flushed offshore and deposited at varying distances from the entrance depending upon sediment size, wave conditions and water depth. Gravel can be transported a maximum of 1 to 2km offshore and sand a maximum of 3.5km offshore (Webber, 1979). Dynamic change of the plan shape of West Pole Sand since the 1960s is a result of variations in the balance between erosion and deposition (Whitcombe, 1995). Sediment sampling by Harlow (1980) revealed a series of sedimentary zones related to current velocity and suggested that wave action could mobilise surficial sediments. The volume of sediment transported offshore by tidal currents was not calculated, but fresh supply could be estimated from littoral drift inputs at the entrance, as tidal current bedload movement of sediment from within the harbour is negligible. Contemporary supply to the tidal channel consists almost entirely of eastward drift of mostly fine gravel and sand from Eastoke Point. Virtually no fresh material is now introduced into the offshore transport pathway in the tidal channel; sediments transported will be those periodically pushed onshore from the tidal delta by storms from the south. The tidal delta is therefore a finite resource and any outputs -in particular, dredging- are likely to represent a net loss to the local sediment budget. It is significant that the Winner lowered by 0.5-1.0m between at least 1926 and the present time, contributing to an increase in the pivotal movement and erosion of East Head, and thus expansion of the cross-sectional area of harbour mouth (ABP Research and Consultancy, 2000). The main channel has deepened, partly in consequence of increased tidal current scour. Whitcombe (1995) calculated that the outer Chichester bar lost some 90,000m3 of sand, 1989-1995. The Winner has also moved northwards in recent decades (Geosea Consulting, 2000). It is presumed that the progressive increase in the width and depth of the Chichester Harbour entrance reflects its adjustment to a more stable condition, in equilibrium with hydrodynamics and hydraulic regime. It is not clear if this has yet been fully attained (ABP Research and Consultancy, Ltd, 2000).
Further discussion, including details of dredging, is contained in the unit covering East Head to Pagham Harbour. See also the unit concerned with Chichester, Langstone and Portsmouth Harbours.
E02 Langstone Harbour Entrance (see Introduction to Estuarine Outputs)
Tidal currents at the Langstone entrance have a similar effect to those at Chichester in causing net offshore transport of sediments entering the channel by littoral drift. Analysis of beach volume changes by Harlow (1980) indicated mean westward drift into Langstone Entrance from Gunner Point of 10,000m3a-1 and eastward drift of 1000m3a-1 from Eastney Point. These inputs to the tidal channel may have ceased or diminished recently for littoral drift has reversed at Gunner Point since 1982 (Hydraulics Research, 1988; HR Wallingford, 1995); over the longer term, net westwards drift has been reduced by modifications to sewage outfalls (Webber, 1982) and coast protection. (See Section 3). Sediment sampling indicates that deposition patterns are determined by sediment size, with fine sands (mean grain size of 0.17mm diameter) being transported to the East Winner bank; very fine sand is moved to the outer limits of the tidal delta; gravel is transported along the tidal channel to an inner bar, and coarse and medium sand is taken to the tidal delta seaward of the inner bar. The Langstone tidal delta may therefore also be a finite sediment store resulting from reduced littoral drift feed to the tidal channel. Whitcombe (1995) has analysed changes in the plan shape and volume of the East Winner, 1976-1992. This reveals fluctuating expansion and regression (accretion and erosion), involving losses and gains of an average of 50,000m3a-1. The cause of these changes would appear to be shifts in the alignment of the outer Langstone entrance channel. See unit on Chichester, Langstone and Portsmouth Harbours for further discussion.
E03 Portsmouth Harbour Entrance (see Introduction to Estuarine Outputs)
The Portsmouth Harbour entrance is the most sheltered of the inlets thus littoral drift input to the tidal channel is very low (Halcrow Maritime, 2000). Although its tidal prism is smaller than for the other harbours the Portsmouth entrance is considered extremely stable and easily capable of flushing out any arriving littoral drift (Harlow, 1980; Halcrow Maritime, 1999; Universities of Newcastle and Portsmouth, 2000). Due to diminished wave energy, the ebb tidal current transports material further offshore than at Chichester or Langstone. Supply from the Gosport frontage is transported offshore to Spit Sand and supply from the Southsea frontage is transported to Horse and Dean Sand (Harlow, 1980; HR Wallingford, 1997) (See Section 3). The Portsmouth Harbour entrance tidal channel thalweg is therefore a fixed boundary between (i) the Selsey-Bracklesham, Hayling and Portsea and (ii) the Hamble to Gosport sediment transport sub-cells (Bray et al., 1995). It is probable that Horse and Dean Sand is the sediment sink for the Bracklesham, Hayling and Portsea cell (Harlow, 1980). Supply volumes have not been computed for this pathway but contemporary supply to Horse and Dean Sand must be minimal because: (a) littoral drift is very weak at Southsea, so input to the tidal channel by westward drift must be negligible (Grontmij, 1973; Harlow, 1980; Webber, 1982; Halcrow Maritime, 2000); (b) the entrance and approach channel is frequently dredged to maintain a depth of at least 12m, thereby entailing output of sediment from the transport pathway.
Harlow (1980) suggests that Langstone Harbour channel may now be the boundary of the Bracklesham/Hayling sub-cell because little material crosses the channel and significant accretion has occurred in recent times immediately east at Gunner Point and the East Winner.
5. SEDIMENT STORES AND SINKS : BEACH MORPHODYNAMICS - References Map
5.1 Sediment Character and Distribution
Hayling and Portsea beaches are mostly composed of fine to coarse flint gravel, which forms a steeply sloping upper (storm) beach. This grades seawards into a mixed sand and gravel mid-beach, with a gently sloping sandy lower foreshore exposed at low water. This sequence varies according to location, for example a wide sandy lower foreshore exists at low tide at Eastoke Point and along much of Hayling Island, whilst Portsea beaches are predominantly gravel. Wave conditions affect sediment distribution and beach morphology with a clear distinction between the coarse upper beach and sandy foreshore during moderate or calm sea states, and intermixing of sediments during and after storm conditions (Harlow, 1980; Hydraulics Research, 1980; HR Wallingford, 1995;1997).
Detailed sampling of beach sediments has been undertaken by Grontmij (1973), Butterfield (1978) and Harlow (1980). The most comprehensive analysis is by Harlow (1980), which involved review of previous research as well as original sampling. His approach tried to identify an "overall ultimate" pattern in spite of short-term variability of surface sediment grading. The Hayling beach surface was sampled after a major storm during which the entire beach face was remodelled by waves (thus the sampling was directed to reveal the pattern to which the grading was trending in the long-term). Analysis revealed that both gravel size and sorting increased downdrift on Hayling and Portsea beaches, a pattern also repeated in groyne compartments. The sandy lower foreshore was sampled by Butterfield (1978) who found the coarsest sand at the east Hayling littoral drift divide, with improved sorting in both directions away from this location. Sand size and sorting declined with distance offshore. Sediment distribution with depth was investigated on the West Pole Sands using a water jetting technique (Harlow, 1980). This indicated vertical sorting with an active "clean" sand layer up to 0.5m thick overlying an inactive, darkened (anaerobic) sand to 1.5m below the surface, beneath which a downward coarsening gravel deposit occurred. Sediments thinned to the west, with only a 0.4m thick active sand layer resting upon bedrock (Bracklesham Beds). Sampling on Gunner Point revealed very well sorted wind blown sand (median particle size of 0.177mm) indistinguishable from sands on East Winner, the probable source area. Sampling of beach gravel at Eastney has proven an absence of angular clasts, with over 50% of pebbles semi-rounded or well-rounded and flatness poorly developed (Grontmij, 1973; University of Portsmouth, 1989-2000). No consistent longshore trends of clast shape and size were identified, which suggested that beach-near shore exchange (ie. cross-shore transport) is more active. However, only indistinct cross-shore sorting of flint clasts was apparent, under a range of wave energy conditions.
Information on beach sediments is therefore available, but as samples were collected on relatively few occasions so it is possible that they were representative only of short term conditions. Harlow (1980) compensated for this by sampling immediately after a storm; and University of Portsmouth data was collected over twelve consecutive years Grontmij (1973) data was collected on three separate sample occasions. However, the timing in most cases was not adapted to prevailing tidal and wave conditions.
The major beach nourishment at east Hayling completely altered the sedimentology of this and adjacent beaches. (See Section 2). The imported material was rapidly sorted by waves and a pattern resembling the previously existing grading quickly became established on the seaward face of the fill. A sandy lower foreshore developed seaward, with a mixed sand and gravel mid-beach and a coarse gravel-dominated upper storm beach. Analysis of 220 samples taken immediately following project completion to check the design specification revealed that gravel clast size increased from the toe of the beach to the crest, and also away from the littoral drift divide (Harlow, 1985). Although sorting of the beach surface was rapid, much of the imported material remained unaffected by wave action and became compacted to 2000 kg/m3 (compared to original specification of 1750 kg/m3) resulting in the development of an upper beach cliffed erosion scarp - see Photo 10 (Hydraulics Research, 1987; McFarland et al, 1994; Whitcombe, 1995 (1986-89); W.S. Atkins, 1998). This crestal scarp initially retreated at 2-3m.a-1, but since reduced to 1.8 m.a-1 (1990-95) and to 0.5m.a-1 after 1995 (Havant Borough Council, 1999; 2000). Problems with compaction and profile variability may result from the high proportion of fines amongst the imported fill, which inhibits sub- drainage (Whitcombe, 1995). The initial rapid sorting on the beach face was modelled with flume experiments which suggested that the armouring of the beach face with coarse pebbles was not necessary, for natural self-armouring was rapid and occurred with a grade of material selected by incident wave conditions (Grant, 1986). Total composition and size-range of sediments has been affected by progressive profile adjustment and beach overtopping at Eastoke under severe storm conditions in 1994 and 2001.
(Post 1996 profile data are available from Havant Borough Council, but are not in any analysed form).
5.2 Sediment Sources
Harlow (1980) calculated that cell-wide coast erosion since the mid nineteenth century could have supplied 1.4 million cubic metres of sediment to Hayling Island, but as net sediment throughput was 15 million cubic metres it was hypothesised that the remainder was provided by wave transported feed from offshore or longshore sources. Ridges composed of hard cemented gravel have been identified several kilometres offshore (Harlow, 1980; Wallace, 1990 Whitcombe, 1995) and it is suggested they are relic possible barrier beaches abandoned by sea-level rise. Before immobilisation these were possibly originally significant sediment sources in the local budget. Gravel on Eastney Beach is similar to offshore gravel deposits revealed by sampling and these could also have been transported onshore by waves (Grontmij, 1973). Although significant supply from offshore may have been a factor in the past, no major feeds from offshore (except from the tidal deltas) are indicated by existing analyses. Supply may have ceased due to exhaustion of offshore deposits, or absence of suitable transport mechanisms.
5.3 Beach Volumes
Surprisingly few estimates of gross beach volumes are quoted in the literature. Instead, most analysis has concentrated on volumetric change, measured using beach profiles obtained from EA ABMS data. Harlow (1980) calculated that the gravel volume of Gunner Point was 15 million m3 and Webber (1979) calculated the total sediment volume of Chichester tidal delta at 25 million m3. Replenishment at east Hayling added 535,000m3 to the depleted existing beach, but it is estimated that up to 200,000m3 of this could have subsequently been dispersed laterally with over 100,000m3 artificially recycled (Havant Borough Council, 1999).
5.4 Beach Accretion/Depletion
East Hayling.The historical trend has been for beach erosion, as determined by map comparisons over the period 1865-1969 (Harlow, 1980). Beach profiles measured at the Beach Club littoral drift divide indicate possible short-term reversals of this trend, with increasing beach levels between 1969 and 1974. After 1976, beach erosion resumed and was attributed to both dredging of Chichester Bar and groyne management of the shoreline of Bracklesham Bay (Webber, 1979; Harlow, 1980). Rates of loss were relatively small because only limited material was available on the beach (Hydraulics Research, 1988). Regular profile measurement of the east Hayling replenished beach have revealed that net erosion has continued, particularly at the littoral drift divide and at its extremities (Harlow, 1985; Whitcombe, 1995; W.S. Atkins, 1998). Losses have been calculated at 30,000 m.a-1, 1985 -1994, (Whitcombe, 1995), but initial loss up to late 1987, was 88,000 m.a-1. (Hydraulics Research, 1987). The impacts of storms have been surprisingly limited, with losses subsequently recovered. Although offshore loss of fines, and compaction of dredged fill were important, the major factors appeared to be the accelerated volume of littoral drift, and offshore transport (Harlow, 1985; Webber, 1987; Hydraulics Research, 1987, 1988; Whitcombe, 1995; W.S. Atkins, 1998; Havant Borough Council, 2000). Losses were at a maximum east of the drift divide. Losses from the replenished beach therefore nourished adjacent beaches and were partly offset by recycling, totalling over 100,000m3 between 1985 and 1993 (Whitcombe, 1995, 1996). Overall, between 1985 and 2000, net losses from the entire Eastoke frontage have been approximately 5,000m3a-1, with a total input of slightly over 20,000m3 a-1 from recycling operations balancing gross loss of 25,000m3a-1. (Whitcombe, 1995; Havant Borough Council, 2000) Beach crest retreat at a mean rate of 1ma-1 since 1986 reflect this loss, in part, although it is substantially .due to the adjustment of profile form to an equilibrium condition. (See also Section 2) Whitcombe (1995) was unable to identify the attainment of equilibrium over the 8.5 years following the emplacement of replenished material. In undertaking analysis of 25 beach profile surveys (out to 150m seawards of mean low water), he noted that steady erosion was not accompanied by seasonal cut and fill fluctuation. A consequence of these fluctuations was that even by 1990, the swash limit of severe storms had migrated back to the walls protecting the gardens of properties (Photo 16).
Eastoke Point.This area accreted between 1985 and 1990 due to the supply of sediment (particularly fines) from the replenished beach (Hydraulics Research, 1988, HR Wallingford, 1995). However, the south-western sector continued to loose material after approximately 1988, and rock armour was introduced in 1992 to offset substantial upper beach crest retreat. (Whitcombe, 1995).
Andy Point.Map analysis indicates a phase of accretion at up to 5,000m3a-1 between 1842 and 1932 followed by erosion at 1,000-2,000m3a-1 between 1932 and 1967 (Harlow, 1980). This was attributed to interruption of northward littoral drift from Eastoke Point by groynes protecting Treloar Hospital, built in 1949 (Harlow, 1980). This sector has not benefitted from updrift renourishment, as transport is directed offshore into the Chichester Harbour entrance channel.
Beachcot.At this location, increased protection to a building on the beach resulted in interception of gravel drift on the upper beach after 1976 (Harlow, 1980). This resulted in accretion updrift (eastward) and depletion down-drift. Subsequent construction of groynes and a timber revetment to the west temporarily shifted the zone of scour further downdrift to the golf club frontage (Harlow, 1980; Hydraulics Research, 1988; HR Wallingford, 1995). However, this area has experienced net accretion since the mid 1980s, partly due to receiving material from the Eastoke replenished beach. This is a cumulative effect, offset by removal of some potential gain due to recycling operations (Havant Borough Council, 1999) further east. (Removal of sediment gain from in front of the Golf Course for recycling is not currently practised (2001) due to landownership limitations).
Gunner Point.Historical accretion has been well documented for this site and began at least 400 years ago. Accretion has been variable and reduced to a mean of 5,600m3a-1 during the period 1842-1977 (Harlow, 1980). Beach profile analysis over the period 1975-1987 revealed a fluctuating pattern with net accretion up to 1982 and significant erosion and recession of the beach crest thereafter (Hydraulics Research, 1988, HR Wallingford; 1995; Havant Borough Council, 1999).
Hayling Bay.Analysis of hydrographic charts covering the period 1842 to 1976 has indicated significant accretion at East Winner and in the western part of Hayling Bay.(Whitecombe, 1995). The central offshore and deep water zones seaward of the 5m contour were, however, relatively stable, with the eastern offshore area, particularly West Pole, eroding (Foss, 1978).
Eastney.Significant accretion has been determined at Eastney from map comparisons over the period 1842-1976 (Harlow, 1980), 1896-1972 (Grontmij, 1973) and 1983-1996 (Halcrow Maritime, 2000). The accretion rate averaged 10,000-12,000m3a-1 for the beach as a whole, but since 1966 accretion at 4,000m3a-1 has been recorded behind the sewage outfall constructed at East Point (Webber, 1982). Northward of this point beaches flanking Langstone Harbour entrance have become depleted due to interception of drift by this outfall (Webber, 1974, 1982). Beach profiles at Eastney and Southsea measured by Portsmouth City Council over the periods 1935-1958, 1970-1972 and since 1983 have revealed up to 1.5m variation in beach level.
Southsea/Old Portsmouth.It was reported by Hydraulics Research (1987b) that the beach between Eastney Barracks and Southsea Castle was relatively stable. Between Southsea Castle and Old Portsmouth beach levels tend to fluctuate, with a small net loss of sediment since approximately 1980. This area is heavily protected by sea-walls and concrete slopes (Hydraulics Research, 1987b; Halcrow Maritime, 2000).
6. SUMMARY OF SEDIMENT PATHWAYS AND BUDGET - References Map
7. COASTAL DEFENCE AND HABITAT INTERFACE ISSUES - References MapIn a fully natural condition this coastline would provide a wide range of mobile and partly mobile vegetated shingle habitats, however, practices of coastal defence together with a historical trend of natural recession, narrowing and steepening of some gravel beaches, have had some negative impacts on habitat survival and development. The key contemporary habitats are vegetated shingle (Gunner Point and Eastney Beach), sand dunes at Gunner Point and Eastoke Point and major intertidal sandbanks exposed at the West Winner. The spits at the harbour entrances influence the tidal regimes of the harbours and limit wave penetration, thus contributing indirectly to habitat maintenance within the harbours. Some 93 ha. of sand dunes have developed on the shingle foreland of Gunner Point/Sinah Warren, including foredune, mobile dune and dune grassland. A large part of the site is managed as a golf course, that has reduced the areas of remaining natural dune grassland. The sand dune habitat here is likely to be maintained or increase slightly in area due to continuing accretion of shingle ridges that act as a substrate upon which dune formation occurs. A relatively mature vegetated dune system of 14 ha. also occurs at Eastoke (Sandy) Point. It is low-lying and protected by defences such that periodic tidal inundations could occur if defences were not maintained and upgraded (Bray and Cottle 2003). A major area of vegetated shingle occurs at Gunner Point thinning eastward towards Eastoke. Successive gravel ridges have accreted, as the shoreline has grown seaward by up to 600m over the past 400 years. Much of the area has since become covered by sand and succeeded to dune grassland with a large area converted to a golf course. The shingle habitats are restricted to a variable discontinuous strip along the back of the beach some 10-50m wide along the majority of the frontage, but increasing to 150m at Gunner Point amounting in total to around 30 ha. Beach ridge accumulation has been sustained in recent years by “spillage” westward by drift of sediments from the beach replenishment operations at Eastoke. A smaller area of accreting vegetated shingle is present along Eastney Beach from Fort Cumberland westward to Lumps Fort. The area of vegetated shingle areas is likely to experience a modest expansion due to continued deposition of shingle ridges at Gunner Point (Bray and Cottle 2003). Disturbance and recreational pressure are intense and these areas may require zoning of activities in future to reduce impacts. Details of appropriate management and habitat creation techniques for this resource have been set out by Doody and Randall (2003), although a valuable first step would be to undertake baseline mapping and species identification within the existing communities. It will be important to ensure that contractors involved in future beach management operations are made aware of the extent and sensitivity of the vegetated shingle communities, perhaps by implementing types of guidance being promoted by the West Sussex Vegetated Shingle Project (2003).
8. OPPORTUNITIES FOR CALCULATION AND TESTING OF LITTORAL DRIFT VOLUMES - References MapThe relatively modest wave energy, partial shelter afforded by the Isle of Wight, complex configuration of nearshore banks, interruptions by tidal inlets with transport occurring by rapid tidal currents, waves and combinations of both and prevalence of groynes means that shorelines of this frontage are not well suited for definitive studies of drift. There are, however, opportunities to improve knowledge of drift and beach behaviour. In particular, the provision of improved monitoring of beach volumes should in future allow crosschecking of modelled estimates of potential drift e.g. HR Wallingford (1995), and budget calculations e.g. Whitcombe (1995) against continuing changes in beach volume. Locations especially amenable to study include:
9. KNOWLEDGE LIMITATIONS AND MONITORING REQUIREMENTS - References MapThe SMP (HR Wallingford 1997) and coastal processes strategy study (HR Wallingford 1995) has reviewed much of the available information and made recommendations for monitoring and research. Some recommendations are in the process of implementation by the Strategic Regional Coastal Monitoring Programme, a consortium of coastal groups working together to improve the breadth, quality and consistency of coastal monitoring in South and South East England (Bradbury, 2001). A Channel Coastal Observatory has been established at the Southampton Oceanography Centre to serve as the regional co-ordination and data management centre. Its website at www.channelcoast.org provides details of project progress (via monthly newsletters), descriptions of the monitoring being undertaken and the arrangements made for archiving and dissemination of data. Monitoring includes wave and tidal recording, provision of quality survey ground control and baseline beach profiles, high resolution aerial photography and production of orthophotos, LIDAR imagery and nearshore hydrographic survey. Not all of these actions are presently planned for this unit. Data is archived within the Halcrow SANDS database system and the aim is to make data freely available via the website.
The recommendations for future research and monitoring here therefore 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:
It should be noted that Havant Borough Council for many years have undertaken regular surveys (profiles, bathymetry, sediment sampling), in addition to the EA ABMS. This is a quality data set awaiting further analysis or publication of results.
10. REFERENCES - Map
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MMIV © SCOPAC Sediment Transport Study - Portsmouth Harbour Entrance to Chichester Harbour Entrance