1. Introduction
Gravel beaches, fronted by mixed fine gravel and sandy foreshores with only a few backshore features of prominence, dominate the character of this low-lying coastline (Photo 1). Tertiary (Eocene) sands and clays outcrop across the foreshore, and provide the foundations of the beach substrate; they are occasionally exposed during storms.
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 mixed gravel and sand spits that have developed in opposed directions. The tidal regime is characterised by a long slack at double high water. 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, and the ebb tidal delta. 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 has been the tapering form of the orthogonal West Winner shoal immediately west of Langstone Harbour mouth; this feature has experienced considerable changes in size and position over the past century and since 2002 has become a barely discernible submerged feature.
Much of this coastline is urbanised (Photo 5), except for the central and western parts of Hayling Island, and is defended along some sectors by a variety of sea walls, revetments and groynes that have constrained beach widths (Oranjewoud, 1991; Halcrow, 2010a). Up until the early nineteenth century (1820-1830), much of the area directly behind the beach backshore 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 shoreward migrating barrier beaches, with probable periodic overtopping and breaching, cutting off former shallow tidal embayments and creeks (Wallace, 1988, 1990; Cope, 2005; Stripling, et al. 2008, Moon, 2010). The harbour entrance convergent spits may post-date the breaching and breakdown of this ultimately segmented barrier, such events occurring as recently as the late seventeenth or eighteenth century (Wallace, 1990; Cope, 2005). 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.
A major new source of coastal data is from the DEFRA-funded National Network of Regional Coastal Monitoring Programmes. These Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org.
The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. (See CCO Annual Survey Reports for further details).
The Southeast Regional Coastal Monitoring Programme measures nearshore waves using a network of Datawell Directional Waverider buoys. The buoy deployed at Hayling Island is in 10mCD water depth. Between 2003 and 2012 the prevailing wave direction was south-by-west with an average 10% significant wave height exceedance of 1.26m and a maximum significant wave height of 3.8m in March 2008. (CCO, 2012b; ESCP, 2012a). Refer to New Forest District Council (2010) for data on the exceedance of significant wave heights, 2003 to 2009 and ESCP (2012a) for predicted maximum significant wave heights and extreme wave and water levels (based on long period swell waves) for return periods between 1 and 1,000 years at five locations along the south Hayling coastline. A wave recorder has also been established at Chichester Bar since 2001 (see website at www.chimet.co.uk).
Wave energy decreases progressively from east to west, largely due to the protective effect of the Isle of Wight, so that modified swell waves only contribute to the wave climate west of Eastney during severe storms (Hydraulics Research, 1984; 1992; Havant Borough Council, 1992a; Halcrow, 2010c). 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. Wave focusing is highest at the eastern end of Hayling Island (specifically Eastoke Point) when swell waves approach this shoreline. Significant wave heights decline westwards along the south Hayling shoreline except at Gunner Point, where they increase (ESCP, 2012a). For a 1 year return period, a maximum significant wave height of 5.19m at Eastoke Point declines to 2.62m at the entrance to Langstone Harbour (refer to H R Wallingford (2009) for calculations of significant wave heights and extreme still water levels specific to Eastoke Point). 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). 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 1.2m, increasing to 1.6m at Southsea Castle, derived from numerical modelling and hindcasting from wind records for 1971 to 1991, is calculated by Halcrow, (2010 a, b and c). Refer to Halcrow (2010 b and c) for details of predicted extreme wave heights and water levels at a range of return frequencies for several locations along the Portsea coastline between the harbour entrance and Eastney. A still water level of 2.7m was recorded during the storm surge of March 10th 2008 (refer to Wadey et al. (2012) for estimated still water levels at return frequencies between 20 and 200 years). Halcrow (2010c) predict an extreme water level of 2.9m with a 1 in 50 year recurrence at the harbour mouth, increasing to 3.04m at South Parade Pier and 3.14m at the entrance to Langstone Harbour. Tidal records from Portsmouth naval base, 1813 to 1998 indicate that extreme water levels increased by 0.92mm per year during this long period, accelerating to 3.4mm per year 1950 to 1998 (Halcrow, 2010a). Pirazzoli et al. (2006) and Pirazzoli and Tomasin (2008) examined the same record and considered that relative mean sea-level rise between 1991 and 2002 to have been 1.37cm per year (+/- 0.52). Land subsidence is considered to account for more than one half of this increase. The maximum recorded combined surge and highest astronomical tide during this period was 549cm, considered to equate with a 1 in 50 year recurrence. Haigh (2004) considers that there is a surge-tide interaction that produces a decrease in surge level at about the time of highest astronomical tide. Ship-generated waves are normally less than 0.40m height, but may be an important local component, as there are over 90,000 annual vessel movements (1998). Recurrent flooding of Old Portsmouth indicates a 1 in 2 year probability of a water level of +2.54mOD, increasing to +3.03mOD for a 1 in 200 year return period. Corresponding wave heights for Langstone Harbour Entrance are 2.40m and 2.58m (Halcrow, 2009c). Extreme water levels here are 2.75mOD (1 in 2 years) and +3.14mOD (1 in 50 years). 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, due to the influence of local bathymetry. Bradbury et al. (2007; 2011) analysed the records from the nearshore Waverider buoy in Hayling Bay, June 2003 to June 2008, to reveal that the island’s eastern and central shoreline experiences bimodal (i.e. swell and local wind) wave period spectra between 6 and 25% of the time during most winter months. During short periods they may be dominant, e.g. during January 2007 there were thirty bimodal events. At least 40% of storms are characterised by bimodal seas, with swell and storm peaks normally out of phase. The ESCP (2012 a) note the 3rd November 2005 event as being most severe in terms of flooding for Eastoke. This storm event was distinctly bi-modal in nature; the resulting flooding to Eastoke was not on the same scale as storm events prior to the 1985 replenishment scheme but did serve as a stark reminder of the very real and prominent threat to this area from overtopping, and the likely effects if beach management were to cease.
Rectilinear tidal currents in the nearshore zone adjacent to the shoreline have characteristic velocities of less than 0.5m per second during spring tides, with only very limited competence to entrain sediment (Hydraulics Research, 1992; HR Wallingford, 1995; Halcrow, 2010a and b). Further offshore, they can attain velocities of over 1.25m per second 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. Tidal currents interact with wave trains in complex ways; their combined 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 on the East and Central Solent.
2. Sediment Inputs
Most of the Hayling (Photo 6) and Portsea (Photo 1 and Photo 5) frontages are at or below mean sea-level and coast erosion has been prevented along specific frontages by 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 (including submergent tidal deltas) 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 2km 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) and other documentary sources (ESCP, 2012a).
2.1 Marine Inputs
F1 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 3km to 4km offshore (Harlow, 1980; Wallace, 1988). Sedimentological analysis of the East and West Pole Sands delta deposits indicate that 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). Analysis of beach volumes on Hayling Island prior to 1985 revealed a 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 per year, sufficient to offset littoral drift losses at the time, was therefore postulated by Harlow (1980). This figure probably conceals significant annual variations, for drift rates fluctuated markedly over this period (see section 3).
Whitcombe, (1995) attempted to calculate a provisional beach budget for the post-nourishment eastern and central sectors of Hayling beach. He calculated that onshore-directed import of some 12,000m³ per year may substantially compensate for losses caused by longshore transport east and west of the drift divide (refer to later sections covering beach nourishment and beach accretion/depletion trends.) This 1995 calculation has not been confirmed subsequently and therefore the volumes have been removed from the F1 arrows.
A high resolution, 100% coverage swath bathymetry survey was commissioned by the Southeast Regional Coastal Monitoring Programme. This extensive survey, covering 194km², extended between Lee-on-the-Solent and Selsey Bill and offshore to abut with the northeast Isle of Wight survey boundary, between Cowes and Bembridge, was completed in July 2013. The ebb delta south of Chichester Harbour entrance comprises a lobe of coarse material extending seawards approximately 2km, surrounded by a sandy seabed. Further to the west the seabed is dominated by coarse-grained sediment, mainly gravel.
F2 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 was indirect, as it derived exclusively from 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 sufficient explanation of erosional loss. Further details are given in the unit on East Head to Pagham Harbour.
The swath bathmetry surveyed in 2013 as part of the Southeast Coastal Monitoring Programme shows numerous dynamic and mobile bars and shoals and bedforms on East Pole Sands which may indicate current driven onshore transport between the eastern flank of the delta and the East Head foreshore. The seabed to the west of Chichester harbour is dominated by sand, a mixture of coarse and fine sediment between the West Pole Sands and Eastoke Point; the coarse-grained ebb delta south of Chichester Harbour entrance, indicates current- driven onshore transport between the eastern flank of the delta and the East Head to Cakeham foreshore (Bray, 2009; Fitzgerald, 2012 - refer to the literature review in the East Head to Pagham Harbour unit for details of accretion patterns since 2005, and their explanation).
F3 Feed from West Pole Sands
Onshore movement of gravel to the beach immediately adjacent 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 (which has decreased in size since then) 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 150m west of Eastoke Beacon.
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 south 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 120m³ per year 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. Analysis of Coastal Monitoring Programme bathymetry data, sediment sampling and divers' observations indicate a stable, immobile seabed, as indicated by poorly sorted sands, gravels and boulders with abundant attached marine benthic flora and fauna. Therefore, the 2004 arrows indicating speculative weed rafted gravel transport have been removed.
F4 Feed from West Winner to Eastney
The entrance to Langstone harbour is a dynamic mobile environment with the large East Winner sandbank found immediately to the east of the deep entrance channel. 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). By 1994 the feature had almost disappeared and by 2002 it is no longer visible from the aerial photography.
Analysis of historic aerial photography indicates that the West Winner bank has been decreasing in size since the earliest aerial photography in 1946. Analysis of Coastal Monitoring Programme 2013 swath bathymetry data indicates that this bank is not discernible. However the East Winner sandbank now extends south over 4km offshore before curving to the southwest; the shoreward 2km of the bank is exposed at spring low tides. The eastern flank of the sand bar rises 2-3m above the surrounding gravel seabed. Horse and Dean Sands do not appear as prominent features in the 2013 swath bathymetry.
There is no conclusive evidence to support the detail for individual F4, F5 and F6 arrows of the 2004 mapping and therefore all three have been replaced by a single F4 arrow.
2.2 Beach Nourishment
N1 Eastoke
A major beach nourishment was completed between April and December 1985 on the 2.2km Eastoke frontage at Hayling Island between Sandy Point and Rails Lane (Havant Borough Council, 2007; HR Wallingford, 2009; ESCP, 2012a). This was in response to progressive loss of both back and foreshore beach height, width and volume at an estimated rate of 13,000m³ per year over the previous 40 years (Hydraulics Research, 1980; Harlow, 1985; Havant Borough Council, 2007; HR Wallingford, 2009; Ruocco et al., 2011). In the fifty years prior to 1980 (and presumably over many previous decades) this frontage retreated at an average rate of 0.6m per year. A concrete seawall was built in sections between 1939 and 1954, (with a return lip added in 1974) and was a contributor to beach drawdown and its inadequate height to increased frequency of overtopping during surge events due to wave reflection and enhanced run-up. 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 design 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 AODN, with a crest 30m in width, (later reduced to 18m in 2007) tapering at each end. 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 (Photo 10) (Mc Farland et al., 1994), but this was initially 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, 2007). Subsequently, sediment sourced from both dredging the entrance channel to Chichester Harbour and licensed offshore aggregate sites was added to that transferred back to this beach by recycling (refer to section 3). Between 1986 and 1993, an annual loss of 30,000m³ per year was experienced, due to longshore drift both east and west from the drift divide, accompanied by profile flattening (Whitcombe, 1995). Between 1994 and 2003, the trend was for annual losses of approximately 38,000m³, partly compensated by gains from recycling and by-passing. If this practice were to be discontinued, this negative budget would mean that the beach would return to its 1993 pre-nourishment volume. Between 2001 and 2011, net loss, after replenishment, was 42,000m³, although there have been major inter-annual fluctuations in response to aperiodic storm and surge events (ESCP, 2012a).
'Top up' (recycled) supplies have been derived from bulges and accretion filets that periodically develop to 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. These features persist, but refer to ESCP (2012a) for specific details of beach morphodynamic and geometric changes since 2004. Further details on sedimentology are given in section 5.
At Hayling Ferry (North of Gunner Point) small quantities of gravel were transported northward periodically (Webber, 1974 a and b, WS Atkins, 1998). On Southsea beach, perceived surplus accretion of gravel on Eastney Beach was transported to the frontage between Southsea Castle and South Parade Pier, according to the scale of loss. This transfer is modest, at less than 1,000m³ per year in recent years (Halcrow Maritime, 2000). A summary of more recent activity can be found in the SCOPAC Sediment Stores and Sinks study.
2.3 Coastal Erosion
E1 Harbour Entrances
Study of Langstone and Chichester Harbour entrances by Hooke and Riley (1987) revealed significant erosion at Eastney outfall (0.48m per year 1870-1932), together with a shortening and thickening of Eastney Spit (Photo 1) and erosion of the proximal point of Black Point, Hayling (0.32m per year, 1910-1968). The SCOPAC Sediment Stores and Sinks report calculated that there are large quantities of sand accretion at Black Point Spit, amounting to 46,700m³ between 2003 and 2012 acting as a final beach store. 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 East Head to Pagham Harbour.
3. Littoral Transport
The 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 Eastoke Beach: 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). This divide is the product of the fact that waves, especially swell waves, reach this frontage at very small angles of incidence; thus variations in wave approach lead to large changes over short timescales of longshore transport directions and rates. To the east, net longshore transport is eastwards, to the west it is directed westwards. The precise location of this divide at any one time (or for short periods) is determined by both immediate and antecedent variations in incident waves, with wave refraction under contrasting sets of prevailing wave periods and directions being particularly important (W. S. Atkins, 1998; Whitcombe, 1995; HR Wallingford, 2009), and on drift rates to east and west. Net potential eastward drift was the lesser volume, and varied between 2,000m³ per year and 12,000m³ per year over the study period, with a long-term mean of 5,000m³ per year (Harlow, 1980). Drift increased at Eastoke Point due to changed coastal orientation; Hydraulics Research (1980) suggested a mean rate of 10,000m³ per year for this sector.
Post-replenishment. Littoral drift rates increased by several magnitudes after replenishment for the following reasons:
- Large quantities of gravel became available for transport on a beach that was previously severely depleted.
- Replenishment completely buried some existing groynes (although others were increased in height shortly after completion of the programme. Subsequently, some groynes were extended back to the pre-existing sea wall).
- The replenishment caused slight changes in beach alignment, especially at the western and eastern extremities.
- The recharge material was relatively poorly sorted and the initial artificial profile was steeper than the natural equilibrium profile. This promoted enhanced sediment mobility due to the reflective nature of this profile, and its progressive adjustment to an equilibrium form.
The replenished beach has been routinely monitored, initially by an intensive series of monthly beach profiles. These were analysed by Hydraulics Research (1987 and 1988) and littoral drift was calculated by assuming that all beach volume changes were solely the result of longshore transfers. Information from this approach may be misleading where significant onshore/offshore transport operates. Analysis demonstrates that a littoral drift divide remained in approximately the same position as before replenishment (Whitcombe, 1995), but that eastward drift increased to 53,000m³ between February 1986 and February 1987 (Hydraulics Research, 1987 and 1988). Further analysis of beach profiles and short-term deployment of tracing experiments using aluminium pebbles revealed that mean eastward drift over the period 1986-1990 was about 30,000m³ per year (Havant Borough Council, 1992b), declining to 11,500m³ per year between 1990 and 1991 (Whitcombe, 1995). Fitzpatrick (1992) also used aluminium pebbles, deployed during a short period of storms of moderate intensity, and calculated an annual eastwards drift of 17,000m³; however, tracer recovery in this case was low. Littoral drift was therefore initially very rapid on the replenished beach but declined as the beach approached an equilibrium. Wave refraction analysis suggests that net eastward movement is promoted by modified swell waves approaching from the south-west and, south-south west. South south-easterly and east south-easterly waves, which occur for less than 7% of the year, set up a net westward movement (W.S. Atkins, 1998). During these short periods, the littoral drift divide is probably inoperative. Groynes were constructed in between 1987 and 1991, to reduce the rate of eastwards drift and beach erosion (Hydraulics Research, 1988; Whitcombe, 1995). Havant Borough Council (1991) calculated that the prevailing drift was 13,300m³ per year in the five years following recharge project completion. W.S. Atkins (1998) proposed the lower rate, of 11,300m³ per year for 1985-1995. Following later replenishment events, Moon (2010) determined a rate of 17,500m³ per year. Thus, as expected, net eastward drift rates declined as the beach progressively attained morphodynamic stability.
Before implementation of the scheme, both Harlow (1980) and Hydraulics Research (1980) assessed previous experience elsewhere 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 30-40,000m³ per year. Data from W.S. Atkins (1998) and Havant Borough Council (1999) indicate a loss of 27,100m³ per year, 1991-1998, declining from 46,000m³ per year, 1991-94 to 25,000m³ per year, 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 (e.g. 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 net accretion at Eastoke Point, particularly of coarse sand and fine gravel. Material has been 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, and subsequent profile modelling to provide further beach stability and enhanced resistance to overtopping (which was experienced in November 2005 when wave run-up elevations exceeded design predictions - refer to section 5.4.) (Atkins, 2006; HR Wallingford, 2009).
The Eastoke Peninsula Beach Management Strategy Plan for 2000 to 2005 (Havant Borough Council, 1999, 2000, 2007) and the South Hayling Beach Management Plan (ESCP, 2012a) set out, amongst several other measures: (i) a continuing programme of gravel recycling, taking accretion excesses downdrift of the drift divide (i.e. to both east and west) to areas of deficit on the existing beach; and (ii) recharge, obtained by dredging the navigable approach channel to Chichester Harbour and/or from offshore aggregate sources. Sediment from dredging the Chichester Harbour bar is more appropriately regarded as recycling, as it derives in part from eastwards littoral drift and in part from onshore to offshore losses from the renourished beach. The initial quantity was 50,000m³, followed thereafter by estimated inputs of approximately 25,000m³. The major advantage of this approach has been that it retains material in the local sediment transport sub-cell. Without recycling, it was considered that rates of loss would probably progressively increase, to at least 30,000m³ per year to the end of the design life of the renourished beach, accompanied by recession (Havant Borough Council, 1999, 2000). Moon (2011) states that the estimated net loss between 2004 and 2010 was 37,000m³, ESCP (2012a) give a figure of 42,000m³ for the ten years, 2001-2011. Since 2005, annual beach recycling has added between 14,500 and 51,500m³ (the latter in 2011), with the addition of small quantities of road imported gravel from terrestrial sources. Dredged sediment from Chichester Harbour provided 58,360m³ in 2007 and 17,400m³ the following year, whilst 90,300m³ of gravel sourced from offshore aggregate resources from a site south-east of the Isle of Wight was added in 2009. Between 1985 and 2009, recharge accounted for 49%, recycling 39% and extraction from Chichester Bar 12%. Nourishment has in part been determined by large volumetric losses due to major storms, as in November 2005 when Eastoke beach was overwashed and its crest lowered by 2m. Thus inputs, in total some 1,160,000m³, have exceeded estimated longshore and offshore directed natural losses, primarily with the objective of enhancing the defence standard of this critical beach given its vulnerability to storm surge induced drawdown (refer to sections 1 and section 5.4 for further detail of storm wave spectra). Losses resulting from winter storms and surge events have tended to be greatest across the mid-beach multi-berm sector, between 2 and 3mODN, with drawdown to the foreshore creating a shallower concave profile.
Sediment tracing (East Solent Coastal Partnership, 2012b) indicates that the drift divide at the Eastoke frontage has remained in a relatively stable position since 1985. The latter study involved RFID tracking the positions of tagged clasts over a two year period. Despite limitations of detection, especially after the first year, a drift divide at approximately the location of the junction of Southwood and Creek Roads was evident. West of this divide, net westwards transport was at an estimated rate of 130m per month, but possibly attaining 200m per month given more energetic wave conditions; results from the Eastoke 1 deployment of the ESCP Tracer Study showed that over a 9 month period in 2010, tracers moved west between 25 and 1900 metres and continued to move in a westerly direction towards the Langstone Harbour entrance channel; eastwards, towards the entrance of Langstone Harbour, it was lower at approximately 60m per month. Walkley (2011) employed the same methodology with a 70% tracer recovery for three complete surveys during a short period of above average significant wave heights approaching from the south and south-west. Mean distances of travel varied with both incident wave heights and pebble shape and size, with smoother, more rounded particles being transported further than those of angular shape. The mean distances travelled produced an estimate of longshore drift of 10,670m³ per year eastwards. There was also an observed tendency towards net onshore transport. Young (2009), however, recorded net offshore movement during a short period of calm conditions, with longshore transfer of 7.8m³ per tide, with an eastwards component. Littoral transport rates at this location are have been estimated from the Coastal Monitoring Programme data as between 3,000 and 10,000m³ per year, but replenishment and recycling has continued along this frontage at regular intervals since the large 1985 replenishment, which makes an accurate assessment of long term annual sediment transport rate difficult.
LT3 Eastoke Point to Black Point (Sandy Point Spit)
Site observations indicate northward drift from Eastoke Point to the south–north aligned Black Point spit (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 8,000m³ per year was indicated by beach volume analysis (Hydraulics Research, 1980; Beard, 1984) but littoral drift was probably, a smaller volume as some of this material was probably diverted to the tidal channel. Studies by Harlow (1980) revealed depletion of the Black Point Spit, at 1,000 to 2,000m³ per year between 1932 and 1967, which was preceded by accretion at 1,000-5,000m³ per year between 1842 and 1932. Thus it appears that potential littoral drift may be between 1,000m³ per year and 8,000m³ per year. Sediment supply has and will be been further regulated with the completion of the terminal rock groyne and rock revetment at Eastoke Point (1990-1992) and the forthcoming Eastoke Point scheme (2013). Dredging of the approach channel to Chichester Harbour provides potential sediment for periodic replenishment. A mix of coast protection structures between Black Point Spit and the western limit of the Sandy Point Nature Reserve currently maintain beach levels, which since 2004 have been increasing. The more recent accretion of a wide sandy foreshore around the distal point may indicate that there has been an increase in the littoral transport rate, although a similar build up of sand at Cakeham and West Wittering points to more large scale morphological changes to the Chichester Harbour inlet system (Fitzgerald, 2012). Sediment stored by these structures might also rapidly increase littoral drift throughput in the event of any breakdown of stability, and increased erosion. The proximal point of the spit supports a small area of sand dunes, carried out by a local programme of stabilisation and stimulus of new growth.
Analysis of Coastal Monitoring Programme data indicates that to the east of Eastoke Point and heading in to the harbour the sediment transport rate reduces to 1-3,000m³ per year, with the lowest sediment transport rates occurring within the harbour towards Black Point. The beach at Black Point has gradually accreted between 2003 and 2012, with northward littoral transport from the beach at Eastoke Point.
LT4 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. Gunner Point is a cuspate foreland composed of closely spaced gravel ridges partly concealed by degenerate sand dunes and a thin sandy veneer. Map evidence (Harlow, 1980) reveals that it has prograded over the past four hundred years (perhaps longer), and continues to prograde in response to both increased drift volumes from the east (presumed to derive from the Eastoke replenished beach) and the welding of discrete pulses of mobile gravel moving from off and nearshore. The latter move westwards and then northwards before being lost to the Langstone Harbour entrance channel. From here they are presumed to be flushed seawards.
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 calculated for nine separate periods between 1842 and 1976, which revealed significant spatial and temporal variability. Generally, the drift rate increased westward, sustained by eastward beach erosion; most transport is considered to take place during periods when high-energy waves approach 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 and the effect of the East Winner on wave attenuation. Maximum drift of over 80,000m³ per year 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 17,000m³ per year was calculated. This increased to 20,000m³ per year one kilometre to the west and declined to 19,000m³ per year at the Beachlands surface water outfall (constructed in 1973 (Webber, 1974b)). Further west, it declined to 13,000m³ 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 14,000m³ per year (1967-1976) and 14,000m³ per year (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 gravel and sand accretion extending back about 380 years. Mean High Water Mark at Gunner Point has moved 195m seawards since approximately 1880 (Havant Borough Council, 2000). HR Wallingford (1995) suggest progradation of 0.8m³ per year since 1910. A total of between 10-15,000m³ per year 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 contemporary Gunner Point foreland and Ferry spit.
Post-replenishment: Volumetric analysis using measured beach profile data revealed that drift was 37,000m³ per year (February 1986 to February 1987) and between 6,000 and 13,000m³ per year (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 9,500m³ per year (Whitcombe, 1995) and 4,800m³ per year (W.S. Atkins, 1998). Whitcombe (1995, 1996) identified the transfer of a "wedge" or bulge 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 source for recycling 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 17 to 20,000m³ per year.
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 was sustained only 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 littoral transport divide 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 this reversal, and therefore a drift convergence eastwards, is simply a short-lived local variability, i.e. an impersistent feature, 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 between about 1970 and 1994 could have reduced the elevation of Langstone Bar and thereby 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. Sediment tracing (East Solent Coastal Partnership, 2012b) on the Hayling Island side of the Langstone Harbour channel, at Gunner Point reported that tracers moved westwards into the Langstone Harbour entrance channel.
Analysis of Coastal Monitoring Programme data indicates that post replenishment recharge/recycling events at Eastoke, the net westward sediment transport rate directly to the west of the management site is more than 20,000m³ per year (Moon, 2010 calculated a rate in excess of 30,000m³ per year) The sediment transport rate decreases gradually towards the west until it reaches Gunner Point, with a littoral transport rate of 10–20,000m³ per year between 2003 – 2012 at this location. This is a reduction from the 2004 suggested rate of more than 20,000m³ per year, which also indicated an eastward drift of material from Gunner Point that has not been identified in more recent investigations. The net, and dominant, direction of sediment transport on the storm berm of Gunner Point is westwards, towards the entrance to Langstone Harbour. Three deployments of pebble tracing confirmed this pathway, at an estimated rate of 60m per month (though detection rates were low, partly due to burial of tracked clasts beyond tracking range). This study was unable to identify the drift divide that was proposed in the earlier editions of this report, but it was limited to a relatively short timescale of investigation (Eastern Solent Coastal Partnership, 2012a and b). It is estimated that 3,000m³ per year of sand and gravel is transported offshore to the ebb delta. An estimated half of this quantity is then transported periodically back onshore to the beach east of Gunner Point, with the remainder retained on the ebb delta or moved permanently offshore.
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). However by 1994 the bar feature had almost disappeared and by 2002 was no longer visible from the aerial photography. Analysis of Coastal Monitoring Programme 2013 swath bathymetry data indicates that this bank is no longer discernible.
LT5 Gunner Point, Langstone Harbour Entrance Channel
Analysis by Harlow (1980) indicated a long-term mean westward drift of 15,000m³ per year to Gunner Point where net accretion of 5,000m³ per year, primarily gravel, was recorded. Much of the remaining 10,000m³ per year of sediment was transported into the Langstone tidal channel and then flushed seaward to accumulate on the bar or on East Winner, promoting its southwards and eastwards growth. 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 welded sinusoidal bulges (preceded by short-term erosion phases) which migrate northward by littoral drift along the eastern side of the Langstone entrance to the Ferry Point spit beach (Photo 3). This trend is confirmed by W.S. Atkins (1998; 2006) for the mid -1990s and both Cope (2005) and ESCP (2012a) up to 2012 using air photo and Lidar imagery. These features were earlier traced on maps (1842-1976) and on aerial photos (1976 and 2001); northward migration was measured at 10m per year. No 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 Point spit. The growth of this spit between the late seventeenth and mid-nineteenth centuries, together with the more recent and ongoing expansion of Gunner Point, accounts for the historical narrowing of the entrance channel to Langstone Harbour (Tubbs, 1999; Moon, 2010).
Beach profile analysis by Hydraulics Research (1988, HR Wallingford, 1995) indicated recent erosion of the apex of Gunner Point and demonstrated continued gravel accumulation on the "bulges" in Langstone entrance suggesting that northward drift still prevailed. If this is correct, it was postulated that a littoral drift divide may have developed at Gunner Point, although its exact location was be established. HR Wallingford (1995) suggest that loss of foreshore material at Gunner Point, between 1982 and 1987, 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 3,700m³ per year.
Analysis of Coastal Monitoring Programme data for the period 2003-2012 indicates that the northward drift rate on the eastern side of Langstone Channel reduces from 10–20,000m³ per year at Gunner Point to less than 3,000m³ per year on the beach defining the Langstone Harbour entrance channel. The sediment builds up along the western coast of Hayling Island as slight but distinct bulges, with coarse to medium grain size sediment finally passing in to the channel.
LT6 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 1,000m³ per year to 2,000m³ per year (Harlow, 1980). Using similar techniques Webber (1982) determined eastward drift of 8,000m³ per year from the littoral transport divide. This drift divide was attributed in part to wave refraction caused by 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. Halcrow (2010c) have confirmed a drift divide, but slightly westwards of the position determined by earlier research (as indicated on the 2004 map) using two different periods of beach response and corresponding wave time series, 1993-1997, as input to beach plan modelling.
Analysis of Coastal Monitoring Programme data between 2003 and 2012 and Eastern Solent Coastal Partnership (2012) tracer study, support a transient drift divide 1,200m further east to that recorded in 2004, in the vicinity of Fort Cumberland. This coincides with the disappearance of the West Winner by 2002. A rate of north-eastward drift between Eastney and the tip of the spit has been re-calculated as 1-3,000m³ per year, a reduction from the 2004 estimates of 10-20,000m³ per year. These rates are in broad agreement with results from sediment tracer study (Eastern Solent Coastal Partnership, 2012) which showed net drift of 600m³ per year west of both outfalls at Fort Cumberland along Eastney Spit. Tracking of individual clasts revealed rates of transport eastwards towards Langstone Harbour of approximately 50m per month, with some by-passing of the groyne at the harbour mouth and the pier downdrift.
In 1966, sewage outfalls at Eastney Point were encased by culverts and an additional outfall constructed (Webber, 1974a, 1984). These structures were believed to intercept littoral drift, causing gravel accretion of 4,000m³ per year (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 (Photo 3). HR Wallingford (1995) calculated a drift rate of 6,800m³ per year, 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). Halcrow Maritime (2000) calculated an updrift input of approximately 13,000m³ per year, with the implication that a significant proportion might move into the mouth of Langstone Harbour entrance channel.
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). By 1994 the feature had almost disappeared and by 2002 it is no longer visible from the aerial photography.
Analysis of historic aerial photography indicates that the West Winner bank has been decreasing in size since the earliest aerial photography in 1946. Analysis of Coastal Monitoring Programme 2013 swath bathymetry data indicates that this bank is not discernible. Horse and Dean Sands do not appear as prominent features in the 2013 swath bathymetry.
LT7 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 probability for material on the lower foreshore to become entrained and lost to the tidal channel, so that progressive rapid 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, however, the ESCP tracer studies have shown sediment passing over outfalls throughout 2011 (Eastern Solent Coastal Partnership, 2012). Analysis of Coastal Monitoring Programme data supports an eastward drift from Fort Cumberland to Eastney Spit, with a rate of 1-3,000m³ per year between 2003 and 2012, with the rate decreasing towards the distal point of Eastney Spit. This is substantially less than earlier estimates e.g. Webber (1982) calculated a potential rate of 15 to 16,000m³ per year, but with substantial losses to the Langstone channel.
LT8 Eastney to Canoe Lake car park
There was some uncertainty over longshore drift rates west of the Eastney drift divide. Harlow (1980) proposed a rate of approximately 2,000m³ per year, whilst Webber suggested 6,000m³ per year and HR Wallingford (1997) calculate an intermediate figure of 3-4,000m³ per year. Rates were determined by locally-generated waves, affected by refraction induced by the projecting form of the West Winner shoal which is no longer effective. Swell waves only occasionally reach this sector of coastline, because of protection afforded by the Isle of Wight and historically by wave energy decay created by the sand and gravel banks of Horse and Dean Sands when prominent. Shoreline advance has accelerated since the 1980s with 145,400m³ natural accretion to beach volumes (16,200m³ per year) between 1983 and 1992 (HR Wallingford, 1995), and a similar quantity between 1996 and 2010. Beach elevation increased in response. The source of supply of this sediment, which was dominantly gravel, is uncertain and was most likely to derive from onshore feed from the Langstone tidal delta, or more specifically from the store of coarse sediment released by the previously extant West Winner shoal. There may also have been some cyclical pattern of supply. Further evidence of weak longshore and cross-shore transport comes from analyses of the sorting of beach clast size and shape. This reveals little discernible grading or sorting (Grontmij, 1973; University of Portsmouth, 1990-2000).
Beach elevation changes between 2003-2012 from analysis of Coastal Monitoring Programme data and the Eastern Solent Coastal Partnership (2012) tracer study show westward movement of sediment from the drift divide to the vicinity of the Canoe Lake car park where there appears to be a drift convergence zone that was inoperative when the 2004 edition of this study was written.
Calculations from between 2003-2012 suggest a net longshore westward transport rate of between 3-10,000m³ per year, with sediment transport convergence at the approximate location between Lumps Fort and the Canoe Lake. These findings are supported by the Eastern Solent Coastal Partnership Sediment Tracer Study (2012b).
LT9 Southsea Castle to Canoe Lake car park
Based on beach elevation changes between 2003 and 2012 apparent from the analysis of the Regional Coastal Monitoring Programme data and Eastern Solent Coastal Partnership (2012b) tracer study, an estimated 1,000-3,000m³ per year of sediment is moving in an easterly direction. There is erosion directly in front of The Pyramids with sediment travelling eastwards as far as the Canoe Lake car park, at estimated rates of between 50 and 60m per month, forming a convergence zone. This represents a change in drift direction which in 2004 was suggested to be westward between Canoe Lake and Southsea Castle. The coastal apex at Southsea Castle is therefore identified as a drift divide.
LT10 Southsea Castle to Portsmouth Harbour entrance
Visual observations of gravel built up against obstructions over the past 30 years along with tracer pebbles deployed between Clarence Parade and Old Portsmouth (Eastern Solent Coastal Partnership, 2012) indicates very weak net northwest drift on the beach between Southsea Castle (Photo 1), Clarence Pier (Photo 5) and Old Portsmouth. 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 all other observations (Atkinson, 2000; Halcrow, 2010a). Harlow (1980) suggested that littoral drift is weak because wave height is limited by fetch and nearshore diffraction/refraction and strong ebb tidal currents in Portsmouth Harbour entrance. Halcrow Maritime (2000) and Halcrow (2010a) note slight beach narrowing since the 1860s, although there have been alternating phases of beach retreat and advance over this period. In recent years, modest quantities (<1,000m³ per year) of sediment have been periodically added to replenish winter losses. A summary of more recent activity can be found in the SCOPAC Sediment Stores and Sinks study.
Beach depletion has been a factor producing overtopping and flooding events during exceptional storm and/or water level conditions at least since the early nineteenth century. Old Portsmouth was especially vulnerable, a problem addressed by the construction of robust defences in 2003-4.
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 to the dredged main approach channel 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, supported by beach monitoring undertaken by the Coastal Monitoring Programme. 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, most recently to -16mCD, 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. This is confirmed by pebble tracer studies (Eastern Solent Coastal Partnership, 2012), though detection and recovery was very low and conducted only over a short timescale. Littoral transport is probably not in excess of 300m³ per year (HR Wallingford, 1995). This is supported by the recent sediment budget analysis undertaken as part of the update of this study which records littoral transport of approximately 1-3,000m³ per year; this material is transferred to the harbour entrance channel and moved seawards by the ebb current or transported directly offshore to adjacent banks. 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 opposing western (Haslar) spit shoreline, and the entrance channel thus forms a well-defined transport cell boundary (Bray, et al., 1995).
The Camber spit on which Old Portsmouth has developed since the twelfth century, and now alienated from natural processes, (Photo 4 and Photo 5) is indicative of long sustained longshore transport into the harbour mouth. However, it may have a complex history involving barrier migration, breaching and subsequent re-orientation. There is no information on its sediment composition.
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, former aggregate extraction from nearshore banks, navigational dredging and shipping movements.
4. Sediment Outputs
4.1 Transport in the Offshore Zone
O1 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. A clockwise circulation may operate around West Pole, inferred by the periodic natural 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. However, analysis of the 2013 swath bathymetry survey data, commissioned by the Regional Coastal Monitoring Programme, provided no conclusive direct evidence of this transport pathway. 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).
O2 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 recently discontinued. However, there is no continuous data on quantities removed from the banks, as all dredging records also include losses from the maintenance of the access channel across the outer bar. Between 2003 and 2012 the volume of the East Winner has increased by 26,000m³, with the accretion showing as west-east oriented bars across the bank
4.2 Estuarine Outputs
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 occurs as tidal currents weaken, thereby creating major sediment accumulations (East and West Pole Sands; East Winner) extending 3km to 4km offshore (Harlow, 1980; Wallace, 1988; Halcrow, 2010a). No transport of coarse sediment across the entrance channels takes place. In the mid nineteenth century Langstone ebb delta was fully by-passing, with shore attached shoals on both sides of the entrance channel and shoals across the inlet. Analysis of successive hydrographic charts reveals that one hundred years later the margins of the delta had retreated, and the channel had deepened (Rossington, 2008; Moon, 2010). It has continued to reduce in size since the mid-1960s. Significant sorting and transport of sediments occurs on these ebb deltas by combined action of waves and tidal streams.
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 induced 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 2004, 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.
Sediment calculations from survey data show approximately 12,000m³ per year is lost off the eastern side of Eastoke each year. Due to channel deepening in the west of Chichester channel, the stronger currents at this location could remove this sediment offshore or on to the Chichester tidal delta.
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 10,000m³ per year and eastward drift of 1,000m³ per year from Eastney Point. These inputs to the tidal channel may have ceased or diminished between 1982 and 2003 when a littoral drift divide was located at Gunner Point.
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. There is evidence to suggest tracer pebbles were carried south once in the harbour entrance (Eastern Solent Coastal Partnership, 2012).
The Langstone tidal delta may therefore be a finite sediment store or sink resulting from reduced littoral drift feed to the tidal channel. Rossington (2008) analysed changes in its volume between 1961 and 1997, derived from hydrographic charts, and concluded that there had been growth during this period. Whitcombe (1995) analysed changes in the plan shape and volume of the East Winner, 1976-1992, which revealed fluctuating expansion and regression (accretion and erosion), involving losses and gains of an average of 50,000m³ per year. The cause of these changes would appear to be shifts in the alignment of the outer Langstone entrance channel.
The Portsmouth Harbour entrance is the most sheltered of the inlets thus littoral drift input to the tidal channel is very low to virtually zero (Halcrow Maritime, 2000; Halcrow, 2010a). Although it’s 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 mostly transported offshore to Spit Sand and supply from the Southsea frontage was transported to Horse and Dean Sand (Harlow, 1980; HR Wallingford, 1997) (see section 3). 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 16mCD, thereby entailing output of sediment from the transport pathway. Previous studies have suggested that this sediment was deposited on the Horse and Dean Sands. Analysis of swath bathymetry data commissioned through the Coastal Monitoring Programme indicates the remains of the sand banks but provides no evidence of continued feed of material or supply onshore. Rossington (2008), however, concluded that the ebb tidal delta increased in volume between 1974 and 2004 (with considerable variations during intervening years), but expressed doubts as to the accuracy of the bathymetric surveys that were used for analysis.
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
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 backshore 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; Halcrow, 2010a). Both fixed and mobile sand dunes occur at Gunner Point/ Sinah Warren and Sandy Point, Hayling Island, fed from adjacent sandy foreshores.
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 had been 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 windblown sand (median particle size of 0.177mm) indistinguishable from sands on East Winner, the probable source area. Samples of beach sediments down to maximum low water at various sites along south Hayling beach are tabulated in the South Hayling Beach Management Plan (ESCP, 2012a). Combined data on particle size for all sites gives a sand and fine gravel fraction of 64% and coarse fraction (>2mm diameter) of 22%. 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 (i.e. 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 most sample data 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) samples were collected on three separate occasions. However, the timing in most cases was not adapted to tidal and wave conditions.
The major beach nourishment at Eastoke 2009 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 2000kg/m³ (compared to original specification of 1750 kg/m³) resulting in the development of an upper beach cliffed erosion micro-scarp - see Photo 10 (Hydraulics Research, 1987; McFarland, et al., 1994; Whitcombe, 1995; W.S. Atkins, 1998). This crestal scarp initially retreated at 2-3m per year, but since reduced to 1.8m per year (1990-95) and to 0.5m per year 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 using 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, fresh imports of recharge material and beach overtopping at Eastoke under severe storm conditions in 1994, 2001, 2005 and 2008.
5.2 Sediment Stores
Harlow (1980) calculated that cell-wide coast erosion since the mid nineteenth century could have supplied 14 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. Further information may be found at http://www.scopac.org.uk/sediment-sinks.html.
5.3 Beach Volumes
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 Eastoke, southeast Hayling in 1985 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). Subsequent additions from both recycling, by-passing and recharge are given in section 2.2 and section 3 (LT2).
5.4 Beach Accretion / Depletion
East Hayling
The historical trend, especially since the early 1940s, has been for beach erosion, as determined by map comparisons over the period 1865-1969 (Harlow, 1980) and 1970-2007 (HR Wallingford, 2009). During the first three decades of the twentieth century some net beach accretion was recorded by observation. 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 following its completion 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 and 2006; Havant Borough Council, 2007; Moon, 2010). Losses were calculated at 30,000m per year, 1985 -1994, (Whitcombe, 1995), but initial loss up to late 1987, was 88,000m per year (Hydraulics Research, 1987). Average annual losses between 1985 and 2003 are estimated at 38,000m³ (ESCP, 2012a). The impacts of early storm events were 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 net 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 recharge and recycling, totalling over 100,000m³ between 1985 and 1993 (Whitcombe, 1995, 1996). Overall, between 1985 and 2000, net losses from the entire Eastoke frontage were approximately 5,000m³ per year, with a total input of slightly over 20,000m³ per year from recycling operations balancing gross loss of 25,000m³ per year (Whitcombe, 1995; Havant Borough Council, 2000). These figures have been recalculated by ESCP (2012a), who suggest a net loss of 38,000m³ per year, 1985 to 2003. Beach crest retreat at a mean rate of 1m per year 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 trends and fluctuations was that by 1990, the swash limit of severe storms had migrated back to the walls protecting the gardens of adjoining properties (Photo 16). Renourishment since 2001, up to 2011, has sought to compensate for ongoing losses, estimated at 42,000m³ per year (ESCP, 2012a). Bradbury et al. (2007; 2011) examined the response of Eastoke beach during several storm events between 2003 and 2008, including that of 3 November 2005 which was characterised by extreme wave run-up, severe crest cut back, backshore overtopping and hinterland flooding. This latter event was associated with a bimodal (wind and swell) wave spectra, a maximum storm-propagated wave height of 3.5m, swell wave heights in excess of 2m and a 0.5m surge. The swell train was recorded at the nearshore Waverider buoy in Hayling Bay, 9km offshore, three hours before the arrival of the storm peak, but the peaks of both wave types coincided. Analysis revealed that approximately 40% of total wave energy during this event was derived from the long period swell fraction. Other storms incident on this beach did not result in overtopping, and were either associated with a bimodal spectra when breaking wind and swell waves were out of phase or when the wave train was unimodal. Mortlock (2012) examined the same storm events for this beach, and concluded that conventional formulae used for predicting overtopping under estimated wave run-up elevations when incident waves are bimodal. There are evident implications for beach management.
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). A ness was built out towards Eastoke Beacon, although there were also losses in the first two years due to storm events (HR Wallingford, 2009.) Accretion continued between 2001 and 2007, (with surplus material - some 30% of volume - removed for updrift recycling) but significant erosion was experienced during the storm surge of March 2008. However, the south-western sector continued to lose material after approximately 1988, and rock armour was introduced in 1992 and subsequently after 2006 to offset substantial upper beach crest retreat and profile flattening (Whitcombe, 1995; HR Wallingford, 2009). Eastoke Point will be stabilised further by Havant Borough Council with a rock revetment, rock groynes and 25,000m³ of replenished material in 2013, funded by Flood Defence Grant in Aid.
Sandy Point
Map analysis indicates a phase of accretion at up to 5,000m³ per year between 1842 and 1932 followed by erosion at 1,000-2,000m³ per year between 1932 and 1967 (Harlow, 1980). This was attributed to effective though partial interruption of northward littoral drift from Eastoke Point by groynes protecting Treloar Hospital, built in 1949 (Harlow, 1980) and later structures. This sector had not previously benefitted from renourishment sourced from updrift, as transport is directed offshore into the Chichester Harbour entrance channel. However, significant accretion of fine to coarse sand around the distal point of Sandy Point since 2008 suggests that this pathway of movement is either selective of grain size or that drift rates have increased. ESCP (2012a) state that the beach between Black Point and Sandy Point increased in volume by 18,790m³, 2004 to 2012.
Beachcot and Beachlands
At Beachcot, increased protection provided by a short seawall to a building (Inn-on-the-Beach) on the beach backshore 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, whilst the beach developed a quasi-equilibrium form. Breastwork defences were removed in 2012 in response to this decline in beach volume. (Harlow, 1980; Hydraulics Research, 1988; HR Wallingford, 1995; Moon, 2006; ESCP, 2012a). The beach to the east of Beachlands has experienced net accretion since the mid-1980s, partly, perhaps mainly, due to receiving material drifting westwards from the Eastoke replenished beach (refer to ESCP 2012a for details of annual variations since 2004). This is a cumulative effect, offset by removal of gain due to periodic, and since 2009 annual, recycling operations (Havant Borough Council, 1999; HR Wallingford, 2009; ESCP, 2012a) further east. Between Beachlands and Beachcot, beach volumes have been declining since 2004. ESCP (2012a) calculate that, taking the south Hayling frontage as a whole there was a gain of volume of approximately 140,000m³ between 2004 and 2012. Subtracting the 180,000m³ introduced as recharge material over the same period, this reveals a net loss of 40,000m³ (5,000m³ per year).
Gunner Point
Historical accretion has been well documented for this multi-ridged gravel cuspate foreland and began at least 400 years ago. Accretion has been variable, at a mean of 5,600m³ per year 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 backshore beach crest until approximately 1999 (Hydraulics Research, 1988, HR Wallingford; 1995, 2009; Havant Borough Council, 1999). Accretion has resumed since then, (calculated to have been 92,250m³ between 2004 and 2012 - ESCP, 2012a) characterised by discrete pulses of gravel moving onshore along the southern shoreline then migrating west and north to prograde the beach delimiting the Langstone channel. Since approximately 1910, Gunner Point has advanced seawards some 200m. From 2003 to 2012 the Gunner Point area increased in volume by 248,000m³ (CCO, 2014).
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 west of Fort Cumberland from map comparisons over the period 1842-1976 (Harlow, 1980), 1896-1972 (Grontmij, 1973) and 1870 to 2002 (Halcrow, 2010a). The accretion rate averaged 10,000-12,000m³ per year for the beach as a whole up to 2000 but has declined since. Since 1966 accretion at 4,000m³ per year has been recorded behind the sewage outfall constructed at East Point, where there was some gravel extraction- of unknown quantity- in the 1960’s (Webber, 1982; New Forest District Council, 2010). Northward of this point beaches flanking Langstone Harbour entrance have become depleted due to interception of drift by this outfall (Webber, 1974, 1982). Halcrow (2010a), analysed beach monitoring data identified a trend towards erosion between Eastney Point and Fort Cumberland, but some widening of Eastney spit. Beach profiles at Eastney 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 but net accretion. There was extraction of backshore shingle at two borrow pits west of the former royal Marines barracks in the 1960’s and early 1970’s, but no records of quantities removed are available.
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 interannual gains and losses of sediment (with overall accretion) between 1980 and 2002. Between the early 1950’s and mid-1970’s the beach between the War Memorial and Clarence Pier was significantly wider than it has been in more recent decades. Drawdown may be related to incremental deepening of the access navigation channel to Portsmouth Harbour, which is close inshore. The frontages west of Clarence pier and around the salient of Southsea Castle are heavily protected by seawalls and concrete slopes related to generations of military defences, and inhibit beach development. The short sector of the “Hot Walls” between the Square and Round Towers accumulates a confined gravel beach that also has a history of oscillating beach volumes (Hydraulics Research, 1987b; Halcrow Maritime, 2000; Halcrow, 2010a).
6. Summary of Sediment Pathways and Budget
- This unit comprises a naturally eroding and transgressive shoreline fringed by gravel barrier beaches and interrupted by the inlets of Portsmouth, Langstone and Chichester Harbours formed by inundation of the low valleys of southward flowing tributary streams of the Solent River. As sea levels have risen, this shoreline has followed a classic transgression model of a coastal barrier migrating across a low-lying hinterland, although its behaviour is complex in detail due to the effects of the major tidal inlets and major constraints imposed by management;
- Net drift operates primarily westward along the shores of Bracklesham Bay, Hayling Bay and Portsea Island and delivers shoreline sediments to the inlets of Chichester, Langstone and Portsmouth Harbours, respectively. Sediments are then flushed seaward by tidal currents and stored within large ebb tidal deltas. Variable quantities are driven ashore from the deltas by wave action, whereupon they may be re-circulated within a short shoreline drift reversal back to the original inlet, or continue to drift westward along the beach to the next inlet in the sequence;
- The ebb tidal deltas form important secondary controls on this coastline due to their storage of sediments and their dissipation of wave energy that provide local stabilising influences;
- Intense cycling of shoreline sediments occurs between beaches, tidal inlets and tidal deltas with most materials being stored over variable timescales within the deltas. Spits have grown across the inlets under the control of the inlet regime and local wave driven sustaining drift pathways;
- Tidal exchange between the harbours and the open coast occurs at narrow inlets generating locally strong currents that intercept drift, flushing sands and gravels seaward to form the ebb-tidal deltas. Drift transports coarse sediments towards each harbour entrance to form double spits around relatively narrow inlets that are maintained naturally by tidal exchange. The spits at the entrances (e.g. Black Point and East Head) provide vital protection against wave penetration and enable deposition of fine sediments within. The extensive ebb-tidal deltas cause shoaling of approaching waves and dissipation of their energy to provide shelter to the spits and tidal inlet.
- On Hayling Island, western parts of the open coast around Gunner Point have accreted strongly and prograded, whereas eastern parts have eroded persistently and the major 1985 replenishment here has required intense beach management to maintain its longevity. The reasons for these changes are: (i) net east to west drift that delivers sediments to Gunner Point (ii) a drift divide at Eastoke creating a short drift reversal that transports beach sediments rapidly into the Chichester tidal inlet, and (iii) failure of return onshore transport at Eastoke to match the drift losses to the Chichester inlet.
- Over the past 30 years the sediment budget of this cell has been strongly modified by artificial inputs and outputs of sediment. Ignoring the 2.3 million cubic metres extracted from the Horse and Dean Sand sediment sink in the early 1970s, the dredged output from the active sediment circulation system was over six times the input by replenishment up to the late 1990s. If this situation were to resume, a sediment shortage may develop leading to reduced beach levels and erosion of sediments from onshore stores, e.g. Gunner Point. The regular, and detailed, analysis of beach volume changes since the latter 1980s (Havant Borough Council, 1992b; 1999; Moon, 2010; ESCP, 2012a) provided an opportunity to derive a quantified sediment budget for this sector and the regular Coastal Monitoring Programme surveys have allowed further detail to be added.
7. Opportunities for Calculation and Testing of Littoral Drift Volumes
Data collected by the Defra-funded National Network of Regional Coastal Monitoring Programmes is pivotal for future improvement in estimating beach change. The Programmes consist of topographic beach surveys, nearshore bathymetry, aerial photography, lidar, coastal hydrodynamics (waves and tides) and terrestrial habitat mapping. Specifications for data collection are consistent for all regional programmes and the data and analysis reports are made freely available under the Open Government Licence from www.channelcoast.org.
The Southeast Regional Coastal Monitoring Programme commenced in 2002. The Lead Authority is New Forest District Council, with data collection, analysis and reporting led by specialist teams at the Channel Coastal Observatory (CCO), Canterbury City Council and Adur and Worthing Councils. Longer term Coastal Monitoring Programme data, when combined with other data sets, academic research (refer to Moon, 2010; ESCP, 2012a) and historical studies may enable sediment budgets, transport rates and directions to be identified and/or validated in the future. Budget calculations for discrete sectors of both beaches and nearshore/offshore sediment stores and sinks may provide an appropriate approach (Moon, 2010). The 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, repetitive beach replenishments to offset short-term volumetric losses and presence of groynes and other control structures means that shorelines of this frontage are not well suited for definitive studies of drift.
8. Knowledge Limitations and Monitoring Requirements
Notwithstanding results from the Southeast Regional Coastal Monitoring Programme, recommendations for future research and monitoring that might be required to inform management include:
- The Hayling-Portsea coastal unit has been researched in some detail and the major pathways and directions of sediment transport are established. Despite this, much of the quantitative information has been estimated from beach volume changes and the mechanisms controlling sediment transport require further research. The numerical modelling of sediment transport that has been undertaken to date has not always been rigorously tested against the recorded changes in beach volumes;
- The effective application of numerical modelling studies of beach behaviour and sediment transport processes requires the input of high quality nearshore bathymetric survey data to improve tidal modelling and the representation of the nearshore wave climate. This is especially important for those sectors of the near and offshore environments with complex landform and sediment associations, especially around the harbour inlets and ebb tidal deltas;
- 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 is often a key constraint in undertaking modelling;
- The scale and periodicity of onshore transport of coarse sediment to Hayling and Eastney beaches from the offshore tidal deltas remains poorly understood. Quantitative information has been exclusively derived from beach accretion so that actual pathways and the nature of this input (i.e. whether pulsed or steady) remain very uncertain. Tracer experiments could be employed to provide original data; the recovery of tracers underwater could identify wave conditions favourable to onshore transport. Further information about onshore feed is necessary as the source areas (e.g. Chichester bar) are subject to dredging yet it is uncertain whether this activity adversely impacts on the overall transport flux;
- Navigational dredging close inshore is not covered by the Government procedure for aggregate extraction, which requires each application to be reviewed according to a rigorous set of guidelines. Ideally, this procedure should be applied to dredging operations on the tidal deltas, for it is possible that subsequent sea-bed modifications could affect wave refraction; this, in turn, could influence littoral drift and hence beach erosion. This may particularly apply to recent changes at Gunner Point, which is located immediately onshore from dredging areas on East Winner and Langstone Bar.
- The replenished beach at Hayling initially lost material at a significantly greater rate than predicted. Although some of this loss results from offshore transport of fines and drift into Chichester Harbour inlet, some loss may have resulted from attrition. Estimation of attrition losses could be undertaken to assess the longevity of this recharged beach, although is relevant to all beaches throughout this unit. Research undertaken by the BERM project (Dornbusch et al., 2002) on abrasion wear of flint clasts in East Sussex provides some methodological guidelines;
- Littoral drift rates on the replenished Eastoke Beach have been more rapid than predicted post replenishment. It remains uncertain why this is and whether it relates to errors in previous studies, or a genuine acceleration of drift along the beach. It the latter is correct this phenomenon needs to be understood in order to inform future management of the beach;
- Both the net direction and rate of littoral drift at Southsea remain variable and difficult to quantify.
