East Head, West Wittering and Cakeham: Interpretation of beach changes

Transcription

1 East Head, West Wittering and Cakeham: Interpretation of beach changes Draft Report to: Chichester District Council and East Head Coastal Issues Advisory Group Malcolm Bray University of Portsmouth March 2010

2 CONTENTS Contents Introduction Background Methods Data Collection Data Analysis Wave conditions and storms Beach and foreshore changes Cakeham West Wittering The Hinge Spit Neck Spit Head Spit Tip Discussion Critique of Analysis and Interpretation Key Insights Management Considerations Sand Accretion at Cakeham and West Wittering The Hinge: erosion indicators and triggers for action Spit Neck: erosion indicators and triggers for action Conclusions Recommendations Acknowledgements References Dune Management Resources APPENDICIES (supplied in separate files) Appendix 1 Original difference mapping produced by Channel Coastal Observatory Appendix 2 Profile analysis summaries extracted from CCO annual reports 1

3 1. INTRODUCTION This report aims to provide an analysis and interpretation of changes occurring along the beach and foreshore of the Cakeham, West Wittering and East Head frontage. It updates and extends the geographical coverage of a similar previous study (Bray, 2007). It is concerned primarily with the period August 2004 to October 2009, but includes analysis of some profile data extending up to December The objectives are as follows: 1. Identify the main changes occurring to the beach and foreshore for covering areas defined by the Bray (2007) study, including potentially sensitive areas such as East Head spit neck and the Hinge; 2. Extend analysis to cover the Cakeham frontage ( ) including assessment of recent sand accumulation; 3. Attempt to provide explanations for the changes occurring, including estimations, where possible of likely trends for the future; 4. Comment upon potential implications for on-going and future monitoring and management at East Head, West Wittering and Cakeham. Although the brief was primarily to evaluate and report upon the beach and foreshore changes within the specified areas and time periods, likely future changes and their implications for management will be discussed where applicable. 2. BACKGROUND The area under consideration comprises the eastern side of the entrance to Chichester Harbour at the western extremity of Bracklesham Bay. It is a site characterised by complex processes of sediment transport powered by waves from the English Channel and Spithead/Hayling Bay and currents generated by the exchange of tidal waters at the harbour entrance. These forcing agents have been investigated and summarised by several previous studies (ABP, 2001a and 2001b; HR Wallingford, 1995 and 2000; Posford Duvivier, 2001). The sediment transport pathways have been studied by HR Wallingford 1995; 2000 and McLaren, (2000) and are summarised by Bray et al (2004). These items, together with monitoring data covering the period 2004 to 2006 were analysed within the study by Bray (2007) and a further interim update by Bray (2008). In summary, shoreline drift feeds sediments towards the harbour entrance from Bracklesham and Hayling Bays where sands and gravels are intercepted by strong tidal flows and preferentially flushed seaward by dominant ebb tidal currents. This transport system has resulted in formation of small spits flanking the entrance (Black Point and East Head Spit) and a very much larger ebb tidal delta seaward of the entrance comprising an estimated 25million cubic metres of sediment. Storms drive sands and gravels back towards the entrance and towards the shore where a significant feed of sand is identified (Figure 1). Review and listings of the key insights generated by those previous studies were provided by Bray (2007). Further developments since that report are summarised as follows: 1. Expert guidance was sought by the East Head Working group in respect of the likely future evolution and the potential options for management at East Head and the Hinge. A report was produced (Brampton et al., 2007) which highlighted the uncertainties associated with making predictions at this location, especially concerning the likelihood of breaching at the spit neck and Hinge and whether any such breach could become permanent. It identified shoreward migration of the nearshore channel seaward of the Hinge that was leading to beach narrowing. It suggested that if the present defences at the Hinge were not maintained, the shore 2

4 would be likely to retreat with the spit neck experiencing a potential increase in wave exposure and sediment supply. It concluded that it was important to undertake a policy of monitoring and adaptive management in order to avoid a breach occurring. 2. Major incursions of sand have been experienced along the West Wittering and Cakeham frontages since 2007 with significant quantities blown to onto the upper beach and backshore. Some tonnes of sand were excavated from the backshore, where it was accumulating adjacent to the rear fences of properties at Cakeham and deposited on the beach a short distance to the west in December Some 1,000 tonnes of sand were excavated from the back shore of the West Wittering frontage and deposited below mean high water mark on the beach at the Hinge in May Some 50 tonnes of sand were removed from the West Wittering backshore in Sept./Oct and placed on the upper beach and defences above mean high water at the Hinge. These two operations can be classified as bypassing since the sediment was transported downdrift artificially. Sand accumulating in front of beach huts on the West Wittering frontage has been excavated and deposited as a berm on the backshore immediately in front extending along a 400m frontage. This is estimated to involve between 3,000 and 6,000 cubic metres of material; 3. A berm set back landward of the Hinge has been constructed using some 13,000 tonnes of sand recycled from the foreshore immediately west of the northern tip of East Head Spit in March Its purpose is to prevent any breach occurring should the defences fail and the Hinge erode catastrophically; 4. An MSc dissertation has been completed by White (2009) who analysed a significant quantity of GPS and LIDAR survey data from 2004 to 2009 focusing upon East Head spit, but extending also eastwards to Cakeham. Some of the surveys are additional to those of the Channel Coast Observatory (CCO) and so offer valuable additional information and the opportunity to verify the results of the CCO data. Some analyses, however, utilised profiles extracted from ground models which are a little less reliable that the directly measured profiles available from CCO. Approx. outer margin of ebb tidal delta Figure 1. Summary of Sediment Transport pathways based on interpretations of Brampton, Bray and Townend (2007). 3

5 3. METHODS The report provides an interpretation of monitoring data measured by Havant Borough Council and managed and processed by the Channel Coast Observatory (CCO). 3.1 Data Collection Data is collected using a differential GPS system operating in kinematic mode with one receiver static on a precisely known point and the other traversing across the foreshore carried by the surveyor and recording planimetric locations (Ordnance Survey eastings and northings) and elevations (Ordnance Datum Newlyn). These types of instrument typically would survey to accuracies of within 2cm to 3cm and CCO routinely audit the quality of their survey data and reject or tag any not meeting strict criteria (Channel Coast Observatory, 2010). Two types of survey plan are adopted: (i) (ii) Area surveys: where an approximate grid is traversed with the aim being to cover all parts of a specified foreshore allowing production of digital ground models to represent the three dimensional morphology. Annual surveys were available in this format for August 2006, July 2007, July 2008 and October It should be noted that whereas six earlier surveys were available between 2004 and 2006 for East Head and West Wittering, area data are only available from August 2006 onward for the Cakeham frontage; Profile surveys: where exactly the same fixed shore-normal lines are traversed on each survey visit enabling two dimensional profiles or sections to be superimposed from one survey to the next. Some 71 profile lines were available extending from 5a00168 (Cakeham/East Wittering boundary) to 5a00233 (tip of East Head spit). Up to three surveys per annum, according to profile, were typically available in this format covering the period August 2003 to Dec The profile data are especially valuable at Cakeham because: (i) they cover the period prior to the start of area surveys and (ii) they extend landward into the accreting dunes beyond the landward limit of past area surveys. Future area surveys will be extended by CCO to ensure coverage of the dunes. 3.2 Data Analysis The project brief was for an analysis of processed area data, however, following discussions with CCO the scope was extended to include the profile data also. This was undertaken to provide improved temporal coverage and to aid interpretation of trends. CCO routinely analyse every third or fourth profile within their annual reports and provide time series plots and summaries of trends in cross section area (Channel Coast Observatory, 2009). Since these data were readily available it was agreed that they would be used in conjunction with the area data. Plots were also obtained from CCO of intermediate (interim) profiles although these were typically surveyed less frequently (usually annually). Area Surveys Staff at CCO prepared ground models from each set of survey data and then overlaid successive surveys and subtracted the respective foreshore levels of each model to reveal areas of erosion (loss) and accretion (gain). The differences between pairs of surveys where contoured at 0.25 m intervals, colour coded and overlaid onto georectified aerial photographs enabling areas of change to be mapped in their true spatial positions with respect to features appearing on those photos. An example is provided in Figure 2, but full copies of the original maps are presented within Appendix 1. It is considered that measurement, surveying and ground modelling errors should be significantly less than 10cm in total per survey, thus any comparison of survey pairs should yield net errors of no more than 0.2m. Since the contour interval selected for mapping of differences is 0.25m, it must be assumed that all changes mapped are real. 4

6 East Head, West Wittering and Cakeham: beach changes Profile Surveys Twenty profiles within the study area were analysed within the most recent CCO Annual Report (Channel Coast Observatory, 2009) comprising 5a00168; 5a00170; 5a00173; 5a00176; 5a00179; 5a00185; 5a00189; 5a00195; 5a00198; 5a00201; 5a00208; 5a00212; 5a00215 (Hinge); 5a ; 5a00225; 5a00229; and 5a00233 (N. tip of East Head spit). CCO analyses provided within the annual report included: (i) an overall summary of net annual change; (ii) plotted time series of profile surveys (back to August 2003) and (iii) trend analysis of all beach cross-section areas back to August Additional analysis of cross section areas was undertaken to calculate areas above -1.84m O.D. (whole profile) and +1.5m (upper gravel beach). This was done as a means to separate trends affecting the gravel upper beaches (e.g. at Cakeham and West Wittering) from those occurring across the wide sandy lower foreshores in front. Interpretation The site area was sub-divided into five foreshore change sub-units based on physical characteristics and a preliminary assessment of changes (see Figure 2). The aim was to define relatively homogenous frontages that had behaved in similar manners. Spit tip Spit head Spit neck The Hinge West Wittering Cakeham Figure 2. Sub-division of foreshore change units plotted on CCO difference mapping. 5

7 Descriptive time series of changes occurring within the six different parts of the site were compiled by cross-referencing the area and profile data. Summaries of wave and storm surge conditions dating back to August 2003 were extracted from the CCO annual report. Reference was made to processes and trends reported from the literature or inferences made from the author s personal experience to provide explanation of the changes identified. It should be understood that the high quality data covers just six years and therefore still provides only a short interval for analysing changes. It is recommended that the report by Bray (2007) also be consulted since it sets changes within a longer term context. 4. WAVE CONDITIONS AND STORMS Summaries of wave conditions and storm events were extracted from the CCO annual reports covering the period July 2003 to May 2009 (Channel Coast Observatory, 2009). The summaries were based upon data collected by the East Solent buoy operated by CCO. It is a Datawell directional wave rider buoy (mark 3) moored in a water depth of 10.2m CD located some 5km due south of Eastoke, Hayling Island. It is seaward and to the south west of the Chichester ebb tidal delta that would not have affected the waves recorded. Waves affecting the study shoreline do, however, have to travel over the ebb delta and would be subject to significant shoaling and refraction transformations in the shallow waters. Larger waves are likely to be induced to break over the delta during most tidal states, so that the maximum shoreline wave height would normally be depth limited. Figure 3 summarises this data. Figure 3. Direction vs. Significant wave height for July 2003 to May 2009 (Channel Coast Observatory, 2009) 6

8 The analysis demonstrates that by far the majority of waves approach from between 180 and 210 degrees, including most, but not all of the higher waves. The very highest waves approach from between 165 and 180 degrees. Wave energy from west of 210 degrees is low because of the sheltering effect of the Isle of Wight. Summary annual statistics are presented within Table 1 enabling a very coarse comparison of forcing conditions during the period covered by topographic surveys. Table 1 Summary annual wave statistics Period Mean Hs (m) Mean Direction Max mean monthly No. storm events (deg) Hs (m) Hs > 2.4m Jul 2003 Jan * (Nov) 12 Jul 2004 Jun (Oct) 3 Jul 2005 Jun (Mar) 11 Jun 2006 May (Dec) 20 Jun 2007 May (Jan) 18 Jun 2008 May (Jan) 11 Hs is height of the highest one third of waves recorded (significant wave height). * potential directional problem with data. The mean wave heights reveal that the period was significantly more energetic than the preceding recording interval of The directional analysis suggests that the annual, directional climate has been relatively steady at the buoy location although it s difficult to relate this to potential shoreline conditions without analysing the significant effects of refraction occurring over the delta. A significant monthly variation is observed in wave heights. Although a peak in October in 2004 correlates with the initial overwashing of the neck of East Head spit on 28/29 th October 2004, the monthly heights in were significantly higher with concentration in December and January. The peak monthly average wave height recorded to date attained 1.41m in November 2010 which was an exceptionally stormy month. A significant annual variation can be seen in the numbers of severe storms where waves exceeded 3m significant wave height (Table 1 and Figure 4). Figure 4 Storm events Hs>3.0m July 2003 to May 2009 (Channel Coast Observatory, 2009). A significant increase in the most extreme events was recorded from 2006 onward and a maximum significant wave height of 3.79m was recorded on 10 th March 2008 in conjunction with a storm surge of 1.09m (recorded at Portsmouth). If all events yielding wave heights 7

9 over 2.4m are tallied the increase in storm activity from June 2006 onward becomes even more striking (Table 1). An important observation made from examination of the collected wave data is that there is significant swell wave activity within the record. The 2008 annual report (Channel Coast Observatory, 2008) identifies instances of bimodal wave activity within the storm events (Figure 5). These are composed of joint occurrences of significant storm waves (locally generated) and swell comprising long period and wavelength waves that penetrate up the English Channel from the Atlantic and which are refracted around the Isle of Wight and subject to focusing by seabed bathymetry upon parts of Hayling and Bracklesham Bays. This swell wave phenomenon is known to surfers who congregate at West Wittering to ride the waves as they shoal and break over the ebb tide delta and nearshore banks. These bimodal events are a concern because they contain significantly greater energy and are often linked with beach erosion, overwashing of spits and barriers and overtopping of defences worldwide. Indeed, they were implicated in some of the post-replenishment beach losses from Hayling Island in the 1980s, although quality wave records were not available at that time (Hydraulics Research, 1987). Figure 5 Occurrences of storm wave events with bimodal distributions due to presence of swell waves (Channel Coast Observatory, 2009). A significant consequence of the more stormy winters and of the increased energy evident from 2006 onwards is that there would have been a greater potential for waves to transport sediments from the ebb delta landward into the harbour entrance and towards its flanking shorelines. Transport directions during calmer periods are likely to be determined by tidal currents so that net transport would tend to be seaward in the main inlet channel during those periods, especially within the main channel where currents are focused. Material presented by ABP (2001a) and HR Wallingford (2000) would generally support a regime involving prevailing ebb current driven seaward sediment transport interrupted by brief, but intense landwards storm wave driven transport. The greater wave energy would also tend to generate greater net littoral drift along shorelines provided that sediments were available for transport. It does not link directly to the potential for upper beach erosion and spit overwashing because that is controlled by the coincidence of the peak wave activity and storm surge with the stage in the high/low and spring/neap tidal cycles. Indeed, the largest combined wave and surge event which occurred on 10 th March 2008 fortunately peaked during low water. A full analysis of the joint wave, surge and tide conditions is beyond the scope of this report. 8

10 5. BEACH AND FORESHORE CHANGES To simplify the complex patterns recorded, changes are presented according to the six shoreline sub-units identified in Figure 2. Excerpts of the difference mapping are provided for each sub-unit (Figures 6 to 12), but readers should consult the plots in Appendix 1 for the full extent of mapping together with keys to contour shading and map scales. Changes occurring within each unit are presented between June 2006 and October 2009 (East Head West Wittering) and (Cakeham). These periods alone remain too short to enable trends to be extrapolated forward reliably so it is important also to consider the earlier changes occurring between 1946 and 2006 that were presented by Bray (2007). 5.1 Cakeham Profiles 5a to 5a00198 Different types of behaviour appeared associated with an eastern (low change) part of the frontage as opposed to central and western parts where changes were much more significant. The overall pattern is of major mid and upper foreshore accretion within linear features within central and western areas with corresponding losses on the lower foreshore. The patterns recorded support the earlier interpretation of Bray (2007) that the changes involved the onshore migration of swash bars of sand from the East Pole Sands. Indeed, the zones of accretion at the seaward extremities of profiles 5a00189 and 5a00194 would appear indicative of the continued emergence of swash bars from sub-tidal areas. Figure 6. Cakeham frontage changes Eastern Frontage (Profiles 5a00168 to 5a00179) Profiles generally varied in level over the study period by less than 0.5m and few distinct features were evident except for small swash berm variations on the upper beach. Limited changes were recorded over the period 2003 to 2006, followed by modest gains in sediment on the mid foreshore reaching a peak in February Small losses then occurred to October 2009 with a sharp recovery being recorded in December Trends were more marked on the mid and lower foreshore seaward of the 1.5m contour than on the gravel upper beach which either remained steady or gained material slightly. The net result tended to be an overall gain of some 5-10% of the profile area due to gains of sand on the lower/mid beach. Backshore scarps or vegetated breaks of slope remained stable and did not alter significantly. 9

11 Central Frontage (5a00179 to 5a00185) Characteristic multiple swash bars were identified on the lower and mid foreshores. On some profiles up to four bars spaced at roughly 100m intervals were present. The bars varied in amplitude from 0.4m to 1.2m and in crosshore width from 40m to 150m. A small bar on profile 5a00179 was observed to emerge at low water in August 2006 and migrate rapidly shoreward at over 100m per annum to weld itself to the mid/upper beach in May The largest bar on profile 5a00185 was observed to emerge in Nov and migrate landward more slowly at around 50m per annum to weld with the mid/upper beach in May Assuming a longshore extent of 1km, width of 150m and thickness of 1m a bar volume of 150,000m 3 of sand can be estimated. The upper beach and backshore altered very little over this period accreting slightly, whereas the mid and lower foreshore accreted more significantly increasing in cross section area by 6-17%. A tendency was noted again for sediment gains to peak in February 2008 and December Western Frontage (5a00185 to 5a001198) Multiple swash bars are again evident migrating landward across the wide low gradient sandy foreshore. It is possible to trace their emergence at low water (in 2004, 2005 and 2008) and their subsequent migration at 50m to 100m per annum across the foreshore to weld with the mid/upper beach. In spite of the obvious potential for sediment input the overall gain on the lower foreshore (seaward of the 1.5m O.D. contour) amounted to no more than 6% to 14% of the profile cross section area. An important difference along this part of the frontage was that the upper beach above the 1.5m contour accreted significantly with the cross section area increasing by up to 70%. The gains started to become evident from summer 2007 onward and appeared to accelerate after winter Accretion in the form of blown sand extended over the backshore behind the beach building a narrow dune berm up to 7m O.D. elevation (representing up to a 1.5m gain in elevation between July 2007 Oct 2009). Significant infilling up to 0.7m depth by blown sand also occurred behind the elevated dune ridge as sand was blown a further 10 15m landward. Locally, even greater depths of sand accumulated against the rear fences of several of the coastal properties. In December 2008 some tonnes of sand were excavated from in front of the fence of two adjoining properties, yet by December 2009 sand had again built up sufficient to overtop the fence in places. Increasing quantities of blown sand and dune growth clearly appear related to the influxes of sand received by the mid beach as a result of the onshore migrating swash bars. Summary and Interpretation It appears that bars form on the sea bed at East Pole Sands beyond the seaward limit of the survey area and then migrate onshore at m per annum before coalescing (welding) with the mid-upper foreshore. This process delivers significant quantities of sand to the shoreline from East Pole Sands and the ebb tidal delta. Similar movements of swash bars are reported widely from sand dominated inlets on east coast of USA and elsewhere in the world and are part of a mechanism by which sediments can recirculate between a tidal inlet, its ebb delta and adjacent shorelines (Fitzgerald, 1996; Fitzgerald et al. 2000). The recorded rates of bar migration of ma -1 are within the range reported within the literature. Migration rates tend to be faster in locations with small tidal ranges and can be as low as 30ma -1 at strongly macro tidal locations such as the west Normandy coast of France (Robin et al. (2009). Migration tends to slow as bars move further up the shore profile and their tidal inundation period reduces. The eastern boundary of the zone of swash bar migration is identified at 5a00179 and the zone itself extends NW into the West Wittering frontage. Some leakage of sand appears to cause slow lower foreshore accretion further east, but the major changes occur to the west of 5a Between this point and profile 5a00185 the swash bars mainly affect the wide sandy lower foreshore leaving the steeper upper beach and backshore relatively unaffected. Westwards of 5a00185 significant mid and upper beach accretion is also recorded as sand is 10

12 driven ashore by wind to form dunes on the backshore. A possible interpretation is that sand moving onshore is driven northwestward by the net littoral drift on the foreshore so that it is deflected towards the western Cakeham frontage by the time that it reaches the upper beach. An alternative explanation is that more time is required for swash bars to weld to the upper beach at central Cakeham, in which case the zone of upper beach accretion and dune growth could possibly extend further eastwards towards profile 5a The strongest accretion occurred after summer 2007 with peaks being recorded in February 2008 and Oct. Dec which appears to correlate with (i) the general increase in wave energy and storm waves occurring after 2006 and (ii) concentration of storm events within the winter months (see Section 4). This corresponds well with research into swash bars that suggests that they form most readily and migrate episodically onshore specifically during high energy wave events (Robin et al, 2009). It appears that individual bars can emerge every 1-3 years on a given profile and take 2-4 years to weld to the upper beach. One large bar was estimated to have a volume of some 150,000m 3 of sand suggesting a possible supply of some 50,000 to 150,000m 3 a -1 by this mechanism. The changes in profile cross section area were integrated with the distances between profiles to calculate approximate volumes of sediment gain for each of the three zones as shown in Table 2. Table 2. Beach and foreshore volume changes along Cakeham frontage *. Zone change April 05- Dec 09 (m 3 ) change July 07- Dec 09 (m 3 ) Total Upper beach >1.5m Total Upper beach >1.5m East 40,000 1,000-4,000 2,300 Central 46, ,000 1,200 West 148,000 32, ,000 28,000 TOTAL 234,000 33, ,000 31,500 % of total *volumes are calculated from profile data and assumes profiles are typical of intervening zones. It is evident that the majority of gains have occurred in the Western zone and that some 62% have occurred since July Only 14% of the gains involve accretion on the upper beach and backshore in spite of the obvious dune growth and the management problems caused by blown sand. All of the significant upper beach accretion occurred within the western zone in the July 2007 period. Beach excavations and artificial movements of sand undertaken for management since 2005 at West Wittering and Cakeham are modest compared to these estimated magnitudes of natural changes. The overall volume of accretion (234,000m 3 compares favourably with the estimated input from swash mar migration (200,000m 3 to 600,000m 3 ) over the four years Indeed, it might be expected that recorded accretion should be less than the total sand input due to losses by drift north westwards towards the Hinge and into the harbour entrance. A potential net north-westward sand drift of 25,000m 3 a -1 has been calculated to operate in this direction (HR Wallingford, 1995 and Posford Duvivier, 2001). 5.2 West Wittering East: Profiles 5a00198 to 5a00201 The earlier difference mapping revealed alternating bands of erosion and accretion across the wide foreshore indicating a progressive onshore movement of swash bars that could be tracked using profile data. Between 2004 and 2005 the most landward bars coalesced into the mid/upper beach resulting in accretion. A notable bar formed at around 500m offshore in summer and migrated 100m landward and 0.8m upward up to summer 2007 and eventually welded to the mid beach in May These events explain the broad area of 11

13 accretion developed on the upper foreshore that is evident from the difference mapping (Figures 7 and 8). Whilst the steep upper beach and immediate backshore changed little on profile 5a00198 the corresponding areas on 5a00201 accreted very significantly gaining up to 25m in width and 1m elevation due to dune growth. Significant erosion was evident at the seaward toe of the profiles along this zone where a nearshore channel appears to be migrating landward. However, in eastern parts adjacent to profile 5a00198 renewed accretion was recorded from Winter 2008/09 to Dec 2009, possibly indicative the emergence of further swash bars. The net effect was for a moderate gain of profile levels due mainly to gains on the mid and upper beach and mixed trends on the lower foreshore. Figure 7 West Wittering: net changes Sept 2005 Aug 2006 Jul 2007 Jul 2007 Jul 2008 Jul 2008 Oct 2009 Figure 8 West Wittering: annual changes West: Profiles 5a00201 to 5a00212 Swash bars were less pronounced on the lower foreshore of this zone and the major trend was for foreshore erosion especially at the seaward extremity where the nearshore channel has migrated landward and narrowed the beach. Erosion was most intense over the period April 2005 July 2007 although it continued at the seaward extremity up to December The upper beach eroded less during the earlier period and since July 2007 has accreted. A sharp berm can be identified on the backshore between 5a00203 and 5a00208 which 12

14 comprises sand that was artificially excavated from the backshore in front of the beach huts that line this frontage. Summary and Interpretation Distinctly different trends have been recorded with: (i) erosion prevailing from April 2005 to July 2007 with especially significant losses occurring at the seaward profile toes and (ii) accretion occurring from July 2007 to December 2009, especially on the mid and upper beach. Volumetric analyses indicated that erosion significantly exceeded accretion over the full time period with the eastern zone experiencing a net gain of 20,900m 3 and the western portion losing 50,200m 3. Profile changes suggested that accretion in the east occurred due to welding of onshore migrating swash bars to the mid/upper beach face, but these changes were not evident to the west of profile 5a It is likely that the nearshore channel (Figure 7) inhibits swash bar formation west of this point, so that the accretion of some 33,800m 3 recorded from July 2007 to December 2009 would need to be attributed to inputs from littoral drift arriving from the east. Previous research on the west Normandy, France coast has suggested that drift shadows develop immediately downdrift of zones of initial swash bar attachment with erosion occurring during a short lag period before the swash bar material becomes distributed downdrift (Monfort 2007; Robin and Levoy, 2007). The trends recorded here could be explained if a similar process were operating Table 3. Beach and foreshore volume changes along West Wittering frontage Zone change April 05- July 07 (m 3 ) change July 07- Dec 09 (m 3 ) Total Upper beach April 05 Dec 09 Total Upper beach >1.5m Total >1.5m East -11, ,500 14,300 20,900 West -84,000-3,000 33,800 21,500-50,200 TOTAL -95,600-2,100 66,300 35,800-29,300 *volumes are calculated from profile data and assumes profiles are typical of intervening zones. To summarise, the data suggests that onshore migrating swash bars deliver inputs of sand along a 1.3km zone between profiles 5a00176 (Cakeham) and 5a00205 (West Wittering). Accretion in excess of 250,000m 3 was recorded with around 70% occurring after July Some 47,000m 3 (18%) accreted on the upper beach above the 1.5mO.D. contour, the remainder accreted on the wide low gradient sandy foreshore. Total sand inputs undoubtedly exceed the amount of accretion recorded due to losses north westward by drift. 5.3 The Hinge Strongly mixed trends and highly variable behaviour were evident with a tendency for annual alternations of accretion ( ; and ) and erosion ( ; The net effect was for significant erosion at the seaward toe of the lower foreshore and for accretion on the mid and upper beach (Figure 9). The upper beach was quite variable where it adjoined the vertical timber revetment, perhaps experiencing cycles of scour and recovery relating to storm events and occurrences of wave reflection from the structures. A bar-type feature was observed on the lower foreshore from 2004 to 2007, but then disappeared The lower and mid foreshore in the three embayments between groynes C21 and C24 accreted significantly and the upper beach showed mixed trends, but was less variable (Figure 10). Between groynes C19 and C21 the lower and mid foreshore eroded and mixed trends occurred on the upper beach. 13

15 Figure 9 The Hinge: net changes Aug 2006 Jul 2007 Figure 10. The Hinge: annual changes Jul 2007 Jul 2008 Jul 2008 Oct The pattern from winter was reversed one year later when strong erosion occurred throughout the the mid and lower foreshore of all areas extending intermittently to the upper beach. Between groynes C21-C24 limited upper beach accretion was recorded Partial recovery of foreshore and upper beach levels were recorded by October 2009, although a proportion of this material was lost from some of the profiles by December Indeed, a clayey substratum was exposed intermittently near the toe of the shingle upper beach immediately to the east of groyne C21 from 9 th December 2009 onward. Although it undoubtedly relates to the exceptional stormy preceding month, this type of exposure is indicative of a depleted profile that is under pressure to retreat landward. The artificially constructed berm installed to avert beaching is evident behind groynes C22-24 at the Hinge. Summary and Interpretation Both annual and seasonal trends occured along this frontage within potentially confusing fluctuations of erosion and accretion that are difficult to explain. Based on analysis of data from profiles 5a00212, 5a00215 and 5a00208 a net loss of just over 30,000m 3 is estimated, of which some 90% of losses were from the lower and mid foreshore (Table 4). Losses from the upper gravel beaches are limited to the period which is immediately following the removal of groyne planking. The groynes appeared to generate some discontinuities in 14

16 the accretion and erosion patterns although the more intense erosion was observed on the lower foreshore at and seaward of the groyne toes and within the updrift groyne compartments (C 19 C21). It suggests that the groynes are generating a stabilising influence at a time when sediments appear in short supply at this location, but the complexity and inconsistency of the foreshore changes make it extremely difficult to draw further conclusions. It is possible that within a few years much larger quantities of the sediments migrating onshore at Cakeham and West Wittering should be transported to the Hinge with potentially beneficial effects upon foreshore levels. Table 4. Beach and foreshore volume changes along The Hinge frontage Zone change April 05- July 07 (m 3 ) change July 07- Dec 09 (m 3 ) Total Upper beach Upper beach April 05 Dec 09 Total >1.5m Total >1.5m All -14,800-3,100-18, ,400 *volumes are calculated from profile data and assumes profiles are typical of intervening zones. 5.4 Spit Neck This comprises the portion that was overwashed in autumn and winter and was reconstructed in Summer 2005 using 13,000m 3 of gravely sand recycled from the spit tip. Aug 2006 Jul 2007 Jul 2007 Jul 2008 Jul 2008 Oct 2009 Spit Head Spit Neck Figure 11 Spit Neck and Head: annual change in elevation (m) The Spit Neck area has experienced accretion events taking the form of subdued swash bars at the profile toe that have migrated landward up the profile to weld to the mid upper beach in front of the reconstructed spit neck (Figure 11). Such accretion intervals occurred in late 2004 to 2005, and Following onshore movement of the bars, notable zones of 15

17 erosion appeared at their former positions in 2006 and Furthermore, once the bars had welded to the reconstructed berm the majority of material was lost and appeared to feed midupper beach accretion further to the north in front of the dunes. Loss by drift northward would explain the occurrence in of a zone of erosion immediately in front of the reconstructed berm. By December 2009 this zone had largely recovered due to accretion by further onshore moving sediments. Whilst the majority of the sediments involved appeared to be sand, a notable swash bar of gravel welded itself to the upper berm in autumn 2008 and then migrated northward towards the spit head. A small quantity of the sand delivered to the upper face of the spit neck is blown further onshore and deposited within a zone of growing dunes immediately landward (east) of the footpath that runs along the reconstructed neck. The reconstructed berm crest has remained remarkably stable and has altered little since construction retaining an elevation of 4.0m to 4.2m O.D. The dunes on the berm have grown since Summer 2006 and have attained widths of 5-10m and elevations of up to 5m O.D. Some cliffing in the upper berm is evident up to a distance of 20m north of groyne C24, but this appears to be a very local process due to scour/outflanking induced in the immediate lee of the groyne. Interpretation and Summary Conditions on the foreshore in front of the spit neck appear connected to the episodic erosion and accretion intervals recorded at the Hinge and suggest that the sediments that periodically accrete on this foreshore are transported around the Hinge. The changes occurring on the lower and mid foreshore are frequent, but over time cause a gain-loss elevation within the range m. Persistent losses of the types observed prior to the 2004 overwashing events have not been observed, instead erosive periods tend to be followed by partial or complete recovery due to onshore moving sediments. The most recent period of recovery appeared in autumn/winter 2009 and was possibly connected to the exceptional wave energy of November 2009 (Section 4). In spite of the strong wave conditions of the period (Section 4) and profile erosion over , the reconstructed spit neck appeared stable and in places gained elevation and width due to dune growth. It has provided an effective defence and with a continuing spread of marram grass that appears instrumental in stabilising the areas of dune growth, its resilience against overwashing appears to be increasing. Analysis of GPS survey data by White (2009) revealed an upper beach gain of 10,100m 3 from 7 th June 2005 to 21 st July 2005 largely as a result of the recycling and berm reconstruction operation. Then from July 2005 to 25 th August 2009 a further 2,300m 3 of sediment accreted. Analysis of the CCO profile data covering the full intertidal profile from November 2005 to December 2009 is presented within Table 5 below. Table 5. Beach and foreshore volume changes along Spit Neck Zone change Nov. 05- July 07 (m 3 ) change July 07- Dec 09 (m 3 ) Total Upper beach Upper beach April 05 Dec Total >1.5m Total >1.5m 09 All -5,500-2, ,100-6,200 *volumes are calculated from profile data and assumes profiles are typical of intervening zones. Overall, the analysis reveals that this zone has lost sand amounting to around 9,000m 3 net from the mid and lower foreshore, but gained around 2,900m 3 net on the upper beach above 1.5m O.D. If it is understood that the November 2005 survey represented the most healthy lower/mid foreshore conditions ever recorded the losses can be seen to represent a net reduction from very high initial levels. Indeed, comparisons with the 2003 and 2004 conditions reveal that foreshore levels remain relatively high in spite of there recorded losses. Nevertheless, it will be important to continue to monitor the foreshore levels because 16

18 they control wave dissipation during critically high combinations of waves, surge and tidal levels and hence provide vital protection to the reconstructed berm. 5.5 Spit Head The mid-foreshore accreting bars observed using difference mapping (Bray 2007) appeared to migrate shoreward and northward and coalesced with the upper beach to result in an accreting zone some 50-60m wide adjoining the dune toes that extended along the full frontage (Figure 11). Along the majority of the dune front trends altered from erosion of some 25-40m ( ) to accretion except for the northern extremity where some dune front erosion persisted. Mixed trends were recorded from the lower foreshore especially at the most seaward extremities where profile data was available beyond the limits of the difference mapping and appeared to indicate areas of elevation gain adjoining the tidal channels. The difference mapping, however, indicated a tendency for elevation loss along much of the central and southern lower foreshore except for extreme northern parts adjoining Chichester Channel Although accretion continued at the dune toe along the upper beach it was weak and confined within a narrow strip. Rather stronger upper foreshore accretion was evident in the southern portion due to the migration northward of sediments from the foreshore in front of the spit neck. A notable zone of erosion occurred at the northern part of the spit head where the strong accretion of the preceding interval dispersed, probably representing northward drift and supply to the Chichester Channel. Dune front erosion and scarp recession was evident adjacent to this area and towards the spit tip, but total recession was only a few metres because upper beach levels recovered rapidly after summer Lower foreshore trends were again mixed although an accreting bar appears to have formed in the mid portion of the foreshore and erosion is indicated along many of the seaward extremities of the survey area Renewed accretion was evident along almost the full frontage of the upper beach and many of the dunes behind the beach accreted seaward by 4-7m and upward by 1-2m. In places, narrow foredunes or embryo dunes appear to be forming in areas previously subject to erosion. A prominent strip of erosion is evident on the mid/upper beach in the south and is indicative of the continued northward drift of a bar of sand and gravel that had previously developed in front of the spit neck. It suggests that the upper beach along the main body of the spit was fed over this period by drift from the south rather by sand moving onshore from the west. The lower foreshore again shows mixed and complex patterns of elevation gain and loss involving maximum variations in level of 0.8 to 1.0m. Once again the seaward extremities of the survey area exhibit the greatest losses and possibly are indicative of shoreward migration of the tidal channel that separates the East Head foreshore from the West Winner banks within the harbour inlet. Interpretation and Summary The trends recorded on the upper foreshore and from the dune front confirm that the change of regime from a predominantly eroding to an accreting condition which began in winter (Bray 2007) has now extended northward along the full west facing extent of the spit. Initially, it involved the development of at least two accreting bars on the lower foreshore followed by onshore and northward migration, but since 2007 it appears to have been sustained by sediments drifting northwards from the foreshore in front of the Spit Neck. Upper beach accretion has in turn protected the dune toes from cliffing and encourages renewed dune accretion. 17

19 In spite of the lengthening survey record the lower foreshore appears to present a complex series of variations in foreshore shapes and level with little obvious consistent activity. Some of the patterns are likely to arise from migration of bars and other bedforms under the joint effects of waves and tidal flow, making broader and longer term trends difficult to discern. Analyses of profile cross section areas were similarly inconclusive showing net gains in and losses from , especially in southern parts. The upper beach accreted significantly over the full period gaining at least 18,900m 3, but the lower foreshore suffered a net loss of around 28,900m 3 (Table 6). All losses occurred within the period and were concentrated in southern and central areas especially at the seaward profile toe. In spite of the uneven distributions of losses and gains, a net loss of less than 8,000m 3 was estimated which is small considering the very large foreshore area under consideration. It should be noted that only four profiles (5a00221; 5a00225; 5a00229; 5a00233) were used for the volume analysis representing a frontage length of almost 700m so that the reliability of the volume analysis could be less than was achieved for other areas. Table 6. Beach and foreshore volume changes along Spit Head Zone change Nov. 05- July 07 (m 3 ) change July 07- Dec 09 (m 3 ) Total Upper beach Upper beach April 05 Dec Total >1.5m Total >1.5m 09 All 32,900 11,500** -40,800 7,400** -7,900 *volumes are calculated from profile data and assumes profiles are typical of intervening zones. **potential underestimate arising from profile area calculation method. 5.6 Spit Tip This zone comprises a sandy gravel foreland (marked F on Figure 12) that developed initially in the 1970s, thereafter its west and north facing flanks fluctuated considerably. It also includes the north and northeast facing margins of the spit head terminating in a small sandy recurve at its eastern extremity. Immediately to the west of the tip of the gravely foreland lies the borrow site for the recycling of sand that was used to reconstruct the spit neck in June 2005 and for construction of the anti-breach berm behind the Hinge in March 2009 (marked rc on Figure 12) Western Flank of Gravel Foreland Erosion of the upper face and crest of the foreland continued from 2004 to summer 2007 with a 23m loss recorded at profile 5a00233 and a further 7m loss from summer with stability thereafter. Nonetheless, observations in December 2009 indicated that the foreland had been overwashed by waves during the preceding weeks. The upper foreshore in front of this foreland face, however, accreted strongly in and in response to northward migrating linear bars of sand arriving from the Spit Head (see Section 5.5). In the strong accretion of the preceding interval dispersed (Figure 12), probably representing northward drift and supply to the Chichester Channel. The lower foreshore representing the flank of the Chichester Channel has fluctuated considerably with strong accretion in , fluctuating levels up to summer 2009, erosion by October 2009 and recovery by December Since this was the general borrow area for the March 2009 Hinge berm the erosive trends could be a cause for concern, however, December 2009 profile data for 5a00233 suggested that there had been a good recovery of levels. Northern Tip of Gravel Foreland Accretion occurred on the foreland tip and its north eastern flank adjoining the Chichester Channel in with mixed trends and losses thereafter. Examination of profile 5a00235 revealed that the maximum northward extent of the foreland crest was achieved in June 2006 with 5m retreat to July 2007 followed by 11m retreat to May 2009 and another 8m 18

20 retreat to December 2009, amounting to 25m retreat in total. This trend is a little puzzling since recession of the foreland crest occurs in close proximity to accretion on the narrow foreshore immediately to the east of its tip. It could perhaps represent the initial stages of a reorientation of the foreland tip involving growth north eastwards rather than northward as had occurred up to rc F Aug 2006 Jul 2007 rc F Jul 2007 Jul 2008 rc F Jul 2008 Oct 2009 Figure 12 Spit Tip: annual change in elevation (m) rc = borrow material site for June 2005 and March 2009 recycling; F = gravel foreland 19

21 North Facing Shore Relatively little change has been recorded along this relatively sheltered segment except for a small but persistent zone of erosion identified originally over the period (Bray, 2007), which has continued to erode. Fresh cliffing was observed on a site visit on 12 th December 2009 and it appeared that the focus of erosion had migrated slightly eastward. Summary and Interpretation Recession of the western flank of the gravel foreland has reduced and lower foreshore levels have fluctuated, but overall healthy levels were maintained. The gravel foreland tip has, however, ceased to extend northward into the Chichester Channel and is retreating, although its eastern flank is accreting which could result in a clockwise rotation of the orientation of the foreland tip. Changes along the gravel foreland and its west and north facing foreshores appear to be controlled by the arrival and dispersion of sediment from south delivered by wave driven drift. Thus, its evolution is highly variable and the very storm waves that can deliver sediment are also capable of pushing that material into the Chichester Channel where is lost from the spit system. Since the foreland crest is mainly gravely, it could be that there has been a relative shortage of gravel within the incoming drift to account to the 25m recession that has occurred at its northern tip. The difference mapping suggests that the lower foreshore along the foreland flanks has fluctuated, but has experienced net accretion over the study period. The capability of the low gravel foreland to continue to project northward into a deep and fast flowing tidal channel (Chichester Channel) suggests that significant quantities of sediment continue to delivered northward along the spit, although it may be that gravel inputs have declined. Probably only a small proportion of the sediment arriving actually remains on the spit. Some could be pushed towards Pilsey sands during storms, but It is likely that the majority would be entrained by dominant ebb currents and flushed westward along the Chichester Channel and then seaward via the Emsworth channel, or possibly directly across the Winner. This circulation could then return the material to the Pole Sands, whereupon it would once again become available for transport back towards the harbour entrance, thus constituting a circulation or natural recycling of material. Sept Aug 2006 Aug Sept