Coastal Change Around The Wash

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the Nene, Spalding on the Welland and Boston on the Witham (Buck, 1997). 2.2.1. Bathymetry ... Offshore from the north Lincolnshire coast the sea bed inclines.
Coastal Change Around The Wash Assessment of past change, prediction of future change and identification of coastal squeeze PART 1: Literature Review

English Nature April 2004 Final Report CONTRACT No. I2.5.2 -160

Coastal Change Around The Wash: Literature Review

English Nature April 2004 Final Report 9P4956

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SUMMARY English Nature has a responsibility to report on the condition of its designated sites, including The Wash, which is part of The Wash and North Norfolk European Marine Site and is a Site of Special Scientific Interest. The Wash is the largest embayment in the United Kingdom, comprising intertidal sandbanks, mudflats and saltmarshes. The saltmarsh and intertidal flat areas are of considerable physical, ecological and recreational value. They support a wide variety of flora and fauna in an extremely dynamic and increasingly scarce ecosystem, and represent one of the United Kingdom’s most important feeding areas for wintering birds, particularly waders and wildfowl. The Wash provides a sheltered, low-energy environment in which tides and waves are the main factors controlling sedimentary processes. This environment favours accretion making the area an important sedimentary sink. Artificial embankments separate The Wash from the land-claimed coastal plain of the Fenland. Prior to the construction of these embankments, the transition from marine to freshwater sedimentary environments in the embayment was gradual and mobile. Intertidal sand and mudflats and saltmarshes would have graded landwards into freshwater-dominated environments and vegetation communities were able to migrate landward or seaward dependent on conditions. Today the coastline is marked by a sharp boundary and there is a clear division between saltmarsh to seaward and enclosed land to landward. The modern coastline of The Wash results from a long history of land-claim of its saltmarshes, which began in Roman times, and has produced around 32,000 ha of agricultural land since the 16th century. The Wash’s natural response to this land-claim is for accretion to occur seaward of the new embankment, creating new areas of elevated saltmarsh in order to establish a new equilibrium. Evidence indicates that over the last 200 years most of the saltmarsh areas around The Wash have advanced seawards, with an associated seaward movement of the high water mark. In contrast, (and anomalous for The Wash as a whole) the saltmarshes at Butterwick Low (along the north-western shore) have retreated landward. The historical position of the low water mark illustrates a high degree of spatial variability with areas of localised advance and retreat occurring simultaneously. The general trend of historic (1828-1995) movement of The Wash low water mark is seaward. However, this general trend masks more recent (last 30 years) localised fluctuations with significant lengths of the low water mark moving landward. This literature review forms the first part of a two part investigation entitled “Coastal Change around The Wash”. This report (Part 1) describes Holocene (last 10,000 years) and historical coastal change around The Wash, and its predecessor, the Wash-Fenland embayment. It identifies the geological, sea level and anthropogenic events that have led to the present position of the shoreline. This understanding of the embayment’s recent past will help to develop strategies for managing coastal change in the future. This review also provides an initial analysis of the historic and recent changes to the saltmarsh communities that have occurred between 1971 and 2002, based upon the results of various saltmarsh surveys carried out in The Wash.

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CONTENTS Page 1

INTRODUCTION

1

2

SETTING 2.1 2.1.1 2.1.2 2.1.3 2.2 2.2.1 2.3 2.3.1 2.3.2 2.3.3

2 2 2 2 3 3 3 6 6 6 7

Geological Setting Jurassic and Cretaceous Pleistocene Holocene Geographical Setting Bathymetry Physical Setting Tides Tidal currents and residual currents Waves

3

CONTEMPORARY SEDIMENTS 3.1 Sediment Dynamics 3.2 Sediment Provenance 3.2.1 Suspended load 3.2.2 Bedload 3.3 Sediment Distribution 3.4 Sedimentary Environments 3.4.1 Saltmarshes 3.4.2 Intertidal flats 3.4.3 Sand banks and channels 3.4.4 Beaches, spits and dunes 3.4.5 Cliffs

10 10 11 11 12 12 14 14 17 18 18 19

4

HOLOCENE COASTAL CHANGE (SIMPLIFIED) 4.1 Sea-Level History and Sedimentary Succession 4.2 Sediment Supply and Sedimentary Succession 4.3 Archaeological Evidence

20 20 22 22

5

HISTORICAL COASTAL CHANGE 5.1 Land-Claim 5.1.1 Wisbech Estuary 13th century land-claims 5.1.2 Post 13th century land-claims and outfalls 5.2 Shoreline Movement 5.2.1 Saltmarsh edge between 1971/74 and 1982/85 5.2.2 Saltmarsh and mudflats between 1994 and 2000 5.2.3 Low water mark between 1828 and 1995 5.2.4 Mean high water spring and mean sea level between 1991 and 2000 5.2.5 Bathymetry between 1828 and 1971

23 23 24 24 25 26 30 33

COMPARISON OF SALTMARSH COMMUNITIES 1971–1985 TO 2002 6.1 Introduction

45 45

6

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6.1.1 6.1.2 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.4 6.4.1 6.4.2 6.4.3 6.5

Potential Limitations in the Comparison Saltmarsh Communities Current Situation Area 3 Area 4 Area 5 Area 6 Observed effects on Management of Saltmarsh Communities Initial Comparison of Change Changes in Pioneer Community Changes in the Low/Middle Saltmarsh Community Changes in the Upper Saltmarsh Vegetation Summary

45 45 47 48 49 50 51 52 52 53 54 54 55

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CONCLUSION

56

8

REFERENCES

57

FIGURES Figure 2.1. Simplified bathymetry of The Wash. Contours in metres below OD. Figure 2.2. Aerial photograph-composite taken on 5th September 1971 showing exposed areas at low tide. The photomosaic was made by Fairey Surveys as part of the feasibility study for the proposed Wash Water Storage Scheme. Figure 2.3. Annual significant wave heights (average height of highest one third of the waves) with a return period of 1 year in The Wash. Contours are in metres. After Posford Duvivier (1996a). Figure 3.1. Distribution of mean sand size in The Wash. After Wingfield et al. (1978). Figure 3.2. Fenland palaeoenvironmental reconstruction in section and plan views. After Wheeler (1995). A = offshore mobile sands and silts, B = sandflat and mudflat, C = saltmarsh, D = estuary, E = brackish open water, F = coastal reedswamp and sedge fen, G = carr (fen woodland), H = mere or lake, I = raised bog, J = upland. Figure 4.1. Relative sea-level curve for the Fenland. Figure 5.1. History of land-claim around The Wash. Figure 5.2. Intertidal width and saltmarsh boundary changes. After University of Newcastle (1998a). Figure 5.3. Saltmarsh boundary changes between 1994 and 2000 along the north-eastern shore (top) and Breast Sand (bottom). After Pethick (2002). Figure 5.4. Comparison of 1994 and 2000 beach profile data from Butterwick Low (top) and Wrangle Flats (bottom) showing changes in elevation of saltmarsh and intertidal flats. After Pethick (2002).

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Figure 5.5. Historical evolution of the low water mark. After Posford Duvivier (1997a). Line 1 shows the evolution between 1828 and 1995. Line 2 shows the evolution between 1971 and 1995. Red = retreat. Blue = stable. Green = advance. Black = insufficient data. Figure 5.6. Mean annual retreat or advance rate of mean high water spring (left) and mean sea level (right) (Gibraltar Point to River Nene outfall) between 1991 and 2000. After Environment Agency (2003a). Figure 5.7. Mean annual retreat or advance rate of mean high water spring (River Witham outfall to River Great Ouse outfall) between 1991 and 2000. After Environment Agency (2003b). Figure 5.8. Mean annual retreat or advance rate of mean high water spring (left) and mean sea level (right) (River Nene outfall to Hunstanton) between 1991 and 2000. After Environment Agency (2003c). Figure 6.1 Estimated area of each Annex 1 Sub-Feature 1971-2002 (After Hill, 1988 and Posford Haskoning, 2002). Tables Table 2.1. Wave attenuation over the intertidal mudflats and saltmarshes of Wrangle Flats, Butterwick Low and Breast Sand. After Cooper (2001). Table 5.1. The total areas of saltmarsh (km2) in 1971/74 and 1982/85 subdivided by shoreline section. After Hill (1988). Table 5.2. Changes in saltmarsh extent (width) between 1971/74 and 1982/85 to seaward of land-claims of various ages (after Hill, 1988). Movement of saltmarsh edge is calculated from the difference in marsh width between 1971/74 and 1982/85 vegetation maps, taking into account the width of any land-claims between those dates. + = seaward movement of saltmarsh edge. - = retreat of saltmarsh edge. Table 5.3. Historical evolution of the low water mark at transects around The Wash coast. After Posford Duvivier (1997a). Table 5.4. Mean annual shoreline retreat/advance rate of mean high water spring and (in brackets) mean sea level. Compiled from Environment Agency (2003a, b, c).

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INTRODUCTION The Wash is the presently active, unfilled part, of what was once a much larger WashFenland embayment. It provides a sheltered, low-energy environment in which tides are the main factor controlling sedimentary processes. This environment favours accretion, making the area an important sedimentary sink. The distribution of surface sediments and the processes controlling their distribution, particularly in the intertidal areas, are important for several reasons: • •

• •

the saltmarshes are important from an ecological (nature conservation) perspective; the intertidal flats are of high environmental value, particularly as feeding grounds for birds. The particle size of the intertidal sediments has a strong influence on the densities of macrobenthos, and thus on the distribution of birds that feed on them (Goss-Custard et al., 1988; Goss-Custard and Yates, 1992); the saltmarshes act as natural flood defence whereby the sea dissipates its wave energy well away from the coast; and the upper saltmarshes are viewed as potential high-grade agricultural land and have until recently been subject to land-claim. However, removal of large areas of saltmarsh has interfered with the natural sedimentary processes and constitutes a significant threat to the environment (Doody, 1987).

This literature review describes Holocene (last 10,000 years) and historical coastal change around The Wash, and its predecessor, the Wash-Fenland embayment. It identifies the geological, sea level and anthropogenic events that have led to the present position of the shoreline. This understanding of the embayment’s recent past will help to develop strategies for managing coastal change in the future. This review also provides an initial analysis of the historic and recent changes to the saltmarsh communities that have occurred between 1971 and 2002, based upon the results of various saltmarsh surveys carried out in The Wash.

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SETTING

2.1

Geological Setting

2.1.1

Jurassic and Cretaceous During the Jurassic, limestones were deposited which now outcrop along the western margin of the Fenland. These were followed by Jurassic mudstones, which underlie much of the central part of the Fenland and The Wash. Lower Cretaceous Chalk, outcropping along the eastern edge of the Fenland, was then deposited unconformably on the Jurassic mudrocks. The rocks have a general eastward dip caused by uplift of the western margin of the North Sea Basin during the late Cretaceous and Tertiary. Erosion of the bedrock during this time differentially removed the softer mudrocks relative to the harder more resistant Chalk beds. The Wash-Fenland Basin was therefore created as part of a large clay vale stretching from Humberside to Cambridgeshire. The easterly dip of the rocks has produced westward facing Chalk scarps, forming the eastern edge of the Fenland, and a shallower sloping western edge.

2.1.2

Pleistocene Over the Pleistocene (last two million years) the climate of the United Kingdom has varied with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age and the actions of the ice sheets have been instrumental in forming the modern Wash landscape. Ice originating in the North Sea widened and deepened the embayment during the Anglian glaciation (Rose, 1987; Clayton, 2000a, b). The ice sheet extended as far south as a line between the Rivers Thames and Severn (Bowen et al., 1986; Boulton, 1992) depositing till, sands and gravels (Perrin et al., 1979) over a wide area of East Anglia and the Midlands. After the ice sheet withdrew a drainage system became established with rivers such as the proto-Trent, proto-Great Ouse and proto-Nene flowing into the newly formed WashFenland lowland (Rose, 1994). The Anglian glaciation was followed by the Hoxnian interglacial, Wolstonian glacial and Ipswichian interglacial. Deposits from these periods are preserved around the periphery of the Wash-Fenland providing evidence for alternate marine and periglacial conditions. The Ipswichian interglacial was followed by the Devensian glaciation, when ice flowed south into the embayment with the ice maximum located along a line between Boston and Hunstanton (Bowen et al., 1986; Boulton, 1992). The catchments of the rivers draining into the embayment were subject to intense periglacial activity and large quantities of gravel were deposited either as terraces in confined valleys or low gradient fans at the margins. In the southern Fenland, the valleys of the Rivers Great Ouse, Nene and Welland contain extensive outcrops of these gravels. The evolution of the Devensian landscape caused lowering of the Fenland surface, independent of the course of the rivers, leaving the Devensian gravels isolated in the southern Fenland as “islands” such as MarchWimblington, Chatteris and Ely.

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2.1.3

Holocene The sediments of the present interglacial provide evidence for the most recent marine inundation of the embayment. They range from sequences dominated by mud and peat, deposited in intertidal mudflat, saltmarsh and freshwater environments, to sequences dominated by sand, deposited in intertidal sandflat or subtidal environments. The sequence of Holocene sediments within the embayment provides the long-term context to the development of the historic and contemporary environments. The Holocene sediments and evolution of the embayment are described in section 3 of this review.

2.2

Geographical Setting Artificial embankments separate the land-claimed coastal plain of the Fenland from The Wash. The total length of the modern coastline (excluding river outfalls) is about 110 km, of which more than 75% is fronted by saltmarshes and intertidal mudflats (Pye, 1992). In the north-western part of The Wash, at Gibraltar Point, the coast is dune-fringed, and on the eastern edge, a gravel storm beach ridge up to 6 m higher than the adjacent sandflat stretches 11 km south from Hunstanton. Four main rivers, the Great Ouse, Nene, Welland and Witham drain the Fenland and flow across the coastal plain in embanked channels, bounded by training walls at their mouths. These rivers drain a catchment of about 12,500 km2 of which the Fenland comprises 3400 km2. Tidal incursion extends up the lower reaches of the rivers and is limited by sluices at Earith on the Great Ouse, Whittlesey on the Nene, Spalding on the Welland and Boston on the Witham (Buck, 1997).

2.2.1

Bathymetry The present-day Wash has an area of about 670 km2 (Davidson et al., 1991) and an average depth of less than 10 m (Figure 2.1). The entrance to The Wash is about 20 km wide. The deepest areas are located within the central part where maximum water depths of 40-50 m are recorded in the Well and Lynn Deeps. These two bathymetric features extend into the North Sea to connect with Inner Silver Pit. The connecting channel has a maximum depth of about 30 m and has well defined shoulders marked by the 20 m isobath. Outside The Wash, to the east of the channel the bathymetry rises to form the shallow areas of Burnham Flats and Docking Shoal, which are covered at low water by only a few metres of water. Offshore from the north Lincolnshire coast the sea bed inclines gently seaward to reach a depth of 10 m around 2-4 km into the North Sea. The Wash has a broad intertidal zone comprising a complex series of sand banks, sandflats and mudflats that are exposed at low tide. An aerial photograph-composite taken in 1971 (Figure 2.2) shows the location of these intertidal areas. Although minor modifications have occurred since 1971, the sand banks are still essentially the same shape and in the same location (Hydraulics Research Station, 1975a). Indeed, many of these sand bodies can be recognised on navigation charts of The Wash from the 1600s and 1700s (Inglis and Kestner, 1958). Boston Deeps is an important control on the low water mark along the north-eastern shore. The positions of the deep and the latest flood embankments means that the intertidal width gradually decreases south-west along this shore.

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Figure 2.1

Simplified bathymetry of The Wash. Contours in metres below OD.

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Figure 2.2.

Aerial photograph-composite taken on 5th September 1971 showing exposed areas at low tide. The photomosaic was made by Fairey Surveys as part of the feasibility study for the proposed Wash Water Storage Scheme.

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2.3

Physical Setting In a coastal and shallow marine area like The Wash, tidal currents and waves are responsible for the distribution of sediment. Hence, the examination of possible future patterns of sediment movement and deposition cannot be achieved without a thorough understanding of the underlying physical processes.

2.3.1

Tides The Wash is characterised by macrotidal semi-diurnal tides with a spring-tide range of around 6.3 m and neap-tide range of 3.0 m. The shape of the tidal wave is modified as it progresses into The Wash from the North Sea: tidal curves show an increase in both amplitude and elevation of high water (Ruxton, 1979). For example, the mean range of spring tides increases from 6.0 m at Skegness to 6.8 m at Tabs Head (River Witham outfall). The tidal curves also illustrate a progressive increase of asymmetry, with the relatively steep flood limb becoming steeper landwards. At Tabs Head the spring tide floods for about five hours and ebbs for about seven hours. The present day tidal prism during a spring tide is about 2.8 x 109 m3 (Ke et al., 1996). At low water, about 45% of the intertidal area becomes exposed (Figure 2.2) (Buck, 1997). Over the last 2000 years the tidal prism of The Wash has decreased because of coastal accretion and land-claim leading to exclusion of tidal waters from wide areas. The western North Sea is subject to tidal storm surges, which often add considerable height to tidal levels on the east coast of England. Storm surges occur when strong winds combine with very low atmospheric pressure and high spring tides to produce an abnormal rise in local sea level. There were two well documented North Sea surges last century; in 1953 (Robinson, 1953; Steers, 1953; Rossiter, 1954; Summers, 1978; Flather, 1984) and 1978 (Steers et al., 1979), both of which caused extensive breaching of sea defences and coastal flooding which affected parts of the Wash-Fenland area (Barnes and King, 1953; Grove, 1953; Summers, 1978; Harland and Harland, 1980). During the 1953 event, high water levels at Boston and King’s Lynn were 5.25 m OD and 5.65 m OD, respectively, about 1.8 m and 2.45 m above predicted levels. The surge of 1978 was of lower magnitude but coincided with high water and so produced the highest flood levels recorded at both Boston and King’s Lynn. The predicted heights of the tide at high water were 4.3 m OD and 4.37 m OD, respectively, but these were exceeded by 1.2 m and 1.55 m, to produce water levels of 5.5 m OD and 5.92 m OD (Steers et al., 1979).

2.3.2

Tidal currents and residual currents The tidal currents approaching The Wash from the adjacent offshore area consist of two systems. The first, and strongest, approaches along the north Lincolnshire coast before turning south-west to enter The Wash (Robinson, 1964). The second moves east to west along the north Norfolk coast, also turning south-west to enter The Wash. Within The Wash, the large tidal range produces strong currents in the tidal channels. In the central deep-water area (Well and Lynn Deeps) the flood velocities are higher than the ebb (Ke et al., 1996), producing residual currents in an onshore (south-west) direction. Mean-depth averaged flood current velocities are typically 0.5-0.7 ms-1 at spring tides and 0.2-0.4 ms-1 at neap tides, while mean-depth averaged ebb current velocities are 0.4-0.6 ms-1 at springs and 0.2-0.3 ms-1 at neaps.

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Boston Deep and the marginal intertidal areas of The Wash are dominated by residual seaward (north-east) water movement (Ke et al., 1996). However, the tidal currents over the intertidal flats also have a strong offshore-onshore component (Amos and Collins, 1978; Collins et al., 1981). The velocity of these currents (both alongshore and offshoreonshore) are reduced as they travel across the intertidal flats and on to the saltmarsh. Maximum velocities of up to 0.4 ms-1 over the outer intertidal flats, 0.3 ms-1 over the inner intertidal flats and 0.1 ms-1 over the saltmarsh have been recorded at Freiston Low and Butterwick Low (Evans and Collins, 1975; Collins et al., 1981; Ke et al., 1994). 2.3.3

Waves The most commonly occurring wind direction in eastern England is south-westerly, whereas the direction of greatest fetch lies to the north-east. For these reasons, annual significant wave heights (wave heights with a return period of 1 year) range from about 1 m inshore to above 3 m across the Wash entrance (Figure 2.3) (Posford Duvivier, 1996a). Wave energy is concentrated at Gibraltar Point and Gore Point compared to locations within the embayment, due mainly to refraction of the waves as they approach the entrance (Halcrow, 2002). The wave heights diminish as they travel into The Wash and are attenuated as they propagate across the shallower intertidal areas around the margins. Amos and Collins (1978) reported wave heights of 0.3-0.5 m on the more sheltered intertidal flats of Freiston Low and Butterwick Low. The eastern coast of The Wash has a harsher wave climate than the south and west coasts (Figure 2.3) (Halcrow, 2002) and is subject to a weak southerly net longshore drift along the upper foreshore between Hunstanton and Snettisham (Pye, 1992). Halcrow (2002) showed that the asymmetry is due to the dominance of incoming waves from the north-east and the greater width and higher elevation of intertidal areas along Wrangle Flats (Cooper, 2001). Gardline Surveys (2000) and Cooper (2001) characterised the wave climate at the mouth of The Wash between May 1999 and May 2000. The significant wave height ranged from a minimum of 0.06 m to a maximum of 2.8 m, with a mean of 0.6 m (standard deviation 0.38 m). The most frequently occurring waves were 0.25-0.5 m high, and wave heights above 1.0 m occurred relatively infrequently. Wave periods were mainly between 2.5 and 4.0 seconds with a mean value of 3.3 seconds. The waves were predominantly from an offshore direction, approaching The Wash from the north to north-east sector, or less frequently, from the north-east to east sector. Halcrow (2002) showed that the waves contained a bimodal distribution, recording both the incoming waves and a reflected wave component. Halcrow (2002) used the offshore wave data and a wave transformation model to derive an inshore wave climate for The Wash. They described three significant conduits for wave propagation within the embayment; Boston Deeps, Gore Point to Breast Sand and Gibraltar Point via Wrangle Flats to Butterwick Low. The modelled incoming significant wave height (1 m) suffered little attenuation as it progressed from the mouth along Boston Deep into the embayment. Between Gore Point and Breast Sand there was 50% attenuation as the waves progressed into the embayment. However, modelled waves moving around Gibraltar Point then across Wrangle Flats (north-eastern shore) were attenuated from a significant wave height of 1 m at the mouth to 0.1 m at Butterwick Low. Cooper (2001) studied wave attenuation over the intertidal flat and saltmarsh surfaces at three locations around The Wash; Wrangle Flats, Butterwick Low and Breast Sand (see Table 2.1). Of the three sites, Wrangle Flats has the highest elevation and Breast Sand the 9P4956/R/DBRE/PBor

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lowest. The Butterwick Low transect has the widest saltmarsh (542 m), followed by Wangle Flats (425 m) and Breast Sand (387 m). Wave heights were reduced by an average of 16% as they passed over the intertidal flats at Wrangle. Wave energy was reduced by an average of 10%. Over the saltmarsh, wave heights were reduced by 91% and wave energy by 97%. Across the entire Wrangle Flats shoreline, these figures equate to an average wave height attenuation of 94% and an average wave energy attenuation of 99%. For Butterwick Low wave heights reduced by an average of 23% across the intertidal flat and 64% across the saltmarsh. Wave energy reduced by 36% across the intertidal flat and 72% across the saltmarsh. Across the entire Butterwick Low shoreline, these figures equate to an average wave height attenuation of 69% and an average wave energy attenuation of 79%. At Breast Sand wave heights were lowered by an average of 36% as they travelled across the intertidal flat and 78% across the saltmarsh. Wave energy was lowered by 56% across the intertidal flat compared to 91% across the saltmarsh. Across the entire Breast Sand shoreline, these figures equate to an average wave height attenuation of 85% and an average wave energy attenuation of 96%. The results show that wave height and energy attenuation are significantly greater across the saltmarsh than across the intertidal flat (see Table 2.1). The tendency is for higher elevation intertidal areas (relative to the tidal frame) to be wider and to be colonised by higher density vegetation, optimising the degree of attenuation. Table 2.1. Wave attenuation over the intertidal mudflats and saltmarshes of Wrangle Flats, Butterwick Low and Breast Sand. After Cooper (2001). Wrangle Flats

Butterwick

Breast Sand

Low Width (m)

425

542

387

Highest

Intermediate

Lowest

Mudflat Wave Height Attenuation (%)

16

23

36

Mudflat Wave Energy Attenuation (%)

10

36

56

Saltmarsh Wave Height Attenuation (%)

91

64

78

Saltmarsh Wave Energy Attenuation (%)

97

72

91

Total Wave Height Attenuation (%)

94

69

85

Total Wave Energy Attenuation (%)

99

79

96

Height

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Figure 2.3.

Annual significant wave heights (average height of highest one third of the waves) with a return period of 1 year in The Wash. Contours are in metres. After Posford Duvivier (1996a).

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CONTEMPORARY SEDIMENTS A factor of fundamental importance to developing an understanding of coastal change in The Wash is the nature of contemporary sedimentation and the possible response of this sedimentation to changing anthropogenic and climatic factors such as potential future rises in sea level. A sound knowledge of the nearshore sedimentary processes and environments is therefore essential to understand the present coastal system and to enhance predictions of future change.

3.1

Sediment Dynamics It is now generally agreed that the sediments of the contemporary Wash are largely of marine origin with relatively unimportant contributions from modern rivers (Evans, 1965; Evans and Collins, 1975, 1987; Wilmot and Collins, 1981; Wilmot, 1985; Dugdale et al., 1987; Chang and Evans, 1992; Plater et al., 1994; Ke et al., 1996). Fine-grained sediment entering the embayment from offshore is transported in suspension by tidal currents onto the intertidal flats and saltmarsh areas (HR Wallingford et al., 2002). Ke et al. (1996) suggested that a net landward flux of suspended sediment into The Wash occurs through the central parts (Well and Lynn Deeps) of the entrance with the main export occurring along the flanking areas via a complex of channels, banks and intertidal flats (Robinson, 1964). Satellite images show a persistently high content of suspended sediment in shallow water off the north Lincolnshire coast extending south into The Wash (Aranuvachapun and Brimblecombe, 1979; Dugdale et al., 1987). Although suspended sediment movement appears to be the dominant mode of transport through the embayment, bedload transport is important in shaping the sea bed sediments into a variety of bedforms, which are particularly well developed along the margins of sand banks (Figure 2.2) and in the tidal channels (Evans, 1965; Amos and Collins, 1978; McCave and Geiser, 1978; Wingfield et al., 1978). Stride (1963) argued that the asymmetry of offshore sand waves indicated a net movement of bedload into The Wash. The annual input of suspended marine sediment into The Wash has been estimated between 6.8 x 106 (Ke et al., 1996) and 5.3 x 107 (Evans and Collins, 1975) tonnes. These figures are one to four orders of magnitude higher than the total annual fluvial input (mainly suspended) which has been estimated at 2 x 105 tonnes by Crown Estate Commissioners (1969), between 4.3 x 104 and 1.73 x 105 tonnes by Wilmot and Collins (1981) and between 2.8 x 103 and 1.77 x 104 tonnes by Dugdale et al. (1987) and Plater et al. (1988, 1994). The computed annual net bedload transport rate across the entrance to The Wash is 1.4 x 104 tonnes (Ke et al., 1996) which is also very small compared to the annual suspended load. The total annual estimates of suspended load (marine and fluvial) are much larger than the estimated amount of deposition (over the last 2000 years) on the muddy intertidal areas of 1.6 x 106 tonnes yr-1 (Evans and Collins, 1975). Only a small proportion of the suspended sediment over the intertidal areas is actually deposited. According to Ke et al. (1996) the highest suspended sediment concentrations in The Wash occur above the intertidal areas (40-600 mg l-1) with intermediate values at the entrance (10-100 mg l-1) and the lowest values in the centre (30 mm yr-1) on the lower-middle marsh in areas with high creek density, reducing to less than 10 mm yr-1 on the upper marsh. Harper (1979) measured rates up to 50 mm yr-1 towards the centre of the marsh (19731976). According to Harper (1979), maximum vertical accretion took place in winter, when most vegetation was either dead or dying, with low accretion in summer. Wind speed and wave height and the related suspended sediment concentrations in the nearshore waters were thought to be more significant than vegetation cover in controlling the seasonal pattern of sediment deposition, a conclusion supported by Hartnall (1984). However, Hartnall (1984) also described micro-morphological variations of the marsh surface caused by different plant communities. For example, a distinct increase in 9P4956/R/DBRE/PBor

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vertical accretion in the middle marsh occurred after the collapse of Puccinellia in autumn, forced by high winds and spring tides. Coles (1979) also showed that the highest rates of saltmarsh accretion in other parts of The Wash took place in winter. Figure 3.2.

Fenland palaeoenvironmental reconstruction in section and plan views. After Wheeler (1995). A = offshore mobile sands and silts, B = sandflat and mudflat, C = saltmarsh, D = estuary, E = brackish open water, F = coastal reedswamp and sedge fen, G = carr (fen woodland), H = mere or lake, I = raised bog, J = upland.

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3.4.2

Intertidal flats The sandflats of The Wash generally occur between the low water mark and about 1.7 m OD, and dip seawards at 1-4o (Pye, 1992). Mudflat sedimentation generally occurs above 1.7 m OD and extends landward to the seaward limit of saltmarsh vegetation at around 2.1-2.4 m OD (Pye, 1992). The boundary between the mudflat and saltmarsh varies around The Wash, from a shallow or steep transition, through to a low eroding cliff. Evans (1958, 1960, 1965, 1975) subdivided the intertidal flats of Freiston Low and Butterwick Low into five environments of deposition, which form zones roughly parallel to the coastline. From landward to seaward these are; higher mudflats, inner sandflats, Arenicola sandflats, lower mudflats (sporadically developed) and lower sandflats. The mudflats are incised extensively by gullies and small creeks up to 0.2 m deep and 0.4 m wide (Ke et al., 1994). The higher mudflat deposits are dominated by laminated silts and sandy silts (Evans, 1958, 1965, 1975). The number and thickness of sand laminae decrease inland, with a complementary increase in the proportion of mud laminae. Small-scale symmetrical and asymmetrical ripple-marks are developed parallel to the shoreline and desiccation cracks and bioturbation are common. They are actively reworked by a network of creeks forming low-angle cross-stratification as a result of channel migration. Over a four-year period (1993-1997), Brown et al. (1998) found a consistently stable mudflat environment around The Wash. Coles (1979) recorded the highest rate of accretion on the higher mudflats during the autumn (1973 and 1975) although accretion was continuous throughout most of the year. The process of mud accretion in the shortterm was considered to be aided by the presence of mucus, produced by diatoms, on the mud surface, which helps trap and bind fine sediment and inhibits erosion. However, Coles (1979) added that long-term accretion of mud was more likely to depend on its ability to withstand erosion during storm conditions. The inner sandflats and Arenicola sandflats are inundated at high water throughout the tidal cycle. The inner sandflats are composed of very fine sands and silty sands whereas the Arenicola sandflats are mainly fine sands (Evans, 1965). Both are heavily bioturbated, with the Arenicola sandflats being characterised by a dense population of the worm Arenicola marina. The low water mark is bounded by a narrow sandflat (the lower sandflat) composed of very fine sands with transient mud drapes. At low water spring tides the lower sandflats are completely exposed whereas at low water neap tides they remain inundated (Evans, 1965). Coles (1979) showed that during 1973-1974 the inner sandflats were subject to erosion during the winter months and accretion during the summer months, but at the end of the year the net gains and losses approximately balanced. Amos and Collins (1978) and Collins et al. (1981) have described the bedforms on the sandflats of Freiston Low and Butterwick Low. On the Arenicola sandflats, they reported low-energy, asymmetrical ripple-marks between 10 and 50 mm high formed by a combined influence of wave-induced and tide-induced currents. On the lower sandflats (and in creeks) they found higher-energy asymmetrical current ripple-marks, which increased in size seawards in response to increasing particle size. On the inner sandflats they found less asymmetric ripple-marks formed by waves, but reactivated by the ebb tidal current. Megaripples have been described on the bed of the bounding tidal channel (inner part of Boston Deep) (McCave and Geiser, 1978). 9P4956/R/DBRE/PBor

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Between the Arenicola and lower sandflats, the surface gradient steepens into the lower mudflats (Evans, 1958, 1965). The lower mudflats are composed of poorly-sorted, laminated sands and silty sands, similar to those of the inner and Arenicola sandflats. However, their surface is relatively smooth compared to the bounding sandy intertidal areas. The lower mudflats are not always present (Collins et al., 1981; Al-Agha et al., 1995). Indeed, McCave and Geiser (1978) suggested that the absence of lower mudflats was the norm, because the mud is kept in suspension by wave action, and that they only exist in the more sheltered inner parts of The Wash. Coles (1978) studied the transition of the higher mudflats and inner sandflats at Wolferton for over 2 years between September 1975 and January 1978. She found a highly dynamic boundary alternately moving landwards and seawards dictated by erosional forces and the balance between mud-accreting diatoms and their macroinvertebrate grazers. She suggested that evidence for permanent progradation of mudflats over the sandflats is difficult to measure in the short term. 3.4.3

Sand banks and channels The dominant morphological characteristic of the offshore zone of The Wash is the presence of sand banks (some of which are exposed at low water) and intervening channels (Figure 2.2). These environments are composed of two main facies: clean sand, and sand with mud laminae (Institute of Geological Sciences, 1974; Wingfield et al., 1978). Clean sand occurs around the outer margins of the banks, where it may be shaped by waves into ridge and runnel structures, and in the tidal channels, where megaripples may be prevalent (McCave and Geiser, 1978). Sand with mud laminae (characterised by over 70% sand) is mainly found on the inner margins of the banks where it is protected from wave action by the outer belts of clean sand. In a landward direction, across the intertidal flats, there is a change from sand with mud laminae to mud with sand laminae (characterised by over 50% mud). Most of the larger bedforms (>5 m spacing) in The Wash occur on the sand banks and intertidal parts of the tidal channels (Figure 2.2) (King, 1964; McCave and Geiser, 1978). The seaward parts of the sand banks are shaped into low ridges, 0.5 m high, 50-100 m in spacing, and with a slight landward asymmetry, which are interpreted as wave-formed ridge and runnel features (McCave and Geiser, 1978). They are covered with asymmetrical ripples usually having a flood orientation (suggesting a net inshore sand transport regime), and layers of mud were found in many of the runnels. The channel bedforms are dominated by megaripples 0.3-0.6 m high and 10-15 m modal spacing, which are dominantly flood oriented (but with some degree of ebb modification).

3.4.4

Beaches, spits and dunes At Gibraltar Point the relatively sheltered environments of the intertidal flats and saltmarsh of The Wash pass north across the River Steeping into more exposed beach, spit and dune environments. The sandy beaches between Skegness and Gibraltar Point are fairly wide, ridge and runnel beaches, backed by dunes (King and Barnes, 1964; King, 1968, 1973). They are built by constructive longer waves approaching from the north-east and the position of the ridges varies depending on weather conditions and the availability of sand (King and Barnes, 1964; King, 1968; Fox, 1978). The ridges tend to diverge slightly offshore southwards from the coast and generally have a straight swash 9P4956/R/DBRE/PBor

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slope, fairly flat crests, and a shorter, steeper landward slope (King, 1973). Sand is pushed over their crests as the tide rises causing a regular inland, and hence southward, migration of the ridges (Barnes and King, 1955; King and Barnes, 1964; King, 1968; 1973). King and Barnes (1964) have measured average landward migration rates of about 40 m yr-1, although faster migration rates may be associated with larger volumes of longshore sediment transfer. Fox (1978) recorded contrasting directions of sand movement on different parts of the beaches associated with differing angles of approach of the waves. He showed that the angle of approach on the more seaward ridges was from the north-east whereas on the ridges behind, it was almost perpendicular. The alongshore and onshore supply of sand causes eastward accretion and formation of new ridges, such that Gibraltar Point is characterised by a series of sand bodies extending parallel to the coastline (with some enclosed marshes) (Barnes and King, 1961; King, 1973; Psilovikos, 1979). As the ridges approach the upper beach they may become stabilised through the growth of the new ridges further seaward, allowing windblown sand to accumulate, and forming foredunes. The accretion is related to the presence of a nearshore, foreshore-connected, linear sand bank (Skegness Middle), created and maintained by tidal current residuals, which has acted as a long-term sediment source to the coast (King and Barnes, 1964; King, 1973). The point of connection of the sand bank to the foreshore has consistently been the location of maximum accretion (King and Barnes, 1964; King, 1964, 1973; Dugdale et al., 1978) which has moved progressively south during the last 100 years (Robinson, 1964; King, 1978). Areas immediately to the north of Gibraltar Point, which were once locations of accretion, are now eroding as the input of sediment onto the foreshore has decreased, and the coast is more exposed to waves during storms (King, 1973). The spit at Gibraltar Point is intimately associated with deposition on the ridges and foredunes to the north, as it represents its southward extension, being built from sediment that passes southwards from the foredunes (King, 1970). It is this accretion that will ultimately control the future growth of the spit. When the accretion has built out sufficiently for sediment to move further offshore along the beach, then the spit will be starved of sediment, and become static as another spit forms seaward of it (King, 1970). On the eastern shore of The Wash, a continuous strip of storm beach gravel, up to 6 m high, occurs in the form of a spit stretching for 11 km from Hunstanton to Dersingham. The spit has grown southwards and provided shelter for growth of saltmarshes on its landward side. A narrow strip of gravel, degraded to a lower elevation than the spit occurs near Wolferton and was probably its former southerly continuation (Gallois, 1994). It should be noted that the literature available about the east coast of The Wash is limited compared to the extensive literature for the west coast. 3.4.5

Cliffs Active cliffs around The Wash are restricted to immediately north of Hunstanton where they are 10-20 m high. They expose Cretaceous ferruginous sandstones known locally as Carstone in the lower part, overlain by the Red Chalk and preceding the white Lower Chalk (Gallois, 1994). Erosion rates averaged over the period 1981 to 1995 are low and increase from the north end of the cliff (0.1 m yr-1) to the south end (0.2 m yr-1). Maximum rates of 0.3 m yr-1 have been measured (Phipps, 1999).

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4

HOLOCENE COASTAL CHANGE (SIMPLIFIED) The contemporary sediments of The Wash described in the previous section are the culmination of Holocene sedimentation in the Wash-Fenland embayment that began about 8000 years ago. The embayment has partially filled with these sediments in response to sea-level change and natural marine and estuarine processes, and, more recently (over the last 2000 years), as a consequence of land-claim of fringing saltmarsh. The Holocene sediments are, therefore, characterised by complex lateral and vertical lithological changes comprising alternations of clastic sediments and peat which contain a detailed record of the sedimentological, geomorphological, climatic and sea-level evolution of the area (Godwin, 1975, 1978; Shennan, 1986a, b; Waller, 1994a; Wheeler and Waller, 1995; Brew, 1997a; Horton, 1997; Holt, 1999; Brew et al., 2000, 2004; Bailiff and Tooley, 2000; Brew and Williams, 2002).

4.1

Sea-Level History and Sedimentary Succession At the end of the Devensian glaciation, the world climate warmed rapidly and global sea level began to rise. About 12,000 years ago, sea level in the North Sea was tens of metres below present and much of the area between the United Kingdom and continental Europe was still land (Jelgersma, 1979; Lambeck, 1995; Shennan et al., 2000b). Sea-level rise was rapid at first, c. 20 m every 1000 years (20 mm yr-1), then slowed after about 6000 BP, by which time, much of the present North Sea was flooded by marine waters. The general trend of the Fenland sea-level curve (Figure 4.1) shows that initially relative sea level rose at an average rate of 4.5 m every 1000 years (4.5 mm yr-1) between c. 8000 and 6000 years ago (Shennan et al., 2000a). After c. 6000 years ago the rate of rise slowed to less than 1.3 m every 1000 years (1.3 mm yr-1). Shennan and Horton (2002) argued for major changes in tidal range in The Wash during the Holocene. They showed a 1.6 m increase in range at the present day compared to 3000 years ago. As a consequence of the change in tidal range, the rate of sea-level rise for the last 3000 years was re-calculated to 0.86 mm yr-1. As a consequence of the early rapid rise in sea level, the shorelines of the North Sea migrated landward, and environments affected by the marine transgression would typically have progressed from freshwater to intertidal to marine (Balson, 1999; Shennan et al., 2000b). The rising level of the North Sea initially created a chain of barrier islands along the edge of the plateau that extended north from The Wash to Outer Silver Pit. Landward of the barriers an extensive area of intertidal flat developed. Due to the low gradient of the offshore area and the rapid rate of sea-level rise, the lateral rates of coastline movement were high, estimated between 30 and 60 m yr-1 (Balson, 1999). The sea entered the Fenland c. 7850 years ago and quickly flooded the central and eastern sectors (Brew et al., 2000). After c. 4400 years ago the sea finally flooded the western and southern sectors. The bulk of the Holocene succession of the Wash-Fenland embayment has been deposited from a well-mixed pool of sediment transported by tidal currents into the embayment from the North Sea (Holt, 1999). Deposition has taken place under the influence of a rising sea level, which has gradually slowed towards the present-day (Figure 4.1). The sands generally fine-upwards, becoming very silty towards the top and the muds have a generally uniform particle size throughout the Holocene (Holt, 1999; Brew et al., 2000). These trends indicate that sedimentation kept pace with, or exceeded, sea-level rise. Higher energy environments were replaced by lower energy environments, by lateral 9P4956/R/DBRE/PBor

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progradation and vertical accretion with the available accommodation space created by sea-level rise being filled due to the ample sediment supply. The continued infilling of the embayment with sediment in combination with a slowing sea-level rise eventually led to development of peats, as brackish environments were transformed into freshwater environments by the relative sea-level fall. Initial formation of peat was localised, being a different age in different parts of the embayment, but by c. 3000 years ago it was on a more regional Fen-wide scale. These peats are covered by clastic deposits which form the final diachronous fill of the embayment. The ages of the top of these peats suggest a complicated pattern of resubmergence starting at c. 2750 years ago in the western Fenland. A cluster of dates between c. 2250 and 1950 years ago in the central and south-eastern Fenland suggest a later, but fairly rapid, reflooding of these areas. Figure 4.1.

Relative sea-level curve for the Fenland. After Shennan et al. (2000a).

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4.2

Sediment Supply and Sedimentary Succession Shennan (1986a, b, 1987a, 1989a) considered that the alternation of silty clay and peat beds was driven by local tendencies of sea-level movement, which, in turn, were controlled by regional eustatic sea-level variations. Waller and Hall (1991) questioned this interpretation and emphasised the role of changes in sedimentary processes, coastal morphology and rates of consolidation as of equal importance, a case supported by recent detailed work on the coastal plain of Belgium (Baeteman, 1999). Zalasiewicz (1985) and Zalasiewicz and Wilmot (1986) argued that there may be a causal link between the siltingup of tidal creeks in the Fenland and the transition from silty clay to peat. They envisaged that the infilling of the creeks caused the seaward sediment supply to cease, and this may in itself have initiated peat formation. This model removes the need to invoke sea-level fluctuations as the direct control determining clay-peat transitions. Indeed, the mechanisms that led to the renewed expansion of tidal flat areas (c. 2750-1500 years ago) appear to be unrelated to sea-level, and another explanation is needed. The formation and subsequent destruction of migrating barriers cannot be used as an explanation. This is because the Wash-Fenland embayment is postulated to have remained “open” to the North Sea with no protective barriers behind which major formation of intercalated peats could have occurred (Shennan, 1986a, b). Seismic profiles (unpublished data) towards the entrance to The Wash support this view with no seismostratigraphic evidence for barriers or remnants of barriers. A process of inundation is put forward here, which is based on the hypothesis of Beets et al. (1994) for the later Holocene evolution of the Dutch coastal embayments. Between c. 7000 and 3000 years ago, the supply of sediment from marine sources to the embayment kept pace with, or exceeded, sea-level rise, leading to sedimentary infilling and progradation. However, after c. 3000 years ago the continued removal of sandy sources from offshore led to a deficit in supply relative to sea-level rise (even at a reduced rate), and, to compensate, new sources were activated. The new supply came from reworking of the previously deposited Holocene sediments of the embayment itself, leading to shoreface erosion and renewed landward migration of the tidal system. Indeed, the modern sea bed in the northern parts of The Wash and immediately offshore is composed of a till or chalk planation surface with very little overlying Holocene sediment. Further into The Wash, the origin of a major seismic reflector within the sequence may be related to this phase of erosion (Brew, 1997a).

4.3

Archaeological Evidence After the latest transgressive phase, the Fenland sediments yield less information about former conditions and the succession of events are interpreted from archaeological evidence. Simmons (1978, 1980) described a fall in the altitude of Lincolnshire salterns between c. 2150 BP (200 BC) and 1750 BP (200 AD) indicating a regressive phase. Shennan (1986b) proposed that this regressive phase began c. 1900 BP (50 AD) and continued until around 1550 BP (400 AD) with the beginning of a further transgressive phase (Churchill, 1970; Hallam, 1970; Salway, 1970; Smith, 1970; Wheeler, 1995). However, there is little stratigraphic evidence for this regressive phase.

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5

HISTORICAL COASTAL CHANGE

5.1

Land-Claim Few areas of the United Kingdom have undergone as great a transformation as that experienced by the Wash-Fenland during the last 350 years. The landscape of intertidal flats, saltmarsh, fen and shallow mere, has been converted by humans, through land-claim, into an almost treeless arable plain cut by embankments and a network of long straight drains (Borer, 1939). Much of the coastline between the Humber Estuary and Hunstanton is protected by artificial embankments, many of which were strengthened and raised after the disastrous flooding that occurred during the 1953 storm surge. The start of major drainage schemes for the Fenland began in the 17th century (Darby, 1940, 1983) initiating a further three centuries of drainage (Figure 5.1). Figure 5.1. (1996a).

History of land-claim around The Wash. After Posford Duvivier

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5.1.1

Wisbech Estuary 13th century land-claims Prior to the 13th century, The Wash extended south to an estuary at Wisbech, about 20 km south of the present coastline (Hydraulics Research Station, 1975a). Much of the modern Fenland was saltmarsh and reedswamp of no agricultural value. However, Saxon farmers were eager to farm the fertile land formed from siltation of the estuary. To speed the process of maturation of the saltmarsh to produce agricultural land, earth embankments were constructed on the upper saltmarsh. These embankments formed the landward limit of marine flooding and defined the new position of the coastline. Hence, the great Sea Bank (Roman Bank) was constructed in the 13th century, and subsequently extended, strengthened and heightened (Taylor and Hall, 1977; Silvester, 1988). The bank formed a continuous barrier to the sea stretching from the Tofts to the eastern Fen-edge. The pattern of land-claim shows that the Wisbech Estuary was wide and trumpet-shaped. It would have had a meandering low-water channel (Kestner, 1976) cutting its way through constantly shifting sands (Alderton, 1984; Alderton and Waller, 1994) between the Sea Bank on its west and east banks. In the 14th century, the Wisbech Estuary began to silt up. The inhabitants of the town of Wisbech believed that the sedimentation was caused by silt brought down the river. They therefore cut a channel to divert the flow of the river from its original course through Wisbech to flow north-eastward from Littleport to Brandon Creek (Fowler, 1934b; Astbury, 1958; Seale, 1979), possibly along the track of an existing Roman canal and from there to King’s Lynn. As a result, the drainage of the whole southern Fenland was altered, so that the bulk of the freshwater discharged at King’s Lynn instead of Wisbech (Seale, 1975a; Hall, 1996).

5.1.2

Post 13th century land-claims and outfalls The permanent reduction of freshwater flow to the estuary at Wisbech had far-reaching accretional effects (Kestner, 1962). Up to this time, the estuary had been prevented from silting-up by fluctuations of its low water channel (Kestner, 1976). The reduction in freshwater flow damped the fluctuation and allowed a broad strip of saltmarsh to accrete along the western side of the channel, reducing the width of the estuary by half. Accretion did not stop in the estuary, but spread seaward in a north-westerly direction, and then westward along the foreshore until a strip of saltmarsh about 4 km wide, had accreted in front of the Sea Bank between Long Sutton and Holbeach (Figure 5.1). It was enclosure of this saltmarsh, starting in 1645, which marked the beginning of major land-claim around the margin of The Wash (Summers, 1976; Robinson, 1987; Hall and Coles, 1994). Since the 17th century, about 320 km2 of the Fenland has been turned into agricultural land (Borer, 1939; Kestner, 1962; Doody, 1987; Robinson, 1987). The remainder of the much smaller Wisbech Estuary was abandoned in stages from 1830 onwards, when the River Nene was trained to flow along a new cut made through land on the west side of the old estuary, which had already been enclosed in 1720 (Inglis and Kestner, 1958). The construction of the new cut caused rapid accumulation of sediment on the adjoining intertidal flats, such that they had vertically accreted over 2 m and become vegetated within 12 years (Borer, 1939). This land was enclosed in stages, culminating in the enclosure of 1867. No further engineering works were carried out in the area for more than 40 years but in 1910 the first of a series of small enclosures began which were continued in 1917, 1925, 1951 and 1954.

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Land-claim between the Rivers Welland and Witham continued into the 19th century with enclosures in the 1860s and 1870s (Figure 5.1). Construction of the River Witham training walls in 1884 led to accelerated accretion on either side of the trained channel and to seaward of them (Inglis and Kestner, 1958) and land-claim was taken a stage further in the 20th century. A new outfall for the River Great Ouse was created in 1853 and small landclaims were made to the north of this outfall in the remainder of the 19th century and into the 20th century. The final land-claim in The Wash took place in mid 1980s and the procedure has now ceased. Land-claim around the shoreline of The Wash has included areas close to the outfalls of the estuaries. These land-claims have had the effect of reducing the tidal prisms of the estuaries causing a reduction in tidal current velocities. The reduction in velocities has, in turn, led to an increase in sedimentation rates around the outfalls and within the tidal channels (Pye, 1992; Posford Duvivier, 1999a; Posford Haskoning, 2001). The estuaries are in the process of adjusting to the new tidal forcing being applied to them. The bed levels of the Nene Estuary currently exhibit a rising trend with seasonal fluctuations, caused by variations in fluvial flows (Posford Haskoning, 2001). The Great Ouse Estuary also currently experiences sedimentation, with the appearance of shoals or bars within the channel (Posford Duvivier, 1999a). However, this may also be due to the process of “rollover” whereby sea-level rise moves sediments from the mouth of the estuary towards the head (hence siltation at Denver Sluice). After the construction of training walls along the Welland Estuary, areas around the outfall experienced rapid siltation. The sedimentation rate and the associated encroachment of the saltmarsh has now slowed, and it is postulated to continue at a slower rate (Posford Duvivier, 1999b).

5.2

Shoreline Movement The large-scale land-claims have reduced the tidal prism of The Wash causing a loss of tidal energy within the embayment, and potentially creating a situation where saltmarsh accretion increases. Indeed, historical evidence indicates that the saltmarshes have, in general, advanced seawards around most of The Wash, associated with a seaward movement of the high water mark (Inglis and Kestner, 1958; Hill and Randerson, 1987; Hill, 1988; Pye, 1995). Lateral accretion is often very rapid, and, according to Inglis and Kestner (1958) and Kestner (1962, 1975, 1979), is greatly enhanced by land-claim. They argued that when a new embankment is built, the sedimentary environment is no longer in equilibrium. The mean current velocity across the intertidal flats to seaward and in saltmarsh creeks is lowered, either by reduction of the volume of tidal water or dissipation of energy caused by interfering with the natural flow. This leads to rapid seaward migration of the boundary between the saltmarsh and mudflat through enhanced deposition of finegrained sediment. The expansion of saltmarsh continues until equilibrium is re-established, and the rate of lateral accretion will depend on the sediment supply, shape of the intertidal profile and the proximity of the low water mark (Hill, 1988). Kestner (1975, 1979) showed that lateral accretion occurs in a cuspate fashion with the seaward-pointing cusps centred on the creeks that supply water and sediment to the marsh. He concluded that the lateral accretion of The Wash shoreline over the last few centuries had resulted from phases of saltmarsh growth, triggered by repeated embankment construction. In contrast, Stoddart et al. (1987) suggested that saltmarsh development in The Wash is controlled by the physical processes of velocity variation and sediment flux on the marshes themselves, and could find no support for the hypothesis of Inglis and Kestner (1958) and Kestner (1962).

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5.2.1

Saltmarsh edge between 1971/74 and 1982/85 Hill (1988) calculated that, between 1971/74 and 1982/85, the net area of active saltmarsh in The Wash decreased by 2% (from 42.41 km2 to 41.58 km2, Table 5.1). However, this decrease is largely due to enclosure of 8.64 km2 (20% of original area). If land-claim is excluded from the calculation, then active saltmarsh area increased by 18%. Table 5.1.

The total areas of saltmarsh (km2) in 1971/74 and 1982/85 subdivided by shoreline section. After Hill (1988).

Figures in sq. km’s

Area

Area

Area

Net

Gain

1971/74

Enclosed

1982/85

Change

Outside Enclosure

Gibraltar Point–Witham

11.12

5.27

9.28

-1.84

3.43

Witham-Welland

8.26

0

8.44

+0.18

0.18

Welland–Nene

12.04

0.59

14.73

+2.69

3.28

Nene–Ouse

6.76

2.78

4.48

-2.28

0.50

Ouse-Hunstanton

4.22

0

4.66

+0.44

0.44

Total

42.41

8.64

41.58

-0.83

7.81

Table 5.1 shows a net loss of saltmarsh along the sections of The Wash coast where large land-claims have occurred. A net loss of 1.84 km2 was recorded along the Gibraltar PointRiver Witham shore and 2.28 km2 between the Rivers Nene and Great Ouse. Along sections of coast where no land-claim has taken place since 1971/74, the area of saltmarsh has remained relatively stable, increasing by 0.44 km2 on the east coast and by 0.18 km2 around the Rivers Witham and Welland outfalls. Changes in the position of the seaward edge of saltmarsh at 46 points around The Wash between 1971/74 and 1982/85, in front of embankments of various ages, are shown in Table 5.2 (Hill and Randerson, 1987; Hill, 1988). The highest rates of lateral accretion have taken place along the north-eastern shore between Wainfleet and Friskney. A rate of 42 m yr-1 was calculated in front of a 1966 embankment, 10-25 m yr-1 seaward of a 1973 structure and 14-27 m yr-1 in front of a 1976/77 land-claim. In contrast, (and anomalous for The Wash as a whole) the saltmarshes at Freiston Low and Butterwick Low (also along the north-eastern shore) in front of 1952 and 1979/80 embankments have retreated by 2-3 m yr-1 and 15 m yr-1, respectively. By comparison, Inglis and Kestner (1958) and Kestner (1962) calculated a mean seaward advance of about 8 m yr-1 between 1828 and 1952 for the saltmarshes in the same area. Prior to construction of the River Witham training wall, the saltmarsh advanced at an average rate of 1.4 m yr-1 (1828-1871) whereas between 1887 and 1903 the average rate was 4.2 m yr-1, peaking at 10.7 m yr-1 between 1903 and 1918. According to Pye (1992, 1995), the recent retreat was due to an insufficient intertidal mudflat elevation at the time of embankment construction, so vegetation was unable to become established. Progressive land-claims may have advanced too far onto the existing saltmarsh and left insufficient width and height in front of the embankment to form new saltmarsh (University of Newcastle, 1998a).

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Along the southern shore, lateral saltmarsh accretion rates of 5-11 m yr-1 were recorded at Terrington and Wingland Marshes where the last land-claims took place in 1955 and 1974, respectively. Coles (1978) recorded a 100-150 m seaward advance of the mudflats two years after completion of the 1974 embankment. These figures compare with lateral accretion rates of over 20 m yr-1 for similar areas between 1917 and 1952 (Inglis and Kestner, 1958). Even earlier, construction of the River Nene outfall in the 19th century altered tidal and current patterns such that the rate of extension of the Wingland saltmarsh was as high as 50 m yr-1 (Kestner, 1962). Between Wolferton and Wootton along the east coast of The Wash a 2-12 m yr-1 seaward extension of the saltmarsh occurred in front of 1960/67 embankments. Inglis and Kestner (1958) and Hill (1988) showed that in the absence of land-claim or largescale engineering works, the saltmarsh edge is relatively stationary. Hill (1988) showed little movement of the saltmarsh edge between 1971/74 and 1982/85 in front of 19th century embankments. At Leverton the saltmarsh edge retreated at a rate of 2 m yr-1 in front of the 1809 embankment. Around Holbeach retreats of 0.5-4 m yr-1 took place in front of an 1838 embankment. Near the Rivers Witham and Welland outfalls, movement varies between a 1 m yr-1 advance and a 2 m yr-1 retreat in front of 1865/70 embankments. University of Newcastle (1998a) compared the width of the intertidal zone with the movement of the saltmarsh boundary (1971/74-1982/85) between Gibraltar Point and the River Witham. They showed that as the intertidal flat width decreased towards the south, the rate of advance of the saltmarsh boundary decreased until, at a point 9 km north of the River Witham outfall, it reverses from advance to retreat (Table 5.2). North of this point the saltmarsh advanced seaward at gradually increasing rates in a northward direction, averaging around 18.5 m yr-1. South of this point the saltmarsh boundary receded at an annual rate of 1.4 m. They showed that saltmarsh erosion commences when the intertidal flat width is about 3.5 km (Figure 5.2). Any decrease in the width of the intertidal flat means that wave dissipation is reduced to a level at which erosion of the saltmarsh begins. With sea-level rise, the point at which saltmarsh begins to erode will move further north along this coast. Assuming sea-level rises of 3 mm yr-1 and 6 mm yr-1, this point (or “node”) will move northwards at 9 m yr-1 and 18 m yr-1, respectively (University of Newcastle, 1998a).

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Table 5.2.

Changes in saltmarsh extent (width) between 1971/74 and 1982/85 to seaward of land-claims of various ages (after Hill, 1988). Movement of saltmarsh edge is calculated from the difference in marsh width between 1971/74 and 1982/85 vegetation maps, taking into account the width of any land-claims between those dates. + = seaward movement of saltmarsh edge. - = retreat of saltmarsh edge.

Point

NGR

Last Land-Claim Date

Extension 1971/74-1982/85

1

550578

1966

+460

2

541573

1973

+270

3

534566

1973

+110

4

523555

1976/77

+215

5

518551

1976/77

+295

6

509542

1976/77

+230

7

497528

1976/77

+155

8

473507

1976/77

+245

9

470508

1809

+100

10

458497

1962

+60

11

447486

1809

+110

12

439476

1809

-25

13

430459

1972

+5

14

425452

1972

-5

15

418445

1972

+60

16

406433

1979/80

-165

17

406423

1976

+60

18

399414

1952/65

-30

19

393399

1952

-20

20

369388(E)

1865

+15

21

369388(W)

1865

-25

22

362379

1870

-20

23

355365

1870

+65

24

351357

1870

-10

25

372352

1949

+75

26

388354

1950

+170

27

412339

1838

-5

28

423337

1838

-40

29

438329

1838

-20

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Point

NGR

Last Land-Claim Date

Extension 1971/74-1982/85

30

453320

1840

+110

31

464303

1875

+90

32

477293

1875

+25

33

482283

1978

-105

34

486275

1865

+85

35

501267

1974

+50

36

504266

1974

+120

37

513263

1974

+110

38

528260

1974

+125

39

546264

1955

+120

40

567262

1955

+50

41

584253

1974

+85

42

601256

1967

+20

43

611268

1960/66

+35

44

617279

1966

+100

45

632289

1966

+130

46

643297

1966

+25

Figure 5.2.

Intertidal width and saltmarsh boundary changes. After University of Newcastle (1998a).

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5.2.2

Saltmarsh and mudflats between 1994 and 2000 Pethick (2002) extended the analyses of Hill (1988) to investigate the position of the saltmarsh-mudflat boundary between 1994 and 2000 along the north-eastern shore and at Breast Sand (Terrington, south-eastern shore). Along the north-eastern shore, most of the saltmarsh advanced at an average rate of 5.6 m yr-1, apart from the southernmost 1.5 km and areas 12-16 km north of the River Witham outfall where retreat took place (Figure 5.3). Over most of the Breast Sand area, the saltmarsh edge advanced seaward at 3 m yr-1 between 1994 and 2000. However, an advance of 16 m yr-1 and a retreat of 1-2 m yr-1 were recorded at the eastern and western ends, respectively (Figure 5.3). Figure 5.3.

Saltmarsh boundary changes between 1994 and 2000 along Breast Sand (below) and the north-eastern shore (overleaf). After Pethick (2002).

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Beach profiles at Butterwick Low show that between 1994 and 2000, the saltmarsh accreted vertically by 8 mm yr-1 (Figure 5.4). In contrast, the intertidal flats experienced erosion, from zero at the saltmarsh boundary to 64 mm yr-1, around 1.5 km from the embankment (average rate of 20 mm yr-1). Similar trends exist for Wrangle Flats, where the saltmarsh vertically accreted at 9 mm yr-1, and the intertidal flats eroded at an average rate of 36 mm yr-1. The erosion rate of the intertidal flats decreased landward to zero at the saltmarsh boundary (Figure 5.4). The saltmarsh at Breast Sand experienced vertical accretion of 20 mm yr-1 between 1994 and 2000. Pethick (2002) argued that saltmarsh accretion rates, both vertically and horizontally, are positively related to the distance from the adjacent subtidal channel, i.e. the width of the intertidal zone. They suggested that a wider intertidal zone was capable of more effective attenuation of wave energy than a narrower one (Cooper, 2001) and hence saltmarsh backing a wide intertidal zone is impinged by lower wave energies.

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Figure 5.4.

Comparison of 1994 and 2000 beach profile data from Butterwick Low (this page) and Wrangle Flats (overleaf) showing changes in elevation of saltmarsh and intertidal flats. After Pethick (2002).

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5.2.3

Low water mark between 1828 and 1995 Posford Duvivier (1997a) analysed the changing position of the low water mark (mean low water spring tide) of The Wash between 1828 and 1995, and between 1971 and 1995. They divided the coastline into 17 shore normal transects with transect 1 at Gibraltar Point and transect 17 at Hunstanton (Table 5.3). The movement of the low water mark was mapped at each of these transects. It illustrates a high degree of spatial variability with areas of localised advance and retreat occurring simultaneously (Figure 5.5). Over the period 1828 to 1995 the low water mark of The Wash has advanced in a seaward direction (or remained stable) apart from a short stretch at Heacham that has retreated landward. The Heacham shoreline is exposed to the most extreme wind and wave conditions in The Wash (Posford Duvivier 1996a). Over more recent times (1971-1995) the pattern of movement has been more complicated with areas of landward movement (e.g. Wainfleet to Butterwick Low, River Nene to Bulldog Sand), seaward movement (e.g. River Welland to River Nene and Bulldog Sand to Dersingham) and stability (Leverton and Snettisham). The general trend of historic (1828-1995) movement of The Wash low water mark is seaward. However, this general trend masks more recent (last 30 years) localised fluctuations with significant lengths of the low water mark moving landward. In addition, the movement of The Wash low water mark is affected by the movement of offshore sand banks. Although a seaward advancing low water mark is observed on the south-western shore between the Rivers Welland and Nene, it is possible that the landward migration of

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sand banks to join the shore produces an apparent seaward advance (Posford Duvivier, 1997a). Table 5.3.

Historical evolution of the low water mark at transects around The Wash coast. After Posford Duvivier (1997a).

Transect 1 2 3 4 5 6 7 7A 8 8A 9 10 11 12 13 14 15 16 17

Location Gibraltar Point Friskney Flats Friskney Flats Wrangle Leverton Butterwick Low Black Buoy Sand Black Buoy Sand River Welland Mare Tail Sand Holbeach St Matthew Gedney Drove Breast Sand Bulldog Sand Peter Black Sand Dersingham Snettisham Heacham Hunstanton

1971-1995 Seaward Landward Landward Landward Stable Landward Insufficient Data Insufficient Data Seaward Seaward Seaward Seaward Landward Landward Seaward Seaward Stable Seaward Seaward

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1828-1995 Seaward Seaward Seaward Seaward Stable Seaward Insufficient Data Insufficient Data Seaward Seaward Seaward Seaward Seaward Seaward Seaward Seaward Stable Landward Stable

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Figure 5.5.

5.2.4

Historical evolution of the low water mark. After Posford Duvivier (1997a). Line 1 shows the evolution between 1828 and 1995. Line 2 shows the evolution between 1971 and 1995. Red = retreat. Blue = stable. Green = advance. Black = insufficient data.

Mean high water spring and mean sea level between 1991 and 2000 Environment Agency (2003a, b, c) analysed beach profile data from around the coast of The Wash to determine changes in the position of mean high water spring and mean sea level (equidistant between high water and low water) between 1991 and 2000 (Figures 5.65.8). The data analysed was collected every 6 months (in summer and winter).

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Figure 5.6.

Mean annual retreat or advance rate of mean high water spring (this page) and mean sea level (overleaf) (Gibraltar Point to River Nene outfall) between 1991 and 2000. After Environment Agency (2003a).

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Figure 5.7.

Mean annual retreat or advance rate of mean high water spring (River Witham outfall to River Great Ouse outfall) between 1991 and 2000. After Environment Agency (2003b).

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Figure 5.8.

Mean annual retreat or advance rate of mean high water spring (this page) and mean sea level (overleaf) (River Nene outfall to Hunstanton) between 1991 and 2000. After Environment Agency (2003c).

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Between Gibraltar Point and Holbeach, the position of mean high water spring has been variable (Environment Agency, 2003a) (Figure 5.6 and Table 5.4). The northern section between Gibraltar Point and Friskney has retreated (1-27 m yr-1) and advanced (1-64 m yr-1). Between Friskney and Butterwick, mean high water spring has generally advanced (0-13 m yr-1), whereas south of this, the Freiston shore has generally retreated between 0 and 8 m yr-1 (with an anomalous advance of 25 m yr-1). Between Holbeach and the River Nene outfall the position of mean high water spring is characterised by general retreat at rates between 1 and 36 m yr-1 whereas between the River Nene outfall and Dersingham it is characterised by advance at rates between 0 and 20 m yr-1 (Figures 5.7 and 5.8) (Environment Agency, 2003b, c). North of Dersingham to Hunstanton a general retreat of mean high water spring occurred with rates ranging from 0 to 6 m yr-1. The Hunstanton and Old Hunstanton frontages have generally advanced up to 1.5 m yr-1 (Environment Agency, 2003c). The results for mean sea level between Wrangle and Butterwick contrast markedly with those for mean high water spring (Figure 5.6 and Table 5.4). Apart from a short section at Wrangle, mean sea level has retreated between 0 and 56 m yr-1. South of Butterwick, mean sea level has been reasonably static (advance up to 1.6 m yr-1 and retreat up to 0.2 m yr-1) until the River Witham outfall when a higher retreat rate occurs (8 m yr-1). Between Snettisham and Hunstanton, there has been a general advance of mean sea level (Figure 5.8 and Table 5.4). This compares to the general retreat of mean high water spring at Snettisham and lower advances at Hunstanton. The comparative movements at Snettisham may be due in part to regular beach recharge activities here between 1991 and 1999 (Environment Agency, 2003c). Table 5.4.

Mean annual shoreline retreat/advance rate of mean high water spring and (in brackets) mean sea level. Compiled from Environment Agency (2003a, b, c).

Location Gibraltar Point Wainfleet Wainfleet Wainfleet Wainfleet Wainfleet Friskney Friskney Friskney Friskney Friskney Wrangle Lowgate Wrangle Lowgate Wrangle Wrangle Old Leake Old Leake Leverton Bennington Butterwick

Profile L3D5 L3D4 L3D3 L3D2 L3D1 L3C6 L3C5 L3C4 L3C3 L3C2 L3C1 L3B7 L3B6 L3B5 L3B4 L3B3 L3B2 L3B1 L3A7 L3A6

Advance

64.14 6.81 15.41 3.85 19.64 4.65 1.01

(11.1) 0.27 (0.2) (2.2) (17.3) (26.7) (21.0) (56.2)

0.73 4.54 7.19 3.28 3.81 (18.2) 1.37 0.14 9.92 0.24 13.47 9P4956/R/DBRE/PBor

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Retreat 1.02 27.04

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Location Butterwick Freiston Freiston Freiston Witham Outfall Holbeach Holbeach Holbeach Holbeach Holbeach Holbeach Gedney Gedney Gedney Gedney Gedney Nene outfall Nene outfall Nene outfall Nene outfall Terrington Terrington Terrington Terrington Great Ouse outfall Great Ouse outfall North Wootton North Wootton North Wootton Wolferton Wolferton Dersingham Dersingham Snettisham Snettisham Snettisham Snettisham Heacham Heacham Heacham Hunstanton Hunstanton Hunstanton Hunstanton Old Hunstanton

Profile L3A5 L3A4 L3A3 L3A2 L3A1 L4C6 L4C5 L4C4 L4C3 L4C2 L4C1 L4B7 L4B6 L4B5 L4B4 L4B3 L4B2 L4B1 L4A3 L4A2 L4A1 N0D5 N0D4 N0D3 N0D2 N0D1 N0C5 N0C4 N0C3 N0C2 N0C1 N0B4 N0B3 N0B2 N0B1 N0A2 N0A3 N0A4 N0A5 N0A6 N0A7 N0A8 N1D1 N1D2 N1D3

Advance (0.2) (1.6) 24.97 1.0

5.7 25.5 6.1 1.4 0.8 6.9 6.6 2.2 15.7 36.0 4.0 7.8 1.8 16.4 20.3 3.6 19.6/30.4? 5.9 5.4 1.0 1.8 15.3 3.3 18.9 0.7 0.3 1.4 0.7 5.6 0.3 (0.52) (0.18) 1.0 1.2 (0.59) 0.3

(9.53) (9.53) (2.92) 0.9 (2.19) (0.28) 1.6 (0.86) 0.3 (1.86) 1.5 (2.56)

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Retreat 1.31 (47.6) 0.09 4.15 (0.2) 7.77 (11.1)

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5.2.5

Bathymetry between 1828 and 1971 Hydraulics Research Station (1975a) described changes in the bathymetry of The Wash and the positions of the outfall channels of the Rivers Nene and Great Ouse, between 1828 and 1971. They indicated that the major banks of The Wash have generally not changed position over the 143 year period, but changes in size occurred. For example Sunk Sand, off Hunstanton, increased significantly in size in south-west and south-east directions, and Thief Sand, Sunk Sand and Ferrier Sand all suffered erosion (about 1.5 km) of their northern ends. The seaward positions of the main channels in the south-east Wash were also fairly stable between 1828 and 1971. However, towards their landward ends the channels changed greatly with regular switching and meander shifts. The channel of the River Great Ouse has switched twice from one channel to another, whereas the outfall channel of the River Nene has remained relatively stable.

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6

COMPARISON OF SALTMARSH COMMUNITIES 1971–1985 TO 2002

6.1

Introduction In order to predict what may happen to The Wash’s saltmarsh communities in the future, it is first necessary to describe their current situation and any historic changes that have occurred. An excellent description of the current saltmarsh communities of The Wash can be obtained by reviewing the information presented in Volume III NVC Survey of Saltmarsh and other Habitats in The Wash European Marine Site (Posford Haskoning, 2002) and National Vegetation Classification of The Wash National Nature Reserve (Ecological Surveys Ltd, 1999). Although not directly comparable, due to distinct differences in methodology and detail of survey, these two reports allow for a high-level comparison against the historic patterns of change described in Hill (1988).

6.1.1

Potential Limitations in the Comparison Initial observations that can be made in terms of the change in the saltmarsh communities have to be considered as a general overview, not an accurate description of the changes that have actually occurred. Aside from the different levels of detail, areas of study and overall aims of the surveys (i.e. comparing change (Hill, 1988), NVC transect surveys (Posford Haskoning, 2002)) is the fact that around The Wash there is considerable spatial variation in numerous biological and parameters that are crucial in determining the nature and extent of the communities present (see Section 5.2). These include, but are not limited to: • • •

Patterns of erosion and accretion; Grazing management; and Effects of reclamation.

Therefore, unless repeated, almost identical surveys are carried out, patterns of temporal and spatial change will be impossible to identify accurately. 6.1.2

Saltmarsh Communities To ensure that the description of the saltmarsh communities present, both historically and currently, can be compared in a way that is useful to English Nature the information is discussed in zones that correspond with those presented in the Regulation 33 advice package for The Wash and North Norfolk Coast cSAC, namely: • • • •

Pioneer/lower saltmarsh vegetation; Low/middle saltmarsh vegetation; Middle saltmarsh vegetation; and Upper saltmarsh vegetation.

Typical defining communities for each of the zones, relatively commonly encountered in The Wash, include: •

Pioneer/lower saltmarsh vegetation

SM6 (Spartina anglica community); SM8 (annual Salicornia community); 9P4956/R/DBRE/PBor

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SM9 (Suaeda maritima community); SM10 (transitional low-marsh vegetation with Puccinellia maritima, annual Salicornia and Suaeda maritima); and SM11 (Aster tripolium var. discoideus community) •

Low/middle saltmarsh vegetation

SM13a (Puccinellia maritima community with Puccinellia maritima dominant); SM14a (Atriplex portulacoides dominant sub community); and SM14c (Puccinellia maritima sub-community. •

Upper saltmarsh vegetation

SM17 Seriphidium maritimum community; SM16 Festuca rubra community; SM21 Suaeda vera saltmarsh community SM23 Spergularia marina – Puccinellia distans community; SM24 Elytrigia atherica saltmarsh community; and SM25 Suaeda vera–Limonium binervosum saltmarsh community In addition to this general overview it is also useful to relate the saltmarsh communities to each of the Annex 1 sub features of the cSAC as presented in the Regulation 33 package. These are as follows: N.B. Annex 1 sub-feature communities are listed in accordance with JNCC Report 312: Appendix 2: Guidance on the relationship between Annex I habitat types and the National Vegetation Classification (NVC)

Salicornia and other annuals colonising mud and sand (Pioneer communities) • • •

SM8 SM9 SM27 Annual Salicornia saltmarsh community

Spartina swards (cordgrass swards) • • •

SM4 Spartina anglica saltmarsh community SM5 Spartina maritima saltmarsh community SM6

Atlantic salt meadows • • • • • • • • •

SM10 SM11 SM12 Rayed Aster tripolium on saltmarshes SM13 Puccinellia maritima saltmarsh community SM14 Atriplex portulacoides saltmarsh community SM15 Juncus maritimus–Triglochin maritima saltmarsh community SM16 SM17 SM18 Juncus maritimus saltmarsh community 9P4956/R/DBRE/PBor

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• •

SM19 Eleocharis uniglumis saltmarsh community SM20 Blysmus rufus saltmarsh community

Mediterranean and thermo-Atlantic halophilous scrubs • • •

6.2

SM7 Arthrocnemum perenne stand SM21 SM25

Current Situation The 2001/2002 NVC survey (Posford Haskoning, 2002), for ease of analysis, split The Wash into 7 components: • • • • • • •

Area 1 – Snettisham Scalp to River Great Ouse (Including Snettisham Scalp Nature Reserve); Area 2 – River Great Ouse to River Nene (including The Was Nature Reserve); Area 3 – River Nene to Lawyers Creek (including Lutton, Holbeach, Gedney and Nene); Area 4 – Lawyers Creek to The Haven (including Gosdyke, Decoy Outfall, Kirton and the Scalp and Cots); Area 5 – The Haven to The Horseshoe (including Freiston Low, Leverton and Toft); Area 6 - The Horseshoe to Gibraltar Point; and Area 7 – Gibraltar Point to Skegness (this area falls outside the scope of this report).

Areas 1 and 2 were not described in the Posford Haskoning (2002) report, as they coincide with the surveys of grazing compartments within The Wash National Nature Reserve, undertaken by Ecological Surveys Ltd (ESL) in 1999. The following description of Areas 1 and 2 is based upon the findings of the ESL surveys. The surveys recorded 12 distinct NVC saltmarsh communities within Areas 1 and 2, the commonest of which were SM6, SM11, SM13a, SM14a, and SM24, which were present in all the grazing compartments surveyed and show a relatively clear transition from pioneer to upper marsh community. Other commonly occurring communities included SM9 and SM8). The upper marsh community SM23 was relatively infrequent and the most infrequently encountered communities were SM10, SM13c and SM17. The most notable absence was that of a true “upper marsh” community i.e. SM16. This can be explained by the relative “newness” of the saltmarsh within the NNR and the long term removal of the upper levels of the marsh as part of on-going reclamation (ESL, 1999). The 1999 ESL survey of Areas 1 and 2 also included a short discussion of the general trends in vegetation, identified by Hill (1988), accompanied by notes on the applicability to the survey of the NNR. The main conclusions are given below: •

Decline in SM14/Atriplex portulacoides: This change was noted in some areas of the ESL survey but, conversely, areas of Atriplex portulacoides expansion were also recorded;

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6.2.1



Increase in SM13a: Again, this trend was also observed in the 1999 survey where SM13a was seen to be increasing at the expense of SM14a, and colonising bare mud in borrow pits;



Decline in upper marsh SM13: This trend is not applicable in the NNR as there is very little recorded upper marsh area;



Decline in SM11, replacement with SM10: Within the NNR the reverse was noted by ESL. SM11 was actually considered to be undergoing localised expansion at the expense of SM10;



Increase in SM24: Same trend observed by ESL; and



Declines in SM8 and SM6: Within the NNR SM6 was showing slight decline. However, SM8 was seen to be increasing.

Area 3 Gedney to Nene The strip of saltmarsh at the base of the seawall at Gedney and Nene is dominated by SM24. This is fronted by a mosaic of SM13 and SM14. This area of marsh also has patches of SM14a. The area just east of Lawyers Creek, consists of a wide band of SM13 and SM14 forming a mosaic which is fronted by SM8. This is interspersed with SM24 occurring on elevated areas, and some SM11 also present. The strip at the base of the seawall forms an interesting assemblage of SM8, SM11 and SM13/SM14 mosaic. The following communities were identified in Area 3: • • • • • • • • •

SM6; SM8; SM9; SM10; SM11; SM13a; SM14a; SM14c; and SM24.

Holbeach The main marsh appears to be more mature than most saltmarsh areas in The Wash, with a mix of E. atherica on the higher areas and A. portulacoides on the lower parts. There are also areas where Spartina agg. and Aster agg. have high abundance. Within this area there is a mosaic of SM13/SM14 around the salt pans. This marsh is not grazed. MOD targets are present on raised areas of ground. The communities present include:

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• • • • • 6.2.2

SM8; SM13a; SM13c; SM14a; and SM24.

Area 4 Fosdyke To the west of Lawyers Creek the marsh is dominated by SM8 interspersed with patches of SM14a, SM11, and some SM6. The strip at the base of the seawall forms the same mosaic present to the east of Lawyers Creek. The following communities were identified: • • • • • • • •

SM6; SM8; SM9; SM10; SM13a; SM14a; SM14c; and SM24.

Kirton Marsh Kirton Marsh is dominated by SM13a with SM14a forming frequent small stands within the wider SM13 community. Extensive belts of SM24 traverse the saltmarsh occurring on elevated areas. There are also isolated areas of SM17 throughout the marsh. SM8 and SM9 pioneer communities occur along creek edges, and on mudflats fronting the River Welland and the coast where small clumps of SM6 are also present. The following communities were identified: • • • • • • • • •

SM6; SM8; SM9; SM10; SM11; SM13a; SM14a; SM17; and SM24.

Scalp and Cots Marsh The south of Scalp and Cots Marsh is dominated by a large area of SM13a with SM24 lining the creek edges, and appearing in small patches on elevated ground. To the north of this site, the saltmarsh is dominated by a mosaic of SM13, and SM14.

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Due to very high tides at the time of the 2002 survey and the intricate network of creeks, it was impossible to access the pioneer SM8 and SM9 communities on the mudflats. However, it was expected that the general pattern of pioneer communities grading into the SM13 and SM14 communities occurs here as at the other marshes around The Wash. The following communities were identified: • • • • 6.2.3

SM6; SM13a; SM14c; and SM24.

Area 5 Freiston Low A mosaic of SM13 and SM14 dominates the majority of the saltmarsh at Freiston Low. This SM13/SM14 mosaic is fronted by SM8 pioneer community along the seaward edge, with occasional SM9 pioneer community also occurring. There are occasional small stands of SM10 transitional low-marsh vegetation and SM6. SM23 is present in thin strips along the base of the seawall, with some occasional patches of SM11 present throughout the middle marsh. Species of interest: • Limonium binervosum (Red Data Book - lower risk - Nationally Scarce); • Hordeum marinum (Nationally scarce). The following communities were identified: • • • • • • • • • •

SM6; SM8; SM9; SM10; SM11; SM13a; SM14a; SM14c; SM23; and SM24.

Leverton This area follows the same pattern of the majority of The Wash marshes with SM8 dominating the mudflats seaward of the marsh. This is backed by a mosaic of SM13 /SM14 middle marsh communities, with SM24 along the seawall. Small patches of SM17 are present on the areas of upper marsh. Species of interest: • L. binervosum 9P4956/R/DBRE/PBor

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The following communities were identified: • • • • • • • •

SM6; SM8; SM10; SM11; SM13a; SM14a; SM17; and SM24.

Toft Marsh Toft Marsh consists of the SM13 Puccinellia maritima/SM14 Atriplex portulacoides mosaic fronted by SM8 pioneer annual Salicornia pioneer community. Within this area of marsh there are patches of SM10 transitional low-marsh vegetation with patches of SM6 Spartina anglica on lower creek edges. The strip at the base of the seawall consists of SM24 Elytrigia atherica with areas of the pioneer SM8 Annual Salicornia and SM6 Spartina anglica communities around the edges of the creeks, which run parallel to the seawall. Species of interest: • L. binervosum. The following communities were identified: • • • • • • 6.2.4

SM6; SM8; SM10; SM13a; SM14a; and SM24.

Area 6 Wainfleet Generally, it appears that the marsh is dominated by SM13/SM14 with SM8 on the lower marsh. SM24 occurs on areas within the marsh where sediments have accumulated, and Aster agg. and S. maritima occur on creek edges and in depressions within the wider marsh. The area at the base of the sea wall is fenced and has been grazed with severe poaching opening up the sward. • • • • •

SM8; SM11; SM13a; SM14a; and SM24.

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6.3

Observed effects on Management of Saltmarsh Communities The 2002 NVC report for The Wash briefly describes observations of the effects of management practices upon the saltmarsh communities encountered during the field surveys. The observations made are as follows: In general terms, the saltmarsh communities of The Wash are not considered to be exceptional and only contained a very few species of note i.e. L. binervosum and H. marinum. One feature that did stand out, however, was the effect of cattle grazing and poaching upon SM13a compared to unmanaged SM13a communities. Those sites where grazing occurred, i.e. Freiston Low, Leverton and Toft Marsh, had a much increased species diversity when compared to ungrazed sites such as Gedney and Nene, Fosdyke and Kirton Marsh. The action of grazing and trampling appears to create a niche for the development of SM23 upper marsh community, as well as the more diverse SM13a. Both these communities contained the nationally scarce species L. binervosum and H. marinum.

6.4

Initial Comparison of Change As mentioned in Section 6.1, any initial comparison of the surveys carried out between 1971 and 2002, is going to be subject to inaccuracy as a direct result of the need to draw comparisons from surveys that were, in terms of aims, coverage and methodology, carried out differently. Therefore the following section should be read as a high level overview, intended to identify change in terms of the Annex 1 saltmarsh sub-features of The Wash cSAC. This high-level overview will be used in Part II of this report to provide an indication of how these changes relate to observed changes in the predominant coastal processes and future evolution of The Wash as a whole. Figure 6.1 provides an initial indication of the areas of each of the sub-features and how they have changed, for various reasons, between 1971 and 2002, based upon the results presented in Hill (1988) and Posford Haskoning (2002).

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Figure 6.1

Estimated Area of each Annex 1 Sub-Feature 1971 – 2002 (After Hill, 1988 and Posford Haskoning, 2002)

Estimated Area (ha) of each Annex 1 Sub-Feature, 1971 - 2002 3500 3000

Area (ha)

2500

1971 - 1974

2000

1982 - 1985 1500

2002

1000 500 4.34 ha in 2002 an

ic

ne

nt

rra

la

i te ed M

At la ss / ra C

or

dg

At

nt

ic

ss ra dg or C

/c er ne Pi o

Pi

on

ee

rC

om

or dg

m

ra s

un

s

ity

0

Sub-Feature

6.4.1

Changes in Pioneer Community Hill (1988) found that the Salicornia and Spartina stands in The Wash have declined compared to previous surveys. During the 2002 survey, large stands of Salicornia were found at the base of all the saltmarshes, covering the mudflats, which are extensive in The Wash. This may account for the large increase in the pioneer community shown in Figure 6.1. In comparison to this, Spartina was only found in relatively small clumps, seaward of Salicornia. However, as with most saltmarshes in the UK, it is expected that SM6, is spreading within the pioneer zone. Suaeda maritima was not recorded by Hill (1988), however, in the 2002 survey it appeared to be fairly extensive above the SM8 annual Salicornia saltmarsh community. The SM11 community occurs frequently, often in old borrow pits, and along creek edges and there is one notable very large stand west of Lawyers Creek. However, the results of the 1988 survey, suggest that the amount of cover of Aster agg. has decreased since the 1971-1974 survey implying either maturation of the marsh, or alteration as a result of grazing.

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The following communities were noted in both the 1988 report and the 2002 survey: • • • • •

SM6; SM8; SM9; SM10; and SM11.

There were no new communities found and no communities were found to be absent between the surveys. 6.4.2

Changes in the Low/Middle Saltmarsh Community Between 1971 and 2002, grazing of the marshes has resulted in changes in the constituent species. In particular, the 2002 survey has highlighted that the SM13a saltmarsh is much more species rich where the saltmarsh has been grazed, as described in Section X.X. This community, although still dominated by Puccinellia maritima, supports other species, which are unable to compete in un-grazed conditions. Where grazing has not occurred, the community is relatively species poor. The presence of grazing allows the stand to be opened up, providing suitable conditions for various species to colonise. Within the marshes, SM14 was notably absent. Where Atriplex portulacoides was present it was mainly clinging to the creek edges. This was also noted by Hill (1988). Potential reasons for this decrease are given as recent cold winters, and a reduction in the use of marshes for grazing. In addition to this, the 2002 surveys concluded that the middle marsh, was on the whole generally species poor with very little SM13c. The following communities were noted in both the 1988 report and the 2002 survey: • • •

SM13a; SM14a; and SM14c.

There were no new communities found, however, SM16 Festuca rubra saltmarsh community was reported in 1988, but was not apparent during the 2002 survey. 6.4.3

Changes in the Upper Saltmarsh Vegetation Notably within The Wash saltmarshes the SM24 community and SM23 community were both very evident. Within the main marsh, Elytrigia atherica was present, in raised areas and adjacent to creeks. As noted in Hill (1988) this trend was also present, as a result of succession on well drained sites in the absence of grazing. This is particularly true for Holbeach, which appears to be a very mature marsh. The following communities were noted in both the 1988 report and the 2002 survey: • • •

SM17 Seriphidium maritimum saltmarsh community; SM23 Spergularia marina-Puccinellia distans saltmarsh community; and SM24 Elytrigia atherica saltmarsh community.

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There were no new communities found and no communities were found to be absent.

6.5

Summary This section describes the saltmarsh communities present in each area of The Wash, in accordance with the 2002 NVC report (Posford Haskoning, 2002). It has also presented an initial high-level overview of the key changes that have been observed in the saltmarsh communities of The Wash between 1971 and 2002, in relation to the key Annex 1 sub-features of the cSAC. As discussed, the results of the comparison cannot be viewed as a definitive description of the change that has occurred, due to the large degree of difference in the source data. Similarly, any conclusions that can be drawn from the comparison can only be considered as an overview as the various spatial variations and mechanisms of change have not been fully described in any of the reports reviewed. The comparison does, however, allow certain patterns to be identified. These patterns can then be compared with the geomorphological analysis, interpretation of historical change and prediction of future change that will form Part II of this study. Based upon a review of the three reports the general observations of the Annex 1 subfeatures are as follows: •

Despite an observed decline, as reported in Hill (1988), the pioneer and transitional pioneer/cordgrass swards communities appear to have increased around The Wash to cover an area roughly just over twice as large as observed in the 1971-1974 surveys. This increase could be influenced by any of the factors discussed in Section 5.2, especially where the saltmarsh has been able to increase seaward, as the extent of intertidal mudflats has increased;



The true cordgrass swards and transitional cordgrass/Atlantic salt meadow communities appear to have declined, except in the south-east corner of The Wash;



Atlantic salt meadows remain the most dominant sub-feature around the whole of The Wash; and



Very little mature, Mediterranean and thermo-Atlantic halophilous scrubs, upper marsh community is present. This is in keeping with the generally observed composition of The Wash saltmarshes, which have historically been kept in a permanently immature state by continuing periods of land-claim.

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7

CONCLUSION This report integrates a considerable number of publications to provide a comprehensive review of literature on The Wash. The review contains detailed information on coastal geomorphology, coastal processes and coastal change within which the predictions of future coastal change that will form Part 2 of the study will be undertaken.

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8

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Borer, O. 1939. Recent coastal changes in south-eastern England: a discussion. IV. Changes in The Wash. Geographical Journal, 43, 491-496. Boulton, G.S. 1992. Quaternary. IN: Duff, P.McL.D. and Smith, A.J. (eds.) Geology of England and Wales. The Geological Society: London, 413-444. Bowen, D.Q., Rose, J., McCabe, A.M. and Sutherland, D.G. 1986. Correlation of Quaternary glaciations in England, Ireland, Scotland and Wales. Quaternary Science Reviews, 5, 299-340. Brew, D.S. 1997a. The Quaternary history of the subtidal central Wash, eastern England. Journal of Quaternary Science, 12, 131-141. Brew, D.S., Holt, T., Pye, K. and Newsham, R. 2000. Holocene sedimentary evolution and palaeocoastlines of the Fenland embayment, eastern England. In: Shennan, I and Andrews, J.E., editors, Holocene Land-Ocean Interaction and Environmental Change around the North Sea. Geological Society of London Special Publication, 166, 253-273. Brew, D.S. and Williams, A. 2002. Shoreline movement and shoreline management in The Wash, eastern England. IN: Proceedings of Littoral 2002, Volume 2, 313-320. Eurocoast-Portugal Association. Brew, D.S., Evans, G., Horton, B.P., Innes, J.B. and Shennan, I. 2004. Holocene palaeoenvironmental evolution and sea level history of the north-western Fenland, eastern England. Quarterly Journal of the Geological Society. British Geological Survey. 1978a. King’s Lynn and The Wash. England and Wales Sheet 145 with part of 129. Solid and Drift Geology. 1:50 000 Series (Keyworth, Nottingham: British Geological Survey). British Geological Survey. 1997. The Wash. England and Wales Sheet 129. Solid and Drift Geology. 1:50 000 Series (Keyworth, Nottingham: British Geological Survey). Brown, S.L., Warman, E.A., McGrorty, S., Yates, M., Pakeman, R.J., Boorman, L.A., Goss-Custard, J.D. and Gray, A.J. 1998. Sediment fluxes in intertidal biotopes: BIOTA II. Marine Pollution Bulletin, 37, 173-181. Buck, A.L. 1997. An inventory of UK estuaries. Volume 5. Eastern England. Peterborough, Joint Nature Conservation Committee. Burd, F. 1989. The saltmarsh survey of Great Britain. An inventory of British saltmarshes. Research and Survey in Nature Conservation, 17. Nature Conservancy Council, UK, 180pp. Chang, S.C. and Evans, G. 1992. Source of sediment and sediment transport on the east coast of England: significant or coincidental phenomena? Marine Geology, 107, 283-288. Churchill, D.M. 1970. Post Neolithic to Romano-British sedimentation in the southern Fenlands of Cambridgeshire and Norfolk. In. Phillips, C.W. The Fenland in Roman 9P4956/R/DBRE/PBor

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Kestner, F.J.T. 1975. The loose-boundary regime of The Wash. Geographical Journal, 141, 388-414. Kestner, F.J.T. 1976. The effects of training works on the loose-boundary regime of The Wash. Geographical Journal, 142, 490-504. Kestner, F.J.T. 1979. Loose boundary hydraulics and land reclamation. IN: Knights, B. and Phillips, A.J. (eds.)., Estuarine and coastal land reclamation and water storage. Saxon House, 23-47. King, C.A.M. 1964. The character of the offshore zone and its relationship to the foreshore near Gibraltar Point, Lincolnshire. East Midland Geographer, 3, 230-243. King, C.A.M. 1968. Beach measurements at Gibraltar Point, Lincolnshire. East Midland Geographer, 4, 295-300. King, C.A.M. 1970. Changes in the spit at Gibraltar Point, Lincolnshire, 1951 to 1969. East Midland Geographer, 5, 19-30. King, C.A.M. 1973. Dynamics of beach accretion in south Lincolnshire, England. In. Coates, D.R. (ed), Coastal Geomorphology. State University of New York, Binghamton, 73-98. King, C.A.M. 1978. Changes on the foreshore and the spit between 1972 and 1978 near Gibraltar Point, Lincolnshire. East Midland Geographer, 7, 73-82. King, C.A.M. and Barnes, F.A. 1964. Changes in the configuration of the inter-tidal beach zone of part of the Lincolnshire coast since 1951. Zeitschrift fur Geomorphologie, 8, 105-126. Lambeck, K. 1995. Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound. Journal of the Geological Society of London, 152, 437-448. Love, L.G. 1967. Early diagenetic iron sulphide in recent sediments of The Wash (England). Sedimentology, 9, 327-352. McCave, I.N. 1987. Fine sediment sources and sinks around the East Anglian coast (UK). Journal of the Geological Society, London, 144, 149-152. McCave, I.N. and Geiser, A.C. 1978. Megaripples, ridges and runnels on the intertidal flats of The Wash, England. Sedimentology, 26, 353-369. Perrin, R.M.S., Rose, J. and Davies, H. 1979. The distribution, variations and origins of pre-Devensian tills in eastern England. Philosophical Transactions of the Royal Society of London, B287, 535-570. Pethick, J. 2002. Coastal Data Analysis: The Wash – Study 3: Long-term inter-tidal profile evolution modelling. Report to the Environment Agency.

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