View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UCL Discovery Published in Hydrobiologia (2011) 672:105–119DOI 10.1007/s10750-011-0760-y Seabed dynamics in a large coastal embayment: 180 years of morphological change in the outer Thames estuary Helene Burningham and Jon French Coastal & Estuarine Research Unit, Department of Geography, University College London, London, UK e-mail: [email protected] Abstract This article analyses the morphological history of the outer Thames seabed, covering over 3,000 km2 from Aldeburgh (Suffolk), to Southend-on-Sea (Essex) and Margate (Kent). The region has been depicted on bathymetric charts since the sixteenth century, and has been formally charted since the eighteenth century. Charts published since the early 1800s incorporate sufficient grid reference or ground control detail for georectification onto a common coordinate system (British National Grid). The morphological history of the outer seabed was thus reconstructed through the digitisation and inter polation of soundings onto a regular grid (3Dsurface). The evolution of seabed morphology was examined using transects, bathymetric change maps and spatial statistics. The results show considerable spatial variability in seabed behaviour. Within the central Thames, banks have experienced significant depth changes can be associated with lateral shifts in individual banks. Some of the outer banks in this region exhibit progressive elongation. Shifts in bank position across the Suffolk shoreface appear to be more subtle, and there is evidence here of both onshore and offshore migration. There is no clear evidence of any regionally coherent response to large-scale historical forcing such as sea-level rise. Keywords Estuary • Sandbanks • Tidal channels •Shelf ridges • Morphodynamics • Geomorphology •Bathymetry • Hydrographic survey • Marine habitats Introduction The outer Thames region, occupying the southwestcorner of the North Sea, connects estuaries from theSuffolk (Orwell, Stour, Deben and Alde/Ore), Essex (Crouch/Roach, Blackwater and Colne) and Kent(Medway and Swale) shorelines, in addition to the river Thames itself (Fig. 1). The substantial spatialextent of the region (the seabed to the 20 m depth contour covers c. 3,000 km2) accommodates a largerange of subtidal landforms. Sand banks and intervening channels, extending up to 80 km in length and 7.5 km in width, link the Thames estuary to the southern North Sea, whilst further north and closer to the main shorelines, a variety of more discrete seabed features are present (Fig. 1). The sedimentology and broad geomorphology of the outer Thames seabed is substantially inherited from the palaeo- (Pleistocene) landscape and modifications associated with the Holocene marine transgression (D’Olier, 1972). Evidence of palaeochannelsrunning west to east is present on high resolution bathymetry, but the present regional morphology is 106 Hydrobiologia (2011) 672:105–119 Fig. 1 Location and bathymetry of the outer Thames region. Dotted lines show the position of transects used in Fig. 3. Insets show the location of Figs. 6a and b. dominated by tidal-stream aligned banks and ridges (Fig. 1). A London Clay (Eocene) basement outcrops in places, but much of the seabed comprises gravel and sand deposits and silt/mud channel fills; sediment texture and deposit thickness is spatially variable (HR Wallingford, 2002). The area is important regionally, nationally and internationally for a number of reasons. Channels throughout the outer Thames provide crucial navigation routes to the ports of London, Tilbury, Felixstowe and Harwich, and the sediments across the seabed provide a major aggregate resource. Sandbanks in the region have become popular sites for offshore wind farm development, with a fully operational site at Kentish Flats (10 km2 area), construction at Gunfleet Sands (10 km2) and consent for a site in the vicinity of Kentish Knock (the London Array, c. 245 km2). The extensive network of flats, banks and channels comprise a significant natural ecosystem that supports a wide range of habitats and species, recognised in the plethora of Marine Protected Areas (including Special Protection Areas (SPAs) and Special Areas of Conservation (SACs)) and inclusion in the Marine Conservation Zones Project. Habitats of note within the coastal zone include blue mussel beds, Sabellaria spinulosa reefs and seagrass beds, but much of the outer Thames is dominated by a complex network of habitats formed in subtidal sands and gravels. Here, wide ranging communities of marine polychaetes, crustaceans, echinoderms and molluscs exist, which support diverse and economically important fishing grounds. Previous studies of tidal channels and sand banks have commented on the organisation of features within the outer Thames estuary. Certainly, the number of channels present is suggestive of mutually evasive tidal streams, a commonly observed characteristic of macro-tidal estuaries (Robinson, 1956; Wright et al., 107 Hydrobiologia (2011) 672:105–119 1975). Robinson (1960) inferred that most of the channels running through the outer Thames were flood channels, and that ebb channels were present only in the far southwest close to the Thames river entrance (Oaze Deep and The Warp). This is supported to some extent by Prentice et al. (1968) who showed that seabed biofacies within the Southend region extended seaward within channels of The Warp and Oaze Deep. Evidence from sand waves, found around the northern extents (‘heads’) of the central sand banks, and at each end of the Suffolk banks and offshore ridges, show abrupt reversals in asymmetry, and associated sediment transport directions, indicating that both flood and ebb tidal currents are actively moving sediment but that dominance is less clear-cut (Prentice et al., 1968; Langhorne, 1973; Caston, 1981; Harris, 1988). Accordingly, there remains some debate over the origin and maintenance of channels and banks in the outer Thames. A commonly mooted assumption is that the gross configuration is associated with underlying topography within the clay basement (Prentice et al., 1968; Harris, 1988). However, Harris (1988) observed that the broad bathymetric highs of the outer Thames banks, and the strong tidal signature of the intervening channels set the system apart from other large estuary mouths such as the Severn (southwest UK) or Moreton Bay (east Australia). In his 3-stage classification of degree of estuary infilling, he placed the Thames in the last (advanced infilling) stage and suggested that there is little vertical accommodation space left for sedimentary accretion, such that only lateral growth (of banks) could follow, with subsequent convergence with marginal tidal flats. Despite a number of studies of morphology, sediment transport and hydrodynamics across the southern North Sea over the last 50 years, there has been very little direct examination of historical seabed behaviour. The aims of this article are first, to describe the recent history (last 100–200 years) of the outer Thames as determined from analysis of published hydrographic charts and second, to evaluate the observed dynamics. Materials and methods Physical context The mean tidal prism of the Thames at Southend-on-Sea is c. 6.1 9 108 m3, which is considerably larger than the combined tidal prism of all other estuaries in the outer Thames (c. 4.2 9 108 m3). Tidal regime is predominantly macro-tidal to the south (spring tidal range of 5.3 m at Southend-on-Sea and 4.3 m at Margate) and low meso-tidal to the north (2.3 mat Orfordness). Tidal currents in the southern North Sea are to the southwest during flood and to the northeast during ebb (McCave, 1979). Tidal dominance is spatially variable (Caston, 1979), but the net sediment transport direction is broadly to the south-southwest (Stride, 1963). Wave climate is bimodal, forced by dominant southwesterly winds and northeasterly storms. To the northeast of the region (West Gabbard), annual average significant height (Hs) and period (Tz) are 1.1 m and 4.1 s, respectively, which decrease to 0.3 m and 2.2 s in the centre of the region (Maplin Sands). Extremes inthe wave record, calculated as the 90th percentile in significant wave height, are about twice the mean: 2 mat West Gabbard and 0.6 mat Maplin Sands. Historical rates of mean sea-level risevaryfrom2.57 ± 0.33 mm year-1 atLowestoft (in Suffolk, to the north of the outer Thames region) to 1.22 ± 0.24 mm year-1 at Southend and 2.23 ± 0.13 mm year-1 at Sheerness (Woodworth et al., 2009). Methods Bathymetric data were obtained from UK Hydro-graphic Office published nautical charts. Chart 100 (published between 1812 and 1848) and subsequently chart 1610 (as published between 1855 and 1954) covered the entire outer Thames region at a scale around 1:150,000. Since the 1950s, chart 1610 has focused on a broader area at a smaller scale and coverage of the outer Thames is split between northern (chart 2052) and southern (chart 1183) extents. Most chart publications comprise soundings associated with previous surveys supplemented with local updates. Furthermore, the frequency of updates and publications is inconsistent over the last 200 years. As such, undertaking a comparison of all published material can generate an ambiguous representation of change. Hence, for the purposes of this study, charts separated by at least a 40 year interval were used to reconstruct the morphological history of the outer Thames. Chart 100 from the 1820s was considered in comparison to chart 1610 from the 1860s, 1910s and 1950s and charts 2052 and 1183 from 2000. 108 Hydrobiologia (2011) 672:105–119 Charts were scanned whole on a large-scale scanner by the National Maritime Museum (Greenwich, UK) at 300 dpi. Raster images (TIFF format) of each chart were then georeferenced in ArcGIS 9.3 using ground control points and grid references (where present) to British National Grid (OSGB36). The RMS spatial error in the rectification process was <100 m for all charts except chart 100 (1824), which had RMS errors of 200 m.