Historical Seabed Mobility in an Outer Estuary - Sea Basin Environment

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Historical Seabed Mobility in an Outer Estuary - Sea Basin Environment H. Burningham and J. R. French Coastal and Estuarine Research Unit Department of Geography University College London London, WC1E 6BT, UK h.burningham@geog.ucl.ac.uk ABSTRACT BURNINGHAM, H. and FRENCH, J., Historical seabed mobility in an outer estuary - sea basin environment. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN Sandbanks and ridges that form in sediment-rich current-dominated shoreface environments play an important role in coastal dynamics (spatially and temporally), influencing the nearshore wave climate as well as providing sediment source-sink functions. In this paper, we describe the marine geomorphology of the outer Thames estuary in the southern North Sea. This region of seabed (over 5000 km 2 ) is characterised by wide estuary mouth banks, headland-associated banks and open shelf ridges. Their historical evolution is assessed through the examination and spatial analysis of bathymetric surfaces derived from nautical charts covering the last 180 years. The study uses principal component analysis to examine the dominant trends in bathymetric change, following recent demonstrations of this approach by BUONAIUTO et al. (2008) and REEVE et al. (2008). Results show that the banks and ridges throughout the outer Thames are dynamic, despite maintaining a relatively consistent topological framework. There is a clear spatial (and classificatory) difference in historical behaviour, and the morphodynamics of each set of features appears to be dependent on a specific combination of tidal current and wave driven sediment transport processes. The central estuary mouth banks show progressive changes associated with lateral shifts in position and along-bank extension. Over the same timescale, headland-associated banks to the north shift in orientation relative to the shoreline but in no consistent direction. Shelf ridges to the east, which are very narrow and long features (<1 km wide, >10 km long), have changed little in comparison. ADITIONAL INDEX WORDS: sandbank, tidal channels, PCA, eigenvectors INTRODUCTION Banks and ridges form across the seabed in continental shelf and estuarine embayment environments, particularly those rich in mobile sediments. DYER & HUNTLEY (1999) synthesised much of the early literature on the morphology and development of these large-scale bedforms and classified these features on the basis of formative environment. Their top level subdivision separated offshore, shelf forms (Type 1) from estuary mouth (Type 2) and inshore, headland-associated (Type 3) forms. Type 1 ridges are long, tall linear forms that are composed of sand and are formed largely in response to shelf (offshore) tidal currents (DYER & HUNTLEY, 1999). Type 2 ridges cover the various formations associated with tidal inlets and estuarine entrances. Sedimentology can vary depending on sediment supply (BURNINGHAM & FRENCH, 2007) and morphology is generally dependent on the relative energy of, and interaction between, wave and tidal current processes (HAYES, 1980; FITZGERALD, 1988). Type 3 ridges relate to inshore features that form in association with coastline physiography, particularly where shoreline orientation changes significantly (e.g. in the presence of a headland or foreland), causing an excess deposit of sediment transported alongshore (DYER & HUNTLEY, 1999). Where significant coastal retreat occurs, these banner banks will develop into alternating ridges that become increasingly detached from their shoreline sediment supply, and modified further as wave and tidal processes also respond to shoreline retreat (DYER & HUNTLEY, 1999). Specific studies of these seabed features are few, owing to the difficulties in obtaining data over the spatial and temporal scales required. Although there is a broad knowledge of their geography, particularly in the southern North Sea, few works have analysed the morphological evolution of the seabed from a geomorphological perspective. An extensive sand bank region northeast of Norfolk has been examined in detail, with specific reference to sediment transport pathways (COLLINS et al., 1995). Further south, banks occupy the inshore region of the Norfolk/Suffolk coasts, and these have been the focus of various studies examining connections between banks and coastal behaviour, often with conflicting interpretations (ROBINSON, 1966; 1980; MCCAVE, 1978). More recently however, studies of the Great Yarmouth/Lowestoft inshore banks on the Norfolk/Suffolk coast have documented historical morphological evolution and modelling of associated hydrodynamics (DOLPHIN et al., 2007; PARK and VINCENT, 2007; HORRILLO-CARABALLO and REEVE, 2008; REEVE et al., 2008). These studies have demonstrated the importance of historical analysis in the understanding of morphology and behaviour of the seabed in the vicinity of banks and ridges, particularly as an aid to understanding changes in patterns of erosion and deposition along neighbouring shorelines. The seabed of the outer Thames estuary in the southern North Sea comprises ridges and banks across the DYER & HUNTLEY 589

2 Historical Seabed Mobility in an Outer Estuary - Sea Basin Environment Figure 1. Banks and ridges of the outer Thames estuary, southern North Sea. Under the classification of DYER & HUNTLEY (1999), Type 1 shelf ridges exist to the east, Type 2A wide estuary mouth banks occupy the central basin, and Type 3 headland-associated banks occur to the north. Wave climates given in Figure 2 refer to locations A) Bawdsey, B) West Gabbard, C) Clacton and D) Maplin Sands. (1999) classification (Figure 1). Shelf ridges (Type 1) are located to the east, on the open shoreface, wide estuary mouth banks (Type 2A) cover the central region between Kent and Essex, and headland-associated banks (Type 3) are located along the Suffolk shoreface to the north (Figure 1). This assemblage of banks and ridges is unusual for a single region that can be considered as one system. Although some studies of hydrodynamics, sediment transport and morphology across the southern North Sea exist, there has been very little direct examination of historical seabed behaviour. The work described here considers the recent history (last years) as derived from published hydrographic charts to assess the geomorphic history of the seabed in this outer estuary - sea basin environment, across a region >5,000 km 2. This paper is part of a wider project examining seabed morphodynamics over the last 400 years. PHYSICAL SETTING The outer Thames region hosts the tidal confluence of a number of estuaries. In addition to the Thames, estuaries enter along the Kent (Medway and Swale), Essex (Crouch/Roach, Blackwater and Colne) and Suffolk (Orwell, Stour, Deben and Alde/Ore) shorelines (Figure 1). The Thames is by far the largest of these, with a mean tidal prism of c. 6.1 x 10 8 m 3 s -1 at Southend-on-Sea: in contrast, the combined mean tidal prism of all other estuaries entering the outer Thames is 4.2 x 10 8 m 3 s -1. The differences in tidal prism partly reflect the large geographical variation in tidal range across the outer Thames. Spring tidal range is 5.3 m at Southend-on-Sea (Essex), decreasing to 4.3 m to the east (Margate, Kent) and decreasing further to 2.3 m to the north (Orfordness, Suffolk). It is clear that tidal flow into and out of the inner Thames exerts a significant control on bank morphology across the outer Thames (Figure 1). But morphology and the general organisation of sediment across the seabed is also influenced by the strongly bimodal southern North Sea storm wave climate and locally generated wind waves (Figure 2). Waves recorded at West Gabbard (buoy B) have an average significant height (H s ) of 1.1m and an average period (T z ) of 4.1s: wave heights >2m account for about 10% of the wave record ( ). In contrast, H s =0.3m and T z =2.2s at Maplin Sands (buoy D), where H s >1m account for only 0.7% of the wave record 590

3 Burningham and French Figure 2. Wave roses derived from records obtained from wave buoys located in Figure 1 [A) Bawdsey, B) West Gabbard, C) Clacton and D) Maplin Sands]. Data source: Cefas and BODC. ( ). On average, relative sea level has been rising in this region by about 2.4 mm yr -1 over the last century. The seabed owes some of its character to the former Pleistocene landscape, particularly in terms of palaeovalleys and associated Holocene shorelines (D OLIER, 1972), which are evident in high resolution bathymetry. On a regional scale, there is little direct association between seabed morphology and this palaeo-landscape due to the development of banks and ridges over the London Clay (Eocene) basement. This substrate is variably organised, ranging from thick sand waves deposits to thin gravel lags and exposed clay bedrock (HR WALLINGFORD, 2002). The region is therefore a very important aggregate resource, which often conflicts with the management of the habitats and species it supports. METHODS Nautical charts covering the outer Thames estuary date by to the 16 th century, but only those published since the early 19 th century contain sufficient internal precision and geographic accuracy to enable georectification, and hence spatially-based analyses. Charts have been published with increasing regularity and spatial scale over the last 200 years. Although far greater detail is achieved in consulting original soundings or the highest resolution charts, this does not necessarily improve the comparison of surveys across the historical period as the resolution of analysis is determined by the lowest resolution data. Surveys from the early 19 th century were published at the scale of 1:140,000 covering the entire outer Thames region (as defined in Figure 1), and for the inclusion of these data in the analysis, comparable charts from across the last 200 years were used. Charts dated 1824, 1864, 1847, 1893, 1910, 1918, 1926, 1934, 1954 and 2003 were scanned whole on a largescale scanner by the National Maritime Museum (UK) at 300dpi. Raster images (TIFF format) of each were then georeferenced in ESRI ArcMap 9.2 ( using all available grid references and ground control points, to the cartesian British National Grid (OSGB36) coordinate system. The average rectification error was 55m, excluding the recent chart which had an error of 5m. Furthermore, the 1824 chart contained a systematic error in grid position affecting east of 1 30 E (giving an error of 209m) so could only be considered for analysis of features west of 1 30 E (BURNINGHAM and FRENCH, 2009). Depth data were digitised from the charts into point layers. There was some degree of re-use of soundings between the 1893 to 1926 editions where only specific bank areas were resurveyed. All depths were converted to metres relative to Ordnance Datum Newlyn (OD) to enable direct comparison between charts. Due to the geographical variation in the relationship between chart datum (CD) and OD, a trend surface model was used to predict tidal levels for the chart-specific datums (LAT, MLWS and 1 foot below MLWS) across the region (BURNINGHAM and FRENCH, 2008). A point-specific CD to OD conversion factor for each point on each chart was used to convert soundings to depths in metres relative to OD (i.e. relative to mean sea-level). Bathymetric data were interpolated onto a 100x100m grid using the nearest-neighbour method in ArcGIS 9.2 3D Analyst. Regions of interest were defined on the basis of bank type (Figure 1). The mean seabed surface was calculated using a proportional averaging, where a weighting was applied to each layer based on the time interval between surveys. A Principal Component Analysis (PCA) was used to extract the main variance in the data into a reduced number of uncorrelated factors (for more examples of the use of PCA-based methods in bathymetric change analysis, see CUADRADO and PERILLO, 1997; REEVE et al., 2008). Bathymetric layers were transformed to remove the mean seabed surface, which would otherwise dominate the first component (BUONAIUTO et al., 2008). The positions of banks in the offshore region (Type 1) on early surveys are problematic and only surveys since, and inclusive of 1893, showed consistency in position. As such, independent PCAs were conducted for the three bank-type zones (Zones 1, 2 and 3 covering bank types 1, 2a and 3 respectively). The PCA results were compared with net change and change between successive surveys through correlation analysis. These analyses were accomplished in ArcGIS 9.2 Spatial Analyst. RESULTS AND DISCUSSION The PCA of temporal change in seabed bathymetry (transformed to remove the mean) is expressed in the form of spatially explicit principal components (PCs) supported by eigenvalues (Figure 3). The analysis explains the variance in the data in a reduced number of independent variables (PCs). The PCs reflect the gradient of each derived variable, shown in Figure 3 as a greyscale gradient: the mid-score (grey) represents the midpoint of the gradient, and high (black) and low (white) scores represent the respective limits of the gradient. In zone 1, the first 3 components explain 93% of the temporal variance in seabed bathymetry (Figure 3). It is not surprising that this is higher than that explained by the first 3 components in zones 2 (78%) and 3 (73%) as only 7 surveys were included in the offshore analysis in comparison with 10 for the other two regions. Figure 3 shows that changes are focused along bank margins where lateral shifts cause large changes in depths. Interestingly, although each PC reflects changes along bank margins, the geographical focus of change within each component is different. This suggests that migration, growth and decay of banks operate over different, and potentially independent, time-scales. In zones 1 and 2, PC1 is well correlated with net change over the period considered (zone 1: 1918 to 2003 R=0.95 and zone 2: 1824 to 2003 R=-0.88). Change in the first half of the 20 th century best explains PC1 in zone 3 (R=-0.84) and PC2 in zone 1 (R=0.89). Change throughout the 20 th century is best correlated with PC2 in 591

4 Historical Seabed Mobility in an Outer Estuary - Sea Basin Environment Figure 3. Principal component analysis of temporal changes in seabed bathymetry for each zone (based on bank/ridge type). Results covering zones 2 and 3 (type 2a and 3 banks) refer to surveys between 1824 and 2003; results for zone 1 offshore (type 1) refer to surveys between 1893 and Results are shown as a greyscale from white (minimum score) to black (maximum score). zone 2 (R=0.86) and change throughout the 19th century best explains PC2 in zone 3 (R=0.86). The temporal expression of the PCs is given in the form of eigenvectors which show how the extracted variance is represented across layers, and so between surveys (Figure 4). The PC1 eigenvector for zone 2 shows a gradual trend over the time scale considered, which is also evident, though markedly stepped, in zones 1 and 3. As PC1 in zone 2 is well correlated with net change, this suggests that the overall changes in bathymetry in zone 2 are expressed relatively evenly across the surveys, thereby suggesting that the nature of this change has been progressive. In zones 1 and 3, variance expressed in PC1 is dominated by changes in the first half of the 20th century, although the full gradient is extended over the entire time period. In zone 1, the second and third components are notably noisy by comparison, with little variance over the turn from 19th to 20th century and significant change in the early 1900s. This is likely to be a result in survey resolution changes across zone 1; for example, Unnamed Bank, the northern-most bank in this zone, was not fully recognised until the mid-1900s (BURNINGHAM and FRENCH, 2008). In zone 2, PC2 eigenvectors suggest some degree of behaviour reversal between the 19th and 20th centuries. As PC3 is centred on zero throughout the 20th century, it is possible that this component is expressing a behaviour that is only evident in the 19th century. Zone 3 exhibits a pattern almost directly inverse to zone 2 in the eigenvectors of PCs 2 and 3, suggesting similar time-scales of behaviour. It is tempting to attribute the progressive trend captured by the first components to the historical rise in sea level, but the spatial patterns of these component scores point more to a long-term lateral migration or along-bank growth -related behaviour. These 592

5 Burningham and French Figure 4. Eigenvectors of each bathymetric layer (survey) for principal components 1 to 3 across the three bank-type zones. shifts are likely to be driven by tidal currents; in zone 2 the Thames tidal prism exerts a clear control over seabed morphology, whereas in zones 1 and 3 tidal processes are governed by shoreface tidal streams. Wave processes are only effective across the bathymetric highs. CONCLUSION The use of PCA in the analysis of bathymetric change does provide a means of reducing the variance contained in these inherently extensive datasets. Here, the first three components are effective in capturing 73-93% of the variance in the bathymetric surveys over the year time scale. PCA has been used to assess similarities and differences in the historical behaviour of different bank types. The analysis suggests that the wide estuary mouth ridges have experienced progressive change across the 200 year history presented, associated with lateral movement and along-bank migration. It also appears that some of this mobility occurs specifically over ~100 year time-scales. Similar shifts are evident for the headland-associated banks, though this zone is dominated by mid-1900s variance. The offshore ridges present a rather more complicated picture, potentially more influenced by changing resolution of surveys and the features identified within them. LITERATURE CITED BUONAIUTO F.S.; BOKUNIEWICZ H.J. and FITZGERALD D.M., Principal component analysis of morphology change at a tidal inlet: Shinnecock Inlet, NY. Journal of Coastal Research, 24 (4), BURNINGHAM, H. and FRENCH, J.R., Morphodynamics and sedimentology of mixed-sediment inlets. Journal of Coastal Research, SI 50, BURNINGHAM H. and FRENCH J.R., Historical changes in the seabed of the greater Thames estuary. The Crown Estate, 33pp plus appendices. BURNINGHAM H. and FRENCH J.R., Seabed mobility in the greater Thames estuary. The Crown Estate, 32pp plus appendices. COLLINS, M.B.; SHIMWELL, S.J., GAO, S., POWELL, H., HEWITSON, C. and TAYLOR, J.A., Water and sediment movement in the vicinity of linear sandbanks: the Norfolk Banks, southern North Sea. Marine Geology, 123, CUADRADOA D.G. and PERILLO G.M.E., Principal component analysis applied to geomorphologic evolution. Estuarine, Coastal and Shelf Science, 44, D OLIER B., Subsidence and sea-level rise in the Thames Estuary. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 272 (1221), DOLPHIN, T.J.; VINCENT, C.E., COUGHLAN, C. and REES, J.M., Variability in sandbank behaviour at decadal and annual time-scales and implications for adjacent beaches. Journal of Coastal Research, SI 50, DYER K.R. and HUNTLEY D.A., The origin, classification and modelling of sand banks and ridges. Continental Shelf Research, 19, HAYES, M.O., General morphology and sediment patterns in tidal inlets. Sedimentary Geology, 26, HORRILLO-CARABALLO, J.M. and REEVE, D.E Morphodynamic behaviour of a nearshore sandbank system: The Great Yarmouth Sandbanks, UK. Marine Geology, 254, HR WALLINGFORD, Southern North Sea Sediment Transport Study (Phase 2). HR Wallingford Report, EX FITZGERALD, D.M., Shoreline erosional-depositional processes associated with tidal inlets. In: AUBREY, D.G. and WEISHAR, L. (eds.), Hydrodynamics and Sediment Dynamics of Tidal Inlets. Springer-Verlag Inc., New York, pp MCCAVE, I.N., Grain-size trends and transport along beaches: Example from eastern England. Marine Geology, 28, (1-2), M43-M51. PARK. H.-B. and VINCENT, C.E., Evolution of Scroby Sands in the East Anglian coast, UK. Journal of Coastal Research, SI 50, REEVE D.E.; HORRILLO-CARABALLO J.M. and MAGAR V., Statistical analysis and forecasts of long-term sandbank evolution at Great Yarmouth, UK. Estuarine, Coastal and Shelf Science, 79 (3), ROBINSON, A.H.W., Residual currents in relation to shoreline evolution of the East Anglia coast. Marine Geology, 4, ROBINSON, A.H.W., Erosion and accretion along part of the Suffolk coast of East Anglia, England. Marine Geology, 37, ACKNOWLEDGMENTS The authors wish to thank the Crown Estate and the National Maritime Museum for ongoing support in this project. 593