DTI Strategic Environmental Assessment Area 2 (SEA2) geological processes (interpretation of multibeam, sidescan sonar, chirp and grain size data acquired in 2001 from the seafloor of the Norfolk Banks and Dogger Bank, southern North Sea)

The focus of this study is on the offshore sandbanks occurring in the mature southern North Sea gas producing fields on the UK continental shelf. New geological interpretations have been made from bathymetric, sidescan sonar and seabed photographic and sample surveys carried out for the Department of Trade and Industry in 2001 over the Norfolk Banks and the Dogger Bank. The survey data were acquired in water depths varying from approximately 10m to 40m or more. The new survey data were collected to fill information gaps on the seabed processes influencing the seabed sand transport directions, composition of the seabed habitat and sandbank function. Interpretative data have been collected for bedforms ranging in size from sand banks more than 20km long and spaced more than 5km apart to sand ripples less than 2m long and spaced less than 60cm apart. Existing geological information has been integrated with the new survey information to present the interpretations of the variety of the modern seabed habitats in their wider historical and regional contexts. The research findings from the seabed study are interpreted from data collected during relatively calm spring/summer wave conditions and without surge-driven tides. • Under the range of seasonal hydrodynamic conditions the sandbanks interact with the flood and ebb tidal currents, wind and wave and storm surge currents so that the seabed on the sandbanks is characterised by superimposed mobile small to very large mobile bedforms. The exceptions found were on and adjacent to the crest of the Dogger Bank and on the crests of the Norfolk Banks all in less than approximately 18m water depth. In these areas the larger transverse bedforms appear to have been destroyed at the bank crests by waves. • The Dogger Bank is a glacial outwash feature that was deposited under conditions of prolific sediment supply when the southern North Sea basin was partly covered by a terrestrial ice sheet more than 14,000 years ago. Although it is now covered by mobile sand at seabed, the bulk of the Dogger Bank in the area of survey is immobile and consists of cohesive muddy and gravelly sediments that are resistant to erosion. • The main changes of seabed mean grain size across the Dogger Bank at the West Patch occur in areas with patchy seabed hollows with an origin from to scour caused by strong near-bottom currents during storms. The seabed sediments occurring outside the scours are predominantly well sorted fine sands. The seabed sediments in the scours are characterised by relict very poorly sorted muddy and gravely sands. • The Norfolk Banks formed following the retreat of the terrestrial ice sheet, marine inundation of the southern North Sea and by interactions between sea-level changes, prolific sediment supply and near-bottom currents. The bulk volumes of the banks consist of predominantly uncohesive submarine sands that are not resistant to erosion by strong currents. • Internal data from the Broken Bank have been interpreted to propose a possible a generic model for the historical development and integral function of the active banks in the region. The model is important to understanding the long-stability of the banks because it indicates that the oldest banks may be segmented into aggrading (heads) and younger prograding tails (NW ends) that are sensitive to changes in position of the heads. On the basis of their internal structure and external geomorphology both the banks heads and tails have migrated to the NE. There is no data for an understanding of the nature of the along-bank transition from aggrading to prograding internal structure. iii • There is an overall positive correlation between the transverse bedform facing directions and the trends of decreasing mean grain size in the medium to fine sand fraction of the seabed sediments on the Norfolk Banks. The correlation is interpreted to infer net sand bedload transport directions across and between the banks to the north and east. • The trends of mean sand grain size with position on Ower Bank B appear to show the existence of segments of decreasing mean grain size to the NE across the bank and decreasing mean grain size along the bank flanks in a clockwise direction. These trends are interpreted to confirm what has been previously inferred for nearshore banks but not calibrated by regional sampling, namely that both across- and along-bank sediment transport processes interact to influence bank seabed composition. • On the basis of the interpretation of the data from Ower Bank B and the consistency of single profiles across other banks a process model is proposed that may be applied to predict segmented systematic variations in the composition and functions of the active banks. The model predicts that the finest seabed sediments within the sand fraction will be observed on the SE flank of the segment under the influence of the process identified. • There is an overall decrease of mean grain size with decreasing water depth on the banks in the region that are outside the influence of the strongest tidal currents. • The interpretations of the new regional seabed sediment textural data have confirmed the uniqueness of the functions of the Norfolk Bank relative to surrounding banks.