The origin, classification and modelling of sand banks and ridges

Sand banks and elongated sand ridges occur in many coastal and shelf seas where there is abundant sand and where the currents are strong enough to move sediment, but they have a wide variety of forms. Their generation requires a source of mobile sediment, either from the local sea bed, or from coast erosion. Most appear to have been created during the post-glacial rise in sea level, but they have been subsequently modified by changing currents and waves, thus losing their relict characteristics. A descriptive classification scheme is developed to unify the approaches of marine geologists and physical oceanographers, which emphasizes the formation and present hydrodynamic setting in their long-term development. Open shelf linear ridges (Type 1) are up to 80 km long, average 13 km wide and are tens of metres in height. They are oriented at an angle to the flow, are asymmetrical and appear to migrate in the direction of their steep face. They appear to be in near equilibrium with the flow. These contrast with linear ridges formed in mouths of wide estuaries, which are aligned with the flow, and which migrate away from their steeper face (Type2A). In narrow-mouthed estuaries and inlets, tidal currents are strong only close to the mouth and waves are more dominant. The banks then form close to the mouth as ebb and flood deltas (Type 2Bi). When the coast is retreating, the ebb delta forms a primary source of sand to the nearshore region, which can become modified by storm flows into 'shore attached ridges' at angles to the coastline (Type 2Bii). Tidal eddies produced by headlands can create 'banner banks' (Type 3A), but when the headland is retreating alternating or 'en-echelon' ridges can be formed which can become isolated from the coast as it recedes (Type 3B). Coastal retreat and rising sea level can then cause the ridges to become moribund. Thus the majority of ridges rely on sea level rise for their origin. Theoretical and modelling studies of the shorter term response to present hydrodynamic forcing are generally confined to Types 1 ridges and 3A banks. The most promising work considers the coupled system of hydrodynamics, sediment transport and morphology on Type 1 ridges, and predicts features such as the ridge spacing and angle to the flow. Type 3A sand banks are clearly related to the flow patterns produced by the headlands, and the models can reproduce the eddy structures and sand bank extents. Nevertheless, the vital role of shoreline processes has not been fully incorporated into the models, and there is little modelling of ebb tidal deltas or other Type 2 banks. There thus appears to be a wide scope for modelling the generation, evolution and stability of sank banks under the scenario of rising sea level and coastal retreat. (C) 1999 Elsevier Science Ltd. All rights reserved.