The origin, classification and modelling of sand banks and ridges

Sand banks and elongated sand ridges occur in many coastal and shelf seas w here there is abundant sand and where the currents are strong enough to mov e sediment, but they have a wide variety of forms. Their generation require s 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 an d waves, thus losing their relict characteristics. A descriptive classifica tion scheme is developed to unify the approaches of marine geologists and p hysical oceanographers, which emphasizes the formation and present hydrodyn amic 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 heig ht. 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 mout hs of wide estuaries, which are aligned with the flow, and which migrate aw ay from their steeper face (Type2A). In narrow-mouthed estuaries and inlets , tidal currents are strong only close to the mouth and waves are more domi nant. 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 int o 'shore attached ridges' at angles to the coastline (Type 2Bii). Tidal edd ies produced by headlands can create 'banner banks' (Type 3A), but when the headland is retreating alternating or 'en-echelon' ridges can be formed wh ich can become isolated from the coast as it recedes (Type 3B). Coastal ret reat and rising sea level can then cause the ridges to become moribund. Thu s the majority of ridges rely on sea level rise for their origin. Theoretic al and modelling studies of the shorter term response to present hydrodynam ic forcing are generally confined to Types 1 ridges and 3A banks. The most promising work considers the coupled system of hydrodynamics, sediment tran sport and morphology on Type 1 ridges, and predicts features such as the ri dge spacing and angle to the flow. Type 3A sand banks are clearly related t o 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 t here is little modelling of ebb tidal deltas or other Type 2 banks. There t hus 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.