Wave-formed sand ripples at Duck, North Carolina

A recently developed acoustic multiple transducer array was utilized to measure small‐scale bed forms in the nearshore and inner shelf regions at Duck, North Carolina. Two populations of wave‐formed ripples were observed: short wave ripples (SWR) with heights ranging from 3 mm to 2 cm and lengths ranging from 4 to 25 cm and long wave ripples (LWR) with heights ranging from 3 mm to 6 cm and lengths ranging from 35 to 200 cm. The SWR were only present sometimes, and their presence or absence was determined by a critical value of the near‐bed mobility number. The SWR were highly dynamic, sometimes flattening during wave groups and reforming over several incident wave periods. The LWR, in contrast, were almost always present. They were longer and lower relief than predicted by models or generally observed previously. Both SWR and LWR were often observed to migrate shoreward but were rarely observed to migrate seaward. The dimensions of the SWR, when they were present, were predictable by the Nielsen [1981] model or the Wiberg and Harris [1994] model to within approximately a factor of 2.