Wave-formed sand ripples at Duck, North Carolina

A recently developed acoustic multiple transducer array was utilized to mea sure small-scale bed forms in the nearshore and inner shelf regions at Duck , North Carolina. Two populations of wave-formed ripples were observed: sho rt wave ripples (SWR) with heights ranging from 3 mm to 2 cm and lengths ra nging from 4 to 25 cm and long wave ripples (LWR) with heights ranging frot h 3 nun to 6 cm. and lengths ranging from 35 to 200 cm. The SWR were only p resent sometimes, and their presence or absence was determined by a critica l value of the near-bed mobility number. The SWR were highly dynamic, somet imes flattening during wave groups and reforming over several incident wave periods. The LWR, in contrast, were almost always present. They were longe r and lower relief than predicted by models or generally observed previousl y. Both SWR and LWR were often observed to migrate shoreward but were rarel y observed to migrate seaward. The dimensions of the SWR, when they were pr esent, were predictable by the Nielsen [1981] model or the Wiberg arid Harr is [1994] model to within approximately a factor of 2.