Field observations of bedforms in high sediment transport conditions

Abstract

In the first reported use of MHz-frequency acoustics to study small-scale bedforms in the near shore zone, Dingler and Inman were able to observe the evolution of transition ripples under energetic groupy waves near the flat bed threshold. They found that these low-relief, long-crested bedforms appeared to be obliterated under the largest waves in a group, and subsequently reformed in one or two wave periods [J. Dingler and D. Inman, Proc. 15th Coastal Conf., Vol. II, 2109–2126 (1976)]. More recently, measurements of linear transition ripples were made during an autumn storm event using scanning wide beam and fixed narrow beam sonars, as well as a high resolution laser-illuminated video system. These observations also indicate the occasional apparent disappearance of these ripples, and also their reformation at the same locations. The question arises as to whether the disappearance is apparent (the result of increased near bed sediment concentration and the definition of bed elevation based on reflection amplitude) or real (planing off of the ripple crests by sheet flow).