Barchan-shaped ripple marks in a wave flume

Barchans, isolated crescent-shaped bedforms, are believed to be formed under almost unidirectional wind or water flows and limited sand supply. The formation of barchan morphologies under the action of purely oscillatory wave motion has not yet been fully investigated. The present study attempted to form barchan topography in a wave flume and to compare this with barchans in the field. Barchan morphologies of ripple size, called the barchan ripples, were generated from a flat bed by the action of waves. The horn width, the distance between horn tips, of the barchan ripples increased linearly with an increase in the total length, the overall length projected on the centre line of the barchan, with a coefficient common to barchan dunes in deserts. The ratio of horn length to horn width of the barchan ripples was smaller than that of barchan dunes, but similar to that of subaqueous barchans in the field. The longer the wave period was, the larger the ratio of the body length to horn width became. Most subaqueous barchans formed under waves (in the laboratory) and unidirectional flows (in the field) had blunter horns than subaerial barchans. The shape of the barchan ripples changed with wave period. The outer rim became rounder with increasing wave period. The relationship between the base area and the height of barchan morphologies seems to be linear, with a constant coefficient for the scale from ripples to dunes. The barchan ripples showed a linear relationship between the height and the horn width, similar to that for barchan dunes. The migration speed of the barchan ripples was proportional to the cube of the flow velocity and was inversely proportional to height. The same relation with a different value of the coefficient was obtained for barchan dunes. Copyright (C) 2004 John Wiley Sons, Ltd.