The Spreading of Sand Tracers Under Water Waves: Laboratory and Field Investigations

A series of Eularian sand tracer experiments using fluorescent dyed sand were conducted in a large laboratory wave flume and in the high energy environment off the Oregon coast in order to determine the longitudinal (streamwise) mass spreading (dispersion) coefficient of sand under the oscillatory motion of progressive water waves. Laboratory wave periods ranged from three to five seconds with wave heights of 38 cm. Wave periods during the field experiment were ten seconds with wave heights of about one metre. Water depths were 3.17 and 16 to 18 m in the laboratory and field, respectively. Mass spreading coefficients, determined from the time-rate of change of variance of the tracer distribution, were found to be proportional to the entrainment rate of tracer at the injection site. A direct relationship based upon random-walk considerations was also found between the sand spreading rate and the wave orbital parameters. A spreading model based upon the continuous release of tracer from a line source was compared with favorable results to the observed tracer distribution patterns.