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Quantitative Analysis of Beach Sand Movement, Virginia Beach, Virginia
Boon, John D.
Boon, John D.
Abstract
The present study describes a new approach to quantitative sediment tracing with dyed (fluorescent) particles, which can be used to obtain estimates of grain velocity, estimates of mobile layer thickness, and estimates of the rate of longshore sand transport.A simplified stochastic-process model was employed Involving the use of statistical procedures to analyze tracer displacement data on a time-stratified basis. The procedures provide meaningful estimates of the direction and magnitude of the movement of beach sand through set Intervals of time. Results of a single tracer experiment are reported from a beach located near Virginia Beach, Virginia. The tracer particle-displacement data obtained wares 1) subjected to preliminary tests to determine the need to separate the data according to two dynamic zones; 2) treated as normally distributed populations in which mean particle displacements were computed for specific times following tracer release and compared with one another using "t" tests to determine significant mean differences;3) time-stratified where successfully tested so that weighted mean displacements representing the time Interval 0 to 50 minutes could be computed; and 4) converted to mean particle velocities by dividing adjusted mean displacements by average travel time. For the morning of September 17, 1966, during moderate wave conditions at high tide, average particle velocity was determined for the swash and surf zone®. In the swash zone, V (X) • 0.44 ft./min. (0.22 cm./sec.) in the longshore direction. In the surf zone, V (Y) * 0.37 ft./min. (0.1# cm./sec.) in the offshore direction. Computed mean grain velocities and certain environmental data gathered prior to the time of the tracer experiment were used to make an estimate of the thickness of the mobile layer (the depth interval in which semi-continuous motion of the grains occurs). The thickness,based on grain velocity in the offshore direction and the measured rate of sand accretion seaward, was about 0.5 in. (1.3 cm.); assuming this to be the thickness of the mobile layer in the swash zone as well,an estimate of the longshore rate of sand transport was then computed using known values of swash zone width and longshore grain velocity.A transport value of 440 yd. 3/day (336 m3 /day)» in a longshore direction was obtained.
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1968
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10625047.pdf
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Virginia Institute of Marine Science
DOI
https://dx.doi.org/doi:10.25773/v5-kpx0-q629