Sand Camera Analysis

A twenty day subset of the sand camera and structured light data was analyzed by Lt. Damon Dixon for his MS thesis. The cross-shore light stripe generated by a laser light source was converted to a timeseries of cross-shore bed height profiles.

Bed profile Evolution

This first figure sumarizes the bed evolution during the measurment period. Each of the six panels shows a 3-6 day period of relative ripple height (cm) vs. cross-shore distance (cm). Offshore =0 while onshore =-60. Red and yellow outline ripple crests while blue and light green represent ripple troughs. The key feature to notice is the evolution from long crested ripples (yearday 252-258) to shorter crested bifurcated ripples (yeardays 260-263).

Current Sand Camera Image

The following figure shows two ripple crests movement vs. time in the top panel. The y-axis is cross-shore distance oriented such that the offshore direction =0 cm and the onshore direction =-50 cm. The Shields parameter plotted vs time in the bottom panel. Bigger waves result in strong bed orbital velocity. It is during these times, that the ripples migrate. Migration is predominantly offshore, except during yearday 249, where the top ripple crest bifurcates resulting in a crest that continues to migrate offshore and the separting crest to migrate onshore.

Ripple migration and forcing

Ripple migration and forcing

If your browser will play an MPEG movie clip you can Click Here to view an animation of the bed profile The movie shows that when the critical Shields Parameter threshold is exceeded the ripples move. At times, however, when the threshold is not exceeded, the bed is active. This is because the Shields parameter is based on a 30-minute average RMS orbital velocity which does not reflect the highly energetic bursts of waves followed by low amplitude waves found in narrow banded wave groups.


Last Reviewed: February 2003
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