Stereo–video observation of nearshore bedforms on a low energy beach

Abstract

A new technique utilising underwater stereo–video was tested for its ability to measure nearshore bed morphology at high-frequency and high-resolution. A stereo–video system was used to monitor small-scale ripple morphology (wavelengths 0.05–0.10 m, heights ∼5 mm) in the nearshore of a low energy beach in southwestern Australia. Waves, currents and suspended sediment concentrations were also monitored. Bed morphology could be measured with a mean accuracy of 2 mm with the stereo–video system. Measurements in the field were limited to ripple crest positions since targets over the entire ripple profiles could not be identified in both the left and right stereo images. This limitation could be overcome in the future by use of laser stripes to illuminate the profiles and the generation of epipolar lines within the analysis software to match corresponding targets in each of the images. Ripple crest positions were measured at 1 Hz and the relationship between ripple movement and near-bed hydrodynamics was investigated to test the applicability of the stereo–video system to make high-frequency bedform observations. Ripple crests were found to oscillate at the same frequency as the swell waves. The magnitude of the ripple movement at swell wave frequencies was an order of magnitude greater than ripple movement at wind wave frequencies. Net onshore migration of the ripples corresponded to onshore wave asymmetry, very weak mean cross-shore currents and net onshore suspended sediment flux measured at an elevation of 0.05 m.