| Summary: | The Coherent Acoustic Sediment Profiler (CASP) uses four acoustic beams at two frequencies (1.3 and 5.3 MHz) to measure the backscatter from sediments and infer concentrations in 1.68 cm bins to one meter range. It also measures three velocity components and sediment concentration in the same intersecting volume (O(1 cm3)) at a sampling frequency of 36 Hz. Empirical calibration of the sediment profiling capability of the CASP has been performed with sediment sampled from the nearshore zone at Duck, N.C. The sediment is characterized by a lognormal distribution with a mean grain size diameter of 0.18 mm, and standard deviation of 1.2. The 1.3 MHz sonar measures sediment concentrations of O(0.0001 kg per m3) to 25 kg per m3, and the 5.3 MHz sonar measures concentrations of O(0.00001 kg per m3) to 8 kg per m3 both with an accuracy of 10%. A gram size discrimination capability, based on comparison of measurements by the two frequencies and the assumption of a lognormal sand size distribution, can detect variations of +/- 0.05 mm mean geometric radius between the calibration sand and the sediment measured in the field. Field data from a day characterized by narrow banded, nonlinear wave forcing during the DUCK94 experiment show that episodes of high sediment concentration correlate well with the maximum onshore velocities within 2 cm of the bottom, but decorrelate above this level. Data is ensemble averaged with a phase mapping technique to illustrate this correlation. Sediment flux calculations reveal strong shoreward transport at th boundary layer, and weaker seaward transport higher in the water column. Distribution
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