Coherent Doppler Profiler observations from a ripple migration event at the Long-term Ecosystem Observatory LEO-15.

摘要

The migration of seabed ripples from yearday 335, December 1, of the 1999 LEO15 experiment is examined. Of primary interest are the suspended sediment profiles which are measured acoustically using a Coherent Doppler Profiler (CDP). The backscattering statistics are examined to improve the confidence in the CDP measurements. Incoherent backscattering, in which the contributions from different scatterers have phases which are random and independent, is assumed in most backscattering estimates of concentration. The velocity measurements, which are also made, require coherence between consecutive pings, preventing this assumption of incoherent scattering from being made. Simulations show that even though there is coherence in the measurements, a linear relationship between backscattered amplitude and the square root of concentration still applies. Turbulence is found to be the most significant physical factor influencing the correlation. The simulations also show that even though the higher order moments change as the correlation varies, the mean is unaffected. This result is essential in order to apply the calibrations made in the lab to the field data.; A computationally efficient, statistically based model is introduced to predict the suspended sediment concentration over a coarse grid to obtain a first order estimate of the important physical processes during the ripple migration. A method of extracting the model parameters from the data is developed. Convection of suspended sediment within the water column, settling of suspended sediment, and diffusion are relatively unimportant compared to the pickup process. The increased pickup over the troughs is consistent with vortex shedding.