Development and field application of a littoral processes monitoring system for examination of the relevant time scales of sediment suspension processes.

摘要

A microcontroller-based system of oceanographic instrumentation providing a comprehensive set of measurements relevant to sediment transport processes has been developed. Analysis of the data provided by the system yields time series of vertical profiles of mean sediment size and concentration, horizontal profiles of bedform geometry, and single location measurements of flow velocity, pressure, turbidity, and water temperature. Details of the system architecture, including capabilities provided by both hardware and software contained within the system are given.; An improved method for the determination of suspended sediment size and concentration from the system's acoustic backscatter intensity measurements is presented. By retaining the size dependence throughout the derivation for an explicit solution for concentration, a new explicit solution to the acoustic backscatter equation results. This new concentration solution improves the technique for determining median sediment size by incorporating sediment attenuation in the calculation. Because this new technique relies on the minimization of the variance in concentration as determined by different frequency transducers, the previous technique of pairing transducers of different frequencies is replaced by a technique making use of any number of different frequency transducers. The new size/concentration inversion technique is tested using both simulated and laboratory data. Numerical precision is shown to be the only source of error with the use of simulated data. Laboratory tests result in less than 20% error in the determination of both concentration and size over a range of nearly one meter.; Finally, suspended sediment concentration data from the nearshore region obtained from an experiment performed in Duck, North Carolina, are examined to find the relevant time scales of sediment suspension. In this location, low frequency forcing mechanisms are as significant in suspending sediment as the incident-band wave forces typically used to model suspension. Like wave groups, this low frequency forcing results from the linear superposition of velocity components in a narrow band of frequencies. When these frequency interactions are considered, coherence greater than 60% is found between the velocity squared and the near-bed concentration across most of the spectrum.