Passive Acoustic Measurement of Bedload Transport in Natural Bedrock Channels

0126131

Slingerland

The objective of this proposal is to design and calibrate a non-invasive, passive acoustic system for measuring spatially-integrated bedload transport rates in a natural bedrock channel. We pro-pose to measure the self-generated noise of bedload clasts in transport. Our experimental design consists of eight hydrophones and eight geophone deployed along a 100m reach, with data recorded to disk after passing through a sixteen-channel analog-to-digital conversion board on a laptop computer. The phones will be epoxied onto bedrock, near the banks, and onto large boulders in the center of the stream. The hydrophone array and the geophone array will be compared to determine which, if either, is preferable and to eliminate signals from fluid turbulence, cavitation, and bubble collapse. Both broadband and narrowband amplitudes will be analyzed for predictive power of bedload transport. The purpose of the streamwise array is to exploit the presence of downstream propagating waves of bedload transport and to determine an optimal sampling design. The system will be calibrated using bedload transport rates observed at low flows by conventional techniques such as chickenwire traps and Helley-Smith samplers, and at high flows by multiple surveys of a delta where the stream empties into a standing body of water The delta surveys will be accomplished using a shallow-water echosounder. Key hydraulic variables also will be measured in the study reach in order to relate these variables to measured bedload fluxes. The result will be a non-invasive method for measuring cross-sectionally-integrated bedload transport rates in bedrock reaches at very short time intervals.