|Rigby Jr, James|
|MURRAY, NATHAN - University Of Mississippi|
Submitted to: International Conference on Fluvial Hydraulics
Publication Type: Proceedings
Publication Acceptance Date: 2/23/2016
Publication Date: 7/13/2016
Citation: Rigby Jr, J.R., Wren, D.G., Murray, N. 2016. Acoustic signal propagation and measurement in natural stream channels for application to surrogate bed load measurements: Halfmoon Creek, Colorado. International Conference on Fluvial Hydraulics. 8th International Conference on Fluvial Hydraulics. River Flow 2016. July 12-15, 2016 St. Louis, MO. pp. 537-538.
Interpretive Summary: Water is the most powerful element shaping the landscape. Streams move tremendous amounts of rock and sediment from mountains and hills to the ocean each year. Some of this material is carried in suspension in the stream while the larger particles are rolled or bounced along the bed by the flow. This material moving along the bed is called "bed load" and is notoriously difficult and expensive to measure directly with the level of accuracy desired for estimating the total sediment load delivered to the river outlet. Methods to indirectly measure this load are attractive because they may be inexpensive and allow estimates that are continuous in time. One such technology is the use of underwater microphones to record the noise generated by rocks as they collide with one another along the bed. The work in this study advances the use of underwater microphones by investigating how for these sounds propagate under water in streams. This will help determine what area is measured by a single underwater microphone so that the right number of microphones can be deployed to measure the entire moving bed.
Technical Abstract: Monitoring sediment-generated noise using submerged hydrophones is a surrogate method for measuring bed load transport in streams with the potential for improving estimates of bed load transport through widespread, inexpensive monitoring. Understanding acoustic signal propagation in natural stream environments is a basic component of developing this methodology, yet the acoustics of these environments have been received much less attention than, e.g., marine environments. A series of acoustic propagation experiments were conducted in Halfmoon Creek, a snow-melt dominated stream in central Colorado, using a known underwater source to determine signal propagation in directions longitudinal and transverse to the thalweg. Results the dominance of flow noise around the hydrophone resulting in low signal-to-noise ratios for measurements with the hydrophone held in a fixed position relative to the bed. “Lagrangian” measurements using a floating measurement platform allowed to move with the flow displayed a significant decrease in ambient flow noise relative to the fixed case and resulted in signal detection at much higher ranges. Complex range dependence of frequency and amplitude are presented and discussed in relation to stream geometry and bed material.