Passive Acoustic (Hydrophone) Measurement of Coarse Bed Load
Prior research has indicated that passive hydroacoustic technology shows great promise to replace conventional bed load sediment transport monitoring that is logistically difficult, costly, and potentially unsafe. The research question is: How can a surrogate method of bed load measurement using passive hydroacoustic technology (hydrophone) be implemented to provide quantitative, accurate, and reliable measurements? In order to answer this primary question, the following inquiry must be answered, which is the goal of this research. What instrumentation developments, interdisciplinary collaborations, and testing are necessary to extend existing passive hydroacoustic technology into a viable new tool that can consistently provide reliable bed load measurements at a lower cost for better management of gravel-bed rivers?
This research proposal is being submitted on the heels of a funded scoping proposal to investigate the current state of using hydrophones to measure bed load, determine potential collaborators, and form partnerships that will advance the science of hydroacoustic bed load measurement. This effort revealed that the Agricultural Research Service (ARS) is interested in pursuing a means to measure bed load with hydrophones. The ARS has planned to collaborate with the University of Mississippi's National Center for Physical Acoustics (NCPA) to advance this research. A collaboration with team members knowledgeable in the fields of sediment transport and acoustics and signal processing is well positioned to advance the application of passive hydroacoustics in the measurement of bed load.
Need and Benefit
Sediment transport in rivers occurs along the bed (bed material) and within the water column (suspended sediment). While suspended sediments are more easily quantified, gravel bed material transport is much more difficult and costly to measure. Existing technology for measuring bed load, such as pressure difference and Birkbeck samplers, is either too costly or infeasible for many reasons. Current methods of bed load measurement do not provide continuous data, which is critical for improving the current understanding of the transport of coarse bed material. Bed load is highly episodic and the transport rate can change rapidly. Collecting physical measurements with pressure difference samplers is often unsafe due to the high depth and velocity that is present when the coarsest fraction of a gravel-bed river is in motion. For these reasons, flows with the greatest effect upon the channel are almost never measured for their bed load transport rates. When bed load is measured with pressure difference samplers, typically less than 1 percent of the total load is actually sampled. Therefore, there is high uncertainty in transport estimates from measured data, even under the best circumstances.
Passive hydro-acoustic (hydrophone) technology can continuously capture the full spatial (i.e., it hears the whole riverbed) and temporal range of gravel-bed load transport, providing an improved understanding of bed load transport in gravel-bed rivers and therefore, our ability to manage them. Deploying passive hydroacoustic sensors and data logging equipment is more easily implemented and cost effective than established methods. As an example of the amount of labor and cost of conventional bed load measurement, the Trinity River Restoration Program spends about $250,000/year to measure bed material load for managing gravel augmentation for fish habitat. A likely cost for deploying a hydroacoustic bed load measurement system might be on the order of $10,000 and can be used continuously for as long as the equipment lasts.
Relevant gravel bed management questions that can be answered with quality bed load data include:
1. At what discharge and/or applied shear stress does gravel start to move at a particular river location?
2. What is the bed material load, in mass per unit time, over a range of flow conditions?
3. How is the movement of coarse bed load related to the timing of a flood hydrograph? Current knowledge identifies hysteresis regarding water and bed load discharges but little is understood about the variability of the hysteresis.
4. How does implementation of given management action change the bed material load transport dynamics?
These questions are basic for any management action on a gravel-bed river, be it flow management, diversion dam construction/ modification/removal, gravel augmentation, bank stabilization, grade control, channel manipulation, habitat restoration, etc. These questions are impossible to answer effectively without bed load field data.
The following documents were not reviewed. Statements made in these documents are those of the authors. The findings have not been verified.
Research Bulletin (interim, PDF,
By Robert Hilldale
Publication completed on June 20, 2012
This information was last updated on May 18, 2013
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