Passive Hydro-Acoustic Monitoring of Gravel-Bed Material Sediment Load for Management of Gravel-Bed Rivers, including Gravel Augmentation to Improve Endangered Fish Habitat Downstream from Dams
Project ID: 3253
Principal Investigator: Robert Hilldale
Research Topic: Sediment Management and River Restoration
Priority Area Assignments: 2011 (Climate Adaptation)
Funded Fiscal Years: 2011
Prior research has indicated that passive hydro-acoustic technology shows great promise to replace conventional bed material load sediment transport monitoring that is extensive, costly, and cumbersome (Barton 2006; Belleudy 2008; and Thorne 2006). The research question is:
What instrumentation developments and field testing are necessary to extend existing passive hydro-acoustic technology for monitoring bed material load in gravel bed rivers into a viable new tool that can consistently provide a greater amount of data and more economical data for better management of gravel bed rivers?
The management of gravel bed rivers includes gravel augmentation, diversion dam construction/modification, dam removal, channel restoration, bank stabilization, etc.
Need and Benefit
Sediment transport in rivers occurs along the bed (bed material) and within the water column (suspended sediment). Suspended sediments are relatively easy to measure. Gravel bed material transport is difficult and costly to measure.
Existing technology is a bedload trap. When measuring, the bedload trap is lowered from a boat to the riverbed and retrieved after a prescribed amount of time during which the sampler fills with bed material load. The bed material is sent to a lab for weighing and size analysis. This process is repeated across the channel to measure total cross sectional bedload. The Toutle River and Elwha models' bedload traps each weigh 160 pounds or more. Samplers of this size require large boats with motorized winches. Bedload is also highly episodic and changes rapidly; sampling during peak flows is often unsafe due to the high depth and velocity. For these reasons, flows with the greatest effect upon the channel, structures, and habitat suitability are almost never measured. These samplers typically measure less than 1 percent of the total load. Therefore, there is high uncertainty in transport estimates from measured data, even under the best circumstances. Bedload sampling is labor intensive and costly and, thus, rarely accomplished.
Passive hydro-acoustic technology can capture the full spatial (i.e., it hears the whole riverbed) and temporal range, providing improved resolution of bedload transport, which will vastly expand our understanding of gravel bed rivers and, therefore, our ability to manage them. Passive hydro-acoustics is much cheaper, so bedload monitoring can be possible in places where it is not currently being done. As an example of the amount of labor and cost of conventional bed load measurement, the Trinity River Restoration Program spends about $300,000/year to measure bed material load for managing gravel augmentation for fish habitat. Relevant gravel bed management questions include:
1. When or at what flows and/or shear stress does gravel start to move at a particular river location?
2. What is the bed material load over a range of flow conditions?
3. How does implementation of given management action change the bed material load transport dynamics?
These three 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 three questions are impossible to answer without bedload field data.
The shape of river channel, bed material size, and the availability of suitable habitat are dependent, in part, upon the sediment size and amount supplied to the river, water shed hydrology, and valley geology. Dams store sediment along with water and reduce or eliminate the natural availability and recruitment of gravel in the riverbed needed for salmonid habitat. In spite of the number of instances where gravel bed material augmentation is being implemented, the full range of benefits and effects is not clearly understood and difficult to document. Example gravel augmentation management questions include:
1. How far downstream from a gravel augmentation point will habitat benefits be realized?
2. How long will it take to achieve downstream habitat benefits?
3. How long will habitat benefits at a specific location persist?
4. What is the amount and frequency of augmentation needed to achieve habitat benefits at specific downstream locations?
5. How do reservoir release patterns influence the answers to questions 1-4?
Examples of management questions related to flow management include:
1. What flows are needed to rework (mobilize) sediment deposits to prevent vegetation encroachment on the channel, channel narrowing?
2. Will transport of certain sizes of material reduce habitat value or result in channel incision, which reduces the potential for providing flows to flood plain wetlands?
Contact the Principal Investigator for information about these documents.
This information was last updated on August 23, 2014
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