Development of a Continuous Bedload Transport Measurement Technique
There is a need to measure bedload downstream of two dams on the Elwha River (Glines Canyon and Elwha) in Washington that are slated for removal. The two major questions related to the proposed bedload measurement are:
* How well did Reclamation models predict the downstream transport of sediment accumulated in the reservoirs?
* At what rate is bedload being transported by the river channel below the two dams over time?
Reclamation will need to track the bedload released from the reservoirs over time so that modifications to the rate of dam removal can be made if necessary. The Elwha and Glines Canyon Dams are scheduled to be removed over a two to three year period.
Need and Benefit
Currently there is no method available for continuous monitoring of bedload on the Elwha River. Moreover, only a few continuous bedload monitoring methods or devices are currently being used around the world. By installing a series of bedload impact sensors (instrumented steel plates) across the channel, bedload can be measured at a specific location as it passes a fixed cross section. These steel plates will be attached to the downstream edge of a sheetpile wall that is being installed for water diversion purposes.
The method of bedload measurement currently available is direct sampling. This method requires a team of individuals to be present during sampling and is very costly. The sampler is usually deployed from a bridge, boat, or cableway and remains on the bed for several seconds to a few minutes, depending on the transport rate. Data collected in this fashion can only be applied to the conditions under which it was sampled, primarily discharge. During dam removal, bedload transport can not be assumed constant for a given discharge due to the temporal changes in sediment supply as the dams are being removed. Therefore, the bedload released from the reservoirs will not be able to be calculated using standard transport curves and direct sampling methods.
The benefit to Reclamation will be a surrogate bedload measurement method that will be calibrated in the lab and in the field. It is expected that the method will be proven reliable and could be applied elsewhere. This research may contribute to efforts by the Federal Interagency Sedimentation Project (FISP) to provide Federal approval for surrogate sediment measurement methods and devices. A tremendous need exists for continuously monitored bedload data, as current understanding of bedload transport is incomplete. Increased understanding of bedload transport will improve our predictive capabilities of sediment transport.
U.S. Geological Survey, Department of the Interior
The following documents were not reviewed. Statements made in these documents are those of the authors. The findings have not been verified.
Development of a Continuous Bed Load Transport Technique (final, PDF,
By Robert Hilldale
Report completed on June 10, 2011