Patterns in gravel bedload transport from impact plates in a laboratory flume

Authors: Wren, Daniel; Kuhnle, Roger; HILDALE, ROBERT - Us Bureau Of Reclamation

Submitted to: Joint Federal Interagency Sedimentation and Hydrologic Modeling
Publication Type: Proceedings
Publication Acceptance Date: 12/10/2018
Publication Date: 6/24/2019
Citation: Wren, D.G., Kuhnle, R.A., Hildale, R.C. 2019. Patterns in gravel bedload transport from impact plates in a laboratory flume. Joint Federal Interagency Sedimentation and Hydrologic Modeling Conference, Reno, Nevada, June 24-28, 2019. 1-5 pp.

Interpretive Summary: The measurement of sediment transported on and near the bed of streams and rivers is an important need for river management, particularly when dams are removed, which creates a need for measuring the release of stored coarse sediments. The use of steel plates on the bottom of a stream, with sensors mounted on the plates, allows the recording of impacts from rocks impacting the plates. The focus of the work here is on using recorded signals from impact plates installed in a laboratory flume to study the effect of different prior conditions on gravel transport rate. The flume was first operated with a bed that was mechanically flattened. Later experiments began with progressively higher flow strengths before making measurements of gravel transported at a lower flow strength. It was found the gravel transport was highest for the case that began with a mechanically flattened bed, followed the case with the highest shear stress before the test case. These results will be useful to agencies, such as the U.S. Bureau of Reclamation, that need to predict when gravel will be moved by flows streams or rivers and how previous flow conditions affect the prediction.

Technical Abstract: Bedload remains difficult to measure, and finding patterns in bedload transport, such as those caused by the passage of a hydrograph or from a change in sediment supply, is particularly difficult since manual sampling methods are laborious and limit temporal resolution and total monitoring time. The use of accelerometer-equipped steel plates installed in the bottom of a channel allows the impact of particles on the plates to be continuously recorded. Using a calibration process, the data can be converted to particle size, and the sum of the particle masses can be used to quantify transport rate over a range of time scales. Impact plates in a 30 m laboratory flume were used to record gravel transport over a range of shear stresses for a gravel/sand mixture. The data were processed to reveal patterns in mass transport and gravel particle size resulting from different antecedent conditions for repeated experiments with the same flow conditions. Both initial transport rate and variability in transport rate were found to be higher when starting from a screeded bed and when following flows with larger excess shear stresses.