CHARACTERIZATION AND SIMULATION OF CHANNEL ADJUSTMENTS IN RESERVOIR DEPOSITS FOLLOWING DAM REMOVAL
Watershed Physical Processes Research Unit
2011 Annual Report
1a.Objectives (from AD-416)
The two primary objectives of this study are to: (1) characterize the erosion resistance to hydraulic and geotechnical forces of deposits in two reservoirs slated for dam removal (Klamath and Matilija), and (2) determine (predict) magnitudes and styles of channel adjustment through these deposits following removal of the dams. The latter objective will be accomplished by coupling the 2-dimensional flow and sediment transport model SRH-2D developed by the Bureau of Reclamation with National Sedimentation Laboratory (NSL’s) Bank-Stability and Toe-Erosion Model (BSTEM). A subsidiary objective is to determine stable bank geometries for these deposits which can be used by the Bureau for channel design.
1b.Approach (from AD-416)
Sediment cores, spatially distributed throughout the reservoir will be obtained by the Bureau. NSL personnel will accompany Bureau of Reclamation staff during sampling and use a series of instruments for in-situ testing of the hydraulic and geotechnical resistance of the materials to erosion. Data on the critical shear strength and unconfined compressive strength of the deposits will be measured at several depths along the core under saturated conditions. Partial cores will be returned to NSL for further testing under drained conditions and for direct shear tests. Bulk samples will also be returned to NSL and remolded for erosion-resistance testing in soil boxes under various pore-water pressure and applied stress conditions. Data obtained from the in-situ and laboratory testing will be used to populate the Bank-Stability and Toe-Erosion Model (BSTEM) to predict stable-bank geometries under saturated and drained conditions.
Working in concert with scientists from the Bureau of Reclamation and the University of Mississippi, computer codes from the Bureau’s 2-dimensional mobile bed model (SRH-2D) and NSL’s Bank-Stability and Toe-Erosion Model (BSTEM) will be coupled. The resulting model will provide a comprehensive tool to simulate channel-adjustment processes including, degradation, aggradation, bar development, channel widening and lateral migration. Model validation will be accomplished using the detailed data set available from the Goodwin Creek, MS, research bendway. Application of the model to channel adjustment in the reservoir deposits will be conducted by the Bureau.
This research includes two separate tasks: (1) determining erodibility characteristics of cohesive, reservoir deposits, and (2) converting the National Sedimentation Laboratory's Bank-stability and Toe-Erosion Model (BSTEM) code to Fortran for integration into the Bureau’s 2-dimensional flow and sediment transport model SRH-2D. Bottom sediments from three reservoirs along the Klamath River System were sampled by clam-bucket dredge and shipped to the National Sedimentation Laboratory (NSL) for erodibility testing. Measurements of critical shear stress and erodibility coefficient were conducted on remolded samples. Torvane shear strength, penetrometer and soil moisture tests were also conducted on each sample. A significant regression relation was developed between critical shear stress and the erodibility coefficient that can be used to calculate the erodibility coefficient from critical shear stress. Reservoir-average values of critical shear stress under moist conditions that retain no less than 67% moisture were equivalent to the stress required to entrain sand-sized particles. Upon drying to moisture contents less than 67%, reservoir-average hydraulic shearing resistance increased dramatically by a factor of 10 to 50. Critical shear stresses of this magnitude are equivalent to those for gravels and cobbles. Reservoir-average shear-strength values for moist conditions ranged from 5% to 15% less than dried conditions. The hydraulic and geotechnical data sets were successfully combined to develop relations between total shear strength and both critical shear stress and the erodibility coefficient. This signifies that these two hydraulic-erosion parameters can be estimated with the Torvane device. This potentially has a great advantage as the Torvane device can be deployed easily and rapidly.