CHARACTERIZATION AND SIMULATION OF CHANNEL ADJUSTMENTS IN RESERVOIR DEPOSITS FOLLOWING DAM REMOVAL
2010 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.
Bottom sediments from three reservoirs along the Klamath River System were sampled by clam-bucket dredge and shipped to National Sedimentation Laboratory (NSL). Testing of reservoir sediments is being conducted in the laboratory to provide quantitative information on the parameters that control hydraulic and geotechnical processes of channel adjustment. Tests are being conducted over a range of moisture contents and densities. The rate of incision/erosion into the deposit by hydraulic forces will be a function of the critical shear stress and erodibility coefficient (k) of the materials. Measurements of critical shear stress are being obtained on remolded samples, pressed initially to a density of 12 kN/m3 using both a Cohesive Strength Meter (CSM) and a mini jet-test device. Torvane shear strength, penetrometer and soil moisture tests are also being conducted on each sample. This latter test will also provide values of the erodibility coefficient (k). Streambank erosion occurs by a combination of hydraulic processes acting along a bank face and geotechnical processes (mass failure) operating on the upper part of the bank mass. To predict potential rates and magnitudes of bank erosion and channel widening using numerical techniques, hydraulic resistance of the surface materials will be defined using the data described above for streambed erosion. Data on geotechnical resistance (shear strength) of the banks that will form via incision into the deposit will be provided by the Bureau. A matrix of bank heights and angles will then be simulated using the Bank-Stability and Toe-Erosion Model (BSTEM) to determine critical conditions for bank failure. At the time of this report, sample testing is about 75% complete. Project monitoring is conducted by conference calls and by meetings at the Bureau’s offices at the Denver Federal Center in Lakewood, CO.