Project Number: 6408-13000-018-40
Project Type: Reimbursable

Start Date: Jul 01, 2008
End Date: Jun 30, 2009

Objective:
The overall objectives of the proposed study are to determine critical shear stress and erodibility values for fine-grained materials comprising the channel boundary of selected tributaries of the Tualatin River and to use these results to estimate how potential changes in flow regime may impact the frequency and duration of sediment entrainment and bank stability. A parallel objective is to test a new, smaller version of the jet-test device that is being constructed by the USDA-ARS.

Approach:
The research approach involves reconnaissance of tributary streams to determine the relative stability and to identify appropriate locations for testing of cohesive bed and banks. Once identified, jet-tests and borehole shear tests will be conducted in the sediment comprising the channel boundary to determine the resistance of the materials to hydraulic forces operating at the bed and bank toe, and geotechnical forces operating on the bank mass. Parallel tests will be conducted with the existing jet-test device and a newly designed device. Analysis of historical flow records in the basin in combinations with channel surveys will be used to calculate flow depths and shear stresses for the range of flows. In the absence of historical data one-dimensional step-backwater flow modeling will be conducted to develop a stage-discharge relation and other hydraulic parameters for the selected sites. These data will be used to compare critical shear stresses with those provided by the flows. This will also be conducted using proposed flow regimes to compare the frequency and duration of flows that exceed the criotical shear stresses of the material. The flow data will also be used in concert with the Bank-Stability and Toe-Erosion Model (BSTEM) to determine critical conditions for bank stability at the selected sites. developed by the USDA-ARS, National Sedimentation Laboratory. BSTEM has been developed and previously used successfully in the Mid South Area and other regions. The frequency that these conditions occur will be determined for existing (historical) flow conditions, as well as for the proposed flow regimes. Differences will be used to identify the net effects of the proposed flow regimes on bank stability. BSTEM will also be run iteratively over an annual hydrograph representing high-flow years under existing (historical) and proposed flow regimes. Volumes of erosion by hydraulic forces and bank failures will be summed for each storm event comprising the annual hydrograph for both existing and proposed conditions. Differences in eroded volumes will provide information on the effects of the proposed flow regime.