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

Start Date: Jul 01, 2008
End Date: Jun 30, 2010

Objective:
The overall objectives of the proposed study are to (1) determine critical shear stress and erodibility values for fine-grained materials comprising the channel boundary of selected tributaries of the Tualatin River, (2) use these results to estimate how potential changes in flow regime may impact the frequency and duration of sediment entrainment and bank stability, (3) test a new, smaller- version of the jet-test device being constructed by the USDA-ARS, and (4) determine the magnitude of root reinforcement provided by native riparian species in the basin.

Approach:
Reconnaissance of tributaries of the Tualatin River will be conducted to ascertain channel stability using Rapid Geomorphic Assessments (RGAs) and to select 50 sites for more intensive data-collection activities. An additional 250 sites will be selected for more limited erodibility testing. At each of the 50 intensive sites, a series of measurements will be conducted to provide the data to perform analyses of erosion potential and bank stability. For erosion potential, submerged jet tests will be performed to determine the critical shear stress and erodibility coefficient of bed and bank-toe materials. With these data, erosion rates under a given boundary shear stress and duration will be estimated. Resistance of streambanks to mass failure will be determined using a borehole shear-test device. Historical flow data will be downloaded from USGS Web sites and used to determine trends in mean-daily and peak flows over the period of record. Discharge data will be used to develop flow magnitude, frequency and duration relations using standardized techniques. Boundary shear stress will be obtained from the discharge data in combination with bed-slope information obtained during the RGAs. Flow and boundary shear stress data will be used in combination with critical shear-stress data to determine under which flows entrainment is predicted at each site. The frequency and duration relations for these discharges will provide an estimate of the percentage of time that erosion will occur. This analysis will be conducted for both existing conditions and for alternative flow regimes. The Bank-Stability and Toe-Erosion Model (BSTEM) will be used to evaluate critical conditions for bank stability. The frequency that critical hydrologic conditions occur will be evaluated from historical gage records and compared to the proposed alternative flow regimes to determine the potential effects of the proposed regimes on the volumes and frequency of mass failure. Tensile strength and root distributions will be determined in a trench or bank face at field sites for five native, riparian species. This will be accomplished by 1) determining the root diameter ¿ tensile strength relation; 2) quantifying the number of roots and their size distribution; and 3) quantifying the distribution of roots with depth. For each species at least three sets of field experiments, representing a range of ages will be conducted. A tentative list of native species has been developed from which five will be selected. Results of the field investigation will be used as inputs into the root-reinforcement model RipRoot to calculate the increase in shear strength due to roots. Simulations of bank stability conditions will be conducted using BSTEM with a range of measured bank-strength data from tests along tributaries of the Tualatin River. This will be accomplished by modeling different bank heights, bank angles and pore-water pressure conditions with and without the effects of vegetation. Comparisons between species and between non-vegetated and vegetated banks will be made. Species assemblages making up the greatest reinforcing effects will be tested and reported.