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Establishing reference condition for streambed mobility: Quantifying the effect of form roughness from stream habitat survey data
Citation:
KAUFMANN, P. R., J. FAUSTINI, D. P. LARSEN, AND M. A. SHIRAZI. Establishing reference condition for streambed mobility: Quantifying the effect of form roughness from stream habitat survey data. Presented at American Geophysical Union Meeting, San Francisco, CA, December 15 - 19, 2008.
Impact/Purpose:
The size and mobility of streambed particles are sensitive to changes in the balance between sediment supply and transport.
Description:
The size and mobility of streambed particles are sensitive to changes in the balance between sediment supply and transport. Therefore, changes in mobility can be an indicator of natural or anthropogenic alterations in this balance. Predictions of the critical diameter for mobility of stream bed particles (Dc) typically depend on estimates of bed shear stress that are based on stream gradient, depth, and the specific weight of water. In complex natural streams, depth at a given discharge increases with hydraulic resistance, but reach-average shear stress exerted on bed particles declines if depth increases are the result of form resistance associated with large roughness elements such as wood and bedforms. Published, practical approaches to quantify the reduction in this component of shear stress are lacking. We derived an adjustment of bankfull shear stress and Dc that multiplies the bankfull hydraulic radius (Rbf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance, (Cp/Ct)1/3. We computed Cp using a Keulegan equation relating resistance to relative submergence of bed particles and Ct from thalweg mean and residual depths and large wood volume. Measured total resistance was determined by dye-transit over a wide range of discharge, residual pools, and wood in natural stream channels. We tested our estimates of Ct using data from a survey of 104 wadeable streams in the Pacific Northwest, USA. Estimated bankfull Ct ranged from 0.002 to 1.68 (friction factors 0.016 to 13). Stream discharges calculated using Ct compared favorably with velocity-area measurements of discharge at low flow, and published data on 1 to 2-year recurrence floods. The median ratio of particle- to total resistance (Cp/Ct) was 6.7% at bankfull flows, showing that in half of these streams at least 93% of resistance resulted from large roughness elements including longitudinal bedforms, pools and wood. Bankfull Dc ranged very fine gravel to large boulders. The median effect of our form-roughness adjustments was a 40% reduction from the unadjusted critical diameter (inter-quartile range 28 to 59%) throughout the region, suggesting a substantial influence of large-scale roughness on particle mobility in wadeable streams at bankfull flow.