HR: 08:25h
AN: H31J-02    [Abstracts]
TI: Quantifying Reductions of Mass-Failure Frequency and Sediment Loadings from Streambanks using Toe Protection
AU: * Simon, A
EM: asimon@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655, United States
AU: Pollen, N
EM: npollen@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655, United States
AU: Mahacek, V
EM: valley _ mountainconsulting@yahoo.com
AF: Valley Mountain Consulting, 1034 Emerald Bay Road, South Lake Tahoe, CA 96150, United States
AU: Langendoen, E J
EM: elangendoen@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655, United States
AB: Streambank erosion represents an important form of channel adjustment and a significant source of sediment in disturbed streams, often contributing 60-80% of the suspended sediment load. Mass failures regularly occur by a combination of hydraulic processes that undercut bank toes and geotechnical processes causing bank collapse by gravity. Little quantitative information is available on the effectiveness of bank treatments on reducing erosion. To evaluate potential reduction in sediment loadings, the hydraulic and geotechnical processes responsible for mass failure were simulated under existing and mitigated conditions using a Bank-Stability and Toe-Erosion Model. Two critical erosion sites were selected from each of three watersheds that contribute the greatest amounts of fine sediment by streambank processes in the Lake Tahoe Basin: Upper Truckee River, Blackwood and Ward Creeks. Blackwood and Ward Creeks represent west-side, steep (0.008-0.03 m/m), coarse-bedded systems where 10-15 m-high terraces sporadically abut the channel. The Upper Truckee River represents flatter (0.002 m/m) sections that meander through grassed meadows. To provide for the driving, hydraulic forces, the 1995 annual hydrograph was selected as a typical high-flow year. The rain-on-snow event of January 1-2, 1997 (a 50- year event in some parts of the basin) was also added. Stage data from gauging stations were discretized into individual events to use with channel gradient to calculate boundary shear stress. Bank-material strength data were collected for each layer using a borehole shear-test device. Species-specific root-reinforcement values were applied based on root distributions using a fiber-bundle model. Hydraulic erosion was simulated using an excess shear-stress approach in the toe-erosion sub model. The new geometry was then exported into the bank-stability sub-model to test for stability of the bank under peak flow and drawdown conditions. In this way, BSTEM was used iteratively for all flow events under both existing conditions and with stone-toe protection. Volumes of material eroded by hydraulic and geotechnical processes were tracked for each event and summed to make comparisons between existing and mitigated conditions. Under existing conditions, total streambank erosion ranged from 472 m³ to 5260 m³ of which 35 m³ to 900 m³ were fine grained (silts and clays). On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures, which occurred about 5 times over the period. Iterative simulations with 1.0 m-high rock-toe protection showed a dramatic reduction in average loadings (87%; std. error = 4.2%). Failure frequency was reduced in most cases to a single episode, coinciding with recession of the January 1-2, 1997 rain-on-snow event. Thus, an almost 90% reduction in streambank loadings was realized by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. Thus, simulations show average load reductions of about an order of magnitude (2070m³ to 127 m³ for total erosion; 292m³ to 21.2 m³ for fines). Results stress the critical importance of protecting the bank toe-region from steepening by hydraulic forces that would otherwise entrain previously-failed and in situ bank materials, thereby allowing the upper bank to flatten (by failure) to a stable slope.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1860 Streamflow
DE: 1894 Instruments and techniques: modeling
SC: Hydrology [H]
MN: 2007 Fall Meeting