HR: 08:20h
AN: H41I-02    [Abstracts]
TI: The Role of Riparian Roots in Resisting Hydraulic Scour at Bank Toes
AU: * Pollen, N
EM: tashapollen@hotmail.om
AF: USDA-ARS National Sedimentation Laboratory, PO Box 1157, Oxford, MS 38655 United States
AU: Simon, A
EM: asimon@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, PO Box 1157, Oxford, MS 38655 United States
AB: Riparian vegetation interacts in a number of ways with the geotechnical and hydraulic processes operating in streambanks. Previous research has shown that roots provide mechanical reinforcement to soil through the production of root-soil matrix, in which the roots resist tensile forces and strengthen banks against mass failure. In addition, plants have been shown to reduce pore-water pressures, and thus increase the stability of streambanks through removal of water by evapotranspiration. However, to date, very little has been documented about the effect of roots on hydraulic scour at bank toes, which when eroded, greatly increase bank instability. In this study a vertical jet-test device was used to measure rates and volumes of scour in soils permeated by switchgrass roots. Twenty tests were carried out over a range of root densities. Results show that the volume of soil scoured during a test declined non-linearly with increasing root volume, per unit volume of soil (r² = 0.84, p < 0.05). The r² for this relationship is fairly high despite the fact that the location of root distributions relative to the shear stress field induced by the jet varied between tests, even under similar root volumes. Root volume per unit volume of soil for the jet tests carried out ranged from 0.0132 to 0.172 cm³roots/cm³soil with a corresponding reduction in the erodibility coefficient (k) from 0.01061 to 0.00011 cm³/N-s for the soil-root composite in the neighborhood of the testing point. Because vertical scour was restricted by roots in some tests, the method for calculating k was modified to use scour volume instead of scour depth. Erodibility coefficients associated with the minimum and maximum rooting volumes were used in toe erosion model runs for silt material. At a shear stress of 2.6 Pa, toe erosion of 0.00007 m² was recorded for the maximum root volume compared to 0.01325 m² for the minimum root volume. In the model runs, as available shear stress increased, the ability of the roots to reduce scour was also reduced, but a doubling of shear stress to 5.5 Pa, still resulted in a 38% reduction in scoured toe material between runs with the minimum and maximum root volumes. Further field and flume experiments would be useful to investigate further the effects of roots on patterns of scour under different root distributions and flow conditions. Such data would be useful in determining suitable planting densities for different vegetation types and species at bank toes.
DE: 1815 Erosion
DE: 1821 Floods
DE: 1824 Geomorphology: general (1625)
DE: 1848 Monitoring networks
DE: 1860 Streamflow
SC: Hydrology [H]
MN: Fall Meeting 2005