HR: 11:15h
AN: NB22G-04    [Abstracts]
TI: Use of a Fiber-Bundle Model to Quantify Root Reinforcement in Streambanks
AU: * Pollen, N L
EM: npollen@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O Box 1157, Oxford, MS 38655 United States
AU: Andrew, S
EM: asimon@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O Box 1157, Oxford, MS 38655 United States
AB: Streambank instability poses a number of economic and ecological problems for river managers and users. Riparian vegetation plays a number of roles in protecting stream banks from erosion by particle entrainment and mass wasting, with the impacts generally separated into those that are mechanical and those that are hydrological. This paper investigates ways of quantifying the mechanical reinforcement provided to the soil matrix of the banks by riparian plant-root networks. Established perpendicular models used to quantify root reinforcement sum the ultimate tensile strengths of the roots in the soil matrix and add these to the ultimate soil strength. However, in reality the roots do not all break at once, leading to overestimation by these root-reinforcement models. First, the assumptions made by established perpendicular root-reinforcement models were investigated, and the results compared to a newly constructed root-reinforcement model, RipRoot, which is based on the fiber-bindle models used in materials science. Results of the two root models were compared to direct-shear tests of root-permeated and non-root-permeated soil samples. Second the root reinforcement values obtained from the two root-reinforcement models were input to a streambank stability model in order to assess the impact of the differences between the root models on streambank factor of safety values. The results show that the simple perpendicular root-reinforcement model overestimated the actual root reinforcement provided to a streambank. The new fiber-bundle model, RipRoot, provided more accurate estimates of root reinforcement through its inclusion of progressive root-breaking during mass-failure of a streambank. In cases where bank driving forces were great to enough to break all of the roots, the perpendicular root-model overestimated root reinforcement by up to 50 %, with overestimation increasing an order of magnitude in model runs where streambank driving forces did not exceed root strength. The difference between the root reinforcement estimates from the perpendicular root model and RipRoot had a considerable impact on streambank factor of safety values. For the highest banks modelled (3 meters) the difference in factor of safety values between the runs with the two models varied from 0.13 to 2.39 depending on the riparian species considered.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1851 Plant ecology
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly