HR: 1330h
AN: H52A-1174    [PDF]
TI: A new Approach for Quantifying Root-Reinforcement of Streambanks: the RipRoot Model
AU: * Pollen, N L
EM: npollen@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, PO Box 1157, Oxford, MS 38655 United States
AU: * Pollen, N L
EM: npollen@ars.usda.gov
AF: Environmental Monitoring and Modelling Group, King's College London Strand, London, WC2R 2LS United Kingdom
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 plays an important role in controlling geotechnical and fluvial processes acting along and within streambanks through the binding effects of roots. Quantification of this mechanical effect is therefore essential to accurately model streambank stability. Until now, most attempts to include the effects of root reinforcement by riparian vegetation have used root-cohesion values estimated using the Wu et al. (1979) equation, requiring the tensile strengths and diameters of the roots crossing the potential shear-plane. However, the Wu et al. equation is a static model that assumes that all roots break, and that they all break simultaneously. Field observations and laboratory experiments have shown that in reality the roots do not all break simultaneously, and that the breaking of roots during mass failure is in fact a dynamic process. Static models such as the Wu et al. equation are therefore likely to produce overestimations of cohesion due to roots. As a response to this concern, a dynamic root reinforcement model (RipRoot) was developed, based on the concepts of fiber bundle models (FBM's) used in materials science. Within the model the root-soil system is loaded incrementally resulting in progressive root breaking and redistribution of stresses from the broken roots to the remaining intact roots in the soil matrix. The redistribution and loading process continues until either all of the roots have broken, or equilibrium is reached where the root network supports the driving force imposed on the bank. The increase in bank cohesion using the static Wu et al. equation are 18% to 38% higher than RipRoot for riparian tree species, including Black Willow, Sandbar Willow, Cottonwood, River Birch and Eastern Sycamore, and 49% higher for Switch Grass. These variations in cohesion values can have a significant impact on streambank Factor of Safety (Fs) values calculated using the Simon et al. (2000) bank-stability model. For example, a 3m high silt streambank had a Fs of 0.98 without vegetation, indicating instability. With the addition of cohesion from 200 River Birch roots this value increased to 1.22 using the RipRoot value (Conditionally Stable) and 1.37 using the Wu et al. equation (Stable). In this example both of the root models produced cohesion values that were large enough to make the bank more stable, but the more conservative value from RipRoot suggests the bank may only be conditionally stable. Results to date indicate that the dynamic nature of RipRoot removes some of the overestimation from the static equation of Wu et al. (1979), therefore producing more realistic values for root reinforcement, which are particularly useful and important in the context of river management and restoration.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1851 Plant ecology
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2003 Fall Meeting