HR: 0800h
AN: H51E-0798    [Abstracts]
TI: Simulating the Fluvial Erosion of Fine-Grained River Banks
AU: * Darby, S E
EM: S.E.Darby@soton.ac.uk
AF: School of Geography, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom
AU: Sarkkula, J
EM: juha@mrcmekong.org
AF: Finnish Environment Institute, P.O.Box 140, Helsinki, FIN-00251, Finland
AU: Koponen, J
EM: jorma.koponen@eia.fi
AF: Environmental Impact Assessment Center of Finland Ltd, Tekniikantie 21b, Helsinki, FIN- 02150, Finland
AU: Kummu, M
EM: matti.kummu@iki.fi
AF: Water Resources Laboratory, Helsinki University of Technology (TKK), P.O. Box 5200, Hut, FIN-02015, Finland
AB: River bank erosion is the product of a suite of specific processes that together contribute significantly to the sediment yielded from river catchments. Many studies have emphasised that hydraulic erosion of bank-toe materials may exert a dominant influence on the long term rate of river bank retreat. Fluvial bank erosion rates are normally quantified using an excess shear stress model of the form E = k(τ_b-τ_c)^a, where E is the erosion rate per unit time and unit bank area, τ_b is the boundary shear stress applied by the flow, k and τ_c are erodibility parameters (erodibility coefficient, k, and critical shear stress, τ_c), and a is an empirically derived exponent (equated to unity in bank erosion studies). This model has the advantage of simplicity, but in practice difficulties in estimating the values of the erodibility and shear stress parameters seriously inhibit its accuracy. We are seeking to improve the parameterization of the excess shear stress model through the use of field measurements and analytical modelling, at field sites on the Mekong River in Laos. Specifically, τ_b is estimated using a new model [Kean and Smith, 2006, J. Geophys. Res., 111(4), F04009, doi:10.1029/2006JF000467] of flow over irregular bank topography. Data from our study sites indicate that the form roughness induced by natural topographic bank features (slumps, embayments, etc) is a major component of the spatially-averaged total shear stress, with the skin friction component (i.e, τ_b) typically an order of magnitude less than the total stress. This indicates that previous bank erosion investigations, that employ estimates of the total shear stress, may grossly misparameterize the true value of τ_b. To estimate τ_c, we have employed a Cohesive Strength Meter [CSM, Tolhurst et al., 1999, Estuarine, Coastal & Shelf Sci., 49, 281-294], a jet-testing device that is normally used in studies of the stability of cohesive sediments on inter-tidal flats, but which has not previously been employed in the context of river bank studies. Our data show that values of τ_c are typically of the order of 1 Pa, again indicating that previous studies may have over-estimated the true critical stress. Moreover, compared to conventional jet-testing devices, the portability and small size of the CSM's sampling chamber, together with the speed of individual tests, allows the collection of large numbers of replicate samples within discrete sedimentary horizons, such that the inherent natural variability of each bank material horizon can be defined. This has allowed us to modify the excess shear stress erosion model to include a probabilistic component associated with the measured statistical distribution of τ_c for a specific bank-toe material. We have used our data to estimate bank erosion rates for a range of flow discharges, with annual erosion rates subsequently determined by integration across the predictable annual (monsoonal) flow regime. To assess the predictive ability of our model we have compared annual rates of bank-toe retreat (assumed to be a good proxy of bank retreat rates) with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery.
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1860 Streamflow
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: 2007 Fall Meeting