HR: 1340h
AN: H53C-1256    [Abstracts]
TI: River Bank Erosion at the Intra-Event Timescale: Implications for Bank Sediment Delivery
AU: * Darby, S E
EM: S.E.Darby@soton.ac.uk
AF: School of Geography, University of Southampton, Southampton, SO17 1BJ United Kingdom
AU: Rinaldi, M
EM: mrinaldi@dicea.unifi.it
AF: Department of Civil Engineering, University of Florence, Florence, 50139 Italy
AB: Bank erosion is often a significant contributor to catchment sediment yield. However, it has proved difficult to gain insight into the relationship between erosion events that supply bank material to river systems, and associated variations in fluvial sediment flux. One of the difficulties has been that there is a mismatch in our capability to quantify processes at relevant timescales. Thus, while fluvial flux can readily be monitored quasi-continuously during competent flow events, studies of bank erosion processes have largely been focused at the event timescale. A lack of sub-event scale data is understandable given the difficulties involved in accessing river banks during competent flows. Nevertheless, bank sediment delivery processes involve a combination of (quasi-continuous) fluvial erosion and (quasi-discrete) mass-wasting processes. Moreover, relatively little research has been focused on the dynamic interactions between these two groups of processes, especially in terms of quantifying the extent to which different bank erosion processes might dominate in different environments. To address these issues, we have been investigating bank sediment delivery processes at two study sites. The Fiume Cecina (central Italy) is an actively migrating (c. 10 m/yr) gravel-bed river, which constrasts with the River Asker (southern UK), a gravel-bed river with relatively slow (c. 0.1 m/yr) retreat rates. At each site we have developed simulation models to predict modes of bank sediment delivery during observed flow events. Our approach follows previous investigations in that event hydrographs are initially discretised into a series of time steps. Finite-element seepage analysis and slope-stability modelling software packages are then used to simulate bank pore water pressure and mass-failure conditions at each time step, to quantify their evolution throughout the flow event. Where our approach differs from previous studies is that we have used Computational Fluid Dynamics (CFD) simulations to estimate the boundary shear stress distribution exerted on the banks in each time step. Consequently, the bank profile is updated in response to any deformation caused by fluvial erosion. As a result, our simulations are able to highlight two contrasting roles that fluvial erosion has in triggering bank failures during flood hydrographs. First, fluvial erosion destabilises the bank with respect to mass failure by steepening the bank profile. However, this bank deformation also results in a modified distribution of bank pore-water pressures relative to cases where there is no fluvial erosion. A possible implication of our results is that conceptual models of bank sediment delivery processes founded on event-scale analyses may be misleading. Previous modelling studies have emphasised mass-failures as quasi-catastrophic events timed to occur on the falling limb of event hydrographs. In contrast, our simulations suggest that mass-wasting might also occur as a series of much smaller-scale episodes, timed at frequent intervals throughout the event hydrograph. It follows that the residence time of bank material debris delivered to the bank-toe may be much shorter than suggested by approaches that do not account for bank deformation processes at the intra-event time scale. Further research is required to investigate this hypothesis.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting