Experimental study of rill bank collapse

Abstract

Rill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre-existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the 'interrill' zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents > > > > >35 per cent (sandy loam) and > > > > >23 per cent (loamy sand), critical bank heights were 0·11-0·12 m and 0·06-0·07 m, respectively. Bank toe undercutting at the outside of the rill bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the rill bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development.slopes beyond the angle of repose (e.g. models for non-cohesive river banks by Wiele and Paola, 1989;Kovacs, 1992; and Kovacs and Parker, 1994). However, river banks usually consist of cohesive stratified silts and clays, with complex localized variations in mechanical composition, strength and hydraulic conductivity. Most studies have focused on such systems, recognizing realistic riverbank geometries and the influence of spatially and temporally variable soil water conditions on soil strength (e.g. Thorne et al., 1981;Osman and Thorne, 1988;Simon, 1989;Simon et al., 1991). Hagerty (1991), in particular, demonstrated the importance of soil water conditions in generating positive pore pressures and bank...

Meandering channel dynamics in highly cohesive sediment on an intertidal mud flat in the Westerschelde estuary, the Netherlands

“…In our case, banks are also undercut by erosive bend flow and by the passage of backward steps in a similar manner as in rills described by Stefanovic and Bryan (2007). Afterward, the failed sediment mass deposits on the channel bed and locally reduces the shear stress and the immediate probability of renewed bank erosion, until this sediment is removed by the flow.…”

“…Purely erosive, backward migrating steps under hydraulic jumps have been observed in various settings, including rills (e.g., Stefanovic and Bryan, 2007), streams with cohesive beds (Simon and Thomas, 2002) and as headcuts on mountain slopes in general (e.g., Parker and Izumi, 2000). They form in nonuniform subcritical flow with a downstream increasing Froude number, such as in the extreme case of a waterfall.…”

Morphodynamics of Gully Development on the Platform–Slope System of Spoil Dumps under Platform Concentrated Flow

“…In the A-layer expansion stage, sidewall expansion dominated the erosion. The flow concentrated in the B layer and eroded the sidewall toe, which accelerated gully expansion [37]. In the B-layer incision stage, gully erosion was again dominated by bed incision, which was mainly due to the step-pools developed on the gully bed [38][39][40].…”

9.8 Bedload Kinematics and Fluxes

“…The character of this supply reflects both short-and long-term patterns of sediment delivery and storage mechanisms that create and rework alluvial deposits within the river valley (e.g., Chaplot and Le Bissonais, 2000;Church, 1983;Macklin and Lewin, 1989;Stefanovic and Bryan, 2007). The character of this supply reflects both short-and long-term patterns of sediment delivery and storage mechanisms that create and rework alluvial deposits within the river valley (e.g., Chaplot and Le Bissonais, 2000;Church, 1983;Macklin and Lewin, 1989;Stefanovic and Bryan, 2007).…”