Rill network development and sediment budgets

Abstract

Threshold conditions for rill incision are well known, but few studies have examined the effect of network geometry on water and sediment fluxes within an evolving rill system. This paper reports the first in a series of studies designed to identify the influence of soil properties on rill network and confluence geometry, and on water and sediment fluxes in rill systems. The object of this study was to prepare detailed rill network water and sediment budgets identifying important sources and sinks. Simulated rainfall experiments were carried out on a Canadian silt loam soil in a 7·1 m × 2·4 m flume on a 5° slope. Rill networks of varying complexity developed, which were ultimately constrained by flume boundaries. Sediment and water fluxes and hydraulic conditions were measured within networks and at a terminal weir. Networks evolved by initial knickpoint incision in the lower flume, as flow shear velocities reached critical levels of 4·5–5 cm s−1 followed by headward migration and tributary development. Microtopography determined tributary location, but the timing and intensity of development were controlled by the incision and migration rate of the main channel, which changed local thalwegs and base levels, raising shear velocities on side slopes above critical levels. Sediment discharge at the weir broadly reflected rill incision intensity and transport-limited conditions, but as active incision moved headward, the linkage became attenuated and identification of discrete erosional incidents (e.g. local bank collapse) in the signal of the weir record became very difficult. Detailed water and sediment budgets showed much more complex patterns of localized incision and deposition within networks, strongly influenced by local changes in thalweg and in water discharge due to seepage or return flow, and by the effect of confluences on hydraulic conditions. Results indicate the value of detailed sediment budgets in interpreting weir water and sediment flux records, and the necessity of linking such measurements for erosion plots and hillslope segments to rill network characterization. Experimental results are consistent with a simple model of rill system evolution based on channel incision, headward migration, and the critical shear velocities for rill initiation. Copyright © 2000 John Wiley & Sons, Ltd.