Development of an empirical model for calculating sediment-transport capacity in shallow overland flows: Model calibration

Abstract

Previous research has demonstrated the poor performance of fluvial sediment transport equations when applied to overland flow. An empirical sediment-transport capacity model, specifically designed for overland flow, has been developed and calibrated. The model separates the portion of the transport capacity due to flowing water from that due to rainfall impact. Model development has been described by Guy et al. (2009a). This paper outlines model calibration results and model interpretation. Tests of the model are presented in Guy et al. (2009b). Model calibration data were obtained in a 1.50 m long by 0.253 m wide flume, with rainfall supplied by a single-nozzle rainfall simulator, and flow provided by a constant head tank. Sediment was injected into steady flow over an impermeable, roughened bed, at a rate that balanced the outflow. Transport capacity and hydraulic measurements were obtained at five gradients between 1 and 12%, with four test materials covering a range of size and density, four simulated rainfall intensities, and a range of base flow rates. Hydraulic measurements obtained with flows with Reynolds numbers in the laminar flow regime indicated that the depth of zero velocity was raised into the moving sediment layer, such that it exceeded the predictions of the velocity profile model. Surface velocities matched model predictions, such that flow resistance did not need explicit consideration in developing a transport model. The transport component due to flow processes depends on discharge, slope, and material properties, and is similar to the Schoklitsch equation. The significance of bed slope and discharge is between that required by stream power per unit bed area and shear stress. The component due to rainfall impact depends on rainfall intensity, discharge, slope, and relative particle density. Particle size was statistically insignificant but this may be due to the relatively small size range examined. The portion of the transport capacity due to rainfall impact varied from 100% at the lower limit of the study data (at flows less than the fluvial transport thresholds) to approximately 50% at the upper limit. The results suggested that sediment transport can be classified into three types: in shallow flow sediment transport (investigated in this study), the rainfall-impact component of transport capacity increases with discharge, although its relative significance decreases. In intermediate flow sediment transport, the rainfall-impact component decreases to insignificance. In deep flow sediment transport, rainfall does not contribute to the transport capacity.