Empirical model for calculating sediment-transport capacity in shallow overland flows: Model development

A reliable sediment-transport capacity equation for overland flow has not previously been developed, and previous studies have indicated that the models that are frequently applied to overland flow perform poorly. In this study an empirical model was developed. Four groups of factors that control transport in overland flow were identified: properties of the flow, the impacting rainfall, the boundary geometry, and the soil. However, lack of detailed knowledge of hydraulic and transport processes in shallow flow led to an empirical modelling approach. Modelling variables representing these four groups of factors were determined. The form of the chosen model was a two-component additive model, which separates the portion of the transport capacity due to flowing water from the portion due to rainfall-impact. The model allows transport in the absence of direct rainfall, allows transport due to rainfall-impact when discharges are lower than the thresholds for particle motion, and allows both flow and rainfall-impact to contribute to transport during rainfall.