Numerical Modelling of Bedload Transport at steep Slopes

Many sediment transport models for rivers are available, but most are limited to low gradient rivers. For torrents and mountain streams with steep slopes only a very limited number of simulation models exist. Most simulation models developed for rivers can’t be applied to steep slopes. Sediment transport capacity formulas derived from experimental flumes without bed forms are taken as reference condition. Observed sediment transport in steep and small streams can be up to three orders of magnitude smaller than values predicted by these formulas. Apart from limited sediment supply, this discrepancy may be partly due to substantial bedform roughness reducing bedload transport efficiencies. Torrents and mountain streams with steep slopes typically have an irregular bed topography and low relative flow depths. The roughness of irregular bed profiles cannot be described sufficiently by a percentile of the grain size distribution. Many authors reported about the effect of step-pool sequences contributing to flow resistance  Using the total channel gradient may result in an over prediction of the streams’ transport capacity. For that reason form roughness losses need to be considered when a sediment transport model is applied at steep slopes. Form roughness can be considered in the SETRAC model.