SETRAC – A sediment routing model for steep torrent channels

Abstract

AsedimentroutingmodelforsteeptorrentchannelnetworkscalledSETRAChasbeendeveloped. SETRAC is the acronym for sediment transport in Alpine catchments. In SETRAC the water runoff is given as inputhydrographsatdiscretepoints.Akinematicflowroutingofthefloodhydrographisperformedinthechannel network. Different sediment transport formulae and flow resistance approaches can be selected in SETRAC.The sediment is transferred through the channel network considering a reachwise sediment budget. Initial sediment stock can be assigned for each channel reach. SETRAC is applied to a well documented case study on a flood event with substantial sediment transport which occurred in a torrent catchment in the Austrian Alps. Using a rough correction procedure to account for form roughness losses which reduce the energy slope available for bedload transport, a reasonable agreement has been obtained between simulated and observed sediment loads. Considering the uncertainty of the input data, the results are encouraging. In contrast to studies on lowland gravel-bed rivers, relatively few have been made on sediment transport in steep headwater channels, with stream gradients steeper than about 0.05. Sediment transport dynamics in these channels may be quite different from low- gradient channels. There is often a strong interaction between hillslope processes and the channel network. Sediment transport may be supply limited rather than controlled by the sediment transport capacity under given discharge and channel conditions. Steep head- water streams are characterized by a wide range of sediment sizes and temporally- and spatially-variable sedimentsources.Bedmorphologyandchannelstruc- tures may be influenced by the presence of large boulders, woody debris and bedrock constrictions. This can result in large variations in channel geom- etry, streamflow velocity and roughness, and thus the applicationoftheoreticalsedimenttransportequations may be problematic (Gomi & Sidle 2003). Quantita- tive measurements of sediment and bedload transport in steep streams are very limited (Rickenmann 2001). Many bedload transport formulas for gravel-bed rivers were developed or calibrated based on exper- iments in laboratory flumes. Measured transport rates in natural streams may differ substantially from values predictedwithsuchformulas(e.g.Bathurstetal.1987, Gomez & Church 1989). Also, measured transport ratesingravel-bedriversandboulder-bedstreamsmay vary by several orders of magnitude at similar (mean)