Flow, Sediment Transport, and Erosion in Steep Mountain Channels: an Alpine Symphony

Abstract:

Steep headwater channels comprise a large fraction of the total channel length in mountainous regions. They control the transport of water, solutes, and sediments to larger rivers downstream, and regulate the erosional development of many mountain landscapes. Flow and transport processes in these steep channels contrast with those in their lower-gradient counterparts. Steep streams have complex bed morphology, with large roughness relative to the flow depth, and the flow is likewise complex, often comprising a variable mixture of air and water. Here we present several examples from Switzerland highlighting recent research into hydraulics, sediment transport, and bedrock erosion in steep Alpine channels. The Riedbach presents a striking natural experiment, in which the channel gradient steepens from roughly 3% to more than 40% in less than 1km, while the discharge, flow width, and sediment transport rates remain roughly constant. Measurements at the Riedbach illustrate the self-adjustment of flow velocity and bed roughness in steep mountain channels. Data from the Erlenbach, an intensively instrumented stream in the Alpine foothills, illustrate how the bed configuration of such streams regulates the relationship between flow and sediment transport. Field studies at the Gornera illustrate how flow patterns control the spatial distribution of bedrock erosion in this glacier-fed stream in the shadow of the Matterhorn. We present an overview of these studies and discuss their implications.