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Influences of woody debris on flow patterns and channel morphology in a low energy, sand-bed stream reach

by Michael Mutz
International Review of Hydrobiology ()

Abstract

In lowland areas, such as the glacial landscapes of eastern Germany, sand-bed streams are the most common stream type. They have low gradients and their hydrological regime is often subdued due to the frequ ent interruption by lakes. Very few is known about the influence of woody debri s in these streams, since nearly all previous studi es are from high-gradient conditions, where streams have coarse bed sediments and harsh hydrological regimes. The research objectives of this study were first to assess the quasi-natural quantity and quality of wood in a lowland sand-bed stream and second to understand the influence of wood on the channel morphology and the flow patterns at base-flow. The three-dimensional stream bed relief was surveyed by electronic distance measurement. The posi- tion and the size of large woody debri s was assessed by close-up photography. An acousti c Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographi c Info rmation System. The standing stock of wood was 1.9 m3 and 39 woody elements per 100m2 of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed defl ected flow and locally caused strong secondary curre nt, high turbulence, and scour of the stream bed at base- flow. Wood rest.ing directly on the stream bed, which contributed the majority of the wood inside the bank-full channel, determined the roughness of the stream bed. Near-bed flow patterns observed were isolated roughness flow and wake interference flow, which was registered inside the accumulations of wood. 68% of the stream bed had shear stress above critical. Hence, the secondary morphological structures of the sand-bed were controlled at base-tlow by the flow which was determined by the woody debris distribution.

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