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

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 frequent interruption by lakes. Very few is known about the influence of woody debris in these streams, since nearly all previous studies 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 position and the size of large woody debris was assessed by close-up photography. An acoustic Doppler velocimeter was used to record the patterns of flow velocity and turbulence. Overlay and analysis of the spatial data was done using a Geographic Information System. The standing stock of wood was 1.9 m3 and 39 woody elements per 100 m2 of stream bed. The flow pattern was clearly controlled by the wood. Woody elements elevated above the stream bed deflected flow and locally caused strong secondary current, high turbulence, and scour of the stream bed at baseflow. Wood resting 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-flow by the flow which was determined by the woody debris distribution.