The Ecology and Management of Wood in World Rivers

Hydraulic Effects of Wood in Streams and Rivers

Michael Mutz


Abstract.—Although submerged wood obviously influences the flow, little information exists on its various hydraulic effects in streams and rivers. This chapter gives a brief overview of the current knowledge about hydraulic effects of circular cylinders and simple tree shaped models and summarizes the few field data on wood induced hydraulics in streams and rivers. The focus is on the flow pattern and other effects of importance for instream ecology. The principal cross-flow field of a singular log perpendicular to flow is determined by the Reynolds number related to the log’s diameter. For the range of Reynolds numbers of logs and branches in streams and rivers (1 ‧ 102 to 1 ‧ 106), the cross flow pattern is symmetrical, vortex streets shed, and a wake with reduced mean velocity develops behind the log. In the vertical confined flow of streams and rivers, the hydraulic effects depend on the blockage caused by the log, its distances to the water surface, and its distance to the streambed. The blockage determines the resistance to flow, the upstream afflux, the local flow acceleration, and the intensity of flow deflection. For logs within distances of 2 diameters to the water surface, the relative submergence and the Froude Number determine the highly variable local cross-flow field. For logs near the streambed, the form and roughness of the bed and the size of the gap to the bed control the hydraulics. Submerged jet-like flows, which cause local scour, are reported, but detailed information on the hydraulics of logs close to a natural streambed is missing. For logs in close contact to or partly embedded into the bed, the principal flow pattern of recirculating vortices attached to the bed develop in front and behind the logs. The extent of these vortices and the extent of the wake behind the logs appear to be larger in sand-bed streams than in flumes with smooth and level beds. Complex dense wooden objects and wood accumulations are comparable to solid structures. Their flow field is determined by the size of the bluff surfaces and the shedding from edges obtuse to flow. Wood spread out at the streambed causes skin roughness, and models based on technical roughness approximate the resulting near-bed flow regime. The general validity of most findings in streams and rivers is still vague since they are supported by only few data. Further flow data from the field and from flume experiments that simulate the complexity of the natural environment are needed.