Modeling the effect of wood accumulation patches on flow and morphology

Numerous rivers have been confined and are eco-morphologically impaired, resulting in an increased demand for river restoration projects. Wood placements are a common and inexpensive measure. To plan and evaluate river restoration projects including wood placements, it is important to understand the interactions between flow, wood, and sediment. Flume experiments were performed to quantify flow and morphologic structures associated with wood accumulation patches, herein defined as an accumulation of less than 20 logs. The objective is to provide data and describe wood accumulation patch characteristics and respective flow structures that can be linked to desirable river conditions (e.g. fish habitat preferences). The first flume experiments were conducted with a single log placed on a movable bed with a uniform grain size diameter. This setup was also extended to multiple logs. During the experiments, the approach flow conditions (flow velocity and flow Froude number) and wood characteristics (log dimensions, number of logs, and log orientation to the flow) were varied. We hypothesize that hydraulic flow variability, as reflected in flow depth and velocity, increases with increasing patch dimensions and patch compactness. We expect that a wood accumulation patch will affect flow variability over 2-3 times its length. The relationship between flow variability and patch dimension is yet unknown. Therefore, we would like to identify: (1) how many logs are required to create backwater suitable for fish shelter, (2) what patch density increases deposition in the patch wake, and (3) what patch setup creates the most heterogeneous flow field up- and downstream of the patch.