DENSITY-DEPENDENT SEDIMENT DYNAMICS NEAR A PATCH OF CYLINDERS

Marsh grasses and other flora in fluvial systems have historically been thought to exacerbate flooding and impede shipping navigation. However, native vegetation is now known for helping restore and maintain the health of streams and rivers by stabilizing the streambed and providing refuge for native fauna. Investigations on the effects of varied packing densities of live plants have been performed to clarify how vegetation influences flow dynamics. Little work has been done concerning whether varying the packing density of artificial plant stems has a significant effect on streambed and scour development.

The purpose of this project was to visualize and quantify sediment transport and the accompanying streambed changes in and around a patch (or cluster) of stiff plant mimics, specifically hollow plastic rods (i.e. tubes) in place of live plant stems. While not identical to live plant stems, this substitution is necessary to capture the effects of plant stem density on erosion patterns without the complexities of using live vegetation. This research tests the hypothesis that in sufficiently dense patches, the cluster of individual shoots will act together as one cylinder. In sufficiently sparse patches, the erosion pattern reflects several independent cylinders.

Based on the six trials completed, the preliminary results agreed with the hypothesis. Net erosion occurred in all three densities and two orientations of the circular base. As density increased, the patch of individual cylinders increasingly acted like a single large cylinder. However, the intermediate density (1106 stems m^-2) produced irregular depositional patterns downstream of the patch. When the base was rotated 22.5° and a sufficient flow blockage was achieved, the amount of sediment eroded was the same for all higher packing densities.

The pursuit of this limited research project has been relatively effective at isolating the effects of packing density on local sediment dynamics. To fully understand the applicability of this study’s results for natural plant patches, a helpful future project would consider how the scour patterns depended on properties such as rigidity specific to PEX tubes. Perhaps other factors (e.g. cross section geometry) play more significant roles in controlling changes to the sediment around a structure.