Polychaete Tubes, Turbulence, and Erosion of Fine-Grained Sediment

The role of polychaete tubes protruding through the benthic boundary layer in promoting or hindering erosion of fine-grained sediment was examined in laboratory experiments. Diver core samples of the top 10cm of sediment were collected west of Trinity Shoal off the Louisiana coast in 10-m depth. Diver cores were used in laboratory experiments conducted in a unidirectional flume. Tubes that were constructed by polychaetes, which comprised 70% of the species from the study area, were inserted into the core sediment surface. The sediment cores were then placed in the 2-m long test section of a small oscillatory flow tunnel and high-speed, stereo particle image velocimetry was used to determine the 2-dimensional, 3-component fluid velocity at high temporal (100 Hz) and spatial (< 1mm vector spacing) resolution. The tubes that protruded above the boundary layer allowed vortices to be initiated. Tubes are made up of shell fragments and fine-grained sediment, allowing for some rigidity and resistance to the flow. Rigidity determines the resistance causing small-scale eddies to form. The small-scale turbulence incited scour erosion, allowing fine-grained particles to be suspended into the water and in some cases coarser particles to be mobilized. Less-rigid tubes succumb to the shear stress, inhibit the formation of small-scale eddies, limit sediment erodibility, and increase the critical shear stress of the sediment. Discussion will focus on a modification to the critical Shields parameter to account for the effects of benthic biological activity.