Skimming flow induced over a simulated polychaete tube lawn at low population densities

Polychaete tube lawns with high population densities are frequent in marine soft-bottom environments. The influence of single tubes on near-bed flow dynamics has been quite well studied, but the critical population density that separates sediment destabilising effects from stabilising effects remains uncertain. This article presents results obtained with artificial tubes in a recirculating flume at a current velocity of 5 cm s–1. Four population densities were tested for their passive effects on the flow dynamics: expressed as percentage of the total surface area covered by tubes, they were 1.1, 2.0, 4.5 and 8.8%. Using a high-resolution 3-dimensional current sensor, horizontal and vertical flow velocity profiles were recorded within the artificial tube lawns. An important deceleration of the current velocity was observed at all population densities, ranging from 38.2% at the lowest population density to 83.8% at the highest. This deceleration, the shape of the vertical profiles, the calculated Reynolds stress values and the direct observation of sediment displacement led to the conclusion that the flow field is modified to gradually raise the effective level of the bottom towards the tube tips, resulting in skimming flow conditions at 8.8% surface coverage. Compared with field conditions, this is still a relatively low population density and thus means that many natural tube lawns have sediment stabilising effects, conditioning the substratum for further benthic succession.