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 d ynamics has been quite well studied, but the critical population density th at separates sediment destabilising effects from stabilising effects remain s uncertain. This article presents results obtained with artificial tubes i n a recirculating flume at a current velocity of 5 cm s(-1). Four populatio n densities were tested for their passive effects on the flow dynamics: exp ressed 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 senso r, horizontal and vertical flow velocity profiles were recorded within the artificial tube lawns. An important deceleration of the current velocity wa s observed at all population densities, ranging from 38.2% at the lowest po pulation density to 83.8% at the highest. This deceleration, the shape of t he vertical profiles, the calculated Reynolds stress values and the direct observation of sediment displacement led to the conclusion that the flow fi eld is modified to gradually raise the effective level of the bottom toward s the tube tips, resulting in skimming flow conditions at 8.8% surface cove rage. Compared with field conditions, this is still a relatively low popula tion density and thus means that many natural tube lawns have sediment stab ilising effects, conditioning the substratum for further benthic succession .