Consolidation of dredged sludge, measured by an acoustic densitometer

Abstract

A field study was conducted on the Cromstrijen dump site in the Rhine-Meuse estuary to find the risks of open-water dumping by estimating the impact effects and spreading of freshly deposited sludge and the consolidation of dumped material. The site is used to dump sludge of acceptable quality from maintenance dredging of waterways and local harbours. The investigated sludge came from a navigation channel and was dredged and transported by a hopper dredge. On the dump site, a small section was marked in which the dump took place. A field version of an existing laboratory acoustic densitometer was used to take a profile of the dry-weight concentration of the sediment bed as measured by the attenuation of high-frequency sound waves. The instrument was calibrated with the aid of homogenized field samples. The acoustic densitometer was useful for obtaining a profile of the bed density. With an echo-sounder, an attempt was made to evaluate the consolidation of the dumped sludge. The use of side-scan sonar at the dump site showed a crater with steep banks, surrounded by a thin layer of excess sediment extending over 8400 m². This was only 20% of the dumped sludge; the remaining material was transported outside the measurement section as a density current owing to near-bed surge and local bed slope. The results of the field study are compared with those of previous work and with estimates of the rate and area of distortion derived from theoretical models for the movement of a flow with negative buoyancy.