CONSOLIDATION OF DREDGED SLUDGE, MEASURED BY AN ACOUSTIC DENSITOMETER

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 th e impact effects and spreading of freshly deposited sludge and the con solidation of dumped material. The site is used to dump sludge of acce ptable quality from maintenance dredging of waterways and local harbou rs. The investigated sludge came from a navigation channel and was dre dged and transported by a hopper dredge. On the dump site, a small sec tion was marked in which the dump took place. A field version of an ex isting laboratory acoustic densitometer was used to take a profile of the dry-weight concentration of the sediment bed as measured by the at tenuation 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-so under, 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 w ith steep banks, surrounded by a thin layer of excess sediment extendi ng over 8400 m(2). This was only 20% of the dumped sludge; the remaini ng material was transported outside the measurement section as a densi ty 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 est imates of the rate and area of distortion derived from theoretical mod els for the movement of a flow with negative buoyancy.