H. Verbeek et Jm. Cornelisse, CONSOLIDATION OF DREDGED SLUDGE, MEASURED BY AN ACOUSTIC DENSITOMETER, Marine and freshwater research, 46(1), 1995, pp. 179-188
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.