Dredging and the disposal of fine sediments in a drinking-water lake
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
P. Vroege, M. Cats, R.W. Stroet, R.T.A.L. Hoofs and M.J. Jeroense
Abstract: Introduction Limburg Water Company has built a new drinking-water treatment plant. The plant extracts its water by pumping groundwater from wells. The scheme includes replenishment of the extracted groundwater through shore infiltration from a nearby lake. However the gravely banks of the lake, reaching as deep as 25 m, were covered with a layer of silt. The latter implied the potential danger of clogging and in turn reduction of the infiltration capacity. To improve the infiltration the silt layer had to be removed by dredging. Complicating factors were the very steep bank slopes, compelling stringent requirements on maintaining the established profile of gravel and the extreme susceptibility to clogging, where only a few mm's of silt, may jeopardize infiltration.
Dredging equipment and method Silt dredging was carried out by means of a wheel dredge with a dredging pump and discharge pipeline. To prevent sediment dispersion and precipitation, two ancillary dredging tools were developed for this project:
- A hood over the dredging wheel to contain the zone of turbidity and forming a suction chamber
- A special dumping pontoon
This dedicated pontoon was constituted by a ring of coupled pontoons forming a discharge trough for the dredged spoil. The trough insides were extended to 10 m below water by a PE curtain to prevent turbidity outside the pontoon
Results
Monitoring and control of the dredging process were made by video images, so as to inspect the state of the cleaned slopes and to consider the necessity of a second turn. Besides turbidity was established by video images and water samples. Through these video images it was established that the dredging had resulted in cleaned slopes with no silt layer and that during the dredging and dumping no turbidity did occur.In addition: By means of pumping trials and measurements of groundwater levels, it was furthermore established that the infiltration capacity had improved considerably. The overall conclusion is therefore that the dredging and disposal project has been very effective and successful.
Keywords: dredging, disposal, discharge trough, hood over dredging wheel, turbidity