Sand/Silt Separation Basin

ERDC TN-DOER-T1

February 2000

Sand/Silt Separation Basin. The Netherlands currently requires sand separation and reclamation

for contaminated sediments prior to disposal of the contaminated fraction. Several methods are

available including various forms of bar grids for the coarse materials, hydrocyclones for sands and

silts, and separation basins such as the one used in De Slufter Confined Disposal Facility (CDF)

(see Hopman and McLellan 1999). De Slufter was completed in 1987 to contain contaminated

sediments dredged from the Port of Rotterdam. It has a capacity of 100 million cubic meters in its

260-hectare site. In an effort to retain as much site capacity as possible, sand is separated from the

contaminated silts on a routine basis. De Slufter managers have continuously looked for innovative

approaches to sand reclamation and are currently looking at several different methods including a

silting basin and hydrocyclones. A sand separation basin is employed that is sized to the velocity

and quantity of material coming into the site. It is a relatively small confined area within De Slufter

that expands in the direction of flow to allow for the sand to settle naturally while the fine-grained

material remains in suspension and is allowed to flow into De Slufter.

Hydrocyclones have also been employed to separate the sand from the fine-grained materials.

According to the managers, the hydrocyclones are responsible for 80 percent of the cost while

supplying 20 percent of the sand; the sand separation basin is responsible for 20 percent of the cost

while performing 80 percent of the sand separation.

Management of Dredging in the River Waal. Shoaling in the River Waal, The Netherlands, is

concentrated along the bendways of the river. Sand waves are common in the river and cause a great

deal of problems for navigation. Several nondredging techniques have been used to control shoaling

in the bendways: armoring the outside slope to force flow into the interior bank; emergent and

submerged groins to train flow to the centerline of channel; and submerged midstream vanes to

reduce the spiral flow near the river bottom, reduce scour on the outside bank, and decrease

deposition on the inside bank. Approximately 1 million cubic meters of material is dredged from

the river, which is estimated to be half to one-third the amount required if only dredging techniques

were used.

Dredging Equipment:

Flexible Spud System. Krupp Fordertechnik has developed a flexible spud system to increase the

wave height a cutterhead suction dredge can operate in while working offshore. The spud is not

actually flexible, but its cable system is designed in such a way that stresses from the wave-induced

motion of the dredge are transferred to the cables and not to the spud. The dredge is allowed to

"rock" back and forth on the spud pin allowing it more freedom of motion without putting additional

stress on the spud. Working conditions of the dredge are increased up to 2 m of wave height.

Hopper Dredge Dragarms. Several innovations to hopper dredge dragarms have been developed

over the past several years including the overflow water recirculating system, the jet cutting systems,

and increased draghead width to improve efficiency. The resulting changes have led to higher

efficiencies. The increase in efficiency is so great that smaller hopper dredges, 3,000-cu-m capacity

or less, are constructed with only one dragarm. The recirculating system pumps the overflow water

from the hopper to the draghead. By eliminating overflow, dredges can operate longer in areas that

will not allow overflow and increase efficiency. Some manufactures claim a 20 percent increase in

efficiency for dredging silty sand using the recirculating system. The recirculating system does