Technology - trdoer50028

1997 to place 1 million m3 in the nearshore zone. Analyses of wind, sedi-

ment, and wave patterns determined that the optimum location, i.e., within

the active movement of the beach, was between the 3- and 7-m contour.

However, practical safety and operating limitations of the dredges re-

quired the material be placed between the 5- and 8-m contour.

Technology

The initial experiment proved to be very successful, with all placed

material remaining within the littoral system after 1 year. In 1998, 3 mil-

lion m3, half of all the material used for beach nourishment in The Nether-

lands, was being placed in the nearshore zone and not directly onto the

beach. To construct the shelf successfully, the dredge must be equipped

with doors that open sequentially, usually bow to stern, and open in incre-

ments. At the 8-m contour, the dredge begins to open it doors and reduces

the bottom elevation to approximately 5 m while reducing the dredge's

draft. As the dredge moves closer to shore, it opens the bottom doors

wider, releasing additional material until the dredge reaches the original

5-m contour. The dredge fills the site until the bottom between the origi-

nal 8-m and 5-m contour is filled to a depth of 5 m.

One placement cycle takes between 6 and 12 min. To place material in

shallower water, the rainbow technique is used (a nozzle located in the

bow of the vessel used to spray the material up to 50 m closer to shore).

Studies of the placement technique have shown that the sediment migrates

shoreward and remains within the littoral system. The Rijkswaterstaat cur-

rently contracts with dredge operators annually; dredges are selected on

the basis of size, draft, hopper volume, costs, and availability. Contracts

are often structured around ongoing maintenance dredging, so that the

beach nourishment can be conducted during favorable weather conditions,

and the dredge can haul maintenance dredge material to the North Sea

while returning to the beach nourishment location with beach quality

sand. Combining projects greatly reduces overall costs.

Potential applications

Currently the Corps constructs nearshore berms at several locations with

great success and cost savings. Most of these operations have been lim-

ited to using split-hull hopper dredges. Construction of the nearshore

shelves indicates that there are applications for dredges with sliding doors

to participate in nearshore placement operations as well, increasing com-

petition. In addition, hopper dredges that can incrementally and sequen-

tially open bottom dump doors could be used to place sand caps on

contaminated material.

Costs

Current estimates in The Netherlands indicate a cost of $1.5/m3 for the

nearshore placement, with haul distances of up to 17 km.

Chapter 4

Detailed Evaluations