Technology - trdoer50035

duce overall dredging requirements. The actions included: (1) eliminating

overflow of the hopper dredges, (2) transporting all dredged sediments to

the North Sea, (3) eliminating the practice of in-channel placement, and

(4) concentrating all dredging at the mouth of the channel. Before making

changes, the Port operated two full-time crews on each dredge. Since tak-

ing action to reduce dredging, the size of the crews have been cut in half

and operation time has been cut almost in half. The Port has been able to

reduce its overall dredging requirement significantly, and is able to oper-

ate with the same budget it had 10 years ago.

Company/Organization(s): Access restricted to U.S. Army Corps of

Technology

The Harwich Haven Port Authorities is located in Harwich, along the

western shore of England. It is a rapidly expanding port that deals primar-

ily in ferry traffic and container terminals. The container portion of the

port channels has been expanded in all dimensions (wider, deeper and

longer) several times within the past 10 years, resulting in increased main-

tenance requirements. Harwich is similar to Tees with a majority of the

shoaled sediment originating from the North Sea. The sediment is primar-

ily fine silts and clays. Ambient water conditions are so turbid that the

port facility routinely has water column turbidity levels of 300 mg/l.

To better understand the effectiveness of dredging and how to reduce

dredge requirements, the Port conducted an analysis of the relationship

between dredging production and tides in the port facility. The detailed

analysis indicated that the best dredging production was obtained during

neap/spring tides. Figure 12 shows the relationship between tidal range,

dredged volume, and removed volume (total survey volume) for May

1996. During these relatively slack tidal periods, the effectiveness of the

dredge is reduced as material quickly backfills the project. Figure 13

shows the same information for a tide event in February 1997. Compari-

son of the tidal events indicates that dredging conducted during February

1997 was more effective in removing material from the channel than the

May 1996 project. The primary difference between the two was the tidal

range, with an additional difference of 1 m in February 1997.

To take advantage of this knowledge, Harwich developed a contract

where hopper dredges are available on demand to dredge during peak tidal

ranges. Dredging occurs four to five times a year depending on need.

The Port enters into a contract for an 18- to 24-month period with the

above-mentioned requirements. The specification also requires the hop-

per dredge to operate in thin cuts at speeds up to 5 knots to supply the re-

quired amount of material to the draghead. The contractor has several

rates for payment (by day if production is low, unit price for high produc-

tion, and hourly rates are also available). The Port estimates that current

costs are approximately $1.0 to 1.25/m3 (US). Figure 14 shows the de-

crease of dredge volume over the past 4 years by implementing the above

management technique.

Chapter 4

Detailed Evaluations