A Potential Alternative to Berth Maintenance Dredging

The South Carolina State Ports Authority's Columbus Street Terminal is located on the right descending bank of the Lower Town Creek Reach of the Cooper River in Charleston, South Carolina. The 3875' pile supported concrete wharf structure is situated parallel to the channel, but the lower 1640' is canted 10 degrees inland. The reach widens on the lower end with the channel flow going away from the wharf. Significant silting was experienced on this lower end. The channel and berth are authorized to –45' MLW. The lower 1640' of berth was dredged to 53' MLW every four months to maintain the authorized depth. The total cost to maintain depths on the lower end was approaching $1 million per year. Each 4 month dredging removed approximately 80,000 cubic yards. The dredging cost averaged about $250,000.00 per cycle or $750,000.00 per year. The material was placed in nearby confined disposal facilities. Maintenance of the disposal areas averaged approximately $250,000.00/year. Dredging operations had to be worked around vessel activity and typically took 4–5 days per cycle. Scheduling the dredge often became difficult and prices could significantly vary with dredge availability. Delays in dredging the berth created operational problems and vessel groundings. Fortunately, the soft bottoms never damaged the vessels. In the late 1980's a new technology was developed by the Navy to control sedimentation in estuarine and fluvial berthing areas. The technology was commercially installed in Terminal #4 at the Port of Grays Harbor in Washington in 1987. Certain sediments will form a layer of fluid mud during periods of slack water between tides. Those sediments may be kept in suspension or re-suspended by introduction of flow into the water column. Kept in suspension, they then would be carried out of the berthing area on the next tidal flow. The new technology consisted of a series of water jets to keep the fluid mud in suspension until the natural water flow carried it out. The jets put energy into the water column during times of slack water when siltation typically occurs. Generally, the technology can best be applied to those sediments that have high silt/clay content and settle slowly from the water column. Sandy type materials tend to quickly settle and are not as easily kept entrained. The technology depends upon the natural flow of the stream. However, the system can be designed to operate in streams flowing as little as 0.25 knots. The systems can operate in fresh or saline conditions and can protect berthing areas up to 250' wide perpendicular to the face of the wharf.