Source: Dredging & Port Construction
An unusual small dredging project to clean contaminated sediment from a Florida waterway required the contractor to keep a lookout for slow-moving marine mammals.
One of the most unique dredging projects ever to take place in Florida created many challenges for its locally-based dredging contractor, Ludlum Construction Company.
The project combined the use of synthetic organic polymers and the removal of over 60,000 m3 of dredged material from an estuary lagoon inhabited by the famous, playful and legally protected marine mammal, the Manatee. With water temperatures rising during the spring construction period, Ludlum anticipated many more of the slow-moving creatures returning to the lagoon.
The $623,000 project cleaned thick black sediment from the mouth of Crane Creek, a body of freshwater that empties into the Indian River Lagoon south of the Melbourne Causeway. Runoff water carried by the creek to the lagoon can originate in drainage ditches as far west as Interstate Highway 95, over 20 miles away. The sediment was pumped into three football-stadium-sized disposal areas using polymers to assist in the settling process and material consolidation. The dredged material was mostly silt and clay, containing fertilizers and heavy metals.
"We're finally doing it, after many years of studies and talks," said Jim Swan, an executive of the St. John's River Management District. Decades of storm-water runoff had carried sediments and pollutants into Crane Creek and other tributaries of the lagoon, where they settled and built up over time. Storm flushed materials smothered sea grass and wildlife while muddying the waters.
Florida's spring weather warms the waters and the manatees return in large schools to the project area, which set a tight window for completion from January through March of this year. With severe penalties looming beyond this date, Ludlum leased an Ellicott Series 370 10-inch discharge cutterhead dredge to perform the work.
"There could be no dredge downtime due to mechanical failures, so we selected Ellicott," said Ludlum's vice president Jim Schwarz.
Ellicott is a reputable lessor of dredging machinery, and it does not permit leased dredges into the field prior to shop reconditioning. With only three months to complete the works, the equipment supplier was a key decision. In any event, there was zero downtime and the job was completed four days early.
In addition to the dredge operator, there were two other men onboard acting as manatee lookouts. They observed some early returnee manatees in the vicinity, but none would come near the dredge's cutterhead. Had a manatee done so, the dredging would have been halted until it left. Many of these endangered mammals are injured and killed each year by powerboat propellers.
After the dredge had removed up to 18 feet of material to reveal a clean, sandy bottom, Ludlum learned that manatees dislike the dirty sediment as much as humans do. Divers saw the creatures rubbing their bellies on the newly cleaned bottom.
This project would not have been possible without the use of liquid synthetic polymers injected into the dredge's discharge pipe to settle the material in the disposal area. Although polymers are not new to dredging, their use enabled settling times 20 times faster than natural means. They also enabled sediment concentrations in the disposal area to increase by five times that in the lagoon bottom.
The polymers were safe for the return water going back into the lagoon. Aquatic biology students from the Florida Institute of Technology performed the water testing required by the Florida Department of Natural Resources for discharge water effluent. As determined by chemical supplier NALCO of Pittsburgh, Ludlum injected the polymer into the dredge's discharge line approximately 200 feet from the vessel. A flow meter onboard determined polymer feed rate. Turbidity was constantly monitored at the dredge and compared to a background reading. The use of polymers increased the project cost by approximately 10 percent, a modest amount considering their effectiveness.
The residents of Melbourne, Florida, including the manatees and other wildlife, now have a restored body of water. The project was so successful that three larger projects are now planned by the St. John's Water Management District for next year. As the Series 370 dredge was being loaded onto a truck for its return to Ellicott, hundreds of people lined a park walkway to see a school of manatees swimming through a marina area into the mouth of Crane Creek.
Reprinted From Dredging & Port Construction