The Nuts and Bolts of Pipes and Pumps

WHAT IF THERE WERE A TECHNIQUE that built hundreds of acres of land in weeks, mimicked natural land-creation processes, and used a renewable resource — Mississippi River sediment — as its raw material?

Some engineers and scientists say pipeline sediment transport is that technique— and that it could play a major role in saving coastal Louisiana.

“Pipeline transport— moving dredged sediment through pipelines to wetland sites — replicates the natural processes that create wetlands,” says coastal oceanographer Dr. Joe Suhayda. “It lets us go out into open water and rebuild marsh in a matter of weeks.”

Worldwide, the technology has been put to many uses: Singapore used dredged material to increase the size of its main island by 20 percent; in the Netherlands, 4.2 million cubic yards of sediment were transported via pipeline to build a 100-mile freight railway. In Louisiana, pipeline transport has already been used in major highway construction and is being developed as a technique for reversing land loss on islands, along shorelines, and in wetlands.

In the Pipeline

Pipeline transport begins where the sediment is located. In Louisiana, that’s primarily at the bottom of the Mississippi River and the Gulf of Mexico. Collecting sediment from these underwater sites for restoration purposes has typically used a pipeline dredge, a specialized vessel capable of removing sediment from depths down to 70 feet.

The dredge carries a suction pipe mounted on an arm that extends into the water. The end of the pipe might be outfitted with a 32-foot-wide dustpan head or an 8- to 9-foot-diameter cutterhead — an array of blades that rotates like a drill bit. As the pipe moves, it sucks up sediment and propels it through the pipeline.

Large amounts of water keep dredged material moving. “A slurry of one part sediment to two parts water works well with silt and fine sand, but with big particles such as gravel or balls of clay, there might be 5 percent sediment and 95 percent water,” says Rick Smith, chief civil engineer with Weeks Marine Inc.

Powerful pumps — one or more located on the vessel, plus boosters along the pipeline — propel the slurry 24 hours a day, seven days a week until work is completed. “To keep material from piling up in the pipeline, we adjust speed based on particle size— fine sand flows at 14 feet per second, while gravel requires 18 feet per second,” Smith says. “We can increase speed by placing the booster pumps closer together. A typical pump can move fine particles about four miles, larger particles two miles. For a long pipeline, we would place pumps every two to four miles, depending on particle size.”

At the placement site, dredged material spews from the pipeline in a muddy rush. Sediment particles collect and pile up; as water drains away, new land is created.

Stuck in the Middle

For Smith, the beginning and end of the pipeline are simple and straightforward: “It’s the middle that’s complicated.”

Say a project calls for dredging sediment at site A and placing it 10 miles away at site B. “Because of rightof- way issues, oilfield infrastructure, the location of canals and so forth, we might have to lay 16 miles of pipeline, which calls for four or more pumps,” Smith says.

“Large boats can’t navigate the canals leading to hard-to-access sites, so we have to fuel the pumps using an endless fleet of small barges,” Smith says. “It’s like trying to build an interstate by hauling dirt with pickup trucks.”

To ease this and other logistical burdens, Suhayda and Smith advocate installing semi-permanent structures— such as temporary canals that will be filled in after a project has been completed — for access to remote marshes. Using pipeline transport in concert with other techniques, such as diversions from the river or hauling sediment in barges, could further increase efficiency.

“We can make this technology even more efficient by using it with complementary techniques, and tailoring each application to the specific environment,” Suhayda says. “The possibilities for pipeline transfer are exciting — if applied correctly, this technology could save the wetlands.”