Sediment Particles Serve as Wetlands’ Building Blocks
The Nitty-Gritty of Silt, Sand and Clay
A muddy blast of slurry plunges from the end of a pipeline into the shallow water of a deteriorated marsh. Engineers and construction workers watch as the grayish-brown rush of water and sediment fills an open-water area. Over the next few weeks, the slurry will separate, sediment settling to form new land as the water runs off into an adjacent marsh.
How do the engineers know how much sediment to pump? How quickly will the sediment settle and dewater? How much will the earth compact beneath it? Understanding sediment’s composition — the proportions of sand, silt and clay particles it contains — lets engineers calculate the answers.
Project Designers Seek Sediment Sources
One of the first steps in marsh creation is identifying a sediment source. Ideally, the borrow site is within five miles of the project area; the closer the site, the less it costs to transport the material.
To evaluate soil type and particle size of the proposed borrow sediment, engineers drill deep into the marsh to collect soil samples. By assessing these borings, engineers can determine the sediment’s performance and settling characteristics. Sand settles faster than silt or clay; heavy particles settle faster than lighter ones.
Roaring from the end of a pipe, one kind of slurry looks much like another: a grayish- brown, muddy rush. But in slurry the proportions of sand, silt and clay vary, and particle size determines how much water must be used to keep the sediment flowing through pipes. A typical slurry is 80 percent water, 20 percent solids. Fine sand and silt particles require less water to keep them moving through the pipeline, while gravel and chunks of clay need as much as 95 percent water to 5 percent sediment.
NOAA
“With inland marsh creation, we have to use whatever the available borrow site offers,” explains Ronnie Faulkner, design engineer with the Natural Resources Conservation Service (NRCS). “If we can determine the sediment’s characteristics, we can work with it and predict its performance.”
For a marsh platform on a barrier island restoration project, sediment characteristics needed determine the choice of a borrow site. “Restoring barrier island beach and dune requires heavygrained sand free from silt and clay particles,” explains Darin Lee, coastal resources scientist with the Louisiana Department of Natural Resources. “That kind of material is often found buried beneath mixed sediments — old deltaic deposits of mud and muck — below the sea floor.”
Engineers prefer to use the muddy material to create the island’s marsh platform and the sand beneath it to rebuild the beach and dune.
“If the island experiences such strong wind and waves that fine-grained, muddy sediment would blow or wash away, we may use the heavier sand in the marsh so it stays in place,” Lee says.
Sediment Properties Shape Project Parameters
Before construction, engineers evaluate the earth underlying the fill area, taking borings to assess soil composition and moisture content. “Under the weight of new sediment, soft, mucky soil compresses more than firmer earth does, so to reach the target elevation in a very wet, spongy marsh we must apply more sediment,” explains John Jurgensen, NRCS civil engineer.
As sediment settles, water flows out of the project area through gaps in the containment dikes. Because particles settle at different rates, a small amount of sediment flows out with the water and is lost during the dewatering process. Engineers take this sediment loss into account when they calculate the amount of slurry they must pump to achieve target elevation.
LDNR
Engineers use computer models to estimate the settlement properties of both the slurry and the underlying soil, then use this data with topographic and bathymetric survey data to calculate the volume of the area they must fill. With that information, Faulkner says, “We can calculate how many cubic yards we must pump and place to achieve the right elevation.”
Knowing the composition and volume of sediment to be pumped, engineers design containment systems to hold the material in place at the project site. “We must allow time for particles to settle before the water runs off, or we’ll lose some of the material we pumped,” Faulkner says.
Understanding how quickly placed sediment will settle and how much the earth beneath it will compact is crucial to achieving the target elevation. “Most of the settling will occur in the first three to five years after construction, but that varies from project to project,” Jurgensen says. Some projects lose a foot of elevation in the first three years and six inches over the next 17 years; others subside continuously throughout the 20-year project life. “Marsh creation presents many challenges,” he says, “but one of the best ways to recover lost acreage is to put more soil out there.”
Engineers must determine not only how much slurry to pump into a marsh creation project, but also how fast to pump it. "We're trying to achieve the target elevation as quickly as possible, but we don't want to breach the containment dikes - doing so wastes sediment, increases cost, slows the project down and potentially infringes on the land rights of neighboring properties," explains the NRCS' John Jurgensen.
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