Posted on October 23, 2023
The U.S. Army Corps of Engineers (USACE) is no stranger to sediment build-up issues. The organization is responsible for maintaining and managing thousands of miles of inland and intracoastal waterways, channels, ports and harbors with a dredging budget of more than $1.7 billion in fiscal year 2023 alone. Presently, USACE researchers are taking on a slightly different challenge and investigating new methods to diminish the accumulation of sediment in lakes and reservoirs caused by dams.
All rivers naturally transport sediment. However, as a river enters a reservoir, most of the sediment settles along the bottom where it becomes trapped. Over the years, this sediment builds up, and the more sediment that builds up, the less the water capacity in the reservoir.
USACE currently manages and operates 419 dams and reservoirs with active water storage requirements. These dams are used to store water, provide flood protection or serve as a source of hydroelectric power.
USACE storage reservoirs were designed to include a sediment storage pool sufficient for 50-100 years of storage. However, with many reservoirs at or over the 50-year mark, some are starting to experience significant displacement of beneficial pool capacity and other sedimentation impacts not addressed by the original designers.
Transitioning to a more sustainable-use model requires a shift in design and operational strategies for dams and reservoirs that will require the development and application of active sediment management strategies not previously employed in U.S. reservoirs. One such method is water injection dredging.
Theory of Operation
Water injection dredging is a method that injects large volumes of water into the sediment so that it can flow into another area.
“It’s not necessarily a new form of dredging,” said Zachary Tyler, a physical research scientist with the USACE’s Engineer Research and Development Center (ERDC). “It’s historically been used on the European continent to dredge a port’s berth or dock in between when ships are moored there.”
Tyler and his team are looking to build a much larger dredge that is capable of fluidizing sediment to create a density current — that is a plume of sediment that has the potential to travel very far — inside a reservoir and out through the dam.
“This dredge is going to be 80-100 feet in length and 40 or so feet wide,” said Tyler. “It will have an injection system capable of dredging very deep — close to 90 feet — which is much deeper than most navigation waterways.”
Water injection dredging and density currents also require very specific environmental conditions.
“The sediment itself has to be fine-grained and composed of silts or clays that settle very slowly,” said Tyler. “If it settles quickly, the density current just doesn’t last very long. The material also has to be capable of being fluidized by a jet of water coming from the dredge. Some sediments, even though they are made of silts and clays, can be very tough or durable.”
“Another environmental requirement is slope,” continued Tyler. “A fairly steep slope is ideal, but as long as there’s some kind of downward slope towards the dam, it’ll promote the density current moving further. And finally, if there’s some way to sort of contain the density current, like an old channel feature, it can help guide — in a rough sense — the material towards the dam.”
Tuttle Creek Lake, a reservoir on the Big Blue River in Kansas, is one USACE reservoir that checked all the required environmental boxes and was selected as the first water injection dredging pilot project.
Tuttle Creek Lake Pilot Project
Tuttle Creek Lake was built as a multi-purpose project with a critical flood control purpose. It also provides a multi-use conservation pool for water supply, fish and wildlife enhancement, recreation, navigation flow support and helps to maintain water quality for downstream communities.
“We are looking for a less expensive option than traditional dredging,” said John Shelley, a hydraulic engineer with the USACE’s Kansas City District. “The volume of sediment in these lakes is just tremendous. By in large, the Corps of Engineers hasn’t managed sediment in lakes, it’s just been accumulating it year after year. And that was okay for a while, but now we’re decades after the dams have been built, and it’s time to figure out what to do.”
In this case, water will be injected into the sediment in an area in front of the dam about 3-5 kilometers, depending on the water level in the reservoir at the time of dredging. The goal is to create a density current that transports along an old river channel that was part of the landscape before for the dam was installed. The idea being that by dredging some distance away from the dam, material moves towards the dam as a density current and then is vented downstream of the dam.
The Tuttle Creek Lake demonstration will be the first time in the world water injection dredging technology has been used in a lake, and there’s question to how effective it will be.
“We’ve done a lot of testing,” said Shelley. “We’ve done testing of the sediment to see how easily it can be fluidized. We’ve done testing to see how quickly it will settle back out and lots of chemical analysis to make sure there’s no contaminants in the sediment that would prohibit it from being passed down stream.”
The study will not only demonstrate the technology, but also give researchers the opportunity to assess the turbidity current that is generated, and how it flows, and what makes it more effective. By collecting lots of data, the group hopes to apply the lessons learned to other USACE lakes and reservoirs.
“We hope that it’ll be wildly successful, and that we will end up being able to apply it to Tuttle Creek long term,” said Shelley. “But the demonstration will be successful just by giving us good information, so that we can make good decisions and potentially apply it at other lakes.”
The team is also looking into the effects releasing the sediment will have on the downstream environment.
Evaluating Downstream Environmental Effects of Releasing Sediment
In order to move toward sustainable sediment management strategies, more information is needed on the potential effects downstream of these sediment releases.
“We are going to study the potential effects on the physical habitat and on the water quality and any potential effects on the organisms downstream,” said Darixa Hernandez-Abrams, a research ecologist with USACE-ERDC.
The team knows that there could be potential benefits or there could be potential negative impacts with the release of the sediment.
“We need to know the duration, magnitude and timing of the release sediments and the relationship between the ecological processes and the physical processes that come into play,” said Hernandez-Abrams. “Not only are you releasing sediment, but you’re probably also changing the hydraulics downstream, and that in turn can have different effects on the downstream biota.”
The goal for the project is to develop tools to try to model those different effects.
“We can inform decision making,” said Hernandez-Abrams. “We can inform potentially adaptive management and environmental impact studies. With this project, the objective is to inform those decision-making processes for USACE.”
“This is a multi-disciplinary, multi-agency effort,” she continued. “We’re trying to take advantage of this opportunity to learn from the effects of water injection dredging in habitats like the Kansas River and Big Blue River downstream of Tuttle Creek.”
Water injection dredging may not be the right method for every reservoir or lake, but the team is hoping it proves a viable option for some.
“If you do nothing, you are choosing to reallocate your storage of water into storage of sediment, and doing nothing has a real cost,” said Shelley. “You are forgoing very significant benefits, and then once it’s full of sediment, it’s full of sediment, and you have a liability and no water storage advantage there.”
“The most important thing is that you start looking at your lakes and figure out which ones need action sooner than others,” Shelley continued. “There’s a host of solutions that could be applicable, and you have to just look at the range of solutions and see what’s going to fit for your particular lake.”
