Maintenance dredging of a navigation channel can be a costly and intensive operation. Water injection dredging is often used to mitigate these concerns and is a common method used in European navigation channels. This technique uses the natural current of a waterway to move sediment away from a channel, and it has financial and environmental benefits.
HDR's Joe Wagner, P.E., D.NE, BCEE, is a leading proponent of water injection dredging in the United States. He’s been involved with the one port that has adopted it in the States — the North Carolina State Port Authority, where he has worked closely with the NCSPA to design, plan, and purchase a custom-built water injection dredging system for use at two NCSPA port facilities (Port of Wilmington and Morehead City) and has advocated nationwide for more widespread use. In this interview, Wagner, a senior dredging engineer with more than two decades of experience in the field, explains the technique and its possibilities.
A. To best understand water injection dredging, it's important to understand traditional hydraulic and mechanical methods.
In hydraulic dredging, the dredge head spins, dislodging sediment and material, which are partially fluidized and sent through a pipeline to a temporary or long-term placement location. A heavy dredge plant production involves numerous pumps directing dredged materials through pipes and many miles of pipelines. The material is either deposited offshore or in a disposal location where it can be dewatered.
A bucket is dropped through the water to the bottom sediment to scoop material in mechanical dredging. This material is placed into a scow or a barge and taken away to be handled again at least once more before it finds its final disposal location.
Water injection dredging, a type of hydrodynamic dredging, is an elegant solution that uses the water’s natural flow to remove the sediment. Water is injected into the sediment at low pressure and high volume and fluidizes the sediment near the bottom, leading the sediment to flow with the natural current. That layer remains close to the bottom. Gravity, currents, normal river flow and other hydrodynamic forces then carry it away. It requires no disposal site — the material follows the natural hydrodynamic forces and keeps the sediment within the system.
Q. What are the benefits?
A. In short: the technique is less expensive, requires less work and finds environmentally friendly uses for what would otherwise be waste.
Much of the cost, time and issues involved in traditional dredging are related to transporting the dredged material to a final disposal location. This effort includes some form of bucket, pipeline, hopper, barge, etc., required to transport that material. In comparison, in hydrodynamic dredging, the water column transports the sediments to where they would have naturally had they been reintroduced. The major work takes place in the preparation and planning, developing an understanding of where the materials will go once they are fluidized by the dredging.
The savings of injection dredging includes mobilizing/demobilizing costs, staff costs, fuel consumption, maintenance for pipes, pumps and the cost of earth-moving equipment or barges to move material elsewhere.
Water injection dredging saves the port the cost of mobilizing/demobilizing, staff costs, fuel consumption, and maintenance of pipes, pumps, earth-moving equipment and barges to relocate dredged material.
The technique can also be more environmentally friendly. A water injection dredging system mimics natural forces, so the ecological elements that interface with the dredging exhibit a more natural reaction. Obviously, it’s not a natural occurrence for something to scoop material out of a waterway, take it up through the water column and dispose of it. Fluidizing material at the bottom surface and letting the current carry it away is a natural, environmentally-friendly alternative.
Q. How do you assess whether water injection dredging is a good fit for a port?
A. When we conduct feasibility studies, we look at three main areas:
Sediment: The type of sediment is key to success. The sediment being fluidized needs to be fine. Fine-grained is the best, and fine sand will work. However, it cannot be prone to clumping, otherwise, it won’t be fluidized correctly.
Hydrodynamic conditions: Something needs to take the fluidized sediment away. That could be a current, a tide or gravity. But standing water will not work for this process.
Downstream area: The area where the sediment is to be deposited must be suitable. In a best-case scenario, this could be a marsh that is losing sediment. It could also be the ocean or another spot that is environmentally appropriate for the material.
All these conditions need to be considered to receive regulatory and legal approval. In some cases, hydrodynamic modeling is sufficient. In other cases, we’ve been able to do a demonstration to confirm predictions.
If the conditions are right, a successful dredging process will disperse the sediment into the system rather than creating a buildup of material.
Q. Where has water injection dredging been used?
A. This is a proven technology that’s been widely used in Europe — roughly 10-15% of maintenance dredging there is done through water injection dredging.
But the U.S. has been slower to adopt the technology for a variety of reasons, including the regulatory environment and simple inertia — ports are used to using traditional methods.
Just one port, the North Carolina State Port Authority, has adopted this method. NCSPA has owned and operated a water injection dredge, the Osprey, since June 2021. In that time, it's dislodged over 270,000 cubic yards of material through regular dredging events, operating about 90 hours.
It's estimated that the port will save roughly $1 million a year using this dredging technology. That estimate of savings doesn't consider the previous cost of transporting dredge material to an offsite location. It also doesn't include the cost of procuring and creating more disposal locations, which would have been needed with continued traditional dredging.
It also saves the roughly 300-acre port from sending its dredge material to a nearby 900-acre site, a disposal site that has grown through the years and otherwise would take up more valuable real estate.
Q. What are the future implications for this technique?
A. In January 2023, the U.S. Army Corps of Engineers issued a “Beneficial Use of Dredged Material Command Philosophy Notice,” outlining an intention to increase its use of beneficial use of dredge material.
The technique fits perfectly with this new directive and its philosophy of working with nature to put materials to use within the natural system.
This is a great technique that has the potential to solve problems for ports in an environmentally friendly way. I am passionate about sharing it, and I hope that this move from the Corps will spur greater use of injection dredging in the U.S.
Advice and Inspiration
Q. How did you get involved in water injection dredging?
A. I may have heard of the concept before, but my first distinct memory of water injection dredging was reading the definition of dredging in a college textbook. I remember asking the professor about a section of the text that explained dredging but barely referred to hydrodynamic techniques.
I got really excited about the technique through projects that weren’t using it. In the early 2000s, I worked on one project where they were taking beneficial material out of a marsh area. That material is valuable economically as well as for the environment. Seeing those instances where clients would pay to dredge material and then pay to store it elsewhere grew my passion for water injection dredging.
As a side note, I was recently asked to review a new dredging engineering college textbook by that same professor, and I was able to encourage the author to expand his definition of dredging to include hydrodynamic dredging. It’s interesting how things come full circle and how that seed of interest grew to become a large part of my whole career.
Q. Do you have any advice for someone starting out in the marine field?
A. My journey to the marine field happened in somewhat of a circuitous route. I started studying hydrology and hydraulics as a civil engineering student but moved to the education field. I earned a bachelor’s in mathematics and a master’s in biology while spending roughly a dozen years in academia. When returning to civil engineering, specifically coastal engineering, I intended to focus again on hydrology and hydraulics. But the biology and teaching background gave me a whole new perspective. Now I was primed for what has become a career in the beneficial reuse of dredged material.
It really benefitted me to start as a generalist and then follow my passion to more specific areas of expertise. Now, when I’m in meetings about various topics that aren’t my specialty, I have at least passing familiarity with a variety of areas. It helps better inform the work I do, especially with dredging. My advice to those starting out would be to get a good baseline of knowledge in a variety of areas, then find your passion and follow it.
