Clearing the Channel: Dredging Operations and Erosion Control

March 1, 2003
Dredging activities are performed as both construction and maintenance functions in various waterways, especially the waterways used for navigational activities ranging from recreational boating on a lake to commercial shipping involving supertankers and container cargo ships. Dredging is also performed as part of various land reclamation programs, including beach renourishment, as well as for wetland creation and environmental remediation. Each program, in its simplest form, involves the removal and relocation of soils from an underwater location to an onshore site or a disposal area far offshore. But to be successful, each project requires a great deal of planning and engineering that includes environmental and logistics analysis and identifying equipment appropriate for moving the materials. In addition, assessment of the dredged materials has to be performed to identify a final disposal site, necessary permits, and other regulatory compliance issues. Sensitivity to the breeding habits of various aquatic species might limit the periods of time that dredging can be performed, forcing work to be done under less-than-ideal weather conditions. Fortunately, ongoing research by the United States Army Corps of Engineers (USACE) Dredging Operations and Environmental Research (DOER) program, as well as private innovations, continues to improve the cost-effectiveness, environmental sensitivity, and productivity of dredging operations.The vast majority of dredging activities are financed and performed under the auspices of federal and state agencies, with local port authorities receiving both benefits and cost. Congress provides a significant portion of the funding for dredging activities; until 1986, these projects were completely paid for with federal funds. “In 1986, the Water Resources Development Act changed the way projects were done,” says Grady Bryant of Gahagan and Bryant Associates, an engineering firm that specializes in dredging projects nationwide. The 1986 act required that projects performed by USACE have a local sponsor because such projects provided a local economic benefit and included a cost-sharing strategy. “Depending on how deep you dug your channel, you would have cost sharing with the government at some ratio,” states Bryant. “From 20 feet deep to 45 feet deep, the local sponsor would pay 25% of the cost, the government would pay 75% of the cost. If you went deeper, beyond 45 feet, then it would turn into a 50/50 deal for the increment below 45. And for beneficial uses of material where you are creating wetlands or environmental restoration, that adds cost sharing if it was associated with a navigation project.” This law had the further impact of increasing constituent pressure on members of Congress and subsequently on USACE to undertake major channel-deepening projects. “Ships are getting bigger, [so] I’ve got to deepen channels, and the sponsors started saying, “Hey, let’s get it going,” says Bryant. “Prior to 1986 the government pretty much paid 100% of it. The sponsor only had to provide the lands for disposal of the dredge material on navigation projects and take care of maintenance of the disposal areas. Over time the owners became more aware that they had a bigger say in this deal: ‘We’re putting up a lot more money than we were.’ Demands for deeper channels kept coming, so the corps started studying projects, [but it takes] a very long time to get through all the steps the federal government proscribes for projects that have federal monies involved.”sponsibility for management and oversight of dredging projects in navigable waterways are assigned to the USACE. According to the publication “Application of Dredge Monitoring Systems to Dredge Contract Administration Quality Assurance” (ERDC TN-DOER-I3), approximately 80% of all dredging under USACE is performed under contract, with each USACE district taking on more of a contract administration role than a technical hands-on role in ensuring the nation’s waterways remain open and navigable. USACE uses two types of dredging contracts: a lease dredge contract and a yardage contract. The lease dredge contract is based on an estimation of the time required to remove an estimated amount of sediment from the project area. The yardage contract provides payment to the contractor based on predredging and postdredging surveys, and the contractor is paid by the volume of material removed for a price-per-unit amount.But determining the type of contract appropriate to the project is the end result of a significant amount of engineering and environmental analysis. “A lot of times the dredging industry gets called up and asked how much it costs to dredge, like it’s a commodity you buy off the shelf,” remarks Bryant. “We provide the detailed practical approach by first beginning with what it is you’re dredging. Is this a navigation channel? And what are you dredging—rock, sand, silt, clay, or mud? That leads to a preliminary engineering investigation and analysis of the soil and its dredgeability and the best equipment.”Disposal of dredged materials must also be considered at the beginning of the project; it might involve placing them in an upland confined disposal facility. In some cases, issues of contamination must be addressed, requiring soil analysis and a plan for disposal. “From there, you are able to put together a plan of how the project should be,” states Bryant. “Then you can provide a cost to the owner.”Pushing the Research EnvelopeAlthough dredging activities have been performed for more than a century, research is ongoing to identify ways to deal with the various issues and technological opportunities continuing to develop. Toward this end, USACE’s DOER program has five focus areas for its research:aquatic placementcontaminated sedimentsenvironmental windowsrisk assessment and managementinnovative technologyAccording to Robert Engler, Ph.D., senior environmental scientist for the Engineering Research and Development Center at the corps’ Waterways Experiment Station in Vicksburg, MS, “This program is currently midway through a 10-year effort that combines both the operational aspects of dredging with the environmental concerns to result in cost-effective operations that meet or exceed environmental standards.” Research in the aquatic-placement area involves determining cost-effective methods of aquatic placement of soil that lead to beneficial uses where it’s placed. “Obviously these have the largest of environmental concerns,” says Engler. “In that focus area we are looking at management of these placement sites to ensure that sensitive areas are not impacted and to further ensure that the site itself will result in some environmental benefit.”
The second focus area involves the management of sediments contaminated by decades of inputs from industrial activities. “Under the contaminated-sediments focus area, we’re aiming at cost-effective approaches in assessing the environmental hazard that will drive us to proper disposal alternatives,” states Engler. “If the sediments pose an environmental hazard, they will be placed in confined disposal facilities. Confined disposal is the most expensive form of dredge material placement. As such, we are stressing research that will allow us to treat the sediments at a confined disposal facility or use techniques either to cause the contaminates not to be available to the biosphere or to result in the contaminates degrading into a nontoxic form.”The third focus area, environmental windows, deals with estuarine, river, lake, or ocean environments where dredging would not be allowed because of the presence of sensitive aquatic organisms. “Environmental windows negatively impact 95% of the operations and maintenance dredging in the United States,” says Engler. “They limit when we can be dredging and result in severe logistical problems for the dredging industry. In many cases the environmental windows themselves are not based on sound scientific fact, just simply fear of the unknown. We know a certain fish is there or a shellfish or a sensitive area with submerged aquatic vegetation, and we know that these sites are more sensitive during certain times of the year. But the quantification of these has been nil, and what we’re doing in this area is quantifying clearly when the areas are so sensitive that we should not be dredging, to make these windows more realistic. Narrowing these windows just a day or two can result in phenomenal savings to the dredging process itself. We’ve also brought in the National Academy of Sciences to do a study overviewing our research and the need for windows. The National Academy of Sciences has endorsed our approach and made some recommendations on how to better negotiate the reality of environmental windows.”The fourth area of focus for DOER is risk assessment and management. “Risk assessment is not a new approach, but it is a relatively new approach to the environmental world,” states Engler. “Within this risk area we are developing the probability models and the computer techniques so our clients can sit at their PCs and use these risk models to do scenario testing and look at alternative risks among various management alternatives. Right now we have definitive guidance on how to do human-health and ecological risks associated with contaminated sediments when the proposed disposal site is aquatic. We’re just wrapping up the same human-health and environmental risk assessments for contaminated material going to land sites. There is no solution to contaminated-sediments disposal that’s risk-free, and what we’re aiming at is that disposal option with the lowest risk.”The fifth focus area involves the identification and transfer of innovative technology. “We’re not developing technology; we’re scouring the world for technology that can be used in the dredging world in an innovative sense,” says Engler. “We’re looking at dredging equipment, software, management frameworks, and environmental controls. With any innovation we can bring in that shows serious promise in causing the dredging process to be less expensive and more environmentally protective, we’ll conduct a field demonstration to see if this really works. This has allowed us to demonstrate a half-dozen new techniques in dredging and dredge-material management.”The Equipment That Moves the MaterialsDredging uses several types of excavation equipment to move the material from the bottom of the channel to the ultimate disposal site. Coupled with various pumps, hoppers, and barges for transporting the materials as either liquid slurry or water-saturated soil, this variety of equipment allows project managers to design appropriate dredging methods to address each project’s environmental and physical characteristics. For example, Wilco Marsh Buggies of Marrero, LA, manufactures a line of tracked Swamp Excavators that can float and move through water. Marsh Buggies of Harvey, LA, provides amphibious track vehicles, such as the 34-ft. Marsh Excavator with metal tracks over pontoons for maneuvering in aquatic and marshland environments. The primary piece of equipment is the dredge itself. “Dredges are material-handling machines in the same way that, say, an excavator or a loader or other kinds of off-road equipment handle material,” says Donald McCaig, vice president of sales for Baltimore Dredges–Ellicott Division in Baltimore, MD, a manufacturer of cutter-suction and hydraulic auger dredges. “The difference in the dredging world is that when you need to move material that is underwater, and it makes sense to transport that material to a remote location in a pipeline, then dredges come into play as a very cost-effective means of moving material in a slurry.”There are several types of dredges in common use. These include the bucket dredge, suction or cutterhead dredge, and dustpan dredge. Each dredge can handle different materials under different conditions and with different disposal options. The bucket dredge is useful in locations where a disposal site is not easily accessible or where the dredged materials may be placed along the sides of a channel. “Let’s say you’re dredging a small, 100-foot-wide canal system,” says Kim Autin, vice president of Marsh Buggies. “[The operator] would go in with a bucket dredge and dig the canal to the specified depth, then take that material and throw it on the edge of the canal and disburse it along the length of the canal—only within the reach of the bucket dredge itself. If the bucket dredge has a 100-foot reach, then he can dispose of that material only 100 feet away. He can’t move it 4,000 feet up the road. Most large bucket dredges don’t really work with dump trucks like you normally see alongside the highway. Instead there are what they call hopper barges.”The second type of dredge is the suction or cutterhead dredge. These are in essence large vacuum cleaners with a fitted head that removes materials. “A cutter-suction dredge uses a basket type of rotating cutter,” says McCaig. “Cutter-suction dredges can also be fitted with wheel-type excavators for harder material. A dredge does two things: It excavates the material and places it in slurry, and then it pumps it to a remote location. A very important component of the process is the excavation. Cutter-suction dredges, because they are large and have a relatively small cutter compared with the overall weight of the machine, can dig hard materials. They are capable of mining virgin material that has never been cut before. Mud Cat dredges, on the other hand, have typically an 8.5- or 9-foot-wide auger, which is the mechanism that presents the material to the suction part of the dredge. But it is not a very good cutting machine for virgin material; it is a very good machine for mud. Another difference between the Mud Cat and the cutter-suction dredges is the size—Mud Cats stop at about a 10-inch pump, which is approximately 4,000 gallons a minute of water, whereas cutter-suction dredges can go up to 33- to 34-inch pumps.” Ellicott manufactures both the smaller Mud Cat line and the larger Dragon and Super Dragon cutterhead dredges.“Some of these dredges might have a 16-inch discharge on the large scale, and a small one might have a 6-inch discharge,” says Autin, noting that the machines range from about 30 to hundreds of feet long. “There are large pumps within the barge or dredge unit itself that pump the material to another location. If they have to pump it really far, they use booster pumps, or small pumps to increase the head pressure within the pipe. You’ve got to realize you are pumping probably 60% water and 40% mud.”Dustpan dredges are a third type of dredge, typically with wide, flat dredge heads and water jets to deal with the cohesionless sediments found in many areas. These high-volume, low-pressure machines can pump dredged material hundreds of feet through rigid pipelines. Using the dustpan dredge to move materials for marsh creation on the lower Mississippi River was an example of the use of innovative technological transfer through the DOER program, notes Engler. “The idea was to be able to pump this material a reasonably long distance and build marshlands with it to use it productively,” he states. “This had never been done before. We had done it with small dredges, hopper dredges pumping over, and cutterhead pipeline dredges, none of which was terribly economically feasible—and they all posed river traffic problems. This dustpan dredge was demonstrated in New Orleans early this year and was a phenomenal success. So successful, in fact, that when after four or five days of pumping the dredge was turned off, we were ready to move the sand to proper elevation for marsh development.”Another developing technology involves an agitation dredge. “It uses a new technology: the combination of water injection and Venturi effect to lift the sediments from the bottom of the navigation channel and let them flow naturally over the sides of the channel,” states Engler. “Our concern was whether the turbidity that could be generated from this would be sufficient enough to result in any harm to the fisheries. We demonstrated this in Galveston District and the Houston Ship Channel and a couple of other areas where fisheries were a concern,” he says. “We demonstrated this as a very inexpensive, effective tool for doing small dredging jobs in the waterway. We found, through very intensive monitoring, that the suspended material stayed very close to the bottom and settled out very rapidly nearby out of the channel, causing minimal to no environmental impact.”For larger projects, it might be necessary to apply a variety of dredging approaches to meet all the project’s objectives. For example, the Port of Houston has had an ongoing project for approximately 30 years. Many of the issues related to dredging operations involve disposal of the dredged materials. “We’ve employed numerous types of dredge plans,” states Grady Bryant. “The primary thing we are doing in the Port of Houston is deepening and widening the ship channel to provide for deeper-draft tankers. In some of the jobs a hydraulic cutterhead dredge, which has slurry pumps mounted on it and an excavating head at the end of it, is lowered down. The dredge swings back and forth and excavates the material and transports the slurry to the disposal facility—in one case, we were dredging clays. Those materials were chosen to be used to construct containment dikes. From there we went to areas that were very soft muds, and with the same dredge we excavated those and pumped them into the area that we had just constructed with the clays. After we dewater that material and settle it, it will become an inner tidal wetland. Another contract had the same muds, but our capacity to [move] that mud into a wetland area was exceeded. A clamshell dredge excavated that material because it was soft and easily excavated. It was deposited in a barge and then towed out into the Gulf of Mexico; the barges were aligned to be dumped to construct topographic relief habitat out in the Gulf.”Dredging on State WaterwaysAlthough USACE is responsible for navigable waterways, states and even private entities may undertake dredging activities as well. For example, the State of Delaware owns and operates two hydraulic cutter-suction dredges. “The primary goal of our program is to dredge and maintain the small boat-navigable waterways within the state outside of the mandated federal projects that are the responsibility of the Corps of Engineers,” states Robert Henry, program administrator for the Shoreline and Waterway Management Section in the Department of Natural Resources and Environmental Control in Dover. “We also do beach nourishment in Delaware Bay with our larger dredge, and we do pond restoration jobs through an agreement with our Division of Fish and Wildlife. We increase the depth and remove noxious weeds to improve the habitat for freshwater fishing in these ponds throughout the state.”Frequency of dredging activities varies according to the specific waterway and the energy regime that it is in. “Much of the small boat channels probably have an average cycle of maybe 15 to 20 years,” states Henry. “Our beach preservation work along the Delaware Bay varies anywhere from four or five years of frequency to up to 12 years. We do a harbor maintenance job for the University of Delaware along the Delaware Bay in Lewes. The harbor there silts in pretty regularly; we probably dredge every four or five years.”
Environmental windows are a big concern to the Delaware program, says Henry. “Depending on the specific project type and location, we have time-of-year restrictions for environmental reasons,” he states. “For example, in the inland-bay areas of our state we have a fairly limited window due to the development of summer flounder and winter flounder. Our limits there are from the first of September through the end of December. In our Delaware Bay jobs we are limited by restrictions on horseshoe-crab spawning and also in some of the lower-bay areas by sandbar-shark pupping. The times of year that are available for us to dredge are generally mid-September through mid-April in Delaware Bay. Wintertime is not a good time to be operating, especially out on Delaware Bay, because of the wave action and the frequent winter storms that affect the area. And every so often we get a particularly hard winter and ice will form on the bay and in the inland bays. That, of course, shuts down our operation also. It’s not a time of year we choose to work in, but it’s the only time that’s available to us as a result of these biological time-of-year restrictions that are imposed through the permitting processes.”Issues Facing the Dredging IndustryFor the industry as a whole, the major issues include funding, environmental restrictions, and the boom-and-starve cycle of program work. “We may see these only increase in the future,” states Henry. His concerns center around both the environmental windows and locating adequate disposal facilities. “Because of newfound knowledge and new concerns over species as more is learned about their habits and life cycles, we may find more time-of-year restrictions, which reduce even more the amount of time that we have in which to do our work.” He also wonders about future disposal sites. “Land is extremely valuable because of the tourism industry—everybody wants to retire near the water, and the price of land is just skyrocketing. It’s becoming harder and harder to find places to lease from people at a reasonable rate in order to dispose of the dredge material.”The boom-and-starve cycle is causing concern for the industry, states Bryant. “Congress has been turning out a lot of work lately and funding it,” he observes. “There used to be a lot more dredging companies around than there are now. There used to be more capacity in the industry, more diversity. Companies waiting for Congress to come out with this work couldn’t hang on, and some of them have just gotten out of the business or went bankrupt. And the remaining industry picked up those dredges, but now there’s still a lot of work coming and dredges are bouncing around from job to job just trying to keep their heads above water. Three or four years from now the dredges will be sitting around looking for work.”DOER and many of the companies maintain Web sites to provide education. The USACE DOER site ( contains extensive reports, studies, and research to provide technical assistance to the industry. “All of our program descriptions and results are electronically available,” says Engler. “We have an interactive Web site where the user community can download our models free of charge. All of our documents are available on a searchable database. The site also has an educational link that we’ve developed to explain the navigational and dredging programs to schoolchildren kindergarten through 12th grade. This educational site came on-line in May and has had more than 1.5 million hits. Almost all are from teachers looking for science information to deal with their students.”