The need to remove contaminated sediments located under a column is increasing. Modern, small hydraulic dredges often are the most logical choice for the task.
Small dredges typically have from 6-inch to 10-inch discharge diameter and float on the water as an integral unit. These dredges usually are configured with horizontal auger or basket-type cutterheads.
Advantages to hydraulic dredging include the fact that the body of water or lagoon does not have to be drained and can remain in service. The entire water surface area can be reached by the dredge, including center portions impossible to reach with a shore-operated dragline or backhoe. Sediment-water mixture is pumped through a leak-proof pipeline system and discharged into a safe disposal area where it cannot return to the dredging location. There is also little or no disturbance to surrounding land by heavy construction equipment.
As early as 1974, EPA was concerned about turbidity during dredging operations and especially contaminated sediments.
Turbidity, the re-suspension of bottom sediments into the water column, appears as a “plume” in the water. This plume usually extends from the surface to the dredge cutter. The extent to which turbidity is caused determines the amount of material that settles to the bottom and is not picked up by the dredge. In any type of contaminated sediment removal, turbidity must be kept to a minimum because there is the danger of interaction with the water column.
Reports on sediments and their removal were published in 1974 and 1976, and the findings of both clearly indicate that small hydraulic dredges using the horizontal auger excavating concept are very effective in the control of turbidity. In later years, modern conventional basket-type cutterhead dredges also became effective tools for this purpose.
In 1989, EPA released an initial report showing 495 rivers, lakes and other waterway segments were contaminated by one or more of the 126 toxins coming from a total of 764 facilities in 55 states and territories. This list continues to grow, and in 1991, EPA began publishing “Contaminated Sediments News” to disseminate information on this complex problem affecting U.S. waterways.
Because a high percentage of contaminated sediments is pumped to sludge de-watering equipment that usually cannot handle high flow rates, the use of 6-inch to 10-inch discharge diameter dredges is excellent because of lower flow rates (500 to 3,000 grams per minute or gpm). These dredges pump a consistently high percentage of solid in the slurry. The use of submersible dredge pumps is playing a major role in increasing solids content and consistency of slurry density. Uncontaminated sediments generally are pumped to safe, properly designed upland dredge disposal areas for natural de-watering and re-use of material.
Surgical dredging is the use of small hydraulic dredges to excavate sediments in very exacting cross-sectional dimensions with disturbance to the immediately adjacent aquatic environment held to an absolute minimum.
Not only must these smaller dredges perform surgical dredging with maximization of sediments removal, with as close to in-situ removal as possible, they must do this with the least amount of turbidity or re-suspension of the bottom sediments back into the water column.
Contaminated and uncontaminated dredging applications include:
- Natural bodies of water such as bays, rivers, inlets, estuary areas and wetland populated with aquatic life and wildlife. Industrial ponds and artificial lagoons (evaporation, holding, cooling, retention, firefighting and sludge treatment ponds.) Drinking water ponds or water supply lakes containing contaminated and uncontaminated sediments that must be dredged while the lake is in service. Environmentally sensitive areas such as recreational lakes, bird sanctuaries, selected dredging in wetlands and flood control reservoirs. Virgin soil that has become contaminated by overlying sediments that present new challenges in excavating hard materials.
Turbidity, measured in nephelometric turbidity units (NTUs) on a numerical scale for cloudiness, is an optical property that depends on fluid and particle characteristics and the instrument used to measure it. Turbidity can be determined from sensors mounted in the vicinity of the dredge cutting head or from water grab samples that are “read” in the laboratory.
The single most important function that a dredge can perform is the maximization of sediment removal as excavated.
Surgical dredging is only possible using small hydraulic dredges. The following are some examples:
Dredging a channel in a water supply lake from the lake center to the water plant intake over 3,000 feet long, 40 feet wide and 20 feet deep, to enable the plant to remain online during drought conditions.
Cutting a channel 9 feet wide and 4 feet deep through a wetland in a bird sanctuary 1,000 feet long to permit water flow during times of low rainfall. Cutting drainage channels in large dredge disposal areas to accelerate water removal for faster drying.
Dredging along lake shorelines and leaving existing aquatic vegetation in selected areas. Cutting deep channels in recreation lakes to prevent fish kills during very cold winters. Dredging sediment around existing marine structures such as piers, mooring facilities and pile clusters where damage to the structures is held to a minimum. High concentrations of contaminated sediments usually are found in these areas.
Removing very thin layers of contaminated sediments from an industrial lagoon ranging in depth from only 2 inches to 6 inches.
When dredging is complete, the lagoon bottom is flat with no ridges of remaining sediment.
Small horizontal cutter dredges can accurately cut to 2 inches in depth. Small basket type cutterhead dredges can control sediment cuts to 4 inches in depth.
Because all cutter functions are hydraulically operated, the cutter hydraulic pressure gauge reading in the operator’s cab can be correlated to the sediment being excavated. For example, should the cutter engage hard virgin soil, the hydraulic pressure will increase instantaneously and the dredge will start to vibrate, indicating to the dredge operator that the cutter must be raised.
Types of Dredges
Basically, there are two classes of small hydraulic dredges: horizontal cutters and basket-type cutterhead dredges.
With the horizontal cutter, the cutting action is forward or reverse with cable travel. The auger flighting is continuous or non-continuous and mechanically feeds the sediment to the intake of the pump. It contains shrouding around the auger to control excessive water intake and turbidity. The dredge pump location can be either in the dredge hull at the waterline or submerged directly behind the auger horizontal dredge.
The nature of the sediment to be dredged plays a predominant role in the dredging project. Gone are the days when project managers observed, felt and smelled the sediment at great risk. Now, geotechnical analysis provides information on sediment type, gradation, liquid limits, plasticity index, in-situ percent solids by weight and low counts for compacted virgin soils- without the requisite hazards.
Horizontal auger dredges are excellent for soft, loose colloidal sediments and sludges with slurry flow rates in the 500 gpm to 2,000 gpm range, but they do not perform well in compacted sand and virgin soil.
Digging depths range from 30 feet to 150 feet with flow rates in the 2,000 gpm to 3,500 gpm range. The draft is 2.5 feet.
Basket-type cutterhead dredges can cut soft, loose sediments and also will dredge compacted sand and virgin soils.
Surgical dredging and control of turbidity during operations using modern, small hydraulic dredges involves many factors such as proper dredge selection, cutter configuration and dredge outfitting.
The material to be dredged is of critical importance, and generally speaking, if the dredge has difficulty excavating the sediment, turbidity will increase substantially.
Geotechnical analysis of the sediment is an absolute requirement and plays an important role in dredge selection.
The use of modern, small hydraulic dredges for operations requiring low turbidity, low resuspension of sediments, surgical dredging procedures, maximization of sediment removal and the ability to dredge accurately is increasing every year. Industry, government agencies and municipalities are required to restore bodies of water containing either contaminated or uncontaminated sediments to meet new water pollution control regulations.