Lake Dredging

The Dredging Feasibility StudyDredging is the removal of accumulated lake bottomsediments ("muck"). The decision to dredge should bebased on sufficient study that shows accumulatedsediment is having an adverse impact on water quality,recreation, or navigation. This decision processincludes defining what you want to use the lake for(such as recreation or water supply) and how you'd likethe lake to appear (water quality), and then determiningif lake sediment is affecting these factors. If so, thestudy should compare alternatives to dredging to see ifsediment removal is indeed the most cost-effectivesolution.

The study should identify the sources of the sedimentcoming into the lake and calculate how much sedimenthas accumulated. Sediment sources from the watershed(such as from construction sites, eroding streambanksand shorelines, urban and agricultural runoff) should becontrolled. Dredging is too expensive to do repeatedly!

Sometimes it is necessary to employ the services of anexperienced professional to conduct a dredgingfeasibility study and help determine if dredging is themost practical alternative. Engineering data and designare almost always needed. The sediment's chemicalcomposition must be tested to determine if contaminantsare present. Disposal sites for the dredged sedimentmust be identified, costs must be calculated, bids mustbe let . . . as you can see, there are many aspects to thedredging decision. This Lake Notes publication isdesigned to give you the background to help make thatdecision.

Why Remove Sediments?Sediments are commonly removed to improvenavigation; restore recreational access for leisureboating, swimming, and fishing; or regain lost storagecapacity in water supply reservoirs. Dredging also isdone to remove nutrient-rich sediments, remove toxic

substances, reduce rooted aquatic plant growth, lessensediment resuspension by winds and waves, and improvefish habitat.

Dredging sometimes can improve water quality byreducing the amount of nutrients available from thesediments, thereby reducing nuisance algae blooms. This can occur through direct removal of nutrient-richsediments, or by deepening the lake enough to allowthermal stratification to develop and thereby limitnutrient movement from deep-water areas to the upperwaters (see the Lake Notes fact sheet “Lake Stratificationand Mixing”). Dredging in areas of rooted aquaticplants controls their growth through direct removal, andalso can limit future regrowth if the new water depthsare deeper than sunlight can reach. For lakes that freezeover in the winter, fish survival can be enhanced byremoving oxygen-demanding sediments and creatingdeeper water areas.

Sometimes, however, sediment removal can dig up theunexpected. Dredging too shallow may uncover fertilesediments and provide a perfect place for aquatic plantsto grow if the bottom gets enough sunlight. Dredgingtoo deep may expose old arsenic treatments (a herbicideused years ago to treat algae) or nutrient-rich wastesfrom a forgotten sewage treatment plant. Beforedredging begins, you'll need to have sediment coresanalyzed by a qualified professional.

Where to Dredge?Sediments can be removed from either the entire lakebasin or just selected spots. Dredging the whole lakeincreases its average depth, but that can be prohibi-tivelyexpensive—and can (at least temporarily) impact thelake's ecosystem. Dredging just in certain areas ("spot"dredging) is less expensive and less ecologicallydamaging since only a portion of the lake bed isdisturbed. Furthermore, a smaller sediment disposal areais needed. Spot dredging can be beneficial in improving

boat and shoreline access, clearing clogged channels orbays, creating hollows for coolwater fishes in thesummer and deeper areas for fish in the winter, andforming boat and fish "cruising lanes" through aquaticplant beds.

How Do You Calculate How MuchSediment to Remove?One method used to determine the accumulatedsediment volume involves setting up a grid pattern overthe proposed dredging area. At each grid point thewater depth and depth to original lake bottom aremeasured using graduated probing poles. A "contourmap" of sediment thickness is created from thesemeasurements. Other methods include the Average EndArea Method and Dobson's Prismoidal Formula. Bothutilize sediment measurements made along transectsacross the lake, providing cross-section or profile views.

For small areas you may be able to do the fieldmeasurements and calculate the accumulated sedimentvolume yourself. For big jobs, more sophisticatedequipment and/or a consultant may be necessary. Resource agencies and consultants can provide guidanceon choosing an approach.

How are Sediments Dredged?For small jobs of a few to no more than 50 to 100 cubicyards, you may be able to do it yourself using buckets,shovels, small pumps, or small excavating equipment. For big jobs, large earthmoving equipment or hydraulicdredges are needed.

MECHANICAL DREDGING can be done either "inthe dry" or "in the wet." Dry mechanical dredginginvolves either partially or completely draining the laketo expose the sediments to drying or freezingconditions. Conventional earthmoving equipment suchas bulldozers, scrapers, backhoes, and draglines are usedto remove the sediment. The equipment either worksfrom shore or moves down onto the dewatered lakebed. The sediment may be stockpiled on shore and thenhauled away in dump trucks to a disposal location, ordirectly loaded into awaiting dump trucks.

Wet mechanical dredging does not involve drawing thelake down. Backhoes, draglines, or grab buckets areused to scoop up the sediment. The equipment caneither work from shore or a floating barge. Wetmechanical dredging usually causes severe sedimentresuspension and turbidity (muddiness).

Mechanical dredging is better at removing hard or rockymaterial than hydraulic dredges. It is useful for smallprojects such as dredging bays and shorelines, and forsmall projects can be less expensive than hydraulic

dredging. It may be the only option if land for asediment disposal basin is not nearby, or if the lake'swater volume is too small to allow hydraulic dredging. Mechanical dredging is ineffective at removing waterydeposits, is labor intensive, and can disrupt lake usage (ifthe lake is drawn down).

HYDRAULIC DREDGING does not involve drawingdown the lake water. The dredging machinery isincorporated onto a floating hull. A cutter with steelblades dislodges the sediments, and a centrifugal pump"sucks up" the muck. This sediment/water mix (called aslurry) is piped to a disposal basin on land where thewater is drained off and the sediments are left to dry. Because lake water is removed along with the sediment(hydraulic dredging slurries are commonly 80-90 percentwater), this type of dredging requires that enough watervolume is maintained in the lake to keep the dredgefloating.

Hydraulic dredging is faster than mechanical dredging,creates less turbidity than wet mechanical dredging, andcan effectively remove loose, watery sediments (greaterthan 70percent water). It is typicallythe most cost-effective methodfor large dredgingprojects (though somesmall projects also maybe more economicalusing hydraulicdredging).

Two types of hydraulic dredges are best suited for mostIllinois lake sediments: horizontal auger and cutterheaddredges.

A horizontal auger dredge utilizes vertical "knives" ona rotating auger to cut and loosen the sediments. The

auger is mounted on aboom at the front of thedredge hull. The dredgemoves across the lake bya winch attached to a tautcable, and it can cutwhile moving forward orbackward.

These types of dredges workwell for soft silts and cause

little turbidity. The auger dredge is smaller than acutterhead dredge and removes sediment in a moreconcentrated form (less water is removed from thelake). The dredge operator can closely control the augerposition and cutting depth. Horizontal auger dredgescan remove sediments in up to 20 feet of water (30 feetwith extensions), and can remove up to 120 cubic yardsof silt per hour.

The cutterhead dredge utilizes a rotating "basket"comprised of smooth or toothed blades (depending onthe sediment consistency) to dislodge the sediment. Thecutter is mounted on a boom ("ladder") at the front ofthe dredge hull. The ladder also supports the suctionpipe. The cutterhead is "swung" through the sedimentto cut it as the is dredge stabilized by one of two spuds(vertically mounted pipes located at the rear of thedredge on each side) pushed into the lake bottom. Aftera swing is completed, the dredge "walks" forward usingthe spuds. Cutterhead dredges are larger, can dredgedeeper (up to 26 feet, or 55 feet with ladder extensions),and have higher production rates (150–350 cubic yardsper hour) than horizontal auger dredges. They can cutcompacted sediments and some original lake bottomsoils, and can work where there is debris and weeds. Aswith auger dredges, the cutterhead dredge operator canclosely control the cutting depth.

Where are the Sediments Put?Croplands and pasturelands are commonly utilized forland-application of trucked sediments or for sitingdisposal basins. Landfills are another option for truckedsediments, although landfill fees greatly increase projectcosts. The closer the disposal site is to the lake, thebetter. In urban areas where open land is scarce, findinga proper site nearby may be very difficult. Generally,dredging is more cost-effective when the disposal site islocated within one mile of the dredging area. However,it is not uncommon to hydraulically pump sediments upto two miles or more, or truck sediments even farther. Dried sediments later may be incorporated into thelandscape at the disposal site. Alternatively, the driedsediments may be removed and used as fill or disposedof elsewhere.

For hydraulic dredging projects, a disposal basin mustbe designed and constructed to hold not only all thedredged sediments once they've dried, but also be largeenough to allow the pumped sediment/water slurryenough time to settle out the solids and return relativelyclear water to the lake. Also, because lake sedimentstend to increase in volume temporarily during hydraulicdredging, a "bulking factor" is added when designingthe disposal basin. A 120% bulking factor is typical. For example, if 10,000 cubic yards of sediment were tobe dredged, the disposal basin would have to be sized tohold 1.2 times 10,000—or 12,000 cubic yards.

Geology of the disposal site must be considered as well. Some sites may be too porous and could allow nearbylakes or streams, or the groundwater table, to beimpacted. Costs increase when the sediments must betrucked far away or the disposal site modified to containthe sediments. If the lake sediments themselves arefound to have elevated concentrations of contaminants(e.g., heavy metals, certain pesticides), special handlingand disposal will be required. Nutrient-enrichedsediments are not considered contaminated and thereforedo not require special handling.

CostsAny costs associated with conducting the dredgingfeasibility study are separate from actually removing thesediments from the lake. As you've probably heard,dredging can be expensive. However, when coupledwith measures to control soil erosion in the watershed,the effectiveness and benefits of dredging can be longer-lived.

Dredging contractors are usually paid on the basis ofcubic yards of sediment removed. Dredging costs canvary greatly—from $5 to $15 per cubic yard forhydraulic dredging (including engineering design andconstruction of the disposal basin), and from $8 to over$30 per cubic yard for mechanical dredging projects(including disposal). It's a good idea to get quotes fromseveral experienced dredging contractors.

As an example, let's calculate how much it might cost toremove 1 acre of sediment 5 feet thick that has clogged achannel. This equals 5 acre-feet or 8,067 cubic yards ofsediment. Using a hydraulic dredging cost of $15 percubic yard for such a relatively small project, thisequates to about $121,000.

Costs can be reduced by scaling back your project,pumping or hauling shorter distances, or, if possible,conducting the work during winter when constructionequipment tends to be idle.

PermitsDredging projects, both mechanical and hydraulic, aresubject to permitting by the U.S. Army Corps ofEngineers, Illinois Department of Natural Resources–Office of Water Resources, and Illinois EnvironmentalProtection Agency. It is best to begin the permittingprocess as early as possible during your project'splanning phase. You can contact Illinois EPA's Divisionof Water Pollution Control, Permits Section, at 217/782-0610 for more information and to request a permitapplication.

Lake Notes is a series of publications produced by the Illinois Environmental Protection Agency about issues confronting Illinois' lakeresources. The objective of these publications is to provide lake and watershed residents with a greater understanding of environmental cause-and-effect relationships, and actions we all can take to protect our lakes.

Hydraulic auger dredge illustration courtesy of Mudcat Division of Ellicott International. Hydraulic dredging operation illustration courtesy of Tennant’s Industrial Dredging. Cover illustration from University of Wisconsin-Extension’s Lake Leaders Handbook.

This Lake Notes publication was prepared by Holly Hudson of the Northeastern Illinois Planning Commission, Chicago, Illinois.Thanks are extended to Peter Berrini, Bruce Yurdin, and Bob Kirschner for their review and comments.

For more information about other publications in this series and to request copies, please contact: Illinois EnvironmentalProtection Agency, DWPC-Lake and Watershed Unit, P.O. Box 19276, Springfield, Illinois, 62794-9276; 217/782-3362.

June 1998. Permission granted to reprint with credit to the Illinois Environmental Protection Agency and theNortheastern Illinois Planning Commission.