Dredging as a Restoration Tool

For Lake Water Quality Improvement

Updated for:

St. Johns River Water Management District

Prepared by:

404 SW 140th Terrace

Newberry, FL 32669

Judith L. Dudley, PhD

Principal Scientist

Ann B. Shortelle, PhD

Chief Scientist

January 2008

Note: The original draft of this paper was produced by Mellissa Warren and Dr. Larry Battoe, and was 

completed July 7, 2003. This draft was updated by Dr. Ann B. Shortelle of MACTEC, September 2007, 

with comments and changes from Drs. Sherry Brandt-Williams and Larry Battoe, January 2008.
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Introduction 

The deterioration of lakes through cultural eutrophication is a global problem. Research on lake 

restoration techniques has blossomed over the past 50 years and lake restoration methods have been 

developed and tested all over the world. A primary purpose of restoration research is to find techniques 

that will improve water quality sufficiently to meet local standards (Sondergaard et al. 2000). In Florida 

and elsewhere, many shallow lakes are considered impaired waterbodies due to excessive external 

nutrient loading and related parameters such as dissolved oxygen. As directed under the Clean Water 

Act, total maximum daily loads (TMDLs) are developed for an impaired waterbody to provide a

“roadmap” to curtail external loading to restore the impaired waterbody by improving water quality. 

Communities with impaired waterbodies have latitude to use various technologies and strategies to meet 

nonpoint source load reductions. Because of this, the primary focus of Florida’s watershed management 

program is management of external nonpoint sources of pollution, especially stormwater, which is 

responsible for greater than half of the loading to Florida’s waterbodies (Livingston and Crane-Amores 

2007). In addition to addressing watershed (external) loading of nutrients to lakes, many restoration 

techniques, including dredging, are “in lake” techniques. For an impaired waterbody, dredging is 

generally considered a technique to increase the assimilative capacity of the waterbody.

Rapid sedimentation (solids being brought into the lake from the watershed by nonpoint sources such as 

stormwater runoff) is one symptom of cultural eutrophication, and dredging is one restoration technique 

that has been used to remove sediments that hold high levels of nutrients. One rationale behind dredging 

to restore lakes is that removal of nutrient-laden sediments will curtail internal recycling of nutrients from 

the sediment layer. Based upon our review of recent literature, although improvement in water quality is 

an often cited objective for dredging, it is commonly not the primary justification for dredging (e.g. Allen 

2000, and others). Other, more common justifications for dredging are to deepen areas for navigation 

and recreation, restoration of flushing, restoration of fisheries habitat, restoration of water holding 

capacity for power generation and/or flood control in reservoirs, and to remove contaminated sediments.

Literature covering the direct long-term effects of dredging on water quality is scarce, and an accurate 

assessment of the results of dredging is difficult. There are three factors that restrict the ability to assess 

the effectiveness of past lake restoration: the unavailability of data and insufficient analysis of data that 

are available, the task-oriented approach to dredging, and the lack of clear project goals and objectives 

(NRC 1992). 

Continual surveillance of lake conditions for a sufficient period of time, both before and after dredging 

has been attempted, and has been conducted on few lakes. In some cases, sufficient data are collected, but 

analysis and interpretation of the data are deficient; therefore, a comparative analysis of lake conditions 

pre- and post-restoration dredging is inconclusive. Pierce (1970, as cited in Peterson 1982) reviewed 49 

projects and concluded that none provided sufficient data to determine the overall effects of lake 

dredging. Currently, more long-term in-lake water quality monitoring data are available, but surprisingly 

few attempts have been made to evaluate these data for changes in water quality associated with dredging. 
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These data sets do not necessarily meet stringent statistical assumptions for analysis of effects of dredging 

on water quality, because the monitoring was not designed specifically for this purpose (Dierberg and 

Williams 1989).

A second problem with the assessment of sediment removal as a water quality restoration technique lies 

within the task-oriented approach to dredging activities. Sediment removal is often seen as the one-time, 

permanent solution to a lake's problems and not as an ongoing process. The potential adverse ecological 

effects of the "activity" are considered short term and secondary to achieving the project goals, and 

environmental impacts are infrequently and unsuccessfully monitored. 

And finally, it is difficult to assess the degree of success of restoration projects when project goals and 

objectives are not clearly defined (NRC 1992). In some cases, the amount of sediment to be removed is 

not accurately calculated. More frequently, sediment volume is defined, but water quality improvement 

goals are not specifically defined, and other environmental parameters are not considered. In most cases 

where water quality improvement is cited as a project objective along with other goals, dredging is 

conducted and the project is declared successful on the basis of the other goals (e.g. restoration of 

navigable depths), and water quality is not specifically monitored post-dredging to determine if project 

objectives related to water quality improvement are met.

The purpose of this paper is to summarize available information on sediment dredging and its potential 

long-term effects on the water quality of shallow lakes. Current documentation of the beneficial effects of 

sediment removal on water quality appears to be equivocal; however, several points were repeated in the 

literature. Successful and unsuccessful dredging projects conducted globally over the past three decades 

have helped to define the possible responses lake water quality will have to sediment dredging operations. 

External loading must be adequately addressed prior to dredging to anticipate a successful outcome (e.g. 

Annadotter et al 1999, Birch and McCaskie 1999, IETC 2000, and others). Setting realistic goals, 

objectives, and cost estimates based on adequate pre-dredging studies of lake ecosystem characteristics, 

combined with correct dredge selection, competent dredge operation, and post-dredging monitoring will 

increase the possibility of sediment dredging operations improving lake water quality. 

Dredging as a Restoration Technique

A major objective of sediment removal is to counteract cultural eutrophication through the removal of 

organic matter and nutrients that stimulate the growth of algae and macrophytes. Cultural eutrophication 

is the rapid and human-induced acceleration of the natural aging processes in lakes (Klein 1997). The 

results of cultural eutrophication are increased lake water fertility due to higher concentration of plant 

nutrients, primarily phosphorus and nitrogen. Elevated nutrient concentrations lead first to an increase in 

submerged and emergent plants, and then to algal blooms, because algae can eventually out-compete 

other plants for nutrients. Increased algae cause turbid water, the shading out of other native vegetation 

and increased sediment production. The new rapidly accruing layer of sediment may itself serve as a 

nutrient source fueling an internal nutrient cycle. Wildlife also is affected with less valuable fish species 
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replacing game fish; this may be attributable to organic sediments covering sandy bottomed littoral zones, 

and loss of native macrophyte fish spawning beds (Allen 2002). 

Both internal recycling and external loading of nutrients contribute to cultural eutrophication, and both 

need to be minimized in order to restore lake water quality. Internal nutrient recycling from aerobic and 

anaerobic sediments, from groundwater seepage, from decomposing macrophytes, and from organism 

activities, can add nutrients to the water column. Lakes in agricultural areas and in urban areas without 

stormwater treatment are likely to receive heavy external nutrient loads. The idea that dredging will 

restore water quality results from the desire to break the internal recycling of nutrients from the 

sediments. Removal may aid in controlling the release of phosphorus and other nutrients into the water 

column and will thus decrease the rate of cultural eutrophication. Studies have shown that in some lakes 

with significant internal recycling, as much as 66% of phosphorus loading can be attributed to sediment 

(Klein 1997). Thus, water quality improvement as a result of sediment removal is commonly listed as an 

objective of dredging projects. 

Lake and Watershed Characteristics Affect the Outcome of Dredging 

Lakes will respond differently to restoration techniques depending on their morphology, hydrology, and 

history of eutrophication (Van Liere and Gulati 1992). The understanding of lake behavior and lake 

characteristics is essential to predicting the effects dredging will have on the lake sediments. Morphology, 

hydrology, and basin land-use history are general lake and watershed characteristics that will influence 

the impact dredging has on water quality. These general characteristics of a lake and watershed are 

reflected more specifically in the sedimentation rate, water residence time, sediment distribution, 

phosphorus release rate, and external phosphorus loading. Understanding these characteristics and how 

they potentially affect the outcome of a dredging project should be evaluated prior to selection of 

dredging as a restoration technology for water quality improvement.

Dynamics associated with phosphorus recycling from sediments are known to affect whether or not water 

quality improvements will be delayed following reductions in external loading in shallow lakes (Boers 

1991). Boers found that the concentration of phosphorus in the sediments and the P:Fe ratio influenced 

the extent of internal recycling, and the retention time of the waterbody influenced any delay in water 

quality improvement following restoration. Residual external loading may also lengthen or prevent water 

quality improvements. Experts agree that the most important factor associated with the success of 

dredging to reduce internal recycling is the control of external loading sources. Sediment accumulation 

rates associated with uncontrolled external nutrient loading will determine the amount of time before any 

potential benefits of dredging are lost. If external nutrient loading is uncontrolled or insufficiently 

controlled, any benefits of dredging will be short lived and the cost-efficiency of dredging will be low. 

Sediment removal would have to continue for long-term restoration, thereby increasing the cost of 

complete lake restoration (Peterson 1982). On the other hand, sediment accumulation without elevated 

nutrient concentrations entombs sediments with higher concentrations of nutrients at depth (GLCS, 1994, 
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and others). In general, the smaller the ratio of watershed area to lake surface area, the greater the 

assurance lake dredging results will last. 

Water residence time or flushing rate will influence the negative impact nutrient release will have in the 

water column. Dredging in lakes with a longer residence time will increase the time re-suspended 

sediments will remain in the water column post-dredging. Short residence times combined with good 

quality water supply may deplete the phosphorus load in the sediments and accelerate phosphorus 

removal (Van der Doeas et al. 1992). Knowing a lake's residence time allows researchers to forecast the 

flushing capabilities of the lake and to predict potential post-dredging results. In dredging operations 

where the goal of sediment removal is to increase lake depth, residence times often decrease. On the 

other hand, in reservoirs and lakes associated with rivers or coastal areas, dredging may restore beneficial 

flushing. In this case, the reinstituted flushing is generally the direct cause of any water quality 

improvement (Cooper Consultants, 1985).

Sediment characteristics and distribution will determine the dredging depth, the equipment used, and the 

cost and direction of dredging. Dredging may be more beneficial when sediments with high 

concentrations are addressed first and when the correct dredging depth is set. Dredging sediments with 

higher nutrient concentrations first, before less-contaminated sediments, could prove more cost-effective 

and take less time. Sediment distribution throughout the lake will influence the dredging depth. Certain 

areas may require greater amounts of removal than other areas. The dredging depth will therefore affect 

the time, cost, and equipment used. If a dredging depth is set deeper than the appropriate depth, greater 

cost will be incurred and alternative equipment may be required. If dredging depth is shallower than the 

required depth (for example Annadotter et al. 1999), costs will be reduced but the amount of sediment 

removed will not be sufficient to restore the lake. Conditions that are most appropriate for dredging are 

small lakes with organically rich sediment, low sedimentation rates, and long hydraulic residence times 

(Graetz et a1. 1988, IETC 2000). Dredging is also most successful in well-flushed systems. 

Sediment phosphorus release rates decrease with sediment depth because, in general, older, deeper lake 

sediment was deposited during a period when the lake basin was less impacted by human activity and 

both the rate of deposition and the nutrient content were lower. Exposing sediment with slower nutrientrelease rates will minimize the adverse impact of nutrients in the overlying water column, and result in a 

decrease of material to be dredged. Phosphorus-release studies from various depths need to be carried out 

before dredging commences to determine dredging depth. If phosphorus release rates from deeper 

sediment are less than release rates from surface sediments, then dredging depth would be minimized, the 

costs of dredging would decrease, and lake restoration might become more feasible. Lakes with enriched 

surface sediment relative to underlying sediment could benefit from dredging (Peterson 1976). If deeper 

sediment layers contain greater phosphorus concentrations, then dredging may expose the lake to higher 

sediment phosphorus recycling (reference we talked about).

Nutrients from sources such as sewage effluent and agricultural runoff enter the water body and stimulate 

production of organic material that creates enriched sediment. Any re-suspension of sediments may make 
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sediment nutrients available again. For example, the external load into Lake Zuideinderplaus, Holland, 

was reduced by 90% and resulted in only a 46% decrease in phosphorus (Van der Doeas et al. 1992). In 

many cases, controlling external loading of nutrients is sufficient for restoration and will result in a 

proportional decrease in lake phosphorus concentration (Sas 19xx). However, in some lakes, the change 

in biota, loss of habitat, physical changes in bottom sediments, and lake hydrology have been so dramatic 

that reducing the external loading will not proportionally improve water quality within the desired amount 

of time (NRC 1992). 

Controlling external as well as internal phosphorus loading is a very important feature in lake restoration. 

Reducing the external load into a lake will slow the re-accumulation of phosphorus in the lake sediment 

in the years after sediment dredging (Recknagel et al. 1994). Kleeberg and Kohl (1999) predict that 

dredging without reduction of the external loading may give only temporary improvement followed by a 

slow return to the present situation. In lakes where internal phosphorus loading is larger than the external, 

dredging is suggested. Failing to stop external loading into the lake in many cases will result in a failed 

restoration effort. Many examples may be found in the literature, of such failures. For example, in Lake 

Eola, dredging with drawdown was conducted in 1972, but the lake continued to degrade. Stormwater 

diversions were not attempted until 1979-1982 (Dierberg and Williams, 1989). See also IETC 2000, 

Jeppesen et al. 1999, Keizer et al. 1992, and others, for additional examples. External loading likely 

contributed to the outcome of some dredging projects in Florida lakes (see case studies below).

A dredging operation conducted by Martin Sondergaard et al. (2000) on Lake Braband, Denmark, showed 

that the dredging of the lake led to an increase in water depth and a reduction in phosphorus release from 

the lake bottom. The internal loading of the lake decreased significantly after the dredging, compared to 

other lakes where only the external loading had been reduced, but water quality remained below standard. 

The turbid state of the water in Lake Braband is due to the high levels of nutrients that continue to enter 

the lake through external sources. The results confirmed that dredging can reduce both the duration and 

the size of internal nutrient loading; however, external nutrients need to be stopped for dredging to be 

considered a long-term restoration technique (Sondergaard et al. 2000). 

The cessation of external sources of nutrients is expected to be a necessary first step in a restoration 

process encompassing further remedial techniques. When the phosphorus input is reduced drastically, 

supplementary measures (e.g., dredging) will be more successful (Van Liere and Gulati 1992). 

Alternatively, once external sources are controlled, internal recycling of nutrients may quickly decrease, 

and less costly, in-lake restoration methods may be as successful.

Pre-Dredging Studies Needed 

Sediment investigation prior to the commencement of dredging operations is essential to ensure effective 

long-term restoration results. In addition to sediment characterization, a thorough evaluation of lake water 

chemistry both before and after dredging takes place is a necessity so that both long- and short-term 
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changes due to dredging may be documented. Turbidity, total dissolved solids, conductivity, dissolved 

oxygen, and pH all need to be assessed. Adequate pre-planning studies will help to define the magnitude 

of the problem and will disclose unique lake characteristics that may help the appropriate use of dredging 

technology while preventing misuse. Feasibility studies for sediment removal of Lake Apopka (Graetz et 

al. 1988) [note to Larry from Sherry for follow-up: we need to look for more recent summary of decisions 

about dredging on these lakes] and Lake Okeechobee (SFWMD 2003) have resulted in decisions not to 

dredge. Both of these lakes are large with significant volumes of nutrient laden sediments. Cost was a 

major factor associated with the feasibility of dredging. One example of the effects of inadequate predredging studies can be found in the attempted restoration of Lake Kasumiguara, Japan. The failure of 

this seven-year dredging project was due to inadequate pre-evaluation studies of nutrient budgets and 

sediment distribution and characteristics prior to restoration implementation. The removal of 300,000 

m

3

of sediment over the seven-year period had no effect on the water quality of the lake. Subsequent 

calculations revealed that an estimated 20,000,000 m3of sediments would need to be removed for a 

15% reduction in organic loading (Graetz et al. 1988; Murakami 1984). This example demonstrates 

the need for preliminary data collection in order to prevent project cost inefficiency, to provide 

realistic goal estimations, and to recognize that each lake contains unique characteristics that can 

affect the efficiency of restoration efforts. The cost efficiency of carrying out a 20,000,000 m3

sediment removal project is questionable. Other restoration techniques might have been considered 

had a pre-evaluation taken place. 

Cook et al. (1986, 1993) consider pre-dredging studies to be the most important aspect of a dredging 

project. Specifically, they suggest that a sediment removal project will be more effective if a thorough 

pretreatment evaluation of a nutrient mass balance – including behavior of the sediment and lake history 

from sediment core analysis – is conducted. 

Peterson (1982) suggests an understanding of sediment characterization is essential before dredging is 

conducted. Sediment characteristics determine particle behavior during dredging and can help predict the 

appropriateness of dredging activities. For example, particle size will determine the nutrient concentration 

and the length and ability of the particle to be re-suspended. Smaller diameter particles often contain 

higher concentrations of contaminants, remain re-suspended for longer periods of time, and are more 

easily re-suspended (EPA 1993). The chemical composition and physical characteristics of the sediment 

also determine the release of contaminants into the water column pre- and post-dredging (EPA 1993). 

Pre-dredging studies need to include an evaluation of the sediment depth and distribution, mineralogy, 

tendency to deposit, organic content, bulk density, and toxicity content. Because of the high cost of 

dredging, dredging depths must be optimized to uncover strata containing less readily exchangeable forms 

of phosphorus. However, since net P release is a function of the P supply via sedimentation, reduction of 

P deposition rates by reducing external P loading may be more effective than dredging in many lakes 

(Kleeberg and Kohl 1999). The necessity for an in-depth pre-dredging study has been documented in 

successful projects and has been suggested as an improvement tool in failed projects
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Understanding the unique qualities of each lake ecosystem before dredging is critical to restoration 

success. A project carried out on the Norfolk Broads, a series of shallow, man-made lakes in Eastern 

England required a thorough investigation to understand the high phosphorus peaks present in the lake. 

The lake experienced patterns of high phosphorus release as a result of periodic development of aquatic 

vegetation. The cyclical nature of aquatic plant growth created periods of peak phosphorus release from 

the sediments, which confused the interpretation of the impact of sediment removal on phosphorus release 

(Phillips et al. 1999). The natural but unusual lake patterns overshadowed the effects of dredging on the 

water quality. Realizing the cause behind the phosphorus peaks enabled a clearer understanding of the 

dredging results. Phosphorus did decrease after sediment removal, but levels remained higher than 

previous years due to cyclic highs. 

Dredges and Equipment 

Sediment removal to retard nutrient release may only be effective if the correct equipment is used. 

According to the U.S. Environmental Protection Agency (USEPA) (1993), there are no remedial 

technologies that can remove or treat contaminated sediment without some disturbance and consequent 

release of contaminants, in this case nutrients. Therefore, the selection of equipment used to dredge a lake 

should be based on its ability to minimize re-suspension. 

Dredging equipment can be divided into four broad categories: mechanical, hydraulic, special purpose 

and pneumatic. All four types of dredges vary in their capabilities according to the sites in which they 

operate most efficiently, their production rates, the sediment re-suspension rates, and operational depths. 

Mechanical dredges include clamshells, dippers, bucket ladder dredges, draglines, and conventional 

earthmoving equipment. Mechanical dredges operate most efficiently in near-shore areas that contain soft 

to stiff mud and have a limited operational depth of no more than 30 to 40m. Although mechanical 

dredging offers the advantage of high-density recovery, it generates high re-suspension of bottom 

sediment, particularly in the fine-grained range. Since the fine-grained sediment is often highly 

contaminated, the higher re-suspension can cause increased contaminant release (EPA 1993). The 

production rate is also relatively slow due to the time-consuming bucket swing, drop, close, retrieve, 

lift, and dump operating cycle. If properly operated, conventional clamshells can remove sediment 

with minimal loss of sediment. Modifications to the conventional clamshell – the closed bucket 

clamshell – are routinely used in contaminated-sediment dredging projects by the U.S. Army Corps 

of Engineers and reduce the amount of re-suspension material by 30% to 70%. 

Hydraulic dredges include portable, hand-held, plain suction dredges, hopper dredges, cutter head 

dredges, and dustpan dredges. Hydraulic dredges generally exhibit higher production rates and lower resuspension than mechanical dredges and, therefore, are more appropriate for environmental dredging. 

Hydraulic cutter head dredges are considered "environmentally cleaner" than other grab-and-bucket 

dredges because of the closed-cycle operation system. Other advantages include the ability of this type of 
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dredge to remove large volumes in short times and to span any horizontal reach because of its floating 

platform (Peterson 1982). They are common in inland lake dredging projects because of this advantage. 

One disadvantage is the operational depth, which is determined by the ladder length or steel boom length. 

Pneumatic dredges are within the forefront of dredging technology. Pneumatic dredges include airlift 

dredges, the Pneuma® and the Oozer®. These types of dredges produce slurries of higher solid 

concentrations than hydraulic dredges and cause less re-suspension of bottom materials. They operate 

most efficiently in soft, viscous sediment commonly found in many lakes and their operational 

depths are greater than 10m. Even though there are several advantages to using pneumatic dredges, 

they have not caught on in the United States or were completely unavailable (Cooke et at. 1993). 

Special purpose dredges were created due to the challenge of the soft lake sediments. The special purpose 

dredges include the cutter-head design used on the Mud Cat®. Solid production rates are increased, resuspension is minimized, and far less water volume must be contained at the disposal site with these 

dredges. This type of dredge is ideally suited to small lake restoration projects with operational depths of 

3 to 18m (Cooke et al. 1993). 

Another special purpose dredge created to be "environmentally cleaner" is the Ecograb dredge. It has 

many advantages, such as removing very thin layers of sediment with high precision. Turbidity and 

dispersion of silt particles are also avoided because all openings are sealed off. The Ecograb is best suited 

for small or shallow areas such as river banks. However, this new dredge has limited use due to budgetary 

constraints. 

Specialized dredging technology has been developed in Japan and the Netherlands to reduce sediment resuspension. Specialized dredges typically have narrow or shrouded dredge head openings to minimize 

suspension and operate with slower productivity at a higher cost. The cost of dredging using conventional 

equipment is high, and lake restoration using this technology is often not economically feasible. 

Specialized dredges will reduce the environmental impact of restoration but with slower productivity, 

they will increase the length of the project and create an even higher cost. If feasibility and conditions 

permit, pneumatic dredges are recommended for environmental dredging. Pneumatic dredges are 

extensively used in Europe and Japan but have only limited availability in the United States. Dredging 

projects in the United States tend to use conventional hydraulic cutter head or horizontal auger dredges or 

mechanical clamshell dredges that cause higher re-suspension rates (Cushing 1998). 

If specialized dredges are not feasible, other measures can be taken to reduce the re-suspension rates 

caused by dredging action. Re-suspension disturbance can be minimized by operational controls, 

including reducing cutter head rotation speed, slowing dredge head movement, reducing the depth of cut, 

and increasing the suction rate (Cushing 1998). Sustar (1979) concluded that the amount of sediment 

disturbance is related to the sediment type. Even well run equipment can cause high turbidity. 
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Unanticipated problems with equipment were encountered in the restoration of Liberty Lake, Washington. 

The equipment operator was not able to locate the top 60 centimeters of light organic sediment, and 

dredging began below the nutrient-rich surface sediment. Post-dredging studies indicated that the 

horizontal cutter head dredge missed areas between the dredged swaths, resulting in a trough-and-mound 

situation and a portion of the dredged area still being covered by nutrient-rich sediments. Only an 

estimated 40% of the dredged area was dredged efficiently. In addition, some macrophytes were left with 

the sediment, resulting in a rapid reestablishment of macrophyte beds. Other problems, including an 

increase of re-suspension, can be encountered when an incorrect dredge size is used. 

Dredging Costs 

Cost comparison among projects is difficult due to the large number of variables such as, type of 

equipment used, productivity rate, dredging time, type and amount of sediment to be removed, dredging 

depth, lake size, ecological considerations, the competence of the dredge operators, and the method of 

sediment transportation and disposal. 

The cost of dredging has increased over the past two and a half decades. Cost analysis research conducted 

by Peterson (1981) on 64 dredging projects revealed a cost range of $0.18 per cubic meter to 

approximately $10 per cubic meter-in: 1975. The cost analysis research found that costs from $0.95 to 

$1.33 per cubic meter were common and could be considered reasonable for hydraulic dredging (North 

American Lake Management Society 1988). More recent ranges are $0.36/m3to $21.00/m3 without 

significant disposal issues (e.g. contaminants present). Because of the wide variability in costs, it is not 

helpful in estimation for a single project. Furthermore, this estimation does not include disposal, transport, 

or monitoring costs. EPA (1993) estimations for dredging costs range from $1 to $25/m3and dredging of 

contaminated sediments is even more costly. The typical range for contaminated sediments is $5 to over 

$25/m3. Simple cost estimators are also available (Cervie et al 1999).

Initial costs of dredging for selected Florida lakes vary widely, by approximately one and one half orders 

of magnitude, and are sometimes as costly as sewage treatment plant effluent diversion projects (Dierberg 

and Williams 1989). Costs are primarily governed by the amount and quality of sediment to be dredged 

along with accessibility of dredge disposal sites. Cost information, when available, is included in the case 

studies presented below.

Environmental dredging usually has a higher cost due to the slower production rates. The slower 

production rates can be attributed to debris removal, shallower cuts, and delays due to sediment treatment. 

A detailed cost estimate could be calculated from specific information on the equipment to be used, the 

method of transporting the dredged material, and current prices for labor and fuel (Cooke et al. 1993). 
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Cost Reduction 

Researchers are continually developing new ways to reduce the cost of dredging. Recently, two 

consulting firms in Florida working on the feasibility of restoring Lake Sippo developed an innovative 

process of dewatering lake sediments using a polymer process to reduce the dredging cost. The process 

reduced the cost of construction of sediment-drying areas and the anticipated dredging period from three 

years to one year. Shortening the dredging time reduced the total anticipated cost and made dredging a 

possibility (MS Consulting 2001). 

Adequate pre-dredging studies will also reduce cost. Dredging is an expensive lake restoration technique 

that requires thorough investigation to ensure that any short- or long-term environmental benefits from 

using this technique counterbalance the cost of operation. Dredging is several times more expensive than 

nutrient inactivation in accomplishing the same short-term goals (minimization of internal nutrient 

recycling) if cost is the only consideration (Cook et al. 1986). Partial dredging (dredging only to a 

certain depth to remove phosphorus) has been evaluated for lakes such as Lake Okeechobee, in an effort 

to evaluate more cost effective dredging scenarios (Reddy et al. 2002, Reddy et al. 2007).

Lake Size 

Technically, there is no reason why a large lake cannot be dredged, although the size of a lake must be 

taken into consideration before dredging begins. The size of Lake Kasumiguara was not adequately 

studied before dredging began and the project failed. Size of a lake will increase the cost of dredging, and 

budgetary aspects will often determine the feasibility of dredging a large lake. Sediment removal by 

dredging was considered for Lake Arre in Denmark, but due to the large surface area of the lake 

(approximately 20 km2) the cost was estimated at $60 million (IETC 2000). Realistically, dredging is 

more efficient when applied to a relative small lake. Most dredging projects are undertaken on small 

lakes. Kampfe Lake, New Jersey was dredged from February 1986 to October 1989 with positive water 

quality results following the dredging. The surface area of the lake was 129,000 m3(Sebetich and Feniro 

1997). As noted above, feasibility studies for dredging of Lake Apopka and Lake Okeechobee resulted 

in decisions not to dredge; costs of dredging these large lakes was prohibitive.

IETC (2000) recommends that a combination of techniques be used in lakes with larger surface areas, and 

not dredging alone. 

Advantage of Dredging 

There are many advantages of dredging and several goals can be accomplished using this type of 

restorative technique on lakes. Removal of toxic substances, macrophyte removal, phosphorus removal, 

and contour alteration for navigational purposes, restoration of fish habitat, and restoration of flushing 

are examples of goals that can be accomplished (Klein 1997). One positive restorative factor associated 
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with dredging is the physical removal of nutrients contained in the sediment. An example of the removal 

of nutrients comes from Lake Trummen, Sweden. This lake was dredged and total phosphorus 

concentration in the lake dropped sharply and remained low for nine years. There is also a rapid long-term 

decrease in internal nutrient loading and nutrient concentration in the water column. Other dredging 

projects have also claimed improvements in water quality (Aron and Carr, ND), although some projects 

have employed more than one restoration technique so the results cannot be attributed solely to dredging 

(Gorini 1987).

Macrophyte control is an important reason dredging should be considered. A project conducted on Lake 

Lilly, WI was deemed a success story due to the control, not eradication, of macrophytes. Prior to 

dredging, Lake Lilly suffered from severe DO depletion and fish kills in the winter months. This was no 

longer a problem ten years after the dredging project and water quality showed an improvement as well 

(Cooke et al 1993); however, 20 years after the dredging, chlorophyll and TP, while still low, are three 

times higher than during the 10 year follow-up. In addition to macrophyte control, general habitat 

restoration, especially for fish, is a common goal associated with dredging projects. In Florida, Florida 

Fish and Wildlife Conservation Commission routinely plans and executes fish habitat restoration projects 

that include sediment and muck removal (Allen 2000). In Palm Beach County, the Lake Osborne Muck 

Dredging Project had stated goals of habitat restoration, improved recreation and water quality (Palm 

Beach County 2007).

One of the main advantages of dredging over other techniques is that dredging does not introduce foreign 

substances into the water body (Klein 1997). Other remedial techniques, such as alum treatment, 

introduce aluminum into the ecosystem, opening up the possibility for increased toxicity and harmful 

effects on the fauna and flora (Gensemer 1998). 

Disadvantages of Dredging 

Analysis of successful and unsuccessful dredging projects has revealed a high potential for negative 

impacts of dredging on lakes and their surrounding (North American Lake Management Society 1988). 

As previously mentioned, the primary concern lies with the possibility for the re-suspension of sediments 

that will result in an increase of nutrients into the water column. Even though Lake Lilly was considered a 

success, there were short-term adverse changes associated with the re-suspension of sediments. There was 

liberation of ammonium nitrogen from the contaminated sediment and total phosphorus increased due to 

re-suspended sediments (Cooke et al 1993).

Additional environmental impacts related to dredging include increased phytoplankton productivity, algal 

blooms caused by the movement of sediment into the euphotic zone of the lake by wind and dredge 

action, the potential for oxygen depletion, lake drawdown, toxic material release into the overlying water 

from contaminated sediments, and a temporary reduction in benthic fish food organisms (Peterson 1982). 
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The length of impact of dredging activities is undetermined and lake-dependent, however most adverse 

impacts are considered transient (Holdren et al. 2001). Temporary increases in turbidity, algal blooms, 

and dissolved oxygen depletions have been experienced during dredging; however, the potential for 

negative impacts can be reduced if the correct dredge is selected and operated by a competent worker, 

pre-dredging studies have been conducted, and external nutrients have been controlled (North American 

Lake Management Society 1988). 

One of the biggest disadvantages of dredging, when compared to other restoration technologies, is the 

need for dredge material (spoil) disposal. Today’s emphasis is on beneficial reuse of dredge materials. 

However, a variety of issues associated with sediment disposal have to be considered in the project 

design, including the possibility of nutrients or contaminants leaching into water from disposal sites 

placed next to waterbodies, dewatering, and other issues.

Cook et al. (1986) and others have found that environmental concerns associated with lake sediment 

removal appear to be less negative than might be expected. Case studies reveal that though adverse 

impacts do occur, they generally are not severe and are of short duration. There have been failures with 

dredging, but most of the failures can be attributed to inaccurate pre-dredging assessments and inadequate 

amounts of sediment removed.

There are few examples where dredging alone has been proven to have a positive effect on water quality. 

Most successful projects incorporate dredging as part of a multiphase restoration effort encompassing 

other remedial techniques such as biomanipulation, alum treatment, and external nutrient divergence. In 

fact, the Lake Trummen dredging project was successful, but the return flow was treated with aluminum 

sulfate. Any positive effects of using dredging alone may be short lived, especially if external nutrients 

are not controlled. A variety of dredging projects have resulted in failure to improve water quality (for 

example: Birch and McCaskie 1999, Breithaupt and Lamb 1982) or to achieve water quality goals 

(Jeppesen et al. 1999).

SELECTED CASE STUDIES 

Banana Lake, Florida 

A second example of partial success is found in the restoration of Banana Lake which has shown 

modestly improved water quality (as indicated by the TSI), but additional restoration activities are 

necessary, as the lake remains impaired.

Banana Lake, historically, the most polluted small (approximately 340 acres) lake in Florida, according to 

trophic state index (Huber et al. 1982). From 1975 to 1987, a WWTP discharged an average of 6.4 MGD 

into a canal draining to Banana Lake. The effluent was diverted away from the lake in 1987. Nonpoint 

sources were not addressed prior to dredging. In 1989-1990, a restoration plan was developed which 

called for hydraulic dredging of the entire lake. Dredging began in October 1990 and proceeded for one 
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year. Approximately 800,000 CY of sediment were removed; however, the lake remained eutrophic after 

sediment removal, and is currently listed as impaired for nutrients (USEPA, 2006). The main benefit of 

sediment removal for Banana Lake was a 50% reduction in chlorophyll a in the two years following 

dredging (Kelly et al. 1994). TSIs post dredging are frequently at or slightly below 80, but were 

consistently above 80 from 1984 until dredging was conducted.

Banana Lake TKN and TP

0

2000

4000

6000

8000

10000

12000 7/25/1984 7/25/1985 7/25/1986 7/25/1987 7/25/1988 7/25/1989 7/25/1990 7/25/1991 7/25/1992 7/25/1993 7/25/1994 7/25/1995 7/25/1996 7/25/1997 7/25/1998 7/25/1999 7/25/2000 7/25/2001 7/25/2002 7/25/2003 7/25/2004 7/25/2005 7/25/2006 7/25/2007

Date

TKN or TP (ug/L)

TKN

TP

Dredging: August 

1990 - 1991 

Source Date: Polk County Water Atlas

WWTP 

Diversion

Battersea Park Lake, London England

The dredging of Battersea Park Lake did not produce the desired improvements in water clarity because 

the significant external loading source was not controlled prior to dredging.

Battersea Park Lake in London, England is the largest lake in the urban park system. It was constructed 

around 1860, and is approximately 8 ha with an average depth of less than 1 m. It was recognized that 

both external loading and recycling of nutrients from the sediments needed to be reduced to improve 

water quality. The lake is used recreationally, and historically was dominated by macrophytes, rather 

than phytoplankton. The lake was dredged in the early 1980s “at great expense, but with no lasting effect 

on water clarity” (Birch and McCaskie 1999). However, at that time, external loading (top off water 

supplied from the nutrient rich Thames River and other sources) were not controlled. By 1994, fish kills 
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were occurring due to low dissolved oxygen. The available water quality data was post dredging, 1993 to 

1996. 

City Park Lake, Louisiana

The long-term water quality trends indicate that the 1984 restoration of City Park Lake was successful in 

short-term improvements in water quality conditions in the lake that have persisted for many years. 

However, the beneficial effects and expense of the restoration have been severely undermined by the lack 

of implementation of long-term watershed and in-lake nutrient management strategies (Ruley and Rusch 

2002), and the lake had returned to pre-dredged water quality by 2000.

City Park Lake in Baton Rouge LA is a shallow (mean depth = 1.2 m), small (surface area of 0.23 km2), 

hypereutrophic urban lake, with an urban watershed covering 1.922 km2. By the late 1970’s the lake was 

suffering from frequent, severe algal blooms and fish kills, prompting a major restoration begun in 1983 

that included dredging and repair of sewer infrastructure to reduce external loading. Combined funding 

for the project was approximately $4.5M. A total of approximately 100,000 m

3

of sediments were 

dredged. Due to concerns about metals in some surficial sediment adversely impacting the disposal area, 

these sediments were moved to one side, deeper, cleaner sediments were dredged, the metal laden 

sediments were then moved to the newly dredged area and covered with cleaner sediments. Overall, the 

mean depth of the lake was increased by approximately 0.3m. Post-dredging monitoring (1984) 

documented immediate improvements in oxygen storage capacity in the lake and improved water column 

transparency, and the frequency of algal blooms diminished for the remainder of the 1980s. The lake 

transitioned from hypereutrophic to eutrophic status. Large blooms of benthic filamentous algae began to 

cause floating scum in the 1990s. Studies in the early 1990s demonstrated that water column total P levels 

fell slightly between pre-dredging and 1991, and algal densities dropped dramatically. Nutrient levels, 

however, remained high. External loading (from a bayou, and continued problems with sewage leakage) 

were identified. By this time, the lake was again considered hypereutrophic. By 2000, the mean summer 

TP and Secchi depth had returned to pre-dredging levels. However, summer chl a and mean summer 

TKN remained much lower. It was concluded that the lake was now nitrogen limited+. 

Cockshoot Broad, England 

The dredging of Cockshoot Broad, a shallow lake in England, as a short-term solution, demonstrates the 

need for additional restoration techniques. The results of isolation and the removal of a meter of 

phosphorus-rich sediment by suction dredge (1982) were almost immediately positive. Total phosphorus 

concentrations quickly fell, and dense submerged-plant communities established in a sheltered part of 

Cockshoot Broad. Total phosphorus remained low from late 1982 until the summer of 1985, when there 

were indications of an increase. There was a decline in aquatic plants in Cockshoot Broad in 1984 and 

1985, and an increased concentration of phosphorus was found in the summer and autumn of 1987 and 

1989 (Moss et al. 1996). Additional measures were needed – and later undertaken – to ensure long-term 

restoration of Cockshoot Broad. 
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Lake Finjasjon, Sweden 

Dredging was determined to be an inappropriate restoration technique for Lake Finjasjon, Sweden, 

because water column phosphorus concentrations were not reduced and chlorophyll a actually increased. 

Initially, external nutrients were reduced and dredging lake sediment was introduced to control internal 

nutrient loading in Lake Finjasjon. After years of unsuccessful results, however, dredging was replaced 

by biomanipulation (Annadotter et al. 1999). Reducing the external nutrients into Lake Finjasjon did not 

assist in the recovery of the lake – there was actually an increase in chlorophyll a – and dredging did not 

decrease phosphorus concentrations. For Lake Finjasjon there was a non-explicit relationship between the 

trophic level and the phosphorus content of the sediment. Instead, the phosphorus concentration in the 

pore water was believed to reflect the trophic level of the lake. Lake managers concluded that continual 

dredging would not produce a clear lake, and dredging was deemed an inappropriate method of 

restoration. Biomanipulation was successfully used as a secondary method to restore Lake Finjasjon 

(Annadotter et al. 1999). 

Lake Hollingsworth, Florida

In Lake Hollingsworth, nutrients and other water quality parameters were lower on average, postdredging, but additional in-lake and watershed restoration efforts have been necessary to further curtail 

nutrient loadings, and the lake remains impaired.

Lake Hollingsworth is one of the most popular recreational lakes in Polk County. On average, over 2,000 

people per day use the lake and shoreline for recreation. Currently, Lake Hollingsworth has an average 

depth of 3.6 ft, with a surface area of 358.15 acres. Lake Hollingsworth serves a drainage basin area of 

1,647.49 acres which includes the Lake Morton and Lake Horney watersheds. The lake experiences high 

bacterial counts, especially during the winter, which curtails its use. Fifty seven stormwater outfall pipes 

discharge into the lake, and a variety of retrofit projects are planned or underway to curtail external 

loading to the lake.

The watershed management plan developed for the lake and watershed recommended dredging of the 

entire lake as a means of restoration (City of Lakeland, 1995). Hydraulic dredging was selected, with an 

estimated cost of $6.4 M dollars (City of Lakeland 1994). Dredging was initiated in 1996. Dredging was 

suspended on March 5, 2001 due to the extreme low water levels in the lake. Two years of extreme 

drought combined with the water loss associated with the dredging process resulted in record low water 

elevations in Lake Hollingsworth. As a result of the dredging through March 2001, approximately 75% of 

the planned dredging volume was removed from the Lake. Two years post-dredging a one time whole 

lake alum application was conducted. The City of Lakeland is planning to resume dredging in the future.
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Lake Hollingsworth TP: Pre - Post Dredge

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700 3/1/93 9/1/93 3/1/94 9/1/94 3/1/95 9/1/95 3/1/96 9/1/96 3/1/97 9/1/97 3/1/98 9/1/98 3/1/99 9/1/99 3/1/00 9/1/00 3/1/01 9/1/01 3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 3/1/06 9/1/06

Dates 3/93 - 12/06

TP ( mg/l )

Dredging begins 

01/97

Dredging stops 

03/01

Alum treatment 

12/03

Data Source: City of Lakeland, 2007

Interpretation of available water quality monitoring data is confounded by the extended drought 

conditions post-dredging and the addition of the second lake restoration technology. However, TP, TN, 

and chlorophyll a concentrations remain lower than pre-dredging conditions, and Secchi depths are deeper 

post-dredging. TSI which averaged above 80 units prior to dredging is now averaging around 70 units. 

Lake Hollingsworth remains eutrophic to hypereutrophic, and stormwater treatment systems were 

considered necessary to curtail external loading. The South Landing treatment wetland was completed on 

the south shore of the lake in 2006.

Lake Maggiore, Florida

Expected benefits from the Lake Maggiore restoration projects which included dredging include: 

improved water quality, improved recreational use and public access, and improved fishery and wildlife 

habitat. The project is only shortly post-dredging with the initial modest effects noted below, and 

monitoring will continue of Lake Maggiore to document these anticipated improvements.

Lake Maggiore is a 380 acre water body located in southern Pinellas County. The TSI of Lake Maggiore 

has historically remained above 80, which ranks it as a highly productive, hypereutrophic lake. 

Development within the watershed primarily occurred prior to implementation of current stormwater 

management regulations. A diagnostic study undertaken in 1989 concluded that water quality was poor 

due to untreated stormwater runoff and large quantities of organic muck on the lake bottom contributing 
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to nutrient enrichment in the lake. A variety of restoration projects have been completed including, but 

not limited to retrofitting of the stormwater conveyance system and addition of five alum injection 

facilities to treat stormwater runoff (built prior to dredging) and dredging of approximately 1.5 MCY of 

sediment from the lake bottom. Expected benefits of the restoration include improved water quality, 

improved recreational use and access, and improved habitat. Routine water quality monitoring was 

initiated in 1996 and continues to date (Smith 2007), and comparisons pre and post dredging to date have 

not shown statistically significant trends among the parameters of interest including Total P, Total N, and 

chlorophyll. 

Lake Maggiore FTSI Score

50

60

70

80

90

100

110 1/1/1996 7/1/1996 1/1/1997 7/1/1997 1/1/1998 7/1/1998 1/1/1999 7/1/1999 1/1/2000 7/1/2000 1/1/2001 7/1/2001 1/1/2002 7/1/2002 1/1/2003 7/1/2003 1/1/2004 7/1/2004 1/1/2005 7/1/2005 1/1/2006 7/1/2006 1/1/2007

Date FTSI Units

Dredging

Started

Source: Smith, 2007

There has been an increase in conductivity (from chloride in the groundwater) which is likely due to 

increased seepage of groundwater following soft sediment removal, an effect that was predicted in the 

preassessment studies (Smith 2007). 

Lake Panasoffkee, Florida

The dredging of Lake Panasoffkee continues to date, and post dredging monitoring continues. To date 

water quality improvements have not been observed, but fisheries habitat restoration and navigation, not 

water quality improvement are the stated goals for this project. 

Lake Panasoffkee at approximately 920 of open water acres and approximately 2000 acres in total, is the 

third largest lake in western Florida. Historically, the lake was known nationally for its redear sunfish 
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fishery. The Florida Department of Environmental Protection (FDEP) designated Lake Panasoffkee as an 

Outstanding Florida Water. The lake is also one of the priority water bodies in the Southwest Florida 

Water Management District's Surface Water Improvement and Management (SWIM) program. In recent 

times, the fishery had declined, and this was attributed, at least in part, to loss of nursery habitat as 

organic sediments covered historically sandy lake bottom and emergent aquatic plants covered areas of 

historically open water. A multiphase dredging plan was developed with the objective of restoring 

fisheries habitat, historic shoreline and improving navigation. Improvement in water quality was 

apparently not a major objective of this restoration, and TSI data have remained relatively constant during 

the dredging to date. The ambitious dredging plan calls for the removal of over 8 MCY of soft sediment. 

Dredging began in July of 2004 and is scheduled to be completed in 2008. The total cost of the 

restoration is estimated to be $26M. 

Lake Panasoffkee: Average In-Lake TSI

September 2000 - January 2006

0

10

20

30

40

50

60

70

80 Sep-00 Oct-00 Nov-00 Dec-00 Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01 Jul-01 Aug-01 Sep-01 Oct-01 Nov-01 Dec-01 Jan-02 Feb-02 Mar-02 Apr-02 May-02 Jun-02 Jul-02 Aug-02 Sep-02 Oct-02 Nov-02 Dec-02 Jan-03 Feb-03 Mar-03 Apr-03 May-03 Jun-03 Jul-03 Aug-03 Sep-03 Oct-03 Nov-03 Dec-03 Jan-04 Feb-04 Mar-04 Apr-04 May-04 Jun-04 Jul-04 Aug-04 Sep-04 Oct-04 Nov-04 Dec-04 Jan-05 Feb-05 Mar-05 Apr-05 May-05 Jun-05 Jul-05 Aug-05 Sep-05 Oct-05 Nov-05 Dec-05 Jan-06

Sample Date Lake Average TSI Value

0

2

4

6

8

10

12

14

16

18 Monthly Rainfall (inches)

TSI LAKE AVG Target TSI = 50 TSI=60 Monthly Rainfall

Source: SWFWMD monitoring data

Dredging

Initiated

Peterson (1982) provides additional examples.

Lake Trehorningen, Sweden 

A multiphase restoration project at Lake Trehorningen, Sweden, was only partially successful because of 

continued external nutrient loading after treatment. Dredging was one phase of a five-phase project to 

restore Lake Trehorningen. 

After dredging, phosphate and total phosphorus decreased by 73% and 50% respectively, between 1975 

and 1978. Organic matter content decreased by 33%, the nitrogen-to-phosphorus ratio increased, and 

chlorophyll a decreased dramatically through eight years following dredging. Although substantial 
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improvements in water quality were observed, the Lake Trehorningen restoration project was rated as 

having only limited success. The areal nitrogen and phosphorus loads far exceeded critical levels after 

restoration. The limited success was attributed to continued high external loads of nutrients. The 

experience at Lake Trehorningen demonstrates once again the need for thorough evaluations of nutrient 

budgets prior to the implementation of expensive dredging activities (Graetz et al. 1988).

Lake Trummen, Sweden 

Lake Trummen is an example of a successful lake restoration project that combined dredging with 

external load reductions and some chemical amendment treatment. The lake was dredged in 1970 and 

1971 to control an internal nutrient loading problem. One meter was removed from the bottom of the 400-

hectare lake using a suction dredge. Approximately 600,000 m3of mud and 300,000 m3of water were 

pumped through settling ponds. The mixture of interstitial and lake water required an alum treatment 

before re-entering the lake. As a result of dredging, organic carbon concentrations decreased from 

approximately 80 mg C L-1and diurnal fluctuations in oxygen, pH, and alkalinity were reduced 

significantly after the restoration. Post-treatment water quality studies were carried out for more than 14 

years, and low concentrations of total phosphorus and nitrogen continued (Graetz 1988). Post-dredging 

interstitial phosphate concentrations decreased to 0.5% to 0.2% of pre-restoration concentrations. Total 

biomass of phytoplankton decreased drastically, and there was a decrease in the type of zooplankton 

commonly associated with eutrophic conditions. Dredging was combined with external nutrient 

divergence and alum treatment to obtain these results. 

Conclusion 

An evaluation of the available literature on dredging and its effects on water quality indicates that 

dredging should be seen as an additional tool for water quality management and not as a primary method 

of restoration. Although many techniques are potentially available to restore degraded lakes, the science 

of lake restoration is inexact and the outcome of applying a given technique to a particular lake is difficult 

to predict (NRC 1992). Understanding the complexity of the transition that takes place within a lake when 

it deteriorates to a eutrophic state highlights the reverse process that needs to take place during 

restoration. 

The failure of projects to restore lake water quality to adequate standards is the result of inadequate predredging studies, the continuation of external nutrient loading, high re-suspension rates, and the 

misunderstanding that dredging alone can restore water quality. Many studies indicate that dredging alone 

will not provide long-term restoration. Sediment removal after source control, and potentially followed by 

other restoration techniques offers the most reliable route to long-term restoration (Phillips et al. 1999). 
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