20 Sediment Removal 20.1 INTRODUCTION Dredging, due to some poor past practices, has received a bad reputation. However, properly conducted, sediment removal is an effective, but expensive, lake management technique. New to this chapter is an extensive case history concerning contaminated sediment removal and the real- ization that formerly named “special purpose” dredges are becoming more common to lake resto- ration, at least in Europe. This chapter describes objectives, environmental concerns, dredging depths, removal techniques, lake conditions, dredge selection, disposal area designs, some case histories, and costs associated with sediment removal (adjusted for inflation to June 2002). Sediment removal, while common, is very limited in documentation concerning the success or failure of most projects. Thus, material in this chapter is not exhaustive, but rather representative of various lake sediment removal procedures. 20.2 OBJECTIVES OF SEDIMENT REMOVAL 20.2.1 D EEPENING When recreational activities are impaired due to shoaling, the only practical means of restoration is lake deepening through sediment removal. According to the United States Department of Agriculture (USDA, 1971), lakes must have a water volume sufficient to exceed water loss by seepage and evaporation, and sufficient depth to prevent complete freezing. In the latter case that means a depth anywhere from 1.5 to 4.5 m, depending on the region of the country. A depth of at least 4.5 m is usually required to avoid winterkill of fish in colder parts of the U.S. (Toubier and Westmacott, 1976). These and other factors, such as intended lake use, availability of a suitable dredged material disposal area, and available funds, must be considered when designing and implementing any lake-deepening project. The reasons for deepening and the means of measuring the success of such a project are the most direct aspects of the sediment removal objectives. Modern dredging equipment efficiently moves large volumes of sediment. Therefore, nearly all dredging projects are considered successful at the time of their completion (Pierce, 1970). However, more recent information from Wisconsin shows that lake deepening can be reversed by sedimentation in 10 years or less (Wisconsin Department of Natural Resources, 1990). Specific examples include the millponds of Bugle Lake and Lake Henry. Therefore, sedimentation rates must be determined before dredging is recommended. Success in terms of deepening is not the only criterion for determining success of a dredging project. Deepening might be accomplished while the overall condition of the lake is actually worsened due to poor dredging techniques (Gibbons and Funk, 1983). Therefore, dredging proce- dure is a critical aspect of the dredging project. 20.2.2 NUTRIENT CONTROL Many shallow, eutrophic lakes do not stratify thermally (polymictic or amictic) making them susceptible to continual or periodic nutrient inputs from the sediment. Deeper stratified lakes might become destratifed when a passing summer, cold weather front depresses the thermocline pushing nutrient rich water into the photic zone of the epilimnion (Stauffer and Lee, 1973). Power boat Copyright © 2005 by Taylor & Francis wakes and bottom fish also are problematic in shallow lakes. Thus, obnoxious algal blooms occur most frequently during peak summer recreation periods. Sediment-regenerated P amounted to approximately 45% of the P loading to Linsley Pond, CT (Livingston and Boykin, 1962). Welch et al. (1979) estimated P inputs to Long Lake, Washington were 200 to 400 kg/yr, or about 25% to 50% of the external loading. Shagawa Lake, MN, experienced summer sediment P pulses of approximately 2000 to 3000 kg during June, July, and August. This compares to an annual P loading from the City of Ely, MN, of 5000 to 5500 kg before advanced waste treatment (AWT) and about 1000 to 1500 kg after AWT (Larsen et al., 1981). Before AWT, sediment P loading to Shagawa Lake was about 28% to 35% of the total loading. The sediment portion of the TP loading to the lake increased to 66% following AWT, even though the total loading decreased considerably after AWT (Peterson, 1981). Sediment-recycled P in Shagawa Lake has been sufficient to produce large summer algal blooms, thus slowing the lake’s predicted rate of recovery (Larsen et al., 1981; Chapter 4). In cases where a significant nutrient loading from sediment can be documented, sediment removal might be expected to reduce the rate of internal nutrient recycling, thus improving overall lake and water quality conditions. However, while dredging rich surface sediments will reduce internal nutrient recycling, this effect might be temporary if external sources are shut off. Kleeberg and Kohl (1999) demonstrated that trophic state in Lake Muggelsee, Germany is controlled more by photic zone production and its associated sedimentation than by nutrient release from the sediment if surface inputs of P are not cut off. Additionally, Sondergaard et al. (1996) found that surface sediment TP in Danish lakes was highly correlated to the external P loading, but only weakly related to other sediment parameters. This strongly reinforces the idea that P input reduction is the first line of defense in lake management and restoration. Consideration of nutrient inactivation is another option for shallow lakes that might not need deepening per se. It is easier, less expensive, and likely to be more successful in terms of nutrient control per se (Welch and Cooke, 1995). 20.2.3 TOXIC SUBSTANCES REMOVAL Toxic substances are a common concern among industrialized nations. Large-scale surveys and improved analytical techniques demonstrate that toxicants are more common to fresh water sedi- ments than previously suspected (Bremer, 1979; Horn and Hetling, 1978; Matsubara, 1979). Many toxicants are recycled from the sediment to the overlying water, where they bioaccumulate in aquatic organisms. Perhaps the most infamous incident of this type (marine water) was mercury pollution of Minimata Bay, Japan, first discovered in 1956 (Fujiki and Tajima, 1973). Other incidents, in the U.S., have involved kepone contamination of the James River, VA (Mackenthun et al., 1979), and PCB contamination of Waukegan Harbor in Lake Michigan (Bremer, 1979). Few occurrences of toxic problems like the one for mercury at Gibraltar Lake, CA, were reported in the past (Spencer Engineering, 1981). However, that has changed in recent times as PCBs and heavy metals, particularly mercury, have been recognized as a more prevalent fish tissue bioaccu- mulation problem (Gullbring et al., 1998; Peterson et al., 2002). The most obvious solution to contaminated sediment is removal, but contaminated sediment removal frequently is complicated by pollution of the overlying water column, through sediment agitation. Most conventional dredges can cause massive resuspension of fine sediment (Suda, 1979; Barnard, 1978). Sediment resuspension while dredging toxic substances must be minimized to prevent secondary environmental damage. Proper selection and design of dredging equipment becomes more important when removing toxic sediment (see the Lake Järnsjön case history in this chapter). 20.2.4 ROOTED MACROPHYTE CONTROL Some rooted aquatic plants in a lake are desirable since they provide habitat for young fish and reduce beach erosion. However, an overabundance of plants may interfere with fishing, boating, Copyright © 2005 by Taylor & Francis and swimming and may be aesthetically displeasing. Respiration by large plant masses in the littoral zone during hours of darkness might significantly reduce dissolved oxygen concentrations. In addition, there is increasing literature concerning the effects of macrophytes on internal nutrient cycling. Their role in this process may be an important reason for attempting to control macrophytes by selectively removing them from a lake. Wetzel (1983) indicated that most of the organic matter found in small lakes may be derived from their littoral zones. Fresh Water aquatic plants extract nutrients chiefly from the sediment (Schults and Malueg, 1971; Twilley et al., 1977; Carignan and Kalff, 1980), but they do not excrete large quantities of nutrients to the surrounding water while in the active growth phase (Barko and Smart, 1980). They do tend, however, to concentrate sediment-supplied nutrients in their tissues. These nutrients are recycled to the lake when plants fruit and during the senescence, death, and decay stages (Barko and Smart, 1979; Lie, 1979; Welch et al., 1979) (see also Chapter 11). Barko and Smart (1979) estimated that in-lake mobilization of P by Myriophyllum in Lake Wingra, WI, might amount to 62% of the annual external P loading. Welch et al. (1979) indicated that much of the “sediment” P loading in Long Lake, Washington probably was due to rapid plant die-off and decay. Current information indicates that any long range lake restoration project concerned with in-lake nutrient controls needs to focus on both macrophytes and sediment (Barko and Smart, 1980; Carignan and Kalff, 1980). 20.3 ENVIRONMENTAL CONCERNS 20.3.1 I N-LAKE CONCERNS Sediment resuspension during dredging is the primary in-lake concern (Herbich and Brahme, 1983). One of the most common problems is nutrient liberation. Phosphorus is of particular concern because of its high concentration in sediment interstitial waters of eutrophic lakes. Dredge agitation and wind action move nutrient-laden sediment into the euphotic zone of the lake, creating the potential for algal blooms. Churchill et al. (1975) reported increased P concentration in Lake Herman, SD, coincident with cutterhead hydraulic dredging, but no increased algal production was noted. This lack of algal increase presumably was due to the high turbidity level. Dunst (1980), on the other hand, found increased algal production in Lilly Lake, WI, when hydraulic dredging began, but it was short lived and never posed a nuisance. While nutrient enrichment due to dredging can become a problem, in most cases the effects are short term and negligible relative to the long- term benefits. Another, and potentially greater, concern associated with resuspended sediments is the liberation of toxic substances. Small-lake toxic sediment removal projects are relatively uncommon, but a few have been undertaken (Bremer, 1979; Matsubara, 1979; Sakakibara and Hayashi, 1979; Spencer Engineering, 1981). Fine particles pose the major concern. Murakami and Takeishi (1977) showed that up to 99.7% of the polychlorinated biphenyls (PCBs) associated with marine sediments are attached to particles less than 74 μm in diameter. This could pose a particular problem for fresh water dredging projects, where particle-settling times are significantly greater than for marine waters. Therefore, added precautions need to be taken when dredging contaminated sediments. Such precautions might include special dredges (see Sediment Removal Techniques section of this chapter and case histories) and special disposal and treatment techniques (Barnard and Hand, 1978; Matsubara, 1979). A common dredging concern among fisheries managers is the destruction of benthic fish-food organisms. If the lake basin is dredged completely, 2 to 3 years may be required to reestablish the benthic fauna (Carline and Brynildson, 1977). However, if portions of the bottom are left undredged, reestablishment can vary from almost immediate (Andersson et al., 1975; Collett et al., 1981) to 1 to 2 years (Crumpton and Wilbur, 1974). Lewis et al. (2001) concluded that small scale dredging impacts on benthos in shallow water bayous were “counteracted” by beneficial effects to other Copyright © 2005 by Taylor & Francis biota due to the removal of sediments and the increase in depth and circulation. In any case, the effect on benthic communities appears to be short lived and generally acceptable relative to the longer term benefits derived. However, partial dredging fisheries benefits must be weighed against the increased potential for nutrient liberation from poorly executed partial dredging projects (Gib- bons and Funk, 1993). These concerns are associated primarily with dredging as a sediment removal technique. Another technique for sediment removal involves lake drawdown (lowering the water level) to expose the littoral sediments, or in some cases (Born et al., 1973) the entire lake basin, followed by removal of sediment with earth moving equipment after it has dried sufficiently. Drawdown accompanied by bulldozer operation is more destructive of the benthic community than dredging. It may also pose additional nuisance problems such as noise, dust, and truck traffic. The section on sediment removal techniques addresses dredging techniques that minimize many of these concerns. 20.3.2 DISPOSAL AREA CONCERNS The major non-lake impact of sediment removal concerns the area chosen for dredged materials disposal. The problem of finding disposal sites in urban areas has become more acute in the U.S. with the promulgation of Section 404 of Public Law 92–500 (The Clean Water Act); this law prohibits the dredging or filling of any wetland area exceeding 4.0 ha (10 acres) without a federal permit. However, Section 404 of the Law was challenged and reversed by a Supreme Court ruling in 2000 that said in effect only those wetlands contiguous with navigable waters are protected by fill permitting. This makes many small wetlands vulnerable to draining, filling and wanton destruction. Flooding of wooded areas with dredged material should be avoided. Flooding kills trees, providing unsightly evidence of improper disposal. Disposal areas may become attractive nuisances in the legal sense and can be extremely dangerous. They tend to form thin dry crusts that, like thin ice, break easily when subjected to the weight of a person or vehicle. Even dewatered and apparently dried disposal areas can be deceiving. Those with strong surface crusting, deep cracking, and vegetation can swallow earth-moving equipment if excavation is attempted too early. Disposal areas covered to depths greater than 1 m should be tested thoroughly to determine their ability to support heavy equipment before any rework on the disposal areas is attempted. It is advisable to fence and post disposal areas for safety. A disposal method used frequently in recent years employs diking in upland areas. A common problem with these sites is dike failure accompanied by flooding of adjacent areas (Calhoun, 1978). Groundwater contamination near upland disposal sites has been identified as a potential problem, however, there are no documented contamination cases involving lake sediment disposal even where monitoring was extensive (Dunst et al., 1984). Upland disposal areas are commonly used for a variety of purposes once they are closed and dewatered. Another lake dredging problem is under-design of the disposal area capacity. Unfortunately, these failings usually become apparent only after the project is fully operational. The problem may be caused by the slow settling rate of suspended sediment in fresh water (Wechler and Cogley, 1977) and reduced ponding depth as the project proceeds. This may result in failure to meet the requirements of suspended solids discharge permits. If that happens there are two choices: shut down until seepage and evaporation allows additional filling, or treat the discharge water. Either alternative adds additional cost to the project. However, increasingly stringent requirements for dredged material return flow waters require innovative settling techniques. A dredging project at Lake Tahoe, CA required that dredge water return flows to the lake be no more than 5 Nephelometric Turbidity Units (NTU), a standard that could not be met by any known technology (Macpherson et. al., 2003). A compromise was reached that allowed discharge at no more than 20 NTU into an adjacent dry marsh. However, even this standard could not be met and the use of polyacrylamides, polymines, aluminum, and iron-based coagulants were discouraged because of potential environ- mental problems. Therefore, a low toxicity, non-contaminant, biodegradable coagulant (chitosan) Copyright © 2005 by Taylor & Francis was tested and used. This product is derived from shellfish shells and marketed under the name of Gel-Floc ® . Gel-Floc placed in the 2,000 gpm recirculation flow consistently reduced dredge water turbidity from 1,000 NTU to an average of 17 NTU. Conductivity, pH, and temperature of the treated water remained unaffected. Disposal areas must be designed for end-of-project efficiency, not average discharge require- ments over the entire use period. Palermo et al. (1978) along with a later section of this chapter summarize important technical information that assists with the proper design, construction, and maintenance of disposal areas for dredged material. Barnard and Hand (1978) describe when and how to treat disposal area discharges if standards cannot be met. Brannon (1978), Chen et al. (1978), Gambrell et al. (1978), and Lunz et al. (1978) provide valuable information that help minimize environmental problems at disposal sites. 20.4 SEDIMENT REMOVAL DEPTH When restoring a lake for sailing, power boating, and associated activities, the deepening require- ments are relatively straightforward. When deepening to control internal nutrient cycling and macrophyte growth, the criteria are less clearly defined. Lake Trummen, Sweden, is perhaps the most thoroughly documented case of sediment removal to control internal nutrient cycling and macrophyte encroachment. Sediment removal depth in Lake Trummen was determined by mapping both the horizontal and the vertical distribution of nutrients in the sediment. Digerfeldt (1972), as cited by Björk (1972), determined that approximately 40 cm of fine surface sediment accumulated from 1940 to 1965. Aerobic and anaerobic release rates of PO 4 – P and NH 4 + – N from sediment surface layers were markedly greater than for the underlying sediment (see the Lake Trummen case study in this chapter). Based on these differences, a plan was developed to remove the upper 40 cm of sediment. Another approach to determine sediment removal depth was proposed by Stefan and Hanson (1979) and by Stefan and Ford (1975). This approach is similar to that developed by Stauffer and Lee (1973), which described thermocline erosion by wind in northern temperate lakes. Stefan and Hanson (1979) used their model to predict the depth to which Hall Lake, MN, must be dredged to control adverse nutrient exchange from the sediment during the summer. In other words, to determine what depth was necessary to establish permanent summer thermal stratification (dimictic condition). The Stefan and Hanson (1979) model assumes stable summer stratification is necessary to prevent enriched hypolimnetic waters from mixing into the epilimnion. Based on that assumption, they calculated that Hall Lake (one of the Fairmont, MN, lakes) would require dredging to a maximum depth of 8.0 m to change it from a polymictic to a dimictic lake. Dredging volume to obtain the 8.0 m depth would be enormous, given Hall Lake’s 2.25 km 2 surface area and 2.1 m mean depth. There was little apparent chemical or physical distinction between shallow and deep sediments in Hall Lake. Phosphorus concentration was relatively uniform from the sediment surface to a depth of 8.5 m (737 to 1412 mg/kg for 37 samples, with a mean of 1,097 mg/kg). It is possible, however, that the P release rates from deeper sediment could be less than those of surface sediments (they were not measured). Nutrient release from the deeper sediment could be slow enough to significantly reduce the adverse impact of nutrients on the overlying water, even though stratification might not be permanent (Bengtsson et al., 1975). If that is the case, surface sediment skimming might produce nearly the same result as deep dredging, and at a considerable saving. Therefore, it would be advisable to conduct incremental nutrient release rate experiments prior to adopting a lake temper- ature modeling approach to determine dredging depth for nutrient control. Dredging will remove rooted macrophytes from the littoral zone of lakes, but there have been few detailed studies to determine the depths necessary to prevent regrowth of nuisance plants. Factors influencing the areas in which rooted macrophytes grow include temperature, sediment texture, nutrient content, slope, and light level (see Chapter 11). Copyright © 2005 by Taylor & Francis Using field data developed by Belonger (1969) and Modlin (1970), the Wisconsin Department of Natural Resources developed a guide to prescribe dredging depths necessary to control the regrowth of macrophytes. The guide was developed by regression of the maximum depth of plant growth in several Wisconsin lakes against the average summer Secchi disc transparency of the lakes. The relationship is described by the equation Y = 0.83 + 1.22 X (20.1) where Y = maximum plant growth depth (m) and X = average summer water transparency (m). Wisconsin lakes with a mean Secchi disc transparency of 1.5 m have few macrophytes growing beyond a depth of 2.7 m. According to Dunst (1980), this relationship was used in Wisconsin as a rough guide to develop dredging plans for macrophyte control. Dunst indicated, however, that dredging depths do not always need to exceed the predicted Y value to achieve control since slight deepening frequently changes plant speciation to less objectionable forms (see Lilly Lake, WI, case study in this chapter and Chapter 11, Table 11.3, for other regression equations for different geographic areas). Work by Collett et al. (1981) attempted to establish the depth of dredging necessary to prevent plant regrowth in the usually turbid Tuggarah Lakes of New South Wales. They bracketed the light compensation depth by dredging three 30 m 2 test plots 1.0 m, 1.4 m, and 1.8 m deep in a 30 × 180 m rectangular area parallel to and about 300 m from the lake shore. Three control plots of the same size (30 m 2 ) were left undredged. Results indicated rapid recolonization (within 4 months) in the plot dredged to 1.0 m. One year after dredging, macrophyte biomass in the 1.0-m plot was about 60% of the pre-dredging level. Macrophytes had not reestablished in the 1.4 m and 1.8 m test plots during the same year. Sediment nutrient levels were found to be similarly high in all test plots, so nutrient deficiency was ruled out as a probable cause of reduced growth. The authors speculated that reduced light penetration at the 1.4 m and 1.8 m depths limited regrowth, but they also noted that deeper plots tended to fill with plant debris and lake detritus, altering the texture of the substrate. Unfortunately, no quantitative measurement of light level or sediment particle size was reported to corroborate their speculations. That macrophytes ordinarily grow to depths up to 2 m (Higginson, 1970) in the Tuggarah Lakes seemed to imply that light alone should not have prevented regrowth at 1.4 m and 1.8 m. The more flocculent sediments in deeper plots may have had a greater influence than indicated by Collett et al. (1981). Their study did not answer conclusively the question of the influence of light on regrowth of plants. It may even raise some question about the rationale for using light level to determine dredging depth. This seems, however, to be a reasonable approach given what we know about macrophyte growth characteristics and light requirements. The maximum depth of autotrophic plant growth depends upon water transparency (Hutchinson, 1975; Maristo, 1941). Canfield et al. (1985) reevaluated the relationship between macrophyte maximum depth of colonization (MDC) and Secchi disc transparency. Duarte and Kalff (1987) confirmed the work of Canfield et al. using several variables from Canadian and U.S. lake data sets. The subject of macrophyte growth characteristics in lakes was addressed briefly in Chapter 2 and covered in much greater detail in Chapter 11. In addition, Duarte and Kalff (1990) is an excellent reference for in-depth coverage on the subject. 20.5 SEDIMENT REMOVAL TECHNIQUES There are two major techniques for sediment removal from freshwater lakes and reservoirs. The first one, lake drawdown followed by bulldozer and scraper excavation, has limited application. It has been used most successfully in small reservoirs (Born et al., 1973). The obvious limitation of this technique is that water must be drained or pumped from the basin. A second drawback is that the basin must be allowed to dewater sufficiently before earth-moving equipment can operate. Copyright © 2005 by Taylor & Francis Despite these problems, plus the added concern of truck traffic to transport the removed sediment, this approach has been used successfully at Steinmetz Lake, NY (Snow et al., 1980). The second, and most common, sediment removal technique is dredging. Huston (1970) reviewed the many types of dredges in use. This chapter addresses only dredges commonly used in lakes and those with special features that minimize adverse dredging effects. Dredges are divided into mechanical and hydraulic types. A third category, “special purpose dredges,” is included to highlight low-turbidity systems for dredging fine-grained and toxic sediments, both of which are relatively common in fresh water lakes and reservoirs. 20.5.1 MECHANICAL DREDGES Grab-type mechanical dredges are used commonly in lake restoration (Figure 20.1). Figure 20.1A shows a clamshell bucket dredge in operation. Figure 20.1B shows a typical Sauerman grab bucket set-up. A limitation of all grab bucket dredges is that they must discharge in the immediate vicinity of the sediment removal area or into barges or trucks for transportation to the disposal area. Their normal reach is no more than 30 to 40 m. Another disadvantage is the rough, uneven bottom contours they create. Production rates are relatively slow due to the time-consuming bucket swing, drop, close, retrieve, lift, and dump operating cycle. Grab dredges commonly create very turbid water conditions due to bucket drag on the bottom as it pulls free from the sediment, dragging an open bucket through the water column, bucket leakage once it clears the water surface, and the occasional intentional overflow of receiving barges to increase their solids content. Another disadvantage is that many lake sediments are highly flocculent, reducing the pickup efficiency of a grab bucket. Grab-bucket dredges have at least two advantages over the other dredge types: they can be transported with ease from one location to another and they can work in relatively confined areas. Thus, their chief use in lake restoration and management is shoreline modification, particularly around docks and marinas. They are readily operated around stumps and trash frequently found in these areas. A grab bucket operates most efficiently in near-shore areas that contain soft to stiff mud. Depth is no impedance, but efficiency drops rapidly with depth, because of the time consuming operating cycle. Silt curtains reduce some of the turbidity-associated problems mentioned above. A silt curtain is a continuous polyethylene sheet (skirt) buoyed at the surface and weighted at the bottom so it hangs perpendicular to the water surface. It may be used to encircle an open water dredging operation or to isolate a length of shoreline (Figure 20.1). The purpose of the silt curtain is to isolate turbidity within the immediate dredging area, protecting clean surface water areas down- stream. Silt curtains, while effective in controlling surface turbidity, are open at the bottom and permit the escape of turbid water near the sediment–water interface. Another means of minimizing turbidity from grab bucket dredging is to use a covered, watertight unit (Figure 20.2). Watertight buckets range in sizes from 2 to 20 m 3 . Manufacturers claim turbidity reductions from 30% to 70% compared to open buckets of comparable size. The dredging process with watertight buckets is cleaner than with conventional buckets, but production is still relatively inefficient compared to hydraulic dredges. 20.5.2 HYDRAULIC DREDGES There are many variations of hydraulic dredges, including the suction dredge, the hopper, the dustpan, and the cutterhead suction dredge. Hopper dredges are impractical for dredging small inland lakes. Cutterless suction dredges have not been used extensively. Attempts to use one at Lilly Lake, WI, in 1978 were abandoned when it was discovered that the partially decomposed plant material in the sediment prevented it from “flowing” to the suction head (Dunst, 1982). A cutterhead suction dredge subsequently was employed. Dustpan dredges are not commonly used in lake restoration, although a “dustpan-like” dredge was used to remove flocculent sediment from Green Lake, Washington in 1961 and 1962 (Pierce, Copyright © 2005 by Taylor & Francis 1970). The device consisted of a 15.25 m suction manifold with slot openings. The total size of the inlet ports was designed to produce inlet velocities of at least 300 cm/s. As sediment consistency increased with depth, some of the inlet ports were sealed to increase flow velocity in the open ones. The dustpan-like suction head was barge mounted and designed to swing in a full 180° arc and discharge into a 50.8 cm diameter pipeline. The discharge distance was about 792 m. This dredge successfully removed 917,500 m 3 of sediment. Björk (1974) indicated that the dredge head used at Lake Trummen, Sweden had a specially designed “nozzle.” The positive experience at Green Lake and at Lake Trummen indicates that dustpan types and other variations of conventional hydraulic suction heads should receive additional consideration for dredging highly flocculent fresh water lake sediments. FIGURE 20.1 (A) Silt-curtain encirclement of an open-water grab dredge operation. (B) Shoreline isolation of a bucket dredge operation, using a silt curtain. (Cooke et al., 1993. With permission.) A Dredge bucket Buoys Shoreline Barge Silt curtain Silt curtain Sediment surface B Sediment surface Buoys Dredge bucket Copyright © 2005 by Taylor & Francis Inland lake sediment removal is most commonly accomplished with a cutterhead hydraulic pipeline dredge. Small, portable, cutterhead hydraulic dredges are the dominant equipment used for inland lake dredging. The primary components of any cutterhead dredge system include the hull, cutter head, ladder, pump, power unit, and a pipeline to distribute dredged material (Figure 20.3). The hull is made of steel and constructed to withstand the constant vibration created by the cutterhead. The hull is the working platform that houses the main power plant, pump, lever room, and the assemblage of winches, wires, “A” frames, etc., that comprise the dredge. At the bow is a steel boom or ladder with a cutter mounted at its distal end. Ladder length determines the practical dredge depth limitations. The ladder also supports the suction pipe and the cutter drive motor and shaft. In some cases, there may be a submersible auxiliary suction pump mounted on the ladder. The ladder is raised and lowered by suspension cables attached at the outer end and to a hull-mounted winch. The cutter or cutterhead typically consists of three to six smooth or toothed conical blades that rotate at 10 to 30 rpm to loosen compacted sediment (Bray, 1979). Cutterheads may be open nose, closed nose, straight vane, ribbon screw shape, or auger-like. Most cutters have been designed specifically to loosen sand, silt, clay, or even rock material. Few, if any conventional hydraulic cutterheads have been designed to remove soft, flocculent lake sediment, so most of them are less efficient than they could be for lake dredging. Spuds, vertically mounted pipes ranging from 25.4 cm to 127 cm in diameter, depending on the dredge size, are located at the stern of the hull on both sides (Figure 20.4). They are used to “walk” the dredge forward by alternately raising and lowering them into the sediment. Operationally, sediment loosened by the cutter moves to the pickup head by suction from the dredge pump, usually a centrifugal type. The sediment slurry is then discharged by pipeline to a remote disposal area. Cutterhead dredges are described by the diameters of their discharge pipes. Hydraulic dredges used for inland lake work usually range in size from 15 to 35 cm, although the one used at Vancouver Lake, Washington was 66 cm (Raymond and Cooper, 1984). Figure 20.4 shows how the cutterhead is moved from side to side, and how pulling alternately on port and starboard swing wires creates the cut path. A major advantage of hydraulic cutter suction dredges over bucket types is that they are not confined in operation by the limitation of cable reaches. Another advantage is their continuous operating cycle. This cycle permits hydraulic dredges to FIGURE 20.2 Open and closed positions of the watertight bucket. (Redrawn from Barnard, W.D. 1978. Prediction and Control of Dredged Material Dispersion Around Dredging and Open-water Pipeline Disposal Operations. Tech. Rept. DS-78-13. U.S. Army Corps Engineers, Vicksburg, MS.) Shell Rod Rod Cover Cover Rubber packing Open position Closed position Shell Copyright © 2005 by Taylor & Francis produce large volumes of dredged material. This advantage, however, is not without its downside. Most hydraulic dredge slurries contain only 10% to 20% solids and 80% to 90% water. This means that relatively large disposal areas, with adequate residence times, are needed to precipitate solids from the dredge slurry. Also, it means that the large pumping capacity of hydraulic dredges might produce unplanned lake drawdowns, unless disposal-area overflow water is returned to the lake. The amount of sediment supplied to the suction head is controlled by cutter rotation rate, thickness of the cut, and the swing rate (Barnard, 1978). Improper combination of any of these FIGURE 20.3 Configuration of a typical cutterhead dredge. (From Barnard, W.D. 1978. Prediction and Control of Dredged Material Dispersion Around Dredging and Open-water Pipeline Disposal Operations. Tech. Rept. DS-78-13. U.S. Army Corps Engineers, Vicksburg, MS.) FIGURE 20.4 Spud-stabbing method for forward movement, and resultant pattern of the cut. (From Barnard, W.D. 1978. Prediction and Control of Dredged Material Dispersion Around Dredging and Open-water Pipeline Disposal Operations. Tech. Rept. DS-78-13. U.S. Army Corps Engineers, Vicksburg, MS.) Cutter Sediment Shaft A frame Cutter motor Ladder Suction Hoist Lever room Gantry Engine house Spud well Spud Floating line Main pump H frame Hull Winch Dredge Advance Cut “A” Spud (up) Spud (down) A B C D Ladder B D AC Front Windrow Starboard swing wire Port swing wire Cutter Copyright © 2005 by Taylor & Francis [...]... average 135 m3/h 30-cm pump = 310 to 420 m3/h, average 365 m3/h 600-m length of discharge pipeline: Copyright © 200 5 by Taylor & Francis 30 cm dredge 35 cm dredge 800 600 400 20 cm dredge 0 00 80 10 0 40 0 50 0 60 0 30 0 20 0 00 10 0 80 0 60 0 0 50 40 20 30 0 0 3 −1 Solids pumped (m hr ) 200 25 cm dredge 500 400 300 200 100 0 0 0 80 60 0 0 40 30 20 0 10 0 0 80 60 0 40 0 30 0 20 10 0 0 Length of discharge... FIGURE 20. 8 Representative production characteristics for various sizes of dredge systems (Modified from Pierce, N.D 1970 Inland Lake Dredging Evaluation Tech Bull 46 Wisconsin Dept Nat Res., Madison.) 2 0- cm pump = beyond effective discharge length; booster pump required 25-cm pump = 60 to 120 m3/h, average 90 m3/h 30-cm pump = 220 to 290 m3/h, average 255 m3/h 800-m length of discharge pipeline: 2 0- cm... Collins Park and several other small lakes in New York State The portability, guidance system, reduced turbidity, and increased solids content resulting from use of these dredges makes them ideally suited to small lake restoration projects New and improved guidance and operating systems on Mud Cat® dredges have been instrumental in successful dredging of lakes in Europe (see case histories in this chapter) ... elevation head, and the friction head in the pipeline Minor head losses usually are not considered 20. 7.1.4.1 Suction Head Since the weight of dredged material (specific gravity of lake sediment is approximately 1 .20) is greater than water, the surface of a column of water equal to the depth of Dead Lake would always have a greater elevation than the surface of an equal sized (diameter) column of dredged... (20. 5) where: V = velocity in the pipeline (m/s), D = inside diameter of the pipeline (m), and v = temperature-corrected kinematic viscosity of water (m2/s × 10−6) (see Table 20. 1) As stated above, the velocity of suction pipeline slurries commonly ranges from 3.0 to 4.0 m/s or greater to maintain the suspension of solids (turbulent flow) If we use the previous assumed slurry velocity of 3.6 m/s and. .. Equation 20. 9: hdv = S2 Copyright © 200 5 by Taylor & Francis Vd2 − Vs2 2( g ) (20. 9) 2 Friction head loss (meters of water per 30.5 meters of pipe) P – 10 LV Hf = 0.015 1 + 100 2gD 10 9 8 7 6 5 Inside diameter of pipe (m) Inside diameter of pipe (m) 0.15 0 .20 0.25 0.30 0.35 10 9 0.40 8 0.45 7 0.50 2 Hf = 0.015 1 + P – 10 LV 100 2gD 0.15 0 .20 0.25 0.30 0.35 0.40 0.45 0.50 6 5 4 4 10% solids 3 2 2 1 20% solids... reasonable assurance of longevity An estimate of sedimentation rates helps determine the infilling rate and, thus the duration of sediment removal effectiveness Although dredging is expensive per unit of dredged material, where costs are amortized over the life expectancy of the project they may look much more reasonable All other conditions being similar, lakes with relatively small watershed-to-surface ratios... to optimize disposal efficiency by minimizing pipeline length 20. 7.1.2 Analyze the Production Capacity of Available Dredging Equipment It is necessary to analyze the production of various sized dredges to determine which equipment might complete the job within the planned 2-year period A survey of equipment reveals that 20cm, 25-cm, and 30-cm dredges are available, so production analysis is limited... near the center of the lake and that transport from this area to the disposal cells will require pipeline transport distances in excess of 600 m Based on that information, the following dredge pump production range analysis was developed, using the minimum, a medium, and the maximum pipeline lengths: 300-m length of discharge pipeline: 2 0- cm pump = 50 to 110 m3/h, average 80 m3/h 25-cm pump = 80 to... discharge elevation head (meters of fresh water), S2 = specific gravity of the mixture being pumped, ED = elevation of the center line of the discharge pipe at the point of discharge (m), and Ep = elevation of the center line of the dredge pump (m) Therefore, when pumping to disposal areas 1 and 2, the discharge elevation head will be hde = 1 .20 (247.7 − 245.0 ) hde = 3.2 m The discharge friction head is the . are relatively common in fresh water lakes and reservoirs. 20. 5.1 MECHANICAL DREDGES Grab-type mechanical dredges are used commonly in lake restoration (Figure 20. 1). Figure 20. 1A shows a clamshell bucket. the production of various sized dredges to determine which equipment might complete the job within the planned 2-year period. A survey of equipment reveals that 2 0- cm, 25-cm, and 30-cm dredges. dredges (see Sediment Removal Techniques section of this chapter and case histories) and special disposal and treatment techniques (Barnard and Hand, 1978; Matsubara, 1979). A common dredging concern