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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SESSION C. OPTIMIZING DREDGING PROCEDURES CHAIR Henry B.Simmons SPEAKERS Thomas Richardson Michael Trawle John Lunz Ellis D.Hart Henry B.Simmons
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings CHARACTERISTICS OF ESTUARINE DREDGING EQUIPMENT Thomas W.Richardson Chief, Engineering Development Division Coastal Engineering Research Center U.S. Army Engineer Waterways Experiment Station Although there is no one type of dredging apparatus designed solely for work in an estuary, the majority of estuarine dredging in the United States is performed by one of three kinds of conventional dredges. In addition, there are many types of nonstandard dredging equipment or techniques that can be used in an estuarine environment. Both conventional and nonstandard dredges can be successful in an estuary if they are matched to the proper job and applied intelligently. This paper will describe briefly the most common conventional U.S. estuarine dredges and will discuss some nonstandard approaches to estuarine channel maintenance problems. CONVENTIONAL DREDGING EQUIPMENT Bucket Dredges The term “bucket dredge” can have different meanings. In Europe, a common type of bucket dredge is the bucket ladder (Figure 1), which excavates the bottom by a chain of buckets that moves around a large pivoting ladder, all in the vertical plane. Material brought to the surface by the buckets is dumped on a chute and falls into hopper barges brought alongside. This type of dredge is virtually unused in the United States except for limited mining applications. Other types of bucket dredges are the dipper dredge, the backhoe, and the dragline, all of which operate like their land-based counterparts (power shovel in the case of the dipper dredge, and all of which usually depend on barges to transport material away from the excavation site. In the United States, bucket dredge most often means a waterborne version of the terrestrial grab bucket or clamshell crane (Figure 2). Such a dredge can be a specially designed piece of marine equipment. In estuaries, bucket dredges can be used in a variety of situations. They are most often employed for slip, pier, and berthing area maintenance dredging, where the ability to excavate close to structures and to handle debris are important. Bucket dredges are classified by bucket volume, which can range from fractions of a cubic yard to 10, 20, or more. The largest U.S. bucket dredge listed in the 20th annual
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 1 Bucket ladder dredge and hopper barge. FIGURE 2 United States bucket grab/clamshell dredge. Directory of Worldwide Dredge Fleets and Their Suppliers (1986) has a clamshell bucket capacity of 50 yd3. As with other types of bucket dredges, the grab or clamshell must depend on hopper barges to carry material away; the farther the distance between dredging and disposal, the more barges are needed to keep a steady flow of excavation.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 3 Modern hopper dredge. Hopper Dredges A bucket dredge is a simple device, utilizing the same fundamental mode of operation as the “spoon and bag” dredge or other types of dredges from medieval times. By contrast, the modern hopper dredge (Figure 3) may be one of the most complex collections of systems and subsystems afloat, excluding naval warships. In principle, it is straightforward. The dredge moves along a navigation channel under its own power, pulling a suction tube (dragarm) deployed from each side. The dragarm extends down to the bottom, where it terminates in a suction fitting called a draghead. Onboard pumps drag a sediment slurry through the draghead, up the dragarm, and into hoppers on the dredge. When the hoppers are full, the dragarms are lifted to the surface and the dredge becomes a transport device, taking its hopper load to a disposal site. Usually, the dredged materal is disposed underwater by opening the hopper bottoms and allowing it to fall through the water column. Some hopper dredges have the ability to pump material from their hoppers through a pipeline, making land disposal an option. What makes a hopper dredge complex is the variety of equipment and crew skills needed to accomplish dredging. In addition to all the usual machinery and systems found on a large ocean-going vessel, a significant amount of the deck and enclosed space on a hopper dredge is taken up by the dredging pumps, motors, reduction drives, piping, valves, davits, and instruments, and by the hoppers themselves. The current industry trend is to instrument and automate as much of the dredging process as possible to increase operational efficiency. As an example, some modern hopper dredges are equipped to automatically shunt low-density suction material overboard (Figure 4), allowing the hoppers to be filled only with more productive higher-density slurry.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings An even more spectacular design trend is toward split-hull hopper dredges, wherein the entire vessel separates along its center line to achieve almost instantaneous release of its load. In an estuary, the hopper dredge is used primarily for major channel maintenance dredging. This is particularly true in entrance channels and the lower reaches of an estuary, where dredged material may be disposed in open water and where wave action, currents, and vessel traffic can be factors in carrying out dredging operations. Hopper dredges are classified by hopper volume. Small hopper dredges may have hopper volumes of 1,000 yd3 or less. The most prevalent size class of U.S. hopper dredges is the 2,000 to 4,000 yd3 range. The dredge fleet directory lists several U.S. hopper dredges of 6,000 yd3 and greater capacity, including one at 16,000 yd3. FIGURE 4 Automatic overboard system for low-density slurry. Pipeline Dredges The third type of conventional dredge most commonly used in U.S. estuaries is the hydraulic pipeline dredge (Figure 5). Some of the earliest applications of such dredges were in the United States in the mid 1800s. Since that time, it has become the most common type of dredge in the United States. Although there are several types of hydraulic pipeline dredges, including the plain suction, dustpan, and bucket wheel, the term usually connotes the cutter suction dredge. This dredge takes its name from the rotating basket-shaped device used to cut bottom material so it can be drawn into the dredge suction pipe. The cutter, its drive motor and gear, and the suction pipe are all mounted on a structural member called the ladder, which pivots in a vertical plane from the dredge hull (Figure 6). Other major cutter
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 5 Hydraulic pipeline dredge FIGURE 6 Cutter suction dredge ladder and cutterhead suction dredge components are its pumps, power plant, swing winches, spuds, and discharge pipeline. The spuds are two large vertical piles located at the stern that can be independently raised or lowered to the bottom. The swing winches carry wires that pass through sheaves on the ladder and that are connected to anchors set out from the dredge to either side. In operation, with the ladder lowered and the cutter rotating in contact with the bottom, the dredge is swung by one of the
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings winches to pivot about one of the spuds, sweeping the cutter and suction in an arc across the bottom. At the end of the arc, the process is repeated in the other direction. By swinging about alternate spuds, the dredge can advance to cover new bottom. Material removed by the dredge can be pumped through the discharge pipeline for a mile or more, depending on a number of variables. In an estuary, the floating portion of the pipeline will usually be supported on pontoons (Figure 7). The floating line will usually lead to a land-based pipeline and eventually into a disposal area. For long pumping distances, booster pumps may be added to the pipeline to provide the required energy. Pipeline dredges are usually classified by the diameter of their discharge lines, although their cutter and/or pump power is sometimes used as well. The smallest pipeline dredges often fall into the portable category, which means that the dredge is constructed to be transported, either as a unit or in modules, from one job site to another (Corps of Engineers, 1983). Portable pipeline dredge units range from 4–8 in. or larger that can be transported whole, to 24 in. or larger weighing in excess of 400 tons. The larger portable dredges may have more than 2,000 horsepower on the dredge pump and more than 700 on the cutter, but figures of 500 and 100 horsepower respectively are more common for medium-sized portable pipeline dredges (12 to 14 in.). For nonportable pipeline dredges, the trend overseas and to some extent in the United States in recent years has been toward larger and more powerful equipment, with discharge diameters of 32 in. or more. The most powerful cutter suction dredger listed in the dredge fleet directory is a U.S. machine with 30,000 total horsepower. Due in part to their range of sizes and features, pipeline dredges can perform a variety of functions in an estuary. With variations in anchoring equipment and pipelines, the larger dredges can work in most wave and current conditions found in estuaries or entrance channels. FIGURE 7 Pontoon discharge pipeline.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings However, due to their pipelines, anchor wires, work boats, and other ancillary equipment, they usually pose more of an obstruction to navigation than a hopper dredge. Since they are not self-propelled, they have little ability for collision avoidance. U.S. Fleet and Costs According to the dredge fleet directory, the United States dredging industry owns approximately 310 cutter suction pipeline dredges of all sizes, 170 bucket grab/clamshell dredges, and 15 trailing suction hopper dredges. In addition, the Corps of Engineers maintains one 20 in. cutter suction dredge and 4 trailing suction hopper dredges (825, 3,140, 6,000, and 8,000 yd3) as part of its minimum fleet. Costs for the three different categories of dredge are difficult to estimate, since so many site-specific factors influence a dredging job. However, in an attempt to indicate relative costs and approximate ranges, bid data from Corps’ dredging contracts nationwide were estimated for fiscal years 1984 and 1985 and summarized in Table 1. The raw data were listed by contract and dredge type. For this paper, only data from those contracts with one type of dredge were used. Since these one-dredge contracts accounted for more than 280 million yd3 of dredging, the results should be approximately representative of nationwide experience. Table 1 indicates that the relative ranking by cost of the three dredge types was the same for both years, for both average and lowest contract bid, with bucket dredge contracts being the most expensive per cubic yard and pipeline contractors the least. Total pipeline dredge TABLE 1 Summary Data for Selected Corps Dredging Contracts, Fiscal Years 1984 and 1985 Dredge Type Year Relative Volume* Low Bid Price $/cubic Yard Average Lowest Bucket 1984 1.00 3.96 1.70 Hopper 1984 6.56 2.15 1.20 Pipeline 1984 24.90 1.20 0.23 Bucket 1985 1.00 3.95 1.90 Hopper 1985 3.35 1.77 0.41 Pipeline 1985 15.89 1.30 0.25 *Relative volume is the ratio of the total volume by dredge type in a year to the total volume for bucket dredges for that year for the contracts that comprise the Table 1 data.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings contract volumes were from 16 to 25 times higher than those for bucket dredges, and almost 4 to 5 times higher than those of hopper dredges. There is little question that both in fleet size and volume of work, the pipeline dredge exceeds all other major dredge types combined in the United States. Highest contract costs per cubic yard can be almost any figure, depending on the particular requirements of a job. Work involving the cleanup of polluted material, small volumes, or long hauling distances can drive contract costs to $10, $15, or $20/yd3 or more, irrespective of dredge type. NONSTANDARD DREDGING EQUIPMENT Agitation Dredges Agitation dredging is a type of dredging that has been practiced in rivers and estuaries for centuries. Although there is no exact definition, it usually involves the suspension or resuspension of bottom material by some type of equipment and the subsequent transport of that material by currents. Richardson (1984) describes a variety of agitation dredging methods and projects. Of those presented, the most promising and applicable to estuaries are rake or beam dragging, propwash, and hopper dredge overflow. The first type involves simply dragging a rake or beam behind a towing vessel to loosen bottom material, with some possible additional augmentation by propeller wash from the towing vessel. This method has been used for a number of years to clean industrial slips and berths in Savannah Harbor, Georgia. Local operators pay the Corps of Engineers an hourly fee for beam dragging operations, on the assumption that the material they remove will have to be redredged by the Corps in the main channel. FIGURE 8 Propwash agitation dredging vessel.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Propwash agitation dredging usually involves a vessel specially modified for such work (Figure 8). By keeping the vessel in a stationary or slow moving mode and deflecting the propwash downward, bottom material can be effectively resuspended in water several times the vessel’s draft. The Corps of Engineers utilizes a propwash vessel, the Sandwick, in the Pacific Northwest to remove sandy shoals in harbor entrances. In fine-grained material such as is often found in estuaries, the effects of propwash can be dramatic in removing localized shoals. Hopper dredge overflow is a type of agitation dredging wherein material brought to the surface by a hopper dredge is immediately dumped back overboard. Historically, this has been the principal means of maintaining the southwest pass of the Mississippi River, where high shoaling rates can make conventional dredging unfeasible. However, the large-scale use of hopper dredge agitation in the Delaware estuary from 1905 to 1954 created an apparent increase in shoaling by resuspending fine material which was then trapped within the estuary to form “fluff” layers of low-density sediment. Costs for agitation dredging are difficult to assess, since there is usually no way to measure its total effects. For localized shoaling, agitation dredging can be several times cheaper than conventional dredging; Savannah Harbor users estimate costs of beam dragging at 1/10 to 1/30 that of other dredging means. Specialized Equipment One of the problems in estuarine dredging is that the bottom material is often of a low density, meaning that it contains a large percentage of water that is costly to dredge and transport and to remove from upland disposal areas. A number of companies and individuals have attempted to improve on this situation, the most popular approach being ways to increase the density of material brought up from the bottom. An example of this type of approach is a pump operated by compressed air that is designed to dredge sediment at its in situ density (Figure 9). Originally called the PNEUMA pump by its Italian inventor, the device has been marketed in several countries, including the United States, and has undergone a number of modifications by various licensees. The basic pump consists of three large cylindrical pressure vessels, each with a material intake on the bottom and a compressed air port and material discharge outlet on top. The pump operates on the bottom and uses the difference between ambient water pressure and atmospheric pressure to fill a vessel with bottom material. Once full, material is forced out of the vessel by compressed air and through the discharge line. A basic version of the pump was tested by the Corps of Engineers in 1978 under a variety of typical maintenance dredging conditions (Richardson, 1982). In general, the tests indicated that it could achieve in situ discharge densities in fine-grained estuarine material but not in sand. Power efficiency was low.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 9 PNEUMA compressed air pump. REFERENCES Anon. 1986. 20th Annual Edition, Directory of Worldwide Dredge Fleets and Their Suppliers. World Dredging and Marine Construction 22(3) 6–101. Richardson, T.W. 1986. Agitation Dredging: Lessons and Guidelines from Past Projects. Technical Report HL-86–6. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Richardson, T.W., J.E.Hite, Jr., R.A.Shafer, and J.D.Ethridge, Jr. 1982. Pumping Performance and Turbidity Generation of Model 600/100 PNEUMA Pump. Technical Report HL-82–8. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. U.S. Army Corps of Engineers. 1983. Survey of Portable Hydraulic Dredges. Technical Report HL-83–4. Vicksburg, Miss.: Waterways Experiment Station.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Positioning systems operating in the VLF, LF, MF, and HF frequencies are operable well beyond the horizon, frequently to ranges of several hundred miles. Those systems operating at VHF and above, in general are limited to line-of-site ranges, not very much beyond the optical horizon. On the other hand, VHF, UHF, and microwave systems are generally, capable of more precise distance or position measurements, than are the lower frequency systems. Other important points to be considered regarding operating frequency include (1) the somewhat larger and more cumbersome antennas which are normally required by the lower frequency systems (MF and HF), and (2) the greater susceptibility of the higher frequency systems (VHF and UHF) to “multipath” problems. HYDROGRAPHIC SURVEY System Components The previous sections dealt with the individual depth and positioning units of a hydrographic survey system. This section deals briefly with these and the other components which make up a complete hydrographic survey system. The purpose is to show how the measured data are incorporated into the system to form the final product: a depth at a point on the survey chart. Control System Figure 3 shows the components that make up a typical automated hydrographic survey system. Note that all components are connected to the data processor, which is the control center of this particular system. In sophisticated systems, control would be maintained by a computer; in smaller, less automated systems, control unit would be a data logger. The system control units and their capabilities are summarized below. Operating under the system program stored in its permanent memory, the control unit (computer or microprocessor) receives range data from the positioning system. It processes this data using coordinate and control information entered at the operator’s data terminal (Figure 3). These data are then correlated with other external data, such as the depth soundings from the digital depth recorder, and used to drive the other peripheral elements and to record the survey information. The control unit has three basic functions: Gather positioning information and compute the position of the boat in its grid coordinate system. Gather other data, such as time and depth, and correlate this information with the position data. The combined data may then be transferred to a magnetic tape, a printer, or a plotter. Compare the present boat position with a preplanned position or course and deliver guidance output that can be used to reposition the boat.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Positioning System The positioning system shown in Figure 3 consists of the transmitter-receiver with antenna, a range console, and the reference stations or transponders. The position information is fed to the control system as described above. Track Indicator Many surveys are conducted along preselected lines. This is made possible by continuously updated position information provided to the track indicator by the control unit. The indicator gives the boat position as left or right of the given line, which is used for maintaining course. The position information is also used to display the boat position with respect to the distance traveled down the preset survey line. Track Plotter and Tape Recorder The track plotter continuously plots the boat position on preselected survey lines drawn onto the chart. It is usually a moving-pen, moving-paper plotter but may be a moving-pen-only type plotter (flat bed plotter). When the control unit is a computer, the depth measured at each position may be printed onto the chart. This constitutes a fully automated, on-line system. As an alternative, the data may be stored on a magnetic tape or cassette for processing and plotting at the land-based computer center as shown in Figure 3. Digital Printer The digital printer is an option which may be included as a unit of the system. It provides hard copy data for instantaneous analysis, which is sometimes desirable. Information provided includes time of day (to the second), range readings, depth, and other parameters if desired. This information is provided virtually instantaneously. Depth Sounder The depth sounding equipment, an integral part of the system was discussed previously. System combinations may vary from independent range and depth units (manual plotting) to a fully automated system that includes a computer with on-line plotting of all survey data. Computer controlled systems offer the greatest capability and versatility, but require relatively large boats. Small-channel surveying requiring smaller boats is normally restricted to data-processing or data-logging type systems,
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings although small boat systems are now being developed that are fully automated, utilizing microprocessors. Satellite Navigation Satellite navigation and positioning methods are new and developing. Existing satellite systems provide navigation information useful to navigators and geodetic and hydrographic surveyors. Present applications include absolute position reference, calibration of localized referenced systems, extension of onshore control to offshore applications, relative position grid development, and marine navigation. The future of satellite surveying appears to be unlimited. In 1970, the U.S. Department of Defense (DOD) began a review of all navigation systems according to cost and need. The result called for a reduction in the number of DOD funded navigation systems and the development of a single Defense Navigation Satellite System. The selected system was entitled the NAVSTAR Global Positioning System (GPS). The best features of other satellite programs were combined to form the basis for this system, which would eventually permit continuous worldwide surveying and navigation in three dimensions with an accuracy not previously obtainable in dynamic situations. The GPS consists of a constellation of satellites, a master-control station, several tracking stations, an upload station, and an unlimited capacity for users. The satellites broadcast information continuously so that any user located in any part of the world is able to compute near-real-time files at any time. A total of 24 satellites, 8 in each of three circular planes and approximately 20,000 km in radius, will comprise the completed space segment. At least 6 satellites will be visible at any given location and time. REFERENCES American Society of Civil Engineers. 1986. ASCE Task Committee on Measurement and Analysis of Bed Forms (in preparation). New York: American Society of Civil Engineers. American Society of Civil Engineers. 1983. Measurement of hydrographic parameters in large sand-bed streams from boats. Task Committee on Hydrographic Investigations. New York: American Society of Civil Engineers. Blanton, J.O. III. 1982. Procedures for monitoring reservoir sedimentation. Technical Guidelines for Bureau of Reclamation. Denver, Colo.: U.S. Department of Interior. Comstock, A.L. 1984. Radiopositioning systems, present and future; their comparative characteristics and applicabilities. Presented to Pacific Congress on Marine Technology, Honololu, Hawaii, April 24–27, 1984. Hampton, Va.: Teledyne Hastings-Raydist.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Downing, G.C. and T.L.Fagerburg. In preparation. Evaluation of vertical motion sensors for potential application to heave correction in Corps’ hydrographic surveys. U.S. Army Engineer Waterways Experiment Station. Vicksburg, Miss: unpublished. Downing, C. 1973. Supplement No. 1 to the Hydrographic Survey Conference, 30–31 May 1973. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Hart, E.D., and G.C.Downing. 1977. Positioning techniques and Equipment for U.S. Army Corps of Engineers Hydrographic Surveys. TR H-77–10. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station, CE. Ingham, A.E. 1974. Hydrography for the Surveyor and Engineer. New York: John Wiley & Sons. Klein Associates, Inc., Undersea search and survey. Unpublished literature. Klein Drive, Salem, New Hampshire. Laurila, S. Electronic surveying and navigation. New York: John Wiley & Sons. Laurila, S. Electronic surveying and practice. New York: John Wiley & Sons. National Oceanographic and Atmospheric Administration. 1977. Positioning Systems. Report on the Works of WG 4146, XV International Congress of Surveyors, Stockholm, Sweden, June 1977. Racal Positioning Systems Limited. Unpublished literature. Kingston Road, Leatherhead Surrey KT22 7ND, England. Raytheon Company. 1966. Underwater acoustics, a brief description, Submarine Signal Division, P451-Aug 66. Portsmouth, R.I. Raytheon Company. Ocean Systems Center. Unpublished literature. Portsmouth, Rhode Island. Von Dyck, S. 1976. Radio aids hydrographic surveying. Course Notes, Hydrography II, Canadian Hydrographic Service.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings CASE HISTORY, SAVANNAH HARBOR GEORGIA Henry B.Simmons U.S. Army Engineer Waterways Experiment Station (Retired) In 1962, I conducted a brief study of Savannah Harbor, Georgia to determine how shoaling rates and patterns in the harbor had responded to several changes in length and depth of the navigation channel. The results of this study were first published in the minutes of the Federal Interagency Sedimentation Conference of the Subcommittee on Sedimentation, Inter-Coastal Waterway Research, held in Jackson, Mississippi, on 28 January–1 February 1963. They were subsequently republished as Technical Bulletin No. 8, Committee on Tidal Hydraulics, Corps of Engineers, U.S. Army, March 1965. Although both of these publications are out of print, copies can be obtained on loan from the Research Center Library, Waterways Experiment Station. Figures 1 through 5 are from the 1962 study. Figure 1 is a location map of Savannah Harbor and shows channel stations throughout the harbor. Figure 2 shows the four channel conditions included in the study. Figure 3 shows the net distribution of flow in the harbor for condtions of the 34 ft channel (1953–1954 conditions on Figure 2). Figure 4 shows the relationship betwen bottom flow predominance and shoaling distribution for the 1953–1954 channel condition, and it may be noted that the regions of heaviest shoaling brackets the location of the point of zero flow predominance in Front River. Figure 5 shows average annual shoaling in the downstream, central, and upstream one-thirds of the harbor for the four channel conditions, as well as the average annual shoaling for the entire harbor for each channel condition. Several significant changes in shoaling patterns and rates were revealed by the study. First, as the harbor channel was progressively deepened and/or extended upstream, the region of heaviest shoaling moved progressively and/or extended upstream. The region of heaviest shoaling moved progressively from the downstream one-third of the harbor to the central one-third and then to the upstream one-third. This major change in shoaling pattern was attributed to intensification of density current action as the channel was deepened or extended, with the result that the nodal point for bottom flow predominance progressively moved in the upstream direction with each change. Second, the total shoaling rate of the harbor increased progresively as the channel was deepened and/or extended. However, it was
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 1 Location of Savannah Harbor. FIGURE 2 Past and present channel depths. FIGURE 3 Distribution of flow in Savannah Harbor.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 4 Relation of shoaling and performance of bottom flow. FIGURE 5 Past and present shoaling characteristics of Savannah Harbor. noted that the latest increase in channel length and depth (1953–1954), which was by far the most drastic of those accomplished during the period covered by the study, increased total shoaling of the harbor by only about 15 percent, compared to much larger percentage increases for the earlier increases in length and/or depth. This finding suggested that shoaling in Savannah Harbor is probably source-limited to about 7
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings million yd3 per annum, and if so, further increases in channel length or depth would probably not be accompanied by major increases in total shoaling. Subsequent to the study just described, the Savannah Harbor navigation channel was deepened to 36 ft, and several years later most of it was further deepened to 40 ft. Dredging accomplished for these greater channel depths verify the conclusions reached during the study; namely, that shoaling of this harbor is source limited at about 7 million yd3 per annum. It is my understanding that, until an improvement plan (to be described later) was implemented, there was some further increase in shoaling in the upstream one-third of the harbor and an accompanying reduction in shoaling in the central one-third. This continuing trend indicates that density current effects were further intensified by these two additional increases in channel depth, with the result that the nodal point for bottom flow predominance moved still farther upstream. At the conclusion of the reference study, a plan of improvement for Savannah Harbor was suggested that was estimated to substantially reduce the cost of maintenance dredging, although no reduction in the total volume of maintenance dredging was expected. Concentration of most of the shoaling in the upstream one-third of the harbor by channel improvements had resulted in an extremely high-cost dredging activity in Front River for several reasons. First, shoaling was so rapid that dredging had to be done frequently, thus requiring numerous expenditures for mobilization and demobilization of dredging plant. Second, the upstream one-third of the harbors is extremely congested because of the many docking facilities and related movement of vessels, so dredging operations were not very efficient in this environment. Finally, the region of heavy shoaling was far removed from suitable disposal areas, so that retention dikes had to be constructed well above optimum height, or long dispoal lines with booster pumps had to be employed to reach satisfactory disposal areas. The improvement plan suggested for Savannah Harbor is shown in Figure 6 and consisted of three principal elements: (1) a large sediment basin to be dredged in lower Back River, with a deep channel connecting the sediment basin to the navigation channel; (2) a dam with tide gates located in Back River just upstream from the sediment trap, the gates to open at the start of the flood current phase in Back River and close at the end of the flood current phase; and (3) a channel connecting Back and Front rivers at the mouth of the Middle River. In operation, the tide gates would remain open during the flood current phase in Back River, allowing the tidal prism of Back River to fill in the normal manner and sediment-laden water to be drawn into the sediment basin from the navigation channel. When the tide gates closed at the end of the flood current phase, the tidal prism of Back River upstream of the dam would flow through the connecting canal and then through Front River, substantially increasing the ebb current velocities in Front River and causing deposited sediments to be resuspended and carried downstream past the junction of Back and Front rivers where they could be drawn into the trap during the next flood current phase. Closure of the tide gates during the ebb current phase would reduce ebb
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 6 Elements of sediment trap plan. current velocities in lower Back River to such extent that an environment conducive to rapid deposition of suspended sediments in the trap area would be created. It was estimated that if this plan were constructed and placed in operation more than one-half of the future dredging in Savannah Harbor would be accomplished from the sediment trap. It is very significant that the trap would be located adjacent to an extensive area that could be developed to accommodate spoil disposal for a great number of years. It was also estimated that the greatest effect of the plan would be felt in Front River, since the significantly increased ebb current velocities should greatly reduce shoaling in this region of high-cost maintenance dredging. The improvement plan just described was completed and placed in operation in 1977. For the next five years (1977–1982), average annual shoaling of Savannah Harbor was 6.8 million yd3 of which 59 percent was dredged from the sediment trap and the remaining 41 percent was dredged from the navigation channel. While I am unable to make direct cost comparisons for maintenance dredging of Savannah Harbor under pre-and post-improvement plan conditions, it is certain that the improvement plan provides substantial monetary savings for the following reasons: Almost 60 percent of the total maintenance dredging is performed in the sediment trap where only limited movement of the dredge is required and there is no interference with dredging operations by ship movements, thus maximizing dredge efficiency. The sediment trap is located adjacent to an extensive area. suitable for spoil disposal. The reduction in navigation channel shoaling effected by the improvement plan significantly increases the time interval
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings between dredging operations and thus reduces the amount expended for dredge mobilization and demobilization. The plan effected a major reduction of shoaling in Front River, which was the highest unit cost dredging area in the harbor. The capacity of the sediment trap is about 9 million yd3, which is sufficient to accommodate more than two years of normal deposition, so dredging to restore the trap can be scheduled to take advantage of efficient scheduling of equipment and favorable weather conditions. Annual maintenance dredging requirements for pre- and post-improvement plan conditions in Savannah Harbor indicate that plan operations did not increase total maintenance dredging; in fact, average annual maintenance dredging appears to be a few hundred thousand cubic yards less following implementation of the improvement plan. This reduction, if it is real, is believed to be attributable to improved dredging efficiency under plan conditions. All dredges tend to agitate and resuspend more sediment than they physically remove from the channel, and this tendency should be reduced to a minimum in dredging from the sediment trap. Any reduction in resuspension and loss of sediment to the system during dredging operations effectively reduces the total sediment source and this should reduce annual maintenance dredging requirements.
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Representative terms from entire chapter: