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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings OVERVIEW OF SYMPOSIUM TECHNICAL SESSIONS SESSION A. SEDIMENT SOURCES AND TRANSPORT PROCESSES Ashish J.Mehta and Hsieh Wen Shen, Session Chair Sediment Sources Knowledge of sediment sources is essential to the design of effective sedimentation control measures. On a long-term basis, suspended load delivered by rivers constitutes the majority of sediment entering estuaries and the marine environment. River sediment discharge is highly episodic. The vast majority of annual sediment load in many tributaries is delivered in a relatively short time period, about 30 days out of a year. It is therefore essential for determining sediment inflow and its temporal distribution that long-term and continuous gauging of river discharge and sediment load be carried out. Clay sediments, such as those containing montmorillonite, illite, and kaolinite minerals, become cohesive when the salinity exceeds one to two parts per thousand, that is one or two parts ocean water in 32 or 33 parts river water. Clays (and silts to a much lesser extent) are thus cohesive well before they enter the lower portions of an estuary. The mixing zone of an estuary, the reach where river and ocean waters mix, provides velocity gradients that promote collision and aggregation of suspended particles and recirculation of aggregated particles. Recirculation is caused by net landward flow of saline water near the bed under the seaward flowing fresher water near the surface. Suspended aggregates carried landward near the bed mix upward into the fresher water where they again are carried seaward and eventually settle back to the near-bed flow. This suspended sediment recirculation is evidenced by the turbidity maximum that is commonly observed along the axis of estuaries. The high concentrations of suspended aggregates that occur in mixing zones further enhance the rate of aggregation of riverborne particles as they enter the mixing zone from upstream. This high concentration also causes rapid deposition in areas that are deepened for navigation within these regions. Studies of transport and deposition in the mixing zones of the Delaware, Savannah, Mississippi, Gironde, and Amazon rivers and San Francisco Bay show that the processes described above are widespread. Shallow bays provide hydraulic conditions that favor the deposition of riverborne sediments during short periods of high sediment discharge. Such bays are subject to frequent onshore breezes during spring and summer seasons, and their long fetches promote the
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings generation of waves. These waves can resuspend the newly deposited material for circulation by tidal currents. The bays thus act as continuing sources of suspended sediment. It is apparent that sediment transport in estuaries is a complex combination of processes and that each estuary is unique by virtue of its configuration, the local ocean tides, the hydrology of its contiguous drainage systems, the character of its sediments, and its exposure to winds. Further elaboration of estuarial sediment transport processes is presented below. Sediment Transport Processes The major hydrodynamic forcing mechanisms for suspended sediment transport are currents and waves. Currents are largely due to tides and river flows. Waves are typically generated by winds, which can also drive currents, particularly in confined water bodies. Tides, freshwater inflows, and winds are the predominant factors responsible for circulation of suspended sediment in estuaries. The effects of all of these mechanisms are complicated by variations in salinity and temperature throughout an estuary, which increases the difficulties of describing suspended sediment circulation and establishing sediment budgets. Physical processes that determine cohesive sediment transport in this environment include suspended particle aggregation, settling and deposition, bed consolidation, bed erosion and bed material entrainment, and convective and dispersive transport of suspended particles. These processes respond to the typically quasi-cyclic, tidally driven environment. Aggregation, settling, and deposition and bed consolidation can progress simultaneously, each at varying rates depending on the ambient hydraulic conditions. As noted above, cohesion of suspended riverborne particles begins where the salinity reaches one to two parts per thousand. The strength of the cohesion increases with increasing salinity and is further affected by organic detritus, microbes, and polysaccharides associated with the mineral particles. Aggregation of suspended particles is the result of particle collisions due to moderate velocity gradients, although Brownian motion and differential settling velocities can become important mechanisms when the concentration of suspended particles is high. These latter mechanisms produce ragged, weak aggregates, however, that are easily dispersed by velocity gradients. Aggregates produced by velocity gradients are stronger and denser. High velocity gradients can break aggregates into smaller component fragments, which can later aggregate in lower velocity gradients. The rate of aggregation is strongly dependent on the suspended sediment concentration and the local velocity gradient. The density of the resulting aggregates depends on the ambient velocity gradients, the shape and density of the constituent particles, and the strength of interparticle cohesion.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings The fall or settling velocity of aggregates is the key parameter needed to determine the rate of deposition and the build-up of bottom deposit. The settling velocity depends on the aggregate sizes and bulk densities formed from the fine suspended mineral and organic material present in a particular system, on the flow conditions, and on the concentration of suspended solids. Limitations of the knowledge of the complex processes that determine aggregate sizes and densities described above make the prediction of settling velocities unreliable, except under limited conditions, and prediction of suspended sediment transport in estuaries now requires field measurements of this important parameter. The presence of suspended sediment affects the flow field, particularly where the sediment concentration is high. The increased suspension density of the commonly observed “fluid mud” layer near the bed affects the velocity profile, as does stable salinity or temperature gradients, except that the fluid mud often has a much sharper upper boundary and the mud can have a higher apparent viscosity. The relations between properties of such mud suspensions and their movement under various hydrodynamic conditions need study in order to develop improved predictive transport relations. Erosion of deposited sediment results from bed stresses applied by currents and waves. The rate of erosion and the depth in the bed to which erosion occurs is strongly dependent on the profile of bed strength relative to the applied stresses. This profile typically shows increasing strength with depth due to increasing consolidation with depth by overburden. The strength is enhanced over time after deposition by the processes of gelling and microbial action. Erosion of deposits can be important to maintenance of navigable water depths because when the concentration of suspended sediment is high, deposition occurs during slack water, and prevention of the accumulation of such deposits requires that the material be eroded by subsequent tidal currents before its strength increases. Knowledge of the erosion processes is also important for the description of sediment circulation and its supply from shallow bays. As noted above, each of the transport processes described deserve further study. In addition, the transport of cohesive-cohesionless mixtures is just beginning to be understood. The hydrodynamics of the tide- and wind-driven, free surface, variable density flows that transport suspended sediment in estuaries should also be studied to improve the accuracy of predictions of sediment transport. Measurements Although the basic transport processes described above occur widely, their relative importance and the parameters for their description vary from place to place. Successful prediction of sediment deposition in proposed harbor facilities and design of facilities having minimum sedimentation rates require a carefully designed and executed measurement program that provides the basic data. Extensive and useful field measurement programs have been
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings mounted by the Waterways Experiment Station, U.S. Army Engineers, and others. These programs have had serious limitations, however, because some of the measurements were instantaneous measurements of widely varying conditions or have been integrated over time and space. Examples include instantaneous bottle samplers and integrated pump samplers for suspended solids determination. Rapid development of methods to improve suspended solids concentration measurement has been in progress the last few years. For relatively low concentrations in the upper part of the water column, devices consisting of pairs of optical turbidity meters, together covering a wide concentration range, have been developed. These devices can be towed or allowed to free-fall through the water column. Radiometric transmittance gauges are required for very dense suspensions. A recent development is a high-speed transmittance gauge, in use at the port of Zeebrugge in Belgium, which allows for the first time measurement of the concentration profile in an undulating mode while underway. It has enabled the generation of maps of density structure of the bed simultaneously with maps of water depth from echo sounder surveys. Such devices have made possible the study of suspended solids distributions in large estuaries such as the Severn and the Rhine. Large data sets from rapid and continuous measurements have been collected. These studies have shown that real estuarial processes, such as sediment entrainment, are highly variable both spatially and temporally. In general, suspended solids loads are typically asymmetrically distributed across an estuary, so that selection of sampling locations in the cross section is very important. Remedial measures that are adopted to combat intrusions of near-bed, dense sediment layers over narrow zones of an estuary are quite different from the techniques required to prevent the incursion of suspensions that are dispersed over the entire water column. Unidirectional and wave-induced oscillatory currents and associated turbulence are strongly influenced by the presence of a high concentration of suspended sediment. Presently, most flow measuring devices either operate in clear water or in waters having low concentrations of suspended solids. Instrumentation for obtaining a reliable measurement of the flow field in suspensions at high concentration are inadequate. Prediction of Sedimentation Four principal techniques are available for estimating sedimentation rates of cohesive materials in proposed harbors or after proposed harbor modifications. These techniques include analysis of field data, physical modeling, mathematical modeling, and hybrid modeling. The analysis of field data has, for instance, been used to predict future delta growth at the mouth of the Atchafalaya River, Louisiana, by extrapolation from past records. Physical modeling is useful for predicting current fields, especially in variable salinity flows. Physical modeling of cohesive
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings sediment transport is limited by the inability to represent aggregation and disaggregation of transported particles and the difficulty of reproducing deposition and erosion processes. Mathematical modeling can be either analytical or numerical. Analytical methods are useful for providing approximate estimates in small, simple systems such as marina basins and short channel segments. Numerical techniques are by far the most powerful. “Zero-dimensional,” e.g., time-dependent and spatially averaged, and one- and two-dimensional models are currently in use. Three-dimensional models have been developed recently and are likely to find extensive use in the future as costs of computation decrease. The hybrid approach makes optimal use of both physical and mathematical modeling by using the physical model to obtain the current field and the numerical model of suspended sediment transport to predict sedimentation rates. This method, pioneered by the Waterways Experiment Station, has been used to study both the Columbia River estuary and Kings Bay, Georgia. The performance of all of these predictive techniques depends very sensitively on the amount and quality of available field data, including hydrographic surveys, suspended solids data, analysis of bed material, salinities, currents, winds, and tides, as well as freshwater and sediment discharges to the estuary. The predictive techniques have advanced to the stage where they are very useful tools for the design of harbor facilities that have minimum sedimentation rates. The models are limited, however, by the limitations in understanding of sediment transport processes described in the previous section. Transport process descriptions that are presently incorporated in models, such as those for deposition and erosion, are based on laboratory studies and include empirical relations. Less empiricism and better establishment of the correspondence between laboratory and prototype processes are needed for more accurate predictions of sedimentation rates. SESSION B. EXISTING FACILITY MODIFICATIONS Scott A.Jenkins, Session Chair Historical Perspective A number of remedial measures, both active and passive, can be implemented in an existing harbor having sedimentation problems. Many harbors are in fact historic harbors, sited in the days of wooden sailing ships. At that time, ship drafts were compatible with naturally available depths of estuaries. With the advent of iron hulls in the middle of the last century, the draft of ships using these harbors increased dramatically. At the same time, the emergence of steam power made it possible for the first dredges to artificially deepen harbors and provide the needed extra depth. One hundred and twenty years of technical refinements have resulted in powerful, highly efficient dredges, largely through advances in diesel and gas turbine engines, centrifugal pumps, and computer control. As a result of such efficiency, dredging has received worldwide acceptance as the primary
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings remedial measure for controlling sedimentation in existing harbors. So what has been the motivation behind recent efforts to seek alternatives to dredging? Although sedimentation is so acute in some harbors that the annual burden of maintenance dredging represents a significant fraction of total port revenues, generally this is not the case. In most harbors dredging costs are actually a small fraction of annual revenue. Instead, the need for alternatives to dredging has grown out of the dilemma now presented by disposal of dredged material. Legislation implemented in the early 1970s placed new environmental constraints on dredged material disposal in ocean and estuarial waters, and many available land disposal sites will reach their maximum capacities in the near future. In addition to the disposal dilemma, the U.S. Navy is faced with the continual problem of maintaining readiness of strategic harbors. As indicated in the first section, sediment supply often occurs episodically and the bulk of annual sedimentation occurs over a short period. Dredging cannot prevent shoaling; however, it can correct shoaling that has occurred, and overdredging can mitigate the effects of shoaling. The loss of access and egress during the time required for a dredge to be mobilized and to remove deposition after a major sediment discharge is an intolerable situation in a military port. Hence, the first structured research and development program to seek out new alternatives to maintenance dredging was initiated not in the private sector, but by the U.S. Navy beginning in the mid 1970s. This search has in part involved a rediscovery of some ancient wisdom. In the fifth century B.C. the Phoenicians apparently learned a passive way of excluding sediments from their harbors. The harbor at Tyre was fitted with a system of proportional weirs which captured wave overtopping and released it in a sudden burst to flush sediments out of the harbor inlet (Inman, 1976). The eigihth century Chinese learned how to actively pass sediments on through their harbors near Canton using a large rolling pin device. This device, translated literally as “muddy river dragon,” was fitted with a number of blades or teeth and was drawn along the bottom by a team of horses pulling from the banks of the river. When this action was synchronized with the ebbing tide, the sediments were resuspended by the teeth of the rolling pin “dragon” and carried seaward. Modern Developments The modern remedial measures outlined by the authors in the session papers utilize either of two ancient approaches: those which exclude sediments from the harbors, and those which act to pass sediments on through the harbors. Some of these methods are active and involve expenditure of man-made power while others are passive and simply manipulate available tidal and river flow energy. The first pioneering efforts in this area were by R.B.Krone. He emphasizes passive measures which involve modification of some of the gross features of waterfront structures. Modifications to cause sediment to pass through harbor facilities are designed to minimize sedimentation rates and to
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings produce frequent local flow velocities that are adequate to scour sediments that do deposit. Modifications to waterfront structures designed by Krone to exclude sediment laden waters utilize quay or sheet pile structures that minimize tidal ventilation into berthing areas. Approaches tested subsequently by J.A.Bailard and S.A.Jenkins invoked systems that could be added to existing waterfront structures without modifying their gross features. Among the new devices tested were (1) a jet array which discharges high velocity water to scour the sediment bottom during the period of ebbing tide; (2) wings and lifting bodies which, when moored near the channel bottom, create turbulent wakes that act to resuspend the sediments; and (3) silt curtains which, when deployed across the open ends of cul-de-sac berths and turning basins, can exclude sediments passively. None of these new approaches can be implemented at every harbor site. As yet there are no universal, generic solutions. This is in part because implementation of these new approaches requires detailed, a priori knowledge of sediment transport processes at a particular harbor site. Once these processes are known, hydraulic and sediment characteristics can be identified and exploited by one or more of the methods outlined in the session papers. For example, some of these methods require bottom currents that exceed a minimum strength while other methods are best suited to situations with no bottom flow whatsoever. The active measures have a limited radius of influence, and because they consume energy, are economical only at shoaling rates that exceed a minimum. If the problem region has a small shoaling rate or large horizontal dimensions, the passive measures are practical. In spite of such limitations, there is generally enough diversity among the shoaling problems found in harbors that local areas do exist where these new approaches are economically favorable when compared with conventional maintenance dredging. Consequently, elimination of all shoaling problems in a harbor is usually not feasible, but reducing them by local control is practical. Although these new approaches may circumvent a number of adverse environmental consequences associated with dredging activities and dredged material disposal, some of them may still conflict with environmental regulations and may require assessment to mitigate potential hazards. Future Concerns and Needs The level of effort in developing new approaches to sedimentation control has been a miniscule fraction of either the annual expenditure in dredging or the revenue that has been generated by harbors being dredged. Research and development have been of limited scale and duration, and have generally served only to validate concepts. Additional prototype testing is needed to realize the large potential for these new approaches, and to further refine engineering methods so that system lifetime can be maximized, maintenance costs minimized, and the highest degree of compatibility with ship and waterfront activities achieved. While prototype testing will refine ideas that have already
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings passed conceptual testing, the search for new yet untested approaches must continue. Modeling can be useful in this search because it permits a wide range of alternative measures with variations to be evaluated at minimal cost. In particular, development of “global solutions” to sedimentation problems of an entire estuarine system is practical only through modeling. To ensure that practical solutions result from this process, theoreticians must communicate with users and carefully define the constraints at each harbor site. This approach will entail a cooperative research and development program where academic and government laboratories receive inputs from port operators, pose solutions, simulate responses, and ultimately return to the port operator to validate the most promising new ideas in prototype operational testing. SESSION C. OPTIMIZING DREDGING PROCEDURES H.B.Simmons, Session Chair The five presentations in this session covered dredging equipment, dredging techniques, environmental considerations of dredging, hydrographic surveying equipment and procedures, and the case history of the Savannah Harbor, Georgia, project in which the cost of maintenance dredging, although not total annual dredging volume, was reduced significantly. Dredging Equipment There are numerous types of dredging plant available to perform new work and maintenance. The most common, and perhaps the least expensive, is the cutterhead pipeline dredge, although costs can rise sharply in restricted channels with much traffic or if long discharge lines with booster pumps are needed to reach disposal areas. The number of self-propelled hopper dredges operating in the U.S. has increased markedly in recent years. They are very efficient in dredging channels in wide bodies of water and in entrances and offshore areas exposed to wave action. The unit cost of hopper dredge work also increases rapidly with distance to the disposal area. Clamshell and dipper dredges are very useful in working in close quarters and near bulkheads and pier faces, although their production rate is small compared to the larger pipeline and hopper dredges. Numerous special purpose dredges are available to work either singly or in cooperation with larger and different types of dredges when needed. When planning a dredging operation, the final decision as to which type of dredge to use should be based on availability of plant, magnitude of the job, economics, and minimizing harmful environmental effects.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Advance Maintenance Advance maintenance dredging is a technique employed frequently in recent years to reduce maintenance dredging costs. This process involves dredging a channel deeper and/or wider than its design or authorized depth, so that substantial shoaling can occur without jeopardizing use of the channel for navigation. Advance maintenance can greatly increase the intervals between maintenance dredging operations, thus reducing the number of mobilization-demobilization actions and their attendant costs. The technique is considered economically sound if funds saved through reduced dredging frequency exceed funds required to remove any additional sediment that may have been deposited as a result of the deeper or wider channel. The two ways to determine whether advance maintenance of a particular channel is likely to save funds are (1) to analyze and extrapolate shoaling data for previous lesser depths of the channel, and (2) use numerical models to simulate estuarine hydrodynamics and sediment transport. If data are not available for analytical studies, and numerical model studies are deemed too expensive or time-consuming, the technique of trying advance maintenance to a modest degree can be implemented and results evaluated. If funds are being saved, the degree of advanced maintenance dredging can be increased; if not, the practice can be abandoned. Environmental Considerations Environmental considerations during dredging operations are numerous and include the nature and location of the material to be dredged, whether materials are contaminated and if so with what, potential resuspension during dredging operations, movement and subsequent redeposition of resuspended sediments, use of the habitat involved by humans and animals, type of dredging equipment available, and method of disposal of the dredged material. Reports by the Waterways Experiment Station on results obtained during the Dredged Material Research Program (DMRP) provide a good source of information about environmental questions. Of special concern is the timing of dredging operations to minimize damage to nursery and/or spawning areas during critical periods for various animals. Environmental consideration in any planned dredging operation should be discussed with all interested governmental agencies as well as the general public as early as possible to avoid last minute objections and criticisms that might delay or cancel the planned operation. It is worth noting that the presence of even a few “hot spots” of contaminated dredged material in material that is otherwise unpolluted can have a disproportionately heavy impact on overall dredging feasibility and disposal costs. Consequently, it may in many cases be useful to consider the possibility of selectively identifying, removing, and managing small quantities of contaminated dredged material as a means of facilitating and reducing the cost of disposing of the remaining cleaner material.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Hydrographic Surveys Hydrographic surveys are important for many reasons; perhaps the most important is to determine when and where a shoal develops, and its rate of growth. Such information is essential to the engineer concerned with how to reduce maintenance dredging. Too often data are not available until the shoal has grown to the point that it impedes navigation. State-of-the-art equipment is available for hydrographic surveying, but it is very costly. Additional research is in progress to improve the equipment and its use. Case History The case history of Savannah Harbor demonstrates that it is possible to greatly reduce the cost of maintenance dredging, even though the amount of dredging required may be reduced very little, if at all. Sediment traps and related facilities that cause shoaling to occur in off-channel sites and near suitable disposal areas offer a method to reduce maintenance dredging cost, provided that total shoaling does not substantially increase and thus offset the savings. This case history also demonstrates that the development of navigation channels in some estuaries has progressed to the point that essentially all available sediment is being deposited. Total shoaling thus becomes source limited, and the navigation channels can be further deepened and widened without major increases in total dredging requirements. A matter that must be given serious consideration, however, is that deeper channels almost always change the location of the region of heaviest shoaling. If such a change moves the heavy shoaling area farther from the principal disposal area for dredged material, the cost of maintenance dredging will likely increase. Conversely, if the location is moved closer to the disposal area, the cost will probably be reduced. SESSION D. DREDGED MATERIAL DISPOSAL CONSIDERATIONS Charles R.Roberts, Session Chair The presentations in this session covered information about location, methods, effects, and regulation of dredged material disposal. Case histories of material disposal in San Francisco Bay and Tampa Bay reveal problems of land and estuary disposal. The process for selecting sites for ocean disposal and the effects of various types of dredging equipment on dredged material are also reviewed. Material Disposal in San Francisco Bay Navigation and recreation are important beneficial uses of San Francisco Bay. Navigation of modern ships in the bay requires continual maintenance dredging. Disposal in aquatic environments has
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings been curtailed since the early 1970s. This has significantly impacted dredging costs in the San Francisco Bay system. Some environmental advocates tend to regard all dredged material as polluted. Even though extensive studies by the Corps of Engineers have shown that only a fraction of bottom sediments is contaminated, disposal of all dredged materials is still severely restricted by many environmental agencies. As an alternative to aquatic disposal, upland disposal has been encouraged. Unfortunately, the areas defined as “upland” have been decreasing, while the areas defined as “wetland” have been expanding around the Bay. Further complicating the situation is the concept of “water-dependent use,” introduced as a criterion for permits to use nearshore land sites. The current definition of water-dependent use does not include use for disposal of dredged material. Despite these developments, land disposal sites have provided good environmental benefits. However, land sites are becoming economically infeasible as existing low cost sites are filled. Unless a beneficial future use of a site can be developed to mitigate the costs, land-based disposal is likely to be expensive. Such opportunities are disappearing. The absence of available sites emphasizes the need for establishment of a water disposal site in lower San Francisco Bay, and an expansion of the definition of water-dependent use to include dredged material disposal. Diminishing land disposal capacity also reinforces the desirability of reserving such capacity for material that cannot be safely or legally placed in open water. This argues for discrimination in characterizing and handling dredged material that may be predominantly clean, but contains a few “hot spots” of contaminated material. Material Disposal in Tampa Bay Tampa Bay is a tropical, full estuary. It is very shallow, with sea grasses and a sandy bottom, and very clean. There are 65 mi of deepened channel in Tampa Bay that require significant maintenance dredging. Historically, disposal has been done by continuous sidecasting. In the late 1960s, dredged material was used to create approximately 2,500 acres of fill areas in Tampa Bay. This activity was halted in 1972 by passage of environmental laws restricting dredged material disposal. One harbor deepening project was grandfathered, and the dredged material was placed on two large islands; one 625 acres, the other 550 acres. Now there is no open-water disposal for new work dredging, and the islands can be used only for disposal of material dredged for maintenance. Furthermore, sand plumes off of the islands that are created by storms have aroused public resistance to continued enlargement of the islands. In the search for land disposal sites, harbor operators have been faced with the following considerations: avoidance of wetlands, limited feasible pumping distances, contamination of fresh water aquifers,
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings avoidance of farmlands, limitations on dike heights, no interference with transportation corridors, endangered species restrictions, volatilization of ammonia, social considerations, and cost of land. An example of the impasse that these considerations create is exemplified in the Alafia and Big Bend channel projects, which require the disposal of 11 million yd3 of silt, sand, and rock. Eight potential land disposal sites were considered, but high costs made the project infeasible. A study showed that cost of land for the most desirable site was $31 million, and the total cost of land disposal would be $80 million. Restrictions on local disposal and the prohibitive costs of exporting dredged material have stopped construction. The National Problem The dredging program managed by the Corps of Engineers includes 25,000 mi of channels serving 400 ports; 130 ports are dredged each year. The average annual amount of maintenance dredging by the Corps is 260 million yd3 at a present cost of $450 million. The total amount of dredging (maintenance and new construction by both the Corps and private dredgers) is 497 million yd3: 14 percent is disposed of in the ocean, 29 percent in coastal waterways, and 57 percent in inland waterways. This amount will increase as projects identified in P.L. 99–662 (The Water Resources Development Act of 1986) and the Navy’s homeporting projects are implemented. Six large projects have disposal requirements aggregating well over a billion cubic yards. About 10 percent of this material is contaminated and will require containment. The 90 percent that is not contaminated can be put to beneficial uses, such as beach nourishment, aquatic habitat development, fill for industrial areas, and sand mining. Even though the projects will be phased over several years, it is essential that planning begin for disposal of dredged materials associated with these new projects, as well as for continuing maintenance dredging nationwide. Dredged Material Properties The character of dredged fine sediments depends on the method of dredging employed. Mechanical dredges, such as the clamshell dredge, typically load barges with excavated material having a density that is nearly that of the bed. Hydraulic dredges, such as cutterhead dredges and hopper dredges, mix water with the dredged material as it is taken in by the cutter head or drags. The material loaded into barges for transport from cutterhead dredges or by hopper dredges is a slurry
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings containing about four parts water to one part sediment (which itself may be 90 percent water by volume). For a given volume of bed material excavated, therefore, the costs of barge transport of material excavated by a mechanical dredge is less than that of material excavated by hydraulic dredges or hoppers. The cost of transporting material from cutterhead dredges is greatly reduced if the disposal site is within reach of its pipeline. Land disposal is only feasible for cutterhead dredges if the dredged material can be pumped to the disposal site. Rehandling of hopper-dredge loads has been practiced in Delaware Bay, but the time that the dredge remains at the rehandling station subtracts from the time that it can be dredging. Rehandling material from a mechanical dredge does not appear to be feasible. Open-water disposal of material from bottom dump barges is a short-lived process. The material falls to the bed in a dense plume. Slurry from hydraulic dredging spreads as it encounters the bed and settles. If there is a strong current, the plume drifts down current, and the material spreading on the bed may be entrained and carried with the current near the bed. Denser material from mechanical dredging falls to the bed where it spreads to a lesser extent. Because of its higher density and strength, it is less easily eroded by currents. In either case the turbidity created at the water surface disappears in minutes, and the impact on the bed occurs over a limited area. It is evident that the requirements of the disposal site be considered in selecting the method of dredging for any project. Multifaceted Approach to Dredged Material Disposal Planning for long-term disposal, including consideration of all options, is essential to the continuation of a viable economy and effective defense. The San Francisco Bay and Tampa Bay experiences might lead to the conclusion that ocean disposal is the way of the future. However, there are places where land disposal is best, places where estuary disposal is effective, and places where ocean disposal is a must. There is no single solution: a multifaceted approach to disposal is needed. The locations of disposal sites and restrictions on their use have had a major impact on the cost of maintenance. Sites available or suitable for future land disposal are frequently limited by environmental considerations, costs, and accessibility, and the capacities of presently used sites are limited. To address maintenance costs, various disposal options must be considered. For example, dredged material islands can be formed in shallow waters where environmental restrictions permit, and sands—other than silt or clay—can be used for beach nourishment in areas with favorable wave climate. Disposal of fine-grained clay and silt in open water in estuaries and in the open ocean, however, remains limited by the perception of adverse environmental effects and the slow response of regulatory agencies to the development of knowledge of the actual impacts of open-water disposal. Much has been learned about the
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings impacts of open-water disposal during the last decade. Studies by the Dredged Materials Research Program of the U.S. Army Corps of Engineers led to the development of a testing protocol for evaluating the release of contaminants from dredged material during disposal and development of numerical models for predicting the fate of disposed materials. These models are already useful and will have greater applicability with further development. Open-water disposal of fine sediments uncontaminated by waste discharges or other pollutant sources appears to have at most a very short-lived impact on the water column. Disposal at high-energy sites, where sediment re-enters the transport regime, has a short-lived impact on the bed unless the site is overloaded by excessive disposal. Even then the impact is contained to a limited area. A means for selecting ocean disposal sites that is sensitive to economical and environmental factors is needed. Some factors to be considered are economic haul distances, public concern, ocean currents and temperatures, structure of the ocean bottom, water depth, transport into an estuary mouth, use of site by finfish and shellfish, and impacts on beaches. An orderly and thorough evaluation of these considerations will lead to the selection of desirable sites. The following techniques are available when contaminated material to be disposed in the ocean must be confined: level bottom capping, man-made pits with capping, containment islands, and catchment pits. All options for disposal of dredged material, together with associated transport costs, should be considered in the planning of maintenance dredging. Such planning should consider disposal over the long term, with phasing of disposal to new sites when the capacities of existing sites are reached. Development of long-term, open-water disposal sites in estuaries and in the oceans should have high priority among the responsible regulatory agencies. SESSION E. NEW FACILITY DESIGN CONSIDERATIONS L.E.Van Houten, Session Chair The presentations in this session covered information about the various factors to be considered when planning and designing a new facility.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Planning Considerations Sedimentation rates should be considered from the initial planning stages of a new port facility and continue to be considered throughout the design and construction phases. The onerous burden of perpetual maintenance can be minimized by selection of a site having hydraulic conditions and suspended sediment loads that can be managed and by designing and constructing the facilities so that sedimentation can be controlled or eliminated. The location of a facility is the single most important factor to consider in managing sedimentation. The facility should be located and planned to minimize disruption to the estuary’s hydraulic regime. Improved channels should follow the natural path as much as possible and should be consistent with vessel maneuvering requirements and economic balance. A facility should be located where suspended sediment concentrations are as low as can be found, consistent with other considerations, and where sediment control practices can be effectively applied. The most desirable sediment control practices are those designed to keep suspended sediment moving through or past facilities. Though seldom fully achieved, these practices can best be implemented where flow velocity is sufficient during a part of a tidal cycle to resuspend any material that has been deposited during periods of lower current velocities. Selecting locations where natural features enhance velocity, such as on outside banks of channel bends, can further minimize sedimentation rates. If an optimum site is not available, or if a breakwater is necessary, alternative practices that keep sediment-laden flows from entering the facilities can be used. For example, water-tight barriers can be constructed that will prevent waters with the highest concentrations of suspended sediments from flowing through the enclosure. These barriers would be applicable in mooring basins carved in shallow nearshore portions of estuaries. However, such sites rarely can be made completely free of sedimentation. Selecting a site and developing a design with the objective of minimizing sedimentation rates and maintenance dredging costs requires a thorough study of all relevent factors. Such a study begins with the collection of the necessary data. Developing a Data Base A data base can be considered to consist of the design constraints to be applied and the physical conditions of the site. Design constraints include economic factors, such as operating costs of maintenance dredging and material disposal, land purchase, and construction costs. Design constraints also include operational requirements on both the water and land side of a facility. As indicated in the earlier sections of this report, field data on sediment supply, sediment transport, and bank stability, currents, tides, waves, water salinity at all levels, weather and meteorological conditions, and
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings environmental characteristics are needed for even preliminary design. The data on physical conditions can be developed from field measurements, sampling and laboratory testing, and published reports. Design for Minimum Sedimentation Rates Design includes evaluating various configurations of port facilities, and usually requires modeling to test the efficacy of various modifications of the original design. Laboratory modeling and testing can be accomplished through the use of physical models, numerical models, and computerized ship simulators. Minimizing sedimentation rates by designing facilities that enhance continued transport of sediments through the facilities can be achieved by minimizing obstruction of currents, providing gradual transitions in the bed in the direction of the currents, orienting wharves parallel to the dominant currents, and presenting the minimum cross section of wharf facilities to the currents. Prevention of sediment-laden waters from entering a dredged basin in regions with weak currents usually requires construction of a watertight enclosure, as described previously. A single entrance should be provided that will admit waters having the lowest concentration of suspended sediments and that will minimize the volume of water that enters the enclosure during a tidal cycle. All of the measures addressed in this session can be costly, and some have significant impacts on vessel operations. The cost of maintenance dredging may be less than the cost of sedimentation rate reduction measures. All factors should be evaluated in making an overall least cost-design. Capabilities for designing harbors with minimum sediment management costs exist in both public institutions and private practice. These capabilities are improving as harbor planners gain experience in applying present knowledge and design techniques and acquire knowledge from research on estuarial sediment transport processes. Additional research would enhance these capabilities significantly. Information on harbor design is published by the U.S. Army Engineer Waterways Experiment Station, the U.S. Maritime Administration, several universities, and private engineering firms.
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Representative terms from entire chapter: