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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SESSION E. NEW FACILITY DESIGN CONSIDERATIONS CHAIR Leonard Van Houten SPEAKERS E.J.Schmeltz, P.E. Jens Korsgaard C.Lincoln Crane Richard Harley Glen Pickering Douglas L.Inman* Scott A.Jenkins * Presentation not made during the Symposium
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings PLANNING CONSIDERATIONS FOR PORTS IN COHESIVE SEDIMENTS E.J.Schmeltz, P.E. PRC Engineering, Inc. Ports located in an estuarine environment dominated by fine-grained or cohesive sediments present problems in port planning that differ in some respects from locations characterized by more coarse-grained material and rock. In general these locales are characterized by shallow water, shifting shoals, and a significant freshwater input. The basic difficulty associated with the installation of port facilities in this type of an environment is that dredging requirements can be very high. This is true both of initial construction and subsequent maintenance. Essentially, sediment deposition results from modifications in the flow regime induced by construction of the port facilities. An awareness of the basic impacts on deposition patterns caused by manmade changes such as channel dredging and installation of port structures is necessary in order to minimize future maintenance while maximizing the safety and operational efficiency of the port. The purpose of this paper is to briefly discuss some of the factors that require attention in the planning of port facilities in this type of location and to provide examples highlighting these considerations. SITE LOCATIONS Ideally, a port location is characterized by protection from wave action, naturally deep water, favorable bottom conditions, access to existing infrastructure, and upland areas that are adequate for the development of shoreside facilities. The site of a port is, however, often dictated by factors other than engineering criteria, such as proximity to existing markets, and diverse economic and political factors. The lack of availability of natural harbors in a specific region may dictate that a selection be made between the lesser of several evils. Port facilities require straight, deep navigation channels with dimensions adequate for safe passage of the largest anticipated vessels. Turns in channels need to be a large enough radius to minimize assistance in navigation. Turning circles adjacent to piers and wharves are necessary, and anchorage areas are generally desirable if not always mandatory. Structures such as piers, wharves, bulkheads and moorings should be easy to construct given the site conditions.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 1 San Francisco, California. FIGURE 2 Kings Bay, Georgia.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Dredging requirements, both in terms of initial construction and maintenance, should be minimal. Many of the world’s finest harbors are located at sites that have many of the above attributes, including San Francisco, New York, and Rio de Janiero to name but a few. Part of the port of San Francisco is shown in Figure 1. Existing conditions in estuaries are often dramatically different from the ideal case. Extensive areas of naturally shallow winding water courses are more characteristic of these areas than straight deep channels. In much of the developed world, the more desirable port sites are already utilized so that many of the alternatives tend to be less than perfect. An example of a less than ideal port site is Kings Bay Georgia Figure 2. In evaluating a port location, it is generally desirable to minimize disruption of the estuarine system induced by man-made improvements. Changes introduced for the port can have a significant impact on circulation patterns, saline intrusion, and sedimentation patterns and rates. The less the magnitude of the required changes in the natural system, the less will be the adverse impacts on port operations. The impacts on sedimentation rates and patterns are most noticeable where sediments are composed of clays and other lightweight material affected by relatively weak currents. Figure 3 shows annual shoaling rates as a function of channel depth for Savannah Harbor. Clearly the result of increasing channel depth is a marked increase in sedimentation rates in the channel. Ultimately a balance must be achieved between initial dredging, future maintenance, port operations, and adverse environmental impacts. For example, available evidence indicates that the majority of U.S. estuaries were in a state of dynamic equilibrium prior to disruption of the entrance bars and shoal areas for navigation purposes (Ippen, 1966). In the majority of these ports, significant maintenance dredging is currently necessary to maintain navigable depths. The following sections discuss planning factors associated with more specific port improvements. CHANNEL GEOMETRY/DREDGING Because of the potential magnitude and extent of its impact on the flow regime in an estuary, dredging is a major factor in the changes in sedimentation rates due to port development. Increases in sedimentation are primarily associated with changes in flow velocity and direction and saline intrusion induced by the introduction of navigation channels, turning basins, and berth facilities. In planning for port facilities the depth and size of navigation channels are controlled by a combination of environmental factors affecting vessel navigation as well as the maximum size of the vessel expected to utilize the port facilities. Detailed discussions are provided by Bruun (1981) and Quinn (1982) among others. Depth of the channel is generally controlled by tides, maximum vessel draft,
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 3 Shoaling rates as a function of channel depth (Savannah Harbor). underkeel clearance requirements, ship motions, vessel squat, and anticipated shoaling. In an estuarine environment vessel draft, underkeel clearance and anticipated shoaling can be affected by flow characteristics and bottom conditions. If a significant freshwater inflow exists in the vicinity of port facilities, vessel draft can change as a result of changes in water density. If this condition is anticipated, an increase in vessel draft beyond that realized in salt water must be accommodated. In ports with hard bottoms, an underkeel clearance of 10 percent of the vessel draft plus anticipated motion is normally desirable in order to preclude damage to the ship from contact with the bottom. In this regard, one of the redeeming values of ports located where bottom conditions are characterized by silts and clays presents itself—soft bottom. As a consequence, a reduction in underkeel clearance requirements and dredging tolerances is acceptable. With a soft bottom, underkeel clearance can be reduced to about 4 ft; dredging tolerances can be decreased. Channel side slopes are generally flatter in fine-grained sediments than in granular materials, increasing initial dredging quantities. Flow velocities are also reduced further due to the increase incross-sectional area. Increases in the channel cross section and changes in alignment generally reduce current velocities in the area resulting in increased deposition and commensurate increases in maintenance dredging. One of the less obvious difficulties encountered in establishing dredged depths/quantities where fine-grained sediments are encountered lies in defining “bottom.” With hard bottoms the process is relatively straightforward, and simple fathometer or lead line survey will suffice. Where fine-grained sediments predominate, a fluid-mud suspension may exist on the bottom, which complicates definition of a navigable
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings depth. Lead lines yield the depth to a reasonably solid bottom while depth recorders do not clearly define the water-mud interface. An assessment of this problem has been undertaken by the Permanent International Association of Navigation Congress. This group defines a nautical depth considered safe to accept as the bed of the channel which precludes damage to vessels and that does not adversely affect maneuverability. At the port of Rotterdam, the group recommends that the “bottom” be defined where the specific gravity of the layer is 1.2. The working group further recommends, however, that the particular value for individual sites be based on local conditions and requirements. The condition also has been noted at the entrances to the Suez Canal at Port Said. In Rotterdam, the thickness of the layer was reportedly 2.5 m. The relationship between depth and density in the fluid mud layer is shown in Figure 4. SALINE INTRUSION While making navigation practical, changes in the hydraulic characteristics of an estuary induced by dredging also increase saline intrusion in the waterway. A saline bottom wedge overlain by fresh water discharging to the sea exists in various forms in all estuarine situations. Flocculation at the interface results in the deposition of material on the bottom. Whether the saline intrusion is highly stratified, partly mixed, or well mixed, the introduction of navigation channels, turning basins, etc. will modify the type and extent of the wedge. Changes in saline intrusion result in, at a minimum, changes in the deposition patterns in the port area and often affect the rate of accumulation as well. Returning to the earlier example of Savannah Harbor, Figure 3 shows the changes in maintenance dredging requirements as a function of depth for an upstream (7.4 mi) and downstream (5.1 mi) reach. Note that increases in channel depth tend in this case to increase relative shoaling in the upstream areas, while decreasing deposition in the lower portions of the harbor on a relative basis. DREDGED MATERIAL DISPOSAL Disposal of both initial dredged material and subsequent maintenance dredging may also prove difficult for fine-grained sediments. For ports in areas with granular materials, the dredged spoil can be utilized as fill in upland areas. However, with fine-grained sediments, dredged material cannot generally be used immediately as fill, if at all. Rather, dewatering is required and a considerable amount of time may be necessary prior to utilization on a site. If not structurally adequate for port facilities, dredged material can be disposed at sea, deposited in holding areas on land, or perhaps
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 4 Bed density survey, Immingham dock entrance, 1975. used as capping material for landfills. In more developed ports, there is the risk of encountering contaminated materials requiring special treatment and handling. Some portions of lower New York Bay are characterized by this type of problem to greater or lesser extents. Finally, dredging of fine-grained sediments results in significant turbidity and can pose an environmental threat to downstream areas. As a consequence it is often necessary to control this problem with sediment screens and special controls at spoil areas. ORIENTATION AND ALIGNMENT Proper orientation and alignment of channel improvements and port structures can minimize the amount of maintenance dredging required for the safe operation of a port. Large-scale dredging in order to realign and deepen approach channels or to provide turning basins generally results in increased sedimentation. In addition, the deepening and improvements in the hydraulic characteristics of the channel will result in an increase in salinity intrusion and the deposition of material within the harbor as noted above. In general, orientation of both channels and berth will be best if they are arranged parallel to the flow of the natural channels. This arrangement tends to minimize disruption of the flow regime and keep sediment moving to the maximum extent possible past the port facilities. Of course, increases in channel depth and/or dimension tends to reduce flow velocities and increase the resultant settling of fine materials. From the perspective of port operations, large-radius turns are desirable, but the introduction of these types of channels may result in a tendency for the channel to meander. A balance must therefore be achieved between the operational needs of the vessels and the stability of the facility. Care must also be exercised in the placement of structures along the banks of the waterways in the estuary. Facilities located to the outside bank of a curve will have a tendency toward erosion; those
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings placed on the inside of the curve will have a tendency toward deposition. In addition to the problem of limiting maintenance dredging at the facilities, caution must be exercised in terms of the effect of dredging on other areas downstream of the facility. The dredging conducted will not only disrupt the dynamic equilibrium of the immediate area, but can result in a sediment trap. Material deposited in the vicinity of the channel will not continue downstream and can starve other areas of their sediment supply. The result is a potentially adverse impact at other locations such as erosion. LAYOUT AND CONFIGURATION The layout of ports for commercial, military, recreational, and other uses have been widely treated in the literature. Detailed discussions are available in Bruun (1981) and Quinn (1982) among others. It is not the intention of this paper to discuss all aspects of port layout and configuration in detail, but rather to highlight particular factors that should be considered for ports in estuarine situations as they differ from those with granular sediments. The effect of relatively small changes in current magnitude and patterns in an estuary can have a marked impact on sedimentation rates. Notwithstanding the large-scale changes from dredging of turning basins and channels, the introduction of piers, berths, and other port structures can induce localized changes that may be equally damaging to the maintenance of the facility. A relatively common structural type for marginal facilities is the relieving type platform shown in Figure 5. These types of structures are particularly common at sites with poor foundation conditions not conducive to gravity structures. In addition, they have the advantage of being economically attractive in many applications. The difficulty with this type of structure, however, is that the pile fields can result in a reduction in current velocity locally with attendant deposition of material in the structure. The result is sediment deposition that eventually spills into the adjacent berth area. Slip-type berthing facilities have also been widely used in a variety of ports and have the advantage of minimizing the length of channel required for a fixed number of berths. However the parallel structures essentially form settling basins where material is deposited on flood tides, but is not easily removed on the ebb tides, particularly fine-grained materials. Examples of this type of deposition can be seen in many ports where slips are the norm, i.e., New York. It is not uncommon for loss of depth in such slips to reach 6 to 8 ft/yr under normal circumstances. Deposition in these types of structures is particularly bothersome given the difficulties associated with dredging in confined areas. Arrangement of the berths parallel to the flow can limit the impact of this type of situation, although the approach is not always practical.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 5 Relieving platform. Solid structures, i.e. caissons, sheetpile cells, etc., can result in local increase in flow velocity and erosion. Restrictions in channel cross section have a similar effect and have been used to advantage, such as at the port of Abidjou where, after a recurring maintenance dredging problem, training structures were added to result in a self-scouring channel (Silvester, 1974). Upland development associated with a port can also have an adverse effect on sedimentation in that increased drainage contributes a greater sediment load to the port. EFFECTS OF VESSEL NAVIGATION Interestingly, the transit of vessels in ports with predominantly fine-grained sediments can serve to clean navigation channels, turning basins, and berth areas of sediments, reducing the amount of maintenance dredging required. This is particularly true in areas where a fluid mud layer exists. Turns in the navigation channels can exacerbate erosion in the channel areas. The problem is induced by vessels navigating the curve in the channel. Changes in power applied entering, through, and leaving the turn can induce erosion of the bottom of the banks particularly where the radius of the curve is relatively small compared to the vessel size. Vessel traffic, even in straight channels can induce bank instabilities which contribute to sedimentation as a result of ship generated waves at the shore. Vessels moving in the channel and in and out of berths can also keep materials in suspension until normal currents can move them. An example of this phenomenon is illustrated in Figure 6 for a location in Bahia Blanca, Argentina. Measurements obtained from October 1976 through September 1977 show a distinctly rapid siltation to a depth of
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings about 8.4 m. At this depth, which was comparable to the draft of the loaded vessels in the area, further deposition seems to have halted (PIANC, 1979). METHODS OF SEDIMENTATION CONTROL Some alternatives exist that can be considered to assist in the control of sedimentation in port areas. Depending on the particular circumstances, a number of different structural and nonstructural solutions have been utilized with varying degrees of success. The approach to limiting sedimentation in specific areas is centered on one of three possibilities: Stop the sediment from reaching the site. Keep the material in suspension through the site. Divert the sediment flow from critical areas. Sediment traps can be utilized to stop sediment from reaching a given area. In their simplest form a pit dredged into the bottom updrift of the port structures can serve to trap some of the material that would normally reach the facilities. Maintenance dredging is facilitated since the location can be controlled. In some instances installation of fixed dredging plants at the location of these traps is possible. The practicality is, however, somewhat contingent on the consistency of sediment flow since the equipment is immobile and significant oversizing may be required to deal with seasonal fluctations. Sediment supply can also be reduced through the use of bank stabilization of navigation channels as well as upstream segments subject to natural erosion. A variety of training structures can be considered to increase local flow velocities, thereby keeping material in suspension through a locale. Walls, skirts below finger piers, and even bubbler systems can be used to accomplish this end. The intent is to locally increase flow velocities keeping material in suspension that would normally settle out of the water column. Obviously periods of slack flow can pose a problem for these types of systems, and substantial analysis is necessary prior to their use. For example, bubbler systems have been used with some success in marinas to minimize sedimentation and to reduce icing in northern locations. Diversion of sediments prior to their reaching a port has also been used to reduce maintenance dredging in harbors. For example, a dike was constructed in the lower part of Newark Bay in the 1930s to divert sediment flow from the Arthur Kill and Kill Van Kull. Significant infilling upstream of the dike is a testament to its early success. In the Dominican Republic consideration was given in a recent master plan to diverting the Haina River to reduce sediment load and flood damage to the Port of Haina. Although the scheme was not adopted, it is shown for illustrative purposes in Figure 7. Investigations indicated that the river was contributing the largest
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 6 Siltation at Bahia Blanca. FIGURE 7 Haina River diversion. proportion of sediments to the port, particularly in the more protected inner areas. The coarse-grained fraction was concentrated in the outer areas of the port, so that a significant saving in maintenance dredging could be anticipated with the diversion of the river. Economically, however, maintenance dredging was a more practical alternative. In each of the methods of control identified above, one factor must be recognized. Whether the material is trapped, maintained in suspension, or diverted, the maintenance dredging problem is not eliminated, it is simply relocated. While the volume of material to be removed may not be significantly altered by these measures, the location of the dredging can be controlled, thus potentially lowering both cost and environmental consequences of maintenance.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings U.S. Army Corps of Engineers. 1965b. Tidal Hydraulics. EM 1110–2–1607 Office of the Chief of Engineers, Washington, D.C. U.S. Army Corps of Engineers. 1961a. General cargo vessels—trends and characteristics. Board of Engineers for Rivers and Harbors, Washington, D.C. U.S. Army Corps of Engineers. 1961b. Study trends in petroleum supply requirements and tanker fleet characteristics. Washington, D.C. U.S. Army Corps of Engineers. 1961c. Trends in dry bulk carriers. Washington, D.C. U.S. Army Corps of Engineers. Continuing series of reports. Wave information studies of U.S. coastlines. Vicksburg, Miss.: Waterways Experiment Station. U.S. Army Corps of Engineers. In preparation. Environmental Engineering for deep-draft navigation. EM 1110–2–1202. Office of the Chief of Engineers, Washington, D.C. U.S. Department of Commerce. 1978b. Merchant fleet forecast of vessels in U.S.-foreign trade. Maritime Administration, Washington, D.C. Prepared by Temple, Barker and Sloane, Inc., Wellesley Hills, Mass. U.S. Department of Commerce. 1978a. A statistical analysis of world’s merchant fleets. Maritime Administration, Washington, D.C.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings BUDGET OF SEDIMENTS FOR ESTUARINE HARBORS Douglas L.Inman Scott A.Jenkins Center for Coastal Studies Scripps Institution of Oceanography Proper siting of a harbor is by far the most important consideration in determining its overall utility, cost of construction, and annual maintenance costs. Experience shows that annual maintenance cost for sediment removal is usually underestimated and frequently, with time, becomes the highest harbor cost. The common problem of underestimation of future dredging costs results from an inadequate assessment of the natural sedimentary regime and incorrect analysis of the effect of the harbor structures on the sediment budget. The purpose of this paper is to discuss the budget of sediment within the context of a natural sedimentary compartment referred to as an “estuarine cell.” Before a realistic estimate can be made of the effect of new construction on the environment, it is first necessary to determine the important natural relationships among the physical driving forces and the sediment responses. Once these relationships are known, we are in a position to begin assessing the natural budget. It is a basic principle of sedimentary systems that they seek various states of short-and long-term equilibria. The effect of new construction on the equilibria can best be estimated when the natural states of equilibria are understood. The concept of the budget of sediment, balanced within the physiographical limits of a sedimentation cell or compartment, has proven to be a very valuable aid in understanding and evaluating sediment management procedures. A sedimentation cell is a coastal compartment or physiographic unit that contains a complete cycle of sedimentation, including sources, transport paths, and sediment sinks. Within a cell the principle of the conservation of mass may be applied to the evaluation and interpretation of coastal and estuarine sedimentation. The procedure, sometimes referred to as the “budgets of sediment,” consists of assessing the sedimentary contributions (credits) and losses (debits) and equating these to the net gain or loss (balance) of sediment within a given coastal segment. The concept of the budget of sediment within a coastal compartment, called a “littoral cell” has led to considerable success in the analysis of the effects of coastal structures in causing accretion and erosion (Inman and Chamberlain, 1960; Inman and Frautschy, 1965; Inman and Brush, 1973; Inman et al., 1986). The intent here is to extend this concept to estuarine systems.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings ESTUARINE VERSUS LITTORAL CELLS It is important to recognize that most estuarine harbors, by nature of their location, may have problems with littoral sedimentation along their entrance and access channels. Generally, estuarine harbors are situated on a bay or river separated from the sea by spits, barriers, tombolos, or headlands (Figure 1a). Examples are San Francisco, Mare Island, Kings Bay and most harbors on the east and gulf coasts of the United States. Littoral harbors are those that border the coast and debouch directly to the sea across the littoral zone. Examples are Los Angeles, Long Beach, and San Diego, California and Port Canaveral, Florida. The sediments and transport processes for estuarine and littoral environments are quite different. Mud and dynamics of fresh-/ saltwater interface are common to estuaries, while sand transported by FIGURE 1 (a) Schematic diagram of estuarine processes. (b) Saline wedge and formation of sediment flocs.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 2 X-ray spectrogram of sediment samples taken at Mare Island, California. The percentage occurrence of a given mineral is proportional to the area under the intensity peak, referred to the basic background level indicated by the dashed line. SOURCE: Van Dorn et al., 1975. waves and tidal currents are essential aspects of the littoral harbor. Harbors such as Kings Bay, Georgia are examples of estuarine harbors with severe littoral problems (Inman and Luftglass, 1979). ESTUARINE SEDIMENTATION Flocculation (coagulation) and eventual deposition of muds occurs when silt-laden fresh water from a river contacts salt water. The river water is usually lighter and rides over the salt water, their boundary forming a missing shear-layer where the dispersed suspended load of the river combines with salt water to form sediment aggregates termed “flocs.” This floc-generating frontogenesis zone may extend a number of miles seaward of the river mouth where the freshwater plume spreads over salt water (Gross et al., 1965; Garvine, 1975; Trefry et al., 1985). Alternatively, the interface may occur landward of the river mouth over the “saline wedge” that may extend along the bottom for many miles upstream (Keulegan, 1966; Krone, 1974).
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 3 Cumulative frequency distribution of particle sizes in uniform dilutions of Mare Island sediment, as function of salinity. Particle sizes are referenced to the settling velocity of a quartz sphere of the indicated diameter. The sample treated with peptizer approximates the basic particle-size distribution before flocculations. SOURCE: Van dorn et al., 1975. Once the flocs are in the bottom layer they may be carried upstream with the moving salt wedge. This recirculation of solids, consisting of downstream transport of wash load and upstream transport of flocs, is an important mechanism for estuarine sedimentation (Figure 1b). COMPOSITION AND PHYSICAL PROPERTIES OF MUDS The composition of the muds from the Sacramento-San Joaquin River systems are shown in the x-ray spectogram of mud from Mare Island Harbor in San Francisco Bay (Figure 2). This mud results from the salt wedge that travels upriver from San Francisco Bay. The clay compositions in percent by weight of montmorillonite, kaolinitechlorite, and illite are 25, 46 and 29 respectively. Reconstruction of the particle-size distribution of the original wash load from which the mud was formed was attempted by adding peptizer of sodium hexameta phosphate to deflocculate the material (Figure 3). The effect of increasing salinity on flocculation was determined for salinities of 0, 12, and 34 percent. The resulting cumulative frequency curves shows the systematic increase in median size of material through 2.4, 7, 9 and 12µ. The actual sizes of the flocs are much larger than shown because of the low density of the flocs (1.22 g/cm−3), which gives them a lower settling velocity than their size would warrant when
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings TABLE 1 Characteristics of Representative Bottom Sediments in Three Navy Harbors SITE SOLIDS % BY WT. VOLATILES % BY WT. BULK DENSITY gm/cm3 CLAY COMPOSITION % BY WEIGHT MONTMORILLONITE KAOLINITE+CHLORITE ILLITE Mare Island 97.5 2.5 1.22 25 46 29 Norfolk 88.0 12.0 1.21 21 48 31 Charleston 96.5 3.5 1.21 52 46 2 SOURCE: Van Dorn et al., 1977. compared with the settling velocity for quartz spheres, which is used as the standard for “size” in Figure 3. The flocs resulting from the nucleation of the smaller particles are larger and, although less dense than the small particles, they have greater fall velocities and settle to the bottom more readily. Since the flocs are loosely combined conglomerates of a number of particles, including water molecules, their physical and chemical properties are varied and change with time. They undergo compaction with a marked increase in viscosity. For example, freshly settled flocs of the material shown in Figures 2 and 3 could be moved (onset of particle motion) by fluid shear stresses of 1 dyne cm−2. After several hours, onset required a stress of 2.5 dynes cm−2, and as much as 3 to 5 dynes/cm2 after several days to weeks (Van Dorn et al., 1975; Jenkins et al., 1981). The detailed mechanics of flocculation and the physical behavior of muds that form from the flocs are also dependent upon the clay minerals that form the flocs. These may differ widely from harbor to harbor as shown in Table 1 and discussed by Krone (1963). Although most muds contain two or more of the clay minerals, Griffin et al. (1968) show that the typical clay mineral is determined by source rock, climate, and weathering. For example, chlorite is characteristically a high-latitude clay that results from erosion of metamorphic rocks. The Saint Lawrence River system is a major source of chloritic clays. Montmorillonite is characteristically formed in volcanic regions, and rivers of the Pacific coast of North and South America usually have medium to high amounts. Kaolinite is formed by intense chemical weathering and is a typical mineral for rivers in low latitudes that drain high rainfall areas. Illite is a general term for a group of stable, micaceous (mostly muscovite) minerals characteristic of detrital sediment runoff from the continents. The clays’ presence and relative abundance are easily determined from an x-ray spectrogram as shown in Figure 2.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SEDIMENT BUDGET FOR AN ESTUARINE CELL The sediment budget for an estuarine cell may be approached in the same manner as that for a littoral cell. The principle difference is in the type of sediment and its mechanics of transport. As we have seen, the muds of estuaries behave in a very different manner from that of littoral sands. Further, the estuarine cell may also include sands, particularly in entrance and access channels where currents are stronger. Because of the many complex interactions in estuaries, it is sometimes necessary to divide the estuarine cell into a series of subcells in accordance with the relative importance of the littoral, lagoonal, and riverine portions of the estuary (Figure 1a). It should be understood that the estuarine cell may include elements of the other three (Pritchard, 1967a,b). In detail, the procedure for estimating the sediment budget for an estuarine system can be handled like that previously described for a littoral cell (Bowen and Inman, 1966). That is, first identify all of the sediment sources, transport paths, and sinks. Then, use all available independent information to evaluate the rates associated with the sources, paths, and sinks. The result is an estimate of the budget of estuarine sediment. For an equilibrium system the three estimates of sources, paths, and sinks must all be equal. If the system is known to be in equilibrium, then balancing the budget suggests, but does not guarantee, that the budget is correct. If the estuary is not in equilibrium, the sources and sinks will not balance. Rather the net will equal the expected accretion or erosion. In this case the confidence of the result can only be assured by the most accurate possible determinations of sources, transport, and sink rates. During early planning, the harbor engineer needs to have preliminary estimates of the sediment budget before all of the detailed studies and measurements have been completed. In this case it is necessary to apply principles of the controlling processes as an aid to obtaining preliminary estimates of transport rates. Examples of this procedure, which lead to first estimates of the sediment budget, are discussed below. LITTORAL SEDIMENTATION The littoral portion of the cell typically includes the shore zone where the principal transport mechanism is the longshore or littoral drift of sand driven by waves and wave—generated currents. This type of sediment transport is responsible for the shoaling of entrance channels and offshore access channels. It is usually the major sediment problem for littoral harbors such as Santa Barbara (Johnson, 1953) and Oceanside, California (Inman and Jenkins, 1983), and Port Canaveral, Florida (Van Dorn et al., 1977). At Saint Mary’s Inlet, the offshore access and entrance channel maintenance dredging for Kings Bay Harbor is estimated to be between 0.8 and 2.8 million m3/yr (Kings Bay Environmental Impact Review meeting, 1986).
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings The longshore transport rate of sand in the surf zone has been shown to be proportional to the energy flux or “stress flux” of the breaking waves. It can be estimated by summing the instantaneous transports obtained from the directional wave spectra (CERC, 1977; Inman et al., 1986). The natural balance of sediment in the littoral portion of the cell requires that the supply of sediment equal the longshore transport of sediment, which in turn equals the sink of sediment: (a) (b) (c) supply=longshore transport=sink For example, at Oceanside, California the supply of sediment (a) is from rivers, the longshore transport (b) is estimated using the stress-flux method, and the sink (c) is sediment lost down submarine canyons. Inman and Jenkins (1983) and Inman (1985) show that separate, independently derived estimates of these three quantities, in m3/yr, are: (a) (b) (c) 213,000 194,000 200,000 It is apparent that these volumes of sediment are remarkably similar, and the budget of littoral sediment is essentially in long-term balance for natural conditions. This gives some assurance that the procedures and volumes are valid. LAGOONAL/RIVERINE SEDIMENTATION Lagoonal and riverine sedimentation are more complex as they include transport and deposition of sandy and muddy sediments. The processes include the important equilibrium between entrance channel cross-sectional areas and tidal prism; the mechanics of meandering tidal channels; saline wedge intrusion; salt marsh deposition; and for large lagoons and bays, littoral transport as well. In the long term, the geologic history of most lagoons is that of deposition and gradual disappearance. Although the continuing rise in sea level is retarding the rate of deposition, the demand for ever-increasing draft (about 20 cm/yr) far exceeds the eustatic sea level rise (about 20 cm/century). The relationship between the cross-sectional areas of entrance channels and the tidal prism behind them (Jarrett, 1976) has proven to be a very useful guide for natural equilibrium conditions. The relation applies to sandy entrance channels and was originally developed by O’Brien (1931) and extended by Inman and Frautschy (1965). However, the relationship also applies approximately to the distributory channels within an estuarine system when the tidal prism is taken as the volume “upstream” from the channel section. For example, in a study of tidal relations along the coast of Vietnam, Inman and Harris (1966) found that natural channel areas near Chu Lai and Vung Tau (Dinh River) were in good agreement with estimates of
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings upstream tidal prisms. Estuarine harbor design usually leads to channel enlargement and deepening. Such channel enlargements have the potential for trapping sediment volumes equivalent to the enlarged section at intervals of only a few spring tidal cycles. Preliminary studies for Kings Bay, Georgia (Inman and Luftglass, 1979) used the channel/tidal prism relationship for estimating maintenance dredging in the interior tidal channels, while the wave energy-flux method was used as an estimate for littoral fill in the entrance and approach channels. Together these procedures gave an annual maintenance dredging for 40-ft depth channels of 2–3 million yd3. For deeper draft Trident submarine maneuvering, the annual dredging was estimated to be much larger, almost 5 million yd3. The saline wedge considerably complicates the budget of sediment by adding a mechanism that reverses the direction of transport from downstream to upstream (Figure 1b). Further, the concentration of sediment flocs in the saline wedge may be between 100 and 4000 g/liter, about 100 times greater than that in the waters above (Van Dorn et al., 1977; Jenkins et al., 1980). In some European estuaries sediment may be introduced from the sea. However in most cases, the flocs in the saline wedge have their orgin in the wash load carried in the waters above the wedge. This is true even though the times of high deposition at any given locality in an estuary may lag by many days the times of high river runoff. The lag occurs because high discharges displace the saline wedge seaward and cause the load to be deposited farther downstream. High spring tides are important in re-establishing saline wedges that then transport flocs upstream. For example, deposition of mud from the flood of February 12–15, 1986, which occurred during a neap tide, did not begin at Mare Island until the following spring tide on February 25. Thus, years with heavy deposition of mud correlate well with years of high river runoff when sediment is supplied to the estuaries (Van Dorn et al., 1977). Accordingly, the episodic climatic behavior that produces the 10-, 50- and 100-year floods becomes an important consideration in estimating the long-term budget of sediment in estuarine harbors. REFERENCES Bowen, A.J. and D.L.Inman. 1966. Budget of Littoral Sands in the vicinity of Point Arguello. Technical Memo 19. U.S. Army Corps of Engineers, Coastal Engineering Research Center. 41 pp. CERC. 1977. Shore Protection Manual, Vols. I and II. U.S. Army Corps of Engineers, Coastal Engineering Research Center. 3rd edition. Garvine, R.W. 1975. The distribution of salinity and temperature in the Connecticut River estuary. J. Geophys. Res. 80(9):1176–83. Griffin, J.J., H.Windom, and E.D.Goldberg. 1968. The distribution of clay minerals in the World Ocean. Deep-Sea Research 15:433–459.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Gross, M.G., C.A.Barnes, and G.K.Reil. 1965. Radioactivity of the Columbia River effluent. Science 149(3688):1088–90. Inman, D.L. 1985. Damming of rivers in California leads to beach erosion. Pp. 22–26 in Oceans ’85: Ocean Engineering and the Environment. Washington, D.C.: Marine Technological Society and IEEE. Vol. 1, 674 pp. Inman, D.L. and B.M.Brush. 1973. The coastal challenge. Science 181(4094):20–32. Inman, D.L. and T.K.Chamberlain. 1960. Littoral sand budget along the southern California coast. Pp. 245–246 in Volume of Abstracts, Report of the 21st International Geological Congress. Copenhagen, Denmark. Inman, D.L. and J.D.Frautschy. 1965. Littoral process and the development of shorelines. Pp. 511–536 in Coastal Engineering. (Santa Barbara Specialty Conf.) New York: American Society of Civil Engineers. 1006 pp. Inman, D.L., R.T.Guza, D.W.Skelly, and T.E.White. 1986. Southern California coastal processes data summary. Coast of California Storm and Tidal Waves Study, CCSTWS 86–1. U.S. Army Corps of Engineers, Los Angeles District. 572 pp. Inman, D.L. and R.W.Harris. 1966. Investigation of sedimentation and dredging requirements, various locations, Republic of Vietnam. Prepared for U.S. Navy, OICC, Republic of Vietnam, under Contract by 79844 with Daniel, Mann, Johnson and Mendenhall, Saigon. 237 pp. Inman, D.L. and S.A.Jenkins. 1983. San Malo Seawall. Letter report prepared for San Malo Beach Assn., Carlsbad, Calif. 3 pp. Inman, D.L. and B.Luftglass. 1979. Summary report on nearshore processes affecting Kings Bay, Georgia. Special Report for Naval Facilities Engineering Command under Contract N00001476-C-0631. 10 pp. Jenkins, S.A., D.L.Inman, and J.A.Bailard. 1980. Opening and maintaining tidal lagoons and estuaries. Proc. 17th Conf. Coastal Engineering, American Society of Civil Engineers 2:1528–1547. Jenkins, S.A., D.L.Inman, and W.G.Van Dorn. 1981. The evaluation of sediment management procedures: Phase IV–VI, final report, 1978–1980. Scripps Institution of Oceanography Reference Series 81–27. 78 pp. Johnson, J.W. 1953. Sand transport by littoral currents. Proceedings 5th Hydraulics Conf., Bulletin 34, Studies Engineering. State University of Iowa. Pp. 89–109. Keulegan, G.H. 1966. The mechanism of an arrested saline wedge. Pp. 546–574 in Estuary and Coastline Hydrodynamics, A.T.Ippen, ed. New York: McGraw-Hill. 744 pp. Krone, R.B. 1963. A study of rheological properties of estuarial sediments. Hydraulic Engineering Laboratory, University of California, Berkeley. Krone, R.B. 1974. Anticipated effects of water diversions on the San Francisco Bay System. University of California, Berkeley. Preprint. 30 pp. O’Brien, M.P. 1931. Tidal prisms related to entrance areas. Civil Engineering 1(8):738–739.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Pritchard, W.D. 1967a. What is an estuary: Physical viewpoint. Pp. 3–5 in Estuaries, G.H.Lauff, ed. Washington, D.C.: AAAS. Publ. 83. Pritchard, W.D. 1967b. Observations of circulation in coastal plain estuaries. Pp. 37–44 in Estuaries, G.H.Lauff, ed. Washington, D.C.: AAAS. Publ. 83. Trefry, J.H., S.Metz, and R.P.Trocine. 1985. A decline in load transport by the Mississippi River. Science 230:439–441. Van Dorn, W.G., D.L.Inman, and R.W.Harris. 1975. The evaluation of sediment management procedures. Phase I, final report, 1974–1975. Scripps Institution of Oceanography Reference Series 75–32, 82 pp. Van Dorn, W.G., D.L.Inman, R.W.Harris, and S.S.McElmury. 1977. The evaluation of sediment management procedures. Phase II, final report, 1975–1976. Scripps Institution of Oceanography Reference Series 77–10, 107 pp.
Representative terms from entire chapter: