SESSION B.
MODIFYING EXISTING FACILITIES

CHAIR

Scott A.Jenkins

SPEAKERS

Ray B.Krone

James A.Bailard

Scott A.Jenkins



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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SESSION B. MODIFYING EXISTING FACILITIES CHAIR Scott A.Jenkins SPEAKERS Ray B.Krone James A.Bailard Scott A.Jenkins

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings REDUCING SEDIMENTATION RATES IN HARBOR FACILITIES Ray B.Krone Department of Civil Engineering University of California, Davis Unbearably high sedimentation rates are experienced in old harbors, where water depths have been increased over the years to accommodate deeper-draft ships, and in recently developed harbors where the possibility of high sedimentation rates was not considered in the harbor design. In these unfortunate circumstances, large investments in physical facilities have already been made and the operator is often faced not only with the onerous costs of maintenance dredging, but also with increasingly costly sediment disposal options. The pressing need to reduce sedimentation rates in such facilities, combined with high costs of significantly modifying an existing installation to effect reductions, pose difficult challenges to harbor engineers. There are two basic strategies for reducing sedimentation rates in harbors: keep the sediment from entering harbor facilities, or keep it moving through the facilities. Implementing these strategies usually requires knowledge of sediment transport processes, sources, physical character, and hydraulic conditions during periods when sedimentation occurs. This information is required in sufficient detail to assess the effectiveness of a proposed remedy. This symposium is concerned with fine sediments that accumulate in tidal waters. A brief description of the relevant transport processes of these sediments is presented so that methods of implementing the strategies can then be explained. A few examples of installations of remedial works are presented to illustrate the approaches. Every problem should be regarded as unique, however, and each solution will depend on application of the principles presented—there are few “handbook” solutions to sedimentation problems. COHESIVE SEDIMENT TRANSPORT Fine sediments are soil materials that have eroded from land, usually from the local drainage, and are carried by runoff waters to the estuary. The particles are transported in suspension in runoff waters, usually as dispersed particles of clay and silt and possibly organic matter. When such particles enter an estuary and the seawater

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings salinity rises to from 1–3 g/liter (Ariathuari, 1974; Krone, 1962), the suspended particles become cohesive: they can stick to one another if they are caused to collide in a modestly stressful environment. Most of the water volume of typical estuaries have salinities greater than a few grams per liter. These fine particles can be regarded as cohesive throughout most of an estuary. Collisions of suspended particles result from thermal motions of water molecules that jiggle small suspended particles continuously, differences in settling velocities of suspended particles, and velocity gradients. Collisions of suspended particles that are slightly displaced from one another in velocity gradients are by far the most prevalent in estuaries. Collisions due to thermal motions and differential settling velocities create loose, weak aggregates that are easily dispersed in velocity gradients. Velocity gradient aggregation, however, produces denser, stronger aggregates that will resist at least the velocity gradient under which they were formed. The frequency of collisions on each suspended particle by any of the three mechanisms increases with the number of particles per unit volume, so concentration is important in determining the rate of aggregation. The frequency of collisions increases in proportion to the velocity gradient; however, as the increasing gradient begins to impose stresses on the aggregate that exceed the strength of the bond between the colliding particles, fewer collisions result in aggregation. At higher gradients aggregates are broken apart. Aggregates may be dispersed and form new aggregates repeatedly as the suspended particles experience changing velocity gradients in an estuary. Aggregates of suspended particles often contain millions of individual clay and silt particles. If such aggregates are dense, their settling velocities are orders of magnitude greater than their individual particles. The settling velocity depends on the history of the aggregate. Fortunately, the history of the immediate past is most important, and under some conditions the settling velocity can be estimated from the suspended solids concentration (Krone, 1962 and 1984). Aggregates that fall to the bed of a channel, basin, or berth and stick there either remain and contribute to continuing accumulation or are torn loose by subsequent hydraulic shear stress. There is no bed load. These cohesive particles are either stuck to the bed or are suspended. The strength of aggregate adhesion to a bed depends primarily on the mineral composition of the aggregates and the structural arrangements of particles in the aggregates. Whether a settling aggregate sticks to the bed or remains suspended depends on the hydraulic bed shear stress. If the shear stress is greater than a critical stress, only a fraction of the suspended material will deposit. If the bed shear stress is less than the critical shear stress, the rate of deposition of aggregates is given by, (1)

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings where h is the depth of the vertically mixed suspension, C is the suspended solids concentration, Ws is the settling velocity of the aggregates, τ is the bed shear stress, and τcr, which can be estimated from the cation exchange capacity of the sediment, is the critical stress for deposition. The expression in parentheses is the probability that the aggregate will stick. The product WsC, the flux of particles settling to the bed, becomes a function of time when aggregation is rapid, and the relation becomes more complicated. The important points, however, are the flux of sediment to the bed depends on the concentration of suspended sediment and the settling velocity of the aggregates, and that the probability of sticking to the bed depends on the bed shear stress. At concentrations above about 10 g/liter, the settling velocity of aggregates is hindered by the interference the particles cause to the escape of the water that they are displacing. A sharp interface develops at the top of the settling particles as particles in such suspensions settle in quiet water, producing the “fluid mud” that is often seen on fathometer traces as a ghost bottom above the more pronounced bottom trace. Fluid muds appear at slack water. The depositing aggregates at the bottom of the fluid mud layer are crushed as overburden accumulates, making the lower portion of the deposit stronger than the aggregates from which it was formed and more able to resist resuspension by subsequent strengths of flow (Krone, 1972). The higher density of a fluid mud suspension inhibits upward mixing during subsequent stronger flows, and high concentrations of suspended solids near the bed may persist for some time. The higher densities of near-bed waters in a mixing zone of an estuary due to salinity can further inhibit upward mixing. High concentrations of suspended sediment are often found near the bed in mixing zones. Erosion of a sediment bed occurs when the applied bed stress exceeds the bonds of individual particles and tears them off one by one as “surface erosion” (Arulanandan et al., 1975). When still higher stresses exceed the bulk shear strength of the sediment to some depth, the sediment is almost instantly suspended to that depth. This process is called “bulk erosion” (Krone, 1962; Parchure, 1975) and is the most important mechanism in estuaries. Typically, a thin deposit formed at times near slack water can be resuspended when the bed stress increases during the strength of flow. It is already apparent that three factors that strongly affect net deposition rates in estuaries are suspended sediment concentration, velocity gradients, and bed shear stress. Implementing the strategies for reducing sedimentation in harbors consists of modifying one or more of these basic factors. KEEPING SUSPENDED SEDIMENT OUT OF FACILITIES The most common implementation of this strategy consists of a wall, such as a harbor enclosure, with an entrance that admits water having the lowest concentration of suspended solids. For example, harbors

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings located in shallow mud flat areas, where a turning or mooring basin is dredged to provide the harbor, often fill rapidly with sediment that is suspended on the mud flats by wind-generated waves. A tight enclosure around the basin, with a single opening in deeper water, can be used to reduce the inflow of waters laden with high concentrations of sediment. The important considerations in designing such enclosures are that the wall be tight, that the waters entering with rising tides have the lowest available suspended sediment concentration, and that the circulation of waters entering through the entrance be minimized. There should be only one entrance, and a baffle at the entrance is sometimes required to minimize circulation inside of the enclosure. Walls are also used where a channel enters a bay and crosses shallow areas to deeper water to reduce the flow of sediment-laden waters across or into the channel. Here again, the effectiveness of a wall depends on the wall being tight and extending sufficiently to admit waters having reduced suspended sediment concentrations. The importance of making the wall tight for sediment control can not be overemphasized: even a relatively small slot can allow the passage of large amounts of water containing high concentrations of suspended sediment into an enclosure. The quiet waters in the basin promote deposition from such flows. Two common ways that these circumstances develop are failure by a designer or a construction contractor to understand the need for a tight enclosure, and erosion of mud from the base of a wall by wave action or currents. An example was provided by the small-craft harbor enclosure at Benicia, California. The location is on Carquinez Strait in the upper San Francisco Bay system. The major drainage to the system is located upstream, and San Pablo Bay is located immediately downstream. Large amounts of suspended sediment are discharged through Carquinez Strait during winter months. Much of this discharge is deposited in San Pablo Bay, resuspending the deposited material and supplying it to the tidal circulation of the waters passing through Carquinez Strait. Large amounts of suspended sediment pass through Carquinez Strait during winter, spring, and summer months. A map showing the region of the harbor (Figure 1) shows shallow areas on either side of the strait. The small-craft harbor is located at the bottom. The harbor was created by dredging a basin in the mud next to an unused ferry pier, and a breakwater was constructed across the outer channel side of the basin by driving peeled log piles in two staggered rows as close together as possible. This construction was permeable to tidal flows. The unused ferry pier had spaced vertical timbers between the piles to break waves, but there was no other obstruction for waters flowing from the adjacent mud flat across the dredged basin and out through the permeable breakwater. The contractor observed a very rapid rate of sedimentation during construction and left the job without completing it. The basin filled 8 ft in less than two years. Sealing the walls (aluminum sheet was most economical) and installing a baffle in the entrance to prevent internal circulation reduced the deposition rate to less than 6 in. per year.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 1 Location of Martinez Yacht Harbor.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Walls are useful where waters containing high concentrations of suspended sediment can be excluded from a facility. An open entrance is feasible in tidal waters when it can be placed so that waters having low concentrations of suspended sediment are admitted. Otherwise a lock is required. At present one such lock in the San Francisco Bay system has been operating since 1966, another will be built when funds become available, and a third is planned. Walls are typically pile-supported timber walls, sheet pile walls, or rock dikes. Walls are expensive to build and maintain, especially for large harbors. In many installations walls serve also as breakwaters, which aids in their justification, however. KEEPING SEDIMENT MOVING THROUGH FACILITIES This strategy consists of minimizing the rate of deposition and maximizing the erosion in the channel, basin, or berth by reducing local velocity gradients, where possible, and maximizing bed shear at the strength of flow. A remedial measure often does both simultaneously. As described above, aggregation of suspended sediment is promoted by the presence of velocity gradients. Sediment-laden waters that pass through piling are subject to vortex formation in the lee of every pile. These vortices extend from the bed to the water surface, so that the entire flow through the piles is subjected to increased velocity gradients. Suspended aggregates coalesce in these down-current vortices and the suspended sediment particles have increased settling velocities. The flux of sediment to the bed down current from the piling is greatly enhanced. At the same time the vortices dissipate energy, reducing the velocities and the bed shear stress in the region in the lee of the piling so that a greater portion of the settling aggregates stick to the bed. Estuaries the world over have old pile-supported wharves placed at an angle to the current, and deposition nearby is rapid. Most modern harbors have marginal wharves or quays. The deleterious effect of piles on sedimentation rates can be minimized by aligning rows of piles in the direction of the prevailing currents, and by minimizing the number of piles supporting the structure. A similar situation results from steep banks of turning basins and channels that are cut across the prevailing current. Flows from the shallower waters over the bank and into the deepened cut develop large eddies, with nearly horizontal axes, in the deepened basin or channel. As in the case with piles, the eddies promote aggregation and dissipate the strength of near-bed flows, both of which promote rapid deposition in the cut. The slope of the upstream and downstream banks can be reduced to 1 vertical on 10 horizontal or flatter to prevent the formation of eddies. Such flat slopes are easily formed by dredging steps with 2 ft risers and 20 to 40 ft treads. Sedimentation will fill the bank side of the treads to form a smooth slope.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings The Richmond Long wharf provides an example where altering the slope of the turning basin reduced sedimentation rates. Patterns of deposition near the banks of the basin are shown in Figure 2. Rapid sedimentation occurred in the approach basin near the south bank and at the berths along the south end of the wharf. Current measurements showed that flood flows across the shallow areas south and east of the facility entered the basin across the south bank and through the piling of the wharf. Revised bank slopes that were recommended after physical model studies are shown in Figure 3. Modification of the banks virtually eliminated deposition in the approach basin. Deposition continues at the berths, however, where sediment-laden flows from the shallow areas pass through the wharf piling. Similar deposition occurs in channels that have steep banks and are aligned at an angle to the prevailing currents. Alignment of channels with the prevailing currents as much as possible or providing flat channel slopes, especially where potentially eroding currents can remove newly deposited sediment, also minimizes such deposition. Marginal wharves, particularly wide platforms supported on piles, have a unique problem. In estuaries where there are large amounts of suspended sediment in motion, sediment accumulates among the piles and creates a mound under the platform. The piles and the mound obstruct the flow of water through the piling and present a blunt upstream end of the wharf to the current. The blunt end deflects the current away from the face of the wharf, and sediment accumulates along the face. An illustration of this phenomenon is provided by the Union Oil Company wharf at Oleum, California. This wharf is located in eastern San Pablo Bay at the west end of Carquinez Strait, as shown in Figure 4. Strong ebb currents flow from the strait past this wharf. Nevertheless, shoaling is a problem at the face of the wharf and along the landward side. A hydrographic survey of the bed near the wharf is presented in Figure 5. Regions of scour flaring away from the faces of the wharf show the deflection of the stronger currents by the blunt upstream end of the wharf, while the area near the face of the wharf is shallower. Obviously, the currents are strong enough to remove any deposition that might occur during times of slack currents; sediment accumulates because these currents are deflected away from the face of the wharf. Scouring observed around bridge piers suggested a remedy. Such scour occurs because the upstream end of bridge piers obstruct the current, creating an elevation of the water surface near the stagnation point where the kinetic energy of the flow is converted to potential energy. This local potential energy creates a downward current at the upstream pier end, and the higher velocity waters on either side of the stagnation point are deflected downward and along the sides of the pier. The requirements appear to be a blunt upstream end gradually curving toward the side of the pier. A small-scale hydraulic model study showed that a vertical wall (Figure 6) provides optimum currents along the face of a wharf (Lee, 1972). With this wall in place, currents in the model immediately adjacent to the face of the wharf were twice those that occurred with the blunt end, increasing the bed shear stress by a factor of four.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 2 Deposition pattern at the Richmond Long wharf, Standard Oil Company of California.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 3 Recommended slopes for banks of dredged cuts. Union Oil Company opted to continue dredging, rather than install the end, and I have not had an opportunity to install such an end on a marginal wharf. It has been used with success on three marina enclosures, however, to maintain water depths along the channel face of the enclosure and across the marina entrance. It has the added advantage of reducing surface currents at the entrance and creating a smooth transition from the higher velocity currents outside of the enclosure to the quiet waters within. An example is presented in Figure 7. These harbors have demonstrated the value of the transition, but one taught a costly lesson. The wall was undercut along its face by the strong near-bed currents, allowing waters having high suspended solids into the enclosure. The wall must penetrate sufficiently far into the bed so that scour along the wall does not undercut it. Alternatively, the bed along the wall at the upstream end can be armored. Additional research is needed to define the distribution of bed shear stress near such appurtenances. CONCLUSIONS These examples illustrate methods of implementing sedimentation control strategies. They are based on fundamental aspects of cohesive sediment transport processes: the control of suspended sediment concentration, aggregate settling velocities, and bed shear stress. Application of these methods to a particular problem requires detailed characterization of the site. Current measurements, hydrographic surveys, measurements of sediment properties, and sometimes numerical

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 4 Location of Oleum Terminal on San Pablo Bay.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 9 Wing and mooring design for field tests are Mare Island Naval Shipyard.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings plates and a horizontal settling plate is free to slide up and down the anchor extension, but float on the soft fluid mud horizon of the upper portion of the bed where the bulk density is approximately equal to ρc. The wing is moored directly to the settling place. The net of the wing buoyancy and the immersed weight of the settling plate is adjusted to float the settling plate near Zc. The mechanical linkage between the wing and settling plate consists of sealed automotive needle bearings from an axle, and an adjustable worm gear bolted to the wing. The worm gear permits final adjustments in the position of the mooring point in order to maintain the desired 12° to 15° angle of attack. Such adjustments are periodically necessary due to trim changes from biofouling or water absorption. As the bottom is 0eroded by the action of the wing, the level of Zc at which the settling plate floats are lowered. Since the wing is connected directly to the settling plate, it too is lowered by the same increment. The anchor extension shaft passes through a clearance hole in the wing. In this way the wing continually maintains its design mooring distance b/2 in spite of changes in the bottom level. In pull tests with a dynamometer, the mooring system in Figure 9 was determined to have a capacity for a 2,500 lb vertical loading. Due to the sensitive military nature of the study area (see Figure 11), a 4:1 design safety margin was required. Therefore, static buoyancy of the wings was restricted to about 625 lbs. This limited the final wing dimensions to a 20 ft span with a 5 ft root cord length and a 3 ft tip cord length. The wings were constructed from 8–12 lbs/ft3 density urethane foam in injection molds. Each wing displaces about 2,500 lbs of water. Therefore, they were ballasted and reinforced to a dry weight of 1,875 lbs using a steel rebarr infrastructure to achieve the design static buoyancy of 625 lbs. The suction side of the finished wings is shown in Figure 10a, the pressure side in Figure 10b. The boundary layer control fences were retrofitted to the suction side to delay merging of the trailing vortex filaments. Field deployments of a small array of wings were performed at Mare Island Naval Shipyard (Figure 11). A study area of 100×100 ft was authorized adjacent to the dry docks just beyond the dredged channel boundary. Generally the bottom in the study area slopes downward toward the channel beginning at −23 ft MLLW near the toe of the concrete quay wall to −35 ft MLLW at the channel boundary line. When dredged prior to installation of the array, the study area had an approximately flat bottom at −30 ft MLLW which rose abruptly forming a mud bank directly adjacent to the quay wall. The dredged bottom contours for several range lines across the study area are found in Figure 12 as dotted profile lines. Between November 1982 and November 1983, 3 to 5 wings were maintained inside the study area. Occasionally some of these were removed for brief periods to retrofit boundary layer control fences and remove biofouling. Two wings were eventually lost due to mooring failures. Within the control areas indicated in Figure 11, between 8 and 10 ft of shoaling was observed. Figure 12 gives the final bottom

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 10 Completed upwashing wings prior to deployment at Mare Island Naval Shipyard. Each wing spans 20 ft and weighs 1,875 lbs.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings contours across the study area at the end of the study period in November 1983. Figure 13 shows an echogram along the axis of the array parallel to the current. Two conclusions are apparent. First, the wings are locally 100 percent effective at preventing shoaling of the dredged bottom, otherwise they would have become buried beneath the sedimentation that has obviously occurred everywhere else. Second, from the scour trail evidenced on both sides of the array, there is some erosion flux which persists for a considerable distance downstream. That erosion flux decays with increasing distance downstream as previously anticipated from Eq. 14. The total erosion flux calculated from the volume of the scour trail over the length of the study period is 1,066 yd3/yr. FIGURE 11 Site plan for the wing experiments at Mare Island Naval Shipyard.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings During the first two months of the study period, all wings in the array were the upwashing type with dynamic lift acting vertically downward against the buoyancy. However, the scour trail was somewhat limited in downstream extent. Since upwash field is directed away from the bottom it was concluded that the resulting shear stress may fall below the critical erosion stress within a short distance downstream. Eq. 14 is valid only for shear stresses that equal or exceed the critical erosion stress. Consequently the array was modified by turning over the solitary wing leading the array in Figure 12 to direct turbulent downwash against the bottom. The result was a dramatic increase in the downstream extent of the scour trail as presently shown in Figure 12. FIGURE 12 Initial and final bottom contours resulting from wing arrays maintained for a period of one year.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 13 Echogram taken with 40 kHz along the axis of the wing array parallel to the current. Note the echo return from a moored wing. Also evident is the erosion of infill sediments down to the basement of undredged consolidated soils.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 14 Comparison of the measured or theoretical erosion flux in the downstream direction. The downstream extent of erosion flux for the modified array was measured from successive volumetric calculations deduced from four separate bathymetric surveys over the duration of the test. The results are plotted in Figure 14 and compared against the theoretical erosion flux calculated from Eq. 14. Here the distance downstream from the array is made nondimensional with respect to the mooring elevation b while the erosion flux has been normalized by the critical erosion flux Qc at which zero net shoaling occurs, that is when We find that the closest agreement with theory was observed for the first survey on February 16, 1983. The theory predicts both the near-field and far-field erosion fluxes with acceptable accuracy. The total downstream extent of zero net shoaling Qf/Qc−1, was about 60 m. However, the intermediate erosion fluxes for the later surveys showed more of a linear than an exponential decay with increasing distance downstream. This higher-than-expected intermediate range erosion flux was possibly the result of erosion on the side walls of the scour trail which grew in extent as the duration of the test increased. The theoretical result of Eq. 14 was based on the assumption of a flat bottom.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings There is an additional hydraulic action performed by an array of wings during the flooding phase of the tide. The action of creating lift also introduces additional dissipation through induced drag. The additional bottom friction per unit area introduced by the presence of the wing may be expressed in terms of the drag coefficient CD (21) where is the two-dimensional section drag coefficient found in Figure 8 and n is a factor that increases with aspect ratio and mooring distance. The rate of dissipation due to this additional bottom friction presented by each wing per unit area is (22) where F is the strain on the mooring and Θ is the angle of the mooring relative to the direction of the mean current u. This dissipation acts to impede the salt wedge circulation. If a sufficient density of wing arrays were placed in estuarine channels then it may be possible to arrest the salt wedge prematurely and thereby avoid the numerous problems created by saline intrusion into inland waterways. To examine the engineering feasibility of this approach, an energy flux survey was conducted around a moored wing as shown in Figure 11. The wing was fitted with a strain gauge and tilt meter on the mooring. A pressure sensor and two-axis current meter were deployed at the centerline of the wing, at both the leading and trailing edges. If (p∞, U∞, w∞) represent the static pressure and the horizontal and vertical velocities at the upstream survey line, and if (po, uo, wo) are the corresponding terms at the wing trailing edge, then the dissipation rate per unit area of the wing doing work against the salt wedge will be found to be (23) Figure 15 gives the time series and spectra for the left- and right-hand sides of the measured terms in Eq. 23. FUSN is the left-hand side for Eq. 23 obtained by multiplying the dissipation rate of the instrumented wing multiplied by 3, the total number of wings in the array at the time. (All three wings were upwashing types.) UCBE is the sum of all terms appearing on the right-hand side of Eq. 23. The

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings time series each represent 2,048 sec of data, or 34 min total record length. Many of the observed current and load oscillations are due to eddies shed from moored ships both on the upstream and downstream ends of the study area. We observe that the two spectra and time series are almost identical, despite what was admittedly a very coarse wake-rake of only three sensors. Estimating the depth of the salt wedge to be 5 m during this record, for which the mean current was 25 cm/sec, it is concluded that each unit area of wing removes 2.9 percent of the salt wedge energy flux through a unit area of stream cross section. FIGURE 15 Spectra and time series of the dissipation rate functions from Eq. 23.

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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings CONCLUSIONS Downwashing wings are required to achieve a critical erosion stress. Upwashing wings increase the erosion flux. The erosion flux is maximized by maximizing the lift and dimensions of each wing. The moored wing arrays do dissipative work against the salt wedge. The rate of dissipation increases as the lift coefficient squared for the individual wings in the array. ACKNOWLEDGMENTS The author is indebted to Dr. E.A.Silva for his support of this work through the Office of Naval Research, Code 421. This paper is the result of laboratory and field experiments conducted by Mr. Jim Palmer, Mr. Joe Wasyl and Mr. David Skelly from the Center for Coastal Studies, Scripps Institution of Oceanography, in cooperation with Mare Island Naval Shipyard divers and Public Works Department. The author wishes to acknowledge their outstanding performance and support and thank them for a job well done. REFERENCES Collins, T.J. 1980. Investigating bridge scour. Railway Trade and Structure 76(4). James, R.M. 1971. A new look at two-dimensional incompressible airfoil theory. Douglas Aircraft Company, Report No. MCD JO918/01, Long Beach, CA. Lambermont, J. and G.Lebon. 1978. Erosion of cohesive soils. J. Hydraul. Res. 16(1):27–44. Liebeck, R.H. 1973. A class of airfoils designed for high lift in incompressible flow. J. Aircraft 10:610–617. Miznot, C. 1968. Etude des proprietes physiques de differents sediments tres fins et de leur comportement sous des actions hydrodynamiques. La Horrible Blanche 7:591. Prandtl, L. 1919. Tragflugeltheorie. Gottingen Nachrichten. Prandtl, L. and O.G.Tietjens. 1934. Applied Hydro and Aeromechanics. New York: Dover Publications. 311 pp. Schlichting, H. 1968. Boundary Layer Theory. New York: McGraw-Hill. 748 pp. Slavert, H. 1948. The Elements of Aerofoil and Airscrew Theory. New York: Cambridge University Press. 232 pp. Stratford, B.S. 1959. The prediction of separation of the turbulent boundary layer. J. Fluid Mech. 5:1–16. Wortmann, F.X. 1955. Ein Beitrag zum Entwurf von Laminarprofilen fur Segelflugzeuge und Hubschrauber, Z. Flugwiss, 3.

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