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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SESSION A. SEDIMENT SOURCES AND TRANSPORT PROCESSES CO-CHAIRS Ashish J.Mehta Hsieh Wen Shen SPEAKERS Ronald Gibbs Ashish J.Mehta Robert Kirby Peter Sheng William McAnally
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings SOURCES OF ESTUARINE SEDIMENTS AND THEIR COAGULATION Ronald J.Gibbs Center for Colloidal Science College of Marine Studies University of Delaware The suspended load delivered by rivers constitutes the majority of sediments entering estuaries and the marine environment. These sediments are responsible for filling the channels, harbors, and waterways of our estuaries and coastal regions. The particles delivered by rivers also transport the majority of the toxic metals and organic compounds that are discharged into estuaries and the oceans. For these reasons, it is critical that we understand the processes related to the source, transport, coagulation, and deposition of these fine-grained particles in estuaries and coastal waterways. The major complicating factor in our present understanding of these processes is that when fine-grained particles first encounter small amounts of seawater, they are attracted to each other to form aggregates or flocs. The coagulation of river sediments has been demonstrated in the laboratory by Whitehouse et al. (1960), and later by Krone (1962, 1972, 1978), Shiozawa (1970), Hahn and Stumm (1970), Edzwald (1972), Edzwald et al. (1974); by Eppler et al. (1983) for Delaware Bay; by Gibbs et al. (1985) for the Gironde estuary in France; by Gibbs and Konwar (1986) for the Amazon River plume; by Kranck (1975, 1979) in Canadian waters; and by Syvitski et al. (1985) for fjords. McCave (1984) presented a review of fine-grained marine sediments erosion, transport, and deposition. It is the purpose of this paper to review the variety of sources pertaining to the concentration, composition, and size of particles delivered to estuaries by rivers, and then to elucidate the processes of what happens to these materials as they are transported, deposited, and resuspended in conjunction with the coagulation process as they first encounter seawater in estuaries. SEDIMENT CONCENTRATION AND DISCHARGE The concentration of suspended materials transported by rivers to estuaries varies widely in the world. This is well illustrated in Table 1, which shows the 20 largest rivers of the world, arranged by water discharge. It can be seen in the right-hand column that the concentration in milligrams per kilogram (the same as milligrams per
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings TABLE 1 The 20 Largest Rivers Worldwide Arranged in Order of Their Water Discharge Drainage Area (103 km2) Discharge (103 m3/sec) Suspended Sediment Yield (kg/km2 yr×103) Concentration (mg/kg) 1. Amazon 5,930 175 55.3 66 2. Congo 4,000 40 14.5 51 3. Orinoco 950 23 82.5 77 4. Yangtze 1,030 22 444 700 5. Bramaputra 560 20 1,179 1,070 6. Mississippi 3,268 18.4 82.5 510 7. Yenisei 2,480 17.5 3.8 190 8. Mekong 390 15.0 395 365 9. Parana 2,300 14.9 31.7 175 10. St. Lawrence 1,300 14.2 2.5 8 11. Ganges 1,060 14.2 1,270 3,400 12. Irrawaddy 370 13.6 744 710 13. Ob 2,440 12.5 5.4 37. 14. Volga 1,350 8.0 12.7 73 15. Columbia 669 8.0 19.0 56 16. Pearl-West 310 7.9 79.8 110 17. Mackenzie 1,700 7.4 2.7 21 18. Indus 1,050 6.8 408 2,200 19. Danube 810 6.2 21.8 200 20. Niger 1,100 6.1 3.8 25 liter) ranges from highs of 3,400 for the Ganges and 2,200 for the Indus, to low values of 8 for the St. Lawrence, 21 for the MacKenzie, and 25 for the Niger. Thus, the variation from the St. Lawrence to the Ganges is greater than a factor of 400 in the concentration of materials delivered. Likewise, the yield of suspended sediments (kilograms of suspended sediments per square kilometer) is shown to vary widely, ranging from over 1,200 for the Ganges to a low of 2.5 kg/km2 per year×103 for the St. Lawrence. Correspondingly, the annual amount of sediment discharge would be the water discharge multiplied by the sediment concentration, and it can be seen that these values would show even wider variations. These wide variations in sediment yield per square kilometer, or in the total amount exiting the river basin into an estuary, are a function of the mean elevation of the area and the amount and distribution of precipitation throughout the year received by that basin. Note that of the high-yielding, high-concentration rivers (the Yangtze, Bramaputra, Ganges, Irrawaddy, and Indus), all are Asian rivers. Table 2, which is a summary of the
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings TABLE 2 Summary of Measured Annual Sediment Yields of Selected Rivers to Oceans Continent Measured Drainage Area (km2) Annual Suspended Sediment Discharge (106 kg) (103 kg/km2) North America 6,380,976 547,908 86 South America 9,890,938 552,872 56 Africa 8,146,755 196,195 24 Australia 1,073,425 42,956 40 Europe 3,515,197 110,622 31 Asia 10,907,016 5,819,303 534 Total 39,913,307 7,269,856 182 SOURCE: Holeman (1968), after conversion to metric units. rivers by continent, also shows that Asia has a very large drainage area of 10 million km2 and that its annual sediment yield is almost 106 kg which is, by far, greater than any other continent. In addition, its kg/km2 erosion is, by far, the highest by almost one order of magnitude: North America runs a very low second at 86 kg/km2, followed by South America, Australia, and Europe with 31 kg/km2, and Africa at only 24 kg/km2. Thus, quite a significant difference exists among continents. If we observe the sediment yields and concentrations derived from some U.S. rivers and estuaries beyond the three or four rivers in Table 2, we can see from Table 3 that the distribution varies quite widely. Table 3 presents the 10 most important U.S. rivers based on large sediment discharge or drainage area. The St. Lawrence River actually carries about 75 percent of the water discharge of the Mississippi River at St. Louis. However, the St. Lawrence transports relatively little sediment because the Great Lakes act as natural sediment traps. Following the Mississippi River in volume of sediment transported are the Copper, Yukon, and Susitna rivers, all of which are in Alaska and drain high mountainous regions. The majority of U.S. rivers have been dammed up over the years, resulting in large sediment traps that are quite effective in reducing the sediment loads delivered to estuaries. Sediments that were once transported to estuaries are now probably filling the reservoirs behind the dams, rather than the channels of the estuaries. The variability of suspended sediment discharges each year must be considered on two scales. First, within any one year most sediments will be delivered in a short time period. This is best illustrated by Figure 1, which shows that during 1 percent of a year (3.6 days), most rivers discharge better than one-half to two-thirds of their sediments
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings TABLE 3 Discharge of Suspended Sediment to the Coastal Zone by 10 Major Rivers of the United States, about 1980. Rivers Average annual sediment discharge (million ton/yr) Rivers that discharge the largest sediment loads: Mississippia 230 Copper 80 Yukon 65 Susitna 25 Eel 15 Brazos 11 Columbia: Before Mount St. Helens eruption 10 (After Mount St. Helens eruption—approximate) 40 Rivers with large drainage areas: St. Lawrence 1.5 Rio Grande .8 Colorado .1 NOTE: ton/yr=tons per year. aIncludes Atchafalaya River. SOURCE: Meade and Parker (1985). for that year. Further, looking at the data for 10 percent of the year (36 days), it can be seen that the vast majority of the sediments of all the rivers in Figure 1 are discharged within this 10 percent period. Conversely, 90 percent of the year represents a very small amount of the sediment-transporting ability of the three rivers, as shown in the top segments of each graph. This variability within one year means that a very high load of suspended material enters an estuary, coagulates, and sediments in a very short period of time; and that during the remaining 90 percent of the year, very low amounts enter. However, the resuspension and redistribution of the material from the shorter time period is continually occurring. Thus, the time scales within the year must be considered far more than most researchers have done in the past. A larger time scale problem is also represented in Figure 1, as can be seen by the rare events that occur in the top figure for the Juniata River. In 1972, Hurricane Agnes produced an increased sediment load. The records of the suspended sediment discharge generated by the hurricane for the Juniata River showed that a ten day discharge was equal to three full years of average sediment transport. For the
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 1 Annual suspended-sediment discharge of three rivers showing the frequencies of suspended-sediment discharges within individual years and the importance of infrequent heavy storms in producing large sediment loads. A. Juniata River at Newport, Pennsylvania. B. Delaware River at Trenton, New Jersey. C. Eel River at Scotia, California. SOURCE: Meade and Parker (1985).
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Delaware River, a two day discharge represented three full years of average discharge. An even more spectacular single sediment discharge event is a storm that struck the Eel River in California. In a three day period, the Eel River carried more sediment past Scotia, California than it had during the previous seven years. In ten days, the transport was equivalent to the previous ten average years. To put this into perspective, the total suspended discharge for the Eel River was 168 million tons that year, which compares with the 184 million tons carried by the Mississippi River past St. Louis during the same year. This tremendous variability, occurring over a period of many years, is exceedingly difficult to sample and to understand because it is normally very expensive to prepare for sampling these types of rare events. However, sudden events are extremely significant in terms of quantity of sediments discharged and in the effects they have on estuaries. Another interesting case comes from the work of Schubel (1974) on the Susquehanna River entering the Chesapeake Bay. Figure 2 shows the sediment concentration during the year 1972. It can be seen that, on the average, the concentration was about 10–20 mg/liter, with a few spikes for storm events during February, March, and into April. During June, Hurricane Agnes caused the sediment concentration to exceed 10,000 mg/liter and the discharge equaled 27,750 m3/sec. This discharge rate continued for a couple of days on a decreasing scale as the torrential rains subsided. Schubel estimated that during this one week period (June 22–28, 1972), the Susquehanna River probably discharged greater than 50×106 metric tons of suspended sediment than had been discharged during the past three decades, and probably even during the past half century. The majority of these sediments were probably deposited in the upper parts of the Chesapeake Bay, and then possibly redistributed by subsequent storms. Hence, it is again illustrated that rare events are significant in the transporting of sediments into estuaries, and could dominate deposition over many years and greatly affect the dredging and shoaling activities occurring in estuaries. Unfortunately, processes usually studied to understand sediment transport are not the rare events, but are of some average conditions over the year. For the Chesapeake Bay, this average would be when the river is discharging 10–30 mg/liter, which is not the period of significant deposition of material. This example should emphasize the need to observe more rare events; or as was shown in earlier figures, at least that 1–10 percent period of the year when high sediment loads occur, and especially when immense storms occur, to understand how materials transport, where they are deposited, and how quickly they are resuspended and redeposited. COMPOSITION OF SUSPENDED MATERIAL In addition to the variability of concentration and yield per year discussed in the previous section, the composition of the material varies greatly by river, particle size, and times of the year. To illustrate this variability, Figure 3 depicts the suspended sediment
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 2 Concentration of total suspended solids (mg/liter) in the Susquehanna River at Conowingo, Maryland, during 1972. SOURCE: After Schubel (1974). composition and particle size at the mouth of the Amazon River, and shows a mean size of about 4 µm with a logarithmic distribution about this mean (Gibbs, 1967). The smallest particle sizes (<1 µm) are dominated by the mineral montmorillonite, the intermediate size (roughly 1–3 µm) is represented by the minerals kaolinite and mica with an admixture of quartz, and the >4 µm size is dominated by quartz, mica, and feldspar. The sand-sized region (>62 µm) represents a small part of the total material that is actually transported.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 3 Weight frequency distribution of various minerals from the suspended material of the Amazon River. SOURCE: Gibbs (1967). Another example is given by Figure 4, showing the particle composition distribution for suspended material near the mouth of the Mississippi River. Again, distribution is quite similar to the Amazon, with smectite (another name for montmorillonite) being the dominant mineral in the <.5 µm fraction. However, the abundance of illite and mica in the higher latitude portions of the Mississippi River is much greater than in the Amazon River; thus the minerals in the <4 µm fraction are dominated by mica, followed by kaolinite. Again, quartz becomes the dominant mineral after about 5 µm, with some other feldspars in the sand-sized fractions. Both distributions are similar, but the portions between kaolinite and illite are different for the two rivers. In the Mississippi River, illite is about three times the value of kaolinite, whereas in the Amazon River the two values are approximately similar in the 1–2 µm size fractions. It can be seen from this example that the composition of material is greatly dependent on fraction size in the river system at any particular time, and that from river to river the overall trends remain the same: montmorillonite is prominent in the smaller sizes, intermediate sizes contain large amounts of illite and kaolinite, and in the >4 µm range quartz and feldspar become the dominant minerals. Thus, if the system contains a dam, lake, reservoir or any barrier that traps the coarser-grained material and eliminates the >10 µm material (which has a fairly high settling velocity in fresh water), half of the quartz would be eliminated in the case of the Amazon and Mississippi rivers. This is important to later discussions about coagulation rates of these minerals when they reach seawater, since minerals coagulate at slightly different rates and at different salinities. Actually, if a river contained <2 µm material (not uncommon in some very slow-moving rivers that have sediment traps in their upper regions), it would be dominated by illite, kaolinite, and montmorillonite, and would have very small amounts of quartz and chlorite.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 4 Size and mineralogic frequency of suspended solids in the mud fraction of the Mississippi River. SOURCE: After Johnson and Kelley (1984). COAGULATION PROCESSES As the particles discussed in the previous section encounter the first traces of seawater in an estuary, the double layer around each particle is compressed by ions in the seawater (especially by divalent ions), which destabilize these particles and create a condition conducive to coagulation. Once the particles are destabilized, the collision between two particles produces an aggregate of two, and the process continues until equilibrium is attained, with the aggregate consisting of hundreds of the original particles. A microphotograph of one aggregate from Delaware Bay in which the particles have just coagulated is shown in Figure 5. Note its high porosity, irregular outline, and very fragile nature.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings FIGURE 5 Photomicrograph of a typical floc from the turbidity maximum. There are three processes in the coagulation of destabilized particles: (1) thermal agitation, (2) fluid shear, and (3) differential settling. The first coagulation process is thermal agitation (Brownian motion or molecular diffusion) in which the random motion of small particles is brought about by thermal effects. The driving force behind this transport is a function of the product of the Boltzmann constant and the absolute temperature. The kinetic energy of water molecules is transferred to small particles during the continuous bombardment of these particles by the surrounding water molecules. Transport by Brownian diffusion depends on the thermal effects only, and is independent of such factors as fluid flow, gravity forces, and salinity. The second process affecting particle transport is fluid shear, either turbulent or laminar. Velocity differences or gradients occur
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Jonsson, I.G. and N.A.Carlsen. 1976. Experimental and theoretical investigations in an oscillatory rough turbulent boundary layer. J. Hydraul. Res. 14:45–60. Kajiura, K. 1968. A model of the bottom boundary layer in water waves. Bull. Earthquake Res. Inst. 46:75–123. Kalkanis G. 1957. Turbulent flow near an oscillatory wall. Univ. of California, Inst. of Energy Res., Ser. No. 72, Issue No. 3. Kemp, P.H. and R.R.Simons. 1982. The interaction between waves and a turbulent current: Waves propagating with the current. J. Fluid Mech. 116:227–250. Krone, R.B. 1962. Flume studies of the transport of sediment in estuarial shoaling processes. Contract Report, U.S. Army Corps of Engineers, San Francisco District. Krone, R.B. 1976. Engineering interest in the benthic boundary layer. Pp. 143–146 in The Benthic Boundary Layer, I.N.McCave, ed. New York: Plenum Press. Lewellen, W.S. and Y.P.Sheng. 1980. Modeling of dry deposition of SO2 and sulfate aerosols. Report EPRI EA-1452, Electric Power Research Institute, Palo Alto, Calif. MacPherson, H. 1980. The attenuation of water waves over a non-rigid bed. J. Fluid Mech. 97(4):721–742. Mehta, A.J., and E.Partheniades. 1979. Kaolinite resuspension properties. J. Hydraul. Div. ASCE 105(HY4):411–416.30. Mehta, A.J., T.M.Parchure, J.G.Dixit, and R.Ariathurai. 1982. Resuspension potential of deposited cohesive sediment beds. Pp. 591–609 in Estuarine Comparisons, V.S.Kennedy, ed. New York: Academic Press. Owen, M.W. 1974. Erosion and Avonmouth Mud. Hydraulic Research Station Report, Int. 78. 25 pp. Partheniades, E. 1962. A Study of Erosion and Deposition of Cohesive Soils in Salt Water. Ph.D. Dissertation, University of California, Berkeley. Pearson, H.J., I.A.Valioulis, and E.J.List. 1984. Monte Carlo Simulation of coagulation in discrete particle-size distributions. Part 1. Brownian motion and fluid shearing. J. Fluid Mech. 143:367–385. Saffman, P.G. and J.S.Turner 1956. On the collision of drops in turbulent clouds. J. Fluid Mech. 1:16–30. Sheng, Y.P. 1982. Hydraulic applications of a second-order closure model of turbulent transport. Pp. 106–119 in Applying Research to Hydraulic Practice, P.Smith, ed. New York: ASCE. Sheng, Y.P. 1984. A turbulent transport model of coastal processes. Proc. 19th Internat. Conf. on Coastal Eng. ASCE, Houston, Tex. Pp. 2380–2396. Sheng, Y.P. 1983. Mathematical modeling of three-dimensional coastal currents and sediment dispersion. Technical Report CERC-83–2. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Also A.R.A.P. Report No. 458, Princeton, N.J. 288 pp. Sheng, Y.P. 1984a. Computation of current- and wave-induced erosional forcing within the Humboldt Bay. A.R.A.P. Tech. Memo #84–18, Princeton, N.J.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Sheng, Y.P. 1986b. Bottom boundary layer and cohesive sediment dynamics in estuarine and coastal waters. Pp. 360–400 in Estuarine Cohesive Sediment Dynamics, A.J.Mehta, ed. New York: Springer-Verlag. Sheng, Y.P. 1986c. Numerical modeling of coastal and estuarine processes using boundary-fitted grids. Pp. 1426–1442 in River Sedimentation III, H.W.Shen et al., eds. University of Mississippi. Sheng Y.P. 1986d. Second-order closure modeling of turbulent flow and sediment dispersion in coastal and estuarine waters. Pp. 1383–1396 in River Sedimentation III, H.W.Shen et al., eds. University of Mississippi. Sheng, Y.P. In press. A three-dimensional hydrodynamic model of estuarine and coastal currents using generalized curvilinear grids. In Three-Dimensional Models of Marine and Estuarine Hydrodynamics. J.C.J.Nihoul, ed. New York: Elsevier. Sheng, Y.P. In preparation. On the effect of suspended sediment Concentration on bottom boundary layer dynamics. Sheng, Y.P. and H.L.Butler. 1982. Modeling coastal currents and sediment dispersion. Proc. 18th Internat. Conf. on Coast. Eng. ASCE. Capetown, South Africa. Pp. 1127–1148. Sheng, Y.P. and W.Lick. 1979. The transport and resuspension of sediments in a shallow lake. J. Geophys. Res. 84:1809–1826. Sheng, Y.P., W.Lick, R.T.Gedney, and F.G.Molls. 1978. Numerical computation of the three-dimensional circulation in Lake Erie. J. Phys. Oceanog. 8:713–727. Sheng, Y.P. and C.Villaret. In preparation. On the determination of sediment erosion relationship. Trowbridge, J. and O.S.Madsen. 1984. Turbulent wave boundary layers. J. Geophys. Res. 89:7089–8007.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings MODELING ESTUARINE SEDIMENT TRANSPORT PROCESSES William H.McAnally, Jr. Hydraulics Laboratory U.S. Army Engineer Waterways Experiment Station This paper describes the methods used by the U.S. Army Corps of Engineers to model the transport, deposition, and erosion of sediments in estuaries. It shows how these methods can be applied to reduce maintenance dredging in estuaries. The U.S. Army Corps of Engineers is responsible for design, construction, and maintenance of federal navigation channels in the United States. The Corps dredges over 250 million m3 of sediment per year in maintaining more than 30,000 km of waterways and about 1,000 harbor projects. About 60 percent of that volume,is dredged in the estuarine and coastal zone (ASCE, 1983). The Corps uses a variety of techniques to predict navigation channel sedimentation, evaluate remedial measures before they are constructed, and evaluate sedimentation impacts of various activities. For small projects and rapid analyses, analytical methods are used (Corps of Engineers, 1984). For large projects, physical, numerical, and hybrid models are employed. This paper will concentrate on numerical and hybrid modeling of fine sediment processes. Numerical sediment transport modeling employs computational techniques to solve mathematical expressions that describe the movement, deposition, and erosion of sediments. Hybrid modeling includes both numerical models and scale physical models in an integrated approach that uses each model for those tasks that it performs best (McAnally et al., 1984a). The hybrid method has been shown to be superior to either numerical or physical modeling alone (McAnally et al., 1984b). The hydraulics laboratory of the Corps’ Waterways Experiment Station conducts applied research in inland and estuarine waterway hydraulics to support the Corps’ navigation responsibilities. As such, it serves as consultant to the Corps’ district and division offices, which are responsible for planning, design, construction, and operation of navigation projects. In that role, it develops analytical and numerical tools that can be used by the Corps in addressing hydraulics questions. The Waterways Experiment Station also performs hydraulics studies for other federal agencies, notably the U.S. Navy.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings BASIC MODELING APPROACH Numerical modeling is usually performed either at one of the Corps’ laboratories or at district offices in consultation with the labs. Physical modeling is usually performed at the Waterways Experiment Station. Some numerical models are intended for use only by the model developers and are not released to the Corps at large. Two fundamental concepts govern use of numerical models by the Corps. Generalized numerical models are developed by the labs for use by Corps engineers who are expert in the physical processes, but not necessarily expert in computer use. The use of standardized models is encouraged so that reviewers do not need to research the model used for a study to note the model used and how it was applied. These two concepts lead to four practical consequences: That the models must be well documented and generalized—i.e., applicable to a variety of situations. That the models employ the systems approach with a collection of related programs to perform various modeling-related tasks. That the approach be basically conservative, limiting revisions to those that have been thoroughly tested and avoiding frequent model revisions that would confuse users and reviewers. That the models be written in modular form, so that when revisions are made, they appear as new options rather than as wholly new models that are inconsistent with previous applications. NUMERICAL MODELS USED BY THE CORPS One-dimensional Models One-dimensional models are rarely used for estuarine sedimentation studies since the processes are strongly two- and three-dimensional. Exceptions are applications of the program HAD-1 to Atchafalaya Bay and the Mississippi River passes. Two-dimensional Models Two-dimensional models include the horizontal (depth integrated) SEDIMENT family and the vertical (breadth integrated) LAEMSED. The SEDIMENT family of models consists of a series of daughter models to SEDIMENT2, developed by Ariathurai, MacArthur, and Krone (1977). The family has developed along two similar but distinct branches. A discussion of the various daughter models is given by McAnally, Thomas, and Ariathurai (1983). One of the Daughters, STUDH, is used extensively by the Corps as the sediment model in the TABS-2 system (Thomas and McAnally, 1985). LAEMSED is a two-dimensional, breadth-averaged model based on LARM2 (Edinger and Buchak, 1983), a reservoir hydrodynamic model. Several
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Corps models have been built on the basic LARM framework, including water quality models. Model LAEM is the estuarine hydrodynamics version of the code and LAEMSED has sediment transport capability added to it (Johnson, Trawle, and Kee, in press). Three-dimensional Models Two three-dimensional models have been developed for use by the Corps: SEDIMENT8 was developed under contract by Resource Management Associates (Ariathurai, 1982), and CELC3D was developed under contract by Aeronautical Research Associates of Princeton (Sheng, 1983). SEDIMENT8 is the offspring of the SEDIMENT family mentioned above. Specialty Models Specialty models are used for some problems that require out-of-the-ordinary treatment. One group of these are the dump models, which reproduce the disposal of dredged material in open water and assist in designing and managing open-water disposal sites. Multiple versions of these models are used to simulate various dumping operations and equipment (Trawle and Johnson, 1986). Though not covered here, these models have recently been applied to dredged material disposal in Puget Sound for the Navy. Modeling Systems The advent of production level numerical models has lead to development of modeling systems—collections of related computer programs that work together to generate the computational meshes, model the processes, and analyze and display the results. Prominent among these systems in the Corps are the TABS-2 system, which has been released to the public as well as the Corps, the Coastal and Inlet Processes (CIP) system, and the TABS-3 system, which is undergoing testing and revision prior to release. TABS-2 and CIP are two-dimensional systems and TABS-3 is a three-dimensional system. TABS-2 has been applied to more than two dozen projects in the last six years. Hybrid Modeling As mentioned above, integrated physical and numerical models used in a hybrid modeling approach take advantage of the strong points of each technique while avoiding its weaknesses. The hybrid technique for sedimentation studies was developed at the Waterways Experiment Station and first applied to the Columbia River estuary, Oregon. Results obtained there were superior to those produced by physical modeling alone. Model predictions of the increase in maintenance dredging after
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings construction of an enlarged navigation channel have been found to be extremely accurate in the first year following completion of the project. CONCEPTUAL AND MATHEMATICAL MODELS The two- and three-dimensional models solve the basic convection-diffusion equation for dispersive transport. In the two-dimensional models the equation is integrated over one of the spatial dimensions, and the resulting smearing of transport is accommodated in the diffusion terms. In both cases turbulent diffusion and dispersion due to spatial and temporal averaging are lumped into the dispersion terms that use effective diffusion coefficients analogous to Fickian diffusion. The sink/source terms in the convection-diffusion equations serve as the means of exchanging sediment between the bed and flow. Calculating these terms represents a significant part of the difficulty in modeling. Settling velocities appear in several of the equations. At present, settling velocities are specified by the user for all of the models. Values may change with time and space if the user chooses. Modifications underway will replace the specified settling velocities with values calculated from other parameters, such as concentration, salinity, and flow characteristics. Bed modules in the two-dimensional models and in SEDIMENT8 are similar, based on Krone’s layering concept in which successive layers exhibit different bulk densities and resistance to erosion. At the user’s option, layers consolidate with overburden and with time, changing resistance to erosion as they consolidate. STUDH offers the option of mixing cohesive and noncohesive layers in the bed and calculating transport of each in turn. It is noted that these numerical models, employing the latest computational techniques on some of the newest and most powerful computers available, are using mathematical models (the equations solved) that are 20- to 25-years old despite the availability of more recent, and presumed more rigorous, equations. The primary reason for this apparent discrepancy is that most of the modeling research effort of the last few years has gone toward making the models practical, i.e., providing computational efficiency, more realistic schematization, and ease of use, instead of more up-to-date equations in the physical descriptions used. This approach has been justified in the past by answers that appear to be quite good, at least in comparison to accuracy of the field data with which model results are compared. Now that the practicality of the two-dimensional models has been established and the models themselves are in the hands of end users, improvements to the physical descriptions are due and are underway. Shear stresses for the sediment equations are calculated from the hydrodynamic results using various relationships between shear stresses and velocity squared. STUDH is most flexible in this regard, offering
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings several options, including Mannings, rough wall, and smooth wall forms. The models permit calculations of shear stresses due to combined currents and short period waves using equations adapted from those proposed by Bijker and by Jonsson (Thomas and McAnally, 1985). HYDRODYNAMICS Accurate computation of hydrodynamics is, of course, essential to accurate computation of sediment transport, but the hydrodynamic calculations are not covered here. STUDH, SEDIMENT8, and the dump models require that water levels and current velocities either be specified or computed by another model. RMA-2 (two-dimensional) and RMA-10 (three-dimensional) are used for that purpose. LAEMSED and CELC3D calculate hydrodynamics and sedimentation with the same computer code. SOLUTION TECHNIQUES A variety of solution techniques are employed by sediment models. STUDH and SEDIMENT8, as well as their hydrodynamic drivers, use a finite element solution technique. LAEMSED and CELC3D employ a finite difference solution. LAEMSED solves on a uniform rectilinear grid while CELC3D allows it to use boundary-fitted coordinate grids that approach the flexibility of finite element computational meshes in mapping complex geometries. SEDIMENT8 transforms the vertical mesh into a rectangular computational domain, and the number of computational points in the vertical can vary gradually in space. It can also change to a two-dimensional depth-integrated solution over part of the mesh. RMA-10, the hydrodynamic model, can accommodate three-, two-, and one-dimensional computational cells in the same mesh. RMA-2 will accommodate both two- and one-dimensional cells. CELC3D also transforms the vertical grid into a rectangular domain, with the same number of cells in the vertical at all points. MODEL APPLICATION EXAMPLES Examples of model applications illustrate the use of the models in reducing maintenance dredging requirements. Corpus Christi Harbor LAEMSED and STUDH have been applied in combination to Corpus Christi Bay and harbor. The broad, shallow bay requires plan-form schematization to show the lateral and longitudinal variations in transport. A depth-integrated model, such as STUDH, can represent that situation well. The navigation channel and harbor exhibit significant
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings vertical variations in currents and sediment transport, which can best be modeled by LAEMSED. Funding and time constraints have not permitted a fully three-dimensional model application. To properly account for the various needs of the Corpus Christi modeling effort, a LAEMSED model was constructed of the channel and harbor and a STUDH model was constructed for the bay. The LAEMSED model was used to examine measures that might diminish sediment-laden density currents into the harbor that contribute to sedimentation. STUDH was used to evaluate the role that open-water dredged material disposal plays in channel shoaling and to identify potential new disposal sites that could minimize return of disposed material to the channel. Models have proven to be very useful in testing proposed remedial measures and identifying potential remedies not previously envisioned. This latter facility points out an important facet of model studies. The scope of work should be flexible enough to allow pursuit of ideas generated by the model results and not restrict testing to just those items planned in advance. Kings Bay The U.S. Navy’s TRIDENT submarine base at Kings Bay, Georgia is being constructed in a shallow, marshy estuary north of Jacksonville, Florida. Areas that were naturally less than 10 ft deep are being dredged to depths on the order of 50 ft to accommodate the Navy’s vessels. Under these circumstances, it is natural that heavy sedimentation is expected. The Waterways Experiment Station has applied the TABS-2 numerical system (and the Coastal and Inlet Processes Modeling System) with a large physical model to Kings Bay in a hybrid modeling approach. Verification of STUDH to available field data showed it to be an excellent predictor for the pre-TRIDENT bay geometry. In fact, the model even predicted the location of the transition zone between sand clays making up the bed near Kings Bay before that location was identified from field data. The models were next applied to predict shoaling in the completed navigation channel and facilities and to evaluate possible maintenance reductions measures. In 1986, shoaling in the partially completed facilities has been less than that predicted. Several possible reasons for the discrepancy have been suggested, including the following: The winter of 1985–86 was drier than usual, reducing the inflowing sediment load to the system. Perhaps a wetter year will produce even greater shoaling than predicted. One of the impacts of the deepened channels as shown in the models is an alteration of circulation in the system of bays and channels. Perhaps the partially-completed dredging has not yet led to that alteration, which will be realized only when construction is complete.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Formation of dense sediment suspensions that eventually may consolidate into shoals may be occurring but are not yet detected by acoustic fathometers. Perhaps the great changes to the system caused by massive dredging has strained the model verification beyond the point at which results diverge from the prototype. Data from the Kings Bay facility is still being analyzed to determine which explanation is appropriate. Plans being tested include sediment traps and installation of a tide gate to regulate the direction of flows through the facility. One series of tests involves evaluation of specialized dredging techniques. An analytic approach showed that agitation dredging or a jet array system could potentially be used to remedy localized shoaling problems. Numerical modeling tests will show the effect of resuspension by those methods on shoaling rates elsewhere in the facility. Mississippi River The Mississippi River estuary is being modeled by the Waterways Experiment Station using the TABS-3 modeling system (with SEDIMENT8) and TABS-2 (with STUDH) in a hybrid approach. Southwest Pass, a major distributary of the river, is the primary navigation channel route. About 20 million yd3/year are dredged from the channel. A two-phase hybrid modeling approach is being used to evaluate the reduction of maintenance dredging that will result from construction of training works and to predict requirements for a deepened channel (from 45 to 55 ft). In the first phase, TABS-2 is being applied in concert with the physical model of the lower 30 miles of the estuary to analyze shoaling in the freshwater portion. The next phase will involve the physical model and SEDIMENT8 in analyzing the salt wedge portion of the estuary, where three-dimensional currents and transport must be reproduced. RESEARCH NEEDS To improve numerical and hybrid modeling of fine sediment transport, research is needed to provide a better description of the physical processes, to provide better input and verification data for the models, and to provide better solution techniques. As mentioned earlier, the Corps’ efforts have recently been concentrated, though not exclusively, in the latter area. Work now underway (albeit slowly) will incorporate other mathematical descriptions of deposition, erosion, and consolidation into the models, as well as internal computations of settling velocities and diffusion. Other research is focusing on techniques to better measure near-bed flow velocities and sediment transport and to locate and define zones of fluid mud. This and other research will
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings make the Corps’ numerical, physical, and hybrid modeling tools even more useful in devising and evaluating remedies for navigation facility sedimentation. CONCLUSIONS Modeling of sedimentation processes by multidimensional numerical and hybrid modeling can be an effective tool in determining approaches to reduce navigation facility shoaling. In cases where the cost of dredging is high or expected to be high, or where remedial measures are expensive, modeling is advised. Localized problems (such as toe scour) may or may not be amenable to modeling solutions and must be evaluated on a case-by-case basis. Further improvements in modeling techniques, in the mathematical description of sedimentation processes, and in field measurement techniques can be expected to increase the utility of these tools. It should be noted that models are rarely either right or wrong: they either lead the engineer to proper conclusions or to improper conclusions. Thus use of model results is as important as model results themselves. In this light, they may be compared to craftsmen’s tools, which can either be productively used or abused. ACKNOWLEDGMENTS Funding for development of the numerical modeling systems was provided by the research and development program of the U.S. Army Corps of Engineers, Office of the Chief of Engineers. The site-specific studies cited here were funded by the respective district offices of the Corps of Engineers. The Chief of Engineers granted permission to publish this paper. REFERENCES American Society of Civil Engineers. 1983. Shoaling processes in navigable waters. J. Waterway, Port, Coastal, and Ocean Engineering 109(2). Task Committee on Causes and Effects of Shoaling in Navigable Waters. New York: ASCE. Ariathurai, R. 1982. Two and Three Dimensional Models for Sediment Transport. Lafayette, Calif.: Resource Management Associates. Ariathurai, R., R.C.MacArthur, and R.B.Krone. 1977. Mathematical model of estuarial sediment transport. Technical Report D-77–12. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Edinger, J.E. and E.M.Buchak. 1983. Development in LARM2: A longitudinal-vertical, time-varying hydrodynamic reservoir model. Technical Report E-83–1. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station.
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Sedimentation Control to Reduce Maintenance Dredging of Navigational Facilities in Estuaries: Report and Symposium Proceedings Johnson, B.H., M.J.Trawle, and P.G.Kee. In press. A numerical model of the effects of channel deepening on shoaling and salinity intrusion in the Savannah estuary. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. King, I.P. 1982. A finite element model for three dimensional flow. Lafayette, Calif.: Resource Management Associates. McAnally, W.H. Jr., W.A.Thomas, and R.Ariathurai. 1984. Multi-dimensional modeling of sediment transport. Proc. Conf. on Frontiers in Hydraul. Eng. New York: ASCE. McAnally, W.H. Jr., J.V.Letter, Jr., J.P.Stewart, W.A.Thomas, and N.J.Brogdon, Jr. 1984a. Columbia River hybrid modeling system. J. Hydraul. Div. ASCE, March. McAnally, W.H. Jr., J.V.Letter, Jr., J.P.Stewart, W.A.Thomas, and N.J.Brogdon, Jr. 1984b. Application of Columbia River hybrid modeling system. J. Hydraul. Div. ASCE, May. Sheng, Y.P. 1983. Mathematical modeling of three-dimensional coastal currents and sediment dispersion: Model development and application. Technical Report CERC-83–2. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Thomas, W.A. and W.H.McAnally, Jr. 1985. User’s manual for the generalized computer program system, open channel flow and sedimentation, TABS-2. IR HL-85–1. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. Trawle, M.J. and B.H.Johnson. 1986. Alcatraz disposal site investigation. MP HL-86–1. Vicksburg, Miss.: U.S. Army Engineer Waterways Experiment Station. U.S. Army Corps of Engineers. 1984. Shoaling predictions in offshore navigation channels: Analytical and empirical methods. Engineer Technical Letter 1110–2–293. Washington, D.C.
Representative terms from entire chapter: