Conversion of a Flood Control System to a Sustainable System: The

The reservoirs of flood control systems will eventually fill with sediment, and all flood control will be lost. Well before complete filling, the loss...
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Environ. Sci. Technol. 2000, 34, 3730-3736

Conversion of a Flood Control System to a Sustainable System: The Energy Requirements for Pipeline Transport of Silt E. HOWARD COKER* Department of Chemistry, University of South Dakota, 414 East Clark Street, Vermilion, South Dakota 57069

TABLE 1. Sedimentation of the Permanent Pool (25% of Design Capacity)a reservoir

yr since closure

% capacity loss

yr until 25% loss

Lewis and Clark Francis Case Sharp Oahe Sakakawea Fort Peck

43.4 46.4 35.4 40.4 45.7 61.5

20.0 17.9 7.6 3.3 4.7 5.6

11 22 80 260 200 210

a

The reservoirs of flood control systems will eventually fill with sediment, and all flood control will be lost. Well before complete filling, the loss of flood control and competition with other system purposes, such as power generation, will force remediation efforts. The energy required to transport the sediments from the reservoirs back to the river channel has been determined using the Pick-Sloan system of the Missouri River as a representative system. Pipeline transport of the silt and clay requires about 3% of the hydroelectric energy generated by the system. This is small enough to justify a thorough study of the full cost and environmental impact of conversion to a sustainable system. Sand and gravel are more economically moved by rail, but they are probably best used as underlayment for silt and clay to mimic the natural process of fertile flood plain creation. The time frame for remediation is much shorter than often presumed. Using the volume of the permanent pool (25%) as the loss of reservoir capacity allowable before sustainability is achieved, the two lowest of the six Pick-Sloan reservoirs will have filled their permanent pools in less than 25 yr.

Introduction The rich flood plains of the earth’s major rivers have been prime sites for human habitation since prehistoric times. However, as agriculture fostered permanent dwellings, a family could no longer simply move its tent to escape the periodic flooding that made the plains so fertile. The cost of flooding has escalated from inconvenience to major national disaster. The response in the 20th century has been to mitigate flooding by damming. China is now embarked upon the building of the massive Three Gorges Dam of the Yangtze River. In 1954, 30 000 people lost their lives to flooding by this river (1). In the United States, the catastrophic floods of 1951 in the lower Missouri River basin produced the nation’s first $1 billion flood (2). Because of sedimentation, any flood control system will have a finite lifetime unless something is done to convert it to a sustainable system. The cost of conversion will eventually be paid since it will be far less than the cost of the flood damage the system was designed to reduce. However, the times for complete sedimentation of the reservoirs often range from hundreds to over 1000 yr, so reservoir sedimentation has largely been viewed as a problem even our great grandchildren will not have to confront. This is not the case. * Corresponding author phone: (605)677-6981; fax: (605)677-6397; e-mail: [email protected]. 3730

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Data are computed for January 1999.

The Pick-Sloan system of the Missouri River will be used to illustrate that the need for remediation is by no means so remote. In part, this is because the system serves secondary purposes of substantial economic and societal import. These include power generation, irrigation, recreation, and navigation. These purposes compete with each other and with the primary function of flood control. For example, low reservoir levels enhance flood control while high levels enhance power generation. The competition increases as the reservoirs fill with sediment. The unusually high runoff in the upper watershed of the Missouri River during the past few years led to nearly complete filling of the Pick-Sloan reservoirs in the spring of 1997 and to the exacerbation of flooding in the reaches below Sioux City, IA. However, the flooding was greatly mitigated. The U.S. Army Corps of Engineers estimates that the dam and reservoir system prevented over $5 billion in damage in 1997 (3). A map of the Missouri River watershed showing the location of the six mainstem dams and their reservoirs is shown in Figure 1 (4). A measure more useful than the time for full sedimentation is a time that signals a significant degradation of the capacity of the system to serve its multiple functions. Although somewhat arbitrary, this has been taken to be the time for 25% sedimentation of a reservoir. This relative amount is the same as that for the permanent pool of each reservoir. This pool’s purposes are to provide the minimum head for power generation and to provide for sediment retention. For January of 1999, Table 1 shows the percent of capacity loss and the estimated number of years for 25% filling of each of the six Pick-Sloan reservoirs. These times were computed from the closure date of each dam, the reservoir capacities in 1996, and the annual influx of sediment into each reservoir from all sources. Lake Francis Case received an additional sediment load from the upper reaches because its dam, Fort Randall Dam, was the first dam closed after Fort Peck Dam. The computation for Lake Francis Case includes this sediment load. The data for the computation were extracted from a summary sheet of engineering data compiled by the Missouri River Division of the U.S. Army Corps of Engineers. The selected data are tabulated in the Supporting Information. The percentages of reservoir filling are consistent with those made by Carlson in 1990 (5). Sedimentation rates vary somewhat from year to year, so the times are to be taken as no more than reasonable estimates. They range from 11 yr for Lewis and Clark Lake and 22 yr for Lake Francis Case to 260 yr for Lake Oahe. Significant raising of the heights of the mainstem dams or the construction of new dams is unlikely to become an acceptable option for the delay of remediation measures. Several instances can be cited to illustrate that sedimentation is currently causing problems of substantial magnitude. The three lower reservoirs serve an important role in 10.1021/es990536o CCC: $19.00

 2000 American Chemical Society Published on Web 07/25/2000

FIGURE 1. Missouri River Watershed. Reprinted with permission from ref 4. Copyright 1989 NRC Research Press. maximizing power generation. When navigation flows are cut in the fall, power generation is greatly reduced at the three downstream dams. Releases from the upstream dams are increased to compensate. The increased winter releases from Oahe to refill the evacuated Lake Francis Case and increase winter power generation have recently been jeopardized by sediment buildup at the head of Lake Sharp. The buildup, exacerbated by winter ice, causes flooding in Pierre and Fort Pierre and has forced substantial reductions in annual power generation (6). Sediment buildup where the Niobrara River enters Lewis and Clark Lake is causing flooding of the towns of Niobrara, NE, and Springfield, SD (7). Because of the current problems caused by sedimentation, the Missouri River Natural Resources Committee, a committee with representation from 10 states, is currently seeking federal funding for a 15-yr study of the Missouri River’s resources, uses, and potential (8). The proposed study would cost $12.5 million/yr. Some smaller reservoirs have been converted to sustainable systems by returning their sediments to their river channels. Hotchkiss and Huang (9) have considered systems that are powered by their hydraulic heads and report several case histories. Larry Hesse and co-workers (10) have suggested the return of the sediment to the river channel as one option for converting the Pick-Sloan system to a sustainable system. A consideration of this option is the focus of this paper. In particular, this paper provides an estimate of the energy required to transport the silt and clay from each of the six reservoirs to the channel of the Missouri River at Gavins Point Dam, the dam whose closure created Lewis and Clark Lake. The energy costs for dredging and for loading the transport systems have not been considered, and no attempt has been made to assess labor or capital costs. Since it is a return to the condition that existed before the construction of the flood control system, return of the sediment to the channel is an environmentally sound option. Hesse notes a number of ecological benefits. The reach of the river from Gavins Point Dam to Ponca, NE, where

channeling begins, has suffered considerable erosion since the closure of the dam in 1955. The erosive, sediment-free waters in this reach have dropped the channel 6-11 ft (11), cutting off oxbow lakes and draining extensive backwater areas that had been important fish and wildlife habitat. Erosion of farmland has also been enhanced. Return of the sediment to the river can reasonably be expected to stop the accelerated channel degradation of about 2 ft/decade and might even lead to some aggradation, thereby preserving and enhancing existing backwater areas. The lowered water levels, which extend about 100 mi below Sioux City, have led to extensions and re-engineering of some city water intakes and the water intakes of coal-fired power plants. Much of Louisiana and Mississippi owes its origin to the sediments of the Missouri River. The deltas of these states have shifted from growth to recedence creating substantial concern. While the reasons for this change are complex, it seems that these two states would be pleased to receive, once again, their annual contributions of sediment from Wyoming, the Dakotas, Montana, and Nebraska. The total volume of sediment, nearly 100 000 acre-ft annually, would be sufficient to create a mountain (assumed to be a paraboloid of revolution) 1 mi wide at the base and 2000 ft high every 5 yr. Dredging with storage of the sediments at sites near the river can provide a partial solution, but the magnitude of the sediment volume would exhaust readily available sites. Alternate uses for the Missouri River sediments will sometimes be appropriate. River sand has many uses. Once loaded onto a system for return to the river, it could be diverted and shipped substantial distances. It might be economic to divert some of the silt and clay to mimic the natural process of overlaying sand near the river to create prime farm land. Efforts to mitigate erosion are important to pursue. The dramatic 40% reduction achieved in sediment load for the Plum Creek watershed in South Dakota is worthy of special note (6). The approach suggested here will actually enhance mitigation efforts. Defining the cost of returning the sediVOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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ments to the river will provide a baseline for evaluation of investments in mitigation programs. The Permanent International Association of Navigation Congresses (PIANC) classifies materials below 2 µm in diameter as clay and those in the range from 2 to 60 µm as silt. The diameters for sand range from 60 µm to 2 mm. The U.S. Geological Survey has classified the components of the Missouri River sediments with particle diameters below 62.5 µm as silt and clay. On this basis, the compositions of the sediments entering the Missouri River reservoirs from the river and its principal tributaries range from 23% to over 99% silt and clay (12). Two means of transport will be considered: pipeline transport whose cost depends on particle size and rail transport whose cost does not. The energy required for pipeline transport of fine particles is substantially less than that for rail transport. As the particle diameter for a sediment fraction increases, energy requirements increase rapidly, and rail transport becomes less costly. The diameter at which rail transport becomes more economical depends on total cost, but the rapid rise with diameter of the energy for pipeline transport enables an approximate definition of this particle diameter. Considering only transport energy requirements, it will be shown that clay, silt, and some of the finer sand can be transported more cheaply by pipeline. Pipeline transport will be considered first. Each variable used is defined in the following text and also in the Supporting Information. A computational supplement detailing several of the key computations is also included in the Supporting Information.

Pipeline Transport The pipeline transport of river and ocean sediments has a long history because of the need for dredging rivers and ocean harbors. Because of the complex nature of turbulent, twophase flow, the early studies were largely empirical. Only recently has detailed, microscopic modeling been attempted. The 1991 monograph of Shook and Roco (13) provides a status report. Another review is provided in the 1996 text by Wilson et al. (14). Microscopic modeling based on particle and fluid properties and conservation laws has yet to be reported in the literature for particles smaller than about 150 µm. However, an estimate of the energy requirements for the transport of fine sand and of silt whose particles are larger than about 50 µm can be made from the data of Roco and Shook (15) on 165-µm sand. Energy is required for the transport of a fluid or a homogeneous suspension because of the irreversible conversion of mechanical energy to thermal energy and the loss of thermal energy by heat transfer. The height or hydraulic head of fluid needed to maintain steady flow is conventionally used to define the energy loss, so this loss is denoted the head loss. Fox and McDonald (16) give the head loss in units of energy per unit mass as

h ) ∆P/F

(1)

where ∆P represents the pressure difference at two points along the pipe and F is the density of the fluid. The pressure change is determined by the pipe diameter D, the distance L over which the pressure drop occurs, the roughness of the pipe e, the average velocity V, the density of the fluid, and its viscosity µ. Dimensional analysis to define a set of unitless ratios of these quantities and the empirical observation that the head loss is proportional to L/D demonstrate that

h ) fLV2/2D 3732

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(2)

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

The friction factor f is an empirically defined function of the Reynolds number Re ) VD/ν and the relative roughness e/D. The kinematic viscosity, ν, is equal to µ/F. Published by Moody in 1944 (17), the friction factor is presented graphically in most texts on fluid mechanics. The Fanning friction factor, more commonly used by chemical engineers, is equal to f/4. Equation 2 was derived for Newtonian fluids and applies as well to homogeneous suspensions. However, as will be shown, it predicts the head loss quite well for fine particle slurries in which substantial concentration gradients exist. The head loss is most commonly expressed as energy loss per unit of weight rather than per unit of mass. Division of the right-hand side of eq 2 by g gives the Darcy-Weisbach equation in which length is the only remaining unit. This length may be taken as the height or head of a column of liquid or slurry required to maintain the flow at the velocity V. The intensive variable h/(gL) is called the hydraulic gradient j. It is a unitless ratio of head loss to pipe length. When aqueous slurries are being considered, the hydraulic gradient is usually expressed in terms of head of water i rather than head of suspension. The gradient i is equal to the product of j and the specific gravity of the suspension. The Reynolds number of a suspension can be computed from the suspension’s density and viscosity. Thomas (18) gives the viscosity relative to that of the pure liquid as

µr ) 1 + 2.5C + 10.05C 2 + 0.00273 exp(16.6C)

(3)

where C is the volume percent. The density of a suspension is given by

F ) Fl(1 + C(S -1))

(4)

where S is the specific weight, the density of the solid relative to the liquid density, Fl. In the laminar flow region, the friction factor is given by 64/Re. Above the transition to turbulent flow that begins for values of Re of about 2000, the friction factor is a function of both Re and e/D. For the purposes of the present paper, pipe roughness may be neglected. Miller’s (19) expression for the friction factor reduces in the smooth pipe approximation to

f 1/2 ) 1.279/(ln(Re) - 1.942)

(5)

Some advantages accrue to considering the energy loss per unit of volume instead of the loss per unit of mass. If one multiplies the head loss by the density of the fluid or suspension and divides by the length of the pipe, one obtains a quantity that will be denoted as the transport index:

T ) F jg ) Fwig ) Ff V 2/2D

(6)

where Fw is the density of water. As slurries become more concentrated, the value of the final expression in eq 6 begins to deviate from the other three. These deviations will be discussed later in the paper. It will be convenient to use the mixed units of kWh acre-ft-1 mi-1. Conversion to these units from metric units of J/m4 is accomplished through multiplication by 0.5513. Since the concern in this paper is the transport of a given volume of settled sediment, it is useful to define Ts for transport of the volume of settled sediment. The two indices are related by the ratio of the volume of percent solids in the settled sediment to that of the slurry. The volume percent in the settled sediment or packing efficiency is taken to be the ratio of the bulk density of the dried sediment to the density of the solid. Thus

Ts ) TFb/CFs

(7)

The bulk density of the sediment is about 1450 kg/m3. For the 35% suspensions considered in this paper, Ts ) 1.56 T. The energy required for transport will be minimized if the velocity of transport is just that required to prevent the formation of a fixed bed of sediment. Because of its economic importance, the literature on this critical velocity is quite extensive (20, 21). One of the better known empirical expressions is the one proposed by Durand (22):

TABLE 2. Relative Hydraulic Gradients for 165-µm Sand Slurries at 20 °C Relative to Hypothetical Gradients for Homogeneous Suspensionsa 10

19

27

34

51.5 263 495

1.15 1.12 1.09

1.29 1.12 1.10

1.36 1.14 1.10

1.16

a

Vc ) F[2gD(S - 1)]1/2

(9)

The Wilson-Judge expression for F was obtained using the drag coefficient for dilute suspensions, so F is not a function of concentration. Expressions for the computation of the drag coefficient, CD, are given by Shook and Roco (25). The temperature dependence of Vc is captured through the viscosity in the expression for CD. Thomas (26) has presented a theory for a lower limit to the critical velocity for particles fine enough to reside within the viscous sublayer. This layer includes a residual laminarflow layer and a layer in which the transition to fully turbulent flow occurs. The laminar layer exists because velocity must be zero at the pipe surface. In the laminar-flow region, no turbulence exists to maintain the particles in suspension. At the Thomas critical velocity, the particles form a moving bed from which they redistribute into the turbulent flow region. The thickness of the viscous sublayer as given by the intersection of the expressions for laminar and turbulent flow is given by

δ ) 31ν/f 1/2/V

(10)

A paper by Laufer (27) shows that turbulent energy is at a maximum at the outer boundary of the viscous sublayer and that it drops to zero at about δ/5. When due account is taken of different definitions of δ, Thomas’ work demonstrates that the diameter below which the critical velocity becomes diameter-independent is about δ/8. His expression for critical velocity may be expressed as

Vc ) 1.1(8/f )1/2(gν(S - 1))1/3

(11)

The relevance of the work of Thomas to the considerations of this paper will be discussed after the implications of the Wilson-Judge equation have been presented. The Wilson-Judge critical velocities were computed from eqs 8 and 9 for 50, 55, and 62.5 µm particles and found to be 0.40, 0.60, and 0.87 m/s at 20 °C in 0.5-m pipe. The corresponding hydraulic gradients and values of Ts were estimated for 35% suspensions from the experimental hydraulic gradients of Roco and Shook for 165-µm sand. This was done by consideration of the variation with pipe diameter and concentration normalized by ih, the hydraulic gradient for the hypothetical homogeneous suspensions. From the equations given above

ih ) (1 + C(S - 1)fV 2/(2gD)

Experimental hydraulic gradients are from ref 15.

(8)

F was originally proposed to be a function of particle size and concentration. Work in progress by Shook and coworkers (23) supports the use of the following expression of Wilson and Judge (24) for F for particle diameters in the range between 50 and 150 µm:

F ) 2.0 + 0.3 log10 [d/(DCD)]

vol % concn

pipe diameter (mm)

(12)

The ratios, given in Table 2, were computed taking account of the variation of the friction factor with concentration. The

TABLE 3. Transport Indices (Ts) for 35% Suspensions at 20 °C in 0.5-m Pipe particle critical hydraulic transport index diameter (µm) velocity (m/s) gradient (m/km) (kWh acre-ft-1 mi-1) 50 55 62.5 165

0.40 0.60 0.87 3.16

0.54 1.11 2.15 22.3

4.54 9.34 18.1 187

data taken from Roco and Shook’s paper (15) were for flow velocities near the critical velocity. The ratios for the 495mm pipe are essentially constant at 1.10. Little data are in the literature to ascertain how the ratio varies with particle diameter. Roco and Shook (28) give hydraulic gradients of 0.12 and 0.18 for 0.52-mm sand near the critical velocity of 1.9 m/s in 50.7-mm pipe at concentrations of 12.2 and 24.8%. The corresponding ratios of i to ih are 1.5 and 1.6, substantially above those for the 165-µm sand. The diameter of the larger sand is in the range for which transport is in part by saltation rather than full suspension. The 165-µm sand is in the range for heterogeneous suspension. The evidence suggests that the ratios for the 50, 55, and 62.5 µm particles should be no larger than 1.1. This value has been assumed. The critical velocities, hydraulic gradients, and transport indices for 35% suspensions at 20 °C are given in Table 3. The transport indices have been computed for suspensions of particles having a narrow particle size distribution. The river sediments are composed of a broad spectrum of particle diameters. When this is the case, the suspension of smaller particles provides a medium of greater density for the suspension of the larger particles. This effect reduces the critical velocity. The complex slurry can be characterized by a single, effective particle diameter smaller than the diameter of the largest particles. Newitt and co-workers (29) have investigated this effect by studying the carrying capacity of mixtures of different sands having narrow particle size distributions. Effects of the order of 10% were observed. Absent more definitive information, it has been assumed that truncation of the particle size distribution for a typical Missouri River sediment at 62.5 µm will lead to a sediment component that can be characterized by an effective particle diameter of 55 µm. As noted earlier, the implications of the work of Thomas must be considered. He obtained experimental critical velocities at 30 °C for 17- and 26-µm sand for concentrations up to 30% by volume. Thomas used the viscosity of water in eq 11, but better predictions of Vc were obtained if slurry viscosities are used. Using the latter, eq 11 was accurate to within 1% for C ) 24%. However, the computed value was 23% low for the 30% slurry of 26-µm sand. Thus, eq 11 cannot be used to computer Vc for a 35% slurry. A computation for a 25% slurry will suffice. With C ) 0.25 for 0.5-m pipe at 20 °C, eq 11 yields a velocity of 0.78 m/s. The corresponding Wilson-Judge velocity is 0.60 m/s. As suggested by Thomas, the two velocities were combined by taking the square root VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Transport and Gravitational Potential Energies of Silt and Clay as 35% Slurries at 20 °C from Each Reservoir to Gavins Point Dama pipeline (GW/h)

gravitational (GW/h)

Lewis And Clark Francis Case Sharp Oahe Sakakawea Fort Peck

0.65 26.6 8.5 82.8 151.4 128.5

0.3 9.0 2.6 20.5 37.3 34.9

total

398

% hydroelectric energy

4.0

105 1.0

a

In the interest of providing overestimates, the sediments have been assumed to consist only of silt and clay.

for the efficiency of the pumps used to create the required hydraulic head. Efficiencies of over 90% can be achieved for larger water pumps, and efficiencies are not decreased for slurries of silica particles less than about 60 µm (30). The scale of the pipeline transport system is small with respect to the total flow of the Missouri River. Assuming the full 92 500 acre-ft of sediment is transported as a 35% slurry, 0.6% of the total flow would be diverted. Still, it will be a very large system at its lower end. Fourteen 1-m pipes would be required. Although the use of 1-m pipes will probably be justified, the analysis was done using 0.5-m pipes because of the danger of extrapolating the Wilson-Judge equation beyond the range of its empirical definition. The equation predicts a lower critical velocity and thus a lower transport energy for 1-m pipe.

Rail and Other Transport of the sum of their squares to obtain 0.98 m/s. For a 35% suspension, the velocity would be substantially higher. Increasing the velocity to prevent the sedimentation of fine particles is inappropriate if a cheaper alternative can be found. Resuspension is a viable option. The energy required for resuspension is quite small and fine particles settle slowly. All that is needed is to raise the settled fine particles by an amount somewhat less than δ. The value of δ for a 35% suspension moving at a velocity of 0.60 m/s in 0.5-m pipe is 1.0 mm at 20 °C. Mechanical devices that could move along the pipe and draw their energy from the flow would be one alternative. This alternative is deemed sufficiently realistic to justify basing the estimate of the energy required for transport on the Wilson-Judge equation alone. The transport index for 55-µm particles in a 35% suspension at 10 °C is only 3.3 kWh acre-ft-1 mi-1. In the interest of providing a conservative estimate, the value obtained of 9.34 kWh acre-ft-1 mi-1 for 20 °C has been used. Dredging will be inhibited during the winter, and summer temperatures are near 20 °C due to reservoir turnover. Use of eq 9 for 55-µm particles is probably pressing its limits since it predicts a critical velocity at 10 °C of 0 for 47-µm particles (41 µm at 20 °C). Some recognition of the likelihood of non-Newtonian behavior is also in order. For reasons both economic and ecological, the river will be expected to transport its own sediment in the historical fashion between Fort Peck Dam and Lake Sakakawea. The computations also do not include pipeline transport for the 87-mi reach between Garrison Dam and Lake Oahe or the 44-mi reach between Fort Randall Dam and Lewis and Clark Lake. Table 4 presents the energies required to transport the sediments of each of the six reservoirs from the head of that reservoir to the Missouri River channel below Gavins Point Dam. These energies assume that the reservoir sediments consist solely of silica particles no larger than 62.5 µm and that the critical velocity for the slurry is that for 55-µm particles. Since the energy requirements are reduced by the gravitational head, the gravitational potential energies for the 35% suspensions have been computed from the average gross head at each dam. Elevation changes in the open reaches are not included because return of the sediments to the river is assumed for these reaches or the reaches are so short as to make their contribution negligible. The average annual hydroelectric energy generated at the six mainstem dams of 9.9 tWh provides a useful point of reference for the energy requirements for transport. For the presumed 55-µm effective particle diameter, the energy requirement is 3.0% of this total. Normalized by the system energy generation, the energy requirements for the 50- and 62.5-µm particles are 0.95 and 6.8%. The three percentages are for the net energy requirements with the gravitational potential energy subtracted. No correction has been made 3734

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A train transport index was obtained using ComputaTrain, a software program produced by the General Motors Locomotive Group (31). The computation was done for a 3000 traction horsepower SD40-2 locomotive with open-top, 34-ton, hopper cars that can carry 100 ton of sand. Beginning with the assumption of a speed of 20 mph, it was found that a single locomotive could pull a train of 25 loaded cars at 18.0 mph while requiring a traction horsepower of 2983. Return of the train at 32.0 mph required 2948 thp. A 1% average grade and a track curvature of 3° were assumed for the round trip. The net energy requirement was found to be 0.103 hph/ton-mi or 263 BTU/ton-mi. In the units used for the pipeline transport, the transport index is 152 kWh acre-ft-1 mi-1. This index is 20% smaller than the transport index for the pipeline transport of 165-µm sand but is larger than that for the transport of silt and clay by a factor of 16. The operating and capital costs of barge transport have not been investigated here, but they would need to be investigated as a part of a more comprehensive feasibility study. The amount of sand and gravel to be transported depends on the percentages of these larger materials within the reservoir sediments. No comprehensive analysis of the particle size distributions of the reservoir sediments has been made. However, estimates for five of the six reservoirs that may be good to within a few percent were derived from data of the U.S. Geological Survey (USGS) (12). Steven Sando of the USGS in Huron, SD, has modeled the sediment composition and flow from the principal tributaries of the three lower reservoirs. The Niobrara River provides Lewis and Clark Lake with sediment that is 77% sand and gravel. These sediments are probably best barged the 25 mi to Gavins Point along with the sediments from the main channel. The sediments of the principal tributaries to Lake Francis Case and Lake Sharp are shown by Sando to be about 1% and 0.5% sand and gravel, respectively. These small amounts are probably best stored locally. The USGS data were not adequate to provide an estimate for Lake Oahe. Data were available from the Bismark office of the USGS in North Dakota and the Helena office in Montana to give estimates for Lake Sakakawea and Fort Peck Lake of 26% and 14% sand and gravel, respectively. The procedures used to obtain these estimates are briefly discussed in the Supporting Information. The amount of sand and gravel in these two reservoirs and the train transport index of 152 kWh acre-ft-1 mi-1 imply a cost for transporting these sediments to Gavins Point Dam that is prohibitive. The best option appears to be the use of the sand and gravel as underlayment for building prime farmland.

Future Research This paper has presented evidence that conversion of the Pick-Sloan system to a sustainable system by a return of

sediments to the channel below Gavins Point Dam is worthy of serious consideration. Unless non-Newtonian effects are a major factor, the estimate for the cost of transport for the silt and clay is probably accurate to within a factor of 2. However, much research is needed to provide the data for a full assessment. This includes basic hydraulic research on the transport behavior of fine particle suspensions consisting of both sharp and broad particle distributions. The composition of the sediment coming into each reservoir needs much better definition. Economic uses of the sand and gravel and the possibilities for local storage must be investigated as does the possibility of using the clay and silt with a sand underlayment for building prime farmland. Major environmental questions are raised by the return of the sediments to the channel. Although there may be little need to restore the system to its original capacity, some sediments, such as those causing the flooding in Pierre and Fort Pierre, must be moved. In other cases, dredging may serve the cause of environmental tailoring. For example, channels could be dredged into areas currently blocked by sediment. Also, sand islands could be created to provide habitat for the endangered least tern and to allow the natural habitat progression from bare sandbars through cottonwood stands to climax vegetation. The overall loss in sandbar habitat between 1892 and 1982 has been 97% (32). Substantial political and legal questions will be raised. Not all fishermen will be pleased with the change in fish populations that will occur. However, most fishermen would be pleased if fish ladders could be constructed in conjunction with the siphon systems for transporting the slurries over the dams. Once sufficient research has been done, research at the pilot plant level will be required. Since the impact of returning the sediments to the river is an important element of the research, Lewis and Clark Lake is the logical site for the pilot plant project. The amount of sediment transported by the pilot project must be adequate to give suitable insight into the engineering, environmental, and economic requirements for converting the Pick-Sloan system to a sustainable system. The primary engineering advantage for the use of Lewis and Clark Lake as a site derives from the relatively short, 25-mi, transport distance. Because the pilot plant would be highly visible to the public, its operations will be informative with respect to legal and political impact as well. Bringing down fine silt and clay from the White River by pipeline as a part of the pilot plant study is worthy of consideration if smaller scale studies show that these finer sediments will allow the pipeline transport of a major portion of the sand from the Niobrara River. Another benefit is that the White River sediments would contribute opacity to the Missouri River and would enable game fish to forage in much shallower waters.

Acknowledgments R. E. Schiller, Jr., Professor Emeritus of Ocean and Civil Engineering, Texas A&M University, provided valuable assistance during the initial stages of the research for this paper. The ComputaTrain software, used to obtain the energy requirements for the transport of sand and gravel, was provided through the courtesy of R. E. Wright of the General Motors Locomotive group, East London, Ontario. The advice of C. A. Shook, Professor Emeritus of Chemical Engineering, University of Saskatchewan, was of critical importance. Steven Sando of the United States Geological Survey in Huron, SD, was especially helpful in providing both data and advice on sediment composition. Carolyn Fuller, Vice President, Van Scoyoc Associates, Inc., Washington, DC, made the suggestion that started my reflections on the subject of this paper.

Supporting Information Available A list of the definition of symbols, one table, various computations, and a description of the procedures used to obtain the composition of the sediments of Lake Sakakawea and Fort Peck Lake (8 pages). This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review May 10, 1999. Revised manuscript received May 8, 2000. Accepted June 2, 2000. ES990536O