Morris & Fan. Reservoir Sedimentation Handbook

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SEDIMENT ROUTING 13.16

FIGURE 13.9 Sediment bypass configuration at Nagle Reservoir in South Africa (afte

nnandale, 198

)

instance the forebay receives runoff from the main stream and from an intake on another

stream. Water enters the storage afterbay only after it has passed through the forebay area,

which acts as a sedimentation basin. Sediment deposited in the forebay is removed by

intermittent flushing.

13.7 OFF-STREAM RESERVOIRS

13.7.1 Technique

An off-stream reservoir is any impoundment constructed off the main river and which is

filled with diverted water of low sediment concentration. Several methods may be used to

reduce the sediment load on the offstream reservoir: (1) sediment-laden discharge from

large floods can be partially or entirely excluded from the reservoir by limiting the

hydraulic capacity of the intake structure or selective operation of the intake gate, (2) the

intake structure can be designed to exclude coarse sediments, and (3) the diversion dam

can be operated as a sediment trap for diverted water, with trapped sediment being

subsequently removed by flushing. An off-stream reservoir can be constructed as a diked

impoundment, using a conventional dam across a small tributary with low sediment yield,

or on the floodplain paralleling the main channel. These strategies are schematically

illustrated in Fig. 13.11. Tributary arms of existing reservoirs may also receive relatively

little sediment compared to the main river branch, thereby acting as off-channel storage

pools within existing impoundments.

Since inflow can be controlled at the diversion point, an off-stream reservoir does not

require a large and costly spillway structure. The reduced sediment loading can largely

eliminate the need to provide a sediment storage pool and reduces future maintenance

costs (e.g., dredging), further reducing costs compared to a conventional reservoir.

Because it never receives high sediment loads, the off-stream reservoir can also provide

consistently high quality water to quality-sensitive users such as municipal water supplies.

SEDIMENT ROUTING 13.17

FIGURE 13.10 Proposed Nan-Hwa reservoir in Taiwan, showing the compartment of the reservoi

into an upstream forebay for sediment tra

ping and diversion, and an afterbay for sediment storage

(after Hwang, undated).

f-stream reservoirs can also provide important environmental benefits compared to

conventional structures. Depending on the design, they may not require a large instream

barrier at the intake site, and the downstream transport of bed material can be maintained.

13.7.2 Application

Of the 33 major reservoirs in Taiwan, 9 are offstream structures designed to bypass

sediment. Data on two of these structures were summarized by Wu (1991), who reported

SEDIMENT ROUTING 13.18

FIGURE 13.11 Alternative configurations for off-stream reservoirs. Limited-capacity intake

diverting flow into: (a) an excavated or diked impoundment, or (b) an impoundment located on a

small tributary with a low sediment load. (c) Use of a barrage to control flow over a diversion wei

and into an off-stream impoundment, as suggested by Urlapov (1977).

hat at Sun Moon Reservoir, 49.5 percent of total streamflow was diverted into the

reservoir but only 3.5 percent of the stream sediment was diverted. At Coral Lake

Reservoir 29.6 percent of the streamflow was diverted but only 12.2 percent of the

sediment. The large difference in sediment diversion at these two structures was attributed

to the configuration of the diversion dam. The diversion dam at the Sun Moon Reservoir

had a higher sediment trap efficiency, and was thus more efficient in preventing sediment

delivery to the reservoir.

13.7.3 Computation of Reservoir Yield and Sediment Exclusion

Whereas yield from an on-stream reservoir can be expressed as a function of storage

capacity alone by a storage-yield curve, in the off-stream configuration reservoir yield is

also affected by the limited hydraulic capacity of the stream diversion and conveyance

channel or pipeline between the intake and the reservoir. A larger diversion capacity will

allow the off-stream reservoir to refill more rapidly, which can be important on flashy

streams. However, the increased intake capacity comes at the cost of higher sediment

loading because a larger percentage of total diversions will consist of sediment-laden flood

flows. Flow into the off-stream impoundment will always be limited to the lesser of

streamflow or the diversion capacity. In the sediment bypass system, this inflow constraint

does not exist because the reservoir impounds the original stream channel and inflow rate

is not restricted.

To minimize sediment loading, the off-stream reservoir should be designed to receive

no additional inflow when it is full. Sediment inflow during floods is thus limited by both

the restricted intake capacity and the higher probability of the reservoir being full during

flood periods. For further reduction in sediment inflow, a gate could be operated at the

stream diversion to manually or automatically select water for diversion or exclusion on

the basis of reservoir level, stream turbidity, and possibly other criteria.

A performance comparison of a conventional on-stream impounding reservoir and an

off-stream reservoir was made using 21 years of daily discharge data from the USGS gage

at Rio Fajardo, Puerto Rico (Morris, 1996). This small (38.6 km

) moist watershed

receives about 2500 mm/yr of rainfall which is rather well distributed throughout the year,

although streamflow is flashy. Sediment yield is 814 t/km

/yr, and a rating curve of

sediment concentration of the form C

mg/L

= aQ

was established for the four available years

SEDIMENT ROUTING 13.19

of mean daily discharge and load data published by the USGS. A daily simulation analysis

was run to determine the yield available on 99 percent of the days in the 21-year dataset,

and to compute sediment accumulation in the reservoir from the average daily suspended

sediment concentration, the volume of water diverted each day, and assuming 100 percent

trap efficiency in the off-stream reservoir.

The simulated system consisted of a stream diversion having various capacities,

combined with various storage volumes in the off-stream storage compartment (Fig.

13.11a). Simulation results are summarized in Table 13.1, expressing both storage and

yield values as the dimensionless fraction of the average daily flow (ADF) over the

simulation period. By comparison, the 99 percent run-of-river flow exceedance at this

gage station for the analyzed dataset is 0.057 x ADF. Sediment inflow is expressed as

percent of total sediment load. For reservoirs on this stream with storage volumes up to 90

days of mean flow (25 percent of mean annual runoff), there was essentially no difference

in yield between a conventional reservoir and an off-stream reservoir with an inlet rated at

3 x ADF. Use of a smaller inlet capacity rated at 1 x ADF produced a small decrease in

yield and a larger decrease in sediment inflow as compared to the larger inlet. The smallest

inlet, rated at 0.5 x ADF, had a significant impact on yield for larger storage volumes but

not for smaller. By comparison, a 100-year sediment pool for a conventional reservoir at

this site with a 90 percent trap efficiency will require 5.6 Mm

of storage volume,

equivalent to 33 days of ADF.

Depending on the configuration, sediment inflow into this off-stream reservoir is 80 to

98 percent less than a conventional (onstream) reservoir at the same location. For a given

yield, the off-stream reservoir can be much smaller than the conventional reservoir

because the sediment pool is practically eliminated. While sediment accumulation is not

entirely eliminated from the off-stream reservoir, the rate of accumulation is reduced to

such low levels that activities such as dredging become economically attractive. This

illustrates that in some streams a dramatic reduction in sediment loading can be achieved

using off-stream storage, without a significant penalty in terms of yield.

TABLE 13.1

Yield and Sediment Load for Conventional and Off-Stream Reservoir, Río

Fajardo, Puerto Rico

Reservoir Configuration

Reservoir

storage

volume,

days at

ADF

yield,

ADF

Yield,

ADF

Load,

Yield,

ADF

Load,

Yield,

ADF

Load,

5 0.145 0.145 2.1 0.145 2.1 0.140 2.1

10 0.220 0.220 3.5 0.220 3.5 0.220 3.5

15 0.285 0.285 5.2 0.285 4.9 0.275 4.6

30 0.425 0.425 10.2 0.385 7.7 0.340 5.9

60 0.545 0.545 16.3 0.500 11.1 0.380 6.7

90 0.610 0.605 20.5 0.530 12.0 0.390 6.8

Notes: ADF = Average daily flow = 1.93 m

/s. Storage volume is expressed as days of discharge at ADF,

where 1 day ADF = 166,700 m

Load = percent of total river suspended sediment load trapped over the simulation period.

A = conventional impounding reservoir, sediment inflow is 100% of river load.

B = off-stream reservoir, inlet capacity = 3.0 x ADF.

C = off-stream reservoir, inlet capacity = 1.0 x ADF.

D = off-stream reservoir, inlet capacity = 0.5 x ADF.

SEDIMENT ROUTING 13.20

13.8 SUBSURFACE RESERVOIR

Deposits of coarse sediment contain voids which can store water, and under favorable

conditions a trap dam may be constructed to accumulate coarse sediment, extracting water

from the subsurface storage by a pipe through the base of the dam and which extends

through the permeable deposits (Fig. 13.12). Use of these systems as for small local water

FIGURE 13.12 Schematic configuration of sediment routing by trap dam and subsurface reservoir.

supply system

s in Africa has been described by Baurne (1984), who also states that

evidence from Libya suggests these types of structures have been used for at least 1000

years. Important advantages include freedom from sedimentation, water quality

improvement for drinking supply as a result of filtration through porous media, and

elimination of evaporative losses once the water level is more than about 0.6 m below the

surface.

The volume of water that can be stored can be estimated from the impoundment

geometry and measurement of the porosity in the local bed material, which may be as high

as 30 percent. Because the water level cannot be visually observed, a standpipe and float,

transparent plastic tube, or pressure gage may be placed at the outlet to monitor the static

water surface within the media. Collection of water at the outlet should be facilitated by

placing a perforated pipe or gravel drain extending into the deposits from the outlet pipe.

Construction of a trap dam to create a subsurface reservoir requires that the stream

carry a significant load of coarse bed material and have an impermeable foundation.

Because only a fraction of the total impoundment volume is occupied by water, a "minor"

seepage loss may represent a significant fraction of the total supply from a subsurface

reservoir. Watertightness is essential. The dam should not be raised faster than the

impoundment can be filled with coarse material, and it may be necessary to raise the dam

in annual increments to match the rate of coarse sediment accumulation. This may be

facilitated by constructing the foundation and abutment elements during the first

construction season, leaving a wide weir to be elevated incrementally during subsequent

years.

Although subsurface storage reservoirs have been reported to date only for smaller

village-size water supplies, they may represent a means to obtain a limited water supply

from sites that are otherwise hopelessly sedimented. For example, reconstruction of

Gibraltar Dam in California to meet dam safety standards occurred during a severe 5-year

drought. There was no source of water for preparing the concrete mix; streams in the area

SEDIMENT ROUTING 13.21

FIGURE 13.13 Well drilled into empty Gibraltar Reservoir, California, during drought. The

photograph also shows dredging equipment left stranded by the drought (G. Morris).

were dry as was the heavily sedimented reservoir. The required water supply was obtained

from wells drilled through the silty reservoir deposits and into the permeable gravels in the

original river bed (Fig. 13.13).

13.9 SEDIMENT EXCLUSION

A sediment excluder is a device or system designed to route sediment along the river,

bypassing the intake. These intakes are an important feature at many water supply intakes

and hydropower diversion dams, and may also be used to supply offstream reservoirs. Use

of an intake configuration which minimizes sediment entrainment is an important

component of design. Sediment exclusion is discussed in more detail by the Committee on

Hydropower Intakes (1995), Raudkivi (1993), and Bouvard (1992). Complex three-

dimensional flow patterns occur in the vicinity of intakes, and physical modeling is

frequently employed in the design of large intake structures.

The distribution of coarse suspended sediment within moving water is not uniform but

tends to decrease exponentially as a function of distance above the bed. Sediment

excluders are designed to take advantage of this vertical variation in sediment

concentration to consistently withdraw water from zones within the flow field

characterized by relatively lower sediment content. The greater the variation in sediment

concentration within the water column, the more effectively an excluder will operate.

Exclusion will not be effective on slowly settling fines that are almost uniformly

distributed across the flow field. Zones of low sediment concentration may exist naturally

in the river or may be created by intake geometry. Suspended sediment exclusion can be

achieved by employing either flow curvature or flow partitioning to reduce suspended

sediment concentration in the intake.

SEDIMENT ROUTING 13.22

13.9.1 Sediment Exclusion by Flow Partitioning

If water level can be maintained at a nearly constant elevation, upstream of a barrage for

example, the flow can be partitioned vertically. For example, vertical partitioning may be

accomplished by a two-level intake across a flow path and

perpendicular to the direction

of flow, at the end of a straight reach. The upper-level intake is used to divert clear water

and the sediment-laden bottom water is discharged downstream through a low-level outlet,

as schematically illustrated in Fig. 13.14. Similarly, a submerged weir running parallel to

the direction of flow will allow surface water to overflow toward the intake while the

deeper water and bed load continues downstream and bypasses the intake.

FIGURE 13.14 Sediment exclusion by vertical flow partitioning. Surface water with

lower sediment content is diverted to use while sediment-laden bottom water is discharged

back to the stream.

13.9.2 Sediment Exclusion by Flow Curvature

When flowing water is diverted at a curve, the water surface will become elevated at the

exterior of the curve because of momentum forces. Water at the surface flows faster than

water at the bottom, and

the greater momentum of the surface water carries it into the

exterior of the curve and forces the slower-moving bottom water out of the curve, creating

a transverse current. When this transverse current is superimposed on the downstream

direction of flow, the resulting current is helicoidal. As a result of this flow pattern, an

intake located at the outside of this curve will be continuously fed with surface water

while bottom water with higher sediment concentration is pushed out of the bottom of the

curve. This principle was employed 2000 years ago in constructing the Dujiangyan

irrigation diversion from the Min River in China.

The basic principle of flow curvature can be employed to construct a variety of intake

configurations, as illustrated in Fig. 13.15. Submerged vanes (e.g., Iowa vanes) can also be

used to produce flow curvature and otherwise modify sedimentation patterns in the

vicinity of intakes and can also modify current patterns to deflect erosive currents

and help

stabilize the river in the vicinity of structures (Odgaard and Wang, 1991).

13.9.3 Gravel Sluice

Gravel is not normally transported in suspension, but its gradual accumulation upstream of

a low dam which diverts water into an intake will eventually block the intake. This block-

SEDIMENT ROUTING 13.23

FIGURE 13.15 Sediment-excluding intake configurations based on flow curvature: (a) helicoidal

flow at a river bend, (b) flow convergence, and (c) pier-type frontal intake.

age can be prevented by periodically flushing gravel from the immediate vicinity of the

intake using a sluicing system such as that illustrated in Fig. 13.16. In this illustration, the

low dam has a vertical sluice controlling flow along a sluice channel that passes

immediately in front of the intake. Opening the sluice removes bed material from the

vicinity of the intake, but no attempt is made to halt or otherwise manage the accumulation

of bed material behind the dam.

13.9.4 Intakes at Large Dams

Intake structures at dams should be designed to minimize problems of sediment

entrainment and eventual blockage by deposits. In some reservoirs, sediment can affect

low-level intakes soon after the dam is closed because of turbidity currents which reach

the dam. As the reservoir fills with sediment, delta deposits will eventually reach the dam,

SEDIMENT ROUTING 13.24

and coarse sediments can be transported to the vicinity of the intake as bed load. Several

strategies may be used to reduce sediment entrainment into intakes at the dam. Low-level

outlets may be established to facilitate hydraulic scour of sediment from the vicinity of the

intake, employing a concept similar to that illustrated in Fig. 15.18. Deep sluices can be

placed immediately below or adjacent to power intakes at the dam, and can be operated on

either a continuous or intermittent basis to reduce sediment entrainment into the intake, as

at Cachi Reservoir (Fig. 19.6).

13.9.5 Sediment Removal from Diverted Water

When sediment exclusion does not eliminate all the grains below the maximum allowable

size, exclusion may be complemented by systems to remove sediment from diverted water.

These systems are generally sized on the basis of the plain sedimentation of discrete

particles, but may also involve circular flow in which centrifugal force also aids in the

removal of sediment. Settling sediments are typically focused into a narrow slot or series

of orifices along the bottom of a sedimentation tank, then removed by either continuous or

intermittent discharge. A conceptual drawing of a rectangular sedimentation tank is

illustrated in Fig. 13.17.

13.10 COMPARTMENTED AND SERIAL RESERVOIRS

13.10.1 Reservoir Compartmentation

Operational flexibility for sediment management can be provided by dividing the total

storage volume into two components by using internal dikes or multiple reservoirs. For

example, use of a single seasonally empty reservoir may be infeasible because some

regulating storage is required during the nominally wet season. As an alternative, it may be

feasible to construct a larger seasonal-use on-stream impoundment which will receive a

heavy sediment load and a smaller offstream impoundment. During the wet season, only

the off-stream reservoir would be used while the on-stream reservoir remains empty for

sediment routing. Both would be available for dry season service. The off-stream

impoundment may consist of an arm of the reservoir that receives limited sediment input.

13.10.2 Reservoirs in Series

When more than one reservoir is constructed along a river, sediment released from an

upstream impoundment will be redeposited downstream unless all sites are managed

together. Sediments should be routed progressively through all reservoirs in series to

minimize the amount of redeposition in downstream impoundments. The reservoir pool

elevation at the downstream site affects the location of deposits, and one operational

objective for multiple reservoirs in series may be to focus deposition into areas which

facilitate subsequent removal. At some sites, such as small mountain hydropower

diversions upstream of a hydrologically large storage reservoir, it may not be feasible to

manipulate the downstream site and the released sediment may simply accumulate in the

downstream pool. This is the case for the reservoirs along the Feather River discussed in

Chap. 22; all the sediment routed through the small hydropower dams simply accumulates

in the large Oroville Reservoir further downstream.

SEDIMENT ROUTING 13.25

FIGURE 13.17 Schematic diagram of rectangular settling tank or desander.

13.11 CLOSURE

A wide variety of routing strategies are available to achieve a sediment balance or to

minimize the rate of sediment accumulation. Routing strategies represent the least

environmentally disruptive method for maintaining long-term storage capacity in

reservoirs while minimizing the environmental impacts on fluvial systems. However,

environmental impacts cannot be entirely eliminated and may continue to be significant,

especially at in-stream reservoirs.

Morris &amp; Fan. Reservoir Sedimentation Handbook

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Morris & Fan. Reservoir Sedimentation Handbook