Morris & Fan. Reservoir Sedimentation Handbook

Подождите немного. Документ загружается.

FLUSHING 15.14

and flip buckets, could result in catastrophic erosion and spillway disintegration.

 Passing sand through power and irrigation tunnels would create severe erosion,

requiring a continuous and extensive maintenance program.

 Sand discharged immediately downstream of the dam would tend to accumulate in

the channel, elevating the tailwater and reducing power production. A continuous

large-scale dredging program may be required.

 A bedrock acceleration of 0. 12g (equivalent to a magnitude 5 earthquake at Tarbela)

can liquefy loose saturated deposits similar to those on the foreslope of the Tarbela

delta. The final slope at rest after liquefaction failure would be about s = 0.005. If the

toe of the delta is within 8 km of the dam, the liquefied sediments could reach, enter,

and clog the intakes, putting the irrigation and electrical generation system out of

service for 6 to 12 months.

The inflowing sediment consists of 59 percent fine sand, 34 percent silt, and 7 percent

clay. Approximately 99 percent of the inflowing load is trapped and accumulates,

primarily in the form of a delta deposit which is advancing toward the dam (Fig. 15.9).

Delta topset beds have a slope of about s = 0.0006 to 0.0008. Most sediments are deposit-

FIGURE 15.9 Advancement of delta deposits toward Tarbela Dam, Indus River, Pakista

(after Lowe and Fox, 1995).

ed on the topset bed as the reservoir fills and levels rise during the wet season, but when

the reservoir is drawn down for irrigation deliveries, the river reworks and transports

these deposits downstream, extending the delta toward the dam. Most sediment is

transported to the face of the delta at the onset of the wet season when the pool level is

still low and discharge is increasing from 1500 to 4500 m

/s.

Several sediment management strategies have been considered. Erosion control in the

watershed, which drains the tectonically active Himalayas, is not technically feasible.

Construction of an upstream dam could temporarily trap sediment, but no such project is

ready for construction. Dredging has been discarded because of the extraordinary volume

of sediment involved, about 150 Mm

/yr. A normally-submerged rock embankment or

dike constructed upstream of the intakes, and submerged at normal pool level, can tem-

porarily prevent encroachment of the toe of the delta into the intakes. By gradually rais-

ing the minimum pool elevation, all delta deposition can temporarily be focused into

FLUSHING 15.15

areas more than 8 km above the dam. These last two measures, used together, can delay

serious sediment problems by as much as 40 years. Nevertheless, continued long-term

operation of the project can be realized only by developing a system to pass sediments

beyond the dam.

Under the proposed long-term sediment management strategy, coarse sediment would

be trapped in the upper part of the pool during impounding periods. The deposited

material would be flushed through a high-level bypass channel around the dam at the

beginning of the subsequent flood season by lowering the pool following irrigation

season deliveries. Because this strategy involves temporary sediment storage and

remobilization, it is classified as a flushing technique rather than sediment routing. The

invert elevation of the flushing channel would be set more than 80 m above the original

streambed elevation, and also above the level of the hydropower intakes. This high invert

elevation reflects cost and technical considerations associated with construction of the

large-capacity (4500 m

/s) bypass channel. A normally submerged dike would be

constructed around the power intakes, thereby allowing delta deposits to approach the

vicinity of the dam and enter the high-level flushing channel without entering the intakes.

The concrete-lined trapezoidal flushing channel would have a 60 m bottom width and 15

m depth, with its upstream invert about 20 m below the dike crest. This layout is

conceptually presented in Fig. 15.10, which shows the normally submerged semicircular

dike which protects the intakes from bed material and also diverts water and sediment

into the bypass channel when the reservoir is drawn down for flushing. The channel

would discharge sediment during a 20- to 30-day period of low reservoir levels at the

beginning of each wet season, and sediment concentration in the diverted water is

expected to run between 1 and 5 g/L based on field measurements of sediment

concentration in the river as it flowed across the delta deposits. Reservoir volume would

be stabilized at a size significantly smaller than the original capacity. All power pro-

duction would necessarily cease during the flushing period. Because Tarbela is a major

source of electrical energy in Pakistan, an alternative power supply is required to sustain

electrical service during flushing periods.

15.5 EROSION PROCESSES DURING FLUSHING

Flushing releases are characterized by both extreme and highly variable sediment con-

centrations. Channel erosion and slope failures within reservoir deposits dominate during

the flushing process, and sheet erosion of deposited floodplain sediment is insignificant.

15.5.1 Slumping at the Dam

When a bottom outlet is first opened to initiate flushing, and if poorly consolidated fine

sediment have accumulated above the outlet, slope failure can be initiated in the vicinity

of the outlet with slumping and plastic flow of the deposits. This slumping is illustrated in

the photograph at the small Santa María Dam in Guatemala (Fig. 15.11). A similar

pattern has been observed by the first author at small reservoirs in Puerto Rico. At

Hengshan Dam in China, mud on the floodplain surface within 350 m of the dam slid

gradually into the channel and was released through the bottom outlet over a period of

several days (Fan, 1985). At the 20-MW Mangaho River project in New Zealand, 59

percent of the storage capacity had been lost after 45 years and the bottom outlet was

buried under 13 m of silt after 25 years without sluice operation. There was no flow dur-

ing the first day the gate was opened, but on the second day silt began to extrude from the

FLUSHING 15.16

FIGURE 15.10 High-level Sediment bypass system proposed at Tarbela Dam (after Lowe and Fox, 1995). The

underwater dike is used to allow delta deposits to reach the high-level bypass channel, without entering the powe

intakes. Elevations and distances in feet.

FLUSHING 15.17

FIGURE 15.11 Slumping of fine-grained deposits near the dam in the small 285,000 m

Santa

Maria reservoir on Río Samala, Guatemala, as sediment is discharged through bottom outlets.

Sediment slumping was accelerated and removal enhanced by closing gates to temporarily

inundate the deposits, then draining the reservoir a second time (V. Galay).

bottom outlet, emptying the reservoir and leaving a crater-like depression above the

sluice entrance. About 75 percent of the accumulated sediment was flushed during the

subsequent month. During this period it was necessary to haul out logs and tree stumps

that blocked the sluice entrance. Thereafter flushing was undertaken annually (Jowett,

1984).

15.5.2 Slope Failure

The erosion of flushing channels produces unstable banks which will slump or slide into

the channel, and bank failure is the primary mechanism involved in the widening of

flushing channels. The main flushing channel may deepen until it encounters the armored

bed of the preimpoundment river, after which further erosion can occur only by widening

FLUSHING 15.18

of the channel by bank failure. The type of slope failure and the stable angle of repose

will depend on the sediment characteristics. Examples of several forms of bank slides

observed during flushing at Sefid-Rud are illustrated in Figs. 23.19, 23.21, and 23.22.

15.5.3 Retrogressive Erosion

A channel erosion process characterized by a zone of high slope and rapid erosion, mov-

ing upstream along a channel having a lower slope and erosion rate, is termed retro-

gressive erosion. This process is illustrated in Fig. 15.12a, based on observations in a

laboratory flume, and in Fig. 24.14 as observed in Sanmenxia Reservoir. The highest rate

of erosion occurs along the steep drop at the downstream end of the deposit, causing this

area of maximum erosion to move upstream through a headcutting process similar to

gully erosion. The point of slope change or headcut may be visually apparent as shown in

FIGURE 15.12 Characteristics of retrogressive erosion from flume tests. (a)

Longitudinal profile in unconsolidated deposits. (b) Transition toward a unifie

slope over time, corresponding to flume test shown in a. (c) Longitudinal

profile in consolidated sediments.

FLUSHING 15.19

Figs. 23.20 and 25.13. The point of slope change is also called the pivot point or the

nickpoint, and the term nickpoint erosion is also used to describe retrogressive erosion.

Multiple headcuts can be formed along the length of an eroding channel.

Retrogressive erosion is the principal method for the formation of flushing channels

through reservoir deposits. The opening of deep outlets – which establishes flow across

deposits having a relatively mild slope, with an abrupt drop or even a waterfall at the

downstream end – initiates retrogressive erosion, creating a nickpoint that can move

upstream rapidly depending on the nature of the deposits and the erosional forces. At

Sanmenxia Reservoir on the Yellow River, a reach extending 80 km upstream from the

dam was affected by retrogressive erosion between October and November 1964, the

longest such phenomenon recorded in a reservoir (Fig. 24.14). In contrast, retrogressive

erosion lasts less than 24 hours in erodible silt deposits in smaller reservoirs in China

such as the 2-km-long Hengshan Reservoir. In more consolidated deposits, retrogressive

erosion will move upstream more slowly. Erosion moved upstream at the rate of 100

m/day in cohesive sediments at Sefid-Rud Reservoir (Tolouie, 1993).

Retrogressive erosion results from the change in hydraulic energy caused by the

discontinuous longitudinal profile, and it is not dependent on any specific grain size in

the deposit, although erosional patterns are influenced by the deposit characteristics.

Retrogressive erosion can occur in coarse sediments on a river delta (Randle and Lyons,

1995) and also in fine-grained and cohesive sediments. In noncohesive or unconsolidated

cohesive sediments retrogressive erosion tends to proceed upstream as shown in Fig.

15.12a. In consolidated deposits the eroding face tends to be more nearly vertical as in

Figure 15.12c. The creation of a stepped erosion face created by alternating layers of

cohesive and noncohesive sediment was observed at Sefid-Rud Reservoir (Fig. 23.14).

As retrogressive erosion proceeds there is a gradual transition of the foreset and topset

slopes to a unified slope, as shown in Fig. 15.12b. The most intense erosion occurs in the

area of highest slope and the pivot point (or nickpoint) continuously moves upstream,

causing the foreset slope to decrease. At the same time channel erosion causes the topset

slope to increase, until a unified slope is achieved. At this point retrogressive erosion has

ended and the erosion process may now be termed progressive erosion. Jiang (1992)

reports that sediment transport computations based on unit stream power have been used

to predict rates of retrogressive erosion.

15.5.4 Progressive Erosion

The term progressive erosion refers to a channel erosion process which occurs uniformly

or progressively along the length of a channel instead of being concentrated at the down-

stream end. In general, when the suspended-sediment concentration in flowing water is

less than the sediment carrying capacity, the flow will entrain sediment from the channel

bed. When clear water enters a zone of erodible deposits having uniform slope and grain

size, it will progressively entrain sediment by eroding the deposit. The rate of bed erosion

will initially be rapid because of the large available sediment-carrying capacity of clear

water. As the flow progresses downstream and entrains sediment, its capacity to erode

and transport additional sediment will decrease, eventually reaching zero. In this manner

progressive erosion can cause a high rate of bed erosion at the upstream end of a deposit

and less erosion at the downstream end. This erosion pattern can be offset in reservoirs by

the tendency for deposits to be coarser and less erodible at the upstream end.

15.6 VARIATION IN EROSION RATE AND SEDIMENT RELEASE

The rate of channel erosion may be limited by either the erosive hydraulic forces or the

available sediment supply in the channel, and in the riverine environment the discharge of

FLUSHING 15.20

sediment of different grain sizes may be characterized as either transport-limited or

supply-limited. The wash load is typically supply-limited, since the load of fine sediment

is almost always limited by the rate of supply from the watershed rather than hydraulic

transport capacity. Conversely, the load of coarse bed material is transport-limited, since

hydraulic transport capacity rather than sediment abundance is the limiting factor. The

same concept applies to the rate of sediment release during the flushing process and can

be used to explain variations in the concentration and grain size of sediment release

during flushing events.

15.6.1 Within-Event Variability

Flushing events are characterized by large changes in the rate of channel erosion and the

suspended-solids concentration in the flow exiting the dam, as illustrated in Figs. 15.5,

19.18, 21.6, 23.15, and 25.8. These variations primarily reflect changes in the availability

and erodibility of fine sediment during the flushing event.

Channel formation and channel maintenance tend to produce two distinct patterns of

sediment release. Material deposited in previous years or decades is continuously scoured

during channel formation, producing sediment concentrations which are initially high and

decline gradually (Fig. 15.5). The maximum concentration and rate of sediment release is

limited by the erodibility of the deposit, which has become consolidated over time.

Concentration variations will occur over the course of a flushing period because of

changing inflow rate and variations in the rate of sideslope failures along the flushing

channel.

During maintenance, flushing the channel will contain only recently deposited mate-

rials which are readily mobilized, including recently reworked sediment. The transition

from drawdown to riverine flow during a maintenance flushing event is characterized by

a dramatic increase in the sediment concentration discharged from the dam as the

unconsolidated fines in the submerged channel near the dam are rapidly evacuated, gen-

erating short-term sediment concentrations of hundreds of grams per liter. However, as

the readily eroded material is washed out, the concentration of fines becomes supply-

limited, depending on the rate of bank erosion and collapse. In contrast, the discharge of

coarse material (e.g., sands) may be transport-limited throughout the flushing event and

may be much more uniform over time. Thus, during a maintenance flushing event, the

release of fine sediment may be initially transport-limited and subsequently supply-

limited, and will typically exhibit extreme variations in concentration and rate of release.

In contrast, the discharge of coarse bed material (including sediment eroded from delta

deposits) may be transport-limited throughout the event and experience much smaller

variations in concentration and rate of release. As a result, in a reservoir having a

significant load of both fine and coarse sediment, short flushing periods may be effective

in removing fines but longer flushing periods and larger flushing flows will be required to

remove the inflowing load of coarse material.

15.6.2 Between-Event Variability

During channel formation the discharged sediment is derived from the erosion of older

and more consolidated deposits. Sediment concentrations remain high during the entire

flushing event as the flow continuously erodes deposits. In contrast, maintenance flushing

erodes the recently deposited and poorly consolidated fines from the channel, producing a

short period of extreme sediment concentration including hyperconcentrated flow, until

the channel sediment supply is exhausted. These contrasting mechanisms can cause wide

between-event variations in sediment concentration. Peak concentrations in channel

maintenance events at Sefid-Rud were almost double the concentrations during channel

formation events (Table 15.2). Variation in the delivery of fines to the reservoir also

FLUSHING 15.21

causes sediment discharge to be highly variable between events, as illustrated by the high

variability between maintenance flushing events at Cachí Reservoir, also shown in Table

15.2. The high load evacuated in 1988 at Cachí reflects increased sediment input from

flooding and a landslide (Jansson, 1992a). The high variability at Sefid-Rud and Cachí

have occurred even though both sites been operated in a consistent manner over the

period covered by the data, subject to normal operational variations caused by natural

events such as floods. These data illustrate the difficulty of accurately predicting

sediment release during flushing.

TABLE 15.2 Variation in Sediment Discha ge and Concentration During Flushing r

Cachí Sefid-Rud

Year

Peak

Concentration,

g/L

Sediment

Removal

Peak

Concentration

g/L

Sediment

Removal

1981 0.477 170* 24

1982 0.386 180* 12

1983 0.545 145* 52

1984 0.665 195 68

1985 344 142

1986 330 46

1987 292 27

1988 430 1.266 305 57

1989 170 0.482 398 54

1990 280 0.653 284 32

*Channel formation stage. All other events are maintenance flushing.

Source: Sefid-Rud data from Tolouie (1993); Cachf data from Ramirez and Rodriguez (1992).

Data from 38 flushing events with full drawdown at the Zemo-Afchar hydropower

station located downstream of the Kura and Aragvi Rivers in the former Soviet Union

were reported by Gvelessiani and Shimaltsel (1968) and again reflect a high degree of

variability. During the first 2 years of operation the reservoir lost 44 percent of its

capacity, another 32 percent was lost in the next 8 years, and 4 percent was lost in the

next 18 years (1937 to 1954). Prior to 1939 the reservoir had been operated with limited

drawdown for flushing, but this was not effective. From 1939 through 1966 a total of 38

flushing events were conducted with full drawdown, for which data from 36 were

reported. Flushing frequency varied from once to as many as four times a year. The rela-

tionship between mean suspended sediment concentration and mean discharge below the

dam (Fig. 15.13) shows that larger discharges tended to remove more sediment, yet

produced lower mean sediment concentrations. The highest mean sediment concentra-

tions were associated with the shortest flushing duration because the highest solids con-

centration occurs at the beginning of the event and then declines. There is a wide range of

variability from one event to another because of factors including flushing duration and

sediment accumulation between flushing events. Peak discharge concentrations, reported

for only two events, were 270 and 370 g/L.

15.6.3 Effect of Discharge on Erosion Rate

Guo and Li (1984) analyzed flushing at the Hengshan Reservoir in China and noted that

lower (base flow) discharges tended to cut a narrow deep channel, but a wider channel

FLUSHING 15.22

FIGURE 15.13 Flushing discharge relationships for 36 flushing events at Zemo-Afcha

reservoir in the former Soviet Union (data from Gvelessiani and Shimaltsel, 1968). Mean

discharge refers to the duration of the flushing event. MCM = 106m3.

can be produced with a larger flow. Actually, the channel may be deepened by using

either smaller or larger flows, but widening is achieved by large flows only. In China it

was found that the flushing effect is maximized if a reservoir is emptied immediately

prior to the arrival of floods so that the flood flow can exert its erosive force on deposits

which have not yet had the opportunity to fully dewater and consolidate after emptying

the reservoir. The phrase used to describe this strategy is "Deepen by small flow, widen

by flood flow." That larger flows widen flushing channels has also been observed in

sediments with a high clay content (Tolouie, 1993) and in coarse noncohesive delta

deposits (Randle and Lyons, 1995). On the basis of work in Algeria, Duquennois (1956)

described the use of a small storage dam upstream of a reservoir to temporarily store and

then rapidly release water, converting a continuous inflow into a series of larger pulsed

flows. The pulsed flow scoured delta deposits and created turbid density currents to

transport sediment closer to the dam.

15.7 FLUSHING EFFICIENCY

5.1.1. 15.7.1 Definition

The term flushing efficiency (F

) is defined as the ratio of deposit volume eroded to the

water volume used during flushing over any specified time interval. For inflow and out-

flow water volumes V

and V

inflow and outflow total sediment concentrations C

and C

(kg/m

), and deposit bulk density ρ (kg/m

), flushing efficiency F

may be com-

puted by:

FLUSHING 15.23





)



(15.1)

During empty flushing when V

and V

, and if C

and ρ are constant, flushing efficiency is

directly related to C

, the total solids concentration in the discharge. An alternative

expression for flushing efficiency is the water: sediment ratio

(Qian, 1982), which is the

inverse of flushing efficiency (1/F

15.7.2 Flushing Efficiency with Partial Drawdown

Reservoir drawdown during flushing may be limited by operational requirements, or by

the elevation of the available outlets if no bottom outlets are provided. When flushing

flow is discharged through outlets located significantly above the level of the deposits,

resulting in a pool of impounded water before the dam, the flushing efficiency is typically

very low. Flushing efficiencies for several reservoirs where sediment was released

through high-level outlets are summarized in Table 15.3. Flushing under conditions of

partial drawdown may erode upstream sediments and redeposit them closer to the dam,

and, if a low-level outlet is opened, some of the eroded sediment may be vented as a

turbidity current, but this is an inefficient means of removing sediment from a reservoir.

Effective sediment removal through a high-level outlet can be achieved only after the bed

of the deposits has risen close to the level of the outlet, as described at Tarbela Dam in

Sec. 15.4.2.

Flushing with partial drawdown may be useful under specific circumstances. For

example, drawdown and sediment release through a high-level outlet was undertaken at

the Guernsey Reservoir in Wyoming to deliver fine sediment to a downstream unlined

irrigation canal. The sediment partially sealed the canal bottom and reduced canal seep-

age losses. Although sediments were scoured from the upper portion of the reservoir

during the 1961, 1962, and 1963 drawdowns, the suspended solids concentration in water

discharged from the reservoir never exceeded 0.8 g/L (Jarecki and Murphy, 1963). The

principal effect of these and subsequent drawdowns has not been to release sediment, but

TABLE 15.3 Overflow Drawdown Flushing

Reservoir

Outflow

situation

Years of

operation

Discharge,

m3/s

Duration

Flushing

efficiency

Water:sediment

ratio

Guernsey,

U.S.A.

Overflow

spillway

1960-1962 56.6 - 198

10-18

days

0.00017 5880

Warsak,

Pakistan

Overflow

spillway

1976-1979,

5 flushings

1410

Total

490.5 h

0.00169 592

Liujiaxia,

China

Overflow

and outlets;

water level

lowered =

4.4-7.8 m

1981,1984,

1985,1988

1660 -

2090

103 -

177 h

0.0023 -

0.0071

435-141

Shuicaozi

China

Overflow

spillway

1965,1966,

1974, 1978,

1980, 1981

21.4-230 3-4 days 0.012 -

0.043

83-23

Source: Fan (1995).

Morris &amp; Fan. Reservoir Sedimentation Handbook

Подождите немного. Документ загружается.

Morris & Fan. Reservoir Sedimentation Handbook