Journal of Hydraulic Research Flushing sediment … · Flushing sediment through reservoirs Chasse de sediments dans des reservoirs HSIEH WEN SHEN, Professor, Department of Civil

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Flushing sediment through reservoirsHsieh Wen Shena

a Department of Civil Engineering, University of California, Berkeley, CA, USA

Online publication date: 08 January 2010

To cite this Article Wen Shen, Hsieh(1999) 'Flushing sediment through reservoirs', Journal of Hydraulic Research, 37: 6,743 — 757To link to this Article: DOI: 10.1080/00221689909498509URL: http://dx.doi.org/10.1080/00221689909498509

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Flushing sediment through reservoirs Chasse de sediments dans des reservoirs

HSIEH WEN SHEN, Professor, Department of Civil Engineering, University of California, Berkeley, CA 94720, USA

ABSTRACT To remove reservoir sediment accumulation for the sustaining of the useful life of reservoir has received increased attention due to the difficulty of constructing new dams. This article is to review current status on the flushing sediment through reservoirs and also to stress the needs of incorporating the risk analysis for the planning of flushing sediment operation through dams.

RÉSUMÉ La reduction de sediments accumules pour maintenir la vie utile des reservoirs recoive de l'attention augmen-tee a cause des difficultes a construire de nouveaux barrages. Cet article met au courant de la chasse de sediments dans des reservoirs et souligne aussi la necessite d'incorporer l'analyse de risques pour le projet et l'operation de la chasse de sediments dans des barrages.

I Introduction

Due to the lack of desirable dam sites and the impacts of dams on stream ecology, attention has been devoted to the reduction of sediment deposit in the reservoirs. In addition to the use of soil control measure to reduce sediment inflow from the watershed into the reservoir, the following approaches have been adopted to reduce reservoir sediment accumulations: (1) to increase the passing sediment through the reservoir during high flows with heavy sediment concentrations, (2) to flush reservoir sediment accumulation through the reservoir, (3) to bypass high flow with heavy sediment concentration from entering the reservoir, (4) to flush sediment from reservoir by density currents, (5) to remove reservoir sediment by mechanical means such as dredging and siphoning. In certain parts of the world, a combination of the above approaches (1), (2), and (5) is an attractive practice. However, due to the concern of fishery and other ecological impacts in many countries such as the United States, it is not allowed to release more sediment from the reservoir than sediment entering the reservoir. In this case, one should dredge sediment accumulation to a certain level and to encourage the passage of sediment through the reservoir during floods. The bypass of flow and sediment from entering the reservoir during flood requires certain special topographical and flow conditions and this approach is not commonly used. We do know the necessary condition but not the sufficient condition for the existence of the density condition and thus it is difficult to rely on the density current to flush sediment. The purposes of this paper are: (1) to present certain current knowledge on flushing sediment through reservoir and (2) to stress the need of using risk analysis to incorporate hydrology in the planning and designing flushing sediment through reservoirs. Approximately, 1% of the storage volume of the world's reservoirs is lost annually due to the sediment deposition (Mahmood, 1987 and Yoon, 1992). Janssen (1999) summarized the estimated rates of reservoir sedimentation by the following Table 1.

Open for discussion till June 30, 2000.

JOURNAL OF HYDRAULIC RESEARCH. VOL. 37, 1999. NO. 6 743

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Table 1. Estimated average rates of reservoir sedimentation as a percentage of total reservoir volume for various countries.

Location

World

Tunisia China

Turkey

Morocco

India

USA

Percent sedimentation rate

1% 2.3%

2.3% 1.2% 0.7%

0.5% 0.22%

Source

Mahmood (1987)

Abdelhadi(1995)

Morris and Fan (1998)

Morris and Fan (1998) Abdelhadi(1995)

Morris (1995)

Crowder(1987)

II Reservoir sediment depositional processes

Sediment particles are carried by the flows into the reservoir and deposited in the reservoir due to the increase of flow area and reduction of flow velocity. As shown in Figure 1, the deltaic deposition consists of four parts: front reach, frontset, topset and tail reach. Due to reservoir operation, some of these particle can migrate toward the dam and also more sediment are brought into the reservoir by the flows, the deltaic deposition pattern may be changed to a wedged type of deposit. For wide reservoir with lateral width much greater than the width of the total lateral width of flow outlets, the lateral distribution of sediment deposit may not be uniform.

III Flushing processes

As discussed by Shen and Lai (1996), flushing processes may include the following two types: (1) the first type is to use flow to remove previously reservoir sediment deposit and (2) the second type is to pass heavy sediment concentrated flow through the reservoir during high flow. When the water stage in the reservoir is high as indicated in Figure 1, only a local flushing cone is formed and the flushing process is not very effective. However when the water stage is low (the topset of sediment deposit is closed to the water stage), the flushing of sediment can be very effective to remove previously reservoir sediment deposit and also to pass sediment in the flow if the frontset of the sediment deposit has reached to a location very closed to the dam. This is commonly known as drawdown flushing and retrogressive erosion can occur. A flushing channel usually occurred during drawdown flushing especially for wide reservoir, and this flushing channel is shown in Figure 2. At the beginning of the drawdown flushing, the top of the flushing flow is above the top of the flushing flow outlet and pressurized flushing flow occurred. At this time the flushing efficiency is not really high and this will be discussed later. However when the top of the flushing flow decreased to a level below the top of the flushing flow outlet, open channel flow occurred for the flushing flow. The flushing efficiency increases greatly. The Tsinghwa University and Northwest Institute of Hydrotechnical Research (1979) and Lai and Shen (1996) showed that the relationship between sediment outflow and flow characteristics in the flushing channel for both field and laboratory data can be summarized by three curves for different sediment sizes as indicated in Figure 3. Janssen (1999) found that this relationship can be applied to both steady and unsteady inflows. Conceivably other curves may exist for large sediment sizes, sediment with cohesive material and for sediment deposit that has been emerged for the water surface. Figure 4 indicates that the sediment outflow dropped significantly for pressurized flushing flow than that for open channel flows through the flushing flow outlet.

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water stage oil react t ^

Zr-:r^--

-flushing cone v-> front reach ,/ji

/rr

J* — — — deltaic deposition

wedge-shaped deposition

original river be

Fig. 1. Schematic sketch of depositional patterns in the longitudinal direction.

Longitudinal section

flushing died

(a)Local Flushing (b)Drawdown Flushing

Fig. 2. Sediment erosion.



1E+3

1E-5 1E^» 1E-3 1E-2

Q™SJ2I B06 1E-1 1E+0

Fig. 3. Relationship between sediment outflow and flow characteristics in reservoir (after Tsinghwa University and Northwest Lab., 1979).

1E-01

1E-02

1E-03:

I 1E^ a 1E-05

1E-0&

1E-07

Field Data

Pressurized Row Condition

' "iË^06 i i rm 1E-05 1E-08 1E-07 1E-06 1E-05 1E-04

Q01'6SW"/ B05

Fig. 4. Sediment outflow and flow characteristics from Lab. data (after Lai and Shen, 1996).

If one wishes to use a numerical model to analyze the flushing flow, the existence of a flushing channel must be considered especially for a wide reservoir. According to Lai and Shen (1996), and Atkinson (1996), that the flushing channel width can be approximated by a geomorphic relationship that B, the width of the flushing channel, is a function of the square root of the flow discharge which is the bankfull flow discharge of the flushing channel. This is shown in Figure 5. It is interested to note that this geomorphic relationship was derived by both field and laboratory data and is very closed to the regime relationship as developed by Lacey (1931) from regime theory.

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• Sanmenxla

- p Goanllng

{•) Guernsey

® Baira

W[= 12.8 Q,0-5

+ ®

1 1 1 1 1000

Flushing dischargg, Q| (m Is)

Fig. 5. Widths formed in reservoir deposits by flushing flows after Atkinson (1996).

One must be very cautious to use the above information. The Figure 4 can only be used when the sediment deposit should have reached to a location closed to the dam and the sediment deposit is more or less the wedged type. When the frontset of the reservoir sediment deposit is located far upstream from the dam, the sediment outflow can be significantly less than that indicated by Figure 4 . As the reservoir sediment deposit occurred mainly away the dam, it would be rather difficult to flush sediment out of the reservoir.. If the sediment deposit contains cohesive material or any difficult to remove material, the sediment outflow can be significantly reduced. Atkinson (1996) has found that in most of the field cases, the sediment outlet discharge were much less than that indicated by Figure 4. A possible reason was that the sediment deposit were away from the dam and the flushing efficiency would be low in these cases. Another reason could be that the flushing time periods were too long and the efficiency could suffer also. Numerous investigations have been conducted to address the problems of flush sediment through the reservoirs but unfortunately most results of these studies are presented in engineering reports and are not readily available. IRTCES (1985), and Albertson etc. (1996) presented comprehensive analysis of flushing sediments by many authors. Some flushing sediment operations in the fields are given in the following Table 2. (after Atkinson, 1996). Chinese engineers have considerable experience with both hydraulic flushing and sediment- pass-through. They have conducted numerous mathematical models and physical models to investigate the flushing sediment through the newly designed Three Gorge Dam on the Yantze River. Extensive amounts of field data have been collected and analyzed by Chinese on the flushing operation in the field. Hopefully some day they can share their wealth of experience with the outside world. Field experience on reservoir drawdown flushing in China have been reported by IRTCES (1985),

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Ding and Long (1985), Shuyou et al. (1988), Guohan and Zhenqui (1989) , Guan et al. (1991), Zhang et al. (1995), Fan and Fan (1996) and Zhang (1996).

Table 2. Examples of Reservoirs that have been successfully flushed after Atkinson (1996).

Reservoir

Baira

Gebidem

Gmund

Hengshan

Honglingjin

Mangahao

Naodehai

Palagneda

Santo Domingo

Country

India

Switzerland

Austria

China

China

New Zealand

China

Switzerland

Venezuela

Reference

Jaggi and Kashyap (1984)

Dawansetal(1982)

Rienossl and Schnelle (1982)

IRTCES(1985)

IRTCES{1985)

Jowett(1984)

IRTCES(1985)

Swiss Nat. Committee on Larqe Dams (1982)

Krumdiek and Chamot (1979)

In other parts of the world for reservoir flushing, Amini and Fouladi (1985) studied the Sefidrud reservoir in Iran, Gvelsini and Shamal"tzel (1971) reported reservoir flushing in USSR, Sen and Srivastava (1995) investigated the flushing operartion related to the Baira Siul hydroelectric project in India, Basson and Rooseboom (1996) described reservoir flushing in South Africa and Bouchard et al. (1996) analyzed the resuspension of sediment in reservoirs and the mud-sliding from the banks. Due to the complexity of flushing operation in various reservoirs, it is difficult to draw comprehensive rules that can be applied to most of these reservoirs. However, the following general rules can be made: 1. Water level in reservoir should be drawn down to improve the efficiency of flushing. 2. Flushing sediment is more efficient in narrow reservoirs than wide reservoirs. 3. For wide reservoir, (or when the total lateral width of the flushing outlets is much less than the res

ervoir width), a distinct flushing channel is formed and retrogressive erosion occurs mainly inside this flushing channel. Sediment may be deposited outside the width of this flushing channel.

4. The width of the flushing channel was found to be a coefficient of about 11 to 12 times the square root of the bankfull discharge inside the flushing channel by Atkinson (1996) from the field data and also by Lai and Shen (1996) as well as Janssen (1999) from laboratory data. Figure 5 describes the variation of the width of the flushing channel with the square root of the bankfull discharge of the flushing channel. This relationship agreed well with the "empirical regime formulae" as presented by Yalin (1992).

Most of the field and laboratory studies of reservoir flushing reported were limited to short term reservoir drawdown. Several numerical models have been used to investigate long term hydraulic

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flushing and sediment- pass- through operations. Nearly all of these modeling studies were based on a one-dimensional modeling. For relatively narrow reservoir, the existence of a flushing channel was often ignored in the numerical analysis because it was assumed that the width of the flushing channel was approximately the same as the reservoir width and the valley width. Peng and Niu (1987) and Ju (1990) have used one dimensional diffusion model to simulate the removed sediment volume and bed profile changes with constant discharge and channel width during fishing. Han and Ho (1996) determined the equilibrium slope of the flushing channel in their analysis of the Three Gorge Reservoir in China. For other wider reservoir and valley, the width of the flushing channel was specified in the numerical model. These analysis were presented by Kitamura (1995), Ziegler and Nisbet (1995) and Chang et al. (1996). Guan et al. (1991) assumed that the retrogressive erosion only occurred in the flushing channel. But they presented an effective optimization procedure to calibrate their model based on Fenhe Reservoir data and the computed reservoir deposit as well as the channel deformation were found to be substantially in agreement the observed field data. Unfortunately not many two dimensional models have been applied to flushing sediment processes. Bechteler and Nujic (1996) presented a two dimensional model to describe the flow and areal sediment deposit patterns for flow and sediment entering a reservoir. Lai (1994) developed a 2-D finite volume unsteady flow and sediment transport model to simulate flushing channel formation during flushing sediment processes. Figure 6 illustrates the development of a flushing channel from this 2-D model by Lai (1994), This is certainly an attractive approach but more development is still needed.

2D Finite Volume Model: (after Lai (1994)

Nonlinear Hyperbolic System:

Governing equations of flow in vector form:

d-i + d-fM + Msl = b(q) o)

dt dx dy

Vectors: q = [h, hu, hv]T; Source/Sink: b(q); Flux vectors: f(q) = [hu, hu2 + gh2l2, huv]T in x;

g(q) = [hv, huv, hv2 + gh2/2]T+ in y

Integrate Eq. (1) over Q by divergence theorem:

ffq,dw = -fF(q)-ndL+jrb(q)dw (2) a so a

where F(q) = [f(q), g(q)]T

Discretize Eq. (2), FVM basic equation is:

Af( =-2FJn(q)LJ + Ab(q) (3)

JOURNAL OF HYDRAULIC RESEARCH, VOL. 37. 1999. NO. 6 749


Bedload Continuity:

t>Zb 1 dt (l-p)

c. Flow velocity field

Fig. 6. The 2-D results after Lai (1994).

Atkinson (1996) proposed the following requirements to analyze the feasibility of flushing sediment from reservoirs: (1) the flow depth for the flushing water channel should not exceed 30% of the flow depth for the normal impounding level, (2) sediment mass flushed annually should exceed the sediment depositing annually, (3) the predicting flushing width is significantly less than the representative bottom width of reservoir then the flushing width can be considered an important constraint and (4) the top width of scoured valley should be twice the value of the actual reservoir top width at full storage if (3) is a constraint and if (3) is not a constraint then the top width of scoured valley approached the value of the actual top width is sufficient. Perhaps these above indices can be modified as discussed below. As shown in Figure 1, there are two arrows at the tail reach and the flushing outlet. At best the flushing flow will remove the sediment accumulation on top of an imaginary line between the two arrows . Of course it is easier to remove the sediment deposit near the outlet channel than that at the tail reach. Thus the first rule is that sediment deposit should be close to the dam. The second index is about the drawdown extent, it

ó(üb, J ó(ab,yr dx dv _

(4)

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would be better to relate the flow depth to the original river flow depth than to the top of the normal depth in the reservoir pool. Perhaps the drawdown flow depth should not exceed the original normal river flow depth. A simplified method was proposed by Scheuerlein (1990) to calculate the sufficient drawdown of water level for flushing sediments in a reservoir.. Thirdly, if a major function of the reservoir is for flood control, then the possibility of flushing sediment and flood water storage should be investigated thoroughly. The fourth index is to consider the width of the reservoir and the width of the total discharge outlets Since the retrogressive erosion usually occurs within the flushing channel and thus the efficiency of the flushing is a function of the ratio between the flushing channel width and the reservoir width.. The fifth index is about the loss of water and this should be investigated in detail by comparing the value of the increase of long-term water storage in the reservoir with the short term loss of water for flushing. Hwang (1996) presented an economic analysis on flushing sediment through Taiwan reservoir. If a purpose of the reservoir is to store water for flood control, then the procedure to operate the dam should be thoroughly investigated for a combined consideration of flood control and sediment flushing.. This will be discussed in Section V.

IV Enginnering analysis of flushing channel operation

For large reservoirs such as the currently under construction Three Gorge Dam, a great deal of detailed analysis has been conducted. Basically the flow and sediment characteristics near the dam are extremely complicated and the design of various hydraulic components and their operations during flushing operation are mainly depend on physical model. Usually one-dimensional numerical models are used to analyze the non-equilibrium transport of the non-uniform sediment following phenomena in different reservoir and flow reaches. Han et al. (1999) made comprehensive analysis on the following concerns in the various reservoir reaches for flushing sediment through the Three Gorge Dam: (1). the increase of backwater level upstream from the head of the reservoir due to the sediment deposition in the reservoir, (2) sediment deposition in the fluctuating backwater reach and the characteristics of its river regime, (3) sediment erosion in the fluctuating backwater reach, (4) navigation in the fluctuating backwater reach and the possibility for improvements, (5) special characteristics in the fluctuating backwater reach in the Three Gorge Reservoir, (6) investigation on the characteristics of density currents and sediment transport, (7) erosion and deposition of bed load, (8) interaction of density currents and sediment deposition, (9) reliability of the data and data collection, (10) confidence in sediment deposition processes and sustainable reservoir volume estimates, (11) comparison between numerical and physical model results, (12) friction of natural river in the reservoir reach, (13) friction factor after sediment deposit, (14) change of friction during erosion process, (15) investigation on the various approaches to solve sedimentation problems in the Three Gorge Reservoir, (16) sediment problem is not a deterring element of the construction of the Three Gorge Dam Engineering Project, (17) the long term operation of the Three Gorge Reservoir to control sediment deposit for maintaining usable reservoir volume, (18) the control of flood water level and the normal flow level at Chungqing City, (19) navigation control in the fluctuating backwater reach and (20) comparison with data collected from existing reservoirs.

These above items are listed here to illustrate the complexity of the problems. Detailed investigations were conducted for each of the above points for the Three Gorge Dam based on the combined knowledge from field data as well as experience, physical modeling, and numerical analysis.

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V Considerations of risk factors

Dams are built to store water for flood control, hydropower generation, water supply etc. and thus water should be accumulated during high flows. Since the peak of sediment inflow, Qs frequently proceeds the peak of flow discharge Q, it would be ideal to open the outlet gates to pass the peak of sediment inflow and then to close the outlet gates before the arrival of the peak flow discharge as shown in Figure 7. Risk analysis should be conducted to investigate the arrival of sediment peak flow in relation to the arrival of peak flow discharge for a given watershed. The efficiency of flushing sediment depends also on the relationship between the time variation of inflow and the time variation of sediment inflow. Both the needs of flushing sediment and water storage should be considered.. This type of analysis has not been conducted adequately in the existing known literature.

Q,Q, / \ / ƒ

Q s / /

/ / y Close Outlet

•

( \ Q \ \ \ \

\ \ \ \ \ \ \ \ \ \^^

Time

Fig. 7. Relationship between the flood peak and sediment peak.

Discharge

Recurrence Interval l4 ►

Downstream Flow Discharge Capacity

Time Fig. 8. Flood waves entering the reservoir and flow releases from die reservoir.

For a large reservoir such as the Three Gorge Dam, a series of flood waves can enter the reservoir during a flood event as shown in Figure 8. The horizontal dashed line is the downstream channel capacity and the downstream flooding will occur if more flow than that indicated by the horizontal line is released from the reservoir. The function of a reservoir is to store the peak flows and not the entire flood. Thus the flow should be released from the reservoir to downstream channel right after the passage of each flood wave so that the reservoir can be used to store the next flood volume. For reservoir operation the flood volume is much more important than the flood peak. It is not sufficient to only test the passing through one high returned flood wave series because it is difficult to con-

752 JOURNAL DE RECHERCHES HYDRAULIQUES. VOL. 37, 1999, NO. 6


struct one flood flow series for representing all cases. The flood volume in each wave and the occurrence time between any two flood waves are all important. There are two risk problems here. The first one is related to the occurrence a series of flood waves and the second one is related to the occurrence of flood events. Perhaps the following analysis as developed by Shiau (1997) for drought can be made for flood events as follows: 1 Flood Event:

A flood event is defined as a collection of reservoir inflow waves or runs which are greater than a truncation level or the downstream channel capacity. Two important characteristics of a flood wave or a run are: (1) the flood duration of a run which is a continuous inflow duration above the truncation level (2) the cumulative flood flow which is the total flood flow volume above this truncation level for each run.

2 Return Period: The return period or the recurrence interval of a flood event is defined as the average elapsed time between the occurrence of two flood events at or exceed a certain flow level. The return period of a flood event is defined as the mean inter-arrival time of flood events equaling or exceeding a certain volume of flow.

Suppose that there occurs a flood event with volume equal or greater than ds at any epoch. Let Nds

be the number of flood events until the occurrence of the next flood event with volume equal or greater than ds. Then, Tds, the inter-arrival time between these two flood events with volume equal or greater than ds is equal to the summation of inter-arrival time of two successive flood events between them as shown in Figure 9. Hence the relationship can be written as:

T„s=2L' (5) i= i

Where L,: inter-arrival time between any two successive flood events; Nds: the number of floods required until the next flood event with magnitude equal or

greater than ds given occurrence of a flood with magnitude equal or greater than ds; Tds: time between two flood events with magnitude equal or greater than ds.

J? T* «i T* > T* -® X X ® ® X ®-

® : severity 2: ds x . severity < ds Fig. 9. Definition of return period of floods.

According to the definition, the return period of a flood event with volume equal or greater than ds is the expected value of Tds. Hence,

E(Tlh) = E J L , = E(Nlh)E(L,) (6)

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The flood inter-arrival time, L, is assumed to have the same distribution, then the above equation can be simplified to:

E(Tds) = E(Nlh)E(L) (7)

Let F&s (ds) denote the cumulative distribution function of flood volume. The probability of a flood event with volume equal or greater than ds is 1 - FAS(ds). Given a flood event with volume equal to or greater than ds occurred, the Nlk-th flood event has the same properties implying that there are Nds - 1 events with volume less than ds. Therefore, Nlh has geometric distribution with parameter 1- FAS(ds), and its probability mass function is:

P(Nds = n) = FDS(ds)"-][l-FDS(ds)], n= 1,2,3,. . . (8)

The expected value of NJs is;

E(N«J = i 1T7T\ (9 )

Therefore, the return period of a flood event with volume equal to or greater than ds becomes:

E{Tlh) = E(Nds)E(L) = E}L], (10) 1 -FDS(ds)

The definition of the return period reveals the relationship between the flood volume and the corresponding return period of the flood events. However, this relationship does not relate to other important characteristics of the flood events, flood duration etc. If the joint distribution of flood volume and flood duration is known, then Equation (8) can be used to determine the corresponding flood volume for certain specific return period of the flood volume. The joint distribution of flood volume and flood duration can be used to estimate the probabilities as well as the statistical properties (e.g. expected value) of flood duration for such flood events. These above analysis can be applied to a series of flood waves or runs. In this case, the flood volume is the total flood volume for a single wave or run and the inter-arrival time would be for the occurrence of next flood wave or run. These analyses can also be applied to a series of flood events and then the flood volume can be the total flood volume for a flood event which is the sum of all the flood volumes in several flood waves within the flood event. An even more complicated case would be to take the flood volume as the total unreleased flood volume from all the flood waves within each flood event. A great deal of hydrological data is needed to accomplish these tasks. Furthermore we should add the task of evaluating the risk of closing the gates for both flood water storage and sediment flushing as illustrated by Figure 7. In addition to these above hydrologie risks, engineers must also consider other factors such as the occurrences of landslides and debris flows to bring in large amount of large rocks at different parts of the reservoir to block the flows etc. In certain cases, dredging may be needed to supplement the flushing of reservoir sediment by flows. For ecological concerns, one should consider the additional turbidity during flushing operation on downstream fishery.

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V Conclusions

An engineering project should be technically sound, economically feasible, and environmentally satisfactorily. This paper is only addressed to some of the engineering analysis on flushing sediment through reservoirs. Flushing sediment has been successfully employed to many reservoirs all over the world. Due to the reduction of reservoir volume by sediment deposit, flushing reservoir sediment will receive more attention in the future. Hopefully, we can share our knowledge to design projects that can sustain the useful lives of existing reservoirs. As stated previously, the purposes of this paper are to review current knowledge and to discuss the risk factors of this operation. The following items were particularly discussed in this article:

1. Drawdown flushing can be a rather useful approach to reduce sediment accumulation in the reservoir.

2. If the width of the dam is less than the width of the reservoir, a flushing channel is frequently formed during retrogressive erosion processes. The behavior of this flushing channel should be included in the analysis of the flushing channel processes.

3. From both field data and laboratory analysis, the sediment discharge in the flushing outlet is a function of the flow discharge in the flushing channel, the flushing channel width and the channel slope.

4. The feasibility of flushing sediment in a given reservoir should be a function of the following factors: the closeness of the reservoir sediment deposit to the dam, the flow depth during flushing processes, the ratio between the total width of the flushing outlets and the reservoir width, and the characteristics of the sediment deposit including the size distributions, the cohesiveness of the sediment particles, and the flow resistance of the sediment deposit.

5. If a dam is constructed to retain flood volume, the combined operation of the reservoir for both flood control and sediment flushing should be considered. If a series of flood waves may arrive in a flood event, the risk of releasing both flood volume and sediment should be carefully investigated.

V Acknowledgement

This writer is grateful to the Taiwan Bureau of Water Resources for sponsoring the effort for the preparation of this manuscript.

VI References

ABDELHADI, M.L. (1995), "Environmental and Socio-economic impacts of erosion and sedimentation in North African Countries" Proceedings: sixth International Sumposium, River Sedimentation , New Delhi, India. CV. J. Varma and A.R.G. Rao (editors),Published by A.A.Balkema.

ALBERTSON, M.L., MALINAS,A., and HOTCHKISS.R., editors (1996), Proceedings of the International Conference on Reservoir Sedimentation, Fort collins, Colorado.

AMINI, A, and FOULADI, C. (1985), "Sediment Flushing at the Sefidrud Reservoir", Proceedings: Second International Workshop on alluvial River Problems", Roorkee, India. Published by Central Board of Irrigation and Power, New Delhi.

ATKINSON, E., (1996), "The Feasibility of Flushing Sediment from Reservoirs" , Report OD 137, HR Walling-ford, November.

BASSON, G.R. and ROOSEBOOM, Albert, (1996)," Sediment Pass-Through Operations in Reservoirs" in Volume 2 of Albertson et. al. (1996).

JOURNAL OF HYDRAULIC RESEARCH, VOL. 37. 1999, NO. 6 755


BECHTLER, W. and NUJIA, M. (1996), "Predicting Reservoir Sedimentation with 2D Model FLOODSIM" in Volume 1 of Albertson et al. (1996).

BOUCHARD, J.P., MAUREL, F., and PETITJEAN (1996), "Numerical Modeling of Sediment Resuspension in Reservoirs" in Vol 1 of Albertson et al. (1996).

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Journal of Hydraulic Research Flushing sediment … · Flushing sediment through reservoirs Chasse de sediments dans des reservoirs HSIEH WEN SHEN, Professor, Department of Civil