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Hydrology in Mountainous Répons, n Artificial R eservoirs; Water an d Slopes

Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 194,1990.

Removal of sediment deposits in reservoirs by means of

lushing

H. SCHEUEKLEIN

Obernach Hydraulics Laboratory, Oskar von Miller-Institut,

Technical University Munich, F. R. Germany

ABSTRACT Removal of sediment deposits from reservoirs by

means of flushing is a widely used method to regain

storage volume. However, its efficiency is very often

overestimated. The importance of supporting auxilliary

measures as water level drawdown is not always recognized

to sufficient extent. Estimation of flushing efficiency by

means of theoretical approach is problematic. The

mechanism is complex and verification of the parameters

involved is difficult. In the paper a straightforward

approach starting from extreme simplifications is

presented. The method allows for a rough estimate on the

prospect of flushing including information on the

necessary drawdown to produce a desired effect. By means

of two graphs the limits and the effictivity of flushing

activities can be judged quickly. The procedure is

demonstrated by means of examples.

INTRODUCTION

Sedimentation of man-made reservoirs is one of the major problems

hydraulic engineers will have to face in future. In spite of the

various methods which can be taken to minimize sediment yield from the

watershed, the intrusion and the deposition of sediment in reservoirs

can never be avoided completely. Sediment enters the reservoir either

as bedload or as suspended load. Usually the amount of sediment

carried in suspension exceeds the bed load transport by a factor of

5 to 10. The sediment deposits in a reservoir are composed accor

dingly. First the coarse material transported close to the bed settles

down forming a delta at the reservoir entrance. Material in suspension

is carried further and deposited more or less uniformly all over the

reservoir.

REMOVAL OF SEDIMENT FROM RESERVOIRS BY MEANS OF FLUSHING

In principle, the mechanism described above takes place similarly in

any reservoir. However, the magnitude of the depositions, the pace of

the deposition process, and the importance of the depositions for the

operation of the reservoir may vary considerably.

In mountain reservoirs, usually the uniformly distributed fine

deposits are of minor importance. It is the gravel and sand settling

at the entrance of the reservoir that causes problems, such as

99

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H. Scheuerlein

100

- occupation of the most active (upper) part of the reservoir,

- raise of water level in the river upstream of the reservoir

(backwater effect),

- degradation of the river bed downstream of the dam due to trapping

of coarse material in the reservoir.

With respect

to the

reasons listed above,

it is

desirable

to

remove

particularly the deposits of coarse material at the reservoir entrance,

preferably by passing it downstream where it is needed to avoid

degradation.

An elegant method

to

solve

the

problem

is to

take advantage

of the

transport capacity of the flow itself without using external energy.

This technique which commonly is called flushing is used throughout

the world, however, not always with the desired success. The

efficiency

of

flushing depends significantly

on the

water level

in the

reservoir during the measure (ACKERS et al, SCHEUERLEIN, 1987, WHITE

et al) . Flushing can be carried out very effectively when the water

level

in the

reservoir

can be

kept

low for

some time while

the

flow

rate

is

high.

As this, on the

other hand, means

a

substantial loss

of

water, effective flushing must be oriented towards minimization of

water level drawdown and flushing time.

Theoretical treatment of reservoir sedimentation and flushing is

difficult for various reasons. The mechanism is complex and the

verification of the parameters involved is problematic due to the

stochastic character of the water and sediment afflux. For any

analysis extensive simplifications are unavoidable.

SIMPLIFIED METHOD TO ESTIMATE FLUSHING EFFICIENCY

With respect to the limited possibilities to describe and to verify

reservoir sedimentation and flushing activities analytically,

sophisticated theoretical treatment seems hardly justified. A

simplified approach has been tried by SCHEUERLEIN, 1989, however, the

procedure presented there proved to be still somewhat troublesome and

unhandy

by

incorporating

the

energy gradient

of the

flow through

the

reservoir. On the other hand, the lateral mixing of the incoming flow

on

the way

through

the

reservoir

was

left

out of

consideration.

In the following another simple method to estimate the possibility

and

the

limit

of

eventual flushing activities will

be

presented. Using

the notations of Fig. 1, and the assumptions

-

simplified prismatic shape

of the

reservoir,

-

inflow equal outflow during flushing,

-

onedimensional analysis,

the continuity equation close to the dam site at a hypothetical

drawdown water level DWL can be written as

B

D

+ B

A . (1)

= V

D 2

h

D

For geometrical reasons,

h

D

H

o

B

D

B

A

B

o ~

B

A

(2)

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101

Removal of sediment in

reservoirs

by flushing

Q A V

original river

drawdown

Longitudinal Sect ion

OWL

operationa

water level

FIG. 1 Flow through a reservoir - definition sketch.

D

A H o

o

V

(3)

substituted in (1) ,

h.

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H. Scheuerlein

102

velocity close to the dam can also be interpreted as the governing

factor for the efficiency of any flushing activity. Considering

V

D

= V

Dc

( 5 )

with

v„ = f(d) (f.i. after HJULSTROM) (6)

Dc

and

Q_ = Q (per definition) (7)

equation (4) changes to

h B - B

^ A

=

V

D c

B

A

+

ÎT ~ V ^

>

h

D

( 8

>

o

z

which after some transformation and with the substitution

«A - B :

9 )

Q

\

finally reads

- ̂ (,„,

H B

o o

Fig. 2 shows the graphic verification of equation ( 10 ) . For easier

handling the HJUSSTROM function (6) has been incorporated in the

auxilliary graphs given in Fig. 3.

With Fig. 2 and 3 a direct and straightforward determination of the

drawdown water level corresponding with a desired flushing effect,

f. i. sluicing of a certain defined grain size is possible. Further

more it is possible to check the limits of flushing at extraordinary

conditions.

The presented primitive method to estimate the margins within

which flushing activities will necessarily have to stay might help to

develop a more realistic feeling of what might be possible and what

even under favourable conditions must be impossible. It is neither

meant nor capable to describe the flushing process realistically. It

rather serves the purpose to bring unjustified expectations towards

the effictivity of flushing (particularly at partly filled reservoirs)

back to the ground.

EXAMPLES

Example : Small reservoir at a run-of-river power plant in the Alps

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105

Removal of sediment in

reservoirs

by flushing

Given:

Q

A

= Q

D

= 400 m

3

/s

B. = B = 60 m

A o

H = 7 m

o

Question:

Necessary drawdown to guarantee flushing of coarse gravel

(d = 30 mm)?

Procedure:

^A 400 , ,, f

3

/ /

q

A

=

B 7

=

~6ÏÏ

= 6

'

6 7

L

m3/s/l

A

^A =

6

>

6 7

= n qs

H 7

u

'

o

q

A

From Fig. 3 for — =0 ,9 5 and d = 30 mm

H

O

q

A

A

= 0 4 4

v

n

H

De o

q. B

From Fig. 2 for 5- = 0,44 and

~ =

1,0

De o A

£ • o «

o

h

D

= 0,44 • 7 = 3,10 jmj

Flushing of coarse gravel would require a drawdown of

AH = H - h„ = 7 - 3,10 = 3,90 [ml

O

D » L J

Example 2: Large reservoir in the Middle East

Given:

Q

A

= Q

D

= 1000 m

3

/s

B

A

= 140 m

A

B = 420 m

o

H 90 m (operational water level)

o

Question:

Up to which grain size will flushing be effictive when the

drawdown shall not drop more than 60 m below operational water

level?

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H. Scheuerlein

106

Procedure:

h

= H - 60 = 90 - 60 = 30 [ m j

D o

^ = 30 = 0 33H 90 '

J

G

h D ,„„

From. Fig.

2 for ^- = 0,33 and

-g-

= y^j = 3

o

A

q

A

0,42

De o

q

A „ ,„

J

q

A 1000 _

n n s

From Fig. 3 for ^- = 0,42 and — -

1 4 0

.

9 0

~ °

5

°»

De o o

d

= 0,3 mm

Even with

a

water level drawdown

of 60 m

flushing efficiency would

be

very poor.

Additional question:

Necessary drawdown when flushing of coarse sand (d = 2 mm) shall

be guaranteed?

Procedure:

q

A

From Fig.

3 for ^ = 0,08 and d = 2 mm

r i

q

A

= 0,21

De o

q. B

From Fig.

2 for ~ = 0,21 and ~ = 3

De o A

^ = 0,18 ->• h

D

= 0,18 • 90 = 16,2 [mj

h

D

o

Necessary drawdown H - h = 90 - 16,2 = 7 3,8m

REFERENCES

ACKERS, P., THOMPSON, G. (1987) Reservoir sedimentation and influence

of flushing. In: Sediment Transport in Gravel-bed Rivers.

J. Wiley & Sons, London, 845-868.

ASCE (1977) Sedimentation Engineering. ASCE Manuals

and

Reports

on

Engineering Practice No. 54 , New York.

SCHEUERLEIN, H. (1987) Sedimentation of reservoirs - methods of

prevention, techniques

of

rehabilitation.

In:

First Iranian

Symposium on Dam Engineering. Tehran.

SCHEUERLEIN,

H.

(1989) Sediment sluicing

in

mountain reservoirs.

In:

International Workshop on Fluvial Hydraulics of Mountain Regions.

Trent, B 77 - B 88 .

WHITE, W.,

BETTESS,

R.

(1984)

The

feasibility

of

flushing sediments

through reservoirs. In: Challenges in African Hydrology and Water

Resources (Proceedings of the Harare Symposium). IAHR Publ.

No. 144, 577-587.

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