evaluation of the useful life of a reservoir brazil: a case...

1 ne Hydrological Basis for Water Resources Management (Proceedings of the Beijing Symposium, October 1990). IAHS Publ. no. 197,1990.

Evaluation of the useful life of a reservoir on the River Manso, Mato Grosso State, Brazil: a case study

NEWTON DE OLIVEIRA CARVALHO ELETROBRAS Centrais Elêctricas Brasileiras SA, Rio de Janeiro, RJ, Brazil

WEUUNGTON COIMBRA LÔU ENGEVDCSA. and Rural Federal University of Rio de Janeiro, RJ, Brazil

Abstract The most important application of the sedimento-metric study in a river is the evaluation of the useful life of a reservoir. This has to be done during the feasibility study and design. It allows evaluation of the dead volume, and consequently the dam height and other values, and also evaluation of the feasibility of the enterprise. The project will not be viable if there is a large sediment load. The commonly used methodology was presented by the US Bureau of Reclamation in the American unit system. In this paper the same methodology is employed, using the metric system for a case study in Brazil. An evaluation of the useful life of a reservoir on the River Manso, Mato Grosso State, Brazil, was performed. The final result of this evaluation shows that the sedimentation at the base of the dam will take almost 1000 years, demonstrating the reservoir's feasibility concerning the sedimentation problem. However, this result does not eliminate the necessity for future care to preserve the conditions that yield a low sediment load in the river. The applied methodology estimates the sediment distribution within the reservoir. This procedure presents a more realistic approach to lake sedimentation than classic methodologies.

Evaluation de la vie utile d'un réservoir sur le rio Manso, état du Mato Grosso, Brésil

Résumé La meilleure application et la plus importante des études sédimentologiques en rivières c'est l'évaluation de la vie utile d'un réservoir. Elle est effectuée pendant les études de fiabilité et la mise au point du projet. Ceci permet d'évaluer le culot du réservoir et par conséquent la hauteur du barrage et d'autres caractéristiques et également la fiabilité de l'aménagement. La méthodologie employée habituellement pour ces études sédimentologiques a été présentée par le US Bureau of Reclamation dans le système américain d'unités. Dans la présente étude la même méthodologie est utilisée en changeant le système d'unités et en utilisant un cas d'application pour le Brésil. On a ainsi effectué une évaluation de la vie utile du réservoir sur le rio Manso (état du Mato

439

Newton de Oliveira Carvalho & Wellington Coimbra Lôu 440

Grosso). Les résultats ont conduit à une durée de vie utile de presque 1000 années ce qui démontré la fiabilité du projet en ce qui concerne la sédimentation. Toutefois ces résultats n'éliminent pas la nécessité de futures mesures pour la conservation des sols du bassin. La méthodologie présentée est plus réalisé pour la sédimentation des lacs que les méthodologies classiques.

INTRODUCTION

The construction of a dam and its utilization always disturbs the natural condition of a stream. The reservoir becomes a natural means for retention of the transported sediment. The detention of sediment is beneficial in some ways, for it permits the cleansing of water for downstream purposes.

However, the continuous sediment deposition may result in an undesirable situation in the reservoir. Maintenance under such conditions of continuous sediment deposition entails a high cost, and it may even be so high that the dam will have to be abandoned.

The number of reservoirs with irreversible problems or with evident processes of sedimentation requiring maintenance is increasing everywhere. At the same time, the construction of a dam has become difficult, more expensive and problematic. The possession of land requires large indemnities due to the increasing population along the rivers. Concerns of the environment become wider. These are some factors which discourage and burden each enterprise.

In this context, sedimentometric studies, evaluation of the useful life of the reservoir, studies and the collection of data on all problems related to lake sedimentation, as well as the protection against erosion on the basin and other means to reduce sedimentation are receiving increased attention.

RESERVOIR SEDIMENTATION

The trap efficiency of sediment is due to the reduction of the velocity of flow in the reservoir. The larger and heavier sediment particles are deposited at the entrance, and the smaller and lighter sediment particles go farther on into the reservoir. A schematic of this sediment distribution in the reservoir is shown in Fig. 1.

Successive layers of sediment particles have different densities, as a result of the sediment particles' shape, size and weight, and also due to the weight of the particles in the layers above, besides other factors.

Classic methodologies that compute sedimentation do not include the sediment distribution along the reservoir. This leads to an inaccurate estimation of the useful lifetime of a reservoir. Recent research by Borland & Miller, presented by Strand (1974), already permits a more realistic and precise evaluation. The methodology presented by Strand is simple and well developed. This present contribution consists of a case study analysed in the metric system to which the constants and graphics were transformed when necessary. The computation of the sedimentation in a dam on the River

441 Evaluation of the useful life of a reservoir on the River Manso

- RESERVOIR DELTA FORMATION

Fig. 1 Sketch of the process of sediment deposition in a reservoir.

Manso, Mato Grosso State, in central-west Brazil, was chosen as an example. This dam is an enterprise for multiple purposes: control of floods in Cuiabâ and other cities downstream, the generation of hydroelectric power, and the elevation of the water level for navigation. The project was developed by the National Department for Sanitation Works, and is now being improved by Sondotécnica, Engenharia de Solos SA (1986) for ELETRONORTE (North Brazil Power Company), to elevate the dam and increase the reservoir, and increase the hydroelectric power capacity from 100 MW to 210 MW.

METHODOLOGY OF COMPUTATION

The computation of the sedimentation time of the dam and the sediment distribution in the reservoir requires knowledge of the sediment discharge inflow, the Brune curve of trap efficiency of sediments, the value of apparent specific weight, the reservoir characteristics, data on the dam project, and curves presented by Strand (1974).


The empirical area-reduction method was used for the computation presented in this paper.

SEDIMENT DISCHARGE

The measurement of the sediment discharge is necessary for the estimation of the volume of the sediment and the sedimentation time. This measurement can be made with a fluviometric station at the place where the dam will be built or upstream, at the headwater of the reservoir when it is already built.

However, not all the eroded material goes into the channel of the river. Some of the eroded material is deposited at natural or manmade barriers within the watershed, and some may be deposited within the channels and their flood plains. These are some reasons why the measurement of the sediment discharge has to be made in the area of influence of the dam.

The sediment discharge measurement is determined by samples taken on a cross section, followed by laboratory analysis and computation by a method compatible with stream and material conditions. With the data of several measurements the sediment rating curve is determined which shows the relationship between sediment discharge and water discharge. If possible, it is better to plot on logarithmic paper two rating curves: one for the rainy season and another for the dry season. The resulting equations will be of the following type:

Qst = K-Q" (1)

where Qa is the total sediment discharge (t day"1), Q is the water discharge (m3 s"1), and K and n are constants.

Several total sediment discharge measurements made on the River Manso during the period 1977-1981, near the place of the construction of the dam, permitted the plotting of the sediment rating curve of the station obtained by visual fit (Fig. 2).

TRAP EFFICIENCY

The trap efficiency of a reservoir is defined as the ratio of the volume of deposited sediment and that of the sediment inflow.

Methods for estimating reservoir trap efficiency are empirically based upon measured sediment deposits in a large number of reservoirs. The commonly used studies are those by G. Brune and A. Churchill presented by Strand (1974). Brune presents a set of envelope curves (Fig. 3), showing the percentage of sediment trapped (the relationship between the volume of sediment trapped and the volume of total sediment inflow) versus the capacity inflow ratio (the ratio between the capacity of the reservoir and the annual volume inflow).


1 000 10 000

TOTAL SEDIMENT DISCHARGE Qst ( t /day)

Fig. 2 Sediment rating curve for the Manso River.

/

' *

—

/

A /

/

•' y

/

/ /

y

t *

Y

y

A 7^

/

c

s?

,/

r \

?

*•' •-"

MEDIAN CURVE

I I I I ENVELOPE CURVES

_.

-

'

-"TT-

_.-.

—

i

i-H— i. !__

0.001 0.003 0.007 0.01 0.03 0.07 0.1 O.Z 0.3 0.5 0.7 1 2 3 5 7 10

CAPACITY INFLOW RATIO (hm 3 o f CAPACITY/hm3 ANNUAL INFLOW)

Fig. 3 Trap efficiency curve (after Brune; see Strand, 1974).

UNIT WEIGHT OF DEPOSITED SEDIMENT

The deposition of sediment in the reservoir is calculated in terms of weight per time (t day"1). To obtain the volume of deposited sediment, a conversion of this unit has to be made. This conversion is possible by means of an estimated unit weight of the deposited sediment.

There are several factors that influence the value of the unit weight of


the sediment deposited in a reservoir. The most pronounced effects are: the way the reservoir is operated, the texture and size of the sediment particles, the compactness or consolidation rate, the action of density currents, the thalweg slope of the incoming stream and the effect of the vegetation in the reservoir headwater area. The way the reservoir is operated and the size of the sediment particles are the main factors of influence.

The classification below (Table 1) came from the researches of Lara and Pemberton mentioned by Strand (1974).

Table 1 Type of reservoir operation (after Lara and Pemberton)

Type Reservoir operation

1 Sediment always submerged or nearly submerged 2 Normally moderate to considerable reservoir drawdown 3 Reservoir normally empty 4 Riverbed sediments

The estimation of the sediment unit weight, or apparent specific weight, is carried out by the following equations:

1i = WcPc + WmPm+ WsPs (2)

T 7 r = y . + 0.4343 K (LT) - 1 (3)

.T - 1

K = KcPc + KmPm + KsPs (4)

where: yt = initial unit weight (t m"3); Wc, Wm, Ws = coefficients of clay, silt, and sand respectively, which can be

obtained from Table 2; Kc, Km, Ks = corresponding coefficients, from Table 2; Pc, Pm, Ps = percentages of clay, silt and sand, respectively, of the

incoming sediment; yT = unit weight after T years (t m"3); T = time of compaction (years); K = a constant which depends upon the size of the sediment, and

the type of reservoir operation; L = Napierian logarithm.


Table 2 Coefficients far use in metric units, transformed from original values of Lara and Pemberton

Type of operation of a reservoir

1 2 3 4

Clay:

w„ c

0.416 0.561 0.641 0.961

K c

0.2563 0.1346 0.0 0.0

Silt:

wm m

1.121 1.137 1.153 1.169

K„ m

0.0913 0.0288 0.0 0.0

Sand:

W, c

1.554 1.554 1.554 1.554

K~ c

0.0 0.0 0.0 0.0

SEDIMENT DISTRIBUTION WITHIN A RESERVOIR

The sediment which is deposited within a reservoir has an irregular distribution, which is a function of several interrelated factors: the diameter and form of the sediment particles, inflow-outflow relations, size and shape of the reservoir, and the reservoir operating procedure.

Research was conducted on several reservoirs, to evaluate the volume of the deposited sediment at a certain time and its distribution, which resulted in appropriate and more realistic methodologies. One of them, called the "empirical area-reduction method", developed by Borland & Miller, presented by Strand (1974), is presented in this paper for the metric system.

The data obtained in North American research indicates that a definite relationship exists between the reservoir shape and the percentage of sediment deposited at various depths throughout the reservoir. This research resulted in the classification of four types of reservoirs according to geometric characteristics, as shown on Table 3.

Table 3 Shape of the reservoir (after Borland & Miller; see Strand, 1974)

Reservoir type Classification m

I Lake or plain 3.5 to 4.5 II Flood plain-foothill 2.5 to 3.5 III Hill 1.5 to 2.5 IV Gorge 1.0 to 1.5

The value of m is obtained from the reciprocal of the slope of the depth versus capacity plot on logarithmic paper, as shown in Fig. 4. The computation of the values of m showed that the reservoir is type II, classified as flood plain-foothill. The values of the coordinates were obtained from the


CAPACITY

Fig. 4 Depth vs. capacity: Manso River reservoir.

elevation-area-capacity curve (Fig. 5). These surveys carried out in reservoirs made possible the development of

dimensionless curves for each of the four reservoir types: reservoir storage design curves (Fig. 6), curves for determining the depth of sediment deposits

noN

S lU

280

230

C

/ /

f

:

A / / *

Area

< '/

(1 50

—

f S

— -

<i s-

)6m

2> -X 100

1

r -

4 çs^

•<S

150

i.°

j ^

-—

200

--• — -

250

? i B.

1 j. .. . 1

i

—

- | -

..... . 1

,

i

1 300 i 350

T 1 «

^

--•»

1-1

•O*

^ V ^ s

, ! .1 : —

.....4 -.

1 i

-J +.

^

v̂

" ~ t """ " T ' T •••-[—'

-H ' H : LT

400 : 450

~\ T 500

3 z

.....

S N V\

'^ \\ '

!

l

j i

j , Volur

1 ne I

1 I 0 J

9

Fig. 5 Manso River dam: elevation-area-capacity curve.


TYPE I

\

\y TYPEH

\ TYPE m

\ TYPEH

2 0 30 40 50 60 70 80

PERCENTAGE OF SEDIMENT DEPOSITED

Fig. 6 Reservoir storage design curves (after Borland and Miller; see Strand, 1974).

at a dam (Fig. 7) and reservoir area design curves (Fig. 8). The volume of the deposited sediment and its elevation relating to the

bottom of the dam is computed by trial and error as shown later using the example. The distribution of the sediment deposited in the reservoir is obtained using Fig. 8.

EXAMPLE OF AN APPLICATION OF THE EMPIRICAL AREA-REDUCTION METHOD

The dam project on the River Manso has the following data: total capacity = 7.337 x 10 mean discharge inflow = 165 m3 s"1

mean annual volume inflow = 5.20 * 109

elevation of the minimum normal water level = 278.0 m elevation capacity zero = 222.0 m elevation of the spillway = 276.25 m elevation of the intake = 264.90 m

9 m3

3 -1 m s


I 000 800 600

4 00

100 80 60

1 ,0 0 ,8 0 ,6

0 ,4

0 ,2

0 , 10 0 , 0 8 0 ,06

0 , 0 4

0 , 0 2 —

L 1

- \

s _ \ — \ U \

-

^

-

1

A

i

\ IV

il

1

"̂̂ —

i

1 1

1

1

1

i

1

1 • '

1 -

-

-

_

—

-

v —

4 0,1 P 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 O

RELATIVE DEPTH p

Fig. 7 Curves for determining depth of sediment deposits at a dam (after Borland & Miller).

The mean annual sediment discharge for the River Manso can be obtained from the sediment rating curve equations expressed by:

12.5228 3 o-l Qst = 0.0047 <3ZDZZ5 for Q $ 200 m3 s

Qst = 0.13469 g18895 for Q > 200 m3 s"1

(5)

(6)

Using the duration curve of water discharge and the sediment equations (5) and (6), the sediment discharge value is 3579.62 t day"1 or 1 306 561 t year"1.

The trapped sediment in the reservoir is 97%, obtained by using the trap efficiency curve (Fig. 3) resulting in 1 267 364 t year"1. This value was doubled in order to prevent premature loss, resulting in a total sediment inflow of 2 534 729 t year:1 This additional volume of sediment can be seen as a provision for sediment space, preventing an anticipated sediment


2,2

2 , 0

1,8

1,6

m 1,2 a:

<

t -<

a:

0 , 6

0 , 4

0 , 2

0 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 ljO

R E L A T I V E DEPTH (P )

Fig. 8 Reservoir area design curve (after Borland & Miller; see Strand, 1974).

accumulation during the ecomonic lifetime of the project, caused by the accelerated process of man's activity in the basin.

The distribution of particle size, for suspended and bottom sediment samples, verified by the corresponding sediment discharge quantities, resulted in an average composition of clay equal to 18.5%, silt 30.9%, and sand 50.6%. The application of equation (2) of y. for the reservoir operation of type I (see Tables 1 and 2), gives the value of the initial unit weight as 1.210 t m~3, where K was computed as 0.076.

Equation (4) was used to compute yT for 100 years, resulting in 1.331 t m"3 and the sediment volume S equal to 190.438 x 106 m3.

The following step is the calculation of the height of the sediment deposited at the foot of the dam in relation to the spillway after 100 years, twice the economic life of the enterprise. To compute this height Table 4 is used and the value relative to the emergency spillway height (H = 276.26 -222 = 54.25 m) has to be considered.

The table shows the values of V H and A H which were obtained from the elevation-area-capacity curve, and the relative depth p. The values in columns 2 and 7 are plotted on Fig. 7. The point of intersection with the

/ /

/ /

/ 2E

/ UX

/

/ '

n

I

1

V !

\i


Table 4 Determination of elevation of sediment at the River Manso dam

Elevation h

(1)

222 225 230 235 240

Ah

P H

(2)

0.0 0.055 0.147 0.239 0.332

VPH

(la m%)

(3)

0.0 0.0005 0.0074 0.0277 0.0713

APH

(10B m2)

(4)

0.0 0.3317 2.4290 5.6722

11.7888

S-VpH (10S m3)

(5)

0.190438 0.189938 0.183038 0.162738 0.119138

HxApH (X 10e)

(6)

0.0 17.995

131.773 307.717 639.542

"p

(5) •• (6)

(7)

CO

10.555 1.389 0.529 0.186

type n curve is equal to pQ = 0.165, therefore the height of the deposited sediment is equal to y0 = p0 x H = 0.165 * 54.25 = 8.95, resulting in an elevation of 230.95.

The spillway sill has an elevation of 276.25 m, showing that for 100 years there will be no problem of sediment load. The present methodology was applied for 500, 1000, and 1500 years. A graph of deposited sediment volume versus time T was made, resulting in a time of 950 years to reach the elevation of the intake (see Fig. 9).

The sediment distribution in the reservoir is calculated using Table 5, which was filled in according to the procedure given by Strand (1974).

A new elevation-area-capacity curve of the reservoir can be drawn

/

y /

y /

/ /I

A

r9

—i ."

50

r32 ! <109n , 3

500 lOOO 1500

TIME (yea r )

Fig. 9 Curve S = f(T) for the Manso River.


Table 5 Sediment distribution in Manso reservoir

ALTITUDE

( m )

( 1 )

276.25

2 7 5

2 7 0

2 6 5

2 6 0

2 5 5

2 5 0

2 4 5

2 4 0

2 3 5

2 3 0

2 2 5

2 2 2

AREA ORIGINAL

do6™2)

( 2 )

254,8987

241,2406

188.6329

140.6330

103,2377

73,6641

50,5757

30.9202

11,7868

5.6722

2,4290

0,3317

0 ,0000

ORIGINAL VOLUME

( 1 0 9 m 3 )

( 3 )

3,9779

3.6422

2,5675

1 ,7443

1,1347

0,6924

0.3818

0. 1781

0,0713

0.0277

0.0074

0 ,0005

0.0000

RELATIVE DEPTH

A h / H

( 4 )

1 .000

0,977

0 ,865

0,793

0 ,700

0 .608

0.516

0,424

0 ,332

0.240

0 .147

0.055

0,000

Ap TYPE CURVE

( 5 )

0.000

0,516

0.958

1 .142

1 .238

1.276

1.268

1.217

1 . 125

0 .986

0.781

0.465

0,000

SEDIMENT AREA

( l O 6 ^ )

( 6 )

0.0000

1.8390

3,4143

4 .0701

4 ,4122

4,5477

4.5192

4,3374

4.0095

3. 5141

2.7835

1,6 573

0.0000

SEDIMENT VOLUME

( 1 0 6 m 3 ï

( 7 )

1 .149

13.133

18,711

21.206

22.400

22,667

22.142

20.867

18,809

15.744

11.102

2,466

ACUMULATED SEDIMENT VOLUME

( 1 0 6 m 3 )

( 8 )

190,416

189,267

176,134

157.423

136,217

113,817

91,150

69.008

48,141

29.33 2

13.588

2 .486

REVISED AREA

{106m2ï

( 9 )

254,8987

239.4016

185,2186

136,5629

98.8255

69.1164

46.0565

26,5828

7.7793

2,1581

---

REVISED VOLUME

00)

3.7875

3.4529

2,3914

1 .5869

0,9985

0,5786

0 ,2907

0.1091

0,0232

----

230.95 3.0452 0,0113 0.165 0,827 K, • 3,0452x10 4 0,827 • 3 .682x10 = A /A p

K2 - 3, 682 X 106 X 190.438 xlO6 - 3 .564 X 106 - K, x S 196,722x106 S,

through data from columns 9 and 10 in Table 5, in dashed lines in Fig. 5.

CONCLUSIONS AND RECOMMENDATIONS

The River Manso basin has good vegetation cover and plenty of rocks on its upper watershed, resulting in a small sediment yield and a long useful lifetime for the reservoir.

The results presented here are based on a small number of sediment discharge measurements, and will be improved with more measurements in the future. It is possible that deforestation in the watershed will result in an alteration of the present results. Better management by the authorities is recommended for the region, to avoid or minimize activities that motivate hydrosedimentologic disequilibrium on the basin.

REFERENCES

Strand, R. I. (1974) Sedimentation. Design of Small Dams, Appendix H. Bureau of Reclamation, Washington, DC, USA.

Sondotécnica, Engenharia de Solos SA (1986) Projeta Executivo da Barragem do Rio Manso, executado para ELETRONORTE, Rio de Janeiro, RJ, Brasil.

evaluation of the useful life of a reservoir brazil: a case...

Documents