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Hydraulics of Dams and River Structures – Yazdandoost & Attari (eds)© 2004 Taylor & Francis Group, London, ISBN 90 5809 632 7

Numerical unsteady flow model simulation during the sluiceclosure of Caruachi Dam

A. Marcano, C. Castro, I. Mendez & E. MartinezC.V.G. EDELCA, Hydraulics Department, Basic Engineering Division, Macagua, Edo. Bolívar, Venezuela

ABSTRACT: Caruachi Hydroelectric Project conforms together with Guri, Tocoma and,Macagua the Lower Caroni Hydroelectric Development. Second Stage of River Diversion atCaruachi included diverting the Caroni River through 18, 5.5 × 9 m sluices in the spillway lowerbody that after being closed by gates permitted the reservoir filling. In order to define the precisetiming for closure of the sluices and to program the reservoir filling, an Unsteady Flow One-dimensional Numerical model was set up based on the model FEQ. The mathematical model wasvalidated under unsteady state conditions using field observations. This paper describes the differ-ent criteria used to program the sluice closure and reservoir filling, according to the requirementsto ensure dam safety and stability during the filling, the criteria to meet the system energy supply,the numerical model set up, its calibration and operation during the sluice closure maneuvers andfilling of the reservoir and, the model–prototype conformity obtained during these processes.

1 INTRODUCTION

Caruachi dam presently under construction is located at the Southeastern region of Venezuela.This – run of the river – 2196 MW is one of the cascade hydropower plants of the Lower CaroniDevelopment and construction schedule required 2 stages of River Diversion. The First Stageincluded river diversion by means of 350 m width natural channel that was created by a cofferdam.The cofferdam permitted building on the dry, the two-body spillway, the powerhouse and, the rightCFRD dam. The Second Stage required river diversion through 18, 5.5 × 9 m sluices located inthe lower body of the spillway structure which allowed building the left embankment dam. Finally,the sluices closure and reservoir filling permit tests and commissioning of the generating units.Sluices closure and reservoir filling processes involve rigorous hydraulic study due to the needof coordinating flow balance to simultaneously fill Caruachi reservoir and, maintain a suitabledischarge and headwater variations in Macagua 2968 MW, located only 22 km downstream, andprovided with a small reservoir. Regulating discharge is done at Guri Dam (9715 MW), located59 km upstream of Caruachi Project that predetermines the hydroenergetic characteristics of theLower Caroni Cascade System, responsible of generating a total of the 70% of the national energyconsumption. Since sluice closure and reservoir filling of Caruachi is a time dependent problem, anumerical unsteady model of all Lower Caroní River was built based on all premises required by thesystem taken in account: energy requirements, hydraulics, geotechnical, structural, constructionand environmental restraints. This paper shows the construction of the mathematical model and themodel–prototype correlation of discharge and water levels during the sluice closure and reservoirfilling of Caruachi Project as part of the Lower Caroni River System.

2 THE LOWER CARONI RIVER SYSTEM

The Caroni River from Guri to its junction with the Orinoco River, comprises a stretch of 110 km(Figure 1). The river runs on a solid gneiss granitic rocky bed, no sediment load is present exceptsome localized loose sand in form of bars that get suspended occasionally, product of miningactivities that still remains at some natural river reaches. River morphology is varied and includes

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MACAGUA PROJECT

CARUACHI SPILLWAY

CARUACHI PROJECT SECOND STAGE OFRIVER DIVERSION

TOCOMA PROJECT FIRST STAGE OFRIVER DIVERSION

0 10

GURI FINAL STAGE PROJECT

SCALE (Km)

5

NE

CU

IMA

CA

NYO

N

GURI

PEROTO ORDAZ

CARONI RIV

ER

TOCOMA

CARUACHI

MACAGUA

MACAGUA RESERVOIR

Figure 1. Lower Caroni River System – December 2002.

very irregular cross sections, wide but shallow and narrow but deep channel reaches, some crosssections are kilometers wide.The following structures are built thus representing hydraulic controlsthat are included in the numerical model architecture: at Guri regulating Dam, two powerhousesare continuously operating with a variable flow pattern flowing through Necuima Canyon andenters a natural river expansion until Tocoma Dam (2160 MW) where First Diversion cofferdams

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POWERHOUSE II (Q vs t)

GURI DAM

POWERHOUSE II (Q vs t)

SPILLWAY(Q vs h)

SLUICES(Q vs h)

MACAGUA DAMPOWERHOUSE I (Q vs t)

ORINOCO RIVER (h vs t)

CACHAMAY BRANCH

POWERHOUSE III (Q vs t)SPILLWAY (Q vs t)

CARUACHI DAM SECOND STAGE OF RIVER DIVERSION

PASO CARUACHI STATION

TOCOMA DAM FIRST STAGE OFRIVER DIVERSION (Q vs h)

MEREY STATION

CHANNELS 1 AND 2 CONFLUENCE

POWERHOUSE I (Q vs t)

Figure 2. FEQ numerical model-schematization for Lower Caroni River System – December 2002.

are built, 18 km downstream of Guri Dam. Caruachi Second Stage of River Diversion located59 km downstream of Guri Dam, receives Guri outflow, and its reservoir level was raised as theprocess of closing the 18 sluices was done. Macagua Project consisting of three powerhousesoperate as – run off the river – powerplants with constant reservoir elevation, 54.50. Manning “n”roughness coefficients for the different river reaches changes from stretch to stretch as obtainedfrom previous investigations. Discharge characteristics of Guri two Powerhouses are specified inthe model by hourly prototype information. Rating curves from Tocoma cofferdams are prescribedfrom physical hydraulic model results. At Caruachi, headwater elevations are specified togetherwith the number of sluices in operation and, spillway overflow information at the final stage ofsluice closure. Outflow hydrographs at Caruachi site are routed through Macagua reservoir underprescribed turbine flows at the Macagua three powerhouses with limitation of the fixed reservoir atEl 54.50 approximately. Water levels at the confluence Orinoco-Caroni River allow knowing waterdepth variation at Macagua downstream reach nearby to the Metropolitan area Puerto Ordaz-SanFelix (Figure 2).

3 FREE SURFACE UNSTEADY FLOW NUMERICAL MODEL-FEQ FORLOWER CARONI RIVER SYSTEM

The Full Equations (FEQ) model for the simulation of one-dimensional unsteady flow was used inorder to know water level and discharge variation throughout control structures atTocoma, Caruachi

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5500

5000

4500

4000

12 24 48 72 96

TIME (hours)

DIS

CH

AR

GE

(m3 /

s)SIMULATED PROTOTYPE

Figure 3. Numerical model calibration at Merey gauge station under unsteady flow conditions.

and Macagua dams. The equation of conservation of mass(

∂Q

∂x

)+

(∂A

∂t

)= 0 (1)

and conservation of momentum(∂y

∂x

)+ V

g

(∂V

∂x

)+ 1

g

(∂V

∂t

)= So − Sf (2)

are used by FEQ to calculate the flow-depth versus time along the reach from Guri Dam to Caroni-Orinoco river confluence, where Q = discharge throughout control volume, x = volume controllength, A = area of volume cross section, t = time, y = water depth, V = average velocity at crosssection, So = channel slope and Sf = energy gradient. The estimation of hydraulic variables inthe stream system results from known initial and boundary conditions with an implicit-differenceapproximation such as Preissman method.

4 MODEL CALIBRATION AND VALIDATION

The adjustment of FEQ model parameters, such as Manning’s n, contraction and expansion coef-ficients, requires knowledge of the hydraulic system in order to simulate observed river behavior.The model calibration aims to reproduce observed data for steady and unsteady flow conditions. Inthe case of steady state flow, the HEC-2 program, which is widely known for gradually varied flowcalculations, was used to simulate rating curves along the river based on 179 cross sections and 49gage stations of water discharge and stage, along the 110 km river length. Runs performed for arange of discharge permitted obtaining Manning’s n values to match prototype known water eleva-tions. To run initial conditions in FEQ program, Manning’s “n” values from HEC-2 fed the inputdata of FEQ, thus the comparison of water profiles from Guri to Orinoco River evidenced similarresults in both models. In the case of unsteady flow, the FEQ model calibration is derived fromGuri power house discharges by means of two hydrograph measured at Merey and Paso Caruachistations which have permanent limnigraphs. Figure 3 shown measured and simulated hydrograph inMerey station where the FEQ reproduces the prototype hydrograph. Paso Caruachi station provideshydrographs to verify the model calibration (Figure 4). Even though additional adjustments may benecessary to reproduce the river behavior, the FEQ results matches the hydrograph trend. However,

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Figure 4. FEQ calibration at Paso Caruachi station under unsteady flow conditions.

they are different in discharge and time. The flood wave simulated arrived 1.48 hours later than themeasured one. This effect is attributed to the volume of off-channel storage in the prototype whichis not correctly simulated by FEQ. On the other hand, computed discharges were different. Never-theless, discharges differences are within 5% of total discharge which, from practical engineeringjudgment results acceptable, taken in account accuracy of water levels and discharge data.

5 OPERATION OF THE MODEL FOR THE SLUICE CLOSURE-STRATEGYFOR SOLUTION

Calculations are based on energetic, hydraulics, geotechnical and construction criteria that can besummarized as follows:

(1) Maintain Macagua and Guri Projects energy generation offer to fulfill the national electricalsystem demand. In addition, Macagua reservoir drawdown should not be less than 52 m.a.s.l.,to avoid turbine malfunction.

(2) Control reservoir rate rising to permit monitoring the performance of the permanent structuressuch as concrete structures, earth dam and rock fill dam. The reservoir rate raising should notincrease more than 0.5 m/day. Additional criteria included allowing time for reservoir shoreforest handling and, fauna rescue on the reservoir islands.

(3) Guarantee Caruachi construction schedule including tests on the first generator.(4) Avoid prolonged high flow velocities in the last sluices to close due to high risk of cavitation

potential for expected maximum velocity in the order of 22 m/s.

FEQ model allows the evaluation of several alternatives in order to get an optimum sluice closureprogram and reservoir filling under the following methodology:

(1) Given the energy system demand during the sluices closure which needed to be supplied bythe Guri-Macagua powerplants.

(2) Compute the flood wave characteristics from Guri powerhouses to Caruachi dam including theeffect created by the sluice closure maneuvers and obtain Macagua energy generation, by FEQnumerical model, maintaining the different adopted criteria.

(3) Compare Macagua resulting generation patterns with required patterns at Macagua. Thisprocedure was repeated until energy supply in the system and criteria adopted are satisfied.

6 SLUICE CLOSURE PLANNING

A preliminary unsteady state FEQ numerical model was set up and different scenarios of clo-sure and their implications on the river system downstream of Caruachi were evaluated using a

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flat hydrograph of constant Q = 5000 m3/s outflowing Guri Dam. Results showed that no specialprovisions were needed when closing the first 13 sluices with regards to downstream implications.However, FEQ model simulations results showed that for every one of the final 5 sluices to beclosed, a instantaneous reduction of 20% (1000 m3/s, in average) of the Caroni River total flowinto Macagua reservoir, thus creating a significant effect on the powerplants normal output gener-ation pattern (Table 1). Case 4 was considered adequate and it contemplates the closure of sluicesN◦ 14 to 17 one every 2 days and, sluice N◦ 18 two hours after sluice N◦ 17, for a total time of6 days, 2 hours. During actual closure according to Case 4, there will be a 1 m drawdown of theMacagua reservoir and a minimum temporary inflow of be 3500 m3/s, the system will normalizein 2 days and 18 hours after the closure of the sluice N◦ 18. The latter was satisfactory for MacaguaPowerplants generation. Improvement of the FEQ Model by using real hourly hydrograph at Guridam and taking provisions of the real conditions of required generation at Macagua powerplants,permitted obtaining the final scheme of closure (Figure 5). This scheme fulfilled all the criteriarequired by the project as mentioned above.

Table 1. Regulation at Macagua during closure of the 5 last sluices unsteady flow simulation.

Results

Caruachi Macagua

Closure duration Maximum Minimum Minimum HWSluices: inflow in Minimum inflow Duration of during the14 AL 18 Caruachi outflow regulated the regulation reguation

Case (d-days, h-hours) (m3/s) (m3/s) (m3/s) (d-days, h-hours) (m.a.s.l.)

1 10 h (a) 5000 300 2000 3d 52.652 5 d (b) 5000 2465 3100 3 d 51 h 53.993 10 d (c) 5000 3315 3850 4 d 53.954 6 d 2 h (d) 5000 2480 3500 2 d 18 h 53.50

Notes: (a) Closure last 5 sluices each every 2 hours.(b) Closure last 5 sluices each every 1 day.(c) Closure last 5 sluices each every 2 days.(d) closure sluices 14 to 17 each every 2 days and sluice 18 two hours after closure of sluice.

Figure 5. Outflow variation at Caruachi and Macagua during closure of 18 sluices.

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7 PLANNING THE FILLING OF CARUACHI RESERVOIR

Raising of Caruachi reservoir starts with the closure of the 18 sluices, from El 58.5 to El 78.8,and proceeds with a spillway operation to complete normal pool elevation, El 91.25 by storingan additional 2200 millions m3 and maintaining the Macagua reservoir and powerplants undernormal operating conditions, releasing an average flow of 4000 m3/s from Caruachi impoundingreservoir to supply Macagua Project. A scheme of reservoir filling was planned based on resultsfrom a numerical unsteady flow model allowing raising the reservoir at a rate of 0.20 m/day, whichresulted in a total filling time of approximately 58 days.

8 NUMERICAL MODEL-PROTOTYPE CONFORMITY

Closure of Sluices N◦ 1 to N◦ 8-Prototype-Numerical Model Correlation: With respect to reservoirelevation at Caruachi, and using a full unsteady state model, water levels computed resulted 0.3 mhigher than prototype, this discrepancy arises as a result of a hydraulic control created by thesubmerged prototype cofferdam that imposed higher headwater levels in the field system, and thisdifference was not contemplated by the numerical model. Macagua average reservoir headwaterprediction results very close to field measurements (Figure 6) with differences of the order of 0.20 m.

Closure of Sluices N◦ 9 to N◦ 18-Prototype-Numerical Model Correlation: Comparison ofcomputed and measured prototype values of reservoir levels at Caruachi following closure ofSluices N◦ 9 to 13, there exists a maximum difference of 0.3 m whereas for the last 5 sluices,sluices 14 to 18, this discrepancy disappears as a result of the hydraulic control been suppressedby cofferdam being fully submerged by the flow (Figure 7).

Filling of Caruachi Reservoir-Prototype-Numerical Model Correlation: Figure 8 shown thatprototype filling time was 62 days after the sluice closure operations ended with the closure ofsluice 18. The planning filling time of 58 days almost predict the real filling time at prototype.Both curves present an average slope of 0.20 m/day that satisfy the reservoir rate raising criteria,ensure water supply to Macagua dam and, deadline scheduled test of the Caruachi first generator.

9 CONCLUSIONS

The FEQ model of the Lower Caroni River System predicted successfully the time sequence forthe sluices closure and program for the reservoir filling according to the requirements to ensuredam safety and stability during the filling and the criteria to meet the system energy supply. Themodel–prototype conformity obtained during the process demonstrated the effectiveness of one

Figure 6. Caruachi and Macagua reservoir elevations – Numerical Model–Prototype correlation duringclosure of sluices 1 to 8.

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. 63,

CARUACHI PROTOTYPE

RE

SE

RV

OIR

ELE

VA

TIO

N (

m.a

.s.l.

)

TIME (hours)

80,00

75,00

70,00

65,00

60,00

55,00

53,000 96 192 288 384 480

Slu

ice

9 C

losu

re

Slu

ice

10 C

losu

re

Slu

ice

11 C

losu

re

Slu

ice

12 C

losu

re

Slu

ice

17, 1

8 C

losu

re

Slu

ice

16 C

losu

re

Slu

ice

15 C

losu

re

Slu

ice

14 C

losu

re

Slu

ice

13 C

losu

re

El. 63,95

El. 66,87

El.75,16

El.78,80

CARUACHI MODEL FEQ

MACAGUA PROTOTYPEMACAGUA MODEL FEQ

Res

ervo

ir S

tabi

lizat

ion

Figure 7. Caruachi and Macagua reservoir elevations – Numerical Model–Prototype correlation duringclosure of sluices 9 to 18.

CARUACHI PROTOTYPE

RE

SE

RV

OIR

ELE

VA

TIO

N (

m.a

.s.l)

93

91

89

87

85

83

81

79

770 192 384 576 768 960 1152 1344 1536 1728

TIME (hours)

CARUACHI MODEL FEQ

El. 78,80 PROTOTYPE FILLING TIME: 62 DAYS

PLANNED FILLING TIME: 58 DAYS

El. 91,25

Figure 8. Caruachi prototype/planned elevations during filling of the reservoir.

dimensional unsteady flow model that allows solving the complexities involved in the processes ofthe sluices closure and reservoir filling of Caruachi Dam, as part of the Lower Caroni hydrosystem.During the processes of sluice closure and reservoir filling it was found vital the cooperationof a multidisciplinary group consisting of hydraulicians, geotechnitians, structural, mechanical,powerplants operators, energy delivery planners and, environmental engineers of EDELCA.

REFERENCES

Marcano, A. (2001). Caruachi Project on the Caroní River–“Key Hydraulic Issues related to Second Stage ofRiver Diversion”. XXIX IAHR Congress, Beijing. Special Seminar SS-2.

Martínez, E., Antonini, M., Pérez, P., Marcano, A. & Castro, C. (2001). Evaluación Multidisciplinaria delimpacto de las maniobras de cierre de los ductos de desvío. V Jornadas de EDELCA, Venezuela.

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