Numerical unsteady flow model simulation during the sluice closure of Caruachi Dam

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1 Hydraulics of Dams and River Structures Yazdandoost & Attari (eds) 2004 Taylor & Francis Group, London, ISBN Numerical unsteady flow model simulation during the sluice closure of Caruachi Dam A. Marcano, C. Castro, I. Mendez & E. Martinez C.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 at Caruachi included diverting the Caroni River through 18, m sluices in the spillway lower body that after being closed by gates permitted the reservoir filling. In order to define the precise timing for closure of the sluices and to program the reservoir filling, an Unsteady Flow Onedimensional Numerical model was set up based on the model FEQ. The mathematical model was validated under unsteady state conditions using field observations. This paper describes the different criteria used to program the sluice closure and reservoir filling, according to the requirements to 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 and filling 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 Caroni Development and construction schedule required 2 stages of River Diversion. The First Stage included 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 right CFRD dam. The Second Stage required river diversion through 18, m sluices located in the 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 need of coordinating flow balance to simultaneously fill Caruachi reservoir and, maintain a suitable discharge and headwater variations in Macagua 2968 MW, located only 22 km downstream, and provided with a small reservoir. Regulating discharge is done at Guri Dam (9715 MW), located 59 km upstream of Caruachi Project that predetermines the hydroenergetic characteristics of the Lower Caroni Cascade System, responsible of generating a total of the 70% of the national energy consumption. Since sluice closure and reservoir filling of Caruachi is a time dependent problem, a numerical unsteady model of all Lower Caroní River was built based on all premises required by the system taken in account: energy requirements, hydraulics, geotechnical, structural, construction and environmental restraints. This paper shows the construction of the mathematical model and the model prototype correlation of discharge and water levels during the sluice closure and reservoir filling 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 except some localized loose sand in form of bars that get suspended occasionally, product of mining activities that still remains at some natural river reaches. River morphology is varied and includes 69

2 MACAGUA PROJECT PEROTO ORDAZ MACAGUA MACAGUA RESERVOIR CARUACHI CARUACHI SPILLWAY CARONI RIVER CARUACHI PROJECT SECOND STAGE OF RIVER DIVERSION TOCOMA PROJECT FIRST STAGE OF RIVER DIVERSION TOCOMA SCALE (Km) NECUIMA CANYON GURI GURI FINAL STAGE PROJECT Figure 1. Lower Caroni River System December very irregular cross sections, wide but shallow and narrow but deep channel reaches, some cross sections are kilometers wide. The following structures are built thus representing hydraulic controls that are included in the numerical model architecture: at Guri regulating Dam, two powerhouses are continuously operating with a variable flow pattern flowing through Necuima Canyon and enters a natural river expansion until Tocoma Dam (2160 MW) where First Diversion cofferdams 70

3 POWERHOUSE II (Q vs t) GURI DAM POWERHOUSE I (Q vs t) CHANNELS 1 AND 2 CONFLUENCE MEREY STATION TOCOMA DAM FIRST STAGE OF RIVER DIVERSION (Q vs h) SPILLWAY (Q vs h) SLUICES (Q vs h) PASO CARUACHI STATION CARUACHI DAM SECOND STAGE OF RIVER DIVERSION MACAGUA DAM POWERHOUSE I (Q vs t) POWERHOUSE II (Q vs t) SPILLWAY (Q vs t) POWERHOUSE III (Q vs t) CACHAMAY BRANCH ORINOCO RIVER (h vs t) Figure 2. FEQ numerical model-schematization for Lower Caroni River System December are built, 18 km downstream of Guri Dam. Caruachi Second Stage of River Diversion located 59 km downstream of Guri Dam, receives Guri outflow, and its reservoir level was raised as the process of closing the 18 sluices was done. Macagua Project consisting of three powerhouses operate as run off the river powerplants with constant reservoir elevation, Manning n roughness coefficients for the different river reaches changes from stretch to stretch as obtained from previous investigations. Discharge characteristics of Guri two Powerhouses are specified in the model by hourly prototype information. Rating curves from Tocoma cofferdams are prescribed from physical hydraulic model results. At Caruachi, headwater elevations are specified together with the number of sluices in operation and, spillway overflow information at the final stage of sluice closure. Outflow hydrographs at Caruachi site are routed through Macagua reservoir under prescribed turbine flows at the Macagua three powerhouses with limitation of the fixed reservoir at El approximately. Water levels at the confluence Orinoco-Caroni River allow knowing water depth variation at Macagua downstream reach nearby to the Metropolitan area Puerto Ordaz-San Felix (Figure 2). 3 FREE SURFACE UNSTEADY FLOW NUMERICAL MODEL-FEQ FOR LOWER CARONI RIVER SYSTEM The Full Equations (FEQ) model for the simulation of one-dimensional unsteady flow was used in order to know water level and discharge variation throughout control structures at Tocoma, Caruachi 71

4 5500 SIMULATED PROTOTYPE DISCHARGE(m 3 /s) TIME (hours) Figure 3. Numerical model calibration at Merey gauge station under unsteady flow conditions. and Macagua dams. The equation of conservation of mass ( ) ( ) Q A + = 0 (1) x t and conservation of momentum ( ) y + V ( ) V + 1 ( ) V = So Sf (2) x g x g t 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 control length, A = area of volume cross section, t = time, y = water depth, V = average velocity at cross section, So = channel slope and Sf = energy gradient. The estimation of hydraulic variables in the stream system results from known initial and boundary conditions with an implicit-difference approximation such as Preissman method. 4 MODEL CALIBRATION AND VALIDATION The adjustment of FEQ model parameters, such as Manning s n, contraction and expansion coefficients, 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. In the case of steady state flow, the HEC-2 program, which is widely known for gradually varied flow calculations, was used to simulate rating curves along the river based on 179 cross sections and 49 gage stations of water discharge and stage, along the 110 km river length. Runs performed for a range of discharge permitted obtaining Manning s n values to match prototype known water elevations. To run initial conditions in FEQ program, Manning s n values from HEC-2 fed the input data of FEQ, thus the comparison of water profiles from Guri to Orinoco River evidenced similar results in both models. In the case of unsteady flow, the FEQ model calibration is derived from Guri power house discharges by means of two hydrograph measured at Merey and Paso Caruachi stations which have permanent limnigraphs. Figure 3 shown measured and simulated hydrograph in Merey station where the FEQ reproduces the prototype hydrograph. Paso Caruachi station provides hydrographs to verify the model calibration (Figure 4). Even though additional adjustments may be necessary to reproduce the river behavior, the FEQ results matches the hydrograph trend. However, 72

5 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 the measured one. This effect is attributed to the volume of off-channel storage in the prototype which is not correctly simulated by FEQ. On the other hand, computed discharges were different. Nevertheless, discharges differences are within 5% of total discharge which, from practical engineering judgment results acceptable, taken in account accuracy of water levels and discharge data. 5 OPERATION OF THE MODEL FOR THE SLUICE CLOSURE-STRATEGY FOR SOLUTION Calculations are based on energetic, hydraulics, geotechnical and construction criteria that can be summarized as follows: (1) Maintain Macagua and Guri Projects energy generation offer to fulfill the national electrical system 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 structures such as concrete structures, earth dam and rock fill dam. The reservoir rate raising should not increase more than 0.5 m/day. Additional criteria included allowing time for reservoir shore forest 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 closure program and reservoir filling under the following methodology: (1) Given the energy system demand during the sluices closure which needed to be supplied by the Guri-Macagua powerplants. (2) Compute the flood wave characteristics from Guri powerhouses to Caruachi dam including the effect created by the sluice closure maneuvers and obtain Macagua energy generation, by FEQ numerical model, maintaining the different adopted criteria. (3) Compare Macagua resulting generation patterns with required patterns at Macagua. This procedure 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 closure and their implications on the river system downstream of Caruachi were evaluated using a 73

6 flat hydrograph of constant Q = 5000 m 3 /s outflowing Guri Dam. Results showed that no special provisions 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 be closed, a instantaneous reduction of 20% (1000 m 3 /s, in average) of the Caroni River total flow into Macagua reservoir, thus creating a significant effect on the powerplants normal output generation pattern (Table 1). Case 4 was considered adequate and it contemplates the closure of sluices N 14 to 17 one every 2 days and, sluice N 18 two hours after sluice N 17, for a total time of 6 days, 2 hours. During actual closure according to Case 4, there will be a 1 m drawdown of the Macagua reservoir and a minimum temporary inflow of be 3500 m 3 /s, the system will normalize in 2 days and 18 hours after the closure of the sluice N 18. The latter was satisfactory for Macagua Powerplants generation. Improvement of the FEQ Model by using real hourly hydrograph at Guri dam 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 criteria required 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 HW Sluices: inflow in Minimum inflow Duration of during the 14 AL 18 Caruachi outflow regulated the regulation reguation Case (d-days, h-hours) (m 3 /s) (m 3 /s) (m 3 /s) (d-days, h-hours) (m.a.s.l.) 1 10 h (a) d d (b) d 51 h d (c) d d 2 h (d) d 18 h 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. 74

7 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 by storing an additional 2200 millions m 3 and maintaining the Macagua reservoir and powerplants under normal operating conditions, releasing an average flow of 4000 m 3 /s from Caruachi impounding reservoir to supply Macagua Project. A scheme of reservoir filling was planned based on results from a numerical unsteady flow model allowing raising the reservoir at a rate of 0.20 m/day, which resulted in a total filling time of approximately 58 days. 8 NUMERICAL MODEL-PROTOTYPE CONFORMITY Closure of Sluices N 1toN 8-Prototype-Numerical Model Correlation: With respect to reservoir elevation at Caruachi, and using a full unsteady state model, water levels computed resulted 0.3 m higher than prototype, this discrepancy arises as a result of a hydraulic control created by the submerged prototype cofferdam that imposed higher headwater levels in the field system, and this difference was not contemplated by the numerical model. Macagua average reservoir headwater prediction results very close to field measurements (Figure 6) with differences of the order of 0.20 m. Closure of Sluices N 9toN 18-Prototype-Numerical Model Correlation: Comparison of computed and measured prototype values of reservoir levels at Caruachi following closure of Sluices 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 suppressed by cofferdam being fully submerged by the flow (Figure 7). Filling of Caruachi Reservoir-Prototype-Numerical Model Correlation: Figure 8 shown that prototype filling time was 62 days after the sluice closure operations ended with the closure of sluice 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 for the sluices closure and program for the reservoir filling according to the requirements to ensure dam safety and stability during the filling and the criteria to meet the system energy supply. The model prototype conformity obtained during the process demonstrated the effectiveness of one Figure 6. Caruachi and Macagua reservoir elevations Numerical Model Prototype correlation during closure of sluices 1 to 8. 75

8 80,00 El.78,80 RESERVOIR ELEVATION (m.a.s.l.) 75,00 70,00 65,00 60,00 55,00 Sluice 9 Closure Sluice 10 Closure Sluice 11 Closure Sluice 12 Closure Sluice 13 Closure Sluice 14 Closure El.. 63,95 CARUACHI PROTOTYPE CARUACHI MODEL FEQ Sluice 15 Closure El. 66,87 Sluice 16 Closure Sluice 17, 18 Closure El.75,16 MACAGUA PROTOTYPE MACAGUA MODEL FEQ Reservoir Stabilization 53, TIME (hours) Figure 7. Caruachi and Macagua reservoir elevations Numerical Model Prototype correlation during closure of sluices 9 to 18. RESERVOIR ELEVATION (m.a.s.l) CARUACHI PROTOTYPE CARUACHI MODEL FEQ El. 91, El. 78,80 PROTOTYPE FILLING TIME: 62 DAYS PLANNED FILLING TIME: 58 DAYS TIME (hours) Figure 8. Caruachi prototype/planned elevations during filling of the reservoir. dimensional unsteady flow model that allows solving the complexities involved in the processes of the 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 cooperation of 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 of River Diversion. XXIX IAHR Congress, Beijing. Special Seminar SS-2. Martínez, E., Antonini, M., Pérez, P., Marcano, A. & Castro, C. (2001). Evaluación Multidisciplinaria del impacto de las maniobras de cierre de los ductos de desvío. V Jornadas de EDELCA, Venezuela. 76