Cerro del Águila Gravity Dam in Perú: Between Feasibility and Challenges

Transcription

1 Cerro del Águila Gravity Dam in Perú: Between Feasibility and Challenges S. M. Sayah 1, S. Bonanni 2, R. Bremen 1, and J. Monaco 3 1 Hydraulic Structure Department, Lombardi Engineering Limited, Via R. Simen 19, 6648 Minusio, Switzerland 2 Chief Engineer, Engineering Department, Astaldi S.p.a., Via G.V.Bona 65, Roma, Italy 3 Project Manager, Cerro del Águila S.A., Av. Santo Toribio 115, Lima 27, Perú s.bonanni@astaldi.com Summary Cerro del Águila project [1] in Perú represents the third step of the Mantaro River major hydropower scheme cascade development. It is situated downstream of the Tablachaca dam constructed in the late eighties. This new 510 MW capacity hydropower scheme will include an 80 m high and 270 m long gravity dam. The central blocks of this vibrated concrete dam are equipped with 4 to 6 mobile gates providing a total capacity of the surface spillway of around m 3 /s. The additional bottom outlets increase the maximum total capacity to m 3 /s. During the initial conceptual study, several relevant criteria related to the concept of the dam and its location were considered. First, a general investigation of the sediment yield to the reservoir due to the high erosion rate occurring in the upper sections of the river was investigated. This investigation also took into consideration the flushing works of the existing reservoir several kilometers upstream of the new dam. This initial investigation showed that the annual sediment yield might vary from several hundred of thousands to several millions of cubic meters of sediment. Moreover, since no sediment trap is foreseen, special considerations are adopted to cope with the impact of this high rate sedimentation. Additionally, to allow adequate flushing of the new reservoir, the capacity of the bottom outlet was significantly increased and its geometry optimized. Annual stepped partial flushing is proposed to allow the transport downstream of the deposited sediments at the upper sections of the reservoir. Other conceptual constraints were related to landslides occurring upstream the reservoir. In fact, this area of the Andes mountain chains is well known for one of the largest landslide that occurred in 1974 in the Mayunmarca area. Therefore, the design of the spillway considered equally the flood generated waves. This paper illustrates the geometry of the Cerro del Águila dam and explains the most relevant features considered during this initial design. Project history and location Cerro del Águila project in Perú represents the third step of the Mantaro river major hydropower scheme cascade development. It is situated downstream from the SAM/Restitución Hydroelectric Development, i.e. the Santiago Antuñez de Mayolo and Restitución power plants 1 (see Figure 1). The SAM HPP is located at the first large and flat curve of the Mantaro river. Originally, another project was planned to develop its second curve practically down to its confluence with the Apurimac river. This project would have included a long low pressure tunnel and a 250 m high dam situated downstream of the Colcabamba river confluence in order to include the discharge provided by the additional catchment area and the additional head between the SAM HEPP tailwater level and the Mantaro river. Figure 1: The confined valley of the Mantaro River where the project is located. Taking into consideration the very large landslide in the Mayunmarca area that occurred in 1974 and apparent vulnerability of the valley side stability, the idea of implementing a very high dam was abandoned and partially substituted by the following: - The Restitución Project developing the 250 m remaining head between the SAM HEPP tailrace and the Mantaro river - The Guitarra Project, developing at short distance

2 downstream of the denominated Guitarra curve. After almost 25 years of consideration, this Guitarra Project (1983) has been redefined and renamed Cerro del Águila Project. Initially, the basic design of this project was elaborated for marketing purposes. After its acquisition by Kallpa Generación S.A., the elaboration of the pre-feasibility study and preliminary geological investigation and analyses were entrusted around the year 2008 to the Peruvian Consultancy firm JByA (Julio Bustamante y Asociados EIRL) in view of an invitation to tender the Engineering Procurement and Construction (EPC) contract. Later on, Fichtner Perú was assigned to revise and further develop the Tender Design. In November 2011, Astaldi S.p.A and their joint venture partner Grana y Montero SAA won the EPC contract to build the scheme. Lombardi SA was chosen later as the project designer. Hydrology and geomorphology - Catchment area: km2 - Gauging station Pongor: EL m asl - Intake: 9 km downstream from Pongor station - Drainage at intake: 9.04 l/s/km2 - Project flood (at the dam site): Q1000 = m3/s - Assumed sediment transport: 1 to 5-6 Mm3/yr - River average slope at the dam site: 0.7%. Artificial reservoir and dam - Dam type: gravity dam (arch form) - Dam height: 80 m from foundation (El El m asl) - Dam crest length: 270 m - Dam crest elevation: El m asl - Exceptional retention water level: EL m asl - Normal retention water level: EL m asl - Total impoundment volume: ~37 Mm3 - Bottom outlet: 6 x 2 slide gates b x h = 4.60 x 6.00 m - Bottom outlet sill level: El m asl - Spillway: 4 x radial gates b x h = x m; 2 flap gates b x h = x Spillway crest sill level: El m asl (radial gates); El m asl (flap gates) - River diversion: 340 m D-shape capacity for El m asl upstream level of 715 m3/s Figure 2: General situation of the Cerro del Águila HPP. The project area is situated around 270 km away from Lima (see Figure 2). It is located at the second curve of the Mantaro River in a stretch where the river flanks are relatively steep between the El and 1250 m asl. It is within the jurisdiction of Pampas, Colcabamba and Salcabamba districts belonging to Tayacaja Province in the Huancavelica region. With regard to the morpho-structural aspects that characterize Peruvian territory, the area is located on the western slopes of the Oriental Cordillera de los Andes in central Peru in the Amazonian watershed. General dam characteristics Dam Layout The new dam main characteristics in reference to the present study are given below (see Figures 3 and 4). Figure 3: Layout of the new dam. A typical section of the new dam is illustrated in Figure 4. The new dam is a conventional concrete gravity arch dam with 18 independent blocks, each 16 m long. The 270 m long dam has a slightly curved planimetric axis (R=400 m), with a crest elevation of m asl. At the lowest point, the base elevation of the upstream toe of the dam is m asl.

3 The maximum dam height measured at foundation is 80 m. The normal operating water level is at El m asl, allowing a total storage volume of around 37 mio m 3. The Maximum Reservoir level corresponding to the Maximum flood is at El m asl. The typical cross section of the dam is designed with inclined faces in order to ensure the required stability during a seismic event. The upstream dam face has an inclination of 1:0.1 (V:H), and the downstream face correspond to an inclination of 1:0.75 (V:H). The maximum width at foundation is around 60 m. The typical width of the crown blocks is 6.2 m. The dam crest is 6.5 m wide and equipped with a concrete parapet at the upstream face. The total volume of concrete of the dam is about m 3. The release of extreme flood events is guaranteed by a gated surface spillway equipped with 4 radial gates and 2 flap gates and a 6 bottom outlet equipped with slice gates. With a total capacity of m 3 /s for the gated surface spillway and m 3 /s for the bottom outlet, the combined total flood discharge capacity of the scheme amounts to m 3 /s. In the next chapters some highlights and an introduction to the main dam features and equipments are given. floods are discharged downstream at the dam-integrated 45 m long chute spillway equipped with a ski jump. The ski jump, located at the bottom end, routes the water to a plunge pool located at the downstream toe of the dam. The ski jump section is an arc with 15 m radius and a subtended angle of 25. A series of 3 m wide and 3 m high deflectors are foreseen at the end of the ski jump in order to open the hydraulic vein and facilitate the air entrainment, favouring the energy dissipation of the water jet before impacting the plunge pool. During a flood event, the latter will constitute a sufficient water cushion that helps protection the rocky river-bed from the jet impact. To guaranty appropriate flow aeration along the spillway chute, air sources are consequently present immediately past the longitudinal step along the six spillway bays at the beginning of the chute. The lateral central guidewalls dividing the spillway chute into three main branches, are in fact interrupted at the intersection with the pillars in order to allow the aeration of the water flux at initial stages. Site Geology The Cerro del Águila Hydropower Project is located in a higher mountainous environment with steep valley slopes (average 30, locally exceeding 60 ) and elevations from 1300 to 3600 m asl. The project is located in two main bedrock lithologies: - Granites and granodiorites (Villa Azul batholith), which characterize the dam area and the southern part of the headrace tunnel (Figure 5); - High-grade metamorphic rocks of the Ambo and Copacabana Group (skarn, hornfels and paragneisses), outcropping in the northern project area (powerhouse, northern part of the headrace tunnel and the tailrace tunnel). Figure 4: Typical section on the central block of the dam equipped with the bottom outlet and radial gate. The major part of the flood discharge capacity of the dam is granted by a gated surface spillway, composed by 6 bays, each 12 m long located at the central blocks 1/3/5 and 2/4/6. Four bays are equipped with radial gates and 2 bays with flap gates designed as well for regulated flow release. The central pillars separating two consecutive bays are 4.0 m wide. The shape of the central pillars and the lateral end wing walls are designed in away to improve the hydraulic efficiency of the spillway. The maximal regulation level of the surface spillway appurtenant structures is located at El m asl. The Figure 5: Granite formation at the dam site.

4 Particularly, the dam site is located completely in granites/granodiorites (Villa Azul batholiths). Rio Mantaro has eroded its current river bed directly into the bedrock. The left-hand and the right-hand slope show however different quaternary deposits: The left-hand side is steeper and directly shows bedrock under a thin cover of colluvial deposits and locally present rockfall/debris flow deposits. On the righthand slope the dam site is on the terrace of a fan of mixed origin and terraced recent alluvial/moraine deposits. Due the erosive environment of Rio Mantaro, only very little current alluvial deposits are present around the river bed. Upstream landslide and hydrology Mayunmarca landslide Some hundred of kilometers upstream of the new am, a massive landslide with a volume of approx. 1.6 x 10 9 m 3 occurred on April 25, The slide created and artificial dam at the Mantaro River and formed a lake 170 m deep. The gradual overtopping occurred in June 6-8, 1974 [2]. This generated a dramatic increase in the breach during the next 6-10 hrs resulting in a final breach approx. 100 m in depth. The peak flow was estimated between m 3 /s and m3/s. The slide material was mostly a mixture of silty sand with some clay resulting in an average size of about 11 mm with some material ranging in size up to 1 m boulders (see Figure 8). Hydrology In Figures 6 and 7 are give the daily flow discharge and the flood statistics as a function of the return periods based the existing measurements station at Pongor Station related to related to 47 years of daily flow data ( ), respectively. Q [m 3 /s] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec average Maximum Minimum Figure 6: Daily water discharge of Mantaro River extrapolated at the dam site. Figure 7: Flow statistics at dam axis. Figure 8: Aerial photos of the Mayunmarca landslide of 1974, situated at approx. 170 km upstream of the future Cerro del Águila dam [3]. During the initial phase of flood definition, the client requested the simulation of the potential impact of the Mayunmarca landslide on the future Cerro del Águila Dam considering a distance from the dam site of around 80 km. In this framework, two distances of eventual landslide from the Cerro del Águila Dam are considered: the real distance of the Mayunmarca event equal to 170 km and the distance requested by the client equal to 80 km. Figure 9 shows the effect of flood wave routing 170 kilometers down the Mantaro River from the Mayunmarca Landslide to the proposed Cerro del Águila Dam. The calculated flood wave peak attenuates almost 30% from m3/s at the slide to m3/s at the proposed Dam. This figure also shows the impact of a hypothetical plug-andbreach similar to the Mayunmarca event but occurring 80 km upstream of the Cerro del Águila Dam. In this case the calculated peak discharge at the proposed dam equals 11'100 m 3 /s.

5 The Maxium capacity of the Cerro del Águila Dam is 12'000 m 3 /s. It can be therefore confidently concluded that the dam can easily accommodate a flood wave due to an event similar to the Mayunmarca landslide happening at 170 km or at 80 km upstream of the Cerro del Águila Dam without overtopping, provided that the reservoir is emptied before the wave arrives. The Mantaro River basin that drains to the Cerro del Águila covers an area of more than km 2. Previous studies provide a rough estimation of the sediment affecting the Cerro del Águila reservoir resulting in a volume between 2 to 10 mio m 3 of sediments. In Figures 11a and 10b are illustrated the intake desander of the Tablachaca HPP situated some hundreds of kilometres upstream of the river. These figures provide a clear idea about the actual sediment transport and sedimentation conditions in the Mantaro River. Figure 9: Routing of a flood wave along the Mantaro River until dam location due to a breach of an upstream dam formed by a landslide. Reservoir sedimentation (a) Site problematic and present situation The Peruvian river basins are affected by high erosion rates (see Figure 10) which result in high bed and suspended sediment transport rates in the fluvial channels. Rough estimations of the suspended sediment yield for the project region might vary between 60 and 100 t/km 2 /yr. (b) Figure 11: Sedimentation and desander cleaning works in the reservoir of Tablachaca situated some hundreds of km upstream of Cerro del Águila dam. Figure 10: High erosion rate of the watershed of Cerro del Águila. The operational conditions of the Cerro del Águila HPP in the long term will significantly depend from the proper management of the sediments carried by the flows of the Mantaro river and depositing at least partially in the reservoir. Except in case of an extreme event it is probable that the importance of this aspect to insure the long term operability

6 of the plant may not arise during the initial operation period (indicatively 2-5 years) of the plant but only in the medium and long term. The proper operation of the plant in the long term will however only be possible in case of a successful sediment management and in particular by strictly including the requirements of the reservoir sediment management into the operational rules. Specific flushing concept and sediment management The design concepts adopted within the present layout of the dam are reflecting some recent design trends developed during the last decades and differing at least partially from design criteria adopted previously. Following some general considerations on the reservoir and river properties with reference to the sediments, the concepts of the proposed sediment management are discussed. Although the characteristics of the Mantaro river are considered, it has to be pointed out that at the present design stage only limited quantitative considerations are possible. The ongoing physical and numerical simulations as well as the collection of the data available on the Tablachaca reservoir will however not affect the design and properties of the civil and hydromechanical works at the dam but rather the frequency, sequence and suitable process of sediment flushing. The recommendations concerning all the operational aspects (from monitoring to the hydraulic flushing process) are thus complete but indicative not considering the results of the ongoing numerical simulations. These simulations will in particular provide indications on the evolution of the trap efficiency of the reservoir and thus on the suitable frequency of the hydraulic flushing. Furthermore the numerical simulations will help to better evaluate the most suitable hydraulic conditions for the flushing operation to be carried out. It is felt that the flow properties of the Mantaro river and the morphology of the reservoir permits to consider the option of periodically removing the sediments deposited in the reservoir to insure to maintain the live storage capacity. Without any doubt, the hydraulic flushing of the sediments represents the most efficient solution for the long term management of the reservoir siltation, despite the flushing operation might result in a temporary plant shutdown. The concept of hydraulic flushing has been increasingly adopted during the last decades in particular in relatively small reservoirs not exceeding in general some millions of m3 capacity especially in Japan, the Alpine and the Andean regions. Most of the experiences of hydraulic flushing in larger reservoirs are concentrated in Asia (China) although the sediment properties and the hydrological conditions (monsoons) differ from the Andean conditions. In terms of hydraulic flushing the case of the new reservoir is thus rather uncommon although not exceptional in terms of the reservoir volume whereas the concepts to be adopted are well known. Also to mention that the height of Cerro del Águila dam is not uncommon for flushing operations. In general terms it is thus necessary to adapt some relatively well known design concepts and procedures to the characteristics of the Mantaro river and in particular the size of the new reservoir. Periodical flushing operations will not remove the totality of the deposited sediments. The purpose of the flushing is thus not to restore pre-reservoir conditions but to limit the silting extent in order not to negatively affect the operational conditions of the plant. The objectives of the sediment management might thus be expressed in the following terms: - Maintain a sufficient trap efficiency for sediment particles in the order of mm. - Practical no reduction of the live storage capacity between the el m asl.(min O.L.) and m asl (M.O.L.) Both aspects are important since the first refers to the capacity of the reservoir to act as desander, whereas the second maintains the required flexibility of the plant. The design of the outlet works, the numerical analyses and the definition of the operational conditions are thus focused in order to fulfill the above mentioned conditions. In general terms, in order to fulfill the above requirements the following conditions shall be satisfied [4]: - Knowledge of the river sediment load - Design of the outlet works taking into account the requirements of the hydraulic flushing, and - Reservoir operation considering the sediment management aspects. Only by considering all the above aspects a successful and efficient sediment management of the new reservoir will be possible. The best design of the outlet structures will be useless if these structures are not properly operated. On the other hand efficient flushing operations are only possible with appropriately designed outlet structures. Figure 12: Schematic representation of the long term equilibrium of the sediments in the Cerro del Águila reservoir. As already mentioned the presence of the reservoir will significantly affect the sediment transport in the Mantaro river. In the long term the flushing operations have to be carried out in order to comply with the project scope. Figure 12 shows schematically long term equilibrium of the sediments deposited within the reservoir. As far as the bed-load in the dam impoundment is concerned, the accretion process entails the formation of a delta at the

7 root of the impoundment as schematically shown in Figure 13. The coarser materials will deposit in the upstream zone of the delta whereas the finer parts will deposit further downstream. It has to be pointed out that the front of the delta moves in the downstream direction as the flow velocities increase progressively in the upstream zone. Figure 13: Schematic representation of a delta forming at the entrance of the reservoir. Further downstream the deposits are constituted by suspended load, with coarser materials depositing more rapidly than the finer ones. Silt and eventually clay deposits might thus be expected close to the dam and in front of the bottom outlet gates. The effective distribution of the deposition within the reservoir of the suspended load will be determined with the numerical models. While opening the bottom outlet gates without any modification of the reservoir level, the inlet of the bottom outlets will be liberated from the deposited silty-clay material. The influence on the sediments will be limited to an area in front of the gates since only in this area the flow velocities will be sufficient to remove the sedimented materials. This corresponds to the partial drawdown. In order to be effective this process has to be carried out at relatively high discharges in the river, compatibly with the capacity of the bottom outlets. High flows will also allow reducing to a minimum the time required to carry out the operation. By a total drawdown (see Figure 14) the sediments will be eroded in the entire reservoir and thus flushed downstream of the dam. However also in this case it is recommended to start with a partial drawdown in order to promote the erosion of the sediments at the entrance of the reservoir and to proceed only in a second step to a total drawdown of the reservoir. flexibility in the sediment management will be possible, the removal of sediments being deposited for many years or decades might be difficult and partially impossible. An accurate management of the sediment is thus of primary importance to insure adequate operational conditions in the long term. As previously mentioned, presently the physical and numerical modeling with regard to sedimentation and flushing processes are on-going. They will provide and strong basis in order to establish the optimum sediment management plan for Cerro del Águila. This plan will be later on confirmed and adapted to local conditions after several years of operations and flushing works of the plant. Conclusion In this paper the general design concept of the new Cerro del Águila dam was presented. It was illustrated that the flood evacuation schemes of this dam took into consideration, in addition to the local hydrology of the Amazonian watershed where the dam is located, the routing of a flood wave generated by an eventual landslide occurring in the upstream. The peak discharge of this flood wave corresponds to the maximum hydraulic discharge capacity of the dam. Moreover, during the design process a special attention was given to the sedimentation problematic. Since the Peruvian Andes are affected with high surface erosion resulting with significant sediment yield in the reservoir, the dam was equipped with large bottom outlets. A specific flushing concept through these organs allows annual and regular partial and complete flushing of the deposited sediments. This concept of flushing will be further investigated by means of physical and numerical modeling and optimized when the scheme is in service. Presently the construction works and on-going under an EPC contract. The commissioning date is foreseen in the second half of Acknowledgements The authors acknowledge the active contribution of Astaldi S.p.A. and Graña y Montero SAA and all their precious suggestions in this initial phase of the project. Furthermore, special thanks are presented to Kallpa Generación S.A., the owner of the scheme. References Figure 14: Erosion and flushing process during a total drawdown of the reservoir. The proper management of the river sediments might only be possible by an operation of the HPP taking into account the requirements of the sediment management. Although some [1] Consorcio Rio Mantaro, Lombardi Eng. Ltd, (2012). Technical report n BC-LOM-3P ITG-001 (unpublished report) [2] Hutchinson, J.N. and Kojan, E., (1975). The Mayunmarca Landslide of 25 April 1974, UNESCO report [3] Fread D.L., (1991). Breach: an erosion model for earthen dam failures, Hydrologic Research Laboratory, National Weather Service, NOAA, Silver Spring, Maryland [4] Morris, Gregory L. and Fan, Jiahua, (1998). Reservoir Sedimentation Handbook, McGraw-Hill Book Co., New York.