The San Roque Multipurpose Dam Project (SRMDP) is situated upstream of the Agno River in the Philippines, about 200 km North of Manila. Once completed, the dam and accompanying hydropower plant will provide up to 345 MW of power, irrigation support and improve the water supply for the local community.
The construction of the SRMDP is being funded by the San Roque Power Corporation (SRPC) under a power purchase agreement (PPA) with the National Power Corporation (NPC) on a build-operate-transfer (BOT) basis. Upon completion of the facility, the SRPC will own the power plant for 25 years after which ownership will pass to NPC. During this time NPC will have ownership of the dam and spillway.
In total the dam and hydropower plant will cost some $1.2 billion. The SRPC has provided the project with 25 per cent equity and a non-recourse debt totalling nearly $600 million. This cost has included the acquisition of the site, resettling of the affected families and social development programmes. In addition to this, NPC has financed the non-power components: irrigation, food control and waste quality enhancement, and the project administration costs, which has come to a total of $600 million.
Raytheon Ebasco Overseas Limited (REOL) is constructing the SRMDP, while United Engineers Inc. is responsible for the design and procurement. REOL has subcontracted to Toshiba for the supply and installation of three vertical-Francis turbine generators, which are rated at 115 MW each at a head of 154 m, as well as the associated switchgear and controls. VA Tech Voest MCE has been contracted to design, supply, manufacture and install some 3200 tons of penstocks including manifold with two bifurcations.
The site, which spans the municipalities of San Manuel and San Nicolas, Pangasinan, was chosen from ten different dam sites and power or irrigation schemes.
Figure 1. The San Roque dam will be one of the largest dams in the world
On completion, the San Roque dam will be one of the ten largest dams in the world with a height of 200 m and a width of more than 1 km. The plant will operate primarily as a peaking power plant, offering every day assistants when base and intermediate load power plants cannot fulfil consumer demand.
The facility can operate all year round at its dependable capacity of 85 MW for a minimum of eight hours a day, even in periods of extreme drought. Whenever surplus water is available, the SRMDP can either provide extra peaking capacity (more than 85 MW) or generate off-peak power, or a combination of the two.
The plant’s estimated annual energy generation is 1000 GWh. Nearly 250 GWh is expected to arise from the its dependable capacity, which is the basis for capacity payments under the PPA. The balance will be made up from surplus power that reduces dependence on imported fuel oil and lowers the variable operating expenses of the power plant.
Benefits to the community
The Philippine Local Government Code required the approval of the 11 local government units that would be affected before construction of the dam could begin.
Although the project has involved the relocation of families, it will also offer benefits to the local communities.
One of the benefits on offer is an irrigation plan. The National Irrigation Administration (NIA) estimates that more than 53 000 farmers will benefit from the irrigation plan, which will provide year-round irrigation benefits for about 70 800 hectares (ha) of farmland downstream of the dam, including parts of Lower Pangasinan, Nueva Ecija and Tarlac, with a partially diversified crop during the dry season. These benefits could be extended to 87 000 ha of farmland by a completely diversified crop during the dry season.
The irrigation plan includes:
- A re-regulating pond that supplies water to the Agno River and the irrigation system during a 16-hour off-peak period when the power component may not operate
- A siphon intake servicing irrigated lands on the east bank in San Nicolas
- Rehabilitating about 47 000 ha of existing irrigation and drainage system
- Developing a new irrigation and drainage system serving some 23 700 ha.
The SRMDP will also reduce the perennial flooding of the Agno River by reducing outflows released downstream by storing a portion of the inflows received from upstream in a reservoir. These floods affect more than 16 local communities around the Pangasinan and Tarlas towns.
However, in the extreme case of a 200 year flood event – a flood that is so strong that it only occurs every 200 years – the dam can have little effect. When this occurs all inflows received from the upstream watershed must be released downstream. In this instance flood conditions will be same as those prevailing had the SRMDP not been constructed. But During typhoons, the reservoir level is allowed to rise above the normal maximum operating elevation of 280 m above sea level (masl) and may reach the maximum exceptional elevation of 290 masl, consistent with SRMDP design criteria.
The SRMDP will improve the quality of the water in the Lower Agno River by serving as a settling basin, trapping sediments transported by the runoff of typhoons or the tailings generated by unregulated small-scale mining activities. The volume of sediments primarily depends on the watershed management plans being implemented by the NPC and the Department of Environment and Natural Resources (DENR). The DENR is developing a regulatory process that will meet the needs of small-scale miners and the environment, as thousands of families in the Agno River watershed depend on mining for their livelihood.
Nonetheless, the design of the SRMDP assumes that sedimentation rates experienced over the past 50 years will continue. The SRMDP reservoir has substantial “dead storage” capacity – representing the volume of water at its lower levels that is not available for power generation, irrigation or flood control.
The reservoir’s dead storage can accommodate the historic sedimentation rates, including the massive volumes released following the July 1990 earthquake, for a period of some 90 years.
Both NPC and SRPC know they have to assure a sustainable livelihood for all project-affected families. This includes both the resettled families, and the non-resident landowners, tenants and tillers and seasonal resource users such as gold panners and fishermen.
The NPC funded numerous skills training and livelihood programmes before construction began, but they have not been that effective in some instances. Many of these programmes were administered via people’s organizations, multipurpose cooperatives and affected local government unit officials who often did not have the required skills. Therefore, much more must be accomplished to assure that each family has the opportunity to be at least as well off after the project as before, and if feasible, better off.
The Office of the President has responded by forming a special inter-agency task force on resettlement and livelihood that is chaired by the Department of Social Welfare & Development. This task force has organized and mobilized the resources of other government agencies and programmes in support of the resettlement action plan developed by NPC. It has surveyed the minimum basic needs of the affected families, assisted them in identifying and establishing community organizations and leaders, and conducted training programmes for these leaders.
A Community Based Sustainable Development Plan is currently being developed in consultation with the community leaders for implementation. Some of the schemes implemented by NPC and SRPC include:
- A community farm serving the resettlement community in San Manuel, Pangasinan
- A nursery managed by a club comprised of young adults (age 16 to 25) from the affected families to support reforestation and agri-forestry projects
- A multipurpose cooperative initially making charcoal from trees felled in the borrow areas
- The making of hand sewn quilts for export and domestic sales, which primarily involves women from affected families working at home
- The making of machine-sewed products for export and domestic sales, many of which feature indigenous fabrics hand-woven in Itogon.
The SRMDP currently employs about 4500 people of which some 80 per cent are residents of affected local government units. About half of the families residing in the San Manuel resettlement community have SRMDP related jobs. This percentage includes manual labours, skilled craftsmen and other SRPC site staff.
Design and construction
The hydropower plant begins with a 200 m rock-fill clay-core dam consisting of nearly 40 million m3 of fill material. The structure consists of an embankment with an impervious central core. The core material is a mixture of silty sand and gravel which is excavated from the abutments of the dam and from borrow areas located near the dam.
Figure 2. The SRMDP will reduce perennial flooding in the area by storing a portion of in the inflow in a reservoir
Filters and drains of sand and gravel are located on either side of the dam’s core to protect it and serve as transition and drain zones between the core and the outer shell. This shell is located on the main section of the dam and is comprised of sand, gravel and rock. This is further protected from waves and erosion by armoured rock which has been carefully cleaned and treated with grout to prevent any leakage of water. A grout curtain controls seepage through the foundation. Galleries (tunnels) are installed below the dam to allow supplemental grouting, and to monitor any water seeping from the foundation area.
The drainage basin of the SRMDP reservoir measures 1250 km2 and yields an average annual water flow of 83.6 m3/s. The reservoir has a total storage volume of nearly 1 billion m3, comprising a maximum active storage capacity of 530 million m3 for power generation, irrigation and flood control purposes at its normal maximum storage surface elevation of 280 masl.
Figure 3. The SRMDP is providing an irrigation scheme to help local farmers, and will also control flooding
Another 140 million m3 of active storage is reserved for flood control purposes at the maximum exceptional surface elevation of 290 masl. The dead storage volume totals 320 million m3.
The gated spillway is located on the right abutment (looking downstream) of the San Roque dam. The spillway is designed to discharge 12 800 m3 per second, which corresponds to the Probable Maximum Flood, an internationally accepted criteria for sizing spillway capacity to handle the maximum possible flood given the watershed characteristics and the climatological properties of the area. The spillway is founded on a competent rock foundation. Most of the material excavated during spillway construction is incorporated into the dam embankment.
The water is conveyed from the reservoir to the powerhouse via a concrete intake structure, a power tunnel through the left abutment of the the dam, a penstock and manifold.
This tunnel measures 8.2 m in diameter and is 722 m long. The tunnel intake is situated 20 m below the minimum normal water elevation of 225 masl to assure the submergence required to operate the water turbines. This intake structure is fitted with trash racks to prevent large pieces of debris from entering the tunnel and damaging the turbines.
Bulkhead gates are installed in a gate shaft about 190 m downstream of the intake. These gates can be lowered to close off the flow of water into the power tunnel for routine inspections or for emergency purposes.
Another 530 m downstream of the gate shaft is the surge shaft. This is a 100 m high vertical shaft which prevents the over pressuring of the power tunnel during rapid opening or closing of the turbine wicket gates, as well as enhancing the stability of the power generated.
The downstream end of the power tunnel is lined with a steel penstock extending to 500 m. The penstock terminates into a manifold that trifurcates the water flow into three smaller penstocks leading to the hydraulic turbines, each of which has a shut-off valve installed to isolate the turbine from the reservoir.
The facility’s powerhouse contains three vertical Francis-hydraulic turbine units, each controlled by a hydraulic governor unit. The turbines drive synchronous generators which have a net rated output of 115 MW each. These turbine generators are capable of independent operation. The switchyard, which is adjacent to the main power transformers, encloses gas insulated switchgear that connects the generating units to a 230 kV double-circuit transmission line. The turbines discharge the water via steel lined draft tubes into tailrace tunnels back to the Agno River.
Figure 4. The 200 metre-high dam and power plant will cost some $1.2 billion
A low level output allows the release of water from the reservoir. Its intake is located adjacent to, and 10 m below the power tunnel intake. It is nearly 1.3 km long and discharges directly into the Agno River 500 m downstream of the powerhouse. It is equipped with wheel mounted and hydraulically operated gates.
The low-level outlet tunnel enables the reservoir to be drawn down to a level below the power waterway inlet in order to allow repair and maintenance of the intake structure and of the upstream surface of the dam. It also allows the release of flows to the Agno River channel in the event the power tunnel being dewatered or if flows cannot be released via the powerhouse.
NPC is constructing 230 kV double-circuit transmission lines from the switchyard to the San Manuel substation approximately 9 km away. SRPC will interconnect the switchyard with the transmission line.
NPC is also constructing a 500 kV extra high voltage transmission line stretching from Labrador, Pangasinan through the San Manuel and San Jose substations to the islands south of Luzon to accommodate the addition of more than 2000 MW of hydro and coal-fired power plants in northern Luzon. This 500 kV transmission line will deliver power generated from San Roque, as well as from other power plants in the north, to Metro Manila.
The completion date for the San Roque multipurpose dam project is 31 December 2002, with commercial operation commencing in 2003. Impoundment of the reservoir will commence at the beginning of the wet season in June 2002.