Reservoir drawdown and sluicing
6.14 Mm³ (original)
The 147 MW Patrind hydropower project in northern Pakistan is likely to face challenges due to the high sediment load of the Kunhar river. Managing reservoir storage and optimising the project’s flushing mechanisms will be key priorities.
Patrind is the first privately-owned run-of-river project in northern Pakistan, and the first generating unit entered into operation in June 2017. The project is located in Patrind village near Lohar Gali, Muzaffarabad, on the border between the provinces of Khyber-Pakhtunkhwa and Azad Jammu and Kashmir.
The Patrind project benefits from the natural difference in elevation between the Kunhar and Jhelum rivers. As shown in the layout in figure 1, the intake upstream of the weir on the Kunhar river diverts the waters towards the headrace tunnel and into the powerhouse on the Jhelum river.
The powerhouse, located about 8 km upstream of the Kunhar and Jhelum river confluence, is equipped with three Francis turbines, each with a 50 MW capacity. Figure 2 shows the aerial view of the Jhelum river downstream the power plant. The net capacity after auxiliary consumption is 147 MW and it is expected to generate 632 GWh of energy annually.
The purpose of the Patrind hydropower project is to deliver renewable electricity to the Pakistani grid, which is currently very hydrocarbon intensive. The project earned certified emission reduction credits under the Clean Development Mechanism (CDM), which enabled it to attract foreign investors. Patrind is owned by Star Hydro Power Limited, a joint venture between the state-owned company Korea Water Resources Corporation (K-water), which owns 80 per cent, and Daewoo Engineering & Construction Co. Ltd., which owns 20 per cent. The project is in line with the Korean government’s current strategy to promote development of CDM projects abroad.
The USD 400 million project features a concrete gravity weir as seen in figure 2, with a crest elevation of 769 masl and a height of 43.5 m. The dam creates a reservoir of 5 km in length with a total capacity of 6.14 Mm3. The reservoir will normally be operated at full operating regime, and the maximum and minimum operating levels are 765 and 760 masl respectively. The designed discharge is 153.66 m3/s and the spillway discharge capacity, including ancillary spillways, is 2,798.80 m3/s.
The Kunhar river originates in the Himalayas and carries a high suspended sediment load, equivalent to around 40 million tons per year, mostly made up of sand and silt material. The mean annual inflow to the reservoir is 104 m3/s and the original projected sedimentation rate at the dam is 4.28 million tons per year.
Two silting basins were designed to protect the turbines from erosion, with the aim of increasing their operational life and reducing the need for maintenance. In the original design, these systems include two sand trap chambers, an outlet chamber and a flushing tunnel. The sand trap system was designed to retain sediments with a grain size greater than 0.20 mm.
The trap was expected to maintain a 100 per cent efficiency for gravel throughout the project’s operational lifespan, while the efficiency for sand particles, clay and silt will initially stand at 100 per cent, but will reduce to 39 per cent in the case of sand and to 8 per cent in case of clay and silt over subsequent years.
However, in order to stabilise the long-term reservoir capacity, a sediment bypass tunnel was included, despite not being part of the original design and the sandtrap structure, therefore, was removed. In addition, the weir location was shifted downstream to increase the length of pool as this section between cofferdam and weir structure will act as a sandtrap. Figure 3 shows the cofferdam and the weir.
The bypass tunnel features a concrete lining and is constructed in a horseshoe cross section, with a slope of 0.02 and an invert elevation at 745 masl. The tunnel has a capacity of 646.34 m3/s to convey coarse particles, with a maximum sediment size of 150 mm at a maximum flow velocity of 8 m/s during flood events. During natural flood events, sediment can be discharged downstream with minimal adverse environmental impacts.
Other future measures planned at the Patrind project include drawdown and sluicing, empty flushing and mechanical dredging of sediments near the intake if required.
In the annual drawdown and sluicing operation, the intakes will be closed and the bypass tunnel opened to flush out the finer particles settled in the reservoir area. The empty flushing operation is expected to last five days, with two radial gates opened simultaneously.
Adaptive measures implemented at Patrind include the use of abrasion resistant coatings and modifications made to the plant’s operating rules. Real-time monitoring is carried out using upstream gauges for streamflow and for suspended sediment concentration.