To combat buildup of sediment in the reservoir that impounds water for the 60-mw Kulekhani No. 1 and 32-mw Kulekhani No. 2 projects on the Kulekhani River, the Nepal Electricity Authority built a series of small dams called check dams in the reservoir. These dams trap sediment before it reaches the powerhouse.
Background on the project
To supply the Kulekhani No. 1 and Kulekhani No. 2 stations, Nepal Electricity Authority built a 406-meter-long, 114-meter-high clay-cored rockfill dam in the Kulekhani River. The resulting 7-kilometer-long reservoir, which
was impounded in October 1981, collects water during the rainy season to use for power generation during the dry season (December through May) through a cascade system of the two powerhouses. In addition, the reservoir receives water from two other intakes.
When the reservoir was initially impounded, it stored 85.3 million cubic meters of water at its full level of Elevation 1530 meters above mean sea level.
Due to the mountainous topography and the seasonal nature of the water flow, rivers in the Himalayan mountain region carry large amounts of sediment. In March 1993, after 11 years of project operations, accumulated sediment in the Kulekhani Reservoir was 2.2 million cubic meters, reducing storage by 220,000 cubic meters.
Together, Kulekhani No. 1 and Kulekhani No. 2 account for about 25 percent of the hydropower capacity in Nepal and contribute greatly during times of peak power demand.
Results of the 1993 flood
On July 19 and 20, 1993, an unprecedented storm hit Nepal. Estimated rainfall in the Kulekhani drainage area was about 400 millimeters for 24 hours. In addition, the Tistung rain measuring station upstream of the Kulekhani watershed area recorded 540 millimeters of rain in one day! Based on the hydrological data available before 1993, this record corresponds to a 1,000-year return period.
This large flood caused as many as 50 landslides in the Kulekhani watershed. As a result, Nepal Electricity Authority was concerned about how sediment deposition would affect the hydro projects. To determine the amount of sediment deposited in the Kulekhani Reservoir by the flood, consultant Nippon Koei Co. Ltd. of Japan conducted a sedimentation survey in December 1993. Data indicated the flood carried 4.8 million cubic meters of sediment into the reservoir. When measured against results from a similar survey performed in March 1993, the reservoir floor in front of the power intake for Kulekhani No. 1 had risen 30 meters. This left only 14 meters of dead storage in the reservoir.
The remaining dead storage volume below low water level in the reservoir was critical to continued operation of the two projects. This was measured at only about 4 million cubic meters. Nepal Electricity Authority determined that the remaining dead storage would be completely filled with sediment if the same scale of flood hit the Kulekhani watershed again.
The December 1993 sedimentation survey indicated that counter-measures were urgently required to mitigate further sediment inflow into the Kulekhani Reservoir.
Investigating options to deal with sediment
To determine the sediment control work to be carried out at Kulekhani Reservoir, in 1994 Nippon Koei completed a Master Plan Study for Sediment Control on the Kulekhani watershed. In addition to analyzing data from the 1993 flood, Nippon Koei assessed the sedimentation process in the Kulekhani Reservoir and estimated future sediment yield of the Kulekhani watershed.
The company estimated that peak discharge during the July 1993 flood was 1,340 cubic meters per second (cms), which corresponded to a 1,000-year return period flood. If no counter-measures were implemented and another big flood on the scale of the 1993 flood occurred, Nippon Koei estimated the remaining life of the intake of Kulekhani No. 1 at 12 years.
Based on major sediment sources in the Kulekhani drainage area, the study assessed seven potential counter-measures for mitigating sediment inflow:
- Dredging sediment in the entire reservoir;
- Dredging sediment around the Kulekhani No. 1 intake;
- Installing check dams to trap sediment entering the reservoir;
- Installing a sand flushing tunnel;
- Dredging around the intake and installing check dams;
- Dredging around the intake and installing a sand flushing tunnel; and
- Installing a sloping intake for Kulekhani No. 1.
Based on its technical and economical feasibility, the sloping intake was selected as the optimum counter-measure to prevent closure of the intake for Kulekhani No. 1. The old intake ran horizontally from its position in the reservoir. The new intake would slope upward, helping to avoid blockage of the intake by sediment inflow.
Once this primary counter-measure was chosen, additional study was undertaken to assess its use in combination with several other counter-measures. In addition to assessing the sloping intake on its own, this study considered:
- Sloping intake with dredging;
- Sloping intake with three check dams;
- Sloping intake with two check dams; and
- Sloping intake with one check dam.
In the end, the study proposed four sediment control structures to be implemented urgently:
- Sloping intake for Kulekhani No. 1;
- Check dam No. 1 at the upstream end of the reservoir on the Kulekhani River, to trap sediment before it enters the reservoir;
- Check dam D-0 on the Darkot River where it enters the reservoir, to decrease the riverbed slope and reduce the sediment carrying capacity of this river; and
- Check dam No. 5 about 8 kilometers upstream from the reservoir on the Palung River, a tributary of the Kulekhani River, to trap sediment.
Building the check dams
Both check dams No. 1 and No. 5 were designed to be above-ground gravity dams, intended to trap sediment. (See Figure 1.) Check dam No. 1 is 14 meters high and has a sediment capacity of more than 1.1 million cubic meters. Check dam No. 5 is 9 meters high and has a capacity of 630,000 cubic meters. Check dam D-0 is a ground sill type intended to hold sediment in a way that changes the slope of the riverbed.
Civil works contractor China International Water and Electric Corporation performed all construction work for the check dams. Work involved in building the dams included diverting the river, excavating the riverbed, pouring concrete, and installing side slope protection. The dams were built with drainage holes consisting of concrete-reinforced pipes. This allows the dams to retain sediment but let water pass through.
Construction of check dams D-0 and No. 5 took six months and was completed in June 1998. Construction of Check Dam No. 1 took eight months (during two dry seasons) and was completed in June 2000. Cost to complete the three dams was US$3.7 million.
To further improve the effectiveness of check dams No. 1 and No. 5, Nepal Electricity Authority developed
a plan to excavate the sand accumulated behind these dams each year, during the dry season. This excavation would create space for sand to be deposited during the next rainy season, as well as generate revenue for local development through sales of the sand for use as a construction material.
Results of the plan
Since they were completed, all three check dams have worked as designed. Surveys performed in November 1998 and January 2005 indicate that the bottom level of the reservoir has remained unchanged at Elevation 1480 meters. In addition, the dams have kept accumulated sediment in the reservoir at a nearly constant level. In November 1998 this level was 2.27 million cubic meters; in January 2005 it was 2.3 million cubic meters.
About 20,000 cubic meters of sand is excavated from behind these check dams each year. One cubic meter of water generates 5.4 kilowatt-hours (kwh) of energy, thus about 108,000 kwh of energy is generated as a result of this removal, which is about 0.06 percent of the annual capacity of Kulekhani No. 1.
Check Dam D-0 also has produced the desired results. It reduced the riverbed slope from 3.3 percent to 1.3 percent, resulting in less sediment inflow into the reservoir from Darkot River.
By Rajendra Prasad Hada, project manager for the Kulekhani Disaster Prevention Project, Nepal Electricity Authority. He may be reached at 121 Clements Street, #1004, Angleton, TX 77515 United States; (1) 979-849-5562; E-mail: [email protected].