Kimathanka Arun HEP

 Background

Kimathanka Arun Hydro Electric Project (KAHEP) is located in Bhotkhola Rural Municipality-2, Sankhuwasabha District of Province-1 in North-Eastern Nepal. The project will utilize the flow from the Arun River for power generation. The generated power will be fed into the integrated power system of Nepal at the nearest national grid substation proposed at Haitar, Sankhuwasabha. The design discharge has been fixed at 143.5 m³/s and the turbine axis level has been further lowered to utilize the available gross head of 379.52m resulting in a project capacity of 454.07MW. The project is conceptualized as a Peaking Run-of-River (PRoR) scheme. The project generates total annual energy 2551.16 GWh that comprises of 1785.95 GWh of wet energy, 485.93 GWh of dry peak energy, and 279.28 GWh of dry off-peak energy.

Accessibility

The total distance from Kathmandu to the headworks site is 665 km. The Project area is accessible from Kathmandu by a black-topped road of 516 km upto Khandbari, followed by an additional 78 km length of earthen road upto Gola. This road falls under the Kimathanka – Jogbani Koshi Highway which will act as a bilateral trade route after its completion. Currently, the track up to Chyamtang Village has been opened, thus ensuring access to the powerhouse area. Track opening beyond Chayamtang towards headwork is being conducted by the Nepal Army.

Topographic Survey

Topographic survey works have been carried out to prepare topographic maps of the entire project area and its components in appropriate scale and details to work out the structural layouts to be surveyed and covered altogether an area of 420 ha at headworks, powerhouse, adit area, camp area and quarry area and a stretch along 29.7 km long access road and 18.5 km long transmission line.

Satellite image processing for generating DEM and contour has been carried out covering the entire project areas whereas Drone Survey has been carried out at headworks, powerhouse and camp areas.

The control points have been established based on a traverse survey with reference to 2 control points established through DGPS Surveying. All the major control points with monumentation have been established in concrete pillars with a cross marked iron bolt or with an iron pin set in rock or boulder or with crosses, chiselled on the boulders.

Hydrology and sediment

The Arun basin is located in the northeastern part of Nepal and southern part of Tibet within Saptakoshi Basin of Nepal. The Arun River originates from a glacier on the Northern slope of Mount Xixabangma in Tibet and is called Pum Qu in Tibet. The catchment area of the Arun Basin at Uwa Gaon outlet  and Kimathanka outlet area is  26,132.79 km² and 24,835.58 km² respectively.

The maximum and minimum mean monthly flow of 562.8 m³/s and 48.0m³/s are estimated in August and April respectively. 1 in 10,000 years flood equivalent to 4,570.5 m³/s is recommended as the design flood. Similarly, 1 in 5 years flood equivalent to 1,493.5 m³/s is recommended as the design flood for river diversion works during construction stage.

PMFs estimated  at the Headworks and Powerhouse location are 6,652 m³/s and  8,887 m³/s respectively. The impacts due to GLOF events in the project location shall be much less compared to impact of PMF due to attenuation.

The average total suspended sediment load from 1986 to 2010 is estimated as 6.61 million tonnes per year. With unmeasured bed load correction, the resulting average annual sediment load has been estimated as 8.26 million tonnes/year which is equivalent to 332.75  tonnes/CA/year.

Geology

The project area lies in the rocks of the Lesser Himalayan zones. In general, the foliation dips towards the southeast direction. The MCT is the major tectonic boundary oriented almost north-south in a direction from Kimathanka to Tumlingtar on both sides of the Arun River, exposed by the Arun Window. Thus, it constitutes several minor shear/weak zones and small-scale local faults that are parallel and oblique to the foliation plane. The main rock types in the project area are banded and augen gneiss and schistose gneiss. The rock is slightly to moderately weathered, moderately to very thickly foliated, and medium strong to very strong, having three plus random joint sets (dip direction/amount-130°/35°, 60°/66°, and 218°/54°). The headworks area and headrace tunnel, including the adit tunnel, are in banded and augen gneiss. The powerhouse and transformer caverns are in banded and augen gneiss. The surge shaft, pressure shaft, tailrace tunnel and main access tunnel are in schistose gneiss with banded gneiss. A quartzite band is in the project area as a marker horizon, but it does not intersect any of the structures of the project. 

The rock mass in the diversion tunnel, bypass tunnel, intake and dam is in class III and VI which is fair to extremely rock mass quality as per Q-system. At the dam axis, bedrock is confirmed at a depth of 65 m below the river bed. Both bank slopes consist of sound rocks with open joints at the surface. The shear/ weak zones fall in very poor rock mass quality.

The main soil types in the project area are alluvial, colluvial, alluvial fan deposits, and residual soil. The most abundant deposit type in the area is colluvium. The colluvial deposits are found at the bottom of steep to moderate slopes, at Chhumsur, Chyamtang, powerhouse area, and surge shaft area. Alluvial deposits are found mainly along the Arun River banks, in the form of recent river deposits and old terraces deposits. Residual soil is occasionally and sporadically observed in the area in the form of patches at Lingam, Chyamtang, and Chhumsur areas.  

Landslide damming is common in the upper stretches of the Arun River, and one historic landslide dam has been identified within the project area. It is interpreted that the thick alluvial deposit encountered during the investigation have been a result of the sediment and debris deposited behind this landslide dam.

Project Description

KAHEP reservoir has a gross storage capacity of about 10.3 MCM and a length of about 2.7 km. It is designed with a live storage capacity of about 3.24 MCM. The live storage volume is selected such that it can satisfy a daily peaking energy demand of about 6 hours for a design discharge of 143.5 m³/s. Full supply level of the reservoir is 2035.0masl with a minimum drawdown level (MDDL) of 2025.0masl.

For the diversion of river during construction, a diversion tunnel along with an upstream and a downstream cofferdam have been proposed. A cofferdam of height 37.6m is proposed 580m upstream of dam axis and an 8.5 m high downstream cofferdam has been proposed about 300 m downstream from the dam axis. A horseshoe shaped diversion tunnel of finished diameter 11.0 m and length 974.9m capable of diverting about 1493.5 m³/s has been proposed. The inlet and outlet inverts of the diversion tunnel are set at 1988.0 masl and 1973.0 masl.

A side intake with invert level at 2010 masl has been proposed to convey the design discharge to the powerhouse. The intake is equipped with two sets of trashracks of size 8.0 m X 17.27 m (W X H) and, stoplog and vertical gate of size 8.0 m X 8.0 m (W X H). The maximum velocity at the trashrack and inlet gates is 0.79 m/s and 2.36 m/s respectively.

The diversion dam of KAHEP is proposed to be a roller compacted concrete (RCC) gravity dam, located between Khyokma and Chumsur Village. The proposed dam has a crest length of about 187.9 m and the deck level is set at 2040 masl while the existing riverbed level at the dam site is at about 1970 masl. From the current geotechnical investigation, the bed rock is expected about 65 m below the existing riverbed level at the deepest point. The maximum height of the dam at the deepest section is 140 m. The dam is equipped with 7 spillways with flip buckets at different levels of the dam body to safely pass the PMF. There are two main spillways of size 11 m x 16 m (W x H) with crest at 2020 masl and three sluice spillways of size 6.5 m x 9 m (W x H) with crest at 1995 masl. A trash evacuation spillway of size 6 m x 3 m (W x H) with its crest at 2033 masl is located on the right side of dam to pass the floating debris. In addition, an unregulated auxiliary overflow spillway of length 20 is provided on the left side of the dam with its invert at 2035 masl.

A circular concrete lined headrace tunnel (HRT) of 8.0m finished diameter is designed to convey the water from the intake to the pressure tunnel. The length of the tunnel is 5918m. To avoid any effect of water hammer in the water conveyance system, the HRT is connected to a restricted orifice surge shaft of 18 m diameter and 59.78 m height.

A pressure tunnel is proposed to convey the water from the headrace tunnel to the powerhouse. The pressure tunnel consists of three portions: upper horizontal tunnel, vertical shaft and lower horizontal tunnel. The pressure tunnel up to the point of first bifurcation has a diameter of 5.75 m, thereby reducing to 5.0 m, 4.0 m, 2.9 m and finally to 2.4 m diameter penstock, which conveys discharge to each unit of the turbine.

An underground powerhouse of dimension 117.5 m X 19.5 m X 40.1 m (L X W X H) has been proposed as the main powerhouse. It is equipped with four Pelton turbines each with a generating capacity of about 113.5 MW and auxiliary and ancillary facilities for control and protection. The powerhouse is four-storied in which every floor can be assessed by two elevators and three staircases. Similarly, an underground transformer cavern of size 102.4 m X 13.0 m X 27.6 m (L X W X H) has been proposed with 13 single phase transformers including 1 spare. The water from the tailwater pit is conveyed through a Tailrace Tunnel (TRT) of 9.0m size. The tail water level at Arun River is at 1652 masl.

An 18.5 km long transmission line with a transmission voltage of 400 kV with double circuit twin Moose configuration will evacuate the power of this plant by connecting it to 400 kV Kimathanka Arun substation proposed at Haitar, Sankhuwasabha.

Construction Schedule and Planning

Total duration for the construction of project is estimated around 60 months from the date of the commencement of work.

SALIENT FEATURES