Puwa Khola -1 Hydro Power Pvt. Ltd

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Introduction

img1Puwa Khola-1 Hydropower Project (PKHP) is a Run-of-River (RoR) type project located in Barbote and Shanti Dada Village Development Committees (VDCs) of Ilam district, Mechi zone, Eastern Development Region, Nepal. PKHP is being developed by Puwa Khola One Hydropower Pvt. Ltd. (herein after referred to as the “Developer/Company”). The Developer has completed the Feasibility Study (FS) for the installed capacity of 5 MW on May 2009. Further, the feasibility study was updated on May 2014 downgrading the installed capacity from 5 MW to 4 MW and Power Purchase Agreement (PPA) was signed for the 4 MW installed capacity. Based on the updated feasibility study report (UFSR), the proposed headworks lies in right bank of Puwa Khola about 100 m downstream from the confluence of Puwa and Jhutre Khola. Desander and initial stretch of headrace pipe lies along the right bank while the major part of headrace pipe, surge tank, penstock and powerhouse have been proposed in the left bank. Geographically, the project area is located within the latitude 26°55’02” N to 26°56’58” N and longitude 87°53’46” E to 87°55’00” E (amended coordinates as per the letter of DoED dated 16 Bhadra 2071). Physiographically, the project area belongs to the Higher Himalayan region and lies between altitudes of about 900 m amsl to 1000 m amsl.

The project area can be accessed from Kathmandu-Charali, Jhapa in the East-West highway from where Mechi highway leads up to Chureghati, Ilam. The road distance from Kathmandu to Chureghati via east west highway is about 685 km. From Chureghati, about 4 km earthen road connects the project’s site office. The existing road needs to be upgraded and extended to join the proposed headworks (about 100 m) and the powerhouse site (80 m). Also, access road needs to be constructed in parallel to the headrace pipe alignment joining the head works and the powerhouse area (about 4 km) for the ease of construction work.

As mentioned above, PKHP has concluded its PPA with Nepal Electricity Authority (NEA) on 9-Magh, 2070 B. S. (January 23, 2014) for the power potential of 4 MW.Based on the updated feasibility study report (UFSR), the design discharge of the project is 4.5 m3/s at 40% of exceedance flow. A 35 m long and 8.3 m high trapezoidal shaped diversion weir has been proposed to divert the design discharge of 4.5 m3/s. The diverted flow passes through three numbers of intake (1.7 m x 1.3 m each) proposed on the right bank and is conveyed by a 22 m long approach canal to the two chambered settling basin (65 m long x 4 m wide x 4.6 m deep each). From the desander, headrace pipe with 1.6 m dia, 6 mm thickness and 3255 m length conveys water to a simple rectangular surge tank of dimension 5 m x 5 m and 15.4 m height. After the surge tank, a 1.6 m dia., 8 mm to 14 mm thick and 403 m long penstock carries the discharge to hit two units of horizontal axis Francis turbines accommodated in a 30 m x 20 m x 9 m (L x B x H) surface powerhouse. The design discharge of 4.5 m3/s has 40% probability of time exceedance and gross head is 117 m. With the design discharge and the head, an installed capacity of 4 MW can be achieved thereafter, the water will be discharged back to Puwa Khola via a 100 m long tailrace canal of width 2.4 m and height 1.9 m. According to the salient features included in the PPA, the power is evacuated by 6 km long, 33 kV single circuit transmission line to proposed Godak substation in Ilam district.

As Based on the independent verification of the annual estimated energy carried out during this study, the total annual energy is estimated to be as 23.11 GWh, of which dry and wet energies are 3.50 GWh and 19.6 GWh respectively.

The Puwa Khola-1 Hydropower Project geologically, lies in the Higher Himalaya Zone, Eastern Nepal, which is characterized by high grade metamorphic rocks. The rocks are highly metamorphosed and crystallized such as gneiss, hornblende-schist, augen gneiss, schistose quartzite etc., with granitic intrusions and schist. Higher Himalaya is sandwiched between South Tibetan Detachment System (STDS) in the north and Main Central Thrust (MCT) in the south. The MCT is the major regional thrust in the Himalaya which lies in about 5 km (aerial distance) south from the project site. This zone comprises mainly high-grade metamorphic rocks such as Kyanite-silliminite bearing gneisses, variegated schist and quartzite.

The headwork’s area lies in the alluvial terrace deposit. The pipe alignment has to pass through mostly alluvium, colluvial deposit with gentle to moderate slope and about 1.5 km of length through rock terrain and forest area with steep slope. The major rock types in the area are quartzite and gneiss. The pipe alignment has to cross Puwa Khola and five Kholsis. The surge tank area lies in the moderate to steep slope comprising of colluvial deposit. The penstock passes through moderate to gentle slope comprising of colluvial deposit in most of the area. Powerhouse area comprises alluvial deposit and boulder zone. The powerhouse foundation may encounter bedrock at about 4 m depth of excavation which comprises of banded gneiss

 

 Head works.

The proposed headworks area lies at moderately wide and flat area at about 100 m downstream from the Puwa and Jhutre Khola confluence. The proposed weir is comprised of exposed bedrock in the right bank and alluvial deposit in the left bank. The right bank is steeper than left bank hill slope.

The intake structure has been proposed in the bedrock in the right bank which collects the design discharge to convey to the settling basin via 22 m long approach canal.The intake and gravel trap requires rock cutting. The approach canal (canal between gravel trap and settling basin), settling basin and headrace canal are proposed along the cultivable land. A flat cultivated land consisting colluvial deposit is selected for settling basin. The selected site is suitable for the settling basin construction. A 35 m long proposed diversion weir is proposed in the alluvial deposit. The rock-outcrops are exposed at the uphill of both bank and about 2 to 3 m upstream of weir at bottom part along left bank. The right bank area comprises steep slope resembling resistive rock.

The exposed rock comprises fine-grained, creamy white coloured, slightly weathered, medium strong to very strong, thinly foliated quartzite with parallel lamination. The lenses of quartz are common in the area along the joint. The exposed rock upstream of weir indicates possibility of bed rock along the weir as well.

The alluvial terrace deposit is unconsolidated permeable material predominantly comprises light grey coloured, gritty, very coarse-grained, angular to sub-rounded, loose with low dry strength boulder, gravel mixed with silt clay. The soil is poorly graded boulders having gravel mixture with fines. The boulders size ranges from 20 cm to 2 m in the area.

Intake

The proposed intake will be built partially in river terrace (alluvial) deposit (on flat land) and exposed bedrock along hill-slope. The exposed rock comprises creamy white coloured, fine- grained, thickly to massive foliated quartzite. Most of the slope is covered with vegetation. The fresh rock mass strength is strong to very strong while weathered rock mass is weak. It is recommended to shift the intake slightly towards hill side such that the foundation of proposed intake shall embed in bed rock. The upstream exposed bedrock checks the velocity of the sediment forming point-bar deposit along its immediate downstream. Thus, forming river terrace where intake, gravel trap and settling basin area has been proposed be constructed. The alluvial terrace is unconsolidated material predominantly composed of boulder to gravely-sand deposit. The soil is light grey, very coarse-grained, loose with fines deposited by adjoining river. The size of the boulder ranges from 20 cm to 2 m.

Gravel trap, Approach culvert and Settling Basin

The proposed gravel trap and settling basin area lies at the right bank along the river terrace deposit. The right bank slope comprises of small talus deposit, which is covered by vegetation. The terrace deposit comprises unconsolidated material predominantly composed of boulder to gravely-sand deposit. The soil is dark grey coloured, coarse-grained, loose gravel with fines deposited by adjoining river. The size of the boulder ranges from 20 cm to 2 m which are of quartzite and gneiss.

Major issues in head works are:

  • Left bank of proposed weir comprises thick loose alluvial deposit
  • Dislodge rock mass were observed at the uphill side of proposed weir
  • Huge boulder of quartzite and gneiss in settling basin and gravel trap

 

Headrace pipe

 

It starts from the end of the settling basin and the initial stretch of about 160 m lies in the flat cultivated terrace in the right bank of the Puwa Khola. After that it crosses Puwa Khola and rest of the total of 3255 m long pipe lies in the left bank. Most of the alignment is buried and passes through the cultivated land. It crosses five major Kholsis and several minor Kholsis. Saddle supports have been proposed on the exposed part to support the pipe.

The proposed headrace pipe alignment crosses numbers of Kholsis, alluvium, colluviums deposit, spurs, valleys and even Puwa Khola in one section.

Settling basin to Puwa Khola Crossing

Alignment starts from the end of settling basin passing along the right bank of Puwa Khola and bend at CH 0+290 which lies in the Shanti Dada VDC ward no.9 immediate downstream to the newly constructed suspension bridge at Puwa Khola. The right bank comprises flat to very gentle slope. The most of alignment passes along the alluvium deposit comprising boulders, pebbles, cobbles on matrix of sand and silt. This is an old terrace buried with thin cover of slope wash-out materials. The area was considered as loose and fragile ground, which could be eroded during flooding.

The proposed headrace alignment crosses Puwa Khola in between bend 1 and 2, comprising in the alluvium deposit at the downstream of newly constructed suspension bridge of Barbote and Shanti Dada VDC. The deposit is recent alluvium.

 

Puwa Khola crossing to Simle Kholsi

The first stretch from the crossing comprises alluvial deposit, which is a flood plain deposit. The rock mass comprising creamy white colored, fine-grained, thickly foliated and weathered rock of quartzite was encountered at the steep slope from CH 580 to CH 600. Numbers of bends were proposed until the Simle Kholsi crossing, which comprises alluvial terrace deposit. The stretch is covered with loamy soil, which was cultivated.

Headrace pipe alignment crosses the Simle Kholsi at CH 1+400 which is the geological boundary between quartzite and gneiss. Morphology of the Kholsi comprises V-shape valley with stable steep (75-80o C) slope. Grey, fine to coarse-grained, thick to massively foliated,

Simle Kholsi crossing to surge tank

The headrace pipe alignment from Simle Kholsi to the surge tank has to cross number of Kholsi and spurs. Minor discharge was noticed in the most of Kholsi and slope of all Kholsi were stable comprising banded gneiss. The landform mostly comprises thin deposit of colluviums along the slope, which is covered by vegetation. Most of slope along the alignment seems stable beside few minor instability due to wet land. Debris deposits of different kholsi comprising of boulder, gravel, and sand with silty clay. The soil is light grey to dark grey coloured, gritty, very coarse-grained, angular to sub-rounded, loose with low dry strength.

The colluvial deposits comprises of gritty, dense, sticky, medium to stiff, silty gravel with clay mixed with boulders which are angular in shape. The soils in these area are mainly light grey to dark grey coloured, moist to saturated clayey to sandy silt with relatively high percentage of organic matter in the uppermost part and there may comprise bedrock at depth of about 50 cm to 2 m (estimation). Most of the pipe alignment has to pass along the moderate to steep slope, which comprises forest land and rocky terrain.

Toribari is a major Kholsi in the alignment and rocks are well exposed in and around the area. The alignment passes through overhang rocky outcrop toward surge tank from the Kholsi. The area comprises of intercalation of quartzite and gneiss covered with thin sheet of colluviums and vegetation. Colluvial deposit comprises of light brownish red colored silt clay with some small angular rock fragments. Some slope failures are also encountered in this section.

Finally the headrace pipe alignment is connected with surge tank that is proposed in rock outcrops. This stretch is stable as it is comprises rock outcrops with very thin colluvial deposits. On surface, sparse vegetation is present. This section consists of grey to dark grey, slightly to moderately weathered, medium to thick foliated, medium to coarse- grained banded gneiss.

Major issues in headrace alignment are:

  • Moderate slope comprises of about 2-3 m thick colluvial deposit.
  • Wet land (sim area), its effect on water way as well as support structure.
  • Rocky terrain with steep slope.
  • River and Kholsi crossing.

Surge Chamber or Forebay

The surge chamber will be made at the downstream end of headrace Pipe. Both Electrical Resistivity Tomography and test pitting has been done to know more on the ground condition of this area. The test pitting, a direct method to observe the below ground material has shown that the proposed location is occupied by thin to medium foliated, slightly to moderately weathered quartzite rock. Only about 0.5 to 5m top layer is loose colluvial deposit in that area. Thus, after removing this top soil the surge chamber can be housed in the rock mass.

 

The proposed Surge Tank area comprises thin colluviums deposit (1-3 m estimation) above the bed rock of gneiss interaction with quartzite with gentle to moderate slope (about 20º – 35º). The colluvial soil is brownish grey coloured, moist, silty clayey and rock fragments of quartzite and gneiss with relatively high percentage of organic matter in the uppermost part which is used for cultivation. The estimated thickness of the soil is about 5 m.

Major issues:

• Gentle slope which about 15º toward southern end of surge tank.

• Expert advice is recommended for slope stability during construction.

 

Penstock Pipe

The penstock alignment proposed along the left bank slope of the Puwa Khola-One will be located partly on boulder and partly on colluvial soil. The purpose penstock alignment can be divided into two sections on the basis of geological and geomorphologic condition.

 

First half section of the penstock lies in the moderate slope (around 30-45º) cover with thin sheet of colluvium deposit and fragment rock of quartzite. Deposit is brownish gray in colour of silty clayey, moist. Rock may encounter at about 1-3 m depth of excavation.

Second half section of penstock pipe lies in the gentle slope (20-30º), comprises of about 1-3 m thick colluvial deposit. The colluvial deposit comprises light grey to brownish coloured, moist clayey to sandy silt with relatively high percentage of organic matter in the uppermost part. The saddles of penstock lies in the spurs of colluviums deposit on the cultivated land. The Y- section of the penstock lies in the alluvium colluviums deposit comprising large size boulder of gneiss and quartzite.

Major issue along the penstock:

  • Moderate to steep slope at the upper section.
  • Thick colluviums deposit in the lower section.

 

Removing boulders from Y-section of the area. 
The last bend and y-wee junction lies at the talus deposit (bearing capacity of soil).

 

Powerhouse

 

Powerhouse and tailrace canal are proposed along the plain of Puwa Khola upstream of the confluence of Sarki Kholsi and Puwa Khola. Thus, flood protection wall extended in the upstream side as well so as to protect structures from the possible threat of flood. The proposed tailrace passes along the cut bank of alluvium deposit just upstream from the confluence between Puwa Khola and Sarki Kholsi.

The proposed powerhouse lies at the left bank of Puwa Khola at the cut bank. It will be founded on the flat and old alluvial terrace (it is mentioned as colluvial in the provided report). The flat terrace comprises scattered huge (ranging from 1-4 m) angular to sub rounded boulder of gneiss and quartzite. The deposits composed of boulders with sands and silt matrix. The thickness of alluvial deposits at the area of the powerhouse foundation is expected to be 5-6 m. The rock mass in the area are exposed in both banks and along the channel, which are light to dark grey coloured, thinly to thickly foliated, fine grained, slightly to moderately weathered, weak to strong rock mass of gneiss. Three set of discontinuities are present which are tight. Attitude of foliation plane in the area is 160º/35º.

Major issue along the powerhouse:

  • Removing huge boulders from the powerhouse area.
  • Powerhouse is proposed in the cut bank area.
  • Extension of proposed protection wall in upstream of river side.

 

Tailrace canal

The proposed tailrace passes along the cut bank of alluvium deposit end just upstream from the confluence between Puwa Khola and small stream flowing from left side. The outlet is just upstream from the confluence along the start of point bar deposit. The area comprises of huge boulders zone covering river terrace deposit like sand (less) gravelly material mixed with fines. Rock mass of gneiss is exposed to the river side at tailrace.

Major issue along the tailrace area:

  • Tailrace area is proposed in the cut bank.
  • Removing of large size boulders of gneiss and quartzite.