A geological report for Hydropower Project

 

A geological report on

Status of

Lankhuwa Khola Hydroelectric Project

In

Sankhuwasabha

December, 2018

Kathmandu

 

 

Evaluation of Geological Studies

 

Geomorphology

The study area is characterized by the medium to low topographic relief with ridges, medium to very steep slope, rock cliffs, river terraces and narrow river valley. Lankhuwa Khola follows from northwest to southeast in the project area. The highest areas of the surrounding project site ranges upto 1250m above mean sea level (amsl). The headworks area situated at around 690 m and the powerhouse site is located at around 560 m. Slope of the headworks area is 25° to 35°at the left bank and 30° to 50° at the right bank. Similarly, slope of the powerhouse site is 30° to 45° at the left bank of the Lankhuwa Khola. Overall drainage pattern of the study area is essentially dendritic. The general layout of project site is shown in Figure

Figure 1: Google Earth image of the project area.

 

 

Figure 2: Topographic map of the project area.

 

Regional Geology of the Study Area

Nepal Himalaya comprises central portion of the 2400 km long Great Himalayan arc with total span of 800 km from east to west. The Himalaya is divided into 5 tectonic Sub-Division according to their evolution and stratigraphic characters.

The purposed LHEP lies in the Lesser Himalaya, Eastern Nepal. The area is mainly composed of quartzite, slate, schist as well as granite in the Lesser Himalaya. The lithostratigraphy of the area is shown in table below. Also, the regional geological map of the area is shown.

 

 

 

 

 

 

Table 1: Lithostratigraphy of Lesser Himalaya, Eastern Nepal (after DMG, 1973)

Zone	Liho unit	Main Lithology	Thickness (m)	Age
Main Central Thrust (MCT)
LESSER HIMALAYA	Lakharpata Formation	Limestone/ dolomite	500-1000	Pre-Cambrian to Palezoic
	Kushma Formation	Quartzite/ Shale/ dolomite	800
	Galyang Formation	Slate	1000
	Naudanda Quartzite	Quartzite	400
	*Ranimatta or Seti Formation	Phyllite, Quartzite	3000+
	*Amarpur and Kabeli Granite Intrusion	Granite	500	Pre-Cambrian
Main Boundary Thrust (MBT)

# Rock exposed in the Project area

 

 

 

Figure 3: Regional Geological Map of the area.

 

Geological Description of the Project Area

The proposed Project area belongs to high metamorphic rocks of the Lesser Himalaya, mainly gneiss and schist. The geological map of the project area is shown below which shows the dominant lithology of gneiss in the area. The detail description of the project area is given below.

Figure 4: Geological Map of the project area (DMG)

 

Headworks Area

The headwork of Lankhuwa Khola is located just above the confluence between Orang Khola and Lankhuwa Khola. Bed rocks are exposed on the right bank. The exposure consists of dominantly gneiss. Many boulders of gneiss, schist and quartzite are seen around the area. The orientation of beds is 35°/35°. Three joint sets dipping NW and SW are observed with average 15 and 55 degrees.

The headwork of Orang Khola is located on the left bank of Orang Khola.  The exposure in the area consists of dominantly gneiss. Also, huge boulders of gneiss, schist and quartzite are seen around the area. The orientation of the beds is 135°/35° and 125°/30°.Two joint sets are observed in the area dipping SE with average of 80 and 18 degrees.  

 

 

 

 

Settling Basin area

The proposed settling basin area at the right bank of the Lankhuwa Khola. The area is covered by colluvium deposits and river terrace. The area is mainly cultivated land consisting of pebbly and sandy soil with some boulders of gneiss.

Headrace Pipe Alignment Area

The proposed Headrace pipe alignment follows along the left bank of Lenkhuwa Khola above bad below the tunnel. The pipe alignment passes through gneiss bed rock, alluvium and residual soil.

Just below the settling basin, the area consists of huge beds of gneiss up to the tunnel inlet area. After the outlet the pipe alignment passes through steep cliff and gneiss beds are observed at the river bank. After the cliff, the alignment passes through colluvial and alluvial deposits consists of boulders of gneiss, schist and quartzite and also river teraace. Just above the surge tank area the pipe alignment passes through residual soil which is cultivable land consisting of pebbly and sandy soil with some gneiss boulders. Gneiss beds are located below the residual soil observed in some small river tributaries.

 

 

 

Tunnel Inlet and Outlet Area

The proposed inlet is located along the left bank of Lankhuwa Khola. This area is covered by thick beds of gneiss. The exposure observed are slightly weathered and joint sets are observed in the upper part. The orientation of beds is 85⁰/15⁰.

 

 The outlet area lies on the left bank of Lankhuwa Khola just opposite of the old suspension bridge. The area consists of huge beds of gneiss with orientation 45⁰/22⁰.

Surge Tank and Penstock Alignment Area

The proposed surge tank area consists of residual soil with some rock fragents. The area is cultivable land.

The penstock alignment area is located on the left bank of Lankhuwa Khola and crosses the Pikhuwa Khola. Initially the area passes through colluvial and residual soil and at the right bank of Pikhuwa Khola gneiss beds are observed. Also, the area is covered by residual soil and gneiss boulders.

 

 

 

 

Powerhouse Area

Powerhouse is proposed to be built on left bank of Lankhuwa Khola. Bed rocks are not exposed in this area and consists of many boulders of gneiss covered by alluvial deposit.  The colluvial material consists of quartzite, gneiss, schist, with diameter ranging from 2-10m. Near the power house area, gneiss beds are observed with orientation 75⁰/18⁰. Also two joint sets dipping NE with average of 30 and 75 degrees.

 

Seismicity Evaluation

Nepal on a regular interval witnesses earthquake along the major active faults in east-west alignment. Historical data and ongoing seismological studies have clearly indicated that the entire region of Nepal is prone to earthquake and it lies in the active seismic zone V. It is evident that the seismic pattern has geographically divided into three clusters of events; viz: western, central and eastern Nepal. The proposed Upper Phawa Khola Hydroelectric Project lies in the seismic zone 2 of the Nepal Himalaya. The soil of the foundation at the dam site belongs to average soil type is quartzite. Therefore, the basic horizontal seismic coefficient is considered to be 0.50. By using the above empirical method, the effective design coefficient according to the seismic design code of Nepal is given by the equation:

Aeff = R*Amax/980

Where, Aeff is effective design seismic coefficient

R = Reduction factor (empirical value R=0.50-0.65)

For the minimum acceleration of 250 gal, reduction factor of 0.50 the calculated effective design seismic coefficient is approximately 0.13.

For the maximum acceleration of 300 gal, reduction factor of 0.50 the calculated effective design seismic coefficient is approximately 0.15.

Hence, the design horizontal seismic coefficient ranges from 0.13 to 0.15 (calculated values). Based on above results the design seismic coefficient for the Project can be taken in the range of 0.13 to 0.15 which is more or less same value represented from the return period of the earthquake. If the structures fall on the different types of the soil (residual, colluvial and alluvial soil), the recommended values of the PGA can be increased by 20%.

Figure 5: Seismic Hazard Map of Nepal

 

 

 

 

Conclusions

Geologically, the Project area belongs to rocks Lesser Himalaya, Eastern Nepal. Thick alluvial deposits are found around the settling basin area and power house area. The surge tan is covered by colluvial deposits and residual area. The present geological investigation shows the good general geological condition of the project area. No major instabilities are observed during the present study besides some steep cliffs.

 

Recommendations

−        Bank erosion should be minimized in the headworks and settling basin area.

−        ERT is recommended for tunnel section Powerhouse area and around Headworks.

−        Tunnel portion can also be replaced by pipe alignment.