Annex 7: Geological Survey Report Lower Jhimruk Project (W-05)
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Annex 7: Geological Survey Report Lower Jhimruk Project (W-05) Nationwide Master Plan Study on Storage-type Hydroelectric Power Development in Nepal Table of Contents INTRODUCTION .............................................................................................................................................. 1 1 LOWER JHIMRUKPROJECT ................................................................................................................ 1 1.1 GEOLOGY OF PROJECT AREA ................................................................................................................... 1 1.2 DAM AXIS ................................................................................................................................................ 2 1.3 RESERVOIR .............................................................................................................................................. 3 1.4 POWERHOUSE .......................................................................................................................................... 3 1.5 WATERWAY ............................................................................................................................................. 3 1.6 INSTABILITIES ........................................................................................................................................ 17 1.7 CONSTRUCTION MATERIALS .................................................................................................................. 17 1.8 FURTHER RECOMMENDATIONS .............................................................................................................. 17 1.9 GPS SURVEY AND PHOTOS .................................................................................................................... 17 Final Report Appendix 5 SEA Annex Annex 7-i Nationwide Master Plan Study on Storage-type Hydroelectric Power Development in Nepal Introduction This Report deals with the engineering geological investigations in the Lower Jhimruk hydroelectric dam projects. This project area was surveyed by Dr Subesh Ghimire, Mr Pratap Bohora, and Mr Ajit Sapkota between 10 and 16 June 2012. Geology and engineering geology of this project are summarized below. 1 Lower JhimrukProject The Lower Jhimruk Reservoir Project is located in Pyuthan and Arghakhanchi Districts of mid-western Nepal. The project covers middle reaches of the Jhimruk Khola. The dam site is proposed near the Chheda village and the powerhouse lies in the lowe reaches of the Jhimruk Khola. 1.1 Geology of project area Figure 1.1 Regional geological map in and around the project area. The project area is comprised of metasedimentary rocks of the Lesser Himalaya with slates, sandstones, limestones, and quartzties as dominant rock types. The geological map prepared by the Department of Mines and Geology of Government of Nepal (Figure 1.1) divides the lithology of the Lesser Himalaya of this area into five formations namely: Sangram Formation, Swat Formation, Syangja Formation, Lakharpata Formation and Galyang Formation separated from the Siwalik rocks in the south by the Main Boundary Thrust. The location of the observation points in the area is shown in Figure 1.2 and Table 1.1. Lithology: The main lithology is comprised of gray to dark, laminated shale with fewer intercalation of gray, medium grained, medium bedded sandstone at the lower part (Figure 1.3). This lithology is comparable with the Ranagaon Formation of Salyan Area (Dhital and Kizaki 1984). The lithology changes to a thick succession of thick bedded, pink, coarse grained sandstone and red-green, laminated shale of Khamari Formation. Sedimentary structures like ripple marks (Figure 1.4) and mud cracks are abundantly Final Report Appendix 5 SEA Annex Annex 7-1 Nationwide Master Plan Study on Storage-type Hydroelectric Power Development in Nepal found in this formation. Further north, the Khamari Formation is overlain by Eocene Beds with predominant sandstones. The Eocene beds are truncated by a fault and the rocks of Khamari Formation repeats to the north. Finally the rocks of Khamari Formation are overlain by thick beds by gray dolomite of Dhurbang Khola Formation. The dolomite is characterized by dome shaped stromatolite (Figure 1.5). Small scale folds are abundantly found in the area (Figure 1.6) Quaternary deposits: Colluvial deposits, and debris flow deposits (alluvial fans) are found on the valley floors of the Jhimruk Khola its main tributaries. The colluvial deposits predominate on upper moderately steep to gentle slopes and spurs. The alluvial deposits are distributed in the lower reaches of the Jhimruk Khola and they are less than 10 m thick. Rock mass condition: The dolomite in the upper reaches of the Jhimruk Khola is comparatively strong but it may suspect the water tightness due to the probable cavern structures. The rocks of Khamari Formation are strong and relatively less permeable whereas the Eocene rocks and the rocks of Ranagaon formation are moderately fractured.. The overall stability of the rock mass is good to very good. The resistant quartzites form steep cliffs, gorges, and sharp peaks, whereas the schist forms a smooth spur and ridge or gently valley. Weathering: Water and air are the main sources of weathering. Apart from them, sunshine (temperature variation) has also played a significant role in weathering of quartzites. The overall weathering depth in rocks reaches up to 5 m and sometimes more than that. Jointing: Three discontinuity sets are seen in the Lowr Jhimruk project area, the three sets are almost perpendicular to each other, especially in quartzites. The shales are characterized by very irregular joints. The joints are straight to slightly wavy in the quartzites, where their aperture ranges from less than 1 mm to 5 mm, but some joints are rather wide (up to 1 cm) and continue for several tens of metres. Stability conditions: The rock soil slopes in the project area are generally stable. The main source of sediments is the debris flow in the Jhimruk River from its tributaries, such as the Chirling Khola, Lasune Khola. Landslides are seen to occur on colluvial slopes and steep rock slopes (controlled by joints). Mainly debris slides, wedge slides, and plane rockslides are found. The highest flood level in the Jhimruk River is about 2.5 to 3 m from the general flow level. Owing to the frequent debris flows from the tributary streams, the water in the Jhimruk River becomes excessively turbid most of the time in the monsoon season. Hence, some measures are required to deal with this problem. On the other hand, it is clean and clear during the dry period. Groundwater conditions: The Jhimruk River, Chirling Khola, and other smaller tributaries are perennial whereas minor gullies are seasonal. The quartzites and shales are quite impervious and the dam constructed on them or the tunnel passing through them should not pose any serious water ingress problems. 1.2 Dam axis The proposed dam axis (Figure 1.7) lies in the pink, medum grained quartzite exposed on the left bank of the Jhimruk River, while on the right bank gray, laminated shale is observed (Figure 1.8). The proposed dam axis (Figure 1.8) has rock exposures on both banks. The rock is categorised as good or fair. Final Report Appendix 5 SEA Annex Annex 7-2 Nationwide Master Plan Study on Storage-type Hydroelectric Power Development in Nepal The area around the dam axis is stable and the Jhimruk River has a more or less straight course to the downstream of that area where the slopes are dry. The dam foundation and the area around the dam axis are impervious and do not pose any serious threat of seepage. It is proposed to construct a rock-fill dam, the quartzites exposed around Simpani and the Chheda village can be utilised to obtain boulders, and the red shales is available in the vicinity of the dam site, located about 2-3 km downstream. 1.3 Reservoir The water level (594 m) in the proposed reservoir extends up to Juda, Angri, Chukaha and Baireni. The engineering geological map of the reservoir and its surrounding areas is presented in Figure 1.9. The stability in the surrounding areas of the reservoir is relatively sound. There are no significant weak or vulnerable zones leading to dam collapse or large failures obstructing the reservoir. The area to be under water consists of bedrock with some minor instabilities. The deep, straight gorge is quite suitable for the dam construction. However, there will be significant sediment transport from the Lasune Khola and Cirling Khola. Hence some check dams and other sediment retention structures are required to prevent the sediment inflow in the reservoir. 1.4 Powerhouse The proposed powerhouse site is located on the right bank of the Jhimruk Khola on its alluvial terrace (Figure 1.10 and Figure 1.11). On the uphill side the lithology is comprised of light gray, red purple shale and orthoquartzite. The powerhouse site is quite suitable for the given project. 1.5 Waterway An intake tunnel (Figure 1.12) is proposed on the right bank of the Jhimruk River, about 3km upstream of the dam. The area is made up of quartzite it has a moderately steep rock cliff. The proposed tunnel passes basically through the quartzite and shales of Khamari Formation. Then it intersects a fault and enters the Eocene rocks. Then the rocks of Khamari formation repeats and are underlain by the rocks of Ranagaon Formation (Figure 1.13). The overburden along the proposed headrace tunnel varies from less than 50 m to