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EXECUTIVE SUMMARY Introduction
Nepal is endowed with vast water resources potential but only tiny fraction of technically and financially viable project are harnessed so far from Government of Nepal and through private sectors. Stef Energy India P. Ltd (herein referred to as Client) is established to develop TR-SGP Hydroelectric Project (TR-SGP) and the Upgraded Feasibility Study service was awarded to SGP IMASS (SGP) (herein referred to as Consultant). As a part of the agreement, SGP IMASS has carried out the Feasibility Study of the said project located in Richowktar, Malekhu, Dhading, Central Development Region, Nepal.
This Feasibility Study Report is prepared by the Consultant as per the Terms of Reference (ToR). Findings from the literature review, site visit, topographical survey, field investigation and analysis of data are the basis of preparation of this report. The feasibility study formed a basis whether or not to conceive the TR-SGP a feasible project to develop.
Technical
A design discharge of 127.91 m3/s for power generation is diverted through a side intake located some distance upstream from the diversion weir. The project is conceived as a small regulating pond created by constructing about 12 m high concrete dam. The head pond will be extended to a distance of about 2 km upstream from the proposed dam. The dam is envisaged to be founded on bed rock but do not have high quality. There are 6 gates will be construction in the dam itself so that there will be free flow spillway type diversion dam available. The extreme flow to be handled is extensively high and thus ungated overflow spillway is designed. The gates will be operated from left bank. Both banks has exposed rock.
The intake will have a flood wall and collection chamber just behind to ascertain smooth transition of flow from intake to bell mouth of the headrace tunnel. The flow than convey through a headrace tunnel of diameter of 8 m finished surface, 3730 m long to the powerhouse. An intermediate adit is located at Bhante Khola, which infact the tunnel is opened at Bhant Khola. Level crossing is proposed to connect the tunnel heading.
Orifice type surge shaft, 20 m at top and 10 m at bottom is designed to address surge effect of the system. A relatively short penstock pipe is designed to convey design flow to the powerhouse. About 30 m long 7300 mm pipe will lead flow from the tunnel and will be branch off once it comes out outside the tunnel. The twin penstock pipe will be about 40 m long 5600 mm diameter will convey the flow to four units of turbine.
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The powerhouse is located in flat land and will be equipped with 4 units of vertical Shaft Type Kaplan Turbine. Each penstock pipe will feed two turbine units. Each unit will have 6.25 MW output and an annual energy generation is 163.32 GWh, with dry energy of 35.35 and wet energy of 127.97 GWh. The gross head of the plant is 29.5 m.
Geology of the tunnelling part is envisioned to be poor and thus a full permanent lining is felt necessary.
Generated power from the TR-SGP will be evacuated through the 132 or 220 kV planned T/L to Naubise where the interconnection point will be at Gomati, Benighat, Dhading, which is about 750 m from the proposed powerhouse.
Environment
An IEE of the project has been approved on Feb 2011 and the findings and observations show that there are no adverse environmental impacts by this project. However, a mitigation measures as proposed in the IEE report should be implemented to ascertain sustainable development of the project.
Economical and financial analysis
Economical and financial analysis of the TR-SGP has been carried out to ascertain the economical and financial viability of the scheme. The technical feasibility of the scheme has been established through studies carried out on each of those aspects. Furthermore, the project cost includes separate costs for major mitigation costs associated to the TR- SGP, land acquisition and community developments in the project area.
The analysis has been carried out based on marginal cost benefit approach for the adopted scheme based on economic and financial parameters deemed to be realistic and standard for analysis of this nature. A sensitivity analysis has been carried out by varying some of the assumed economic and financial parameters.
Economical analysis of the project was also carried out to ascertain economical prospective from the nation. It was looked a project life span of 50 years excluding taxes, royalties, but with the inclusion of very tentative benefit accrued from the project. It is extremely difficult to quantify the exact benefit that is accrued from the project since people living in and around the project area will be benefited in many respects during construction of the project too.
The total project construction cost is estimated as MNRs 5,489.62. For a 70:30 debt equity ratio and 7% interest over debt financing, the total project financing cost is comes out as MNRs 5,864.68. Construction period is assumed to be 3 years. The financial
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The sensitivity analyses shows that the project is financially and economically sound if two or more different unfavourable conditions as discussed in sensitivity analysis will not occur simultaneously.
Feasibility study report structure
The Updated Feasibility Study Report is prepared as the last deliverable for the study conducted by the Consultant. It complies and presents data/information collected, findings, project layout, main design parameters and financial indicators of the project. The report comprises of main report including appendices to support main report. The feasibility report is presented in the following format:
Update Feasibility Study Report
! Volume I Main Report
! Volume II Detail Geological Report
! Volume III Annexure
• Annex A Data on Hydrology
• Annex B Cost Estimation
• Annex C Financial Analysis
! Volume IV Appendix E Drawings
! Volume V Field Investigation Report
Conclusions
The 25 MW TR-SGP project is found technically feasible, economically and financially viable, and environmentally friendly. The headworks area of the project is almost close to the all season blacktopped road. All key project areas are connected to main road by track opening, a minor upgradation will avail accessibility to the project site.
Recommendations
The project is recommended to pursue further. However, following activities are recommended during different development stages of the project:
! The TR-SGP is found technically sound financially attractive. However, wherever possible two extreme cases should be avoided to ascertain better project attraction.
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! The uncertainties as identified during filed geotechnical filed investigations are recommended to be pursued during detailed design phase of the project.
! It is highly recommended to establish a gauging station most likely around powerhouse area and later on move it upstream of dam, once the project is decided to implement.
! Minimum three sets of permanent points to be established at key project area and the points are to be transferred from 2nd and 3rd order grid available in the area;
! Model study of the project is suggested to carry out during detailed design phases of the project.
! Customised design will be required for the powerhouse electromechanical equipment in order to minimise the cost of powerhouse and also to other components of the power plant. It is thus recommended to award the Electro- mechanical contract to a highly experienced manufacturer/supplier.
! Preliminary assessment on power transmission to 132 or planned 220 kV transmission line passing close by the project showed a better option to the project financial attractiveness. This issue should be discussed with NEA and sorted out the possible power evacuation mode of the project. The study, however, recommend to evacuate the generated power at Gomati using PI connection.
! Regarding environmental issues, there could be only short-term problems may be encountered during construction of the project. An effort has been made to minimise those problems in the design. Moreover, additional issues that may be raised by the IEE study will be incorporated in the detail design and construction phases. The cost associated with those short terms problem is estimated and included in the project cost estimation.
! Pre-construction activities such as financing negotiations and documents for detailed design stages should be initiated.
! A full risk analysis for project life cycle (RAMP) should be carried out to determine the associated risk and its effects on the project before detailed design.
Table of contents
Page no.
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EXECUTIVE SUMMARY ...... I List of Figures ...... x List of Plates ...... xi List of Tables ...... xi List of Abbreviations/Acronyms ...... xiii 1. INTRODUCTION ...... 1 1.1 Background ...... 1 1.2 Objective ...... 2 1.3 Scope of services ...... 3 1.4 Study approach ...... 3 1.5 Codes and standards ...... 4 1.6 Project description ...... 4 1.7 Report format ...... 5 1.8 Salient Features ...... 5 2. PROJECT AREA ...... 10 2.1 Location ...... 10 2.2 River basin ...... 10 2.3 Basin physiography ...... 10 2.4 Climate ...... 11 2.5 Access ...... 12 2.6 Transmission line ...... 12 3. TOPOGRAPHY AND CARTOGRAPHY ...... 13 3.1 Background ...... 13 3.2 Available maps ...... 14 3.3 Survey and mapping ...... 14 3.3.1 Traverse Survey ...... 15 3.3.2 Control Survey ...... 15 3.3.3 Detailed survey ...... 16 3.3.4 Topographic mapping ...... 17 3.4 Errors and accuracy ...... 17 3.5 Future survey work ...... 17 4. GEOLOGY AND GEOTECHNICAL ...... 18 4.1 Introduction ...... 18 4.2 Regional Geology of the Project Area ...... 18 4.3 Previous Studies ...... 19 4.4 Detail Geological Studies of the Project Area ...... 19 4.4.1 Reservoir Area ...... 20 4.4.2 Weir Axis Area ...... 20 4.4.3 Portal Inlet Area ...... 20 4.4.4 Tunnel Alignment Area ...... 21 4.4.5 Surge Tank and Penstock Alignment Area ...... 21
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4.4.6 Powerhouse and Tailrace Area ...... 21 4.4.7 Adit Area ...... 21 4.4.8 Access Road from Intake to Powerhouse Area ...... 22 4.5 Results of the 2D-Electrical Resistivity Survey ...... 22 4.6 Geotechnical Studies of the Project Area ...... 22 4.6.1 Reservoir Area ...... 23 4.6.2 Weir Axis Area ...... 24 4.6.3 Tunnel Alignment Area ...... 26 4.6.4 Assessment of Initial Support Requirement for Tunnel Alignment ...... 27 4.6.5 Surge Tank/Penstock Alignment Area ...... 28 4.6.6 Powerhouse and Tailrace Area ...... 29 4.7 Seismicity ...... 30 4.8 Construction Materials Survey ...... 30 4.9 Borrow Area ...... 30 4.10 Laboratory Test ...... 31 4.11 Spoil Disposal Area ...... 32 4.12 Conclusions and Recommendations ...... 32 4.12.1 Conclusions ...... 32 4.12.2 Recommendations ...... 33 5. HYDROLOGY AND SEDIMENTOLOGY ...... 35 5.1 General ...... 35 5.2 Trishuli River Catchment ...... 36 5.2.1 Catchment Phisiography ...... 36 5.2.2 Drainage ...... 38 5.2.3 Climate ...... 40 5.2.4 Ecology and geology ...... 40 5.2.5 Water use ...... 40 5.3 Available data ...... 41 5.3.1 Stream Flow Data ...... 41 5.3.2 Rainfall data ...... 41 5.4 Flow measurement ...... 42 5.5 Hydrological Analysis ...... 43 5.5.1 Discharge Measurement ...... 43 5.5.2 Methodologies for ungauged catchment ...... 43 5.5.3 Catchment Correlation ...... 43 5.6 Long-term Hydrology ...... 44 5.6.1 General ...... 44 5.6.2 Mean Monthly Flow ...... 44 5.6.3 Flow duration curve (FDC) ...... 47 5.7 Extreme Hydrology ...... 48 5.7.1 Low flow ...... 48
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5.7.2 Flood flows ...... 49 5.8 Rating curve ...... 49 5.9 Sedimentology and mineralogy ...... 50 5.9.1 General ...... 50 5.9.2 Sources of sediment ...... 50 5.9.3 Sediment analysis ...... 53 5.9.4 Mineralogy ...... 54 5.10 Conclusions ...... 55 6. WATER STAKEHOLDERS ...... 57 6.1 General ...... 57 6.2 Irrigation requirement ...... 57 6.3 White water rafting ...... 58 6.4 Environmental release ...... 58 6.5 Conclusion ...... 58 7. OPTIMISATION STUDY ...... 59 7.1 Background ...... 59 7.2 Options considered ...... 59 7.2.1 Diversion Weir Crest Elevation ...... 59 7.2.2 Flow exceedence level ...... 60 7.2.3 Waterway ...... 63 7.2.4 Penstock pipe ...... 64 7.2.5 Number of turbine ...... 64 7.2.6 Head loss ...... 64 7.3 Conceptual design ...... 65 7.4 Plant Capacity ...... 65 7.5 Possibility of using headpond as PRoR ...... 66 7.6 Transmission line ...... 66 7.7 Conclusion ...... 66 8. SELECTED PROJECT ...... 68 8.1 General ...... 68 8.2 Headworks design criteria ...... 68 8.2.1 General ...... 68 8.2.2 Principal of headworks design ...... 69 8.2.3 Design assumption ...... 70 8.2.4 River diversion ...... 70 8.2.5 Diversion weir ...... 72 8.2.6 Head pond ...... 73 8.2.7 Stilling basin ...... 73 8.2.8 Under sluice ...... 73 8.2.9 Intake ...... 74 8.2.10 Collection Chamber ...... 74
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8.2.11 Settling basin ...... 75 8.3 Waterway ...... 75 8.3.1 General ...... 75 8.3.2 Headrace tunnel ...... 75 8.3.3 Intermediate Adit ...... 79 8.3.4 Bhante Khola Crossing ...... 79 8.4 Surge Shaft ...... 80 8.5 Penstock ...... 81 8.5.1 General ...... 81 8.5.2 Support piers/anchor blocks ...... 82 8.5.3 Surface drain ...... 83 8.5.4 Slope stabilisation ...... 83 8.6 Powerhouse ...... 83 8.6.1 General ...... 83 8.6.2 Main powerhouse floor ...... 84 8.7 Tailrace ...... 85 9. ELECTRO-MECHANICAL STUDY ...... 87 9.1 General ...... 87 9.2 Turbine type selection ...... 87 9.3 Design Criteria ...... 89 9.3.1 Governor ...... 89 9.3.2 Turbine inlet valve ...... 90 9.3.3 Other system accessories ...... 90 9.3.4 Lifting arrangement (EOT Crane) ...... 91 9.3.5 Cooling water and service water system ...... 91 9.3.6 Drainages and dewatering system ...... 91 9.3.7 Compressed air system / Nitrogen (N2) accumulator ...... 92 9.3.8 Oil treatment and transfer system ...... 92 9.4 Mechanical workshop ...... 93 9.5 Ventilation and air conditioning system ...... 93 9.6 Fire fighting and protection system ...... 93 9.7 Diesel generating set ...... 93 10. HYDRO-MECHANICAL STUDY ...... 94 10.1 General ...... 94 10.2 Steel Penstock ...... 94 10.3 Gates, Stoplogs and Trashrack ...... 94 10.4 Gates and Stoplogs ...... 95 11. TRANSMISSION LINE ...... 97 11.1 Background ...... 97 11.2 Objectives and scope of work ...... 97 11.3 Output ...... 97
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11.4 Methodology for study ...... 97 11.4.1 Basic Approach ...... 97 11.4.2 Desk study ...... 98 11.4.3 Data collection ...... 98 11.4.4 Interconnection with the national grid ...... 98 11.4.5 Route selection ...... 99 Route selection criteria ...... 99 11.5 Transmission line design ...... 100 11.5.1 Transmission voltage level selection ...... 100 11.5.2 Design criteria ...... 100 11.5.3 Conductor Size ...... 101 12. ENVIRONMENTAL ASPECTS ...... 102 12.1 General ...... 102 12.2 Projected environmental impacts ...... 102 12.2.1 Adverse impact ...... 102 12.2.2 Beneficial impacts ...... 104 12.3 Consideration of alternatives ...... 105 12.4 Mitigation measures ...... 105 12.5 Environmental Management Plan (EMP) ...... 106 12.5.1 Environmental monitoring plan ...... 106 12.5.2 Environmental auditing plan (EAP) ...... 107 12.6 Cost of recommended mitigation measures ...... 107 12.7 Conclusions ...... 108 13. CONSTRUCTION PLANNING AND SCHEDULING ...... 109 13.1 General ...... 109 13.2 Access road ...... 110 13.3 Material supply ...... 110 13.4 Construction activities ...... 113 13.4.1 River diversion ...... 113 13.4.2 Headworks ...... 113 13.4.3 Switchyard ...... 115 13.4.4 Electromechanical equipment ...... 116 13.4.5 Transmission line ...... 116 13.5 Construction power ...... 116 13.6 Construction material ...... 116 13.7 Contract package and construction schedule ...... 118 13.7.1 Contract package ...... 118 13.7.2 Construction schedule ...... 118 14. COST ESTIMATION ...... 120 14.1 General ...... 120 14.2 Unit rate analysis ...... 120
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14.3 Assumptions ...... 120 14.4 General Methodology ...... 121 14.4.1 Main civil works estimate ...... 122 14.4.2 Electro-mechanical equipment ...... 122 14.4.3 Penstock and hydro-mechanical ...... 122 10.1.1 Switchyard and transmission line ...... 122 14.4.4 Engineering and Administration Costs ...... 123 14.4.5 Owner’s Cost ...... 123 14.4.6 General Items ...... 123 14.5 Contingency sums ...... 123 14.6 VAT/taxes and duties ...... 124 14.7 Project cost estimate ...... 124 15. FINANCIAL ANALYSIS ...... 126 15.1 General ...... 126 15.2 Project evaluation ...... 127 15.2.1 Assumptions ...... 127 15.2.2 Project benefits ...... 128 15.3 Monthly energy ...... 129 15.4 Sensitivity analysis ...... 129 15.4.1 Variation on discount rate ...... 130 15.4.2 Variation on interest rate ...... 130 15.4.3 Cost variation ...... 131 15.4.4 Variation in energy ...... 131 15.5 Economical analysis ...... 131 15.6 Conclusion ...... 132 16. CONCLUSIONS AND RECOMMENDATIONS ...... 133 16.1 Conclusions ...... 133 16.2 Recommendations ...... 136 REFERNCE ...... 138
List of Figures Page no. Figure 1.1 Project location in river diversion network ...... 2 Figure 4.1 Regional Geological Map along the Trishuli River around Malekhu Area (after Stocklin and Bhattarai, 1977) ...... 18 Figure 4.2 Satellite Map of the Project Area (Source Google Map) ...... 19 Figure 5.1 Location of the Project Area in River System ...... 37 Figure 5.2 Graphical presentation of the mean monthly flow at the proposed H/W, m3/s ...... 45 Figure 5.3 Adopted mean monthly hydrograph at the proposed headworks ...... 47 Figure 5.4 Adopted Flow Duration Curve ...... 48
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Figure 5.5 Stage discharge curve just upstream of the proposed headworks area ...... 50 Figure 5.6 Stage discharge curve at the location of tailrace outlet ...... 50 Figure 7.1 FDC @ the proposed headworks area ...... 60 Figure 7.2 Flow and power duration curve at 30-percentile exceedence level of flow .... 61 Figure 7.3 Flow and power duration curve at 40-percentile exceedence level of flow .... 61 Figure 7.4 Flow and power duration curve at 50-percentile exceedence level of flow .... 62 Figure 7.5 Flow and power duration curve at 60-percentile exceedence level of flow .... 62 Figure 7.6 Preliminary optimisation of the plant at index price level ...... 63 Figure 7.7 Preliminary optimisation of the plant at index price level ...... 64 Figure 8.1 Upsurge and down surge scenarios for oscillation steps of 3 sec ...... 81 Figure 9.1 Turbine Type Selection Monogram ...... 87 Figure 11.1 Typical Loop-in-loop-out system ...... 99 Figure 11.2 Transmission Line Voltage Selection Using Still's Formula ...... 100 Figure 13.1 Construction schedule ...... 111
List of Plates Page no. Plate 3.1 Location of headworks area ...... 13 Plate 3.2 Survey team works in powerhouse site ...... 16 Plate 11.1 Location of intake and tunnel portal area ...... 102
List of Tables Page no. Table 1.1 Salient Features of the project ...... 5 Table 2.1 Geographical co-ordinates of the project ...... 10 Table 3.1 Available topographical maps ...... 14 Table 3.2 Coordinates of control points and permanent benchmarks ...... 15 Table 4.1 Rock Mass Classification ...... 22 Table 4.2 NGI Tunnelling Index ‘Q’ values of the Tunnel Alignment Area ...... 26 Table 4.3 Designed Tunnel Rock Support Class and Respective Rock Support ...... 27 Table 4.4 Assigned Rock Support in Respect with Rock Mass and Rock Support Class ...... 28 Table 4.5 Volume and Location of the Construction Materials ...... 31 Table 4.6 Summary of Results of the Construction Materials ...... 31 Table 4.7 Distribution of Muck Disposal Area ...... 32 Table 5.1 Division of catchment area ...... 37 Table 5.2 Catchment Characteristics According to the Topography ...... 38 Table 5.3 Gauging stations used to generate stream flow data at the proposed intake ... 41 Table 5.4 Rainfall stations in and around the catchment area ...... 41 Table 5.5 Rainfall stations in and around the catchment area ...... 42 Table 5.6 Catchment parameters ...... 43 Table 5.5.7 Mean monthly and annual flow, m3/s ...... 44 Table 5.8 Mean monthly flow (m3/s) at intake using MIP ...... 45
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Table 5.9 Adopted mean monthly flow at the proposed headworks, m3/s ...... 46 Table 5.10 Flow duration values for Trishuli River at the proposed headworks site ...... 47 Table 5.11 Low flows at the proposed intake site, m3/s ...... 48 Table 5.12 Adopted extreme flood flow at the proposed headworks ...... 49 Table 5.13 Flood flows at tailrace, m3/s ...... 49 Table 5.14 Sediment yield of different river system catchment ...... 51 Table 5.15 Sediment transport rates in some rivers in Nepal ...... 52 Table 5.16 Average monthly sediment loads, tons/km2 ...... 53 Table 5.17 Summary of mineralogical composition of sediment samples (Refer FS Study) ...... 54 Table 7.1 Energy and revenue generation ...... 65 Table 8.1 Tunnel inverts level at key areas ...... 76 Table 10.1 Gates and stoplogs ...... 95 Table 12.1 Cost Estimates for Mitigation Measures ...... 107 Table 13.1 Contract packages ...... 118 Table 14.1 Total project cost showing various items of the cost ...... 124 Table 15.1 Results of financial analysis at base case ...... 128 Table 15.2 Annual Energy Generation ...... 129 Table 15.3 Sensitivity Analysis on Varying Discount Rate for a fixed interest rate of 10% ...... 130 Table 15.4 Sensitivity analysis on varying interest rate ...... 130 Table 15.5 Sensitivity analysis on different project cost ...... 131 Table 15.6 Sensitivity analysis on varying currency inflation rate ...... 131 Table 15.7 Economical indicators of the project ...... 132
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List of Abbreviations/Acronyms
BOOT Build Own Operate and Transfer
DHM Department of Hydrology and Meteorology
DoED Department of Electricity Development
IMA SGP IMASS.
INPS Integrated National Power System km Kilometre kv Kilovolt m metre
MVA Mega Volt Ampere
MW Megawatt
NEA Nepal Electricity Authority
PPA Power Purchase Agreement
RFP Request for Proposal
SHC Santoshi Hydropower Company P. Ltd
TR-SGP Trishuli Hydroelectric Project
VDC Village Development Committee
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1. INTRODUCTION 1.1 Background Nepal is endowed with enormous water resources potential, which can be harnessed for hydropower generation including irrigation, water supply, and recreation and so on. It is estimated that about 42,000MW can be generated economically, technically and environmentally friendly way from numerous north-south flowing rivers and rivulets. However, from the latest information revelled that this potential has gone up to 57,000 MW. The advent of small hydropower development in Nepal dates back to 1911. However, yet Nepal could generate about 640 MW of hydropower to date which in reality is about 1% of the total potential. Only 40% of the total population has access to electricity. It is therefore, majority of the population are still using traditional sources of energy such as fuel wood, agriculture by-products and animal wastes to meet the growing demand. Although, fuel wood is still available free in many rural areas, the extensive uses of resources together with increasing population pressure have resulted in depletion of forest resources causing irreparable environmental damages. Looking this fact, only recently, the urge to develop more hydropower projects was felt and a number of projects have been launched.
Water resources is the most important natural resources and free gifts by nature available in abundance and it has been rightly considered a major source for the economic development of the country. Hydropower development could play an important role for the overall economic development of the nation through poverty alleviation. However, due to several technical and financial reasons, the momentum of hydropower development in the public sector could not be maintained because of low load factor and high operation and maintenance cost. The pragmatic policy decisions of government of Nepal that emphasise increased private participation in hydropower sector were implemented. The endeavour led to a rapid increase in private sector participation in the development of hydropower projects as well as increase in power purchase agreement with the public utility, Nepal Electricity Authority (NEA). To grab this unique opportunity, Santoshi Hydropower Company (SHC) was established and decided to participate in the hydropower development sector and there by contribute to the country’s economic growth. As its 1st venture, the Company has developed Thoppal Hydropower Project and this is the 2nd project in pipeline. SHC obtained the survey license from Development of Electricity Development (DoED) in 2005 to develop the TR-SGP Hydroelectric Project (TR-SGP). The TR-SGP is located at Malekhu, Dhading District, Central Development Region of Nepal. The location of the project site in the map of Nepal can be seen in Figure 1.1 below. Further development on the management of the project was taken by
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Stef Energy India Private (SEIP) Limited from beginning of the 2011. SEIP then decided to update the previous Feasibility Study and was asked the SGP IMASS to update the project FS.
Prefeasibility Study was conducted by SHC and study license was based on this proposition. Previous Feasibility Study was done by Infrastructure and Management Associates (SGP) in 2006. The SEIP has decided to continue the study through SGP IMASS and the contract was signed in April 2011 to update the project feasibility study. The report is an outcome of the study. Environmental Social study is beyond the scope of service and thus has not been covered except a brief summary for quantification of the project cost for preliminary level.
Figure 1.1 Project location in river diversion network
Project Location
The project area is located at Trishuli River, just downstream of the Thoppal – Trishuli River confluence at Malekhu, Dhading and is about 75 km from Kathmandu and on the way to Pokhara. The headworks site can be reached some 150 m upstream from the Arbastar-Richowktar suspended bridge over Trishuli River. The project site is accessible from all season roads. More importantly, there is 132 kV transmission line passing through the project area and thus it makes more attractive to evacuate the generated energy into the national grid.
1.2 Objective
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The objective of the TR-SGP study is to update the previous feasibility findings supported by data and information as available after completion of the Feasibility Study.
1.3 Scope of services The prime task of the Consultant is to update the previous feasibility study including the following; but not limited to:
! Review of previous FS;
! Topographic survey and mapping;
! Update and collection of hydrological, meteorological and sedimentological data;
! Geological and geo-technical field investigation works;
! Update project design, preliminary cost estimates and benefit evaluation to determine optimum scale of individual structures and the overall project capacity;
! Construction methods, planning and schedule;
! Cost estimation,
! Financial and economic analysis, and
! Preparation of updated feasibility study incorporating all data and result of the study.
1.4 Study approach Relevant literatures, maps, desk study report relevant to the project were collected, reviewed and analysed. Daily discharge data for the Trishuli River and Tadi Khola were collected from the Department of Hydrology and Meteorology (DHM) and then analysed to arrive at flood flows, low flows and long term monthly average flows. Moreover, flow measurements of the Trishuli River at the proposed project area were measured and validate with the transposed flow of the project.
Team of professionals were visited the site to acquire required data and information and at the mean time to select the best project alignment. Discussions with the local people were made. Drilling points and 2DERT for the geo-technical investigation of the project area were identified. Topographical survey of the key project area was carried out. The approved district rates, district profile and village profile for the district headquarter were gathered. Upon completion of data acquisition, project optimisation as well as project configurations were then proceeds and thereafter financial analysis of the project performed.
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1.5 Codes and standards The design works were carried out using engineering practices, relevant codes and lesson learnt from the similar projects constructed in the past. Relevant publications and literature were extensively used to produce technically feasible, economically sound and environmentally friendly design.
The consultant has used realistic cost estimation and appropriate construction methods suitable to local site conditions.
1.6 Project description The topography and terrain condition of the project area and study conducted by the Consultant determined an appropriate project configuration at the proposed project site.
The project is a run-of-the-river but snow fed type project in which water is diverted from the Trishuli River at a point about 1500 m downstream from the Thoppal Khola - Trishuli River confluence. A gravity structure is constructed as a diversion weir and it is raised about 8 m from the river bed to ensure more head. A maximum discharge of about 127 m3/s is diverted for power generation through a side intake on the left bank of the Trishuli River. The project location is shown in the district map of the Dhading district.
This project utilizes a gross head of 29.5 m in the Trishuli River between the normal water level at diversion weir at an elevation of 348 masl and the tail water in the Trishuli River at an elevation of 319.5 masl. The total length of waterways is about 3800 m whereas the tailrace canal will be about 100 m. Besides, the major components of the TR-SGP can be visualised as the combination of the following hydraulic structures:
! Diversion Weir,
! Side intake,
! Sluiceway,
! Tunnel,
! Gully crossing,
! Penstock,
! Powerhouse,
! Tailrace canal,
! 132 kV transmission line and substation
The existing track opening could be used as an access road to the project though it is little longer. The road is extended from Malekhu Dhading road. Construction of bridge
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right at diversion weir could shorten the road length significantly. Extension of concrete pier from the diversion weir will reduce the construction cost of the bridge significantly.
1.7 Report format The Updated Feasibility Study Report is prepared as the last deliverable for the study conducted by the Consultant. It complies and presents data/information collected, findings, project layout, main design parameters and financial indicators of the project. The report comprises of main report including appendices to support main report. The feasibility report is presented in the following format:
Update Feasibility Study Report
! Volume I Main Report
! Volume II Detail Geological report
! Volume III Annexure
• Annex A Data on Hydrology
• Annex B Cost Estimation
• Annex C Financial Analysis
! Volume IV Appendix E Drawings
! Volume V Field Investigation Report 1.8 Salient Features The salient features of the project are summarized in Table 1.1 below.
Table 1.1 Salient Features of the project
Descriptions Parameters
Project Name TR-SGP Hydroelectric Project (TR-SGP)
Location
Latitude 27° 48' 08” N to 27° 49' 24” N
Longitude 84° 46' 54”E to 84° 50’ 00” E
District Dhading, Central Development Region
Type of power plant
Type Run-off- the-river snow fed type
Hydrology
Catchment area at intake site 6,035 km2
Long term annual average flow 315.65 m3/s
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Descriptions Parameters
Average minimum 1 in 2 year flow 62.02 m3/s
Design flood at intake (1 in 100 yrs) 7,122 m3/s
Design flood at powerhouse (1 in 100 yrs) 7,195 m3/s
Diversion weir
Type uncontrolled Ogee type gravity diversion structure with bottom undersluice
Crest level 348 m
Length 120 m
Height 12 m above natural bed
Radial gate 6 nos. of hydraulically operable gate
Intake
Type Side intake fitted with 15 numbers of mechanised gate vertical type gate
Size of opening 6 x 3.1 m clear opening 15 numbers
Invert level 345.80 m
Collection Chamber
Type Concrete lined trapezoidal open chamber behind side intake
Length ~80 m
Width Varies (Min 4-max15 m
Height 4 m
Headrace tunnel
Shape Horse shoe shape
Length ~3750 m
Height to spring line 4.0 m
Diameter 8 m
Cross-sectional area 50.3 m2
Bed slope 1:1000
Surge Shaft
Type Orifice
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Descriptions Parameters
Upper shaft diameter 20 m
Bottom shaft diameter 10 m
Upsurge level 355.22
Down surge level 333.32 m
Top of the surge shaft 360.22
Gully crossing 1, Bhante Khola where adit is proposed
Penstock
Type Surface
Material Steel pipe
Numbers 2
Discharge through each unit 64 m3/s
Diameter 5.6 m, 13-20 mm thick, welded metal strap along the periphery and supported by anchor blocks
Length ~ 40 m
Powerhouse
Type Surface
Size 64 m long, 43 m wide and 34 m high
Gross head 29.5 m
Net head 22.8 m
Design flow 127.9 m3/s
Capacity 24.95 MW
Tailrace canal
Shape Rectangular but covered type
Length ~100 m
Cross-sectional area 6m wide X 6 m depth and a free board of 1.5 m
Bed slope 1:1000
Turbines
Type Kaplan
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Descriptions Parameters
No of units 4 Nos. (each of 6.25 MW capacity)
Generators
Type Brushless Synchronous
Capacity 7.4 MVA
Voltage 13.8/132 or 13.8/220 kV
Transmission line
Length ~ 1000 m
Voltage 132 or 220 kV
Transformer
Type Three phase, oil immersed
Rating 32 MVA
Cooling ONAM
Power factor 0.8
Frequency 50 Hz
Energy generation
Mean annual energy per year 163.35 GWh
Dry energy 35.5 GWh
Wet energy 129.97GWh
Access road
Length ~ 8 km long track opening road, up-gradation is required
Type Gravel road, single lane
Bridge One number of Belly bridge over Trishuli River (Optional)
Construction period
Construction period from award of civil 3 contract, yrs
Economic Indicators
Project cost, MNRs 5,864.68
Construction cost, MNRs 5,489.62
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Descriptions Parameters
Cost per kW, USD 2,794
Interest rate 7%
Internal Rate of Return (IRR), % 14.75
Payback period, yrs 5
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2. PROJECT AREA 2.1 Location The TR-SGP Hydroelectric Project (TR-SGP) area lies in Salang and Benigaht VDC of Dhading District, the Central Development Region of Nepal, and stretches from the powerhouse at Majhimtar of Salang VDC, to the intake 2 km further east at Janghare, boarder between Richowktar of Benighat VDC and Arbastar of Salang VDC (see Figure 1.1). Most of the project area lies in the Salang VDC except power evacuation point which is located Gomati village of Benighat VDC. The project is located some 75 km from Kathmandu, the capital city of Nepal, towards west and on the way to Pokhara along Prithivi Highway. It is a blacktopped all season road and has to take off at Richowktar and thereafter an earthen road heading towards the proposed headworks located at Janghare, Trishuli River. The project is located some 1500 m downstream of the Thoppal Khola – Trishuli River confluence.
The geographical co-ordinates of the Project area are shown in Table 2.1 below.
Table 2.1 Geographical co-ordinates of the project Latitude, N Longitude, E
Project area boundary 27° 48' 08” to 27°49' 24” 84° 46' 54” to 84° 50' 00”
2.2 River basin The Gandak River Basin is one of the largest river basins amongst the three river basins of Nepal. The major tributaries of the Gandak River are Trishuli, Kali Gandaki, Budhi Gandaki, Marsyangdi, Seti, Madi, Daraundi and many other small tributaries. The Trishuli River is originates from Tibet of China and thus it is a snow fed type of river.
2.3 Basin physiography Trishuli River is the main tributary of the Gandak River system, the largest among the three river basins of Nepal. It is a gauged river and the gauging station is located some 45 km upstream from the proposed headworks area at Betrawatii in Nuwakot District. The proposed project is lies at 27° 48' 08” N to 27° 49' 24” N latitude and 84° 46' 54”E to 84° 50’ 00” E longitude and at an elevation of 438 m. The catchment area of the project is 6035 km2 and almost 40 of it lie on Tibet of China.
The major tributaries of Trishuli River until the location of headworks are Thoppal, Malekhu, Tadi, Kolphu, Mahesh Khola etc. Some 3 km downstream from the headworks, Budhi Gandaki meets Trishuli at Benighat.
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Topographically, the project area lies within the Central Middle Mountain Region of Nepal surrounded by Mahabharat hills in all sides. The elevation within the project area ranges from 400 m to 1000 m. The river gradient at the headworks area is 1 in 26 whereas it is 1 in 125 at the powerhouse area. The project area is cultivated whereas with terraced fields in the steep areas.
Geologically the site seems to be the transition zone of Malekhu Limestone and Benighat Graphitic Slate. The project area lies approximately 5km north of Mahabharat Thrust (MT) which is said to be root thrust of the Main Central Thrust (MCT). It is therefore the project area is envisioned to be located on high seismic zone. Benighat graphitic black slate, cracked but sound unweathered with continuous bands of quartz is the main geological formation available at the project site. The area is mostly covered with cultivation land and most of the land community land.
2.4 Climate The seasonal climatic pattern is similar throughout the basin in the project area. In general, the climate ranges from sub-tropical in the Terai to moderate subtropical in the midlands and tundra in the high mountains. Since altitude is the guiding factor in the climate of Nepal, five different climates on the basis of topographic elevation have been recognised. These are as follows:
Tundra climate, High Himalayas, Alpine climate mid Himalayas (High Mountain) Cool temperate climate Lesser Himalayas, (Mahabharat) Warm temperate climate Mahabharat and Churia Sub-tropical monsoon climate Churia and terai The rainfall pattern in Nepal can be best described as variable, both seasonally and geographically. Heavy rainfall occurs during the monsoon from mid-June to mid- September. The remaining eight months are more or less dry.
The proposed TR-SGP lies in sub-tropical and temperate climate zone. The average annual temperature varies from 3.2º C and 35.3º C recorded at Station no 809 located at Gorkha. The average annual precipitation is 1900 mm in the river catchment but it is more than this in the project area. The relative humidity varies from 94% to 42% over the year. Similarly, the data series of Index no 1005, Dhading, 24 hours maximum precipitation is 253 mm which was occurred in 1962. The figures revelled that the project area is experienced warm and humid climate during the months of June-October and mostly dry and cold temperature during November-December
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2.5 Access The project is accessible from all seasoned black-topped road. Moreover, the project area is also linked with all season roads by track opening road. The project area therefore is availed only seasonally. The headworks area is linked through an earthen road from Richowktar, Malekhu but it requires strengthening of the existing road. This road is extensively used for sand mining. Busses are plying at regular interval and thus there would not be any problem to reach the project site. It is also a driving distance because of which anyone can go and back from the site to the capital city within a day time comfortably. Moreover, there are two suspended/suspension bridges over Trishuli River in the project area which infact make the project accessible round the year.
2.6 Transmission line There is 132 kV transmission line passing some 500 m from the powerhouse area and in the other side of the Trishuli River. The transmission line is just passing near by the Prithivi highway and thus transportation of material and equipment to erect transmission towers and accessories would not be a problem.
The panned substation at Naubise is another option to evacuate the generated energy into the INPS system. Likewise, government has given due consideration to develop Budhi Gandakai Storage project, if this project materialised, the alternative power evacuation point could be Budhi Gandaki Substation. To enjoy this facility, the Budhi Gandaki Proejct should come first.
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3. TOPOGRAPHY AND CARTOGRAPHY 3.1 Background Adequate knowledge of the project area topography is pivotal to the planning and design of any sizes hydropower projects. For a given site, the topography defines the available gross head, one of the major determinants of the hydropower potential of the site. It plays an important role in sizing of the plant capacity, positioning and alignment of the different components of the project and their auxiliaries and therefore has a bearing of the project economics.
In the light of these facts, the consultant has collected all existing maps and literatures published by different organizations on the project area and reviewed them. This section of the report describes the methodology adopted in carrying out topographical survey works of the TR-SGP River Hydroelectric Project. The survey works covers headworks (location of headworks area – Ref Plate 3.1), Adit tunnel area and powerhouse including surge shaft and tail race area. Prior to pursue topographical survey, field visit was made comprising of hydropower expert, geological expert, survey expert to ascertain optimal project alignment. Moreover, additional area as deemed necessary from the site visit prior performing topographical survey is also carried out. Primarily, focused was given to select project alignment and thereon topographical survey of the project area was carried out. Survey crews were mobilized to site in mid of March and had completed the survey. The survey team worked in the site almost one month.
Plate 3.1 Location of headworks area
Headworks Access road to Intake site
The main objective of the topographical survey works was to define the available head more accurately and prepare topographical maps of the project area for sitting major components of the project such as headworks, waterway, powerhouse and tailrace.
The detailed topographical survey of the project area was conducted in March 2011. One survey crew were deployed to the site. Survey equipment: Topcon Total Station was used
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to carry out the topographical survey. Most of the control points were established either on rock or monumented in the ground with concrete. Description cards have been made for those control points, which will be needed for future use together with details of coordinates and levels and included in Volume II.
3.2 Available maps Initially, reconnaissance study was carried out using available topo maps (Table 3.1). Prior to the field survey, the desk study was carried out using the available maps and detail information about the area for the survey work was noted with due consideration on selection of major project components such as headworks, waterway and powerhouse area.
Table 3.1 Available topographical maps
S. N. Sheet no. Name of maps / Scale Published by Location
1 2784 04C, 2784 04 D, Dhading, Nuwakot, 1:25,000 Department of 2784 08A, 2784 08B, 2784 Kathmandu, survey, Nepal 04 A, 2784 04 B, 2785 01 with Finish A, 2785 01 B, 2785 01C, International 2785 01D, 2785 05 A, Development 2785 05B, 2785 02 A, Agency 2785 02 B, 2785 02C, (FINNIDA), 2785 02D 1994
2 2885 13, 2885 14, 2885 Rasuwa, Dhading, 1:50,000 15, 2885 11, 2785 10 Sindhupalchowk
3.3 Survey and mapping The main objective of the survey and mapping works was to prepare the topographical maps of the project area for sitting of major components of the project such as headworks, headrace tunnel, surge tank, powerhouse and tailrace. All the survey works were conducted by marking the two points in fin map and take their co-ordinates and also cross-checking with GPS for horizontal control and for vertical control, level is transferred from Thoppal powerhouse area reference point BM 152.
Initially, reconnaissance study was carried out based on the available topo map/s and the tentative project alignment was marked on the topo map at the initial stage of the feasibility study. Marking of project auxiliaries help in determining the survey corridor
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required for topographical survey. Prior to the field survey, desk study was carried out using these maps and detail information about the area for the survey work was noted.
The fieldwork in a topographic surveying consists of three parts: establishing horizontal and vertical control, locating the contours and locating details. Establishment of horizontal and vertical control points is the most essential part, and is the 1st step in topographic survey.
3.3.1 Traverse Survey Traverse survey is the foremost important part of the survey to ascertain control points at structurally important area. Moreover, traverse survey linking headwork’s and powerhouse and formed a control loop is the process adopted prior to carrying out detailed topographical survey and errors then occurred was distributed to each traverse points. Data and information on traverse is included in Volume II.
3.3.2 Control Survey Altogether, there were 11 bench marks established and controlled survey was performed. Both faces reading were taken and errors if occurred were adjusted accordingly. Table 1.4 shows the co-ordinates of the traverse loop used to established traverse loop and to establish control points for further persuasion of topographical survey.
A control survey was carried out transferring the level established at the Thoppal powerhouse area. Level transferred by the total station and mirrors is holding with tribach to minimise the possible error.
The detailed topographical survey of the project area was conducted in March 2011. Permanent benchmarks and traverse points were fixed at the locations of intake, tunnel portal, tunnel adit, surge shaft, powerhouse and tailrace sites and slightly along the waterways alignments. The survey works have been conducted at these locations and finally connected to a established point at the headworks site.
Survey equipment: Topcon total station was used to carrying out the traverse work, establish the control points (BMs) as well as topographical survey. All permanent control traverse station points for centring and elevation reference were marked on the ground with 16mm dia rods having x-marked at the top inserted in the concrete pillar. The points are also marked with red enamel paint so that they are easily visible. Local person Mr Roj Prasad Tripathi (Sahila Dai) was one of the survey crew which helps to find easy in the future to determine the established point. The coordinates and levels of the permanent control points (benchmarks) are presented in Table 3.2.
Table 3.2 Coordinates of control points and permanent benchmarks
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S.N. Easting (m) Northing (m) Elevation (m) Remarks
1 582278.800 3077601.000 355.740 BM2
2 582296.40 3077682.000 346.066 BM 3
3 581209.222 3077459.545 338.56 BM 4
4 581022.565 3077322.722 435.360 BM 5
5 581058.994 3077673.371 376.810 BM 6
6 579799.138 3078062.072 356.000 BM 7
7 579556.477 3078080.940 369.610 BM 8
8 579708.357 3077954.090 342.076 BM 9
9 578314.827 3077478.752 334.130 BM 10
10 579982.482 3077234.233 332.540 BM 11
11 578046.317 3077626.441 337.310 BM 12
3.3.3 Detailed survey The detailed survey (Plate 3.2) at the headworks area covered about 2000 m upstream and about 200 m downstream of the proposed diversion structures. In order to include the areas for camping site and helipad to intake, the survey area also included 20 m above the existing bed level on the left bank of the Trishuli River. However, no survey work some distance upstream of the headwork along the right bank was carried out because of topographical difficulties.
Plate 3.2 Survey team works in powerhouse site The detailed survey of the tunnel area covered both sides of the gully and regions along the tunnel alignment. The survey area also covered the waterway, surge tank, gully crossing area/s, penstock, powerhouse site and tailrace.
The detailed survey of the powerhouse area covered sufficient area both upstream and downstream from the proposed powerhouse location. Topographical maps are presented in Drawings, Volume IV.
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3.3.4 Topographic mapping The survey data were produced in a digital format and civil engineering software Land Development was used for this purpose. The topographical surveys were carried out for headworks, waterways and powerhouse area in 1:1,000 scale with 1 m contour interval in overall. Intake site, Tunnel adit site and powerhouse site topographical maps were prepared in 1: 500 scale having contour interval 1m. Besides, necessary river profiles and river cross sections were produced for both headworks area and powerhouse area and included in Topographical Maps.
3.4 Errors and accuracy Possible errors were distributed while taking observation. Both faces of result were recorded. This is discussed in Volume II, respective sections.
3.5 Future survey work Surveying and mapping carried out to date has been of adequate accuracy to be used as a basis for geological mapping and for detailed design of the tunnel. The following work will be required at a later stage of the project.
" Detailed survey of the Trishuli River bed in the area of intake as a basis for the physical model study, and " Transmission line alignment survey;
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4. GEOLOGY AND GEOTECHNICAL 4.1 Introduction This report summarizes the finding on the project geology, tunnel lining and support prediction further investigations requirement, assessment on geology of key structures. More discussion is made on Volume II, Field Investigation Report.
4.2 Regional Geology of the Project Area The project area is located between Benighat and Malekhu. It lies within the Lesser Himalaya of Central Nepal, consists of limestone, Adi
Tunn Inta quartzite, slate, dolomite and el Surge limestone. Structurally, the Reser Mahabharat Thrust (MT), a part of the Main Central Powerh Thrust (MCT) is situated in the north of the project area (Stocklin and Bhattarai, 1977) (Figure 4.1).
Figure 4.1 Regional Geological Map along the Trishuli River around Malekhu Area (after Stocklin and Bhattarai, 1977) Figure 4.1 shows the major geological structures around the project area. The Mahabharat Thrust (MT) and disconformity are major geological structure of the area. The MT is also equivalent to the part of the Main Central Thrust (MCT). The disconformities are developed between the rocks of the Dhading Dolomite of the Lower Nawakot Group and the Benighat Slate of the Upper Nawakot Group.
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The proposed area lies on the left limb of the Mahabharat Synclinorium (Figure 4.2). Around the project area, regional folds are not reported and but minor and local folds can be seen within the rocks of the Benighat Slate.
Dhading Dolomite (Dh)
Benighat Slate (Bg)
Malekhu Limestone (Ml)
Figure 4.2 Satellite Map of the Project Area (Source Google Map) The foliation planes are parallel to the bedding plane. The strike of the foliation plane of the project area is northeast to southwest dipping towards northwest. General trends of the foliation plane are northwest direction.
4.3 Previous Studies Stocklin and Bhattrai (1977) and Stocklin (1980) has studied detailed geology of the central Nepal Under UNDP project. According to them, the Central Nepal has been subdivided into the Nawakot Complex and Kathmandu Complex based on the metamorphism of the rock. The Nawakot Complex is again subdivided into the Lower and Upper Nawakot groups. The Lower and Upper Nawakot groups are separated by the disconformity. Talalov (1972), Bordet et al. (1971), Hagen (1968) Auden (1935) and Kizaki et al. (1982) has also studied the geology of the Central Nepal.
4.4 Detail Geological Studies of the Project Area The project area belongs to the Lesser Himalayan rocks, located in north of the MT zone. The lithounit is comprised of thick-bedded, dark grey to dark black slate and minor bands of the limestone as well as the calcareous slate of the Benighat Slate, Upper Nawakot Group. The dip directions of rock are ranged from 0100 to 3200 and dipping towards north
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(600 to 800). The locations for the powerhouse and tailrace as well as weir structures are to be constructed on the alluvial deposits along the river bed as well as old alluvial deposits which are expected to be more than 10 m in thickness. Colluvial deposits are found very less in the project area..
4.4.1 Reservoir Area The reservoir area extends upto old suspension bridge over the Trishuli River which connects the Dhading besi Road from Malekhu Bazaar. Geologically, the area belongs to the rocks of the Benighat Slate some of the upper reaches of the reservoir area falls in the rocks of the Malekhu Limestone. Dark grey to black slates are well exposed along the both banks of the reservoir area. Thickness of the bedding plane range from 1 cm to 50 cm. some area of the reservoir has covered with calcareous slate also. The reservoir area is wide (more than 150 m width). Left bank has comparatively gentler slope than the slope of the right bank. The left bank is composed of old alluvial deposits of the Trishuli River. The river has straight course and very low gradient of the river morphology.
4.4.2 Weir Axis Area The proposed weir axis area is located about 100 m downstream from the confluence between the Chiti Khola and Trishuli River on the right bank of the Trishuli River at Malekhu Bazaar. The proposed area is located about 50 m upstream from the suspension bridge at Trishuli River connects Malekhu Bazaar and Majuwatar. Geologically, the area is located in the Benighat Slate of the Upper Nawakot Group. Around the proposed intake area, dark grey to black, thinly foliated slate with intercalated calcareous slate are exposed on the both banks of the Trishuli River. Superficially, thick alluvial deposits on the bedrocks of the Benighat Slate can be seen on both banks of the Trishuli River. Individual thickness of slate varies from 0.1 to 0.5 m. River valley of the Trishuli River is comparatively narrower than upstream and downstream and the river morphology is straight course can be found around the proposed weir axis area.
4.4.3 Portal Inlet Area The proposed portal inlet area is located on the right bank of the Trishuli River, downstream from confluence between the Trishuli River and Chiti Khola and about 300 m upstream from the suspension bridge at Trishuli River. The proposed structure lies in the rocks of the Benighat Slate. Around the proposed portal inlet area, thick to thin bedded, black to grey slate is exposed. Individual thickness of slate varies from 0.1 to 0.3 m. Topography is steep are seen around the proposed area.
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4.4.4 Tunnel Alignment Area The proposed tunnel alignment follows on the right bank of the Trishuli River and also lies on the rocks of the Benighat Slate. Black to dark grey slate and calcareous slate with thin limestone can be seen along the proposed tunnel alignment. Thin layers (< 2 m thick) of colluvial deposits are found on the hill slope. Individual thickness of slate and calcareous slate varies from 0.2 to 0.3 m and 0.1 to 0.4 m, respectively. The tunnel alignment crosses the Badare Khola, Bhante Khola and Chibedi Khola and other small tributaries. The topography is more or less gentle slope because of the nature of the rocks. In general, between Chiti Khola and Bhante Khola slate is dominant whereas between Bahnte Khola and Majuwatar linestone and calcareous slate can be seen.
4.4.5 Surge Tank and Penstock Alignment Area The proposed surge tank area lies geologically in the rocks of the Benighat Slate, Lesser Himalaya and located on the right bank of the Trishuli River at Majuwatar village on the right bank of the Trishuli River, just opposite to Benighat Bazaar. Around the proposed surge tank and shaft area, grey to black slate and calcareous slate can be observed. But, superficially thin layers colluvial deposits have covered the proposed area. Individual thickness of slate varies from 0.2 to 0.3 m. Topography is steep around the proposed area.
4.4.6 Powerhouse and Tailrace Area The powerhouse and tailrace lies on the right bank of the Trishuli River at Majuwatar village about 2 km downstream from the confluence between the Bhante Khola and Trishuli River or about 1 km upstream from the confluence between the Budhigandaki River and Trishuli River. Geologically, proposed area belongs to the Benighat Slate, Lesser Himalaya. The rocks (slate and calcareous slate) of the Benighat Slate around the powerhouse and tailrace area are exposed on hillside. But, superficially, thick old alluvial deposits have covered the proposed area along the river bank. Individual thickness of rocks is ranged from 0.1 to 0.3 m.
4.4.7 Adit Area The adit area is proposed at the Bhante Khola, this location is nearly half of the tunnel alignment. The area is located geologically in the rocks of the Benighat Slate. Thick to thin bedded limestone and slate can be seen around the proposed area. Both banks has vertical topography. Thickness of limestone is ranged of 0.5 m to 1 m whereas thickness of the slate 1 cm to 20 cm.
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4.4.8 Access Road from Intake to Powerhouse Area The newly constructed road from Bungchu Khola along the Dhading Besi-Malekhu Road connects the project area. The road is partly runs on the colluvial as well as on the alluvial deposits and also partly in rocks. The road follows initially the right bank of the Thoppal Khola then follows the right bank of the Trishuli River and join the intake and powerhouse area. The newly constructed road is earthen road. 4.5 Results of the 2D-Electrical Resistivity Survey Sixteen lines covering 3,225 m were applied for the analysis of the subsurface condition of the project area. Three lines in powerhouse area, two lines in surge tank area, five lines in the tunnel alignment, and five lines in the headworks area were done in the project area. In general, there is thick (less than 10 m thick) old alluvial deposits are found and below 10 m depth there is high possibility to find the bedrocks of slate. The surge tank and the proposed penstock alignment is covered with thin (less than 4 m thick) overburden materials. Along the tunnel alignment at the shallow depth bedrocks are found. At Badare Khola and Bhante Khola the rocks are weak and may be crack zone and there will be high possibility of ground water effect. Along the headworks area the ERT shows that there is less than 10 m thick overburden materials can be found. In general there is high possibility to find the bedrock even the weir axis area in the middle part of the weir axis area.
Detailed discussion is made in Volume II.
4.6 Geotechnical Studies of the Project Area Rock mass classification was carried out based on the NGI “Q” and CSIR “RMR” system. Based on the computed “Q” and “RMR” values the rock mass could be classified into good, fair, poor and very poor rock zones. Classified rock masses are given in Table 4.1. The calculated values can be used for rock support in the headrace tunnel. The rock mass is classified into the very good to excellent, good, fair to good, poor, very poor, extremely poor and exceptionally poor rock mass. All the stability analyses in the rock were done using Geotechnical Software DIPS.
Table 4.1 Rock Mass Classification RMR ≈ 9× lnQ + 44 (Bieniawaski, 1989); RMR = 15 × logQ + 50 (Barton, 1995)
Descriptions Range of Q-values Range of RMR-values Rock Quality Minimu Maximu Minimu Maximu Description Class Descriptions m m m m
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RMR ≈ 9× lnQ + 44 (Bieniawaski, 1989); RMR = 15 × logQ + 50 (Barton, 1995)
Very good to Class I 100 1000 85 100 excellent Massive, stable rock Class II Good 10 100 65 85
Class III Fair to good 4 10 56 65 Massive to blocky, Class IV Poor 1 4 44 56 competent stable rock
Class V Very poor 0.1 1 35 44 Jointed to Class VI Extremely poor 0.01 0.1 20 35 fractured rock
Heavily jointed, Class Exceptionally poor 0.001 0.01 5 20 fractured rock VII with clay bands
4.6.1 Reservoir Area The proposed reservoir area is extended about 1,300 m in length and with range from 150 to 200 m in width and the length is more than 2 km. Almost the right bank is covered with bedrocks whereas left bank is covered with old alluvial deposits. The area on right bank faces 60°/140°. The exposed rock beds are less competent and are favourably dipping against slope face direction. The attitude of the bedrock is 68°/328° (dip/dip direction). One major (63°/260°) and other minor joint sets (53°/152° and 52°/034°) are observed in the exposed area .The joint surfaces are slightly to moderately altered with average joint spacing of 0.1 to 1 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
Rock Classification
Geo-mechanical classification for jointed rock mass of the headwork using CSIR classification was carried out based on the detailed surface discontinuity. Most of the Rock Mass Rating (RMR) of the reservoir area falls in the range 35-40 and it indicates that the rock mass of reservoir site is categorized as Class IV types, which is defined as the poor rock.
Weathering and Strength
Rock mass in the reservoir area is fresh to slightly weathered in slate. Generally, the rocks along riverbank are fresh rock and slightly weathered at higher hill slope. At the lower downstream of the reservoir area slate are found. So, the rocks are slightly weathered found at that place. These rocks are easily erodible in water because of its low strength hardness and content of fine materials.
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Slope Stability
Slope stability assessment analysis of the both banks hill slope was carried out on the basis of aerial photos interpretation and geological observations. An analysis of foliations to determine the stability of the rock mass due to the presence and orientations of the foliations in the rock mass at the intake site was done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal area net. In general the rocks is oriented oblique to the extension of the reservoir. So there is very less possibility in occurring failures along the foliation plane. The wedges formed by the planes were then analysed with respect to the hill slope surface. The internal friction angle has been adopted as 30 degrees for the stability calculation. The dipping of the foliation plane is favorable to the natural hill slope and the relation between them is opposite so very less possibility to occur failure. The wedge formed by the intersection of the joints (J1 and J2) are very less possibility to fall down because they are directed opposite to the hill slope. The thickness of colluvial deposits in the hill surface less than 1 m at most places.
4.6.2 Weir Axis Area The proposed weir axis site belongs on rocky as well as the alluvial deposits the length of the weir axis shall be more than 100 m. The weir axis is located at appropriate site due to the exposure on both banks and straight river course. The persistency of the discontinuities is low. The boulders and gravels of the recent alluvial deposits are generally sub-angular in shape. Thickness of deposits is expected less than 10 m at the headwork sites. Most of the boulders are limestone and schist as well as quartzite. Both banks has the topography is steep slope the proposed intake and weir axis area. The alluvial deposits are comprised of boulder, cobble and pebble (> 80% limestone and 40% schist and limestone). Maximum diameter of the boulder is more than 5 m. The exposed rock beds are less competent and are favourably dipping against slope face direction. The attitude of the bedrock is 80°/330° (dip/dip direction). One major (72°/260°) and other minor joint sets (61°/169° and 72°/028°) are observed in the exposed area. The joint surfaces are slightly to moderately altered with average joint spacing of 0.4 to 2 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
Rock Classification
Geo-mechanical classification for jointed rock mass of the weir axis using CSIR classification was carried out based on the detailed surface discontinuity. Most of the Rock Mass Rating (RMR) of the weir axis area falls in the range 37-42 and it indicates that the rock mass of weir axis site is categorized as a Class IV type, which is defined as the poor rock.
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Weathering and Strength
Rock mass in the weir axis area is fresh to slightly weathered. Generally, the rocks along riverbank are fresh rock and slightly weathered at higher hill slope. The exposed rocks along the river banks are easily erodible in water because of its low strength hardness.
Slope Stability
Slope stability assessment analysis of the left bank hill slope was carried out on the basis of aerial photos interpretation and geological observations. An analysis of foliations to determine the stability of the rock mass due to the presence and orientations of the foliations in the rock mass at the intake site was done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The wedges formed by the planes were then analysed with respect to the hill slope surface. The internal friction angle has been adopted as 30 degrees for the stability calculation. The dipping of the foliation plane is favourable to the natural hill slope and the relation between them is opposite so very less possibility to occur failure. The wedge formed by the intersection of the joints (J1 and J2) may occur failure due to formation of central wedges. The thickness of colluvial and alluvial deposits in the hill surface exceeds 1 m at most places.
Portal Inlet Area
The proposed portal inlet area is covered with bedrock just above the river course. The rocks are slightly to moderately weathered is exposed. The persistency of the discontinuities is low. Thin layer of overburden materials (colluvial deposits) less than 1 m in thickness is found on uphill side of the proposed portal inlet area. Alluvial deposit is composed of slate. The land use pattern is grass land and forest. The area on right bank faces 65°/179°. The exposed rock beds are less competent and are favourably dipping against slope face direction. The attitude of the bedrock is 47°/332° (dip/dip direction). One major (60°/144°) and other minor joint sets (45°/070°) are observed in the exposed area. The joint surfaces are slightly to moderately altered with average joint spacing of 0.1 to 1 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
Rock Classification
Geomechanical classification for rock mass tunnel alignment using CSIR classification was carried out based on the detailed surface discontinuity measurements on exposed rock outcrops around the portal inlet.
Weathering and Strength
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Rock mass is less competent, fresh to slightly weathered, with some fresh to slightly weathered rock exposed at riverbed.
Slope Stability
The stability of tunnel has been analyzed on the basis of geotechnical and geological observations on the surface of the hill slopes through which the tunnel is aligned. An analysis of foliations to determine the stability of the rock mass due to the presence and orientations of the foliations in the rock mass at portal inlet was done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The planes and wedges formed by the planes were then analysed with respect to the natural hill slope.
The internal friction angle has been adopted as 30° for the stability calculation.
4.6.3 Tunnel Alignment Area The proposed tunnel alignment passes through slate of Benighat Slate of the Lesser Himalaya. The orientation of the foliation plane of the exposed rock is directed toward northwest, which is nearly oblique to the tunnel alignment. Other characters of the exposed rock along the alignment are thin-bedded, containing two to three joint sets. Individual thickness of the bedrocks varies from 0.1 to 1 m in the slate/phyllite and limestone. Superficially slate and limestone is covered by thin layers of colluvial deposits. The exposed rock beds are less competent and are favourably dipping against tunnel alignment direction. The attitude of the bedrock is 77°/342°-59°/323° (dip/dip direction). One major (47°/200°-63/291) and other minor joint sets (45°/175°-33°/135° and 43°/004°- 45°/049°) are observed in the exposed area. The joint surfaces are slightly to moderately altered with average joint spacing of 0.6 to 2 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
Rock Classification
Geomechanical Classification for rock mass tunnel alignment using CSIR classification was carried out based on the detailed surface discontinuity measurements on exposed rock outcrops in the tunnel alignment. The results of the rock mass classification are presented in Table 4.6. Most of the alignment is covered with the poor rock mass. The Q value of rock mass is given in Table 4.2 and shows poor to fair rock mass.
Table 4.2 NGI Tunnelling Index ‘Q’ values of the Tunnel Alignment Area
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Weathering and Strength
Rock mass in most of the tunnel alignment is fresh to moderately weathered. The rocks are soft and easily erodible in water.
Slope Stability
The stability of tunnel has been analysed on the basis of geotechnical and geological observations on the surface of the hill slope and friction angle through which the tunnel is aligned. The data measure on the slope has been extrapolated to the tunnel locations. An analysis of foliations to determine the stability of the rock mass due to the presence and orientations of the foliations in the rock mass along the tunnel was done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The internal friction angle has been adopted as 30° for the stability calculation.
4.6.4 Assessment of Initial Support Requirement for Tunnel Alignment Rock support in the tunnel and underground cavern is provided to improve the stability and to safeguard the opening with respect to safety of the working crew. The guiding principle of rock support design is that it is capable to response the actual ground conditions that is encountered in the tunnel and the safety requirement at the tunnel face is met. This requires provision of flexible rock support methods that can be quickly adjusted to meet continuously changing heterogeneous rock mass (Table 4.3 and 4.4).
Table 4.3 Designed Tunnel Rock Support Class and Respective Rock Support Rock Mass Quality Rock Support Assigned Tunnel Rock Support Description (RS) Class Fair to good rock RS III 25 mm diameter 3 m long systematic grouted rock mass bolts at a spacing of 1.5 m x 1.5 m and 15 cm thick steel fiber shotcrete. Poor rock mass RS IV 25 mm diameter 3 m long systematic grouted rock bolts at a spacing of 1.3 m x 1.5 m and 20 cm thick steel fiber shotcrete. Very poor rock mass RS V 25 mm diameter 3 m long systematic grouted rock bolts at a spacing of 1.3 m x 1.3 m and 20 cm thick steel fiber shotcrete. Extremely poor rock RS VI 25mm diameter 3 m long systematic grouted rock mass bolts at a spacing of 1.2 m x 1.2 m and 20 cm thick steel fiber shotcrete. Steel ribs at a spacing of 1 meter to control plastic deformation. Advance pre-
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injection grouting is provisioned to control water inflow into the tunnel. Exceptionally poor RS VII 25 mm diameter 3 m long systematic grouted rock rock mass bolts at a spacing of 1.1 m x 1.1 m and 20 cm thick steel fiber shotcrete. Steel ribs at a spacing of 1 meter to control plastic deformation.
Table 4.4 Assigned Rock Support in Respect with Rock Mass and Rock Support Class Chainage Assigned rock support measures Rock Mass Rock Support Class Class
0+000- 25 mm diameter 3 m long systematic grouted 1+760 rock bolts at a spacing of 1.3 m x 1.5 m and 20 Class III-IV RS IV cm thick steel fiber shotcrete. Concrete lining is required 1+760- 25 mm diameter 3 m long systematic grouted 2+580 rock bolts at a spacing of 1.5 m x 1.5 m and 15 Class II- III RS III cm thick steel fiber shotcrete. Concrete lining is required 2+580- 25 mm diameter 3 m long systematic grouted 3+740 rock bolts at a spacing of 1.3 m x 1.5 m and 20 Class III-IV RS IV cm thick steel fiber shotcrete. Concrete lining is required
4.6.5 Surge Tank/Penstock Alignment Area The surge tank and penstock alignment area are located on intercalation of slate and limestone of the Benighat Slate. Superficially, the rocks are fractured and moderately weathered but it is hoped to find fresh and intact rock with few meters depth from the surface. Three sets of the joints are visible in the rocks exposed. Individual thickness of the bedrock is less than 10 cm. The persistency is low. Thin layer (less than 0.5 m) colluvial deposits have covered the slate in steep slope whereas thick colluvial and alluvial deposits can be found along the penstock alignment area. The land use pattern is grassland to cultivated land. The exposed rock beds are less competent and are favourably dipping against slope face direction. The attitude of the bedrock is 61°/288° (dip/dip direction). One major (70°/183°) and other minor joint sets (41°/111° and 65°/024°) are observed in the exposed area. The joint surfaces are slightly to moderately altered with average joint spacing of 0.1 to 1 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
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Rock Classification
Geomechanical classification for rock mass surge tank and penstock alignment using CSIR classification was carried out based on the detailed surface discontinuity measurements on exposed rock. Most of the Rock Mass Rating (RMR) of the headwork area falls in the range from 37 to 42 and it indicates that the rock mass of penstock alignment and surge tank site is categorized as a Class IV type, which is defined as the poor rock.
Weathering and Strength
Rock mass is fresh to slightly weathered, with some slightly moderately weathered rock exposed. Two to three sets of the joints are predominant. They are rough and stepped. The persistency of the exposited rock is low. The rock is not competent and not hard.
Slope Stability
The stability of surge tank and proposed penstock alignment has been analysed on the basis of geotechnical and geological observations on the surface of the hill slopes. An analysis of foliations to determine the stability of the rock mass due to the presence and orientations of the foliations in the rock mass along the tunnel was done using Lower Hemisphere Projection of the foliation planes in Schmidt’s equal area net. The planes and wedges formed by the planes were then analysed with respect to the natural hill slope.
4.6.6 Powerhouse and Tailrace Area At the proposed location, the alluvial terrace is thick. The ground surface of recent alluvial deposits of the paleo Trishuli River is almost flat. It is assumed that thickness of alluvial deposits more than 10 m thick at the proposed powerhouse. The tailrace also lies on thick alluvial deposits and is more than 10 m from the riverbed. The riverbed is composed of boulder (> 40%) and fine materials (< 60%). More than 70% of the boulders of quartzite and gneiss are found and remains of sand and pebble and remaining are of the schist. The material found at the proposed sites is fine- to medium-grained, grey sand with rounded to sub rounded gravels. Uphill side is covered with bedrocks. The area for the proposed powerhouse and tailrace is flat and wide. The attitude of the bedrock is 61°/288° (dip/dip direction). One major (70°/183°) and other minor joint sets (41°/111° and 65°/024°) are observed in the exposed area. The joint surfaces are slightly to moderately altered with average joint spacing of 0.1 to 1 m. The joint surfaces are rough to smooth and have silty clay fillings in the exposed areas.
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4.7 Seismicity Due to fragile geology and relatively Young Mountain, seismicity is one of the project development hindrance but proper address of the situation, it can be address. Adoption of g value in designing the project structures is utmost importance to address the situation. The Nepal Himalaya has experienced several large earthquakes over the past centuries. The earthquakes of larger magnitudes that have occurred in Nepal Himalaya are summarized below in Table 4.9. Moreover, on the basis of the project design in the similar region and adopted g value indicates that value 0.2-0.3 g can be adopted for the design
4.8 Construction Materials Survey A construction material investigation was conducted in the vicinity of the headwork and powerhouse site as well as along the Trishuli River and Budhigandaki River as well as the Malekhu Khola in the project area. The investigations focused on locating prospective borrow areas of non-cohesive materials, which are to be used mainly as an ingredient of concrete. The prospective borrow sites were identified as sources of coarse aggregates.
The requisite quantities of construction material like boulders, cobble, gravel and sand are generally available in and around the project. Point bar and braided bar deposits of the Trishuli River and Malekhu Khola as well as along the Budhigandaki River are the main source of construction material. These deposits predominantly consist of gneiss boulder, cobble and gravel including some limestone and quartzite. The boulder, gravel and sand deposits in the point bars in and around the powerhouse site along the Trishuli River and Budhigandaki River can be used as construction material. Sand found in these areas is fine- to medium-grained.
Construction materials such as sand and aggregates can be found abundance in the project area. However, boulders are to be transported from quarry sites,
4.9 Borrow Area At the intake site, construction material can be extracted from the alluvial bar deposit on the both banks as well as upstream of the Trishuli River between Malekhu and Benighat villages and also from Malekhu Khola. Material excavated during the construction of weir/intake structures can be used as construction material as well. At several locations deposited materials along the Trishuli River can be used as construction material for the nearest structure. The quantity available within 4 km distance from the intake and powerhouse sites is estimated to be sufficient for the proposed construction works. The location as well as the expected volume and composition of the materials and laboratory
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tests are presented in Tables 4.5. Sands can be brought for the project from Belkhu Khola which is about 15 km from the project area.
Table 4.5 Volume and Location of the Construction Materials SN Location Percentage Volume Composition Stability Source of Land of clasts (m3) condition sediments Use 1. Malekhu Boulder-30- Unlimited Gneiss/granite Stable River bed Barren Khola 40%, (70%), both Cobble and quartzite sides pebble (20%), schist 50%, (10%) sands-10% 2. Trishuli River Boulder- Unlimited Gneiss (70%), Stable River bed Wet 20%, quartzite cultivated Cobble and (20%), schist pebble (10%) 70%, sands-10% 3. Along Boulder- Unlimited Gneiss (70%), Stable River bed Lower Budhigandaki 60%, quartzite part wet River Cobble and (20%), to dry pebble Limestone cultivated 30%, (10%) land and sands-10% upper part forest 4. Belkhu Khola Sands 100% sands Stable Riverbed Barren to cultivated land
4.10 Laboratory Test All laboratory tests were carried out at well-equipped EMES Geotechnical laboratory in Lalitpur and result is shown in Table 4.6. Details are discussed in Volume II.
Table 4.6 Summary of Results of the Construction Materials
Grai Spec Mica Abs Clay Ph Crush Sulphate Los Aggreg Quar Impa Dens n ific Conte orpt Conte i ing Soundn Angel ate tz % ct ity size gravi nt ion nt value ess es Reactiv value ty ity TRA- 2.69 0.17 5.62 30 14.9 2.1 31.0 28.5 73 18.6 1.69 1 TRS- 2.64 1.27 0.21 1 MKA 2.67 0.44 3.57 30 16.9 2.7 35.0 68 19.7 1.75 -1 MKS- 2.63 1.19 0.92 55-60 1
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4.11 Spoil Disposal Area The excavated materials from the tunnel and surge tank area can be deposited along the river banks of the Trishuli River but the available space along the riverbed is wide Table 4.7). The excavated materials can be also deposited along the Trishuli River. Transportation of the excavated materials to the Chibedi Khola, Bhante Khola and Badare Khola valleys are economically considerably cheap and sufficient area is avilable. So, it is better to arrange along the both banks of the river beds of the Trishuli River and other small tributaries where the wide area is available.
Table 4.7 Distribution of Muck Disposal Area SN Location Available Area Land use Hydrology Instability Remarks m2
1 Downstream from 400x40 Barren Wet Stable weir axis
2 Downstream from 300x25 Barren Wet Stable powerhouse
3 Chibedi Khola 200x10 Grass Dry Stable
4 Badare Khola 400x10 Grass Dry Stable
5 Bhnate Khola 200x 10 Grass/barren Wet Stable
4.12 Conclusions and Recommendations 4.12.1 Conclusions Following conclusions are drawn from the limited data availability of the project area geology.
! Geologically, the project lies in of the slate and calcareous slate of the Benighat Slate, Upper Nawakot Group, Lesser Himalaya, Central Nepal. About 70 km west from the capital city and lies along the Prithvi Highway.
! Structurally, the project located about 3 km north of the Mahabharat Thrust (MT) zone, the effect of the MT is considered as minimum. But the tunnel alignment crosses three small tributaries.
! The foliation plane of the rocks extends northeast and no regional folds in the project area.
! Thick alluvial deposits are found around the powerhouse and some part of the intake and reservoir area. Thickness of the alluvial deposits is more than 10 m
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around the proposed powerhouse area. Thin colluvial deposits are found along the surge tank and tunnel alignment.
! The slope stability of the project area is good. The dipping of the foliation plane is oblique to the natural hill slope so favourable condition.
! The sound rocks area found in the surge tank, tunnel alignment as well as in intake area but the powerhouse area has possibility to find the bedrocks.
! The exposed rocks mainly slate are slightly weathered condition and limestone but hope to meet the intact rock mass within few meters depth and the spacing of the discontinuities range from 0.5 to 0.3 m, roughness and waviness of discontinuities plane is rough and steeped, the joints are filled with sandy clays
! The RMR values of the rocks exposed around the project area are poor to fair and the Q values of the rocks along the tunnel area show the fair to poor rock. From Chiti Khola to Badare Khola (1.76 km) and Badare Khola to Bhante Khola is poor rock mass and remaining of the length of the tunnel from Bhante Khoa to Chibedi Khola (1.76 to 2.58 km) has fair rock mass. Last portion from Chibedi khola to Majuwatar contains of poor rock (2.58 to 3.74 km). The slope stability condition of the tunnel alignment is good because the tunnel alignment passes nearly perpendicular to foliation plane and some of the wedges formed by the foliation and joints are critical.
! Sufficient quantity of the construction materials are found along the Trishuli River and Malekhu Khola riverbed and sands can be extracted from the Belkhu Khola river bed.
! Sufficient space shall be available for the mucking along the Trishuli River and other small tributaries.
! The values of the tested materials fall within the standard values so the materials can be used.
4.12.2 Recommendations Following recommendations are made for further understanding of the project area in regards to project geology and geotechnical issues and concerns.
! Presence of the old alluvial deposits on the left bank and powerhouse area so it is better to manage the river training works in powerhouse area.
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! Drilling at proposed project area to know geotechnical parameters of the subsurface bedrock is highly recommended during the detailed engineering design. ! The seismicity factor should be considered in the detail engineering design because the zone is also seismic gap and passes about 3 km south of the MT.
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5. HYDROLOGY AND SEDIMENTOLOGY 5.1 General An accurate assessment of long-term hydrology is of key importance to any hydropower project. The longer the hydrological record the more reliable is the estimation of design parameters for the project. In the case of ungauged (i.e. either limited or no stream flow records) river, direct measurements of hydrological parameters are not available; however, this is not the case for a gauged river.
This section of the report contains an overview of the hydrology sedimentology aspect of the Trishuli River catchment at the proposed intake site located nearby Trishuli River Bridge at Malekhu, Dhading. The main objective of the hydrological study is to study rainfall pattern, to pertain discharging capacity of the catchment, to predict design discharge, high flow and low of the river. It is therefore, the overall aim of the hydrological, meteorological and sedimentology study of the project and to generate mean monthly design flow for the required capacity of the hydroelectric power plant, to estimate floods and low flows of the river.
The uppermost part of the Trishuli River basin lies in Tibet, China and passed through Higher Himalayas, then lower Himalayas and flows down into the mid hill areas and finally the river debouches over into the Terai alluvial plain in Chitwan district. The river is a snow feed type and many rivers and rivulets of monsoon type meets in its way flowing down. The Trishuli River is a gauged River and the gauging station is 447 located at Betrawati, Nuwakot and one of its tributary Tadi Khola is also gauged (Station No 448). The distance between the Gauging Station no 447 and to the proposed intake area is about 46 km where as it is 165 km from its source to the Gauging Station 447. More importantly there are many rivers and rivulets that are joining Trishuli River in between and thus makes difficult to generate mean monthly flow at the proposed intake. It is noted here that the river is snow fed type and thus catchment correlation between the parent river and the area below 447 to the proposed project is not possible. It therefore requires a similar catchment to correlate flow so as to ensure a realistic generation of flow in the area between the Gauging Station 447 and the proposed intake where tributaries of the Trishuli River are contributing. It is now essential to determine a suitable and appropriate catchment so that realistic flow estimation is possible.
The catchment characteristics of the Gauging Station 448 located at Tadi Khola, Belkot Trishuli catchment and the remaining catchment in between the Gauging Station no 447 and the proposed headworks is envisioned to be similar in many respects and thus the
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same catchment has been used to generate mean monthly flow. This is simply because of processed long term daily flow records available for the period of 1969 to 2005.
The hydrological study of the project area comprises the field investigation including desk study, collection of meteorological data and various literature reviews. Briefly the methodology of hydrological study is stated below:
a) Literature review b) Collection of available topographical map of scale 1:50,000 and 1:25,000 produced by Department of Survey, Government of Nepal. c) Determination of effective catchment area for both headworks area and powerhouse area. d) Collection of hydro - meteorological data, e) Discharge measurement near by the proposed headworks area, f) Development of rating curve at head work site g) Long term hydrological assessment h) Low flow and flood flow computations i) Adoption of flow duration curve for fixation of design flow The catchment area for respective gauging station is referred on Climatological and Meteorological published data book from DHM and the remaining area is measured fro available topographic maps.
This report summarizes the analysis and understanding of the available flows in the basin and revised hydrological study of the previous study as was carried out for Feasibility Study of the Project.
5.2 Trishuli River Catchment 5.2.1 Catchment Phisiography The Trishuli River catchment lies within the Gandaki River Basin. The catchment is partly located in Dhading, Nuwakot and Rasuwa District Central Development Region of Nepal and partly in Tibet China. It is, therefore, extremely difficult to determine the percentage contribution of flow from Tibet with respect to the total catchment and basin physiography of the area that lies in Tibet because of lack of data. The catchment of Trishuli River lies in Nepal is shown in Figure 5.1 below.
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Proposed headworks site
Figure 5.1 Location of the Project Area in River System To make the hydrological analysis for better understanding and for ease of calculation, whole catchment with respect to the proposed headworks is divided into three zones. Zone 1 is a gauged catchment and a mother river where as Zone 2 is partially gauged and partially ungauged. The ungauged catchment of Zone 2(b) is correlated with Zone 2(a) gauging station (Stn 448). The catchment area of the project is divided as follows (Table 5.1):
Table 5.1 Division of catchment area
Zone Catchment area classification Catchme Coverage nt area, km2 Agri Forest Snow Rock / meadow
1 Catchment area with respect to 4110 203 433 642 2833 gauging station no 4471
2 (a) Gauged catchment - Catchment 653 308 164 0 181 area with respect to gauging station no 4482
1 Source: Hydrological estimations in Nepal, Department of Hydrology and Meteorology, Nepal 2 Source: Hydrological estimations in Nepal, Department of Hydrological and Meteorology, Nepal
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Zone Catchment area classification Catchme Coverage nt area, km2 Agri Forest Snow Rock / meadow
2 (b) Ungauged catchment - catchment 1272 750 302 0 219 area with respect to the proposed headworks excluding catchment area with respect to gauging station 447 and 4483
Total catchment with respect to the proposed 6035 1261 899 642 3233 headworks
Catchment with respect to percentage 21% 15% 11% 54% coverage
It has elevation ranging from 350 m to above 6500 m of which about 11% of the catchment is covered by snow whereas 54% is covered by rock and meadow. It is therefore could be believed that a direct surface runoff is expected at early but these areas are lies quite a distance from the proposed headworks and thus the time lag to reach the study area would take relatively longer period. A scattered distribution of flood flows can be expected. As per the physiographic regions, the catchment lies in between higher Himalayas and middle hills.
5.2.2 Drainage The Trishuli River is one of the main tributaries of the Gandaki River System, which is one of the biggest river systems of Nepal and flown north-south from the central part of Nepal. The Trishuli River flows with an average river slope of about 1 in 26. However, it is about 1 in 20 or has even mild slope in the project corridor.
The river is a snow fed type river originates from Tibet, China. The catchment area lies above snow line and thus can be expected a reliable base flow due to dry season contribution from the snowmelt. The study conducted by WECS and DHM have categorized that the catchment area above Gauging station no 447 is belongs to the hydrological Region 1 whereas catchment area with respect to gauging station 448 is partly in Hydrological Region 1 and partly in 3. However, area excluding Gauging Station 447 and 448 with respect to the proposed headworks area lies on Hydrological Region 3.
According to the topographical map of the area and the Climatological record book, the catchment of the River for both intake area and Powerhouse Area are characterized as shown in Table 5.2 below.
Table 5.2 Catchment Characteristics According to the Topography
3 Estimated from available map
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Area zone Intake area Powerhouse Area Shape Elevation, masl factor km2 % km2 % Above 5000 613 15 613 15
Between 5000 masl and 3000 2664 65 2664 65 Zone 1 1.53 Below 3000 833 20 833 20 Total Catchment area 4110 100 4110 100 Above 5000 0 0 0 0
Between 5000 masl and 3000 61 9.34 61 9.34 Zone 2 (a) 1.19 Below 3000 592 90.66 592 90.66 Total Catchment area 653 100 653 100 Above 5000 0 0 0 0
Between 5000 masl and 3000 0 0 0 0 Zone 2 (b) 1.53 Below 3000 1272 100 1280 100 Total Catchment area 1272 100 1280 100 Total catchment wrt the headworks and 6035 6042 powerhouse
The catchment area difference between the proposed headworks and the powerhouse is insignificant in comparison with the total catchment area. Moreover there are small gullies and rivulets joining in between the area and but contribution of flow would not be that critical in regards to the parent river and its tributaries joining upstream form the headworks. The shape factor of the catchment is found elongated. It is therefore the stream flow at the point of interest will be available after a long time of interval. Shape factor of Zone 2 (a) is more or less circular and thus its contribution towards flood flow would be taken place in relatively shorter period.
Zone 1 and 2 (a) is the gauged catchment and thus historic data is available for 30 years and 32 years for Gauging Station 447 and 448 respectively. However, there are no measured data available for remaining area of Zone 2 (b). A catchment correlation with 2(a) or empirical approach could be applied to generate time series flow data for Zone 2(b). Hydrological parameters such as Monsoon Wetness Index and Mean monsoon Precipitation value of 1600 mm and 1800 mm could be used respectively. These values are depicted from the Hydrological Study conducted by WECS and DHM.
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5.2.3 Climate As per climatology of Nepal, the catchment area lies in the sub-tropical reason. Because of large catchment area, there are numbers of rainfall measurement station available in the catchment. Precipitation records of different stations have been used to generate annual average precipitation using Theissen Polygon method. The annual average precipitation is estimated as 1900 mm from Theissen polygon method. It has been noticed that the South-East monsoon during June-September contributes majority of annual rainfall, about 80%.
The minimum and maximum temperature as well as relative humidity figure is referred from Meteorological Station No 809 Gorkha and 1005 Dhading has been used. The minimum and maximum temperatures are 3.2º C and 35.3º C respectively according to the data series of Index 809. The relative humidity varies from 94 to 42% over the year. Similarly, the data series of Index no 1005, Dhading, 24 hours maximum precipitation is 253 mm which was occurred in 1962. The figures revelled that the project area is experienced warm and humid climate during the months of June-October and mostly dry and cold temperature during November-December.
The project area is located alongside of the busy highway, i.e. Prithivi Highway and thus during peak hour of traffic there is air and noise pollution. However, the air and noise pollution levels are almost nil rest of the time.
5.2.4 Ecology and geology The catchment lies in the subtropical region. Forest, shrub and bushy land are the major vegetational cover within the lower level of catchment area of the Trishuli whereas rocks/meadows are found in the upper reaches of the catchment area. The percentage catchment coverage by shrub/vegetation is only 15% whereas it is 54% by rock/meadow. Geologically, the upper part of the Trishuli catchment lies in migmatite. The middle part of the catchment is covered by schist; phyllite and lower part is filled with phyllite.
5.2.5 Water use There is no consumptive use (e.g. irrigation) of water in the project area, so all the flow as available at the proposed headworks will be available for generation of electricity. Water quality of the Trishuli River seems relatively good. Springs are common in the project area but not commonly exist in and around the proposed construction sites. However, Trishuli river flow in the project area is used for rafting.
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5.3 Available data 5.3.1 Stream Flow Data Gauging Station 447 at Betrawati and 448 Tadi Khola, Belkot has the long term time series data and thus has been used for further analysis. The catchment area is divided into three zones for simplicity for analysis and is shown in Table 5.1. The Zone 2(b) is correlated using catchment correlation with the Gauging Station 448. Details of gauging stations are presented in Table 5.3.
Table 5.3 Gauging stations used to generate stream flow data at the proposed intake Station name Station Latitude Longitude Elevation Catchment No. m Area, km2 Betrawati, Nuwakot 447 270 58’ 08’” 850 11’00” 600 4,110
Tadipul, Belkot, 448 270 51’ 35’” 85` 08’18” 475 653 Nuwakot
Long term time series gauged data for Zone 1 is available for 30 years (from 1977 to 2006) at Gauging Station no 447. Likewise 32 years of data for Tadi Khola, Belkot, Station No 448 is from 1969 – 2005 (has some years missing data) is available. Therefore, the hydrological analysis for mean flows and flow duration curve were limited to these available data.
5.3.2 Rainfall data Precipitation data is needed for the analysis of surface runoff and to know the nature of the catchment with respect to the river flow. To further understand the effects of the elevation and location, the rainfall around the vicinity of the project area were also studied.
There are many rainfall stations and are spread in the Trishuli River basin (Table 5.4). For the hydrological study, all stations as noticed in the area as well as close to the catchment area were used and Theissen polygon method was applied to derive average precipitation in the catchment area.
Table 5.4 Rainfall stations in and around the catchment area
S. Station name Station Lat Long Elev Annual Monsoon Max 24 No no m precip, precip, hr mm mm rainfall 1. Rampur Chitwan 0902 270 37’ 840 05’ 256 1909 1583 228
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S. Station name Station Lat Long Elev Annual Monsoon Max 24 No no m precip, precip, hr mm mm rainfall 2. Daman, 0905 270 36’ 850 05’ 2314 1741 1339 373 Makanwanpur
3. Timure, Rasuwa 1001 1900 957 718 140
4 Arughat, Gorkha 1002 280 03’ 850 49’ 518 2639 2191 140
5 Trishuli, Nuwakot 1003 595 1742 1441 139
6 Nuwakot 1004 270 55’ 850 10’ 1003 1871 1567 178
7 Dhading 1005 270 52’ 840 56’ 1420 2186 1763 240
8 Kakani, Nuwakot 1007 270 48’ 850 15’ 2064 2773 2345 161
9 Thankot 1015 270 41’ 850 12’ 1630 2005 1594 157.4
10 Dubchachaur, 1017 270 52’ 850 34’ 1550 2409 2010 127.8 Sindhupalchowk
11 Dhunibeshi, 1038 270 43’ 850 11’ 1085 1501 1202 212 Dhading
12 Thamachit, 1054 280 10’ 850 19’ 1847 981 738 70 Rasuwa
13 Dhunche Rasuwa 1055 280 06’ 850 18’ 1982 1915 1314 138
14 Pyansayakhola, 1057 280 01’ 850 07’ 1240 146 Nuwakot
15 Tarkeghyang, 1058 280 00’ 850 33’ 2480 159 Sindhupalchowk
16 Budhanilkantha, 1071 270 47’ 850 22’ 1350 1962 1576 101 Kathmandu
17 Piagutang 1072 4091 1602 1374 69
The weightage average annual precipitation in the basin is estimated as 1900 mm.
5.4 Flow measurement Flow measurements in the Trishuli River were made on different dates and the measured flow is summarised in Table 5.5. Measurement sheet and calculation is presented in Volume III.
Table 5.5 Rainfall stations in and around the catchment area
S. No Date Measured flow, m3/s Remarks
1. 11 March 2006 49.270 Current meter measurement
2. 16 January 2011 45.155 Current meter measurement
3 21 January 2011 61.133 Current meter measurement
4 2 March 2011 46.280 Current meter measurement
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S. No Date Measured flow, m3/s Remarks 46.496 47.462
5.5 Hydrological Analysis As stated earlier that the catchment area has 447 and 448 gauging station located and has long term time series data available. Gauging Station 447 and 448 has been used to generate historical time series data for Zone 1 and Zone 2(a) respectively. However, Zone 2(b) is not possible to establish a long term time series data due to lack of gauging station. Due to catchment similarity, Gauging Station 448 has been used to establish long term time series data for Zone 2(b). Due to inconsistency in the gauged flow, some of the year flow has been discarded. Gauged flow of Gauging Station 447 and 448 is included in Appendix B for reference purpose.
5.5.1 Discharge Measurement Discharge measurement of the Trishuli River could also be a basis to generate time series data but due to few measured data it is not possible to establish a long term time series flow. However, this could be used to validate the generated data.
5.5.2 Methodologies for ungauged catchment As the measured stream flow data of the Zone 2(b) area are not available sequentially, they can only be used as the input parameters for cross-checking. Various methodologies, common to ungauged catchment, were also used to determine the hydrology of the Zone 2(b) of the catchment. The previous feasibility study has applied all empirical approaches to determine the long term time series data for Zone 2(b) and thus no further assessment has been made using those empirical approaches except using catchment correlation with Zone 2(a).
5.5.3 Catchment Correlation Since there are no availability of long term hydrological data for the Zone 2(b) of the project area, an attempt is made to establish long term time series flow data using catchment correlation with Zone 2(a). The catchment parameters are tabulated in Table 5.6 for comparison of the catchment characteristics.
Table 5.6 Catchment parameters
Parameter Zone 2(a) Zone 2(b) Remarks km2 % km2 % Catchment Area 653 1272
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Parameter Zone 2(a) Zone 2(b) Remarks >5000 m 0 0 3000-5000 m 61 9.34 <3000 m 592 90.66 1272 100
5.6 Long-term Hydrology 5.6.1 General Water availability assessment of run-off-the-river projects is made on the basis of long term hydrology of the proposed area. The long term hydrology depends on climate, topography and geology of the area. Due to mountainous topography and inadequate hydrological investigations downstream of the Gauging Station 447, long-term hydrology is difficult to establish for Zone 2(b). Major approaches of defining long-term hydrology are mean monthly flow and flow duration curve which play an important role in arriving a realistic estimation of flow and in the economic viability of a project.
5.6.2 Mean Monthly Flow Estimation of mean monthly flow can give an indication whether the flow at the proposed site seems adequate for power generation or not. Availability of longer period of data year, better the result and will be the case for this project. Generation of mean monthly flow for all region described earlier is discussed hereunder.
A. GAUGED FLOW The mean monthly flow generated from time series data for Zone 1 and Zone 2(a) and 2(b) is shown in Table 5.7.
Table 5.5.7 Mean monthly and annual flow, m3/s
Zone 1 Zone 2 (a) Zone 2(b) Available flow @ H/W Month 3 3 3 m3/s m /s m /s m /s
January 42.34 8.84 9.40 60.58 February 37.57 6.74 7.17 51.48 March 38.00 4.99 5.30 48.29 April 48.12 5.33 5.67 59.12 May 88.80 9.03 9.60 107.43 June 231.23 33.23 35.34 299.80 July 497.67 95.66 101.74 695.07 August 564.86 128.02 136.15 829.03
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Zone 1 Zone 2 (a) Zone 2(b) Available flow @ H/W Month 3 3 3 m3/s m /s m /s m /s
September 369.60 89.67 95.36 554.63
October 156.03 40.31 42.87 239.20 November 77.36 20.48 21.78 119.61
December 52.54 12.43 13.22 78.20 Annual average flow 183.68 37.89 40.30 261.87
The flow as estimated in this study is slightly less than that of FS study estimation. The reason is due to less flow as shown from the field measurement and trends of less precipitation.
The graphical presentation fo the flow is shown in Figure 5.2.
920 880 840 800 760 720 680 640 600 560 520 480 440 400
Flow, m3/s 360 320 280 240 200 160 120 80 40 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months
Figure 5.2 Graphical presentation of the mean monthly flow at the proposed H/W, m3/s B. MIP METHOD In this method, mean monthly flow was derived using measured flow in the month of January 22, 2011 and 2 March 2011. The average measured flow in the month of March 2, 2011 was 46.746 m3/s whereas 21 January 2011 flow was 61.133 m3/s, which has been used to generate mean monthly flow. Calculation flow using MIP method is presented in Table 5.8.
Table 5.8 Mean monthly flow (m3/s) at intake using MIP
Month Jan 2011 measured flow Mar 2011 Measured flow Average flow
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Month Jan 2011 measured flow Mar 2011 Measured flow Average flow
January 61.133 78.730 69.932
February 46.396 59.751 53.073
March 36.298 46.746 41.522
April 27.292 35.147 31.219
May 33.023 42.528 37.776
June 198.409 255.521 226.965
July 496.160 638.979 567.569
August 744.240 958.469 851.354
September 570.666 734.931 652.799
October 248.080 319.490 283.785
November 107.529 138.481 123.005
December 82.693 106.497 94.595
Annual 220.993 284.606 252.799
ADOPTED FLOW
Catchment correlation flow as derived from gauged flow of station 447 and 448 is adopted for further analysis. The adopted mean monthly flow in Gregorian Calander and Nepali Calendar is shown in Table 5.9 and Figure 5.3.
Table 5.9 Adopted mean monthly flow at the proposed headworks, m3/s
Gregorian month Flow, m3/s Monthly flow in Nepali month Nepali month 60.58 January 55.40 Magh 51.48 February 51.51 Falgun 48.29 March 53.66 Chaitra 59.12 April 78.55 Baishak 107.43 May 202.29 Jestha 299.80 June 496.22 Ashad 695.07 July 786.63 Srawan 829.03 August 696.94 Bhadra 554.63 September 390.00 Aswin 239.20 October 171.82 Kartik
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Gregorian month Flow, m3/s Monthly flow in Nepali month Nepali month 119.61 November 98.70 Mangsir 78.20 December 60.72 Poush
900
800 English month 700 Nepali month 600
500
Flow, m3/s 400
300
200
100
- Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Figure 5.3 Adopted mean monthly hydrograph at the proposed headworks 5.6.3 Flow duration curve (FDC) A FDC is a probability discharge curve that shows the percentage of time a particular flow is equalled or exceeded. In a run-off-the-river hydropower project, it is useful to know the variation of flow over the year so as to make ease to select the most appropriate turbine configuration as well as for project optimisation.
The derived Flow Duration Curve (FDC) is summarised in Table 5.10 and presented in Figure 5.4.
Table 5.10 Flow duration values for Trishuli River at the proposed headworks site
5% 10% 15% 20% 25% 30% 35% 40% 45% 50% Zone 1 633.25 513.40 425.00 347.65 273.70 209.10 151.30 116.45 91.80 75.99 Zone 2(a) +2(b) 282.71 212.55 172.93 143.42 111.64 82.13 61.29 48.08 36.11 29.10 Combined 915.96 725.95 597.93 491.07 385.34 291.23 212.59 164.53 127.91 105.09
55% 60% 65% 70% 75% 80% 85% 90% 95% 100% Zone 1 65.79 57.29 51.68 47.60 44.37 41.65 39.02 36.55 33.83 4.25
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Zone 2(a) +2(b) 24.97 21.46 18.41 16.26 14.32 12.20 10.28 8.58 6.48 1.20 Combined 90.76 78.75 70.09 63.86 58.69 53.85 49.29 45.13 40.31 5.45
1,000 950 900 850 800 Zone 1 Zone 2(a) + 2(b) Combined 750 700 650 600 550 500 450 Flow, m3/s 400 350 300 250 200 150 100 50 - 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Exceedence level
Figure 5.4 Adopted Flow Duration Curve 5.7 Extreme Hydrology 5.7.1 Low flow The low flow information is generally used to assess the reliability and the economics of the proposed project. If the occurrence of inadequate flow is too much frequent, a particular project might prove to be uneconomic and unreliable. Knowledge of minimum stream flow is therefore essential in the planning of any hydropower projects. The probable low flows for different return period is presented in Table 5.11.
Table 5.11 Low flows at the proposed intake site, m3/s Duration Return Period, yrs 1 day 7 days 30 days Monthly 2 52.64 54.59 58.35 59.61
10 44.26 45.40 48.42 50.16
20 42.53 43.44 45.96 48.08
Flow measurement carried out on 2nd March 2011 measured 46.496 m3/s revelled that it could be about 8 years 1 day return period flood.
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5.7.2 Flood flows Extreme flood flow plays an important role in design and location of hydraulic as well as infrastructures. Basically, such flow is considered in design of diversion weir, settling basing, gravel chamber, drainage works, canal or pipe crossings, tailrace canal or tunnel, and powerhouse. The value of flood flow acts as safety cushion to ensure safe design and safe location of hydraulic structures as well as superstructures.
The extreme flows can be adopted as determined in the Feasibility Study Report. For reference purpose, it is summarised in Table 5.12 and 5.13 for headworks and powerhouse side respectively.
Table 5.12 Adopted extreme flood flow at the proposed headworks Return period years Area 2 5 10 20 50 100 200 500 1000 Combined flow 2,622 3,701 4,469 5,240 6,291 7,122 7,988 9,195 10,159
Table 5.13 Flood flows at tailrace, m3/s
Return period years 2 5 10 20 50 100 200 500 1000 2,635 3,725 4,503 5,284 6,351 7,195 8,077 9,306 10,289
The catchment area as depicted from the available map shows that there is insignificant catchment area added in comparison to the catchment area @ headworks site. More importantly, there are no Khola and rivulets that can contribute significant amount of flash flood. As such there is one dry Khola exactly at the middle of the project alignment and looking its wideness and boulders that lies around the bank shows that there is no significant impact due to flood.
5.8 Rating curve Rating curve is a stage discharge curve established at known point of cross-section and for a known value of flow. This will gives an idea about stage discharge particularly at the point of interest so as to ensure maximum water level with minimal effect to the surrounding. Looking in view y = 252.46x2 - 169590x + 3E+07 22,000 2 of the safety of the project 20,000 R = 0.9999 location as well as 18,000 16,000 minimisation of problem, 14,000 12,000 rating curve at headworks 10,000 8,000 6,000 Discharge, m3/s 4,000 2,000 0 HydroPower_Nepal.docx 49 15 Jan 2012 335 337 339 341 343 345 Elevation, m
area and powerhouse is prepared and discusses in the subsequent section. Rating curve in a mobile bed would simply give an indicative gauge-discharge relationship. The rating curve at the proposed headworks area and powerhouse area is shown in Figure 5.5 and 5.6 which is taken from the Feasibility Study report.
Figure 5.5 Stage discharge curve just upstream of the proposed headworks area
y = 83.68x2 - 54080x + 9E+06 16,000 R2 = 1 14,000 12,000 10,000 8,000 6,000 4,000 Discharge, m3/s 2,000 0 322 324 326 328 330 332 334 336 338 elevation, m
Figure 5.6 Stage discharge curve at the location of tailrace outlet 5.9 Sedimentology and mineralogy 5.9.1 General Sediment transport in most Himalayan Rivers is a natural phenomenon. The sediment transport in the river is complex phenomenon. It is therefore a careful study of sediment inflow and assessment of deposition is of major importance in the planning of major water resources project no matter whether it is storage project or run-off-the-river projects and particularly in regions of high erosion rates such as young and fragile geological region. The major effects of sediment for the run-off-the-river projects are two folds:
" Severe abrasion, wear and tear of turbine depending on the type and nature of sediment over time, and " Effects on morphology of the river downstream of the dam because of concentrated release of river flow. The 1st effect will reduce the life of the hydro-mechanical equipment whereas the 2nd effect would cause degradation of the river downstream of the dam resulting instability in either side banks. Thus a realistic and objective assessment of sedimentation is necessary for both project economic and environmental consideration.
5.9.2 Sources of sediment Seasonal variations in erosion rate are caused by factors such as in crease in supply of readily erodible sediment after a prolonged dry period, change in soil cover and seasonal
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variation in rainfall intensity. Particle size may range from fine sand to big boulders as the Trishuli River is one of the snow fed river and rounded boulders indicates that these are rolled form quite a distance upstream. Major causes of the sediment production are as follows:
" Sheet and rill erosion, " Stream channel erosion, " Gully (ephermeral and permanent gully) erosion, " Flood plain scour, and " Landslides, mass wasting The estimation of sediment is not an exact science, largely due to various processes which produce the sediment. The best measures of sediment yields from a catchment are:
" Observation of sediment volume deposited in reservoir or river meandering parts, and " To take samples of suspended sediment to obtain annual yields. Infact there are no reservoir in the river itself but one can visualise sediment type and nature form the outer bend of meandering section of the river. According to the study made in this type of river, estimation on sedimentation yields has been made by different researcher. Upland cultivation, and sloping terraces is resulting soil loss between 20-200 tonnes per hectare annually (Chapter 1, Hydro India 2003, Conference paper, Bhusal J. K). Debris torrent in gullies of hilly watersheds provides major sediment inputs to rivers. Depositional areas are upland valleys and flood plains, lakes and stream channel in plain areas.
The sediment transport studies are mainly based on data collection on suspended sediment concentrations. Based on data collected by DHM and information available from different other sources, researchers, have analysed data and have generalised sediment yield of different catchment. The sediment yield of catchment from where river is originates is presented in Table 5.14 below:
Table 5.14 Sediment yield of different river system catchment
S. No River system/watershed Concentration, tons/km2/yr 1. Rivers entering from Tibet 500-1000
2. Rivers in and from high Himalayas 300-1000
3. Rivers in and from middle mountains 3000-8000
4. Rivers in and from Siwaliks 5000-15000
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Looking at the catchment characteristics of the Trishuli River at the proposed intake, all four categories of river system lies in the Trishuli River. It is therefore the sediment study of the system is complex if there are no sediment measurement stations in the river itself. As a matter of fact, the river has a sediment measurement station located some 45 km upstream from the proposed intake.
Sediment transport rates are high in Himalayan Rivers, and specific sediment transport rates exceeding 10,000 t/km2/year have been reported (WECS, 1987). A few examples are shown in Table 5.15 but it is worth to note here that the data from Kulekhani are based on field measurements in a reservoir. It should be also noted that data from Sunkoshi, Arun and Tamur are based on more than 30 year’s data.
Table 5.15 Sediment transport rates in some rivers in Nepal4
S. No River system/watershed Concentration, tons/km2/yr 1. Kulekhani 12,400
2. Sunkoshi 3,800
3. Marsyangdi 7,700
4. Arun 4,025
5 Tamur 6,800
6 Spatakoshi 2,670
Sediment load in the river may vary from year to year. Therefore, for design purposes a long-term data base is utmost important. Fluctuations in the annual sediment load are usually much larger than flow variations. Larger seasonal variations are usually seen in the sediment load. Most of the sediment transport takes place during the monsoon season (usually assumed to be 80% to 90%). High sediment concentrations must, however, is expected during relatively small pre-monsoon floods.
Removal of sediment particles from the diverted water is very important for any hydropower plant. Suspended sediment particles cause severe abrasion to the runner and other mechanical parts of a turbine and thus drastically decrease its life.
4 Sthapit 1996, Chaturvedi, and Asthana 1996, Kayastha 1996
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5.9.3 Sediment analysis As mentioned earlier that the river has sediment measurement station located at Gauging Station no 447, Betrawati, Nuwakot. The average monthly sediment load in the river is shown in Table 5.16 below.
Table 5.16 Average monthly sediment loads, tons/km2
Area Ja Fe Ma Ap Ma Jun Jul Aug Se Oct No De Annua , km2 n b r r y p v c l
4110 1.9 2.4 1.8 4.1 15. 89. 299. 436. 74 36. 5.7 2.8 4.0 8 7 5 2 2
The catchment area at the proposed intake is 6035 km2 which in fact is about 35% more area then that of catchment area at station 447. This indicates that there is more sediment concentration at the proposed headworks site. However, temporal variation in precipitation, it is unlikely to occur simultaneously since the catchment area is significantly bigger in size. It is therefore, approximately 35% more concentration of sediment at one event would be highly optimistic. Considering 90% of the sediment concentration over three months (July-September) is passing and out of which 85% is considered as suspended load.
The average monthly monsoon sediment is relatively higher than that of rest of the months of a year.
The sediment concentration of the river at the proposed site is not available though there is recorded sediment measurement at the Station 447. It is therefore, a tentative estimation of the sediment transport is made using empirical formulae as suggested for hill irrigation system. 1.534 Q = s A0.264
Where,
3 2 2 Qs = annual silting rate, (Mm /100 km ) from 100 km of basin area
A= total basin area, km2
Considering the density of sediment as 1.5 ton/m3, the annual sediment load for Trishuli River at the proposed headworks is tentatively estimated as 14 million ton. Assuming that 60% of the sediment load is transported during three months of monsoon, the total
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monsoon mass is estimated to be 8 million tons. The mean discharge of 823 m3/s at the proposed intake are for the 3 monsoon months (i.e. July, August, and September), the sediment concentration is comes out to be 1250 ppm approximately. However, there are many small rivers and rivulets joins in between the Station 447 and the proposed headworks carrying significant amount of sediment and thus the above figure cannot be adopted.
Using Table 1.16 concept of sediment concentration, Zone I could contribute 40% of the area at 1000 tons/km2/year and rest area to 3000 tons. Similarly, Zone 2(a) and 2(b) contribute the same concentration level of 3000ton/km2/year. The sediment concentration contribution according to the area is (40% x 4110 x 1000 +60%x4110 x 3000+ (653+1272) x 3000) giving 14,817,000 ton/yr. Assuming that 90% of the sediment passing three months of a year between July-September and out of which 85% considered as suspended load, the concentration shall be (14817000 x 90% x 85%)/2134 Mm3 of water) 5312 ppm. The average sediment concentration for the design of the settling basin shall thus be not less than 5500 ppm. Hence for design of the project auxiliaries, the sediment concentration is taken as 5.5 kg/m3.
5.9.4 Mineralogy The mineralogical analysis of the sediment sample is necessary to determine the presence of hard and soft mineral contents. As the rock types of project comprises of quartzite, slate, marble, phyllite, etc and thus the content of quartz in the sediment is expected to be dominated. Sediments collected from the samples of fine and coarse deposits are usually used for mineralogical analysis. Individual sediment samples can also be used if there is sufficient.
During this study, two samples from river bed were taken for mineralogical analysis to find the mineral composition. The results of these samples are presented in Table 5.17. More detail result can be referred to FS Study.
Table 5.17 Summary of mineralogical composition of sediment samples (Refer FS Study) Sample 1 from Belkhu Type of Percentage, % Mohr’s Remarks minerals hardness Quartz 50-55% 7 Feldspar 35-40% 6 Mica 5-10% < 3 Other minerals <5% >6 Tourmaline, garnet, rock fragments
Sample 2 from Malekhu Khola
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Type of Percentage, % Mohr’s Remarks minerals hardness Quartz 55-60% 7 Feldspar 30-35% 6 Mica 6-8% <3 Muscovite and biotite Other minerals >6 Tourmaline, garnet, calcite
The above mineral content study shows a high content of quartz in suspended sediments. Since higher the percentage of the quartz content, there will be higher probability of wearing of turbine. Besides, a more rigorous settling criterion should be adopted. In present case, the criterion adopted for the design of settling basin is exclusion of 85 % of all particle size greater than 0.15 mm and 90% of particles size greater than 0.2 mm. Since the plant is low head high flow, the abrasion rate could not be as severe as it would be for the high head plant.
5.10 Conclusions Based on the above studies on hydrology and sedimentology, followings points can be concluded for the design of hydropower project components:
! The 100 year return flood was estimated as 7,122 m3/s at headworks whereas it is 7195 m3/s at tailrace outlet;.
! 65% dependable flow is 70.10 m3/s whereas it is 164.53 m3/s for 40 percentile exceedance level of flow.
! The compensation flow to maintain the river ecosystem at downstream of intake is 4.8 m3/s, which is 10 % of minimum adopted mean monthly flow, unless otherwise stated by environmental study.
! The mineralogical composition of sediment sampling shows that about 85% of the sediment is of quartzite.
! The settling basin shall be designed for 5,500 ppm sediment concentration whereas the particles to be settled will be 85% and of 80% of particle size greater then 0.2 mm and 0.15 mm particle sizes respectively. Sediment sampling at the proposed headworks area would give better understanding of the sediment yield in the river and thus suggested to be carried out.
! Installation of Gauging Station around the project area would give an understanding of the estimated time series data so as to ascertain the flow availability for power generation.
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6. WATER STAKEHOLDERS 6.1 General Trishuli River is one of the main tributaries of the Gandak River system of Nepal, one of the biggest river system of Nepal, which is flowing north-south and lies at the Central part of Nepal. There are many rivers and rivulets join to the Trishuli River. Trishuli River is a snow fed type River which originates form Tibet, China. The proposed project is located at Janghare, a border between Richowktar and Arbastar. Richowktar is located on the left bank of the Trishuli and lies in Benighat VDC whereas Arbastar lies on right bank and in Salang VDC. There are no development right issues related to the river use within the project area. The project area is accessible at the left bank of the Trishuli River via Richowktar, Malekhu, whereas there is no access on the right Bank. It is therefore, people living on the right bank are much optimistic to see the implementation of the project since upon implementation of the project, there is a hope to have a road available in their areas. As a matter of fact, the construction site except headworks is not accessible from motorable road. Moreover, two suspended bridges over Trishuli River make the project area accessible.
The project is a RoR but snow fed type project and thus there is no issue of resettlement at the headworks area. There is no specific issue at headworks area even raising of dam height significantly. The right bank is a rocky area whereas the left bank is cultivable land but no production because of sand deposit. Diversion of Trishuli River through a tunnel and thereafter open canal as a waterway might need to deviate in some areas so that resettlement can be avoided. In totality, there is no need of resettlement due to the presence of project.
The project will be seriously affected if there are provisions of basin transfer project located upstream of the TR-SGP. However, it is not noticed to have basin transfer project located upstream of the proposed project area and thus it will not be an issue furthermore.
6.2 Irrigation requirement As mentioned earlier that the river is flowing incised way so that there are no uses of Trishuli River flow for irrigation purpose at the proposed project corridor and even upstream except utilisation of river flow to generate power. It is therefore there are no water shearing issues in regards to irrigation purpose especially from the Trishuli River. However, there is consumptive use of tributary rivers of Trishuli, for example Thoppal Khola, Mahesh Khola, Belkhu Khola etc.
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6.3 White water rafting The river is famous for white water rafting. Many locals except project area are involved in this business as an employee, according to the site visit findings. The white water rafting basically starts from Baireni, Glachi, Dhading and end at Charaundi. But few rafters also move forward to further down. Construction of diversion weir will obstruct to the rafter and thus the rafting business could hamper. The EIA study of the project should address the issues and possible mitigation measures.
6.4 Environmental release In the reach between the proposed intake site and powerhouse area there are no tributaries feeding the Trishuli River especially in dry season. However, there is one spring located on the left bank of Trishuli River; close to the suspended bridge to Richowktar-Arbastar. It was estimated about 10 lps in the month of February, which infact is negligible looking at the flow in the Trishuli River. It is therefore, the flow contribution from these tributaries is almost zero. Hence, minimum downstream release will need to be provided for environmental reason and to maintain river ecosystem. The downstream release will be 10% of the driest mean monthly flow, unless the EIA study will recommend otherwise.
6.5 Conclusion The EIA study of the project should address all these and the other water right issues, if any, and propose the best way-out for optimal use of the natural resources. The developer should work in close co-operation amongst the Trishuli River water users and stakeholders for the best utilisation of the Trishuli River flow. It is advisable to have close contact and mutual co-operation with the Trishuli rafting business group so that development activities directly or indirectly related in the use of Trishuli River would be carried out for the benefits of the local peoples, stakeholders as well as the TR-SGP.
There is no flow contribution between headworks and powerhouse area except on spring (~10 lps) and thus 10% of the driest month’s flow need to be released to maintain river ecosystem.
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7. OPTIMISATION STUDY 7.1 Background The section of the report summarizes the previous feasibility study findings, applicability and new approach to conceive the project so as to ascertain the best resource utilisation. Minimisation of environmental foot prints and maximisation of available resources is sought while arriving at optimal plant capacity.
The general approaches for the optimisation studies of the project, which determine the optimum size of the project, are as follows;
• Selection of parameters to be optimised, identification of their range and establishment of a series of alternatives. • Carry out the conceptual design and cost estimate of each alternative. • Estimation of benefits from each alternative. • Comparison of cost and benefits. 7.2 Options considered The plant is a low head high flow type of project. Elevation difference between the planned headworks to the proposed outlet is about 22 m. It is therefore the project optimisation is solely depends on the following parameters:
• Diversion weir height/crest elevation with minimal environmental foot prints upstream; • Exceedence level of flow; • Waterway, and • Minimisation of system losses; 7.2.1 Diversion Weir Crest Elevation The plant is a low head plant thus it would be better to increase the diversion weir height to its optimal level. The study found the previous proposed dam crest level is still the best elevation to ascertain better utilisation of the available area upstream of the dam with minimal foot print. It is therefore the dam crest elevation is fixed at 348 masl. The diversion dam height from the river bed level would be little higher than 8 m. Moreover, due to space problem to locate surface settling basin, it would be more appropriate to set diversion weir height to its maximum elevation so that it would be possible to create a headpond upstream of dam from which relatively less sediment free water can be abstracted without settling basin. With a diversion dam height of about 8 m, the headpond will be about 2-3 km long.
It is important not to construct any kind of obstruction in the diversion weir. A free flow diversion weir would be a better option to ensure free passage of flash flood. Moreover, to maintain the present river bed level, it is suggested to fix gats in the dam itself and to be
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operated hydraulically from either end of the river bank. Segment type of gate is to be affixed in the dam itself and to be operated remotely. This will function as a bottom outlet and would help to reduce d/s stilling basin length significantly. Detailed needs to be discussed during detail design phase.
The tail water level from the recent field survey verification shows 318.0 masl, which is slightly higher than that of the present water surface level in Trishuli River whereas level at planned intake area is 340 m. The total available head is 22 m.
7.2.2 Flow exceedence level The overall flow pattern in the river is found little less than it was estimated in the Previous Feasibility Study but it is not that significant. The adopted FDC at the proposed headworks is shown in Figure 7.1. The corresponding 30, 40, 45 and 65-percentile level of flow is 291, 164, 128 and 70 m3/s respectively.
1,000 900 800 700 600 500 400 Flow, m3/s 300 200 100 - 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Exceedence level-%
Figure 7.1 FDC @ the proposed headworks area A range of flow between 30 and 65 – percentile have been used to determine project optimality level.
Flow and power duration curve for different exceedence level of flow is shown in Figure 7.2 – 7.6.
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1000 80 Flow Dura on & Power Dura on Curve 900 Flo 70 800 w 60 700 600 50 500 40 MW
Flow, m3/s 400 30 300 20 200 100 10 0 0 0% 20% 40% 60% 80% 100% Exceedence level-%
Figure 7.2 Flow and power duration curve at 30-percentile exceedence level of flow
1000 Flow Dura on & Power Dura on Curve 40 900 Flow 35 800 MW 30 700 600 25 500 20 MW
Flow, m3/s 400 15 300 10 200 100 5 0 0 0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 70.00% 80.00% 90.00% 100.00% Exceedence level-%
Figure 7.3 Flow and power duration curve at 40-percentile exceedence level of flow
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1000 Flow Dura on & Power Dura on Curve 25 900 Flow 800 MW 20 700 600 15 500 MW
Flow, m3/s 400 10 300 200 5 100 0 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Exceedence level-%
Figure 7.4 Flow and power duration curve at 50-percentile exceedence level of flow
1000 Flow Dura on & Power Dura on Curve 16 900 Flow 14 800 MW 12 700 600 10 500 8 MW
Flow, m3/s 400 6 300 4 200 100 2 0 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Exceedence level-%
Figure 7.5 Flow and power duration curve at 60-percentile exceedence level of flow Higher the adoption of the design flow more is the generation but due to fragile rock type and huge flow to be handled, it is wise to set an optimal level in a moderate way so that there is no underutilisation of available resources in one hand and better financial attraction from the developer’s prospective. A preliminary resource optimisation study shows that 40-percentil exceedence level of flow gives highest return (Figure 7.6).
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350 1.645
300 1.640 250
200 1.635 B/C 150 Flow, m3/s 100 1.630 50 Design Discharge B/C - 1.625 30 35 40 45 50 55 60 Flow exceedence level-%
Figure 7.6 Preliminary optimisation of the plant at index price level As seen from the above figure that the project is seen more attractive at 40-percentile exceedence level of flow. However, due to higher diversion flow might end up with higher geological uncertainties and more geological risks and it is thus envisaged to be adopted little lower level of design flow. Moreover, to be aligned with the recent government hydropower development vision, it is set the optimal plant capacity of 25 MW and ascertains the design flow accordingly. In this way, the design flow will be 128 m3/s which is 45-percentile exceedence level of flow. This is not far from the optimal plant capacity as assessed from the preliminary optimality point.
7.2.3 Waterway The waterway as conceived in the previous feasibility was a combination of shorter length tunnel, open canal and penstock pipe. This alternative requires relatively big area of land acquisition. Moreover, it could invite slope instability due to wider open canal, as well as system interruption due to longer open waterway system. This issue was discussed with the Client but their inclination still was with this option and thus no tunnelling option was adopted.
In the present study, it is suggested to go for tunnel as waterway and shorter penstock pipe. The tunnel is optimised for the design discharge of 128 m3/s and the result is present in Figure 7.7.
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16000 Headrace Tunnel Optimisation
14000 Optimum Diameter
12000 Net Cost (Construction Cost + Revenue Loss) [Mill NRs] Construction Cost (Tunnel + O&M cost) [Mill NRs] 10000 NPV of Revenue Loss [Mill NRs] Annual Cost Cost Annual 8000 5.2
6000
4000
2000
0 8 5.0 5.2 5.6 5.8 6.0 6.2 6.6 6.8 7.0 7.2 7.6 7.8 8.0 8.2 8.6 8.8 9.0 9.2 9.6 9.8 5.4 6.4 7.4 8.4 9.4 11.0 11.2 11.6 11.8 11.4 10.0 10.2 10.6 10.8 12.0 12.2 12.6 12.8 13.0 13.2 Tunnel Diameter [m] 10.4 12.4 13.4
Figure 7.7 Preliminary optimisation of the plant at index price level The optimal tunnel finished surface is 8 m and will be horseshoe type tunnel. The total length of tunnel is about 3730 m and will have one intermediate adit at Bhante Khola. A covered crossing at Bhante khole is proposed so that we will end up with shorter tunnel length. This can also be used for flushing tunnel if requires.
Due to weak rock type, it is proposed to have a full concrete lining. This will also be beneficial to reduce headloss in the system.
7.2.4 Penstock pipe The optimal average penstock pipe size is 7600 mm and the thickness varies between 19-28 mm.
7.2.5 Number of turbine An initial assessment indicates that two Kaplan units will be applicable for the given head and available discharge. The turbine centre line is set at 319.5 masl and tailrace water level is 318.5 m.
7.2.6 Head loss The waterway length includes headrace tunnel (3750 m) and ~150 m long penstock pipe to convey design discharge. The head loss in the waterway system for the proposed configuration is about 5 m.
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7.3 Conceptual design The project is envisaged to have the following project features:
! Bottom sluicer diversion weir but un-gated spillway; ! Orifice type side intake to abstract surface water; ! Headrace tunnel of 8 m diameter finished size; ! Intermediate adit at Bhante Khola; ! Shorter steel penstock; ! Surge shaft; ! Surface powerhouse; ! Kaplan turbine (2 units); ! 66 kV transmission line; 7.4 Plant Capacity The plant capacity and energy generation on monthly basis is present in Table 7.1.
Table 7.1 Energy and revenue generation
monthly Design Total Generation Dry season Wet season Month Net head flow flow Headloss capacity energy energy
(m3/s) (m3/s) m m (kW) (kWh) (kWh)
Apr 59.12 54.295 1.22 28.78 13,243 4,578,11 4,483,72
May 107.43 102.601 4.35 25.65 22,304 15,808,21
Jun 299.80 127.912 6.94 23.06 25,000 16,929,01
July 695.07 127.912 6.94 23.06 25,000 17,493,31
Aug 829.03 127.912 6.94 23.06 25,000 17,493,31
Sept 554.63 127.912 6.94 23.06 25,000 17,179,56
Oct 239.20 127.912 6.94 23.06 25,000 17,493,31
Nov 119.61 114.783 5.45 24.55 23,889 16,176,49
Dec 78.20 73.373 2.23 27.77 17,271 6,169,64 6,042,43
Jan 60.58 55.748 1.29 28.71 13,566 9,692,39
Feb 51.48 46.653 0.91 29.09 11,505 7,424,23
Mar 48.29 43.460 0.79 29.21 10,761 7,688,47
Maximum Power Generation, kW 25,000
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monthly Design Total Generation Dry season Wet season Month Net head flow flow Headloss capacity energy energy
(m3/s) (m3/s) m m (kW) (kWh) (kWh)
Total seasonal Energy, kWh 35,552,831 129,099,35
Annual generation, GWh 35.55 129.10
Total energy, GWh 164.65
The optimal plant capacity at 45-percentile is 25 MW and annual energy generation after system deduction such as consumptive use, transmission line losses, dry and wet season outage and the net annual energy production is 164.65 GWh.
7.5 Possibility of using headpond as PRoR Though it was mentioned in the previous Feasibility Study that the plant could be developed as a PROR project but now, without settling basin, the intake level will be well above and thus there will be no sufficient water available for peaking plant. If it is desired for short period of peaking generation, still this can be considered but need to discuss during detailed design phase. Moreover, it will depend on availability of differential tariff..
7.6 Transmission line Transmission line is a power evacuation system connected to the substation located at one end and national grid at the other end. The length of the transmission line is about 40 m (to planned substation at Naubise) from the proposed substation, according to the connection agreement. However, it could be an option to connect nearby existing 132 kV transmission line, as proposed approach in the previous Feasibility Study. A separate discussion is made in the respective section of the report.
7.7 Conclusion After carrying out an optimisation of the project and evaluation of the previous Feasibility proposition, it is concluded to adopt the following for further analysis:
! To adopt 45-percentile exceedence level of flow which is about 128 m3/s;
! Crest level of diversion weir to be set at 348 masl and provide bottom sluicer and un-gated spillway;
! Head pond could be used as a surface settling basin and thus no separate settling basin is proposed;
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! Orifice type side intake is proposed to abstract surface water from the headpond, meaning that the orifice opening area will be much bigger;
! Tunnel option would be an better option to minimise environmental foot prints;
! Optimal finished tunnel diameter is 8 m;
! Optimal average penstock is 7600 mm;
! Two units of Kaplan turbine seems appropriate;
! Optimal plant capacity is found at 40-percentile exceedence level. However, due to insignificant benefits difference between 40 and 45 –percentile exceedence level of flow uses, it is suggested to adopt 45-percentile exceedence level of flow to size plant capacity, which will also help to reduce underground uncertainties to some extent.
! Alternative power evacuation option as proposed in the previous feasibility study could be an option;
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8. SELECTED PROJECT 8.1 General TR-SGP is a run-off-the-river but snow fed type project. The proposed system of the power plant will be running at its full capacity of about 25 MW for about 7 months. The design discharge of the proposed plant is 127.9 m3/s and has about 45-percentile probability exceedence level of flow. The river gradient is 1 in 26 at the headworks area and 1 in 150 around tailrace area. Because of having moderate river slope, bigger boulders are not seen lying on the river bed. The proposed project layout is the best option available in the licensing area and as identified during field visit. There is sufficient area available along the right bank as well as left bank of the Trishuli River infrastructure facilities. The diversion weir on the both banks is founded on rock and it is envisioned that rock can be expected at bed as well at shallow depth. The project is designed as run-off- the-river (RoR) project but it could be also possible to develop as peak run-off-the-river (PRoR) and operation time could be about 2 hours a day. If he power purchase agreement (PPA) is favourable in the days ahead, this option can be considered but at this stage of study, it is omitted.
The configuration of the project mainly consists of diversion weir, side intake, collection chamber, headrace tunnel surface steel penstock, surface powerhouse and open/box canal tailrace. Besides that there is Bhante Khola crossing. The design basis for the individual components of the project is described herein in the subsequent sections.
8.2 Headworks design criteria 8.2.1 General The main function of headworks structures in run-of-the-river schemes is to divert water into the water conveyance system during all the various flow conditions and facilitate safe passage of all floods. The headworks must allow the sediment and debris loads carried by the river to pass the weir, exclude coarser particles from the withdrawn water and return the coarser sediment charge to the river before the water is released to the plant.
Many hydro schemes in rivers with severe sediment transport suffer from poor sediment control at the intake and poor techniques for removal of sediments in the withdrawn water. This may result in blocking of intakes, deposition of silt, sand and gravel in channels, ponds and forebay and wear of turbines runners, valves and other components. Maintenance costs as well as production losses grow high after a few years of operation. Success may turn to failure in hydro projects when poor sediment-handling techniques are applied.
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Limited sediment and flow data may easily lead to one of two extremes, if conventional procedures for planning of large reservoir schemes are followed without considering how short the available database is:
! Failure after a short due to "unexpected" high floods or "unexpected" high sediment loads.
! Unnecessary high concentration and operation costs due to "over design".
Neither of these alternatives results in optimised hydro projects. It is however, not feasible to extend the period of field observations considerably in order to obtain sufficient data for most small and medium run-of-the-river schemes.
The nature and events creating due to mass wasting will not be represented in the available statistical material. And, consequently, the available data may not be used to estimate such events determining "design sediment concentration" at the intake site. A fair chance of over estimation of sediment is most likely since data available by using statistical analysis because of time-series available for hydrological and sediment analysis was too short.
It is therefore important to obtain a headworks design, which is flexible with respect to sediment loads and flushing capacity as well as flood discharge capacity. Design and operation must furthermore be linked together in such a way that experience gained through the first years of operation is used to trim the headworks components and the operation procedures.
8.2.2 Principal of headworks design In order to secure reliable withdrawal of water and the safety of the headworks structures, the Consultant has applied the following basic principles in design.
Passage of the sediment load All the sediment load of the river must pass the diversion structure. The diversion structures shall preferably not disturb the natural sediment transport pattern in the river.
Passage of floating debris All floating debris must pass the diversion structures and not accumulate in front of the intake. It might, however, be necessary to install trash rack cleaners to remove the trash and secure a continuous operation of the intake.
Passage of hazard floods A hazard flood situation may occur in any Himalayan river resulting from landslide or GLOF. The headworks will have a free overflow weir to survive a hazard flood without serious damage.
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Sediment control at the intake The riverbed shall not be allowed to build up in front of the intake in any flow conditions and permit bed load to enter the waterways of the scheme.
Settling basin control
The content of the suspended sediments in the water supplied to the plant must meet the design criteria selected for the project. Water shall be supplied to the plant continuously during flushing of the settling basins. Monitoring of the sediment load in the water downstream of the settling basins should be carried out so that the plant may be steeped down or closed hydropower if the content of suspended sediment load becomes higher than the adopted standards during extreme situations
8.2.3 Design assumption The design assumptions for the headworks are:
! The intake must be accessible for clearing during monsoon floods in the event of blockage,
! Structures should not be vulnerable to hazard floods, i.e. a free overflow weir is required,
! 90% of all sediment particles with fall diameter of 0.2 mm shall be excluded from flow downstream of the settling basin. All suspended sediment larger than this size shall be returned to the river. 10% of the minimum mean monthly flow will be released to maintain river ecosystem,
! Spillways are to be provided to discharge water entering the intake back to the Trishuli River.
! Design discharge of 127.29 m3/s at 55-percentile exceedence level of flow.
8.2.4 River diversion The temporary river diversion selection primarily represents a compromise between the cost of the diversion facilities and the amount of risk involved. Proper layout and arrangement of diversion structures will minimise potential risks such as damage due to floods to the work in progress at a minimum expenses. The following factors are envisioned to be important in determination of best diversion scheme:
! Stream flow characteristics,
! Size and frequency of diversion flood,
! Method of diversion,
! Specifications requirements,
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In selection of design flood of the diversion weir, the following parameters are to be considered:
! Safety of workmen and downstream inhabitants in case of failure of diversion works,
! Length and time of work to be considered and also has to determine the number of flood seasons that will be encountered,
! Cost of possible damage to work completed or work still under construction stage and it is flooded,
! The cost of delay to the completion of the work, including the cost of idle time of contractor’s equipment.
Two cofferdams are required viz: diversion weir and side intake area since these are located quite a distance and extension of one cofferdam is found not economical.
Looking at the cost of incurred for protective work to handled progressively larger floods in one hand is compared with the cost of damages resulting from such floods without the increased protection work. A judgement is made to determine amount of risk that is warranted. Construction of coffer dam is the responsibility of the contractor however; possible alternative is highlighted below for consideration. The design flood could be 1:5 years to 1:20 years depending on the river type and morphology. The later one could be adopted for coffer dam design flow.
Coffer dam at intake
River width is the intake area is much wider and thus will not have space problem. Cofferdam parallel to river flow and curved section both upstream and downstream to link the natural ground will be constructed. 1:1 side slope and 2.3 m top width constructed of river material and boulder armouring or gabion works along river face could be adopted. Height of the coffer dam could be around 4-5 m from the river bed to ensure round the year construction of the project.
Coffer Dam at Diversion Weir
The river width at the proposed headworks area is over 100 m and thus there is availability of plenty of working space in many respects. The diversion of the river during construction of headworks will be carried out in two stages. The 1st stages will be diversion of river towards left bank and then start construction of diversion weir, and sluiceway. In the second stage, flow to be diverted along the left bank and complete the remaining works. The second stage river diversion might need coffer dam both upstream and downstream of the diversion weir. Abundance of sand and boulder in the river bed
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could be used to construct coffer dam and boulder armouring along river face. The top width of the embankment cofferdam will be 3.5 m and side slope of 1:1.5 (V:H) where possible otherwise it will be 1:1 side slope.
8.2.5 Diversion weir The diversion weir is a combination of controlled and uncontrolled overflow weir of 100 m long of which 6 number of gates of a thickness of 5 m to be fitted into the weir. Gates will not be exposed and hidden in the dam itself. It will be spread along the length. More numbers of gates can be placed but has to be decided during detail design phase. It is operated hydraulically from one end of the diversion weir. It is expected that the diversion dam will be founded on rock outcrops in all sides. The purpose of providing bottom sluiceway is to reduce length of the stilling basin.
The top width of the diversion weir will be 2.0 m and upstream side of weir top 2 m is made 1:1 slope. A standard ogee shape diversion weir to be designed where the diversion length has no gates located.
The radial gate is of 5.75 m wide by 3.0 m high six numbers radial gates is designed as bottom outlet. It has two functions: flushing of sediment deposition and killing of energy that could generate while falling from 8 m high diversion dam. There must minimum of 2 m concrete on top of section of the gate location to ensure structural stability of the weak section. In between the radial gates, a concrete wall, almost the same opening width has been conceived. The diversion weir is so designed to divert the design discharge of 127.9 m3/s. This is indeed a low head high flow plant and thus every cm of head has its own significance. The crest level of diversion weir is set at 348 masl, about 8 m above from the river bed. This will also increase reservoir/head pond water volume. Moreover, this could be used at base load case if it is opted to operate the plant during dry season. The diversion weir is designed for 1 in 100 years return period flood (7100 m3/s) and free board is provided for a flood with 1 in 1000 years frequency (10,159 m3/s). The sediment in the Trishuli River is quite significant and thus severe abrasion and damage to the overflow spillway and stilling basin is expected. Therefore, to minimize such problems in weir, the provision of opening at the weir will help in excluding bed load whereas floating debris and flash flood will pass over the diversion weir. High strength concrete of +C35 has to be provided at stilling basin and outer part of the weir whereas C25 concrete with 40% boulders to be infill the diversion weir. Boulder/gravel material and geotextile is recommended to be placed as a filter material. Grouted rock bolts are also suggested to increase bond between rock and concrete faces.
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Due to exposed rock outcrop at the both the banks along the diversion weir, no bank protection structures are required. Shotcrete along right bank and extension of protection work from the exposed rock outcrops along left bank will be required.
A fish passage is located along left bank of the river but details to be carried out during detailed design phase.
The calculated water surface elevation for 100 years and 1000 years flood are 354.6 and 357.0 masl respectively. So the bank protection work along left bank to be extended to the level specified above. Details on flood level and weir geometry will be required during detailed design phase.
8.2.6 Head pond Upon construction of 8 m high diversion weir, there will be a back water of Trishuli River extended to a distance of about 2 km upstream, which will be close to the Malekhu Khola- Trishuli River confluence. The right bank is rocky whereas it is old terrace of Trishuli on the left bank. The average width of river is 150 m and river gradient is about 1 in 125. The average depth of flow will be 5 m. The total head pond water volume will be about 1.2 Million m3. With the design discharge of 127.9 m3/s it is possible to run plant in full swing for 3.0 hours or so. Because of the head pond, some section in the left bank has to construct bank protection work since the material in this stretch is all sandy soil. Moreover, there are no trees and the bank slope in general is about 300.
8.2.7 Stilling basin The diversion dam is designed in such a way that the bottom sluicer will kill the residual energy as generated from a falling head of 8 m plus flow depth. A 30 m long concrete floor and baffle wall along with end sill s proposed for stilling basin. The height of the end sill is 500 mm from the invert level.
8.2.8 Under sluice Under sluice has its significant importance to prevent entry of bed load of the river to the intake and buildup of bed load in front of the intake. In the present study, provision of undersluice has been avoided and its function is envisioned to be performed by radial gate which is fitted in the diversion weir. All together there are 6 numbers of radial get having opening size of 5.75 x 3.0 m. The radial will be hydraulically operable and all operating system will be located at one bank of the river. The main function of the under sluice will be to flush sediment deposition accumulated infront of the diversion weir and help to kill energy generated from a falling head of 8 m or so and subsequently will reduce the length of the stilling basin.
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As such there is no undersluice but possibility of using undersluice to be made during detailed study phase.
8.2.9 Intake An orifice type side intake is located about 500 m upstream from the diversion weir axis and on the right bank of the river. The intake invert (345.80) is located some 3.2 m below the crest level of the diversion weir, which is 348.0 masl. The opening of the intake will be 5.0 m wide by 3.1 m high and the total number opening shall be 15 to ensure limiting velocity through intake. Coarse trash rack has been provided at the intake and sloped at 1:5 (H:V) so as to make ease of cleaning the trash rack.
The invert level of intake is 345.80 masl. Water surface at the intake will be 100 mm below the level of diversion weir crest. The opening area is 30% more than that of required opening area to allow space for coarse trash rack. The incoming velocity to the intake is taken as 1 m/s which is limiting velocity through the intake. The water surface level at intake during normal flow condition will be 348.0 masl.
It is expected that the invert level of the undersluice will be 3.2 m above the river baed and thus chances of getting big boulders into waterway system is less. Moreover, the flow velocity in the headpond will be significantly low and thus it is envisioned that there will be less sediment entrant into the system. The intake plate form will be constructed well above 1000 years flash flood level and will be 357.10 m. The intake gate needs to be operated in such a way that it will ensure diversion of design flow into the system.
A vertical sliding type gate is designed to control flow. Likewise, trash rack will be of fixed type. Rack cleaning mechanism is conceived and will be done mechanically. Safety fencing around intake plate form is provided.
A detail rating curve needs to be established during detail design phase and has to be validated during very 1st year of plant operation.
All concrete structure shall be of C35 grade and structural reinforcement will be provided. Intake wall top width will be 400 and a tapered section as it goes to river bed. Foundation is to be located well below the scour depth. It is envisioned that no bed rock for foundation. Bank protection works are to be carried out both upstream and downstream.
8.2.10 Collection Chamber Right behind the intake, a collection chamber is designed. This is required to ascertain a better transition between tunnel inlet dimension and intake opening. The minimum width of the chamber at extreme end is kept 4 m where as it is 15 m at the center. The slope of the chamber is sloped to downstream end so that dredging or mechanical removal of
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accumulated sediment could be possible. The extreme downstream is connected by an access road for taking mechanical equipment for sediment removal. The width of the access road is 3.5 m min. The invert level of the collection chamber at lowest level is 2.5 m with respect to the invert level of the side intake. Due to headpond, it is not possible to have natural flushing from collection chamber. A concrete floor of min 500 mm is designed but need to check during detail design phase of the study.
The back slope of the collection chamber requires protection work. Moreover, catch drain in uphill slope is requiring to control over surface run off coming to the chamber.
8.2.11 Settling basin Due to space problem, it was not possible to locate settling basin. However, it is anticipated that particles during normal operating conditions sediment will be settled in the head pond. However, it is not the case during wet months since flow in the river is high and more suspended load.
8.3 Waterway 8.3.1 General Waterway is meant to convey design discharge to the forebay. Looking at the climatological condition of the project area, it is designed the canal in such a way that there is minimal head loss in one hand and no reduction of water volume. Possibility of using water for irrigation of the land located downstream side of the canal. Siphoning of water abstraction is possible. The width of the canal is big enough and thus it is possible to have evaporation losses in the system. Keeping this in mind, an economical best efficient section and allowable depth to minimise evaporation losses condition has been met in the design.
Right bank of the Trishuli River along the lower terrace of Salang VDC has been used to convey water down to the powerhouse.
Looking at the topography and geological conditions, part of the water way is through tunnel and part of it as an open canal. The open canal water way lies on alluvial Trishuli River fan where sand and boulders can be seen almost every part of the project alignment. The tunnel part lies in the weakest geological areas and thus a careful consideration in designing should be given though rock outcrop can be seen at the inlet tunnel portal area where as no rock can be seen on outlet side.
8.3.2 Headrace tunnel The headrace tunnel starts immediately after the collection chamber. The tunnel portal is located on exposed rock whereas the outlet is at upslope of Majhigaoun. An intermediate
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adit is envisioned at Bhante Khola. The tunnel is designed with minimal pressure flow condition. Because of the size of the tunnel, it is expected a complete concrete lining as secondary support. The tunnel will be a low pressure tunnel. Invert level elevation is shown in Table 8.1.
Table 8.1 Tunnel inverts level at key areas Down S. No Particulars Chainage Tunnel invert level Slope Drop at the start of tunnel with 1 0 333.87 respect to pipe invert level 2 Junction point at Bhante Khola 1700 1:1000 332.17 3 HRT outlet 3730 1:1000 330.14
As the investigation of the rock conditions along the tunnel alignment is limited, a careful lining system has been proposed to ensure leaking out of tunnel flow. The tunnel location shall comply with the following criteria:
(γ w HF ) dmin = + 20m (γ r Cosβ )
Where,
3 γw = density of water = 1000 kg/m
3 γr = density of rock overburden = 2500 kg/m
H = water head in tunnel,
F = factor of safety = 1.6
= average incline of hillside tunnel level and uphill
H d = w − δf + 20m min Cosβ
Where,
Hw = groundwater level above tunnel level,
δf = deduction due to curved equi-potential lines in a valley sides,
+20 m is due consideration to weathered rock
Head loss in the tunnel is calculated by using Manning’s formula and is given below:
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2 ⎛ ⎛ q ⎞ ⎞ ⎜ ⎜ ⎟ L ⎟ ⎜ a ⎟ h ⎝ ⎠ = ⎜ 4 ⎟ ⎜ 2 3 ⎟ ⎜ M R ⎟ ⎝ ⎠
Where,
A = pipe cross-sectional area, m2
Q = design discharge, m3/s
h = head loss, m
L = total penstock length, m
M = Manning’s number = 1/n
R = Hydraulic radius of pipe, m
The shape of the tunnel will be inverted “D” type. The bottom width of the tunnel will be 8 m and depth upto the spring line will be 4 m, tunnel diameter of 8 m would give a cross- sectional area of 57.1 m2 (clear opening area). Additional excavation may need for lining requirement. The bed slope of tunnel is taken as 1 in 1000. The length of headrace tunnel is about 3730 m and giving a head loss of about 4.6 mm.
It is envisioned that the tunnel is passed through weak geological rock condition and thus a full concrete lining of 350 mm thickness as a permanent support and fibre shotcrete and rock bolting as temporary support. The shotcrete thickness shall be 100-150 mm and will be applied depending on the site conditions. If the tunnel part is found poor, steel strap will be applied and fit into the tunnel itself and after its application, a full concrete lining will be provided. The worst combination of final tunnel lining shall be concrete and steel strap.
The inlet tunnel portal will be a concrete structure whereas outlet portal could partly in the open cut.
Fences will be erected both at tunnel inlet and outlet. About 30 m of tunnel outlet portal will be through soil and thus an open cut is needed to construct portal. Open cut may lead instability problem upstream but it is not that serious because the cover is very low and terrain upstream is relatively flat.
TRANSITION ZONE Right after the side intake and before tunnel inlet, a transition need to be constructed since one end of the transition width is 8 m and the other end is over 70 m. The width of the collection chamber varies between 4 and 15 m and thus a uniform transition will be
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required. The tunnel is located almost at the half length of the side intake opening so that one can attain a smooth transition. Moreover, the collection chamber is sloped towards downstream end to facilitate easy removal of deposited sediment.
ROCK/SAND TRAP As there is no settling basin, it is expected heavy sediment deposition in the tunnel invert level. A sand trap is envisioned at eh end of the head race tunnel before penstock pipe start. A minimum distance is maintained and will be located some 10 m upstream from the surge shaft-headrace tunnel junction. It is extended the full width of the tunnel and the length is about 15 m. Two 300 mm steel pipe is conceived to remove deposited bed load will be a continuous flushing especially during wet months. The sand trap is covered by concrete precast slab of 300 mm wide and 250 mm depth and will be extended full width of the tunnel. If the tunnel width is divided in two sections, 2 sets of flushing pipe on each section is required.
From the preliminary understanding of the tunnel, it is envisioned that the tunnel will be a full lined tunnel and thus the concept of rock trap may not be applicable. However, it can be used as a sand trap and is the most to construct.
SPOIL TIP ARRANGEMENT The tunnel spoils from the both ends of the headrace and other spoil from surface excavation should be managed properly. There will be two headings and thus excavation is possible from both ends. The total volume of tunnel muck is about 272,300 m3 with a bulging factor of 1.3. There are three headings for tunnel muck out. Almost half of the tunnel muck will be taken out from Bhante Khola and the remaining is from inlet and outlet portal area. and thus the tunnel muck from each heading is It is therefore half of the tunnel muck will be taken towards headworks area and rest towards the end of the rectangular canal end side. A proper disposal is required and thus toe of the spoil tip area is protected using gabion mattress wall.
Required slope of the spoil tip area is maintained as 1 in 1.5. As per site condition, there will be sufficient height of gabion structure for preventions from the stones rolling down. In additions sufficient drainage systems has also been proposed.
SAFETY FENCING Safety fencing has to be provided around the transition zone and portal area.
SLOPE STABILIZATION There is a huge excavation at the tunnel inlet portal and thus it might lead an instability problem in the slope. Provision of slope stabilization has to be made so that one can reduce operation and maintenance of the hill slope.
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8.3.3 Intermediate Adit To shorten the tunnel length, it is proposed to have an exposed waterway along the tunnel alignment. Bhante Khola is found the most prominent site to do so. Opening of tunnel from this point will reduce the tunnel length significantly. The invert level of tunnel at this point is 332.17 masl. The excavation of Bhante Khola at this point is quite significant. It is therefore this job has to be finished during dry months. Completion of river works and opening access for tunnelling works is planned at one side of the river, preferably along left bank is envisioned to be a better option.
Once tunnelling is complete, bulk head door of steel is planned to allow for future access. Safe access from possible Bhante Khola flash flood is required.
It is also important to make a provision for tunnel flushing and this need to be fitted in the bulkhead door.
Proper management of spoil disposal is required. Landscaping of the area is also important and need to be performed.
SLOPE STABILISATION Slope stabilisation around adit area need to be carried out. Moreover, maintaining of Bhante Khol ais also important.
CATCH DRAIN Catch drain around adit is also important and to be constructed. Safe discharge of catch dain flow to be made. Stone masonry wall constructed in 1:4 C/S mortar is proposed and a minimum cross-sectional area is provided as a catch drain. Necessary gabion wall at major concerned area has been made in the cost estimation.
8.3.4 Bhante Khola Crossing This indeed is an adit point. River will be excavated to required depth and where excavation of tunnel to both upstream and downstream is planned for. An access to adit will be constructed and the remaining rive width is maintained for river flow. It is suggested to go for the required tunnel invert level and start concreting for an equivalent tunnel section of 8 m dia tunnel. One the construction is complete, backfilling of the crossing will be done and thereon maintained the river gradient. C30 concrete coud be used to construct concrete structure. It is expected that about 50 m long concrete section is required to pass the Bhante Khola.
Bank protection and maintaining of the river gradient is required Excavated river material will be stockpiled alongside of the river and will be used to construct embankment.
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8.4 Surge Shaft A surge analysis for different scenarios has been conducted as per the procedures recommended by Mosonyi to allow for both upsurge and down-surge effects. The surge shaft has been designed for the critical case of down-surge. Using the equations of the mass balancing, the surge analysis of the system has been checked.
An orifice type of surge tank with a 25 m diameter at the top and 10 m at the bottom has been proposed for the stability of the surges. The top of the surge shaft will be at 360.22 masl with a freeboard of 5 m. This surge analysis showed that the down surge will reach 333.32 masl, which is slightly higher than that of the soffit level of the headrace tunnel. The result of the surge analysis for the total instantaneous closure of all units shows that the upsurge will reach to 355.22 masl. A steel wire mesh cover and manhole on top of the surge shaft will be provided for maintenance access. Based on the surge analysis, the study proposes a surge shaft with following features:
! Shape of surge shaft : Circular;
! Diameter: 25 m at top and 10 m at bottom;
! Height: 30 m of which 7 m is 10 m diameter;
! Static Water Level: 343.25 masl;
! Maximum upsurge level: 355.22 masl;
! Maximum down surge level: 333.32 masl;
Figure 8.1 shows the upsurge and down surge scenarios for the assumed conditions.
360 355 350 345 340 335 330 Elevation (m) 325
time (sec)
Upsurge
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350 345 340 335 330 325 Elevation (m) 320
time (sec)
Downsurge
Figure 8.1 Upsurge and down surge scenarios for oscillation steps of 3 sec The surge shaft will be an open type structure with the top end exposed above ground surface. Minimum 5 m of the surge shaft are to be constructed above ground level and to be fenced in. Gratings on top and a gate in the side wall will be provided to ensure monitoring of the water level in the shaft.
Surge shaft is about 10 m off from the HRT to ensure no disturbance to construction of the HRT.
8.5 Penstock 8.5.1 General The penstock alignment is chosen considering the topography, geological conditions, and stability of the area. It is taken right after the forebay and conveys the design discharge to the turbine. The following formula has been used to calculate the head loss in the pipe.
2 ⎛ ⎛ q ⎞ ⎞ ⎜ ⎜ ⎟ L ⎟ ⎜ a ⎟ h ⎝ ⎠ = ⎜ 4 ⎟ ⎜ 2 3 ⎟ ⎜ M R ⎟ ⎝ ⎠
Where,
a = pipe cross-sectional area, m2
q = design discharge, m3/s
h = head loss, m
L = total penstock length, m
M = Manning’s number = 1/n
R = Hydraulic radius of pipe, m
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The thickness of the pipe is determined on the basis of maximum pressure to withstand at the given stress of the pipe material. Negative pressure on pipe has also checked to ensure bulking effect on pipe.
Looking at the size of the penstock pipe, two sets of pipe has taken directly from end of headrace tunnel down to the powerhouse. The penstock will be a surface penstock pipe. It is therefore a single pipe has to convey 1/2 of the design flow i.e. 64 m3/s. The length of the penstock pipe is about 100 m and has internal diameter of 5660 mm and having thickness 13 mm upto 20 mm. There will be a metal strip of equal angle welded on the surface of penstock pipe to increase buckling of the pipe and will be wrapped at a distance of 1-5-2 m.
Since it is a continuation of the tunnel and thus there is no need to provide additional submergence to penstock pipe. A smooth transition between tunnel face and penstock is required. Due to twin penstock pipes, relatively wider tunnel section and the width of tunnel will be about 11.5 m. Alternatively, two tunnel outlet at the end of HRT may require since the opening of the tunnel is coming too big. The other alternative is to take a single pipe and bifurcation of the penstock outside the tunnel portal, and this is coming out economical option. The single pipe economical diameter is 7600 mm. The thickness of 15 mm and concrete encased pipe is designed. The length of the pipe will be minimum 20- 25 m and out of which 20 m will be inside the tunnel. Manhole at the beginning of the penstock is provided for access provision.
The HRT tunnel outlet pipe invert level is 330.14 masl. The anchor block is located just outside of the outlet and a bifurcated pipe is designed.
Manning’s’ roughness coefficient has taken as 0.013. The total head loss in the pipe line is expected in the order of 1.0 m.
8.5.2 Support piers/anchor blocks An anchor block is a gravity structure keyed to the penstock so that movement of pipe due to imbalance forces that occur in normal operational mode of the plant will held in position.
There are all together 2 anchor blocks, one is at just outside of HRT. This block will hold the bifurcated pipe. Moreover, expansion/contraction joint is located just outside of the block. The anchor blocks will experience both vertical and horizontal deflection angle. The anchor blocks are to be constructed of C15 concrete with 40% plums and nominal reinforcement. Hoop reinforcement is required around the pipe. The size of the anchor block is about 10 x 13.2 x 9 m (L:B:H). A key is required to maintain sufficient concrete work to maintain down slope. A clear separation between two penstock pipes is
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maintained 1000 mm. Pipe will be laid minimum of 300 mm above the ground so that construction and or maintenance would be possible.
Likewise, there will be one anchor blocks located close to the powerhouse where the block will experienced vertical bend. Due to inward resultant force, the block is designed to hold the penstock pipe and will be relatively smaller in size. A detail to be designed during detailed design phase.
8.5.3 Surface drain To avoid gully formation along the penstock alignment, boulder packing is proposed with a minimum thickness of 400 mm. However, no storm flow shall be allowed to flow in boulder lining.
8.5.4 Slope stabilisation Slope stabilisation is not a major work in the penstock alignment. However, care to be given to stabilise hill slope since the penstock require almost 15 m of wide bench along the slope.
8.6 Powerhouse 8.6.1 General The proposed powerhouse is located at terrace on the right bank of the Trishuli River and also on the right bank of the small gully at Majuwatar, Salang VDC, Dhading. The intention of powerhouse location at this location is to keep the things in safest place from Trishuli River. The area is thick alluvium layer having sand and boulders whose bearing capacity is in the order of 200 kN/m2.
The powerhouse is a surface structure to accommodate two generating units of capacity 12.5 MW each. The powerhouse consists of a RCC structure that will house the machine floor and control building. The machine floors are inlet valve floor, turbine floor, generator floor, maintenance and unloading bay. An overhead traveling crane is provided to facilitate installation and maintenance of equipment. The superstructure of powerhouse will be constructed with RCC columns and beams and precast concrete block masonry wall. For lighting and ventilation purpose, windows will be provided. There will be two doors, the smaller opening will be for daily access purpose whereas the bigger opening will be for transportation of equipment. The roof of the powerhouse is constructed of steel truss structures rested on RCC columns and covered with corrugated galvanized iron sheet.
Fences will be erected around the powerhouse. The main entrance of the powerhouse will be faced towards main access.
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The powerhouse is 64 m long, 43 m wide and 34 m high building. The elevation of the turbine axis is set at 319.50 m elevation. The powerhouse will have the following major floor area available and are discussed in subsequent sections.
8.6.2 Main powerhouse floor The main powerhouse floor consists of:
• Machine floor • generator floor • erection bay; • control building, • inlet valve floor, • workshop, store, rest room, and common room MACHINE FLOOR This level of floor is mainly to house turbines and their auxiliaries. Firefighting arrangements and other accessories of turbines including lubricating systems will be kept. This floor will be located at an elevation of 315.00 m.
GENERATOR FLOOR Only generator cover will be exposed at this floor. This is the main floor area where most of the local controlling and protection system are located. This floor will be located at an elevation of 321.4 m. It is accessible through stair case.
ERECTION BAY The erection bay is meant for maintenance of powerhouse equipment. Overhead traveling crane, large shutter opening for access is available. The elevation of this floor level will be same as generator floor level. Store room will be located.
CONTROL BUILDING Control building will have two floors. The ground floor will serve battery room/generator room where as the 1st floor will contain office room, rest room, control room, visitors room etc. Office and control room will have window facing to the generator floor side and will be constructed of sound proof style where possible. It is wise to keep AC in office and control room.
INLET VALVE FLOOR The purpose of this floor is to house penstock pipe outlet, inlet valve, spiral casing, draft tubes, controlling equipment for inlet valve, drainage pump, cooling water system etc. Drainage pump will be fitted here to pump water through sump well.
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SWITCHYARD An outdoor switchyard is planned to construct adjacent to the powerhouse. The switchyard area will be 50 m x 50 m. The area will be protected by a fence. Proper drainage system will be constructed along the periphery of the switchyard and all area will be covered by 600 mm thick river aggregates.
PERIPHERAL DRAIN Drains are provided around the periphery of each floor level and directed towards the sump well from where a pump will through out the collected flow.
Powerhouse drain water to be diverted to the gully located on right side of the powerhouse. Stone masonry drain structure is proposed.
WATER SUPPLY AND SEWERAGE The supply of potable water to the powerhouse can be use from the natural stream. There is no such source available near by the powerhouse but the source as the public are using could be taken. However, a public consensus has to be take prior commencement of construction.
A septic tank will be provided with the provision of a soak pit in the powerhouse area to manage the sewerage.
SPOIL DISPOSAL The excavated earth material will be dumped either at eth left gully where forebay spoil is dumped or alternatively alongside the existing gully so that an earthen embankment could be possible. Alternatively the outside meandering part of the Trishuli River could be used for spoil disposal.
8.7 Tailrace The tailrace alignment form powerhouse down to the Trishuli River is proposed RCC lined rectangular box section on the basis of quantity of flow to be diverted. The tailrace shall also function a role of natural gully since it has to take this flow down to the River. The tailrace alignment normally would follow the existing gully course. In doing so, the developer has to purchase part of the required land on either side of the existing gully bank. The size of the tailrace canal is calculated using Manning’s formula and is given below:
1/ 2 2 / 3 ⎛ h ⎞ Q = A.M.R ⎜ ⎟ Where, ⎝ l ⎠
A = Cross-sectional area, m2
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Q = Design discharge, m3/s
H = Available head, m
L = Length of canal, m
M = Manning’s number = 1/n =1/0.013
R = Hydraulic radius of culvert, m
6 m wide 6 m deep and about 100m long RCC lined rectangular box canal has been provided. Freeboard of 1.5 m is provided. The concrete grade shall be C30. The slope of the canal will be 1:1000. The outlet part of the tailrace canal is left at exposed rock so as to reduce the cost in one hand and to attain a sound foundation at the toe on the other hand. However, necessary bank protection is required.
After the generation, water will be discharged back to the tailrace canal via draft tube. The tailrace will function as a pressure conduit. Provision of stop-logs and vertical gate is made at the end of the draft tube. Similarly a hydraulically operable vertical gate is also provided at the end of the tailrace canal which will act as an emergency control over backwater effect of the Trishuli River when the plant is not in operation. Water level immediately after the tailrace outlet is 318.75 m during the normal operation period. The measured water level in dry season in the Trishuli at the proposed tailrace outlet is 318.0 m.
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9. ELECTRO-MECHANICAL STUDY 9.1 General The powerhouse electro-mechanical equipment have been studied in conjunction with electrical equipment, civil works and hydro mechanical equipment of the project and the study of the same will cover the following equipment and works in this phase:
9.2 Turbine type selection The selection of turbine type depends on the parameters such as available discharge, net head and the capacity of the generating units. The objectives of the study are to select an appropriate type of hydraulic turbine and its accessories for efficient, economic, reliable and safe operation of the power plant.
The selection of type of turbine primarily depends upon the net lead available and design discharge. For the rated net head of 23.06 meter and design discharge of 127.9 m3/s vertical Kaplan turbine will be the best one. Kaplan Turbine is found appropriate for this particular site. Kaplan turbine arrangement has been selected based on the Figure 9.1 (monogram here under).
Figure 9.1 Turbine Type Selection Monogram The calculated specific speed for the net head of 23.06 m and unit flow of 31.975 m3/s against synchronous speed of 250 rpm shall be 466 rpm. This range of specific speed is best for Kaplan runner. Therefore, Two numbers of Kaplan turbines are proposed for the rated head of 23.06 m and design discharge of 127.9 m3/s. Each turbine is capable of
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handling 63.95 m3/s discharge (design) at a rated net head of 23.06 m, which results in the electrical power of 6.25 kW at an average overall efficiency of 0.86. The size and speed of the turbine is such that the total costs of civil, electrical and mechanical works will be minimized. Number of units’ selection is done by taking consideration of transportation of the heaviest E/M components (30 Ton Rotor) through the local bridges in Nepalese highway.
Turbine instrumentation such as control boards, governor control cabinets will be located close to the relevant units of the turbine floor. Equipment arrangements will be adequate with respect to space and access requirement for transportation, installation, commissioning, operation and maintenance. The principal characteristics or rating of the Vertical shaft type Kaplan turbine for the project will be:
! Installed capacity 25 MW
! No of units 4
! Rated discharge(Q) 31.975 x 4 m3/s
! Rated net head (H) 23.06 m
! Rated speed (N) 250 rpm
! Number of pair of generator poles 12
! Specific speed (ns) 466
! Runaway speed 500 rpm
! Turbine Centre Line 319.5 masl
! Tail water Level 318.5 masl
! Generator Floor Level 325.9 masl
! Seal level of Tailrace gate 316.5 masl
! Powerhouse Size (L x B x H) 64m x43m x34mDesign criteria
For the available flow, it is found economical to adopt two units of 12.5 MW Vertical Shaft Type Kaplan Turbine. However, due to weight of the heaviest unit of the auxiliaries, the Client has suggested adopting 4 units of Vertical Shaft Type Kaplan unit and the same has been adopted for further analysis.
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9.3 Design Criteria The general design and performance specifications for the mechanical equipment are of the current standards, practices and based on the following criteria.
! Each turbine has installed capacity of 6.25 MW
! Each mechanical component shall safely cope with all normal and fault conditions; avoid any over stressing of material and equipment.
! Equipment shall be of standard design, providing satisfactory degree in safety, reliability and ease of operation.
! The turbine setting should minimise cavitation,
! The governor should be of the Proportional Integral Differential (PID) electronic hydraulic type permitting independent unit operation,
! Governor’s timing should be selected to optimise pressure and speed rises.
9.3.1 Governor Each turbine will be equipped with PID type electronic digital governors with an electro hydraulic servo system for guide vane control to permit independent unit operation. The governor will ensure stable governing in parallel operation. In case of frequency fluctuation in the system, an automatic switch over to speed control will be initiated to stabilise the power system frequency. In addition, a simplified opening controller will enable a continuation of the turbine operation, if the speed and power controller with the electronic feedback device fail. The required regulation data are presented as follows for flywheel effect of generator.
! Speed rise during full load rejection 25%
! Pressure rise in the penstock, just upstream of the turbine, will not exceed 20% of maximum static pressure during full load rejection. Moreover, the Governor shall have adjustable time setting provision for closing and opening of guide vane so that the surge effect is very minimal.
The oil pressure unit for the governor system will be of sufficient capacity to drive the governor and actuate the oil hydraulic servomotors of the associated turbine inlet valve. The rated pressure of the oil pressure system will be suitable size.
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9.3.2 Turbine inlet valve One turbine inlet valve will be provided for each turbine unit. The inlet valves will be located inside the powerhouse, immediately upstream the turbines. Each valve will serve to fully cut or give free water flow for the related turbine. The valves, however, will not be control valves to regulate the water flow to the turbine, which is done by the guide vanes of the turbines. The valves shall be operated automatically or manually. The valves shall be standard design butterfly or spherical type, which offers the most smooth flow into the turbines. The diameter of the valve will be selected to optimise valve cost and hydraulic loss through the valve.
The valves will operated under the following conditions: ! Normal operating conditions
Normal valve operation, i.e. closing and opening, will take place under balanced water pressure conditions with the turbine guide vane closed.
! Emergency operating conditions
In emergency cases, the valves will able to close under 2 times the maximum turbine discharge and head at the turbine inlet at the specified minimum closing time without causing the pressure in the penstock to rise above its design value. The closing time of the valve will be so selected that the valve safely can shut off the water supply to the turbine from full flow without exceeding the maximum allowable pressure rise. The closing time will be set during commissioning by changing a fixed orifice furnished with the servo motor.
9.3.3 Other system accessories The following other accessories are required for safe and economical operation of the turbine under any operating conditions and their details will be prepared during detail study of the project.
! Cooling and service water system,
! Drainage and dewatering system,
! Compressed air/N2 accumulator system,
! Oil treatment and transfer system,
! Ventilator/air conditioning system,
! Fire protection system,
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9.3.4 Lifting arrangement (EOT Crane) Electric Overhead Travelling (EOT) crane has been proposed inside the powerhouse for lifting and placing of turbines, generators and other heavy equipment. The crane shall be designed in such a way that all movement take place smoothly and positively without slippage or creeping of the loads.
Selection of two units of machines inside the powerhouse result smaller the size of generator rotor. The crane hook capacity is dependent upon the heaviest component to be handled. For this site condition of head and discharge the approximately weight of the 8 MVA generator rotor is 25 ton. The lifting capacity of the main hoist shall be at least 30 ton and that of the auxiliary hoist 15 ton. The crane shall be designed to be capable of travelling with its full load suspended in any position.
The Cranes will be of standard steel girder all welded box construction, complete with all necessary mechanical and electrical equipment.
9.3.5 Cooling water and service water system The cooling and service water system will supply the water in sufficient quantity to the following components.
! Main generators coolers
! Bearing oil coolers
! Fire fighting system
! Washing and clearing points
! Air conditioning system if necessary
The system shall tap the cooling water either from penstock or from tailrace pit by pumping or from other sources. To provide service water during complete stand still, with the valves closed, smaller tap-off will be made from upstream of the turbine inlet valves.
9.3.6 Drainages and dewatering system The drainage and dewatering system will serve the following purpose:
! to drain the powerhouse seepage water
! to dewater the power conduit
! to drain the powerhouse in the event of emergency
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The system will consist of close drains (to prevent evaporation inside the powerhouse) leading to the drainage pit. Three drainage pumps and three dewatering pumps with associated valves and pipe works will be located in the drainage sump. These pumps will discharge to the tail water. To dewater the penstock & power conduit, a drain valve with piping work just upstream of main inlet valve will be provided for each unit.
9.3.7 Compressed air system / Nitrogen (N2) accumulator Separate compressed air system will be provided for,
! service compressed air system
! governor compressed air system
Three compressors of suitable rating will feed the both system. Under normal operating conditions, one compressor will act as duty and the other as stand by.mechanical brakes of the generator Ring piping in the powerhouse for connection rating and system sizing of the compressed air system or of N2 Accumulator will be carried out in next phase.
9.3.8 Oil treatment and transfer system The oil transfer treatment system will consist of the following main parts:
! three clean oil storage tanks
! three dirty oil storage tank
! one clean oil transfer pumps
! one dirty transfer pump
! one oil purify equipment
! supply and return piping
! fire extinguishing equipment for the treatment room
! control an safety device
Storage tanks, oil purifier and transfer pumps will be installed near maintenance bay in the turbine floor. Separate piping set up, tanks and transfer pumps will be provided for supply and return lines in order to avoid mixing of non-purified oil. Sufficient storage capacity will be provided for the supply of oil volume equal to the entire hydraulic oil volume of one power unit. The capacity of oil transfer system will be determined in detail engineering study of the Project.
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9.4 Mechanical workshop A mechanical workshop will be equipment with machine tools and devices appropriate for trouble free maintenance and repair of all mechanical components and machining of the smaller components of the mechanical, electrical equipment and hydraulic steel structure. All the appropriate list of the workshop equipment, tools and meters shall be in the scope of electro-mechanical equipment supplier.
9.5 Ventilation and air conditioning system The main purpose of the ventilation system for powerhouse will be as follows:
! to provide adequate fresh air for the personnel
! to remove heat generated by mechanical and electrical equipment.
! to provide smoke exhaust ventilation in case of fire to minimize the circulation of smoke and produce of combustion.
The underground powerhouse ventilation system will have a ventilation room probably at the top floor above the control room. The system consists of a fresh air handling unit and an air conditioning unit. The fresh air handling unit is installed inside the ventilation room and consists of air filters and three air admission fans, three “on duty” and one” stand by”. The unit sucks air from outside and distributes it via appropriate ducting to different locations of generator floor, turbine floor or other places such as control room.
The controlled environment will be provided inside the control room. For this, three suitable size of Air conditioning unit are provided and one will work as “On duty” and other will be “stand by.” The air requirement and system sizing of A.C. & ventilation system will be determined in next phase of study.
9.6 Fire fighting and protection system A suitable fire protection system will be installed at different location of turbine, generator and control room and other places as required by design. The details or fire protection system will be studied in next phase of study.
9.7 Diesel generating set One 250 kVA diesel engine generating set complete with the necessary switchgear and accessories will be provided to supply emergency electric for operation of cooling water pump system, governor oil system, battery charger, communication facilities, powerhouse lighting & ventilation, black start and any other system as deemed necessary.
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10. HYDRO-MECHANICAL STUDY 10.1 General Hydro-mechanical components of a hydropower project constitute of gates, trashracks, stoplogs, penstock and other similar steel structures, which face hydraulic load of water and transfer it to the civil (concrete) structures. Such components need to be designed to achieve optimum structural safety by adopting most cost-effective design and minimum hydraulic losses during detailed engineering phase.
Requirements of hydro-mechanical structures at various locations of the water conveyance system, perform structural design calculations, preparation of technical specifications and selection of major structural parts of the components in order to complete the overall project layout need to be assess at the detailed engineering phase of the project.
10.2 Steel Penstock The optimized diameter 7.6m of the penstock has been calculated by using various formulas like ASCE manual, ESHA guideline, DOED guideline, P.J. Bier formula. Besides this, the energy loss against the prevailing PPA rates has also been taken into account. The water velocity of 2.82 m/s is within the standard limit.
However, being the large diameter of 7.6m and short length of only 40m, two units of the penstock can be provided. So, two double lane pipes of internal diameter 5.6m and thickness from 15 to 22 mm is recommended from fabrication, erection and handling point of view. Moreover, double lane pipe provision would be beneficial at part flow situation and at maintenance time as well. Other factors such as corrosion allowance, water hammer effect, factor of safety, material specification, welding requirements and criteria need to be clearly mentioned in technical specification in the tender documents.
10.3 Gates, Stoplogs and Trashrack Radial gates six numbers. are considered at diversion dam. The gates shall be used for closure, inspection and maintenance purposes of the headworks and other structures of the water conveyance system including turbines, and also be used for water filling operation into the tunnel, penstock etc. after they have been dewatered. All the radial gates are to be fitted inside the dam structure and to be operated from the left bank by putting all hydraulic units on the left bank. Gate wise stoplogs are to be located in upstream face of the dam. The size of the gate shall be design so that hoisting mechanism can be fitted inside the dam, no outside exposure. Similarly, stop logs to be fitted accordingly. Trishuli river flood is too high and thus un-gated diversion weir is
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required. The steel support shall be used for raising the gate and hoist by using a handling tool at the time of their maintenance. The gate shall be designed to be leak proof in closed condition.
Similarly, another 16 numbers of vertical roller gates and stoplogs are considered at intake area. The provisions of stoplogs are necessary for the maintenance purpose of gates. All gates and stoplogs shall be with guide frame, hoist, sealing arrangement, controls, steel support with handling tool, and appurtenant parts complete with necessary accessories and worked out at detailed engineering phase.
Coarse trashracks are necessary at intake whereas fine trash racks are envisioned at tunnel inlet and penstock inlet area. The bar spacing are 35 mm and 150 mm for fine and coarse trashrack panel respectively. The position, location and sizes of hydro-mechanical components are illustrated in drawing Headworks, Diversion Weir Plan and Sections. At tunnel inlet, fine trash rack will be installed. Between the intake and tunnel inlet, we will have a pond to ascertain transition between the intake opening and tunnel width. A sloping access to the invert of this chamber to be taken from the right bank.
There is a bulkhead door at Bhunte Khola where we have planned to locate intermediate adit. At the end of the HRT we will have tunnel and penstock transition. There will be two steel pipes and will have manhole on each pipe and will be 4 numbers.
Surge shaft will have an inspection gates above upsurge level and steel gratings on top. There will be four Kaplan unit inside the powerhouse and draft tube gates. Tailrace gate is also required to control back water effect to the powerhouse.
10.4 Gates and Stoplogs Feasibility study level size, type, and location of the hydraulic steel structures viz. gates, trashracks, stoplogs and penstock are summarized in the following table here under.
Table 10.1 Gates and stoplogs SN Item Type Quantity Remarks 1. Gates at Dam (5.75m x 3m) Radial 6 nos Hydraulically operated 2. Stoplogs for Radial gates Vertical 6 nos Electrical (5.75m x 3m) sliding monorail hoist 3. Intake gates (5.5m x 3m) Vertical 16 nos Electrical sliding monorail hoist 4. Intake stoplogs (5.5m x 3m) Vertical 16 nos Electrical sliding monorail hoist
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SN Item Type Quantity Remarks 5. Draft tube gates (4m x 2.5m) Vertical 4 nos Electrical sliding monorail hoisting 6. Tailrace gate (7m x 3m) Vertical 1 nos Electrical hoist sliding
7. Tailrace stoplogs (7m x 3m) Vertical 1 nos Electrical hoist sliding
8. Intake Trashrack (5.5m x 3m) Coarse 16 nos
9. Trashrack at Tunnel inlet (8m x Fine 1 no 8m) 10. Trashrack at Penstock Inlet (5.4 Fine 2 nos x 5.4m) 11. Man hole at transition of HRT 4 nos. and penstock 12. Bulkhead door at Bhunte Khola 1 nos adit 13. Inspection gate at surge shaft Sliding 1 nos spindle (1.5m x 1.5m) 14. Steel penstock pipe (ID 2 x 5.6 40 m x4 nos If single lane is m diameter, thickness Varies adopted, ID from 15 mm to 22 mm) 7.6m, thicknes varies from 18 mm to 30 mm 15. Miscellaneous items (steel As per lot gratings, Railings, Galvanized requirement hatch covers, ladder etc.)
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11. TRANSMISSION LINE 11.1 Background THP is a simple run off river project with the estimated installed capacity of 25 MW. The power generated from THP is to be evacuated to the nearest possible interconnection point to the national grid, safely and efficiently via a power evacuating transmission line.
This chapter presents the power evacuating transmission line from THP switchyard to the possible interconnection points (delivery point), and the interconnection scheme. Design of basic parameters and selection of components of the transmission line and interconnection scheme are also discussed hereunder.
11.2 Objectives and scope of work The objective of the study is to summarize the possible power transmission into the national grid with a techno-economical basis of the system.
• Determination and analysis of possible interconnection point alternatives • Design of transmission line i.e., selection of optimum transmission voltage level, choice of optimum conductor size and other line parameters and components • Determination and analysis of optimum transmission line route and interconnection scheme • Cost estimation of transmission line and interconnection scheme. 11.3 Output The output of the study shall include: • Selection of the interconnection (delivery) point • Recommended transmission line route and interconnection scheme • Description of selected route • Plan of the proposed transmission line route in appropriate scale • Transmission line parameters including voltage level, conductor size and components. • Cost estimation (BoQ) of the transmission line and interconnection scheme. 11.4 Methodology for study 11.4.1 Basic Approach The basic approach to the study consisted of the collection of basic data and fixing the route alignments. The basic data collection included: • Collection of information regarding the existing transmission system in the project vicinity
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• Latest edition of topographical maps prepared by Nepal Government (NG)/ Topographical Survey Branch in 1:25,000 and 1:50,000 scale. • Digital maps and district maps of the area. • Existing national grid coordinates of control points around the project area.
11.4.2 Desk study In the initial phase of the designing of transmission line, desk study was carried out. On the basis of collected data and thorough research, basic plan was prepared for the interconnection point alternatives and tentative route alignments. Extensive review of previous report and government plan and program to upgrade the transmission line corridor has been reviewed.
11.4.3 Data collection Necessary data were collected through meetings with related personnel and study of maps. Following maps were studied for the collection of necessary data:
• District map of Nuwakot and Rasuwa districts • Topographical maps of the region in 1:25,000 and 1:50,000 scale • Digital data of the area
11.4.4 Interconnection with the national grid Loop in loop out (pi connection) scheme, with the proposed NEA 220 kV transmission line:
Nepal Electric Authority (NEA) 132 kV transmission line passes through the vicinity of the proposed THP switchyard location, at a distance of about 750 m. The existing line has already been saturated and cannot transmit surplus power. Therefore, NEA is planning to replace the existing 132 kV line by a 220 kV transmission line along the same alignment. In that case, a Loop in loop out scheme with the proposed 220 kV transmission line would be the optimum choice for the delivery of power from THP switchyard to the national grid. The length of 220 kV line will be approximately 750 m from the TNHP switchyard.
Description of the Loop in Loop Out scheme: In this scheme, a transmission line passing near by a switchyard (or substation) will be broken and the two broken ends of the line are connected via two separate line bays to a bus bar in the switchyard. This scheme of interconnection forms a Pi (π) model and is often termed as Pi connection scheme. In this scheme, a transmission line instead of passing along straight through, is connected to a switchyard in the middle and collects surplus power from the switchyard for transmission.
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This scheme requires two separate line bays to be constructed at the switchyard along with complete protection scheme including circuit breakers (CBs), isolators (DS) and earth switches, current transformers (CTs), potential transformers (PTs), lightning arrestors etc. and can be seen from figure below. Transmission line The loop in loop out scheme (Figure 12.1) should DS also have provision of by passing the switchyard DS DS
in order to ensure continuous operation even CB CB when the power plant (or switchyard) is not in DS DS operation. This can be facilitated by installing a
disconnecting switch in the broken section of the Bus bar at switchyard original transmission line.
Figure 11.1 Typical Loop-in-loop-out system 11.4.5 Route selection Route selection criteria
For the selection of proposed transmission line route, a basic set of criteria was defined at the outset of the study. The criteria are as follows:
• The route alignment should be as far from residential area and cover minimum forest area and should kept straight and short as possible. • As far as possible, the following areas should be avoided: o Heavy forest areas or expensive right of way o Buildings and residential areas o Swampy areas o Main road crossing o Highly productive land and difficult terrain • Situation calling for abnormally long spans and high deviation angle should be avoided. • Towers should be located on geologically stable ground. • River crossing should be done at the narrowest possible river width. • The number of angle points along the route should be kept to a minimum and Angles should be located on high points wherever possible. • The route should be conveniently accessible for construction, operation and maintenance work. • Potentially adverse environmental impacts due to the proposed transmission line should be kept to a minimum.
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These criteria were not assigned any order of priority. Also, they were not expected to be incorporated in totality in the field. The selected alignment would have to be a trade-off between the idealized conditions and the actual site conditions, considering both technical and economical aspects.
11.5 Transmission line design 11.5.1 Transmission voltage level selection The standard transmission voltage levels in Nepal are 66 kV, 132 kV and 220 kV. The 66kV level is phasing out lately due to lack of use and availability of equipment in the international market. The most economical transmission voltage level is selected from among the standard voltage levels. The voltage level in use in the project vicinity is also a major factor to consider while making the choice, since shorter the transmission line, lesser is the investment.
The most economical transmission voltage is estimated using the empirical formula. One of such formula is Still's formula, and is mathematically given as following.
V=5.5 * √ (0.6*L + P/100) Where; L - Length [km] P - Power to transmit (kW) V - Transmission voltage (kV) Figure 11.2 Transmission Line Voltage Selection Using Still's Formula The Figure 1 shows the variation of the most economical transmission voltage with power, plotted using Still's formula for the length of 5 km. The most economical transmission voltage for 25 MW installed capacity for 5 km Long transmission line would be between 87 kV. The obtained voltage level is not of common standard therefore, the nearest standard voltage level of 132 kV or 220 kV should be selected. The choice of voltage level also depends on existing interconnection facilities. In our case, a loop in loop out interconnection at the nearest 220 kV transmission line has been proposed for power evacuation purpose.
11.5.2 Design criteria Unless otherwise specified, the following design criteria are to be applied for the design of the transmission line:
1. Electrical system voltage
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(a) Power system AC 50 Hz, 3-phase, 3-wire system (b) System voltage: Rated 220 kV Maximum 245 kV
2. Quality of Supply The quality of supply is governed by grid parameters i.e. voltage and frequency. For planning purpose, following variations in voltage and frequency are permissible:
1. Voltage variation in normal operation: + 5% of nominal value 2. Voltage variation during emergency: + 10% of nominal value 3. Frequency variation during emergency: + 5% of nominal value 4. Transmission loss: < 4.5 % of the received energy
11.5.3 Conductor Size Since this option has a loop in loop out scheme with NEA’s proposed 220 kV line, the conductor to be used shall be in compliance with the one used in the proposed line. To date, there is no 220 kV transmission line in operation in NEA system. NEA is under constructing 220 kV transmission lines are using ACSR “Bison” conductor. Therefore, the power evacuation and interconnection scheme has been designed with ACSR “Bison” conductor under consideration and sized for:
• The load it is expected to carry. • The voltage drop remaining within the permissible limits. • Optimized capital and annual revenue costs.
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12. ENVIRONMENTAL ASPECTS 12.1 General TR-SGP lies within the Benighat and Salang VDCs in Dhading District, Central Development Region of Nepal. Richowktar, Malekhu and Benighat Bazaar are the nearest market places from the project area. The proposed ~ 25 MW electricity generating plant is located about 75 km road distance to west direction from Kathmandu and is on the way to Pokhara and alongside of the Prithvi Highway. The total catchment of the Trishuli River with respect to the proposed headworks is 6035 km2. The Trishuli River is a snow fed type river originates from Tibet, China. It is one of the main tributary of the Gandak River system. There are many small river and rivulets joins the Trishuli River.
Plate 12.1 Location of intake and tunnel portal area According to the client, the IEE of the project has been approved. However, just to recall for preliminary cost estimation for possible implementation of mitigation measures, the following subsection tries to figure out possible impacts and mitigation measures adopted.
12.2 Projected environmental impacts Environmental impacts are categorised on three aspects viz. physical, biological and socio-economic and cultural environment and thereby impact during construction phase and operation phases are mentioned herein. Data as obtained from literature review and sit visit findings were used to assess project environmental impacts. The magnitude, extent and duration are beyond the scope of this study but the following subsection briefly discuss the likely impacts.
12.2.1 Adverse impact The adverse impact due to the project is mentioned herein according to physical, biological and socio-economic and cultural environmental prospective. The adverse impact could be characterised based on the affected and most affected area due to the project.
PHYSICAL ENVIRONMENT
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Likely impact that could arose during construction and operation phases of the project to the physical environment are mentioned herein.
CONSTRUCTION PHASE ! Impact on noise quality; ! Impact on air quality; ! Impact on water quality; ! Impact on muck disposal; ! Impact on land use; ! Impact due to stock piling of construction materials; ! Impact on slope stability, sedimentation, soil erosion ! Impact on structures due to blasting activities; ! Impacts of toxic materials; ! Impact due to solid and construction waste; ! Impact on natural beauty due to transmission line; ! Any other issue likely to arise during EIA study; OPERATION PHASE ! Change in surface hydrology including ground water level changes along the tunnel alignment; ! Impacts on downstream river and river ecology due to sedimentation and sediment flushing from the settling basin; ! Change in water quality due to reduced flow in downstream; ! Impacts on topography; ! Impacts on slope stability, sedimentation, soil erosion; ! Impacts on micro-climate in reduced flow zone in downstream area and impoundment area in upstream area; ! Change in river morphology downstream of the diversion weir; ! Impacts due to muck disposal; ! Impact on natural beauty due to transmission line, ! Radiation effect form substation and transformer; ! Impact due to leakages/spillage of transformer oil, ! Impacts on birds due to transmission line, ! Any other issues arising in EIA study. BIOLOGICAL ENVIRONMENT
Likely impact that could arose during construction and operation phases of the project on the biological environment are mentioned herein.
CONSTRUCTION PHASE ! Disturbance to aquatic habitat due to construction of weir and river canalising; ! Impacts on aquatic ecosystem due to washout of disposal of spoil and muck in the river; ! Any other issues likely to arise during EIA study including biodiversity aspects.
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OPERATION PHASE ! Impacts on aquatic habitat and obstruction to fish migration in flow reduced area; ! Increased poaching and fishing along the stretch between intake and powerhouse including in the weir area; ! Any other issues likely to rise during EIA study. SOCIO-ECONOMIC AND CULTURAL ENVIRONMENT
Likely impact that could arose during construction and operation phases of the project on the socio-economic and cultural environment are mentioned herein.
CONSTRUCTION PHASE ! Impact on social structure, culture and traditional practices of the rural people due to increased outside project staffs and workers; ! Pressure on the existing infrastructure facilities such as drinking water, health post, education institutions, health centre, and sanitation etc. due to large numbers of outside work force; ! Impact on law and order in the project area due to large work force; ! Impact on occupational health and safety; ! Impact on the existing rafting practices in the Trishuli River ! Impact on the existing local religious sites; ! Impact on people’s behaviour due to increased economic activities; ! Impact on local economy and inflation due to large numbers of outside work force; ! Impact to the local people which is associated with the land acquisition and compensation; ! Any other issues likely to arise during EIA study. OPERATION PHASE ! Impact on safety and movement of people due to sudden change of water volume in downstream of dam in lean season; ! Impact due to safety and movement of people due to presence of long and wide open canal; ! Impact on people’s behaviour due to withdrawal of economic activities; ! Impact on the existing local religious sites; ! Impact on the existing rafting practices in Trishuli River ! Impact on women; ! Impact due to withdrawal of economic activities and employment opportunities; ! Impact on livelihood; ! Impact on the existing water use rights; and ! Any other issues likely to arise during EIA study. 12.2.2 Beneficial impacts The proponent shall also identify, predict and evaluate impacts on issues raised during public consultation in particular to the above mention issues. Furthermore, the proponent shall identify, predict and evaluate beneficial impacts as mentioned below.
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! Employment opportunities for local people both skilled and unskilled during construction and operation phase; ! Potential improvement of public facilities such as health posts, schools, and social services; ! Business opportunity for local people and economical benefits; ! Provide a basis for future rural electrification program; ! Impact on livelihood; ! Link the project affected area to the main highway i.e. Prithivi Highway, which runs in the other bank of the Trishuli River, ! High value of the unproductive land located close to the headworks area at Richowktar; ! Possibility of using upstream of headworks for fish harvest, ! Impact due to tourism; and ! Impact on rural economy. 12.3 Consideration of alternatives Various alternatives regarding design, location and source of construction material, transmission line alignment were considered and the best amongst them is selected. The selected alternative or options considered various options such as design, location, technology, operation procedure, schedule, raw materials availability, power production and project cost. A detail analysis of structures features are t be figure out and to be addressed during detail design phase.
12.4 Mitigation measures Mitigation measures are aimed at reducing the effects of the project which infact generates form the implementation of the project. Cost effective and practically acceptable mitigation measures are stated here under for preliminary assessment purpose. The proposed mitigation measures will minimize the adverse impacts and enhances the beneficial impacts. The mitigation measures for some of the issues identified during feasibility study are listed below.
! Minimisation of diesel engine and proper maintenance of all equipment so as to ensure air quality and reduction of noise level, ! Tree plantation and watering in and around the construction sites where dust is likely to be a problem, ! Safe disposal of waste materials and organic garbage, ! Construction check dams, lined canal will be provided to prevent wasting of spoil tips, ! Minimum of 10% of the lean season flow will be released to maintain river ecology, ! Monitoring of fish habitats and spawning grounds need to be done throughout construction and during the first few years of the operation of period to identify appropriate measures to improve fishery situation, ! To ensure health and sanitation conditions for the workforce and their children
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at each construction sites, ! Raising of social awareness program to discourage antisocial behaviours, ! Encourage local people to conserve their cultural values ! Community development program ! Afforestation to the open barren lands ! Skill development and self-employment generating programs ! Adequate compensation to acquired land and properties ! Amicable solution and establishment of good co-operation with the Rafting business groups However detail mitigation measures to be referred in the EIA study report.
12.5 Environmental Management Plan (EMP) Most development projects leave behind a large number of environmental effects which occur primarily during the construction phase. Therefore special attention must be given to minimise the impacts and protect the project area’s natural resources during the construction phases. A large number of construction activities will be taking place in different locations with a large labour forces and machinery. To ensure that all impacts are minimised and the mitigative measures are followed in an effective way, an overall environmental management plan (EMP) will be prepared considering all the activities related to construction work. All environmental issues related to construction should be resolved satisfactorily by the owner of the project. An awareness raising program will be conducted for the local people regarding the different environmental issues.
The project owner has to run a program to monitor the environmental impact throughout the construction and operation period. The monitoring will ensure that the environmental guidelines are followed and take remedial action to mitigate any unexpected impacts occurring. Monitoring will be the responsibility of the Project Manager who should organise routine monitoring, audit and special studies.
12.5.1 Environmental monitoring plan The environmental monitoring plan has to be prepared in terms of baseline, compliance, and impact monitoring plan to assess the actual effects on physical, biological, socio- economic and cultural environment during construction and operation phase. This monitoring plan will indicate parameter, indicator, location, schedule and methods of monitoring. The monitoring programs will describe the parameters and indicators to be measured, the required sampling stations or location, the frequency of sampling and methods and agencies to be consulted during monitoring activities. The cost of monitoring, activities, manpower requirement to carry out the proposed activities, and organisation set up to carry out the proposed monitoring activities will be included in the environmental monitoring plan.
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12.5.2 Environmental auditing plan (EAP) The environmental auditing plan has to be prepared to assess the actual environmental impacts, the accuracy of predictions, the effectiveness of environmental impact mitigation and enhancement measures and functioning of monitoring mechanisms. This auditing plan will describe indicators to be measured, the required sampling stations or location, the frequency and methods of sampling, responsible authority to carry out auditing and agencies to be consulted while carrying out this auditing plan.
The cost estimation of a comprehensive auditing program and feedback will be included in the overall auditing plan. Timing as well as the appropriate agency for audit shall be suggested as per the EPR 1997.
12.6 Cost of recommended mitigation measures The estimated costs in US$ for land purchase, compensation and mitigation measures are shown in Table 11.7. A reserve fund has included allowing for other needs which may be identified. The reserve fund should be held by the owner, and be available for appropriate compensation and mitigation during the construction, commissioning and early operation of the project. It is envisioned that the total land occupied by the project could be about 13 hectare. Land requires for transmission line is not included.
Table 12.1 Cost Estimates for Mitigation Measures
Item no Description Amount, US$
1 Clinic facilities
1.1 Charges for Health Professionals hiring from Malekhu Hospital 2,000,000
2 School facilities
Upgrading existing school facilities for workers children, 2.1 4,320,000 including staff and material cost for 3 years (lump sum rate)
3 Community and environmental program
3.1 Personnel, materials, training for 3 years 2,160,000
3.2 Forest guards, 3 numbers for 3 years 1,440,000
4 Rural electrification 3,960,000
5 Monitoring and studies
5.1 Environmental monitoring/studies 2,880,000
6 Reserve fund 3,600,000
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Item no Description Amount, US$
Total 20,360,000
Total mitigation cost, Million NRs 20.36
12.7 Conclusions This is a preliminary assessment on project environment as gathered from the previous studies and understanding from the recent site visits. However, this is not presenting a full summary of the project environment and is prepared only for preliminary cost estimation proposes. However, details environmental findings on impact and enhancement benefits can be assess on approved IEE report. Summary of findings is summarized below and which will help to provide an comparative assessment between the approved IEE report and findings in this study.
! The project alignment does not pass through the forest areas.
! The area is food deficit area and thus food grains are brought from outside to their workers. People are employed in different organisations but still unemployment rate is significantly high.
! Due to diversion of Trishuli River flow, there might be a problem to the rafting business.
! Probable impacts due rising of dam height at 348 masl;
Despite of environmental issues in the project area, there are beneficial impacts to the project area people. Beneficial impacts are listed below:
! Employment opportunity,
! Access road available,
! Electrification,
! High business transaction;
! Possibility of fishing,
! Possible to establish resorts alongside the headworks area etc.
Looking at the beneficial impacts and environmental issues, the project is found more rational from environmental prospective and has least environmental impact.
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13. CONSTRUCTION PLANNING AND SCHEDULING 13.1 General This section of the report describes the anticipated construction technology that could be applied to undertake different construction activities at the possible shortest span of construction time. As it is envisioned that the construction of the project could be completed within 36 months’ time but it will depend on the commencement of the construction. If the construction is schedule on season, it is possible to complete the construction in 30 months, otherwise off season start delay six to twelve more months. Construction schedule has been prepared accordingly for the major construction activities and where possible minor activities areas are also taken into account. Critical activities as well as milestone have been identified.
Presently, all project sites area accessible by earthen road in the form of track opening, which is linked to the Prithivi Highway at Richowktar, Malekhu, Dhading as well as Dhading road. Upgradation of the track opening will avail accessibility to key project sites.
The contractors will be required to construct camps for its workforce as well as office buildings purposes. It is envisaged that there will be two camps and will be located at headworks and powerhouse area. The workforce camp will be temporary type whereas the permanent type building upon completion of construction work will be used for office building purpose. Moreover, permanent type building will be used as employee quarter upon completion of construction of the project. These are required during operation and maintenance phase of the project. The contractor shall established potable drinking water supply facility to the camp area.
As there are distribution line passing near by the project areas and thus these lines could be used to supply construction power as well as to supply electricity to the camp area/s too. Reliability of these lines is not high and thus back up system will be required.
The average length of daylight in the project area is roughly 10-12 hours so that the surface construction activities have been assumed to extend over 10 hours a day. A margin for time lost due to adverse weather or other unforeseen delaying conditions has been allowed in the adopted production rate. To expedite the construction advance rate, mechanised system is applied where found appropriate. For example, construction of headworks needs to be completed before monsoon starts. For the underground tunnelling works, artificial lighting, and ventilation system will be required to assure uninterrupted 8 hours work 3 shifts a day and 26 working days in a month.
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The construction work shall be pursued from four different sites simultaneously and the site will be as follows:
! Construction of headworks, ! Tunnel construction (four headings), ! Construction of penstocks and allied works; and ! Construction of powerhouse The details of construction activities are presented in construction schedule and shown in Figure 12.1.
13.2 Access road As mentioned earlier that the access to the project is possible via existing earthen road heading form Richowktar down to the headworks area. Likewise road extension from Dhading road to the proposed powerhouse area is also available in the track opening form. It will therefore require upgrading of the existing track road. To shorten the accessibility to the powerhouse side, construction of road linking to the Prithivi Highway would be possible seasonally using Ferry.
13.3 Material supply Transportation of material can be done in two ways: local supplies and import of material via nearest sea port/dry port at Birgunj. Local supplies shall be carried out using track opening tot eh project sites via Prithivi Highway and Dhading black topped road. Location of local construction materials such as gravel, sand and red clay is available in abundance nearby the project area.
Regarding transportation of electro-mechanical equipment, the nearest port is Calcutta if these are brought from countries other than India. From Calcutta, it has to take upto Birgunj via truck service or rail service.
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13.4 Construction activities 13.4.1 River diversion The intake and diversion weir are located quite a distance. It is therefore there will be two temporary coffer dam. Construction of diversion weir has to be finished during low flow season whereas intake and allied work in that area can be continued throughout the year.
Diversion weir area
Construction of the headworks on the river will require the working area dry during construction. For this purpose, the flow in the river will be required to be diverted and thus it is required to construct a temporary diversion structure. The diversion is required in two phases. In first phase, it is required to construct cofferdam around the working area at right bank of the Trishuli River and to let the flow from left bank. Construction of bottom sluiceway, and parts of the ungated spillway of the diversion weir will be constructed in the 1st phase. Once the construction of the 1st phase is completed, then the 2nd phase will start to construct remaining portion of the river diversion section. The river diversion structure is constructed in such a way that it can safely pass 1 in 5 years return period flood. Canalisation after river diversion would make no back water flow in the working areas. The river diversion will be an embankment type and the materials will be river bed sand and boulder mixed soil. It is suggested to excavate some depth of the river bed to ensure a safe bank.
Intake area
The river width at the planned intake area is more than 150 m or so. Construction of cofferdam parallel to the river flow is possible and will allow round the clock construction of intake and allied works. The construction material could be the river materials and gabion or stone armouring along river face. Details to be decided during detailed design phase.
13.4.2 Headworks The diversion will be constructed by excavating the right riverbank up to the required depth as determined from geo-technical investigation. Excavation will be carried out to the required depth in stage and construction of gravity dam to proceed. It would be better if some of the bottom sluice construction complete to change the river flow otherwise high cofferdam will be required.
Excavation around intake area could be done simultaneously. Opening at intake is quite long to get allowable velocity through trash rack. Completion of intake wall will allow fixation of gates and stoplogs. Little wider opening at back side of the intake is envisaged
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to get required geometry between tunnel inlet and intake opening. Concreting to be done and drop shaft to tunnel inlet to be proceed. Moreover, slope stabilisation needs to be performed parallel. Construction tunnel inlet portal can be started simultaneously. Disposal of tunnel muck can be placed in flat area located downstream along right bank. Moreover, excavated materials could be used to construct coffer dam around intake area.
At the left end of the diversion weir, a fish ladder is constructed so as to ensure a safe passage for migrating fishes in the river.
Tunnel This will be a separate contract. It is therefore the tunnel construction can be started right after the agreement made. Construction of the tunnel falls on the critical path. The total length of the tunnel is about 3800 m. As it is mentioned earlier that the geology is very weak and thus not possible to advance tunnel excavation rate as it should be tunnelling in sound rock condition. Due to bigger tunnel diameter, it is expected that the tunnel advancement will be 12-15 m/week. Because of the terrain, there is one intermediate adit provided at Bhunte Khola. Tunnelling length from headworks site is 900 m whereas from intermediate adit to headworks and tunnel outlet is 900 m and 1016 m respectively. Likewise it is 1015 m long from tunnel outlet side. With a 2 m advancement rate, it may take 75-85 weeks to complete tunnel excavation and couple of more weeks to finish lining and allied works.
The construction camp for the tunnelling crew is located on the right bank of the Trishuli River and close to the headworks area at Arbastar. Likewise at Bhunte Khola and tunnel outlet area could be a potential camping site for workers
The tunnel muck disposal area at headworks area could be at the previously proposed settling basin area. Likewise, it could be around Bhunte Khola along Trishuli River bank to increase bank height. Bank protection measures to be applied to control erosion of disposal site.
Surge shaft Open type surge shaft close to tunnel outlet has been proposed. To allow construction simultaneously with headrace tunnel, it is located little off from the headrace tunnel. It will be a cylindrical section and erected about 5 m above from the ground for safety reason. Steel gratings will cover the shaft.
Excavation from top as well as from bottom would be possible since height of the surge shaft is shorter. A pilot hole from the top would make the construction easier.
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Penstock Surface steel penstock pipe of 2 numbers will be laid some 30 m inside the tunnel outlet area. The pipe will be incased into concrete. Once it is outside the tunnel, series of support pier will hold the pipe and taken to the surface powerhouse. The penstock is surface type. Each pipe will feed the turbine separately.
Powerhouse The construction of the powerhouse consists of two main parts: civil works and electromechanical works. The construction of civil works solely depends on the detail as made available by the equipment suppliers. The civil work can be completed within one year. However, if electromechanical equipment suppliers supplied the equipment delayed, the construction time to complete would also delay. It is not in a critical path and thus traditional excavation method by deploying local human resources could be of use. The powerhouse floor level is well below the present ground level to ensure more head. It is therefore there will be two story building of powerhouse. The capacity of the powerhouse super structure is designed for 20 ton so that a mechanised gantry crane could be used for the purpose of operation and maintenance of turbine and its auxiliaries.
The roof of the powerhouse will be CGI roofing materials and will be supported by steel truss. The powerhouse is provided a workshop bay so that a minor maintenance could be possible at the powerhouse itself.
Tailrace The total length of tailrace canal is about 120 m and is a box type rectangular section. The tailrace canal is constructed of concrete at invert and stonemasonry side wall or all with concrete. The capacity of the tailrace would be the design discharge. This does not fall in critical path and thus can finish simultaneously with the construction powerhouse superstructure. At the end of the tailrace canal, bank protection work is required and this could be finished within 3 months and it will be carried out parallelly with the construction of tailrace canal.
13.4.3 Switchyard The outdoor switchyard will have surface area of 50 m x 50 m. Thus bus ducts connecting the generator to the step up transformer in the switchyard will be accommodated in a cable from the control bay.
Site preparation for the switchyard by levelling can be carried out independent of other works. The civil works for the switchyard will be completed in about 10 weeks.
There will be an interconnection arrangement either at Benighat or Naubise, depending on the connection agreement, to evacuate the generated energy for the project into INPS.
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Construction time of transmission line depend on the interconnection location and thus govern the construction start/completion time. A flexible construction time is estimated in Figure 11.1.
13.4.4 Electromechanical equipment After the civil works contract has been awarded, the works for electro-mechanical equipment like turbine, generators, transformer and auxiliaries will commence. After the preparation of technical specification, tendering will be called at the very day so that possibility of completion delay could be avoided. The successful tendered party will take about a year for design, fabrications and delivery of the equipment and thereafter about four months for erection, testing and commissioning.
13.4.5 Transmission line Transmission line length is very short in comparison with the other projects. It is envisioned that the transmission line length is about 500 m long and need to cross to the other side of the Trishuli River right close to the 132 kV transmission line that is passing through the project area via Benighat. Construction of transmission line is not in a critical path if allowed to connect around Benighat area. If it is not allowed to connect and need to take to Naubise, early start of transmission line construction should be made.
13.5 Construction power The construction power to this project shall be supplied from the distribution line passing through Benighat-Malekhu line. It is therefore, will require couple of 100 m long transmission line to be constructed to supply construction power and for internal consumption purpose.
Construction power requirement for the project is estimated as 1MW hydel and 500Kw diesel (To take care of essential load) i.e. total installed load of 1.5MW of construction power is to be on central place on 11 KV with a step down to 0.440KV, three phase star system, which will run to all the appurtenant features with an AMF control on the sub station.
13.6 Construction material The construction materials required for the project will be procured from the domestic producers/ suppliers. The main construction materials required are as follows:
! Blasting materials and detonators, ! Cement, ! Brick,
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! Sand, ! Steel pipe and angles, ! Stone, ! Reinforcement bars, ! Timber, ! Fuels, ! Coarse and fine aggregate, ! Cohesive materials, ! Backfill and rock fill materials, ! Rock bolts, ! Mechanical and electrical items such as conductor wires, ! Admixtures, ! Steel fibres, etc. The river is famous for aggregates and sand mine. Such materials are found in sufficient quantity in the river but only need to sieve it to obtain in required grade. Alternatively, there are quartzite band close to the project area and thus it can be used as a quarry site. The materials for backfill and rock fill as required for construction of cofferdam will also processed from the excavated materials. As per site condition and materials required, some materials will be imported from other localities. Cement can be either imported from India or Nepali cement can be used. Regarding steel plate to fabricate penstock pipe, material will be imported from India. Wires and other auxiliaries of transmission line will be locally supplied where possible and those items not available in the local market will be imported from aboard. Sand, red soil and aggregates can be available in and around the project area.
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13.7 Contract package and construction schedule 13.7.1 Contract package The construction work is classified into civil and electro mechanical works. Contract package is depending on the time required to complete the construction of the project and resource available. It is envisioned that the project is completed within three years from the date of construction commencement. It is therefore, to expedite the construction activity, the project construction work is explicitly divided into 4 packages and each package is dealt by single contractor. There shall be one main contractor and part of the work can be sublet to sub-contractor. The packages as deemed necessary to complete the construction work within the stipulated time frame are divided according to the Table 12.1 below.
Table 13.1 Contract packages
S. No Contract Packages Activity
1. Contract C1 Civil contractor (Civil works except tunnelling works)
2. Contract C2 Civil contractor, Tunnel
3. Contract C3 Electro-mechanical Contractor
4. Contract C4 Transmission line and switching substation
13.7.2 Construction schedule The construction time for the different activities has been set at a rate adjusted to the local conditions, available information on the rock quality and country’s present situation. The methods of construction described in respective Section have been used for planning, and form the basis for the construction schedule. The anticipated construction schedule for the project is shown in Figure 11.1. The construction schedule shows that the power production can start approximately 3 years from the project construction commencement date. Following assumptions were made in the preparation of this schedule
! One team will construct the diversion weir, ! One team will construct intake area, ! Four team will construct tunnel and shall start from each heading,
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! One team will construct surge shaft, ! One team will start powerhouse construction, ! One team will start penstock area ! One team will construct tailrace canal, ! The overall estimated construction time is about 3 years. In preparation of the schedule, additional contingency time has also been allowed for training local labour to work on the project, for festivals and lost time caused by the monsoon season. Further, for the construction schedule of the project major critical activities are also anticipated. These critical activities are listed below:
! Construction of diversion weir (due to construction difficulties), ! Construction of side intake, ! Construction of headrace tunnel, ! Connection of transmission line to the national grid To shorten the construction period, the different contactors have to work in a co-ordinated way. In doing so, it is of sure that the project completion could be enhanced within 3 years’ time as stipulated in the schedule. The most critical event is the tendering and contract award of electro-mechanical equipment fabrication, supply, testing and commissioning since normally it took about a year for design and fabrication. An efficient project management and good co-operation between the locals also expedite the construction work and lessen the construction time. As a project management, the developer could break the work in such a way that a good co-operation amongst contractor could established. However, the contractor might choose other methods as construction approach so as to reduce the construction period.
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14. COST ESTIMATION 14.1 General Cost estimates are required by the Project Developers to assess whether a project meets their investment objectives in order to decide that the project should be implemented based on the project parameters recommended in the Feasibility Study Report.
This section of the report describes the methodology used for the derivation of the project costs. The quantities and absolute costs in this section are different from the cost estimation as used in the optimization study where relative rates may have been used. The final costs also depend on the construction schedule and planning as well as efficient project management.
The costing of the project has been carried out on the basis of the feasibility study carried out by the Consultant and experience gained in this field and on similar projects. Current costs of equipment and materials have been acquired from manufacturers and suppliers, where possible. Where these were unavailable, costs have been taken on the basis of past projects carried out as well as unit rate analysis applicable for hydropower projects.
All prices and cost data were calculated in NRS, where appropriate, and in US$ for those services and goods which are to be procured from abroad. The currency conversion rate was taken as 78.60 RM per 1 US$. To arrive at the total project cost, the quantity of various items was estimated separately for each work in accordance with the related drawings, as presented in Volume II.
14.2 Unit rate analysis Unit rate analysis for the various jobs has been carried out as per norms published by Ministry of Works and Transport (MoWT). The rates of the locally available materials such as sand, boulders, aggregates, softwood and labours are taken from the approved district rates for the running fiscal year. Regarding electromechanical equipment costs, rates from manufactures/suppliers is sought.
14.3 Assumptions The following criteria and assumptions are the basis of the cost estimate:
! The cost estimate and financial analysis have been based on the US dollar. ! The exchange rate used for cost estimate is US $ 1 = NRs 78.60 ! Price level of Feb 2012, The cost estimate has been made at the price level of 2011. All costs have been first estimated on unit cost basis for each of the components. These have been added to obtain the entire project cost. Lump sum costs have been
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allocated for components where a detailed breakdown of costs is not available or worthwhile.
! Material price and labour cost Material costs reflect real costs incurred at other projects of similar size or having similar scope of works. The prices have been calculated for 2011. It is assumed that the bulk of the construction material can be obtained in the local market whereas some of the steel items and all of the electromechanical equipment need to be imported.
! Semi-skilled, unskilled and some skilled manpower can be available locally. ! Indirect cost The unit costs include profit, and overhead, which the contractor would charge. Along with that, 13% Value Added Tax (VAT) as applicable to all construction material procured is added on top. As per the facilities provided by the Electricity Act 1993, all taxes, including VAT, have been excluded from electromechanical equipment, and all plant and machinery, which the contractor would import for the completion of works and custom duty of 1% is added on the cost estimation. A contingency sum has been added to the total civil and electromechanical cost as shown in detailed cost estimate presented in Volume III.
Unit item rate of electro-mechanical equipment is based on similar project cost and or thumb rule projection of the cost is taken. However, where unit rate are not available, experience from similar project has been used to make unit rate estimation.
14.4 General Methodology The detail drawings were prepared and the major activities were breakdown in different components to estimate the different work item quantities. The project is divided into number of major components for the cost estimation process and is categorically divided as follows:
! Main civil construction works ! Diversion weir, intake, sluiceway, fish ladder, ! Tunnel, tunnel intake and portal, ! Gully crossing, , ! Penstock, powerhouse and tailrace ! Powerhouse electro-mechanical equipment ! Turbines, ! Generators,
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! Transformers, ! Auxiliary equipment, ! Switchyard and transmission line, ! Gates, and trashrack, ! Construction camp and access road, ! Engineering and management costs, ! Resettlement, land acquisition and environmental provisions, ! Owner’s development costs ! Contingencies 14.4.1 Main civil works estimate The cost estimation of the main civil works was prepared on the following basis:
! Breakdown of the total project into a number of distinct structures in measurable units, ! Identification of distinct construction tasks or measurable pay items, ! Calculation of quantities from prepared maps and drawings, ! Development of unit rate analysis based on prevailing market rates appropriately adjusted for the project ! Calculation of cost for each activity by summing up costs of different works, ! Human resources cost, ! 6 days 10 hour work shift and thus prepared construction schedule accordingly, ! Construction material as divided into two category viz. locally available materials and imported material such as steel, plant and equipment etc. from abroad.
14.4.2 Electro-mechanical equipment The cost includes supply, installation, testing and commissioning of all electro-mechanical equipment. Experience and similar project cost has been used in the analysis.
14.4.3 Penstock and hydro-mechanical Fabrication, transportation and erection costs of the penstock, gates, stoplogs, trashrack etc are included in this package. All the rates used are based on information provided by local manufactures/suppliers.
10.1.1 Switchyard and transmission line This cost package includes all electrical and civil costs of the proposed ~750 m long 220 kV transmission line joining the powerhouse with the NEA’s 220 kV transmission line that is passing through the project area at Benighat.
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The cost estimate of the switching substation and necessary auxiliaries required to evacuate power in the 220 kV grids is based on information as supplied by the relevant parties consulted.
Alternative to this, cost comparison to planned substation at Naubise switchyard is also looked at.
14.4.4 Engineering and Administration Costs
2 percent of the total construction costs have been allocated for engineering fees, and project management that may be required for additional studies, all detailed design and construction phase management of the project to be carried out. The costs will cover the following activities:
! Further site investigations such as transmission line alignment survey, access road survey and production of maps and allied structures; ! Preparation of tender design and documentation and detailed engineering design; ! Contract and tendering including preparation of specifications; ! Management of procurement and project administration; ! Reviewing and approval of contractor submissions; and ! Associated costs of the owner for project management. 14.4.5 Owner’s Cost The Owner’s administration costs and pre-project development costs were estimated.
14.4.6 General Items Project cost has been also included likely cost which could be incurred as general items which include insurance, mobilisation demobilisation, and bond. License and permits and so on.
14.5 Contingency sums Contingency sums as practicable to be included on different items depending on the risks associated to the project components. The contingencies shall cover any unforeseen cost that could incur during detailed design phase of the project as well as construction phase of the project. The more information on underground works and foundations beyond the limit of the investigations made during the feasibility study shall be accounted for. The physical contingencies adopted in the feasibility study report are as follows:
Headworks and headrace tunnelling work 15% of the capital cost estimation Electromechanical equipment 5% of the capital cost Hydro-mechanical equipment 10% of the capital cost Transmission line 5% of the capital cost Gully crossing 5% of the capital cost
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14.6 VAT/taxes and duties The amount of VAT payable has been considered as 13% of the total project costs which exclude equipment to be imported from outside country. Custom/duty, taxes and godown charge is lumped together and taken as 2.6% of the estimated cost of the plant and equipment.
14.7 Project cost estimate The total project cost estimate summary is shown in Table 13.2. The costs in Table 13.2 are the outcome of above assumptions and various fees, contingencies as described in previous sections.
Table 14.1 Total project cost showing various items of the cost
VAT complying S. VAT NRs No Particulars % NRs Total complying equivalent 1 Civil works 2,390,860,936 Contingency sum 15.0 358,629,140 Sub - Total 2,749,490,077 90% 2,474,541,069 2 Electromechanical works 1,517,816,908
Contingency sum 5.0 75,890,845 Sub - Total 1,593,707,753 12% 191,244,930 Penstock and Hydromechanical 3 works 126,481,020 Contingency sum 10.0 12,648,102 Sub - Total 139,129,122 90% 125,216,210 4 Transmission line works 98,986,000 Contingency sum 5.0 4,949,300 Sub - Total 103,935,300 75% 77,951,475 5 Access Road 94,656,051 Contingency sum 5 4,732,803 Sub - Total 99,388,853 90% 89,449,968 Socio-environmental mitigation 6 costs 22,043,000 Contingency sum 3 661,290 Sub - Total 22,704,290 90% 20,433,861
7 Infrastructure development costs 24,000,510 Contingency sum 5 1,200,026 Sub - Total 25,200,536 90% 22,680,482
8 Land acquisation and direct costs 39,900,000
Contingency sum 5 1,995,000 Sub - Total 41,895,000 10% 4,189,500 TOTAL CONTRACT COSTS 4,775,450,931 ENGINEERING FEES 2.0 95,509,019 100% 95,509,019 TOTAL CONTRACTS & ENGINEERING COST 4,870,959,949 TOTAL VAT COMPLYING US$ EQUIVALENT 3,101,216,514 VAT 13 403,158,147 TDS on Engineering fees 1.5 1,432,635
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Total Taxes (1% custom duty, 1.5 % local tax & 0.1% godown charge ) 2.6 44,650,023 TOTAL TAX AND VAT 449,240,805 TOTAL CONTRACTS & ENGINEERING COST INC. VAT & TDS 5,320,200,754 Owner's development costs 169,418,179 TOTAL PROJECT COST (Nearest NRs1000) 5,489,619,000
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15. FINANCIAL ANALYSIS 15.1 General Economic and financial evaluation of the project is carried out in order to determine viability of the project. The financial analysis will evaluate the acceptability of the investments made in the TR-SGP as a source of energy supply from the viewpoint of developers. The technical feasibility of the scheme has been established through study carried out on the technical aspect. Apart from the technical, environmental and socio- economical and cultural aspect of the project, the financial analysis provides the most important indicators for the acceptability of the TR-SGP for investment. The economical and financial evaluation is aimed at giving potential investors in the project an overview of the risks and benefits associated with financing the project. Investors and lenders to be reassured that the arrangement for the development of the project will remain constant, to ensure continuity of construction and completion, and that the income projections for the scheme are robust.
Financial evaluation uses the real term monetary values of the cost and benefits and is inclusive of taxes transfers, duties and escalation. The financial evaluation concerns with the developer of the project and its impact on its accounts. Hence, from the perspective of a private developer, financial evaluation is the most important aspect of the project to determine whether to finance it or not.
The financial analysis was undertaken to determine whether the project can meet the expectations of investors (i.e. a satisfactory return on equity) and lenders (i.e. an acceptable debt service coverage ratio for every year of the loan repayment). Results of the financial analysis indicate that the annual revenues that accrue to the project are sufficient to cover the expected annual expenditures over the project life as well as recover the capital investment. A financial model was developed for the project to explore its financial viability from the perspective of the project company over the project life. The financial model generates cash flow projections of costs incurred and revenues received, and produce cash flow statements, loan disbursement and debt service schedules for each year of the project. Performance measures are calculated, including levelized tariff, project payback, return on equity, and debt service coverage ratios. Sensitivity on the financing part can also be conducted to accommodate the appropriate capital structure, debt and equity ratio for financing the Project. Wherever possible, the financial analysis has been carried out on the basis of known information. Where financial parameters are not known they have been estimated by extrapolation from known data or reasonable
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assumptions have been made. These assumptions are, on the whole, conservative estimates and figures considered to be realistic and standard for analysis of this nature.
Project investment decisions are taken for the ensured return on investment. A viable project is that which ensures the return equivalent to the opportunity cost or more. Financial analysis is to determine the project viability from investor’s perspective. The financial analysis and its sensitivity are discussed in this chapter. The financial analysis consists of a cash flow during the project life, a financial evaluation, which suggests the Net Present Value (NPV) and the internal rate of return (IRR) of the project. Sensitivity Analysis has been done to determine robustness of the project.
The economical analysis of the project has been carried out on the basis of 50 years plant life and from the national prospective. All applicable tariffs, royalties, possible exemptions and social benefit accrued from the project are incorporated at feasibility study level.
Varying some of the assumed economic and financial parameters has carried out a sensitivity analysis.
There is a little possibility to develop project as a peaking plant but the generation hours is insignificant. However, it would be beneficial to run the plant especially during peak hours demand of dry months if the buyer has willingness to pay higher tariff. However, in the analysis, no attempt has been made at this stage due to absence of legal provision. However, if there is a favourable PPA ground at latter date, this could be explored further. At this stage, only RoR concept of plant is further dealt in analysis sensitivity analysis.
15.2 Project evaluation 15.2.1 Assumptions A financial analysis has been carried out for the base case on the basis of the following assumptions:
• Capital cost for the project will be disbursed during the project construction period as follows: 25 % in Year 1, 40% in year 2 and remaining 35% in Year 3. • The equity debt ration is taken as 70:30, • Discount rate of 7%, • Project completion period: 3 years from commencing construction, • Economic life of the project is 25 years. • Salvage value of the project at the end of the economic life is zero. • Annual operation and maintenance cost is estimated at 2.25% of the capital cost.
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• Energy selling price is taken as 4.80 and 8.40 NRs/kWh for wet and dry period respectively. 5 months is assumed dry period and remaining 7 months as wet period. • Average interest rate is borrowing is assumed to be 7%, fixed. • Loan Repayment period will be 7 years from the time of generation. • Electromechanical replacement cost at 25 years of operation period, • Transmission losses including internal consumption is assumed to be 1.5% • Taxes and royalty have been taken as per the current Electricity Act. • Exchange rate of 1 US$ = NRs 78.60 Similarly, for economical analysis, analysis is made from the prospective of government. All taxes, royalties, VAT etc are taken out and also include social benefit accrued from the project implementation. The life of the project is taken as 50 years and other conditions are according to the assumptions made in financial analysis.
The analysis is performed assuming that the transmission line is the most to evacuate the generated energy into the national grid.
15.2.2 Project benefits For the financial analysis, the principal project benefits are revenues, which can be derived from the operation of the project. In the analysis three important economic indicators: Net Present Value (NPV), Benefit Cost (B/C) ratio and Internal Rate of Return (IRR) were used. Project benefits have been calculated using the following assumptions:
• Project base construction cost will be MNRs 5,489.62. • Energy generated by 25 MW power plant is estimated to be 35.35 GWh dry energy, 127.97 GWh wet energy and in total it is 163.32 GWh energy generation annually after 1.5% losses in transmission line and for consumptive use. • Assumed rate of sale of energy shall be 8.40 and 4.80 NRs/kWh for dry and wet energy respectively. • Capacity benefits have not been included in the analysis. • 3% annual escalation on energy price • The electro-mechanical replacement cost is taken as MNRs 630 and replacement is made after 25 years of plant operation. Based on the above assumptions the financial analysis yields the following results and is shown in Table 15.1 below.
Table 15.1 Results of financial analysis at base case
S. No. Indicators Value
1 Project cost, MNRs ‘000 5,489.62
2 Construction cost, MNRs ‘000 5,864.68
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3 Project cost/MW, MNRs 219.58
4 IRR, % 14.57
5 Payback period 5th year
A typical economical and financial analysis is presented in Volume III Annex
15.3 Monthly energy The energy generated from on monthly basis is presented in Table 15.2. In calculating annual energy, 1.0 % has been allocated for transmission loss and for consumptive use. The annual energy as calculated in Table 15.2 is after the allowances made on energy which is available for PPA. Result shown in Table 15.2 is a net annual energy generation.
Table 15.2 Annual Energy Generation
Total Dry D/S Available Head Net Design Oper Generation season Wet season Month Flow release Flow loss head flow ating capacity energy energy (m3/s) (m3/s) (m3/s) m m (m3/s) days (KW) (KWh) (KWh)
Jan 60.580 4.829 55.751 1.290 28.21 55.751 31 13,626 9,638,265
Feb 51.480 4.829 46.651 0.910 28.59 46.651 28 11,556 7,382,689
Mar 48.290 4.829 43.461 0.790 28.71 43.461 31 10,811 7,646,735
Apr 59.120 4.829 54.291 1.220 28.28 54.291 30 13,303 4,552,814 4,458,942
May 107.430 4.829 102.601 4.350 25.15 102.601 31 22,357 15,687,457
Jun 299.800 4.829 294.971 6.940 22.56 127.900 30 25,000 16,759,621
July 695.070 4.829 690.241 6.940 22.56 127.900 31 25,000 17,318,275
Aug 829.030 4.829 824.201 6.940 22.56 127.900 31 25,000 17,318,275
Sept 554.630 4.829 549.801 6.940 22.56 127.900 30 25,000 17,007,664
Oct 239.200 4.829 234.371 6.940 22.56 127.900 31 25,000 17,318,275
Nov 119.610 4.829 114.781 5.450 24.05 114.781 30 23,917 16,033,917
Dec 78.200 4.829 73.371 2.230 27.27 73.371 31 17,336 6,130,878 6,065,120
Total 35,351,382 127,967,546
Annual generation, GWh 35.35 127.97 Total, GWh 163.32
15.4 Sensitivity analysis The returns on the project are sensitive to various parameters that are assumed during the study. To better understand the effect of each of these parameters and their impact
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on the returns, a sensitivity analysis has been carried out. The sensitivity analysis looks at varying discount rates, interest rate, cost variation, variation in energy.
The different parameters considered in the sensitivity analysis have been described in more detail below.
15.4.1 Variation on discount rate A discount rate of 10% has been considered for the base case where as sensitivity analysis has been carried out also for discount rates from 9%, 10%, 11% and 12% and keeping interest rate as 10%. Table 15.3 shows the results of sensitivity analysis for different discount rate. In all cases, project seems to be financially viable.
Table 15.3 Sensitivity Analysis on Varying Discount Rate for a fixed interest rate of 10%
Interest rate, % 6 8 9 10 11 Parameters Value Value Value Value Value
IRR, % 14.73 14.41 14.25 14.10 13.95
Payback period, yr 4 5 5 6 6
As the discount rate increases, the attractiveness of the project is declining but there is not much variation in the soundness of the project.
15.4.2 Variation on interest rate An interest rate of 10% has been considered for the base case where as sensitivity analysis has been carried out also for discount rates from 7%, 9%, 10%, 11% and 12% and keeping discount rate as 10%. Table 15.4 shows the results of sensitivity analysis for different interest rate. In all cases, project seems to be financially viable as per BCR and IRR.
Table 15.4 Sensitivity analysis on varying interest rate
Cost overrun, % -20 -10 0 10 20 Parameters IRR, % 18.79 16.47 14.57 12.96 11.59 Payback period, yr 3 4 5 5 6
As the interest over debt financing decreases, the project is found more attractive. If the loan is received with an interest rate of over 11%, the robustness of the project should be thought seriously whether or not to take the loan. For instance, with 7% of the interest over debt financing, attractiveness of the project is found very attractive. .
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15.4.3 Cost variation A sensitivity analysis was carried out varying the project costs over the construction period. For the present study, the project cost was decreased by 10% and 20% respectively on the positive side and increased by 10% and 20% on the negative side. Discount rate and interest on loan is taken as 10%. Table 15.5 indicates that the project is feasible in both cases even the fluctuations in the cost were considered in between - 20% to + 20%.
Table 15.5 Sensitivity analysis on different project cost
Cost overrun, % -20 -10 0 10 20 Parameters IRR, % 18.79 16.47 14.57 12.96 11.59 Payback period, yr 3 4 5 5 6
If the project cost is increased in the range of 20%, the project is still found feasible. However, the decision maker has to think seriously whether or not to pursue the project if the base figure of IRR is less than 4% difference over the loan interest rate.
15.4.4 Variation in energy A sensitivity analysis was carried out assuming that there is decrease or increase in annual energy generation. 10% increase or decrease in annual energy generation was checked for the robustness of the project. It is seen that with a decrease in energy by 10% could be a problem to get the project robustness. Table 15.6 below shows the financial parameters due to changes on energy generation.
Table 15.6 Sensitivity analysis on varying currency inflation rate
-10% 0% 10% Parameters Value Value Value IRR, % 12.78 14.57 16.30 Payback period, yr 5 5 4 A 10% decrease in annual energy generation would result about 3% decreases in IRR for an interest rate over debt financing of 10%. The project is found still attractive for a 10% decrease in energy generation, meaning that there is less flow available for power generation.
15.5 Economical analysis Economical analysis of the project is done on the basis of the country’s prospective. All duties, taxes, royalties, etc. are taken off from the project cost and where possible social benefit accrued from the project to the nation has to be incorporated so as to obtain economical indicators. Economical indicators as derived from the analysis are presented
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in Table 15.7 Socio-economical assumptions have been made to arrive project benefit at very preliminary level. Detail on calculation of such benefit to be gathered and then economical benefit to be derived.
Table 15.7 Economical indicators of the project
S. No. Indicators Value
1 Project cost, MNRs ‘000 5489.62 2 Construction cost, MNRs ‘000 5,864.68 3 Project cost/kW, MNRs 219.58 4 IRR, % 14.57 th 6 Payback 5 year
15.6 Conclusion The sensitivity analysis shows that the project is attractive both financially and economically. It is therefore the project shall be pursued further. The viability of the project will be questionable if more than two uncertainties are associated at the same time. The developer has to ascertain that there should not be any cost overrun and debt financing is available for a lower interest rate.
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16. CONCLUSIONS AND RECOMMENDATIONS 16.1 Conclusions The TR-SGP Hydroelectric Project (TR-SGP) has been studied to the feasibility level. After detailed field investigations like topographical mapping, geological/geo-technical investigations and hydrological data collection and analysis, layout and design works were carried out and appropriate project cost estimate was developed. Economical and financial evaluations were made to determine the viability of the project.
The feasibility study found that the project is technically feasible, economically and financially viable and environmentally friendly. Based on the various studies as described in this report, following conclusions are drawn:
! The Trishuli River is a run-off-the-river but snow fed Type River originates in Tibet, China. The base flow of the river is very pronouncing and thus reliability of power generation will be high. However, present hydrological analysis result shows that there is little reduction in river flow in comparison to the previous FS study.
! The project area is accessible through all season road upto Richowktar via Prithivi Highway and thereafter about 2 km earthen road to reach the proposed headworks site. Moreover, there are track opening and all key project area are linked to main highway.
! 132 kV Marsyangdi-Syuchatar line is passing near by the proposed powerhouse area. Initial assessment shows that the line is saturated and thus NEA has planned to construct 220 kV line instead. This line is located some 750 m away from the proposed powerhouse area.
! Preliminary assessment on environmental study shows that the project will obstruct rafting business.
! The Trishuli River is gauged river and long term time series data are available. The analysis used a gauged flow of Tadi Khola and Trishuli River to analyse time series hydrological flow for the project
! The minimum flow for the month of March is about 48.29 m3/s, 95-percentile reliability level of flow is 40.31 m3/s where as it is 70.09 m3/s and 164.53 m3/s at 65 and 40-percentile reliability level respectively.
! The 5-yrs, 20 yrs and 100 year return period flood flow is 3,701, 5,240, and 7,122 m3/s respectively.
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! There is no consumptive use of Trishuli River flow at the proposed project area. Hence all flow as available in Trishuli River will be available for power generation.
! Benighat graphitic slate, cracked but sound and unwethered with continuous bands of quartz is the main geological formation available at the project site.
! The mineralogical analysis shows that about 85% is the quartz and feldspar whose Mohr’s hardness is 7. 5000 ppm is taken as sediment concentration level for designing settling basin. However, because of low head, it is envisioned that wear and tear to the turbine will be negligible.
! Malekhu, Tamaraha sand and Trishuli Project area’s aggregates is found best for the use of construction materials. Red soil is readily available in and around the project area but no block stone available near by the project area.
! Test pit result shows that the bearing capacity of the soil is good and in the order of 200 kN/m2.
! The tunnel is assumed to pass through weak geology, though the surface geological mapping indicates that it is good but because of bigger size tunnel and shallow cover make it questionable.
! The project is found optimal at 8 m high diversion weir and 45-percentile exceedance level of flow.
! 120 m long 8 m high diversion weir will be constructed across the Trishuli River. It will be a small head pond from where relatively sediment free water will be abstracted for power generation.
! The diversion weir will be combination of controlled and uncontrolled structure. 6 numbers of hydraulically operable radial gates will be erected which infact help to flush bed load whereas flash flood will pass over the diversion weir.
! 70 m by 4 m intake opening including divide wall is proposed to divert required flow. Provision of intake gate, coarse trash rack and stoplogs has been made. Velocity through the intake opening will be about 1 m/s.
! About 3730 m long tunnel is conceived a waterway. The optimal tunnel size is found 8 m diameter horseshoe type finished surface for unlined section and relatively smaller in section for lined tunnel has been designed. At
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intermediate adit is planned at Bhunte khola, which will be a level crossing conduit will connect the upstream and downstream heading of the tunnel.
! Surge shaft of 20 m dia at top and 10 m dia at bottom, an orifice type, is proposed at the end of the headrace tunnel. at the end of waterway is provided to accommodate upsurge effect and a provision of spillway has also been made to ensure safe handling of upsurge water volume. 4 cross drainage works are required in the course of waterway. All of them are designed super passage.
! There will be two penstock pipe of diameter 5600 mm and will be laid at surface. The length of penstock pipe will be about 40 m. About 7300 mm pipe diameter, approx. 30 m inside the tunnel will lead the flow and will be branch off into two sections once it comes out outside the tunnel.
! The project will have four vertical Shaft type Kaplan Turbines with an optimum installed capacity of 6.25 MW together with a total annual generation of which generate 163.32 GWh of annual average energy.
! The power generated from the project will be evacuated to the existing 132 or 220 kV Marsyangdi-Syuchatar line at Gomati of Benighat VDC. A PI connection is found financialyl attractive and technically good and thus has been pursued for further study.
! The project is expected to be completed at a cost of MNRs 5,489.62 and within about 3 year from commencement date of the project construction.
! Based on the financial analysis of the project, the project is still found attractive in economic terms for a minimum selling price of 4.8 NRs/kWh and 8.4 NRs/kWh for wet and dry energy respectively and 3% increment on tariff annually for 1st nine years.
! The financial analysis also shows that the project has a B/C ratio of 1.197 (for base case), an IRR on equity of 14.57 % and. The cost of the project per kilowatt is determined as US$ 2794.
! The sensitivity analysis shows that the project is sound financially and economically even there is a minor variation in discount rate, cost and energy and not more than two negative events are acted simultaneously.
! It should be noted here that in addition to the financial benefits, the project also has a lot of social benefits that would accrue to the communities living in the project area.
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16.2 Recommendations Though the project is very attractive from the present level of study, it could be better to refine the investigations part of the project to arrive at more refined construction cost of the project. Some recommendations as felt important to fine tune the project cost and technical details are mentioned herein.
! The TR-SGP is found technically sound. However cautions should be taken while digging a tunnel which is relatively bigger in size and passing through phylltic type of rock. A short pull length to ascertain a control blasting would be required. Sufficient side cover to the terrain slope is required.
! The project for base case is found financially attractive. However, wherever possible two extreme cases should be avoided to ascertain better project attraction.
! The uncertainties as identified during filed geotechnical filed investigations are recommended to be pursued during detailed design phase of the project.
! It is highly recommended to establish a gauging station most likely around powerhouse area and later on move it upstream of dam, once the project is decided to implement.
! Minimum three sets of permanent points to be established at key project area and the points are to be transferred from 2nd and 3rd order grid available in the area;
! Model study of the project is suggested to carry out during detailed design phases of the project.
! Material testing result shows that the material available in and around the project area is good. It is thus sand from Malekhu and gravel from Trishuli River could be used as a construction materials and which is available in abundance in the area.
! Customised design will be required for the powerhouse electromechanical equipment in order to minimise the cost of powerhouse and also to other components of the power plant. It is thus recommended to award the Electro- mechanical contract to a highly experienced manufacturer/supplier.
! Preliminary assessment on power transmission to 132 or planned 220 kV transmission line passing close by the project showed a better option to the project financial attractiveness. This issue should be discussed with NEA and sorted out the possible power evacuation ode of the project. The study,
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however, recommend to evacuate the generated power at Gomati using PI connection.
! Regarding environmental issues, there could be only short-term problems may be encountered during construction of the project. An effort has been made to minimise those problems in the design. Moreover, additional issues that may be raised by the EIA study will be incorporated in the detail design and construction phases. The cost associated with those short terms problem is estimated and included in the project cost estimation.
! Pre-construction activities such as financing negotiations and documents for detailed design stages should be initiated.
! A full risk analysis for project life cycle (RAMP) should be carried out to determine the associated risk and its effects on the project before detailed design.
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