Revised Feasibility Report of Phawa Khola Hydropower Project

PHAWA KHOLA SMALL HYDROPOWER PROJECT (5MW) TAPLEJUNG DISTRICT

REVISED FEASIBLITY STUDY

Volume 1 of 2

Shibani Hydropower Company (P.) Ltd. Kathmandu MC-32, Anamnagar, Nepal

Tel: +977 1 4466405

Prepared By:

Mahalaxmisthan, Lalitpur 5 977 1 5523784, email: [email protected]

May 2011

PHAWA KHOLA SMALL HYDROPOWER PROJECT TAPLEJUNG DISTRICT

REVISED FEASIBILITY STUDY

MAIN REPORT Volume 1 of 2

SHIBANI Quality Control Name Initial Date

Prepared by

Checked by

Approved by

Volume 1 of 2: Main Report Volume 2 of 2: Drawings

May 2011

Shibani Hydropower Company (P.) Ltd. Kathmandu MC-32, Anamnagar, Nepal

Tel: +977 1 4466405

Phawa Khola Small Hydropower Project Revised Feasibility Study Report

Table of content Page no.

1 INTRODUCTION ........................................................................................ 1-1 1.1 GENERAL................................................................................................... 1-1 1.2 OBJECTIVE OF THE STUDY ..................................................................... 1-2 1.3 SCOPE OF WORK ..................................................................................... 1-2 1.4 PROJECT AREA ........................................................................................ 1-3

2 TOPOGRAPHICAL SURVEY AND MAPPING ........................................... 2-1 2.1 FIELD SURVEY WORKS ............................................................................ 2-1 2.2 MAP PRODUCTION ................................................................................... 2-2

3 GEOLOGICAL AND GEOTECHNICAL STUDIES ...................................... 3-3 3.1 CONSTRUCTION MATERIALS .................................................................. 3-3 3.2 LABORATORY TEST ................................................................................. 3-3 3.2.1 Aggregate test ............................................................................................ 3-3 3.2.2 Soil test ....................................................................................................... 3-5 3.3 PROJECT SEISMICITY .............................................................................. 3-6 3.4 objectives of present study .......................................................................... 3-6 3.5 geological study and field investigations ..................................................... 3-7 3.5.1 Regional Geological Setting ........................................................................ 3-7 3.5.2 Seismicity ................................................................................................... 3-7 3.5.3 Geomorphology and Drainage Pattern ........................................................ 3-8 3.5.4 Landslide and Slope Stability ...................................................................... 3-9 3.5.5 Geology of the Project Area ........................................................................ 3-9 3.5.6 Conclusions and recommendations .......................................................... 3-14

4 HYDROLOGICAL REVIEW ........................................................................ 4-1 4.1 AVERAGE MONTHLY FLOW ..................................................................... 4-1 4.2 FLOW DURATION CURVE ........................................................................ 4-4 4.3 FLOOD FLOWS .......................................................................................... 4-5 4.4 SEDIMENT CONCENTRATION ................................................................. 4-6

5 CAPACITY OPTIMIZATION, DESIGN AND DESCRIPTION ...................... 5-1 5.1 CAPACITY OPTIMIZATION ........................................................................ 5-1 5.2 DESIGN AND DESCRIPTION .................................................................... 5-1 5.2.1 Headworks .................................................................................................. 5-1 5.2.2 Headrace structures .................................................................................... 5-4 5.2.3 Forebay ...................................................................................................... 5-5 5.2.4 Penstock ..................................................................................................... 5-5 5.2.5 Powerhouse and tailrace ............................................................................. 5-5 5.2.6 Electromechanical equipments ................................................................... 5-6 5.2.7 Transmission line and switching station .................................................... 5-13 5.2.8 Hydro-mechanical works ........................................................................... 5-14 5.2.9 INFRASTRUCTURES AND OTHERS ....................................................... 5-18

6 POWER, ENERGY AND BENEFIT ASSESSMENT ................................... 6-1 7 CONSTRUCTION PLANNING .................................................................... 7-1 7.1 CONSTRUCTION METHODOLOGY .......................................................... 7-1 7.2 ORGANIZATIONAL STRUCTURE ............................................................. 7-1 7.3 DETAILED DESIGN .................................................................................... 7-3 7.4 CONTRACTORS AND SUPPLIERS ........................................................... 7-3 7.5 COMMUNICATION ..................................................................................... 7-3 7.6 TRANSPORTATION ................................................................................... 7-4

i


7.7 CONSTRUCTION POWER ......................................................................... 7-4 7.8 CAMPS AND OTHER FACILITIES ............................................................. 7-4 7.9 RIVER DIVERSION .................................................................................... 7-5 7.10 DEWATERING ........................................................................................... 7-5 7.11 MAIN CONSTRUCTION ............................................................................. 7-5 7.11.1 Headworks .................................................................................................. 7-5 7.11.2 Headrace .................................................................................................... 7-5 7.11.3 Forebay ...................................................................................................... 7-6 7.11.4 Penstock ..................................................................................................... 7-6 7.11.5 Powerhouse and tailrace ............................................................................. 7-6 7.11.6 Electromechanical works ............................................................................ 7-6 7.11.7 Hydro-mechanical works ............................................................................. 7-6 7.11.8 Transmission line and switching station ...................................................... 7-6 7.11.9 Access road and construction road ............................................................. 7-6 7.12 CONSTRUCTION SCHEDULE ................................................................... 7-7

8 OPERATION AND MAINTENANCE PLAN ................................................ 8-1 9 SOCIO-ENVIRONMENTAL STUDY ........................................................... 9-1 10 COST ESTIMATE ..................................................................................... 10-1 10.1 CRITERIA AND ASSUMPTIONS .............................................................. 10-1 10.2 METHODOLOGY ...................................................................................... 10-1 10.3 COMPONENTS OF PROJECT COST ...................................................... 10-3 10.3.1 Pre-construction cost ................................................................................ 10-3 10.3.2 Construction cost ...................................................................................... 10-3 10.3.3 Land purchase and lease cost .................................................................. 10-4 10.3.4 Environmental mitigation and monitoring cost ........................................... 10-4 10.3.5 Engineering, administration and management cost ................................... 10-4 10.3.6 Contingency cost ...................................................................................... 10-4 10.4 PROJECT COST ...................................................................................... 10-5

11 PROJECT EVALUATION ......................................................................... 11-1 11.1 COST ESTIMATION ................................................................................. 11-1 11.2 BENEFIT ESTIMATION ............................................................................ 11-1 11.3 FINANCIAL ANALYSIS ............................................................................. 11-1

12 CONCLUSION AND RECOMMENDATION .............................................. 12-1 12.1 CONCLUSIONS........................................................................................ 12-1 12.2 RECOMMENDATIONS ............................................................................. 12-1

Appendices Cost estimate

ii


List of Figure Figure 1.1: Project location ..................................................................................... 1-3

Figure 3.1: Location of project area in seismic hazard map of Nepal (Department of mines and Geology, GoN - 2002) .................................................................... 3-8

Figure 4.1: Flow hydrograph of adopted flow .......................................................... 4-4

Figure 4.2: Flow duration curve .............................................................................. 4-5

Figure 4.3: Instantaneous flood flow curve ............................................................. 4-6

Figure 5.1: Weir and intake location ....................................................................... 5-2

Figure 7.1: Organizational Structure of Project Implementation Team .................... 7-2

Figure 7.2: Construction schedule .......................................................................... 7-7

Figure 8.1: Structure of operation crews ................................................................. 8-1

Figure 10.1: Construction cost distribution (%) ..................................................... 10-5

Figure 11.1: Project FIRR ..................................................................................... 11-2

Figure 11.2: Return on Equity of project ............................................................... 11-2

Figure 11.3: Project cash flow .............................................................................. 11-6

Figure 11.4: Equity cash flow ............................................................................... 11-6

iii


List of Table Table 2.1: Considered control points from Department of Survey ........................... 2-1

Table 2.2: Traverse points ...................................................................................... 2-2

Table 3.1: Estimated quantity of construction materials .......................................... 3-3

Table 3.2: Laboratory test result of aggregate ........................................................ 3-4

Table 3.3: Mineralogical composition of fine aggregate sample .............................. 3-4

Table 3.4: Laboratory test result of soil ................................................................... 3-6

Table 4.1: Monthly average flow ............................................................................. 4-1

Table 4.2: Flow estimation by MIP method ............................................................. 4-2

Table 4.3: Rainfall data of Taplejung (Station no. 1405) ......................................... 4-3

Table 4.4: Flow duration curve data ....................................................................... 4-4

Table 4.5: Instantaneous flood flows ...................................................................... 4-5

Table 5.1: Turbine specifications ............................................................................ 5-7

Table 5.2: Governor specifications ......................................................................... 5-8

Table 5.3: Generator specifications ........................................................................ 5-9

Table 5.4: Step up transformer specifications ......................................................... 5-9

Table 5.5: Circuit breaker specifications ............................................................... 5-10

Table 5.6: Vacuum Circuit Breaker specifications ................................................. 5-10

Table 5.7: Isolator specifications .......................................................................... 5-11

Table 5.8: Station service transformer specifications ............................................ 5-12

Table 6.1: Power, energy and benefit calculation ................................................... 6-1

Table 7.1: Manpower required for project implementation ...................................... 7-3

Table 8.1: Manpower required for plant operation .................................................. 8-1

Table 10.1: Basic rates at site .............................................................................. 10-2

Table 10.2: Cost summary ................................................................................... 10-4

Table 11.1: Input parameters ............................................................................... 11-3

Table 11.2: Parameters calculation ...................................................................... 11-3

Table 11.3: Calculation table ................................................................................ 11-4

Table 11.4: Financial indicators ............................................................................ 11-7

iv


Acronyms AC Alternating current ACSR Aluminium conductor steel reinforced AVR Automatic voltage regulator BC Ratio Benefit cost ratio BM Bench mark BoQ Bill of quantities cm Centimetre d/s Downstream DC Direct current DDC District Development Committee DHM Department of Hydrology and Meteorology DoED Department of Electricity Development FIRR Financial internal rate of return GoN Government of Nepal GWh Giga Watt hour HFL High flood level Hz Hertz IEE Initial Environmental Examination INPS Integrated National Power System km Kilo meter kN/m2 Kilo Newton per square meter kV Kilo Volt kVA Kilo Volt Ampere kW Kilo Watt kWh Kilo Watt Hour LT Low tension m amsl Meters above mean sea level M US$ Million US$ m Meter m/s Meter per second m3 Cubic meter m3/s Cubic meter per second mg/l Milligram per liter mm Milimeter MoWR Ministry of Water Resources MVA Mega Volt Ampere MW Mega Watt NEA Nepal Electricity Authority NPV Net present value NRs Nepalese Rupees O & M Operation and maintenance PCC Plain cement concrete PKSHP Phawa Khola Small Hydropower Project PPA Power purchase agreement PPM Parts per million PVC Polyvinyl chloride RCC Reinforced cement concrete RoR Run of the river rpm Revolution per minute RRM Rock mass rating T Ton ToR Terms of Reference

v


u/s Upstream US$ United States Dollars V Volt VAT Value aided tax VCB Vacuum circuit breaker VDC Village Development Committee WECS Water and Energy Commission Secretariat Yr Year

vi


Salient feature General Development region Eastern

Zone Mechi

District Taplejung

District headquarter Phungling

VDCs Dumrise and Thechambu

Project location (same as before)

Longitude 87 45 27 to 87 46 22 East

Latitude 27 16 37 to 27 19 03 North Type of scheme Run of river (RoR)

Source river Phawa Khola

Hydrology Catchment area 98 km2 at intake site

Mean annual precipitation 2029 mm

Design discharge 2.09 m3/s (Q66%)

Compensation flow 0.080 m3/s

1 in 100 years return period design flood

410.00 m3/s

1 in 5 years return period operation flood

167.00 m3/s

1 in 2 years return period diversion flood

100.00 m3/s

Power and energy Gross head 310.00 m

Net head (for 2.10 m3/s) 298.70m

Installed capacity 5000 kW

Dry season energy 9.79 GWh

Wet season energy 26.43 GWh

Annual energy 36.21 GWh

Project components Weir

Type Concrete broad crested

Crest level 898.0m amsl Length of weir 27.00 m Spillway type Free overflow

vii


Intake

Type Gated side orifice intake with course trash rack

Nos. of opening 3 nos. Opening size (single orifice) 1.50X2.15 m

Flood Spillway

Type Ogee shaped spillway

Flow capacity 6.10 m3/s Overflow spillway length 10.00 m

Overflow spillway capacity 3.50 m3/s Gravel trap

Type Continuous flushing hopper type

Overall length 10.00 m Width 2.50 m

Effective depth 2.22 m Particle size to be trapped 5 mm

Design flow 2.60 m3/s Flushing flow 0.50 m3/s

Settling basin

Type Double chamber, intermittent gravity flushing type

Settling zone length 42.00 m Inlet transition length 18.125 m Single basin width 4.80 m

Overall depth 4.42 m Particle size to be settled 0.15 mm with 90% settling efficiency

Design flow 2.09m3/s Headrace canal

Type Box type RCC

Length 4793.0 m Width 1.35 m

Overall depth 1.40 m L-slope 1:1000

Headrace pipe

Type Low pressure, mild steel pipe

Length 350.0 m

viii


Internal diameter 1.20 m Thickness 6 mm

Design flow 2.09 m3/s No. of anchor blocks 7

No. of support piers 44 Forebay

Type RCC tank

Storage period 60 s Effective length 21.75 m

Width 5.00 m Effective depth 1.30 m Effective storage 126 m3

Max. operating level 890.453 m amsl Min. operating level 889.153 m amsl

Penstock

Type Surface, mild steel circular shaped

Length 742.715m Internal diameter 1.10 m Thickness 6-18 mm

Design flow 2.09 m3/s No. of anchor blocks 15

No. of support piers 70 Powerhouse

Type Surface type, RCC structure

Length 25.50 m Width 11.75 m

Height 12.00 m Tailrace length 100.00 m

Turbine

Type Pelton (2 units) Rated capacity 2700 kW one unit

Turbine axis level 588.00 m amsl Design flow 0.53 to 1.050 m3/s for one unit Rated speed 750 rpm

Generator

Type 3 Phase brushless synchronous (2 units)

Rated capacity 3125 kVA each

ix


Rating 6.3 kV, 50 Hz, 750 rpm Governor Servo motor actuated PID electronic flow

governor

Overhead crane Lifting capacity 15 T

Step up transformer

Type 3-Phase, ONAN cooled, Outdoor type Rating 2 x 3150 kVA, 6.3/33kV, 50 Hz

Transmission line 33 kV single circuit, 3 phase, 50 Hz, 3 km long

Connection with grid Switching station at Bhaluchok

Access road Earthen road, 25 km long

Cost and finance Project cost (with IDC, at 80 NRs/US$) 11.65 M US$

Cost per kW (with IDC) 2118 US$/kW

Debt equity ratio 70/30

FIRR 18.94%

NPV 259.50 Million NRs

BC ratio 1.40

Return on equity 25.52%

x


1 INTRODUCTION

1.1 GENERAL Shibani Hydropower Company (P.) Ltd. got survey license from Department of Electricity Development (DoED) for development of Phawa Khola Small Hydropower Project (PKSHP) located at Phawa Khola of Taplejung district. The company had conducted detailed feasibility study of the project and plant capacity proposed by the study was 2080 kW. After completion of detailed feasibility study and further river flow measurement by more accurate method of current meter, the company decided to explore possibility of capacity upgrading of the project by increasing head if possible and increasing the design flow which was seen possible as shown by actual measurement of the flow of Phawa Khola. Furthermore projects financial soundness could have increased by increasing the power output as marginal increase in energy revenue could be higher than marginal increase in project cost. This could make project more feasible in current situation of construction cost increase and stagnant energy price.

In this connection, a site visit of Phawa Khola Small Hydropower Project was made with representatives of the company, a geologist, a detailed topographical survey team and a hydropower professional from Small Hydropower Promotion Project (SHPP), GTZ from 12 to 18 Shrawan 2065. The main objectives of the site visit were shifting weir and intake upstream location so that additional head could be gained for more power, project layout identification for new weir and intake location, geological observation of landslide area and general geological observation of the project area.

An appropriate weir and intake location was identified at about 1500 m upstream from the previously proposed weir and intake location at an elevation of 898.0 m amsl. Turbine axis level proposed in detailed feasibility study was at an elevation of 590.0 m amsl. Thus gross head available is 308.0m against the gross head of 161.86 m considering the project layout proposed during detailed feasibility study. Furthermore, 65 percentile probability of exceedence flow concluded before was 1.63 m3/s but actual 65 percentile probability of exceedence flow from the monthly hydrograph concluded in the detailed feasibility study is 2.20 m3/s. Thus the project capacity was upgraded to 5000 kW considering Nepal Electricity Authoritys criteria of flow not exceeding 65 percentile probability of exceedence flow against the installed capacity of 2080 kW proposed earlier.

In this connection, a team of experts again visited the site during detail engineering of the project since 28 Chaitra 2067 to 9 Baishak 2068. The team found geological instabilities in middle portion of the water conveyance along left bank of the river. So the geological and topographical features to make the water conveyance system along right bank is examined vigorously and found more stable geological conditions as well as stable and mild topographical features which are favourable conditions to construct headrace canal. A stable crossing place after some 608 m from left bank to right bank is found. Similarly a stable flat cultivated land just downstream of the confluence of Phawa Khola with Kabeli river. After considering all these aspects, it is decided to make the project with settling basin and 608 m headrace canal along left bank and one steel truss bridge over Phawa Khola to cross with steel pipe and then headrace canal along the right bank and powerhouse on right bank of Kabeli river. Hence this Revised Feasibility Study is carried out and Revised Feasibility Study Report is prepared to complement the works carried out during detailed feasibility study phase. This report addresses the issues attracted due to the change in weir and intake location, water conveyance system with plant capacity of the Phawa Khola Small Hydropower Project to 5000 kW.

1-1


Project area and survey boundary is same as applied for survey license before. The survey boundary lies between 87 45 27 and 87 46 22 East in longitude and 27 16 37 and 27 19 03 North in latitude. Difference lies in weir and intake location, headrace alignment, design flow and installed capacity. Thus only relevant information that differs from the detailed feasibility study is included in this report and following chapters illustrate the issues regarding revised feasibility of the project.

1.2 OBJECTIVE OF THE STUDY The main objective of the study was to carry out design and analysis to complement the detailed feasibility study by addressing the issues of capacity upgrading from 2080 kW to 5000 kW of Phawa Khola Small Hydropower Project. Evaluation of technical as well as financial implication to the project was also an objective of this revised feasibility study. Revised Feasibility Study Report of Phawa Khola Small Hydropower Project is prepared after study and analysis of consequences. This report documents study methodology and findings related to the objectives of the study. This report will be submitted to Bank for financial closure and other concern authorities for getting clearances and approval as applicable.

1.3 SCOPE OF WORK The scope of work of revised feasibility study is consistent with the requirements of capacity upgrading issues of a small run of river (RoR) hydropower project. The scope of work for the study are summarised in the list below:

Study and review of available data, information and reports

Hydrological analysis review

Conduction of topographical survey of the project area which is not covered by previous survey works

Field identification of landslide stability and recommendation on alternative headrace alignment and possible stabilization measures

Power potential assessment and energy computation

Project layout preparation and design and description of project components

Power evacuation study

Construction plan and schedule preparation

Operation and maintenance plan preparation

Bill of quantities (BoQ) and cost estimate preparation

Financial analysis conduction

Report writing and drawings preparation

This scope of work was accomplished in required depth with professional integrity and result of the study works is based on high professional standard and practice. Initial Environmental Examination (IEE) was carried out and got approval from concerned authorities since other project parameters remain same. Similarly detailed geological investigation was not carried out and references were taken from the previous study. However field identification work was carried out identifying landslide vulnerability and remedial measures concerning its stabilization as it is a major concern of the project for selection of the best headrace alignment.

1-2


1.4 PROJECT AREA Project area and survey boundary is same as applied for survey license before. The survey boundary lies between 87 45 27 and 87 46 22 East in longitude and 27 16 37 and 27 19 03 North in latitude. Proposed new weir and intake is about 1500 m upstream from the weir and intake site proposed in the detailed feasibility study report and is at an elevation of 898.0 m amsl. A contour headrace canal will convey design flow along left bank upto chinage 0+608m and pressure steel pipe will convey water from left bank to right bank after which headrace canal along right bank will convey up to forebay from headworks area. A mild steel penstock pipe will convey the flow from forebay to powerhouse which is located at just downstream of the confluence of Phawa Khola with kabeli river. It is at a flat paddy field at an elevation of 588.0 m amsl.

The project area lies in Dumrise and Thechambu Village Development Committees of Taplejung district. The project location in map of Nepal is shown in Figure 1.1. The major construction area of the project is located in Thechambu VDC but weir and intake area is situated at Thechambu (right bank) and Dumrise (left bank) VDCs. Proposed 33 kV transmission line of the project will be connected to Integrated National Power System (INPS) of Nepal at Bhaluchok, Amarpur VDC of Panchthar District in a switching station to be built during project construction. However Nepal Electricity Authority shall extend 33 kV transmission line up to Bhaluchok by the time of Phawa Khola Small Hydropower Project completion date. Access road has already been constructed by the company from Kabeli bridge via Khalte of Thechambu VDC to headworks and powerhouse area.

Figure 1.1: Project location Altitude of watershed area varies from 570 to 3800 m amsl. Phawa Khola flows North to South at project area. The average gradient of the river in project area is about 6% and river width varies from 10 to 40 m. The topography is generally favourable for implementation of the hydropower project. The slope of the banks along the river varies from 25 to 75.

The project site is accessible from Birtamod to Charchali 7 km black topped East-West National Highway, Charali to Ilam 78 km black topped Mechi Highway and Ilam to Phidim 65 km black topped Mechi Highway. Similarly from Phidim to Kabeli 61 km gravel road and from Kabeli about 15 km earthen road has been constructed to reach

Project location

1-3


to project site. The proposed project site is situated at about 26 km southeast from Phungling, the district headquarter of Taplejung.

1-4


2 TOPOGRAPHICAL SURVEY AND MAPPING This chapter describes the methodology adopted for topographical survey and mapping works of PKHP area. Detailed topographical survey was conducted to produce the topographical maps of the project site for project layout and design of project components. The survey works covered weir and intake area, waterway alignment, forebay area, penstock alignment and powerhouse area. Separate survey works were conducted for access road and transmission line.

Following activities were carried out during the topographical survey works:

New traverse stations were established from existing traverse stations which were tied with national grid provided by the Department of Survey.

A close traverse survey was carried out to establish necessary ground control points at various locations in the project area.

The entire major ground control points and benchmarks were established on concrete pillars or marked on permanent boulders.

Topographical maps of headworks area, headrace area, penstock area and powerhouse area were produced with accuracy of 1 m contour interval.

Alignment survey of road alignment and profile survey of transmission line was conducted separately.

2.1 FIELD SURVEY WORKS Topographical survey and mapping of the project site was conducted according to standard norms of the survey work required for hydropower project. The survey works was conducted with total station. The coordinates and elevation were transferred from the nearest permanent survey control points established by the Department of Survey. The control points considered in the survey work are presented in Table 2.1.

Table 2.1: Considered control points from Department of Survey Grid sheet alignment

Control points Elevation

Coordinates Easting Northing

167 3298 - 576537.35 3019765.55 167 3299 - 576227.20 3019429.02 BM 220 (Kabeli bridge)

220 548.175 - -

Seventeen permanent traverse points (BM-3 to BM-19) were established in the project area during detailed topographical survey of revised feasibility study phase to control the survey works. Furthermore additional permanent traverse points were established during detailed topographical survey for revised feasibility. The details of the traverse points are shown in Table 2.2. Elevation was transformed from BM 220 of Department of Survey located near Kabeli Bridge.

2-1


Table 2.2: Traverse points

SN Station Northing Easting Elevation

1 BM-3 3018816.757 575122.270 887.093

2 BM-4 3019047.036 574817.894 896.105

3 BM-5 3019199.111 574712.453 903.905

4 BM-6 3019764.072 574774.062 894.085

5 BM-7 3019923.036 574763.637 891.222

6 BM-8 3020053.457 575020.744 1024.236

7 BM-9 3020077.530 575167.748 1026.130

8 BM-10 3020199.820 575291.602 1040.793

9 BM-11 3020618.231 575464.017 991.992

10 BM-12 3020684.741 575432.318 1021.454

11 BM-13 3020948.553 575482.726 1053.869

12 BM-14 3021176.681 575525.310 1017.823

13 BM-15 3021209.530 575499.045 1031.066

14 BM-16 3021339.876 575396.940 1068.739

15 BM-17 3021621.329 575383.649 1060.034

16 BM-18 3021857.417 575485.880 999.382

17 BM-19 3021975.046 575559.154 988.763

Sufficient points were picked up to produce 1 m contour interval. For that every undulation greater or equal to 1 m was considered during the detailed survey. Boulders with average size greater or equal to 3 m were picked up and plotted in topographical map. Permanent and semi permanent built up areas, land use detail, remarkable big trees and all geological features of the survey area were picked up and plotted in topographical map.

The survey work covered at least 200 m on both upstream and downstream side of proposed weir axis and 20 m from centre line of the river in vertical direction. The survey work covered at least 20 m in both uphill and down hill side from the centre line of headrace alignment. The longitudinal slope of headrace alignment considered during the survey work was 1 vertical to 700 horizontal. Sufficient survey area was covered at powerhouse and at least 200 m in both upstream and down stream stretches from powerhouse location of river and 20 m above from the river centre line on either sides of the river was considered in survey work.

All survey works were carried out using the high accuracy total stations with least count of 1. Hence, closing error of traversing was within error limit. This closing error was distributed according to common correction practice.

2.2 MAP PRODUCTION The data of the topographical surveys conducted in the field were processed to prepare topographical maps in required quality. Contour and topographical details are plotted in AutoCAD compatible format and extensively used for project layout and design.

2-2


3 GEOLOGICAL AND GEOTECHNICAL STUDIES The Phawa Khola Hydropower Project (PKHP) is located on Thechambu and Dumrise VDCs of Taplejung District, Mechi Zone, Eastern Development Region of Nepal. The Phawa Khola is one of the major tributary of the Kabeli Khola which finally joins to the Tamor River of Koshi Basin. The major structures of the Project are located on the right bank of the Phawa Khola. The Head works area is located at about 400 m downstream from the confluence of Phawa Khola and Siwa Khola. The Powerhouse is located on right bank of Kabeli Khola at about 100 m downstream from the confluence of the Kabeli Khola and Phawa Khola.

This report presents a description of the regional geology, Project area geology that were observed and investigated during the field visit to the Project area.

3.1 CONSTRUCTION MATERIALS Substantial amount of boulder and cobble for block stone and gravel and sand for concrete aggregate is available at Phawa Khola. In fact, the riverbed materials found distributed along the river course and flood plains of the Phawa Khola and are commonly a heterogeneous mixture of boulder, cobble, gravel and sand. Thus it should be sorted to obtain their proper sizes. Such construction materials can also be brought from nearby Kabeli River. Khibuna Bagar can be a good reserve of construction materials in addition to the sites mentioned above. However the material from such quarry site may require cleaning to remove silt and clay particles.

Estimated quantity of the construction materials available in the project site or its vicinity calculated from the topographical maps and field measurement are tabulated in Table 3.1.

Table 3.1: Estimated quantity of construction materials

SN Location Measurement Area Assumed depth Volume

1 Alluvial terrace at previous intake site 50 m x 60 m 3000 m2 2 m 6000 m3

2 Khibuna Bagar 50 m x 60 m 3000 m2 2 m 6000 m3

The locations described above are within the project area but sand availability is not sufficient. Thus sand should be brought from Kabeli River.

3.2 LABORATORY TEST Different geo technical laboratory tests were carried out during detailed feasibility study of the project to find the geo technical properties of construction materials and other geo technical parameters. The test details are presented in sections below.

3.2.1 Aggregate test Grain size analysis, specific gravity test, water absorption test, soundness test and Los Angeles Abrasion tests were carried out to find suitability of aggregate for concrete production. The tests are described in following paragraphs and test result is presented in Table 3.2.

Grain size analysis Grain size analysis was conducted on soil sample collected from three pits namely from previous headworks, forebay and powerhouse sites in order to determine proportion of clay, silt and sand. The test pits were excavated up to 1.50 m depth and 1 m x 1 m in plan and the samples were collected representing whole section. The

3-3


fraction larger than 60 mm were weighed separately and only the finer fraction were supplied to laboratory for sieve analysis. The D50 values obtained from smaller fraction were corrected in consideration of the original sample content. The uncorrected D50 value obtained for the collected soil sample is 28 mm.

Water absorption test A sample from the previous headworks site was tested to determine water absorption characteristics of coarse and fine aggregates. The test showed that the water absorption of the aggregate is 0.81% which is within a range of good quality aggregate and can be used for concrete production.

Soundness test A sample from the previous headworks site was tested to determine the soundness of the aggregate. The value of the test result is 1.53% which shows that the quality of the aggregate is very good for concrete production.

Los Angeles abrasion test Similarly a sample from the previous headworks site was tested to determine the Los Angeles Abrasion value. The result obtained is 43.20% which shows the aggregate is high abrasion resistant and appropriate for concrete production.

Table 3.2: Laboratory test result of aggregate

SN Soil classification D50

value (mm)

Average apparent

specific gravity

Water absorption

(%) Soundness

Test (%)

Los Angeles abrasion

(%)

1

Boulder, cobble, gravel and sand mixture

28 2.73 0.81 1.53 43.20

Petrographic analysis A soil sample from previous intake site was collected for petrographic analysis for determining the mineralogical composition of the soil sample. The analysis showed that more than 72 % of the minerals are quartz which reveals the strength of the rocks in the project area. This also shows that sediment carried by river flow is highly abrasive to turbine buckets. The mineralogical composition of the soil sample deducted from petroghraphic analysis is presented in Table 3.3.

Table 3.3: Mineralogical composition of fine aggregate sample SN Minerals % Constituents 1 Quartz 72.50 2 Fedlspar 2.50 3 Muscovite 3.00 4 Phlogopite 7.00 5 Biotite 3.50 6 Chlorite 1.50 7 Garnet - 8 Tourmaline - 9 Kyanite - 10 Magnetite - 11 Calc fragment - 12 Beryl


13 Serictie schist, muscovite & biotite schist

10

14 Limonite Occasionally visible 15 Organic material, grass root Occasionally visible

3.2.2 Soil test The soil samples from previous headworks site, forebay site and powerhouse site were collected and tested for grain size distribution, moisture content, specific gravity, Atterberg limit, cohesion and angle of friction. The tests were performed in the laboratory of ESLA Consult (P.) Ltd. The conducted tests were briefly discussed below and test results are presented in Table 3.4.

Grain size analysis Soil samples from three test pits, each from previous headworks site, forebay site and powerhouse site were examined to determine the grain size distribution and soil type. The soil in the headworks area is predominantly composed of sandy soil and possesses good gradation. It is designated as SW in Unified Soil Classification. Similarly, the tested sample from forebay indicates that the soil type is SC and that of powerhouse is GM.

Moisture content Samples from the project area were investigated for the moisture content and moisture content value varies from 7.26% to 16.41%.

Specific gravity The specific gravity determined on the soil samples indicates that there is a very slight variation in the values ranging from 2.65 to 2.67. The values are similar to the ordinary soil.

Atterberg limit The test is applicable only for the soil sample collected from the forebay site. Samples from powerhouse and headworks constitute high proportion of non-cohesive constituents; therefore the test did not give any plasticity result. Plastic limit, liquid limit and plasticity index for the soil sample from forebay area are 19%, 31% and 12% respectively.

Angle of friction and cohesion Direct shear test was conducted for the soil samples from previous headworks site, forebay site and powerhouse site in order to determine angle of internal friction and cohesion. Since the soil samples from the headworks site and powerhouse site possess very little or negligible clay content, cohesion determination is limited only to the soil sample from forebay site. The cohesion for soil sample of forebay site is 1.00 kN/m2 whereas angle of internal friction varies from 29 to 30 for all three sites.

Field permeability test Field permeability tests were also performed at seven places adopting changing head method. The tests were conducted two at previous settling basin site, one at the forebay site and four along headrace alignment. At every test site a bore hole of 10 cm diameter and about 50 cm deep was made using a hand auger having a capacity to penetrate up to 2.25 m depth. A polythene pipe with an internal diameter of 8 cm was inserted tightly up to the bottom of the hole. Then the pipe was filled with water up to its upper end. The fall of water level was recorded for a period of about an hour.

3-5


The permeability test results obtained in this way for the settling basin site, headrace alignment and forebay site are 6.2x10-3 cm/sec to 1.86x10-3 cm/sec, 1.50x10-3 cm/sec to 3.98x10-3 cm/sec and 1.50x10-3 cm/sec respectively.

Table 3.4: Laboratory test result of soil

SN Location Soil classification Moisture content

(%) Specific gravity

Atterberg limits (%) Cohesion

Angle of

friction Plastic

limit Liquid limit

Plasticity index

1 Previous headworks area

Well graded sand 7.26 2.65 - - - - 30

2 Forebay area

Clayey sand, sand-clay mixtures

16.41 2.67 19 31 12 1.00 kN/m2 29

3 Powerhouse area

Silty gravels, gravel-sand-clay mixtures

13.60 2.66 25 - - - 30

3.3 PROJECT SEISMICITY The Great Himalayan Arc is evolved as a result of collision between the Indian and Eurasian Tectonic Plates over a distance of 2800 km from Pakistan in the west and Burma in the east. The Himalayas are located near to tectonic plate boundary. Therefore, the Himalayan region is considered to be seismically active zone. Thus, being a part of the Himalayas, Himalaya of Nepal also falls in active seismic zone. Nepal has already experienced a number of large earthquakes over the past few decades, which has caused considerable damage to life and property. Furthermore, the existence of tectonic joints such as Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Fault (HFF) further increases the degree of seismic risk. Therefore, nearness of a project to such structural joints is vital while assessing seismicity of the project area.

The records of seismic activities are limited in the Nepal Himalaya and hence correlation of seismic events of adjacent Himalayan region would be a useful source of information for designing the hydraulic structures. Several seismic studies have been carried out for various projects in Nepal during engineering design phases and seismic design coefficients are derived for those projects.

In order to evaluate the seismic coefficient for the project structures, seismic map of Nepal prepared by Building Code Development Project (BCDP, 1994) has been referred. This map was prepared after the detailed analysis of the earthquake activity and tectonic structure of Nepal. The country is divided into three seismic risk zones based on allowable bearing capacity of three types of soil foundation. The Phawa Khola Small Hydropower Project is located in the second seismic risk zone of Nepal and the soil foundation at weir site belongs to average soil type. Therefore, the basic horizontal seismic coefficient is considered to be 0.06. By using empirical method, the effective design coefficient according to seismic design code of Nepal is calculated as 0.13 and recommended this value to consider while conducting detailed structural analysis of project components.

3.4 OBJECTIVES OF PRESENT STUDY The objective of the present geological and geotechnical investigation is to prepare a geological map of the project area, evaluate the slope stability condition along the proposed water conveyance system, assessment on suitability of structure locations with respect to the existing geotechnical condition of the proposed site, evaluation and recommendation of appropriate geotechnical properties of soil and rock mass for

3-6


slope stability and foundation analysis/ design purposes. Finally, the availability of construction material in the project area has also been investigated and the location of borrow areas for different types of construction material are shown in separate drawing.

3.5 GEOLOGICAL STUDY AND FIELD INVESTIGATIONS

3.5.1 Regional Geological Setting Broadly, Nepal has been divided into five lithologic units, from north to south they are Tibetan Tethys unit, Higher Himalayan unit, Lesser Himalayan units, Siwalik unit and the Terai plain. The Tibetan Tethys Unit exposes only occasionally within the territory of Nepal, while the other four units are extended from east to west throughout the country.

Geologically the Phawa Khola HP is located in Taplejung Window. This is an important eastern most tectonic window in Nepal Himalaya exposing the low grade meta sediments broadly belonging to Nawakot Complex lying below the crystalline thrust sheets. Bashyal (1970) mapped the Taplejung area and divided the rock in to six units.

The Phyllites, representing the lower most unit; the rocks above the Phyllites sequence represented by chlorite-schists, quartz, biotite-schists, feldspathic quartz-biotite-muscovite-schists, garnetiferrous schist, and kyanite schist. Tourmalines bearing granitic bodies have also intruded into the lowest observed sequence of the Taplejung Window; best exposed south east of Taplejung Bazar along the Phawa Khola section, and have been mapped under the Phawa Khola Granite. No remarkable geological hazards have been encountered in the project area.

3.5.2 Seismicity The major causes to produce seismic hazard to any construction project are the existing tectonic contacts between the different geological units defined above in regional geology. Among them the major contacts are the MBT and MCT located more than 10km from the project site. Location of project area in seismic location map is shown in Figure 3.1: Location of project area in seismic hazard map of Nepal (Department of mines and Geology, GoN - 2002) below.

3-7


Figure 3.1: Location of project area in seismic hazard map of Nepal (Department of mines and Geology, GoN - 2002) The seismic hazard map of Nepal prepared by Department of Mines and Geology, Government of Nepal has been studied to find the level of seismic hazard in the project. The project area falls around the contour indicating 250 300gal of seismicity as shown in figure 4.1. The design of the project structures should be done considering this level of seismicity.

3.5.3 Geomorphology and Drainage Pattern The project area has rugged nature and sharp crest steep topography. Vertical cliffs are also observed. The valleys are deeply incised. The erosion at the hill slopes is at initial stage.

The project area aligns almost through moderately steep slope. The slopes are covered with thin colluvial soil and bedrock is also exposed at places as well as along the bank of the river. The vertical cliffs are rocky cliff. The moderately steep topography is covered with vegetation whereas cultivation and settlement is low which is concentrated to flat and gentle slopes.

The main river, the Phawa Khola flows almost from North to South direction. The tributaries are more or less perpendicular to the major river. The Phawa Khola is a major tributary of the Kabeli Khola which mixes with the Tamor River of Koshi Basin in eastern Nepal. The Phawa Khola, itself is a third order river with almost straight river channel and moderate gradient. The Phawa Khola and its tributaries are emanating from Himalayan mountain range; this ensures the fact that the discharge of the Phawa Khola will vary during the year from a maximum during summer to a minimum during winter season.

Project area

3-8


Photo 3.1: Drainage Pattern at confluence of Phawa Khola and Kabeli River

3.5.4 Landslide and Slope Stability The mountainous slopes along the Project alignment are moderately sloped due east. Although, the Project area aligns through moderately steep slopes, no any sign of instability was visualized during the field visit. Mostly, the steeper slopes were either covered by thin Colluvial soil layer or exposed with rock. The Colluvial soil layer is thicker in more gentle slopes, and no any sign of slope instability is visualized on those areas at the time of field visit. The major foliation joint of the rock masses observed in the vicinity of the project area, dips obliquely inward to the right bank mountainous slope of the Phawa Khola which also ensures the right bank mountainous slope is more stable. Instead, the left bank mountainous slope shows more instability as compared to the right bank mountainous slope; but the preventive measures for the stability should be taken during the cutting of slope although the slopes are considered to be safe during the field study.

3.5.5 Geology of the Project Area Geologically the Phawa Khola HP is located on the lower sequence of Taplejung Window, on the Phyllites sequences. The exposed Phyllites are green to lead-gray colored, and are hard and massive. At few places the Phyllites are interbeded with green Quartzites. Within these Phyllites sequences the granitic bodies have intruded. Actually, the Project boundary lies at the region transition between the lower sequence of Phyllites and Phawa Khola Grinite. At some places, at very near vicinity of the Phawa Khola Granite and Phyllites sequences, the granitic bodies show gneissic structure, resembling with the banded Gneiss.

Besides the exposed rocks, the Project area also contains the colluvial soil and alluvial soil as well. The colluvial soil is one of the major surface deposits found in the headrace canal alignment; whereas the alluvial soil is found in the Head works area and Powerhouse area.

Headworks site The Headworks area of Phawa Khola HPP is located at about 400 m downstream from the confluence of Phawa Khola and Siwa Khola. The right bank of Phawa Khola around the Head works site constitutes the rock exposure. The exposed rock is Phawa Khola Granite and the green-gray Phyllites. The exposed granite is massive in nature and moderately to sparsely jointed and has high strength, whereas the exposed Phyllites sequence is moderately to closely jointed with moderate

Phawa Khola

Kabeli

3-9


strength. Close spacing of joints within the Phyllites sequence seems localized at places. In general, the Phyllites sequence is moderately jointed.

Photo 3.2: Headworks Area Viewing Downstream

The left bank of the Phawa Khola constitutes the alluvial soil. The alluvial soil is composed of rounded to sub-rounded gravels to boulders of granite, phyllites within the matrix of medium to fine sand. Within the alluvial soil there are mainly the large sized boulders of granite.

The proposed settling basin area and headrace canal alignment along left bank is covered with thin to thick colluvial deposits having natural slope less than 40 degree in general. The colluvial deposits are loose debris deposit slope of eroded mass and landslide materials as well as accumulated weathered rock fragments. These consists silty soil, angular to sub angular gravel, pebble, cobble and boulder of various types of rock. These slope materials are covered with vegetation and the slopes seems to be stable..

Headrace Canal alignment The headrace canal alignment runs through the left bank hill of the Phawa Khola. The hill slope is mainly covered with the colluvial soil and bed rock exposures at places as well. The colluvial soil is composed of angular to sub-angular fragments of moderately to completely weathered gravels to cobbles of granite, Phyllite etc. with fine to medium sandy, silty matrix. The deposit is loose to moderately compacted and moderately permeable.

A good exposure of rock with high strength is seen around the proposed river crossing of headrace pipe, and is more stable and safe. The moderately steep right bank hill slope of Phawa Khola seems stable and shows no sign of instability at the time of field visit.

3-10


Photo 3.3: Canal alignment of Phawa Khola Hydropower Project

Besides, the colluvial soil the hill slope of tunnel alignment also possesses the rock exposures at places. The exposed rocks are Phawa Khola Granite and the green-gray Phyllites. The exposed granite is massive in nature and moderately to sparsely jointed and has high strength, whereas the exposed Phyllites sequence is moderately to closely jointed with moderate strength. Close spacing of joints within the Phyllites sequence seems localized at places. Locally, these closely jointed Phyllites sequence may cause the stability problem on tunnel opening. At some places bands of green-gray Quartzite are interbed with the Phyllites sequence, increasing the strength of the sequence.

A good exposure of rock with high strength is seen around the proposed outlet portal of the tunnel, and is more stable and safe as compared to the proposed inlet portal. The moderately steep right bank hill slope of Phawa Khola seems stable and shows no sign of instability at the time of field visit.

The proposed canal alignment along the right bank of the river runs through the moderately sloped cultivated land of the Damphe Danda and Khalte village. The area is covered with the colluvial soil. The colluvial soil consists of angular to sub-angular fragments of moderately to completely weathered gravels to cobbles of granite, Phyllite etc. with fine to medium sandy, silty matrix. The deposit is loose to moderately compacted and moderately permeable. The area seems stable, no sign of instability is visualized at the time of field visit, but consideration should be taken in making the cut slopes.

3-11


Photo 3.4: A view of canal alignment

Fore bay site The proposed forebay site is located on the more or less north facing moderate slope of Khalte village. Currently, the area is practiced as cultivated land. The area is covered with colluvial soil which constitutes the angular to sub-angular fragments of moderately to completely weathered gravels to cobbles of granite, Phyllite in fine to medium sandy, silty matrix and the colluvial soil is in loose to moderately compacted state and is moderately permeable. At present condition, there is no sign of instability but adequate retaining structures should be given to support the new added load of structure and load of water as well.

At about 150 m south east of the proposed fore bay site, there exist a flatter region covered with sparsely populated trees, which is more safe and stable location for the fore bay site. To pour the water to new location from the proposed fore bay site; the water way will run through hard rock exposure of about 30 m length, which should be chipped out. After the rock exposure the water way runs through moderate slope of colluvial soil and endmost length of about 60 m of water way would be continued through the aqueduct (bridging). This new location of fore bay will reduce the length of penstock and spillway as well. If rock chipping and bridging of the water way is economically viable, this new location is recommended for the fore bay site.

Proposed canal

3-12


Photo 4.5: Photo showing forebay location

Powerhouse site The Power house site is located on right bank of Kabeli Khola at about 100 m downstream from the confluence of the Kabeli Khola and Phawa Khola. The area is gently sloped older river terraces of the Kabeli Khola. The area possesses the alluvial soil. The alluvial soil is moderately compacted, moderately permeable and constitutes the rounded to sub-rounded gravels to boulders of Granite, and Phyllite with fine sandy silt matrix. The power house site seems stable. No any signs of instability are visualized during the field visit.

Photo: Closer view to Power house site

Site for Fore bay

Proposed Powerhouse site

3-13


3.5.6 Conclusions and recommendations Conclusions 1. The whole project area lies in the lesser Himalayan rocks. The area consists of

high quality and sound rocks. Cliff areas have high quality and massive rock exposure.

2. The foliation of the rocks exposed in the project area has oblique relationship with the hill slope and it is favourable for structures siting.

3. The most of headrace alignment passes through the colluvial deposits and few areas in rock outcrop.

4. The exposed rocks around the project area are slightly weathered to moderately weathered and consist of several sets of cracks.

5. The project is attractive on regards of favourable geological and engineering geological condition. The engineering geological and geotechnical condition of the project site is expected as fair to good.

Recommendations 1. The project is feasible from geological point of view. However, appropriate

protection measures at headworks area from debris flow from upstream and at other critical locations on the headrace alignment is required.

2. Detailed geological and engineering geological study is required in some critical locations. Detailed measurement of the rock joint orientation (joint analysis at the rock exposure and geotechnical investigation of soil) is necessary in order to suggest the specific slope stability condition at the project area.

3. Study should be carried for west flowing gullies and Kholsis for proper understanding of the stability condition of the project area which could help in minimizing damage from anticipated debris flow. Gully protection works need to be constructed to protect headrace structures from debris flow.

4. Headrace structure should pass through rock outcrop at this slide location to have stable foundation and the structure should be covered from top to protect it from probable debris flow.

5. All engineering structures should be designed based on the extreme rainfall condition to minimize damage from such events.

3-14


4 HYDROLOGICAL REVIEW Rigorous hydrological analysis was carried out during detailed feasibility study to work out different hydrological parameters and average monthly flow. However the hydrological analysis was not complete as spot measurement were not conducted sufficiently and the design flow reported 65 percentile of probability of exceedence was actually 76 percentile. Thus power plant was under sized. Hence this hydrological review has been carried out to review the hydrological issues related to average monthly flow and design flow. Furthermore hydrological parameters relevant to detailed design of the project are also incorporated in this report.

4.1 AVERAGE MONTHLY FLOW The concluded average monthly flow of Phawa Khola at intake site during detailed feasibility study is shown as river flow ADOPTED in Table 4.1. This flow was concluded from different river flow estimation methods and a spot measurement by current meter on 23 September 2004. Later on an additional flow measurement by current meter method was carried out on 07 May 2005 to verify the flow estimated during detailed feasibility study. Then the both actual measured flow data were used to estimate monthly average flow using Medium Irrigation Project (MIP) Method although September is not an appropriate month of flow measurement to use MIP analysis. The flow estimation by the MIP method is shown in Table 4.2. The average monthly flow computed by this method is summarised in Table 4.1 as MIP flow VERIFICATION. The flows measured on 23 September 2004 and 07 May 2005 were corrected to their wetness or dryness from 25 years long rainfall data of nearby meteorological station Taplejung (Station no. 1405) as shown in Table 4.3 before using the measured flows in MIP analysis. Although wetness and dryness from the average of rainfall data has not direct correlation with river flow, this technique could help on accounting dryness and wetness of the river flow little bit. Hence this technique adopted in this hydrological review.

Table 4.1: Monthly average flow

Month MIP flow (m3/s) VERIFICATION

River flow (m3/s) ADOPTED

Baishakh 1.38 1.37

Jestha 3.30 3.34

Ashadh 7.87 8.09

Shrawan 15.17 16.3

Bhadra 15.94 17.20

Aswin 9.41 9.85

Kartik 4.65 4.91

Mansir 2.76 2.92

Paush 2.11 2.14

Magh 1.61 1.72

Falgun 1.19 1.35

Chaitra 0.88 0.97

4-1


From the Table 4.1, one can see that the flow difference between adopted by detailed feasibility study and estimated later on during flow verification is minimal and not more than 7% even in wet season. This shows a very good match between flows adopted and flow verification. Hence the flow adopted in the detailed feasibility study is reasonable as hydrology is not an exact science and certain level of uncertainty is obvious in average monthly flow estimation. Thus average monthly flow recommended by the detailed feasibility study is correct at this stage and again recommended to use for power and energy computation. Flow hydrograph of the adopted flow is shown in Figure 4.1.

Table 4.2: Flow estimation by MIP method Flow measured date 23 Sep '04 07 May '05 Measured flow (m3/s) 9.29 1.55 Wetness correction factor 1.14 1.12 Corrected flow (m3/s) 10.59 1.74 Non-dim. hydrograph ordinate 14.31 2.19 April flow (m3/s) 0.74 0.80

Month Non

dimensional hydrograph for region 1

Flow from 23 Sep '04 measurement (m3/s)

Flow from 07 May '05 measurement (m3/s)

Average flow from MIP method (m3/s)

MIP flow in Nepali calendar (m3/s)

Jan 2.4 1.78 1.91 1.84 Magh 1.61

Feb 1.8 1.33 1.43 1.38 Falgun 1.19

Mar 1.3 0.96 1.03 1.00 Chaitra 0.88

Apr 1.0 0.74 0.80 0.77 Baishakh 1.38

May 2.6 1.92 2.07 2.00 Jestha 3.30

Jun 6.0 4.44 4.77 4.61 Ashadh 7.87

Jul 14.5 10.73 11.54 11.14 Shrawan 15.17

Aug 25.0 18.51 19.89 19.20 Bhadra 15.94

Sep 16.5 12.21 13.13 12.67 Aswin 9.41

Oct 8.0 5.92 6.37 6.14 Kartik 4.65

Nov 4.1 3.04 3.26 3.15 Mansir 2.76

Dec 3.1 2.29 2.47 2.38 Paush 2.11

4-2


Table 4.3: Rainfall data of Taplejung (Station no. 1405) Year Annual rainfall (mm) % wetness from average 1983 1768 -13% 1984 2130 5% 1985 2473 22% 1986 1765 -13% 1987 2253 11% 1988 1897 -7% 1989 2132 5% 1990 2436 20% 1991 2082 3% 1992 1497 -26% 1993 1752 -14% 1994 1835 -10% 1995 2159 6% 1996 2161 7% 1997 2094 3% 1998 2101 4% 1999 1981 -2% 2000 1874 -8% 2001 1912 -6% 2002 2173 7% 2003 2505 23% 2004 1746 -14% 2005 1795 -12% 2006 2147 6% 2007 2055 1%

Average 2029

It is to be noted that MIP method is superior to other indirect methods of river flow estimation in Nepalese context as at least one measurement is an actual measurement and other monthly flows are calculated on the basis of that measurement. However for further assurance of flow data, additional flow measurements in January, February, March and April need to be carried out and use of MIP method for average monthly flow estimation from the measured flow is recommended for coming dry season. Furthermore the measured flows should be corrected to their dryness or wetness as explained earlier.

Compensation flow recommended earlier in detailed feasibility study was 0.08 m3/s which is valid at this stage of study as well.

4-3


0123456789

101112131415161718

Bai

shak

h

Jest

ha

Ash

adh

Shr

awan

Bha

dra

Asw

in

Kar

tik

Man

gsir

Pau

sh

Mag

h

Falg

un

Cha

itra

Flow

(m3 /

s)

River flowDesign flow 2.10 m3/s

Figure 4.1: Flow hydrograph of adopted flow

4.2 FLOW DURATION CURVE Flow duration curve data is worked out from the adopted average monthly flow adopted in detailed feasibility study considering the number of days of months as shown in Table 4.4. Flow duration curve of the same data is also presented in Figure 4.2. 65 percentile probability of exceedence flow from the flow duration curve is computed to be 2.200 m3/s. Thus the design flow for the plant could be as high as 2.200 m3/s according to the provision of Nepal Electricity Authority. However 2.09 m3/s is taken as design flow for the project so that the project capacity could be upgraded to 5000 kW although optimum installed capacity could be higher if provision of limiting design flow to 65 percentile probability of exceedence is exempted.

Table 4.4: Flow duration curve data Month Days Cum. day % time River flow (m3/s)

Bhadra 31 31 8% 16.17 Shrawan 32 63 17% 15.32 Aswin 30 93 25% 9.27 Ashadh 31 124 34% 7.60 Kartik 30 154 42% 4.62 Jestha 31 185 51% 3.14 Mansir 30 215 59% 2.74 Paush 29 244 67% 2.01 Magh 29 273 75% 1.61 Baishakh 31 304 83% 1.29 Falgun 30 334 92% 1.27 Chaitra 31 365 100% 0.91

4-4


0123456789

1011121314151617

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110%

Probability of exceedence

Riv

er lo

w (m

3 /s)

River flowDesign flow 2.10 m3/s

Figure 4.2: Flow duration curve

4.3 FLOOD FLOWS Different methods were used to estimate flood flows for different return period during detailed feasibility study. After those all analysis flood flows estimated using DHM method was recommended as reasonable flood prediction. Hence the flood flows for Phawa Khola at intake site for different return period worked out by DHM method is recommended to consider in designing different hydraulic and other structures in this report too. The flood flows are presented in Table 4.5 and other flood flow not given in table should be worked out from Figure 4.3, instantaneous flood flow curve.

Table 4.5: Instantaneous flood flows

Return period (years) 2 5 10 20 50 100 200 500 1000 Flood flows (m3/s) 100 167 218 271 347 410 475 570 647

Design flood flow for hydraulic structures of the project is taken as 410 m3/s which is 1 in 100 years return period. All structures including headworks and powerhouse should be safe in this amount of river flow with repairable damage. However damage from the flood flow in Himalayan River could not be estimated properly. Hence risk of wash away of hydraulic structures by floods always remains even if the structures are designed considering the design flood. That is due to steep, high scouring and high sediment carrying nature of a typical Himalayan River. As weir and intake construction will be completed within a dry season of a year, river diversion flow will be taken as 1 in 2 years return period flow, the value of which is 100 m3/s. Similarly operation flow is also recommended to be 1 in 5 years return period i.e. plant will be closed if the flow at river is greater than 167 m3/s.

4-5


0

100

200

300

400

500

600

700

1 10 100 1000

Return period (years)

Floo

d flo

ws

(m3 /

s)

Figure 4.3: Instantaneous flood flow curve

4.4 SEDIMENT CONCENTRATION The annual sediment load estimated by different methods during detailed feasibility study varies from 3668 to 9,096 T/km2/year. Since all these values are based on limited data, it cannot be said with certainty, which of the methods is more reliable. Again other more reliable sediment estimation techniques are also not available. Hence mean of these two values 6400 T/km2/year is taken as total sediment load in Phawa Khola at project location.

Now, sediment concentration by mass is given by

C=Tm/Qtm

=3670 ppm

Where

C=sediment concentration (tons/m3)

Tm=mass of sediment carried in three months (tons)

Qtm=water volume during three months (m3)

For catchment area 96 km2

The sediment concentration is worked out as 3670 mg/litre assuming mean monsoon discharge 13.59 m3/s and 60% of sediment is transported within three months period of monsoon. This is an average sediment concentration in river during monsoon season. Peak sediment concentration can be more than three times of the above value but all the sediment at the river cannot enter into the intake. Some of the sediment could be excluded from gravel trap, some from settling basin and remaining goes to the turbine. For the purpose of settling basin design about 1.5 times of the average sediment concentration is assumed for sediment storage volume calculation and flushing frequency computation. Thus the sediment concentration to be considered in settling basin design is 5000 ppm.

4-6


5 CAPACITY OPTIMIZATION, DESIGN AND DESCRIPTION Detailed walk over survey was carried out during site visit of 28 Chitra 2067 to 9 Baishak 2068 to identify appropriate weir and intake location upstream of the previously proposed location and to finalize the best project layout considering that new weir and intake location. An appropriate weir and intake location for the new project layout was found some 1500 m upstream of the previous weir and intake location. Project layout according the new weir and intake location is presented in Drawing no. PK-10-F01 in Volume 2 of 2 of this report.

Capacity optimization of the project was not carried out due to the limitation of study scope to below 65% flow. Hence only plant calculation is done to have it 5000 kW. However individual components of the project were optimized to have optimum size. Trade off between cost and benefit was the basis for component optimization. Following sections describes project component design criteria and result.

5.1 CAPACITY OPTIMIZATION The main objective of plant capacity optimization is to determine the plant capacity with maximum net benefit in the given head and flow condition. But plant capacity optimization scope is not available at this project as design flow is already fixed to 65 percentile probability of exceedence. Hence rigorous optimization study was not carried out and only plant capacity of 5000 kW was worked out considering following project parameters.

Capacity (kW) 5000 Generator efficiency 96.00% Design discharge (m3/s) 2.09 Turbine efficiency 90.00% Minimum release (m3/s) 0.080 Transformer efficiency 99.00% Available head (m), forebay 302.453 Combined efficiency 85.54% Net head at full flow (m) 298.7

5.2 DESIGN AND DESCRIPTION Weir crest level is fixed at an elevation of 898 m amsl and turbine axis level is fixed at an elevation of 588 m amsl. Thus gross head for this new project layout is computed as 310 m.

The project components lie in left bank and also in right bank of Phawa Khola except weir. The Headworks is at left bank and the headrace canal of length 608 m is also kept in left bank of Phawa Khola. There is a river crossing in Phawa Khola then the headrace runs along the right bank of Phawa khola. However access road runs at right side of Phawa Khola in Thechambu VDC. Transmission line crosses Kabeli river from powerhouse to Bhaluchok. Detailed description of project components design, design criteria and assumptions are presented in following sections.

5.2.1 Headworks Headworks consist of weir and intake, intake canal, gravel trap and settling basin. All components of headworks are presented in Drawing no. PK-20-F01 and its components are described below.

Weir A 27 m long Concrete broad crested weir with free flow spillway is proposed for this project. The weir inside core is made of C15 40% plum concrete and is encased with the 50 cm thick Rcc slab at bottom and 65 cm thick Rcc slab at top. The concrete slab is made of C25 concrete. The crest level of weir is fixed at 898 m amsl. The top

5-1


surface of weir is lined with 40 cm thick hard stone lining to prevent from the deterioration of the weir surface. The slope of weir surface in u/s of axis is 1:2(V: H) and at d/s the slope is 1:4(V: H). The crest length of weir is 27 m and 2 m width. The invert level of stilling basin is 890.928 m amsl. The stilling basin is lined with 1.5 m size boulder. A 0.750 m thick graded filter should be placed beneath the boulder lining so that boulder laying could be easier and piping due to seepage and bed material wash away could be controlled. Weir crest level is set to 898 m amsl and weir length is designed as 27.00 m.

A random reinforced boulder armoured plum concrete end toe wall will be constructed at the end of the boulder lining to group the boulders upstream. Two lines of maximum big boulders that could be laid by excavator available at site will be laid to support the end toe wall at the downstream most part of the weir structure. Details of weir are shown in Drawing no. PK-20-F01 to PK-20-F07 in Volume 2 of 2 and weir axis location is shown in Figure 5.1.

During detailed design, care should be taken whether a clay blanket is required to control seepage through weir. Furthermore an additional upstream cut off wall may require protecting weir proper from scouring. This arrangement also needs to be worked out during detailed design stage.

Figure 5.1: Weir and intake location RCC flood walls (normally C25 but C35 at contact of sediment movement) will be constructed at both ends of the weir. Top level of flood wall is 903.00 m amsl (at weir crest location) which correspond to 1 in 100 years return period flood of 410 m3/s magnitude. Although flood walls are designed for 1 in 100 years return period flood sustainability of weir and intake at this flood could not be guaranteed as Small Himalayan Rivers have peculiar characteristics of wilderness. Power associated with such a high flood in such steep large sediment carrying river estimation is beyond the capacity of engineering practice in current context. Scouring action due to high flood could be disastrous at this location at Phawa Khola. So it does not fall under the scope of current study. Furthermore, current weir and intake location is not in favourable location as it is at steep stretch of river and sufficient safe and flatter land

Intake location

Weir location

5-2


is not available to optimize the location of headworks structures. Thus headworks area always remains in threat of flood impact. This site is selected because other better site is not available to achieve this much of head at this area.

Intake A side intake with gated orifice will be constructed at the left bank of the river immediately upstream of the weir axis (See Drawing no. PK-20-F08 and PK-20-F09 of Volume 2 of 2). A course trash rack of 20 mm thick flat mild steel bars with 100 mm clear spacing flushed with river side face of the intake orifices will be placed to control entry of large trashes and boulders inside intake. The trash rack should be removable so that it could be replaced in case of damage. Conventional divide wall and under sluice system of intake layout is not selected as the river is fast flowing and carries large boulders during flood at intake location. Conventional intake arrangement can not operate smoothly in such condition as flood will damage under sluice structures, gates and divide wall itself. However intake clogging could be a challenge and large sediment particle will enter into the intake during high flow season in intake arrangement without under sluice. Thus intake design is done keeping in mind that plant could be operated uninterruptedly throughout year with sufficient flow entry even in clogging of intake orifices. With such consideration 3 nos. of each 1.50 m long and 2.15 m deep intake orifices are designed to abstract design flow of 2.09 m3/s flow in lean season. Sill level of the lower intake orifices is 897.30 m amsl. Allowances of 0.25 m is made to address the head due to weir crest erosion.

The intake orifices are to be lined with 12 mm thick mild steel plate to protect it from erosion. Furthermore base slab and side wall up to 50 cm from invert level should be lined with hard stone so that erosion by gravel and boulder movement could be minimal.

Intake chamber will house gates and acts as transition from intake orifices to intake canal. A slab will be constructed on top of intake chamber from where gates will be operated. Manholes and gratings will be placed at the slab and rungs are fixed to intake chamber wall to descend to intake floor for large sediment removal and repair and maintenance. Intake gate will be closed and plant operation is stopped if the flood at river exceeds 167 m3/s a 1 in 5 years return period operation flood.

Spillway and spillway canal A 10.00 m long spillway will be constructed in after the intake . This spillway will be capable of spilling 3.50 m3/s additional water entered through the intake. Hence only desired flow of 2.60 m3/s will be available along water conveyance structure making them more efficient. The adopted head over the spillway crest is 40 cm. The spillway is safe from possible flood back flow through the spillway. Base slab and side wall up to 50 cm from the invert of intake canal will be lined with 30 cm thick hard stone to protect the canal from erosion due to gravel movement. The canal is capable of transporting maximum possible sediment particle of 150 mm that could enter in to the canal hence sediment deposition is avoided. The spilled water will convey through spillway canal located just d/s of spillway. The spillway canal is 1.5 m wide and 1.5 m deep. The spillway canal is 41.632 m long. The bottom surface of spillway canal is also lined with the 30 cm thick hard stone lining. A detail of the intake canal is shown in Drawing no. PK-20-F10 of Volume 2 of 2.

Gravel trap A gravel trap will be constructed at the end of intake canal for trapping gravel entered though intake. The design flow for the gravel trap is 2.60 m3/s and the flow velocity is reduced below 0.60 m/s to settle gravels. The gravel trap should be capable of trapping 5 mm size particle and flushing the largest sediment particle of 150 mm that could enter through the intake. Hopper type gravel trap without sediment storage

5-3


provision and continuous flushing system is proposed as a gravel trap. The gravels entered through the intake will directly be excluded through the gravel trap with gravity action without retaining them. For that 0.50 m3/s additional flow will be supplied to the gravel trap for flushing gravel settled in the gravel trap.

Overall length of gravel trap is 10.00 m, width 2.50 m and overall effective depth is 2.22 m. Longitudinal and side slope of the gravel trap is designed in such a way that gravels will roll down to the gravel flushing orifice. A detail of the gravel trap is shown in Drawing no. PK-20-F11 of Volume 2 of 2.

Settling basin Settling basin has double chamber. It will settle 0.15 mm size sediment particle with 90% trap efficiency and it will be capable of flushing 5 mm particle size efficiently. Flushing system will be gravity flushing system and flushing of each chamber will be carried out one at a time emptying one for flushing while another is in operation for power generation. Design flow for the settling basin is 2.09 m3/s. Inlet and outlet gates will be provided for both the chambers for flushing operation.

Settling zone length is 42.00 m and inlet transition length is 18.125 m. basin with of a chamber is 4.80 m and the overall depth is 4.42 m. The bottom longitudinal slope of the basin is 1:50 (V: H). A detail the settling basin is shown in Drawing no. PK-20-F12 to PK-20-F15 of Volume 2 of 2.

Storage volume of the settling basin is worked out considering sediment concentration of 5000 ppm and flushing interval 12 hours. Flushing operation for a single chamber is required 60 minutes. A fine trash rack will be installed at the inlet of headrace canal to check the entry of trashes.

5.2.2 Headrace structures Headrace structures consist of headrace canal, headrace pipe and its appurtances. The headrace canal is divided in two stages i.e stage-1 and Stage-2. These structures are described below and details are presented in Drawing no. PK-30-F01 through PK-30-F16 of Volume 2 of 2. Total length of headrace canal is 4793 m.

Headrace canal Headrace canal in stage-1 will be constructed from chainage 0+000.00 to 0+608.016. The length of the headrace canal in stage-1 is 608.016 m. The headrace canal will be constructed with reinforced C25 concrete in base slab and wall and also C25 concrete in top slab. It will be of closed rectangular duct type with longitudinal slope 1:1000, width 1.35 m and overall depth 1.40 m including 0.20 m free board. RCC wall and slab thickness will be 0.25 m. Design flow for the canal is 2.09 m3/s. The headrace canal traverses cultivated land, steeper soil slope covered by vegetation and steep rock slopes and cliffs.

From the outlet of pipe, again a Stage-2 headrace canal will convey the design discharge to forebay along the right bank of river. Some part of headrace alignment in right bank seems steeper but not so difficult to construct the headrace canal. The length of headrace canal in this stage is 4184.5 m. Details of the headrace canal and its appurtances are presented in Drawing no. PK-30-F01 to PK-30-F16.

Headrace pipe Headrace alignment is brought down to avoid the land slide and kholsa crossing making it a river crossing in Phawa Khola and making further alignment in right bank. The total length of pipe is 350 m and the pipe is 1.2 m internal diameter and 6 mm thick. Design flow for headrace pipe is 2.09 m3/s. The details of the headrace pipe are presented in Drawing no. PK-30-F02 and PK-30-F09 of Volume 2 of 2.

5-4


Anchor blocks of nominally reinforced C15 (with 40% plum) will be provided to each bends and an expansion joint will be installed immediately downstream of an anchor block. The number of anchor block required for headrace pipe is 7 and that of support pier is 44. Stone masonry support piers founded on 10 cm C15 PCC and caped with C15 PCC on top will be provided at spacing of 8 m centre to centre. Anchor bars will be embedded on cap concrete and saddle plates will be mounted on it. A HDPE sheet will be inserted in between saddle plate and headrace pipe to act as wear plate. RCC column will require in some critical locations as support pier.

A 25 cm diameter wash out valve will be installed at the lowest part of the siphon alignment. Manholes will be fixed at spacing of 150 m for inspection, repair and maintenance.

There will be pipe inlet and outlet with well profiled bell mouth. The inlet and the outlet maintain submergence required for smooth hydraulic and act as transision between canal and pipe. A major crossing of the pipe line will be at Phawa Khola at where pipe will be crossed through the metal truss bridge.

5.2.3 Forebay A reinforced concrete forebay will be constructed at the end of headrace alignment to dissipate the surge pressure created in to penstock pipe due to load fluctuation or rejection. This structure will also serve as water storage for additional flow for short period and provide sufficient submergence to penstock inlet to avoid vortex. The plant could be operated in isolated mode as forebay is constructed as surge intercepting structure. The maximum and minimum operating levels are 890.453m and 889.153m amsl respectively. The location of forebay and its detail is shown in Drawing no. PK-40-F01 and PK-40-F02 of Volume 2 of 2.

The forebay is capable of supplying full design flow for 60 s from its effective storage which is 126 m3. Its effective length is 21.75m and width is 5.00 m. Effective depth is taken as 1.30 m. A fine trash rack will be installed at forebay. Further a 6 diameter washout valve will be installed at the forebay.

5.2.4 Penstock Mild steel penstock will convey design flow of 2.09 m3/s from forebay to turbine. Its layout is shown in Drawing no. PK-50-F01

Documents

Revised Feasibility Report of Phawa Khola Hydropower Project