International Conference on SHP Kandy Srilanka All Details-Papers-Technical Aspects-A-A22

International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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Headworks of Sunkoshi Small Hydropower Project - A Model of Headworks in Steep and Highly Sediment Loaded Small Rivers

Hari Shankar Shrestha1) and Tuk Prasad Paudel2) 1) Engineering Division, Sanima Hydropower (P.) Ltd., Kathmandu, Nepal 2) Sanima Hydropower (P.) Ltd., Kathmandu, Nepal E-mail: [email protected] ABSTRACT Government of Nepal has given priority for developing small hydropower projects and local developers in Nepal have been involved mostly in the small discharge with high head Run-of-River small power plants in steep and highly sediment loaded rivers. The most challenging job for such schemes is the successful planning and design of headworks. It is difficult to develop detailed conceptual planning and design criteria for the headworks, as each headworks arrangement will be site specific. All headworks shall, however, meet some performance standards. The most basic and general performance criterion is: The plant shall remain in operation during all normal situations in order to secure a safe and regular power generation. Although the planning, design, construction and operation of headworks is a very comprehensive task, it is being done upto date on the basis of past experiences and some guidelines. There is a lack of design, construction, maintenance and operation guidelines on the headworks of the Run-of-River hydropower schemes on very steep and highly sediment loaded small rivers. Case studies are often useful in order to identify the performances and thus to improve headworks designs. Sunkoshi Small Hydropower Project promoted by Sanima Hydropower Pvt. Ltd. (in operation from last three years), is developed in quite steep (1 in 20) and highly sediment loaded river which carries relatively big boulders. The headworks of the project consists of a low head (about 2m height) boulder lined diversion weir, a double stepped orifices on the right bank, 60 m long hard stone lined inlet culvert , Gravel trap, 36 m long steel truss cossing, 200 m long approach culvert, double chambered settling basin and forebay. The performance of the headworks of the plant is fully satisfactory. The design and layout concept of this project can be referred as a model for headworks in the similar rivers. 1 INTRODUCTION 1.1 Sunkoshi Small Hydropower Project

Sunkoshi Small Hydropower Project is located in the Sunkoshi Khola about 88 km north east of Kathamndu in Sindhupalchok district of Central Nepal (see fig. 1). The Project is run of the river type, which will divert a design flow of 2.7m3/sec through 2.6 km long 1.3 and 1.2 meter diameters GRP pipes to the powerhouse (see fig. 2). The water diverted from the river first reaches to the Settling basin through approach canal and it enters to the headrace system through forebay. Utilizing a rated head of 117.5 m, the project generates 2.5 MW of power and supplies average annual energy equivalent to 14.38 GWh to the Integrated National Power System (INPS).

Administratorback


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Sanima Hydropower (P.) Ltd. (SHPL) is the promoter of the project. This company has been established by a group of Nepali citizens, especially engineers and geologists, who own and operate international trading business overseas. The main objective of SHPL is to develop hydropower sector in Nepal. The company aims to participate in overall development of the country using hydropower as an entry point. Sanima Hydropower (P) Ltd. plans to divest its investment in infrastructure, power and industrial sector.

Figure 1: Project Location map

Figure 2: Project Layout (Picture: Google Earth) 1.2 Development of Small hydropower Project in Nepal Nepal Government is pursuing the hydropower development in Nepal from different approaches. Firstly, to develop small hydropower projects to meet local demand in remote area, secondly, to develop small and medium projects to meet the national demand. Thirdly, large scale projects to export to neighboring countries. Government of Nepal (GoN) has identified Hydropower Development as one of the potential sectors and given emphasis for enhancing economic growth of the country. Keeping this in mind, the Hydropower Development policy 1992 and related laws, Electricity Act 1992 and its Rules 1993, were promulgated and enacted in support and to promote participation of private sector in Hydropower development in Nepal, both for domestic use and export related. Further to promote and facilitate the foreign investment and technology transfer in industrialization of the country, the Foreign Investment and One Window Policy 1992 and Industrial Policy 1992 were promulgated by GoN. In line with these policies, the Foreign Investment and Technology Transfer Act 1992 and Industrial Enterprises Act 1992 were also enacted to provide a transparent legal framework required for the participation of foreign investment in industrial sector.

Weir

Pipe and road alignment

Settling Basin Powerhouse


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Government of Nepal has given priority for developing small hydropower projects. A special priority is given for projects up to 1 MW in size by waiving royalty and income taxes to operate such plants. Nepal Electricity Authority (NEA) will purchase all energy produced by such power plants at a standard rate. To promote the small hydropower developers NEA will also purchase energy produced by plants in the 1 to 5 MW range at same rate. Local developers in Nepal have been involved mostly in the smaller run-of-river power plant without any storage reservoir, mainly in the small discharge with high head power plants in steep and highly sediment loaded rivers. Small hydro technology is mature and proven. Civil works and installation of equipment involve simple processes which offer good opportunities to local people for employment and use of locally available materials. Simple and proven design concepts helps to suit to local conditions.

2 HEADWORKS LAYOUT IN STEEP AND HIGHLY SEDIMENT LOADED RIVERS The most challenging job for Run-of-River hydropower scheme is the successful planning and design of headworks. The selected design shall allow minimum of the sediment load in the river to bypass the diversion weir/barrage during all the seasons of the year. Although the planning, design, construction and operation of headworks are very comprehensive task, it is being done up to date on the basis of past experiences and available guidelines. Preparation of design, construction, maintenance and operation guidelines on the headworks of the Run-of-River hydropower schemes on Nepalese context shall be the key aspect and reference for future towards enhancement of headworks lay-out in such rivers.

2.1 General Arrangement

The headworks comprise all structural components required to abstract water from the river to the waterways of the power plant. The main components of the headworks at a run-of-river hydropower plant are: - The diversion weir (or dam) including spillways - The intake - The gravel trap/bed excluder - The settling basins The most challenging job is the successful planning and design of headworks. It is difficult to develop detailed conceptual planning and design criteria for the headworks, as each headworks arrangement will be site specific. All headworks shall, however, meet some performance standards. The most basic and general performance criterion is: The plant shall remain in operation during all normal situations in order to secure a safe and regular power generation. 2.2 Performance Standards of Headworks

The headworks of run-of-river hydropower projects shall be planned and designed to ensure safe and regular power generation from the hydropower plant under normal conditions. For this purpose, headworks arrangement must meet the following performance standards (Haakon (2003)):

a) Passage of floods, including hazard floods b) Passage of ice, trash and floating debris c) Passage of sediments d) Bed control at the intake e) Exclusion of suspended sediments and air

A plant failing to meet standard (a) will be dangerous during floods and it will have poor safety. A plant failing to meet standards (b) to (e) will perform poorly also during normal operation situations. Operation and maintenance costs will be higher and the power generation regularity will be lower than needed. It may be necessary to protect the sluiceways and other components of the spillway system to increase the resistance against sediment-induced wear. 2.3 Influence of River on Selection and Design of Headworks

The selection and design of intakes and headworks obviously depends on the character of the river, on its size and on the scale of hydropower project. The factors to be taken into account include the following:The steepness of the river.


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The bed material exposed. The overall river stability, in plan and section. The sizes and concentrations of sediment in suspension. The degree of bed movement and the sizes of material in motion The permeability of the material below the channel bed. The accessibility of the site to vehicles and plant.

In Nepal, rivers are responding to highly variable conditions of flow and sediment supply because the landscape is geologically very young. There are many landslides providing irregular supplies of sediment and rare events such as the failure of glacier or moraine dams. Thus Nepalese rivers are not in equilibrium, but subject to considerable variability. River character can strongly influence intake site selection. Thus mountain and steep hill rivers can provide favourable conditions for intakes in terms of permanence and lack of interference of sediment in normal conditions. However, these sites can be vulnerable to high velocities, turbulence and movement of boulders during floods. Choice of site is dominated by seeking the presence of rock outcrops which are likely to control the local geometry of the river and may also protect an intake. Irrespective of their size, Nepali rivers carry large amounts of bed load and suspended load. The sediments frequently damage the gate sills, spillway ogee surface, glacis and downstream aprons of diversion structures. Greater damage is witnessed at the undersluice gate sills, ogee surface and downstream part of energy dissipaters than at higher levels of the spillways. In some projects, even mild steel plate armoring of the undersluice surface has been worn out or torn away by the sediment-laden rivers. This problem is typical to all the projects, and the project owners are spending huge sums on the repair of the damages thus caused. It is necessary to protect gate sills, spillway ogee surface, glacis and downstream aprons of diversion structures and other components of the spillway system to increase the resistance against sediment-induced wear (see figures 3 and 4).

Figure 3: Concrete erosion/abrasion after first

year of operation

Figure 4: Hard stone lining erosion/abrasion

after four years of operation Some of the methods applicable to increase the structures resistance to sediment-induced wear are (Haakon (2003)):Boulder lining

High quality and abrasion resistant concrete Steel-fibre reinforced high quality concrete Steel lining Steel rails embedded in concrete Dressed hard-stone masonry lining Epoxy coating Rubber lining Wood lining

Some power plant owners prefer plain concrete structures with an additional thickness of the concrete, which may be worn down over time and then repaired with relatively low costs. It is much more difficult to maintain and repair.


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3 HEADWORKS OF SSHP The headworks of Sunkoshi Small hydropower consist of a boulder lined weir with concrete cut-off, side intake with orifice openings with steel lining followed by hard stone lined intake culvert up to the gravel trap, truss bridge pipe crossing, approach canal concrete closed conduit up to settling basin and fore bay adjacent to settling basin. 3.1 The River At the Headworks Site The Sunkoshi River at the headworks area is steep, with a gradient of approximately 1 in 20 (figure 5). The riverbed contains numerous very large boulders (greater than 4 m size). Catchment area of the Project at the intake area is 81 km2. The catchment area is almost circular in shape with diameter 10 kilometers. There are several landslides at its tributaries. The river carries large amounts of debris, bed load and suspended load during the monsoon even in normal floods.

Figure 5: River at weir area before construction

Figure 6: River at weir area after construction 3.2 Diversion Weir The location of the weir is selected at the rock outcrop at the right bank and a big boulder at the left bank (see Fig 6, 7 and 8). It is a permanent weir in the active channel lined with boulders of 3-5 ton weight. Boulder lining will be done in between the concrete cut-off wall and the weir crest level will be kept at 952.5 masl, which is approximately 2.50 m above the existing bed level. 500 mm thick clay blanket is placed up stream side of cut off wall. Two RC concrete and one plum concrete toe walls are constructed to down stream side of cut off wall to make boulder lining stable.

Figure 7: Longitudinal section of weir

Figure 8: Boulder lining process

Figure 9: Two layers of Intake orifice


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3.3 Intake The location of the intake is on the right bank of the River where the existing rock outcrop provides good protection for a permanent intake structure. Four openings with two layers of orifice openings of size 1.5 m long and 0.3 m high in each are provided (see Fig 9). Upper layer of openings have been provided incase the lower openings are chocked during monsoon to ensure smooth operation to divert required flow into the intake canal. It is aligned parallel to river flow. The invert level of opening is 951.50 masl (50 cm above the river bed level). The design flow would be 3.5 m3/s of which 2.7 m3/s will be used for power generation, 0.8 m3/s for gravel flushing. It is assumed that the sediments would destroy trashrack placed at the intake during monsoon season. So the intake would be just an orifice opening on the intake headwall without any trashrack. The provision for stoplogs has been made. The maximum flow to the intake at 100 year flood is 8.0 m3/s, excess shall be flushed out through gravel trap. The minimum free board at this flow is 25cm. 3.4 Inlet Culvert Intake culvert is aligned on the hillside along the right bank of the river (see Fig. 6). The length of the canal to the gravel trap is 55.0 m. The entire canal is in excavation. There is a gabion wall at the hill side of the excavation to protect the slope. The size of the culvert is 2.2 m by 1.2 m with a freeboard of 0.25 m (at 100 years flood) and the canal gradient of 1 in 40 at the beginning. The culvert can accommodate maximum 10.9 m3/s flow. The discharge, which passes through orifice for 100-year return period flood is 8.0 m3/s. The excess discharge at 100-year return period flood will be spilled and flushed through gravel trap. The velocity varies from 2.8 m/s to 3.2 m/s depending on the flow in the culvert. The culvert can transport up to 0.35m diameter sediments. Stone armoring on canal/culvert bed and sidewalls up to 0.5 m height is proposed to protect the concrete from abrasion due to the high velocity gravel laden flow. 3.5 Gravel Trap A gravel trap of size 5.0 m wide by 18.4m long is provided at the end of the intake culvert (see Fig.6). The transition lengths with horizontal expansion and contraction of inlet and outlet zone are 8.4m and 4.0m respectively. The average velocity in gravel flushing structure will be 0.6 m/s. The structure has a flushing gate of size 0.60m by 0.75m. Discharge for gravel flushing is 0.8 m3/s. A chute spillway is provided towards riverside on to the exposed rock surface. Flushing of gravel can be done continuously or intermittently as required. The most vulnerable areas in gravel trap as well as in flushing channel exposed to wear and tear due to high velocity are lined with dressed hard stone. At the end of parallel section, just before the outlet transition zone, a coarse trashrack is placed to prevent passing of debris and coarse particles to the headrace pipe. 3.6 Gravel flushing Channel A gravel-flushing channel is designed to flush bedload and gravel from gravel trap. The flushing canal is a rectangular section of 1.0m width and 1.5m depth, lined with hard stone. The canal ends on existing rock surface and freely discharges flushing and spilled discharge during the annual floods in the river. The canal bed slope is 1 in 44. Energy dissipation is not required. 3.7 Crossing Pipe and Approach Canal

From the gravel trap water is conveyed through 1300mm ID 6mm thick steel pipe over the Sunkoshi River with a 36m span steel truss bridge (see Fig. 6) from where the 199.42m long approach canal with rectangular cross-section of size 1.8 m x 1.6 m (depth x width) conveys water to the settling basin. The bed slope of the canal is 1:540. The canal is designed hydraulically to convey a flow of 3m/sec with flow velocity of 1.07 m/sec including flushing discharge required in the settling basin.


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3.8 Settling Basin The settling basin was designed to trap 90% of 0.20 mm particles size. It has two chambers of 4.5 m wide and 45 m long (parallel section) settling zone, each with half the required settling capacity. The maximum flow velocity in settling zones is 0.2 m/s. The 34.8m long inlet transition zone has horizontal and vertical expansion of 1:6.8. The bottom slab of the settling basin has a slope of 1 in 100. Settling basin wall top level is fixed at 950.20 amsl. The normal water level in the chambers is 949.9 amsl with a freeboard of 0.30 m. There are two 0.6m by 0.5m sluicing gates for sediment flushing. Hydraulic flushing system is proposed. A concrete lined 1.0m wide by 1.5 m deep open channel of rectangular section is proposed for sediment flushing purpose. The flushing discharge is 0.6m3/s. The channel is 22.5m long with the bed slope of 1 in 40 and ends to the natural gulley, which is lined with 1m boulders. 3.9 Forebay Basin The dimensions of the forebay basin are 12 m x 6m x 4.20 m (length x width x depth) having a total capacity of 270 m3. Two forebay gates with dimensions of 1.1m in width and 1.0 m in height prior to the entrance of the penstock liner are proposed. During sudden opening of the turbine valve, the maximum drop of water level from the minimum water level (MWL) will not exceed the permissible value, i.e. the upper edge of the mouth of the penstock liner shall always be under a hydrostatic pressure of 2.0 m magnitude (required for submergence), and thereby, entrance of air into the penstock shall be avoided.

Figure 10: Settling Basin, Forebay and weir area view

The forebay is designed to accommodate surge variations within the chamber. However provision for emergency spillage of design discharge has been considered. 4 OPERATION OF HEADWORKS

The technical and financial success of a hydropower plant depends on a good design and installation as well as on a proper operation and maintenance. Neglecting operation and maintenance of the plant may have severe consequences. Figures 11 to 14 show the deposition of sediments in front of intake, gravel trap and settling basins. The depositions were occurred not properly following the operation manual. Efficient and continuous running will only be possible with skilled operation of the plant and a well-planned maintenance program. Operation and maintenance procedures must be planned and put into action in the initial stages of the plant operation in order to prevent from breakdowns and reduced power outputs. Improper operation and maintenance of power plant will decrease the life of plant with decreased return.

Weir

Settling Basin

Forebay


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Figure 11: Sediment deposition in front of weir

Figure 12: Removing of deposited Sediment in

Gravel Trap. Safety in the place of work is also a major consideration. The use of good operation and maintenance practice enhance the personnel and property. It ensures the choice of the correct spare parts and tools. Safety equipment should be installed in the plant and should be correctly rated, and never overridden or bypassed. To achieve all above mentioned good operation and maintenance procedure clear and concise operation manual is must. A simple operation and maintenance manual was prepared for the operators in order to assist them in day to day operation of the Sunkoshi Small Hydropower Plant.

4.1 Weir

The water and sediment levels should be read and record in the weir every hour during the rainy season (May to September). If the boulders are accumulated in front of the intake and it disturbs to extract the designed discharge from the river the accumulated boulders should be removed. When the water level increases above 953.00 metres above msl or debris/mud flow is observed in the river, the intake gates should be closed and power plant should be shut down. The bed load of the river will damage the top part of these structures. Record of the damage after every flood season should be recorded. It is possible that some of the boulders from boulders may be displaced or taken out by flood. This should be monitored and recorded after every flood season. As cutoff wall is inside boulder lining there is low possibility of its damage but its top boulder lining with infill concrete may be eroded during flood season. So the concrete filling level of the weir crest before flood season and after flood season should be recorded. If the filled concrete is eroded by the flood concrete should be in filled to require level after the flood season is over. 4.2 Intake There are four no of upper orifices and four no of lower orifice (size 1.5X0.3 m). There is a 1.5 m deep hoper in front of intake. The orifices and flow guiding pier are lined with steel. Bed of the intake is lined with hard stone. There are four gates. It is possible that the hopper may be filled with sediment during flood season clogging lower as well as upper orifices. Trashes can also clog the intake orifices. The intake should be monitored in every hour during flood time. If found the problem immediate action should be taken for clearing it. Carefull monitoring of the intake during first flood should be done and recorded the behaviour of the intake orifice. Modification and mitigation measure should be applied on the basis of this monitoring. This monitoring program and mitigation measures should be continued in flood season of each year. The lower orifices gates should be opened fully. If gate is partly opened seal will be damaged due to the abrasion by sediment and high velocity. During closing, gate should not be tightly closed. It should be closed loosely with gate sill. If less than four lower orifices gate opening is enough to extract the required

Figure 13: Sediment deposition in Settling basin

Figure 14: Sediment deposition in Settling Basin


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discharge from the river, upstream gates should be opened to avoid the sediment (boulder) accumulation in front of the intake. If lower orifices are clogged by trashes or boulders, the upper gates should be opened to extract the required discharge from the river. When the discharge in the river is decreased, the accumulated trashes and boulders should be removed with the help of lifting or scraping equipment. The conditions of the gate should be inspected daily. If it is highly tightened, the seal will be damaged due to the high stress in it. The leakage through the gates is check every year after the rainy season. If the leakage is small try to stop the leakages with silt and sand or clinker (if available) so that maintenance of gasket can be postponed. If the leakage is still there then make necessary repair. When the water level increases above 953.00 metres above msl or debris/mud flow is observed in the river, the intake gates should be closed and power plant should be shut down. Erosion/abrasion of steel lining should be recorded after every flood season. If major erosion/abrasion is observed it should be repaired. It is expected that there will be no sediment deposition inside intake. But it may enter the particle size greater than 300 mm and settle behind the intake orifice. These particles should be removed by lifting it from the gratings provided with the help of some scraping equipment.

4.3 Intake Culvert Intake culvert is of rectangular shape with flood wall at river side. It is hard stone lined. Big sediment particles may settle. Hard stone lining may be damaged due to heavy bed load. The intake culvert should be monitored during the major shut down period and every year after the rainy season. If any damages are observed and operation will not be disturbed by the damage it should be repaired at major shut down period. The seal of expansion joint may be damaged due to high velocity and bed load sediment. It should be monitored and repaired at major shut down or every year after and before the rainy season. 4.4 Gravel Trap

The sediment filling level in the Gravel Trap should be monitored and recorded daily during rainy season and weekly during dry season. If the level is observed at elevation 948.20 m above msl (1.0 m below the invert level of the Gravel Trap outlet), Sediment should be flushed out by opening the Gravel Trap flushing Gate by using the part of the flow. During flood season when there is excess discharge, gravel flushing channel should be fully opened. The water level should be maintained at crest level by opening the gravel flushing gate. 4.5 Pipe Crossing and Approach Canal High flood level below the bridge should be recorded in every year. Inspection of the approach canal from inside should be done during major shut down. There is no great complexity in its operation and will not have significant repair work. 4.6 Settling Basin and Fore Bay

Sediment level should be inspected and recorded in every 2 hour during the rainy season (from May to September) and once a week during the dry season (From October to April). Sediments from the settling basin should be flushed when the sediment level reaches at 947 .70 m above msl. Flushing of sediment at settling basin should be done alternatively. One chamber supplies water to fore bay while another chamber will be in flushing. During flushing process entry of water should be controlled by adjusting the settling basin inlet gate. The outlet orifices should be closed to check entry of water from forebay. After draw down of the level of water the discharge from the inlet gate is so adjusted that there will be open channel flow and maximum sediment transporting capacity.


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5 CONCLUSION The planning, design, construction and operation/maintenance of headworks is a comprehensive task and requires proper consideration of river characteristics, size and scale of the project. The river slope (steepness), exposed bed materials, bends, rock-outcrops etc. are the key parameters while planning and layout whereas site accessibility availability of local materials play important role for adoption of construction technology. The main components of headworks that require adequate attentions for RoR scheme are: diversion weir (dam) including spillway arrangements, the intake, gravel/bed load excluder, and the settling basin. To ensure that the plant remains in operation during all normal situations it is required that following performance standards are met (Haakon (2003)):

a) Passage of floods, including hazard floods b) Passage of ice, trash and floating debris c) Passage of sediments d) Bed control at the intake e) Exclusion of suspended sediments and air

The plant failing to pass design floods poses risk to its safety whereas non-fulfillment of rest of the criteria directly influences operation of the plant resulting in huge outages. Each Hydropower project is site specific especially for steep and sediment loaded rivers, and experiences of the previous plants with similar nature should be taken as references. There are few guidelines on planning, design, operation and maintenances of the plants which are very important to be referred with. Further, preparation of those guidelines and avail them for users could be the step forward towards enhancement of headworks design and operation in such rivers. Generally the headworks operators in small hydropower plants are local unskilled/ semi skilled persons to be engaged in and trained. In such circumstances, an easy and simple Operation & Maintenance manual is the most to operate the structures. The case study of SSHP presented here is one example to demonstrate how it went up and being operated. Adoption of proper technology to suit the site condition for the better operational performances is one important aspect for the life of the structures and regular/periodic monitoring and timely carrying-out of maintenance and repair works is the other aspects to operate them over the design duration. Sunkoshi Small Hydropower Project is in operation from last three years. The performance of the headworks of the plant is fully satisfactory even passed 50 years flood this year. The design and layout concept of this project can be referred as a model for headworks in the similar rivers. 6 REFERENCES American Socity of Civil Engineers (1995) Guidelines for Design of Intakes for Hydroelectric Plants Design Manuals for Irrigation Projects in Nepal (1990), M.7. Headworks, River Training Works and

Sedimentation, Department of Irrigation, Ministry of Water Resources, Government of Nepal Galay V., (1987), Erosion and Sedimentation in the Nepal Himalaya. WECS, Kathmandu, Nepal. Haakon Stle, Dagfinn Lysne, Brian Glover and Einar Tesaker (2003) Hydraulic Design, Hydropower

Development, Department of Hydraulics and Environmental Engineering, NTNU. Sanima Hydropower (P.) Ltd. (2004) Sunkoshi Small Hydropower Project, Detail Design Report, SHPL,

Kathmandu, Nepal Sanima Hydropower (P.) Ltd. (2004) Sunkoshi Small Hydropower Project, Operation and Maintenance

Manual for Civil Structures, SHPL, Kathmandu, Nepal

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International Conference on SHP Kandy Srilanka All Details-Papers-Technical Aspects-A-A22