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Lower Solu Hydropower Project
Location
LSHPP is a medium scale power project on the Solu River, in Solukhumbu Districts in eastern Nepal. Solu is a major tributary of Dudh Koshi with its origin from Dhudh Kunda in the Numbur Mountain. The project area lies between 1750 and 1266m masl. Physiographically, the project is in the Middle Mountains of the lesser Himalayas.
The headwork is located about 18km upstream from the confluence. The project lies in Dudhkunda Municipality, Solukhumbhu..
Geographically, the project area is stretched between 86° 34′ 31” E to 86° 37′ 34” E longitudes and 27° 24′ 49” N to 27° 26′ 41” N latitudes.
Project Details
The project layout was initially assessed for different options on left and right banks. Assessment considering technical aspects, cost and survey license boundaries and interfaces with upstream developer in the upstream revealed that the only viable option was along the right bank.
The main project components are as follows:
- Barrage, intake structure and with gravel flushing arrangement on the left bank.
- Underground desilting basin (desander) on the left bank.
- Headrace tunnel from Salme to Wassigaun.
- Underground restricted orifice type surge tank at near Wassigaun.
- Surface penstock and manifolds from surge tank to powerhouse;
- Surface powerhouse on the right bank of Solu at Bhadaure
- Tailrace discharging back to Solu River
- (Under revision to be replaced by 4 adits) tunnel portals and internal access roads;
- Switchyard adjacent to the powerhouse;
- 132 kV a double circuit transmission line from powerhouse to Lamane.
- Allied temporary and permanent facilities.
Physical, Climate and Flow Features
Drainage basin of Solu Khola is confined in Nepal. The northern catchment extends to the Higher Mountains covering snow and glaciers.
The drainage divide is formed by the Numbur Himal, which is characterized by steep alpine slopes covered by snow at higher elevations. Two other major basins flank the LSHPP catchment area namely Likhu in the west and Dhudh Kosi on the right. The maximum altitudes within the basin range from 1992 to 5900m masl. Lower levels are under the influence of southwest monsoon and the primary source of flow during the wet season. Snow and glacial melt is the main source of water during the dry season. The ICIMOD study of 2001 indicates that these glaciers do not pose any hazard to the Solu basin.
Solu basin is sandwiched between the Likhu basin in the west and Dhudh Kosi basin in the east and is confined entirely to the northern parts of Solukhumbu districts. The total drainage area up to the proposed intake is 411.km2. The terrain of the basin varies from 7000m in the Numbur heights to 6,50m masl at its confluence with Dhud Kosi at Necha Betghari VDC.
The project area lies in Salleri gneiss and schist formation. Minor crystalline schist intercalation with quartzite and phyllite layers is part of the geological formation.
The climate of the basin varies dramatically with change in altitudes. At higher altitude, the climate is more of alpine whilst at lower elevations more temperate.
Between 600 to 1300m, the climate has hot summers and cool winter. From 1300 to 1800m, the climate experienced has warm summers and cold winters. Higher up, the summer is could and winter cold. The climatological measuring station at Chialsa (Index no. 1220) records show that a mean daily temperature of 0.7º C and 16.3º C in January and July respectively. Extreme temperatures recorded for corresponding months are of -9.8º C and 23º C. Average relative humidity varies from 44% to 98% respectively.
In general, the precipitation and temperature decrease from south to north. The air temperature varies from subzero in winter to 35°C in summer. The coldest months are December, January and February. Snowfall is experienced at the elevations above Naya Bazar Phaplu during the months of November through April. Recorded annual rainfall near the headworks site is about 1760mm.
Solu is a perennial stream fed by snow melting, springs and monsoon rains. The mean annual specific discharge at the intake site is about 60.44 litres per second per km2. Mean monthly flow at the headwork site is 24.85 m3/s approximately, while the minimum monthly flow in March is about 5.99 m3/s. 69.06% of the annual flow occurs during monsoon period from June to September, 14.81% occurs during post monsoon season of month October to November, 9.43% flow occurs on winter period of month December to February and 6.70% flow occurs on premonsoon season of month March to June.
Maximum floods occur during the months of July through September, when the monsoon rainfall is fully active in its catchment. The maximum and minimum observed flow from DHM published data are 166 and 4.67 m3/s for the years 2004 and 2001 respectively.
Solu is a typical Himalayan river with a gravel-boulder bed and relatively steep slope. Theoretically such rivers have the potential to carry large amount of suspended sediment and bed loads during floods. The sediments are derived mainly from the erosion of banks and riverbed degradation. Landslides and rock falls are usually frequent such types of basins.
Observations by the Consultants over a number of years at the sediment deposits in the desilting basins of Salleri Chialsa Small Hydel Plant indicate that the sediment concentration is low. This observation is vindicated by the sediment analysis presented in the Feasibility Report for Lower Solu Small Small Hydropower Project 2008 which report sediment concentration within 500 ppm even in the rainy season.
Salient Features
| S.N | Description | Details |
| A | Main Feature | |
| Installed Capacity | 82 MW | |
| Gross Head | 491.0 m | |
| Net Head | 477 m | |
| Location | Solu Dudhkunda-11,Tingla, Solukhumbu |
| S.N | Description | Details |
| 1 | Headworks Structure | |
| a | Type | Barrage |
| b | Dimension | 62.5X73X7.5 |
| c | Special feature | radial gated |
| d | Gate Nos | 6 Nos |
| e | Design flood discharge | 717m3/s |
| f | Design discharge | 20.42 m3/s |
The headworks comprises of gated barrage across Solu Khola, intake, feeder tunnels, underground desilting chambers, access tunnel, silt flushing tunnel. The size of barrage is 62.5m X 73m X 7.5m consisting of six radial gates. The designed flood discharge is 522 m3 /s (1 in 100 Years) and the designed discharge for the power generation is 20.42 m3 /s.
| S.N | Description | Details |
| 2 | Desilting Basin | |
| a | Dimensions | 104.8mX6.5mX11.6m |
| b | No. of bays | 2 Nos |
| c | Type underground/surface | Underground |
There are two underground desilting chambers, directly connected with two independent feeder tunnels. The finished size of Desilting Chamber is 6.5 m (W) x 11.6 m (H) above duct. The length of Desilting chamber is 104.8 m. The height of crown is 2.3 m, height of vertical portion is 6.5 m and height of sloping portion is 2.8 m. The maximum ground cover for Desilting Chamber is about 100 m. The excavation is carried out by pilot tunnel, side slashing and benching down. The flow system is free flow and accordingly, the top level of concrete lining is proposed up to EL 1764.5 m i.e. maximum static water level in Desilting chamber
| S.N | Description | Details |
| 3 | Headrace tunnel | |
| a | Length | 4312.9 m |
| b | Shape | D-Shaped(excavated) and Horse shoe shape finished |
| c | Size | 3.2m X3.2 m |
| d | Lined/Unlined | Lined |
| e | Percent of Lining | 100% |
| f | Geological quality | Gneiss,Schist(most in weak geology) |
| g | Total Length of water conveyance (intake, desilting, feeder, Hrt, pressure shaft, penstock, Tailrace) | 6894.5 |
The Head race tunnel of the project is a pressure tunnel of diameter 3.2m. The excavated shape of Headrace tunnel is D-shape and its finished shape is 3.2 m diameter Modified Horse-Shoe shaped. Total length of unit headrace tunnel and headrace tunnel is about 4312 m and this total length is sub divided into two sections i.e., one portion of HRT lying at left bank and other portion of HRT lying at right bank of solu river. The length of unit headrace tunnel lying on left bank of solu river is 202 m. And the length of HRT on right bank of solu river is 4110.87 m. which start from chainage 492.337 m (RD=0.00) to chainage 4603.21 m (RD= 4110.37). The HRT on right bank ends at Surge shaft. At right bank of HRT, the slope is 1V:331.808H from RD 0.00 to RD 2080 and 1V:197.130H from RD 2080 to RD 4110.87. The invert level of headrace tunnel on right bank start (RD=0.00) is at EL 1756.60 m and at end (RD=4110.87) is 1738.8 m. The flow system in headrace tunnel is pressurized flow. Concrete lining is proposed in the entire periphery. The maximum static water level is at EL 1764.5 m i.e., FRL at barrage and maximum upsurge level is at EL 1781 m.
| S.N | Description | Details |
| 4 | Surge shaft | |
| a | Type | concrete lined vertical shaft |
| b | Dimension | 72m *8m dia |
The location for the Surge Tank has been selected at Gajendanda area. The upper surface of this area is cultivated land comprising of colluvial materials. The proposed area is gently sloping with adequate space for the construction of surge tank. However, the upper surface of this area is covered with about 10 m boulder mixed colluvial materials and is sloping towards Solu Khola. The upper portion of the proposed surge tank will be exposed to atmosphere and the lower portion underground. It is fully lined with concrete. It works as a relief valve for instantaneous opening and closure providing extra supply of water in case of opening and releasing the pressure to control the damage from water hammer in case of closure.
Prior to the selection of the type to be proposed, a comparative study between Surge Tank and Forebay was carried out. The study was mainly focused on topographical condition of the site, suitability of type of structure and the comparison of cost. The proposed location is in close vicinity of BH-5. At this location construction of both either Surge Tank or Forebay is feasible. But after a careful comparison of the cost, it was found that the cost of the Surge Tank was almost 1.76 times cheaper than that of Forebay. In addition, in case of Forebay, about one kilometer long drainage facility will also be required to be constructed to flush out the spilled water. Also there might be some other social issues against the provision of an open structure within Wassigaun residential area. Hence, considering all the topographic, hydraulic, social and economic issues a semi underground Surge tank was found to be more favourable compared to a RCC Forebay structure.
Dimension of Surge Shaft: 6.0 m / 8.0 m diameter, 72m deep.
| S.N | Description | Details |
| 5 | Penstock Pipe | (Under revison to be replaced by underground pressure shaft) |
| a | Type | Surface(pit Type) |
| Dia | 2.4 m | |
| b | Length | 1475 m |
| c | No. of Anchor Block | 15 Nos |
| d | Thickness of Pipe | 10mm to 36mm |
| S.N | Description | Details |
| 6 | Powerhouse | |
| a | Type (surface/semisurface/underground) | Surface(pit Type) |
| b | Dimension | 52.5mX36.2mX30.7m |
| c | No of Units | 2 units of 41 MW each |
| d | Switchyard position | Indoor 132KVA GIS |
| e | Transformer type | Single phase |
| Nos of transformer | 9 Nos (One Spare) | |
| f | Turbine type | Vertical pelton |
The 52.5m X 36.2m X 30.7m surface powerhouse with 2 units of 41 MW with vertical Pelton turbine is located on the right bank of Solu khola.
This structure is subdivided into three bays one is Service bay of size 20m Long, 18m Wide and 13.9m height. Second is Machine Hall of size 32.5m Long, 18m Wide and 29.5m height. Third is control room located just downstream of Machine hall, the size of control room is 52.525m Long, 8.5m Wide and 18.2m height. The Machine hall, Service bay and control room are all separated with an expansion joint of 25mm. In this report the only the machine hall and control room is covered. The service bay is located at left side end of the Power House. The entry to the Power House is planned through service bay. The Service bay and machine hall floor are placed at same elevation (EL. 1282.60m). Below the machine hall the Generator floor has been proposed at EL.1278.10m. Below that the MIV floor has been provided at EL.1273.50m. In machine hall Two Generating units of 41MW each has been proposed. Two vertical shaft Pelton turbines have been placed with 15m centre to centre. In control room at EL. 1273.50 CW equipment’s have been placed. The EL.1278.10m is used as cable spreading area. EL.1282.60m is occupied for placing the control room, workshop and Compressor room. Floor at EL.1287.40m is used for placing Conference room, Canteen, Toilet, Ventilation room & Battery room. The Machine foundation inside the Machine hall are constructed stage wise. The floor slabs and beams come at EL.1282.60 & 1278.10m are connected with the machine foundation. Both the control room and machine hall columns are founded over the First stage concrete of machine foundation. At downstream of control room, the tail water are dropped back into the river through tail race duct. An EOT crane of capacity 80/25/5T has been mounted over the power house frames at EL. 1290.70m, which run throughout the machine hall and service bay end to end. A steel roof truss has been proposed at EL.1296.50m to cover the Machine hall and Service bay. But the control room is cover with RCC roof slab at EL.1291.70m. At the upstream side of the Power House, Transformer bay & GIS hall have been proposed.
The surface valve house structure is located downstream of the surge shaft and the constructed structure is presently under thick cover of slided mass comprising rock blocks, debris and provided support system.
| S.N | Description | Details |
| 7 | Tailrace | |
| a | Type (surface/ underground) | Box Culvert and Tunnel combined |
| b | Length | 230m |
| c | Dimension | 4mX4m |
| S.N | Description | Details |
| 8 | Transmission Line | |
| a | Voltage Type | 132KVA /double circuit |
| b | Length | 4.12 KM |
Project Progress
Works Completed
- Labour camps, stores, offices, roads are completed
- Senior Staff building construction is completed.
- Junior Staff building construction is completed.
Works Remaining
- The upgradation of the road is under progress.
- The camps and infrastructure shall be added as per the requirement.
Civil Works Completed
- Slope stabilization at barrage completed.
- Bailey bridge erection completed.
- All 6 Bays construction at Barrage completed.
- HRT Intake portal for both intake construction is completed.
- Excavation as well as concrete lining is completed for both feeder tunnel.
- Excavation as well as concrete lining works completed for both Desander.
- Head race Tunnel excavation as well as concrete lining at left bank completed
- River protection works completed.
- Silt flushing tunnel excavation and concreting lining completed.
- Access Tunnel excavation and supports completed.
- Guide Walls at barrage completed.
- Drift Tunnel excavation completed.
Civil Works Remaining
- Guide walls between barrage D/S and Bailey Bridge U/S.
- Wing wall construction for Bailey bridge.
- Construction of Control Building.
- Construction of Power Pack room.
- Intake structure modification.
- Desilting Basin Silt Flushing duct modification and rectification.
- Erection for Pipe conduit Bridge
- Civil works for Pipe conduit at river crossing
Hydromechanical works completed at headworks:
- Radial gate – Gate erection is completed.
- Crest Stop Log Gate – Gate erection is completed.
Hydromechanical works remaining at headworks:
- Radial gate – Hydraulic Hosting and electrical control system is remaining.
- Crest Stop Log Gate – Installation of Stop Log Panel and Gantry crane erection is ongoing.
- Intake Service Gate – Gate leaf and Hosting arrangements is ongoing.
- Intake Bulk head Gate – Gate leaf and Hosting arrangements is ongoing+.
- Intake Trash Rack – Installation of TR panel and TRCM is remaining.
- DC Stop log Gate – Gate erection and Hosting arrangements is remaining.
- Silt Flushing Valve- Installation is remaining.
Works Completed
- HRT – Excavation Completed
- Concrete Lining completed for 2418m
Works Remaining
- Concrete Lining for 1688m remaining.
- HRT Adit Plug Gate – remaining
- Grouting, repairing and Cleaning
- Slope excavation and Slope stabilisation works completed
- Surge shaft excavation and concrete lining completed
- Contact and Consolidation Grouting completed
- Steel liner Proposed in Surge shaft. Assessment under progress.
- Excavation and Slope protection work was completed
- Portal for pressure shaft was completed
Landslide triggered in upper reaches of valve house area which damaged portion of portal protection work with uprooting of some SDA. There was no damage to the valve house foundations.
The debris were removed and shotcreting for slope stabilization was completed after the landslide. However, the landslide occurred again in the same area destroying the freshly sprayed shotcrete.
Due to the multiple landslide being occurred in the valve house area, it has been planned to shift the location of valve house to underground at existing Adit 2 location. The design for the same is evaluated from Consultant.
Civil Works Completed
- Slope stabilization completed
- Slope Protection/River Protection work completed
- Following works have been completed in powerhouse building construction
- Construction of Power house complex.
- Construction of transformer bay is completed.
- Construction of transformer oil tank completed
- Oil Sumpand TRC key concreting completed.
- Selection for the Quarry site has been completed.
- Column concreting and reinforcement binding for crane beam at GIS building completed.
- Oil Sump Concreting Completed.
Civil Works Remaining
- Following works have been completed in powerhouse building construction
- Concreting of GIS building and ODY structures is ongoing. .
- Construction of saddles for Fire Fighting tank under progress
- Flood protection wall construction under progress
- Construction of fire water tankin progress.
- Part of GIS building Staircase concreting is remaining.
Electromechanical works completed at Powerhouse:
- Main erection activities of Turbine & Generator for both unit is completed
- GSU Transformer, station auxiliary transformers have been installed in the position
- Most of the power house equipment & auxiliaries have been installed in their position.
Electromechanical works remaining at Powerhouse:
- GIS, MIV, PIV installation remaining
- Part of auxiliary works are remaining
Works completed
- Cut and Cover portion for TRC
- Excavation and Concreting of Tailrace Tunnel Completed
- Tailrace Gate erection completed.
Works completed
- Entire civil works and 132 KV Transmission Line tower erection works have been completed
Works remaining
- Conductor stringing is pending
Substation
Interconnection substation is situated at Tingla which is 4.1 km far from powerhouse of Lower solu hydropower project
Works completed
- Entire civil foundation works, 90% of gantry erection works has been completed
Works remaining
- 10% mechanical structure erection works, installation of switchgear equipment, cabling, BUS bar & ACSR conductor connection and control room work is still pending
Background
The significant movement were noticed at AB3 (Anchor Block #3) and at the intermediate supports between AB3 and AB4. Maximum Anchor Blocks are not founded on rock and rock is assumed at depths of partly >30m. The movements at AB3 to AB4 are related to creeping slope movements pushing onto and below the ABs from the right uphill side (looking in the direction of the flow). These movements can be expected to be greatest during monsoon when the slopes are highly saturated.ABs up to and including AB10 are also not founded on rock.Rock is partly found laterally close to the ABs, esp. AB9 and AB11. Only from AB11 onwards the penstock/ABs are founded on rock, although this rock is mostly weathered and distressed with open and partly filled joints. The present alignment of the penstock may only reliably be maintained downstream of AB11. This alignment would still require substantial foundation improvements and tie backs.
From AB11 upstream a new alignment would need to be found to possibly connect to the AB1 at the valve house. However, at the foundation of the valve house and AB1 rock is currently assumed at larger depth only. An alternative location for the valve house could be in Adit 2.
Therefore, existing surface penstock cannot be used and vertical shaft may be drilled to a depth from where a sub-horizontal pressure tunnel can be driven towards the powerhouse.
Design, survey and investigation under progress for new alignment of pressure shaft.
Works completed
After nearly 100% completion of Anchor block construction and Penstock pipe erection, the design consultant decided to change the alignment in considering the existing geological challenges likely to impact the project components.
Works remaining
- The abandoned penstock is planned to be replaced by underground pressure shaft for which a detailed geophysical investigation is under progress and layout has been finalized. Work has been awarded to the new contractor which is preparing for mobilization at site.