Three natural hazards in fewer than two years had significant effects on the 45-MW Upper Bhote Koshi facility in Nepal. Discover how the plant owner dealt with each event, current status of the project, and lessons learned that can be applied at other hydro projects.
By Michael Bruen, James Witnik and Bikram Sthapit
In fewer than two years, three natural hazards significantly damaged the 45-MW Upper Bhote Koshi Hydropower Project in Nepal. The impacts of the hazards presented a number of challenges to reduce future hazard risks and bring the facility back on line.
Background on the project
Upper Bhote Koshi, the first privately funded run-of-river hydroelectric project constructed in Nepal, is owned and operated by Bhote Koshi Power Company Private Limited. It is located on the Bhote Koshi River, a tributary of the Sunkoshi River, in the Sindhupalchowk District of central Nepal, near the China-Nepal border.
The project has a concrete gravity dam, gated spillway, river intake and desanding basin (see Figure 1). Water is conveyed to a surface powerhouse through a 3.3-km-long, 4.5- to 5.5-m-wide headrace tunnel and a 430-m-long, 3-m-diameter steel surface penstock. The two-unit powerhouse contains Francis turbines that operate with gross head of 145 m and average flow of 36.8 m3/s.
The project was built under an engineering-procurement-construction (EPC) contract and placed into commercial operation in January 2001. MWH, now part of Stantec, was the owner’s engineer during initial operation. China Gezhouba Construction Group Corporation completed the construction at a cost of $76 million.
Figure 1 – Bhote Koshi Site
The 45-MW Upper Bhote Koshi Project features a concrete gravity dam, gated spillway, river intake and desanding basin. Water is diverted to a two-unit surface powerhouse.
Geologic setting
The project is in a transitional zone between the Midland – consisting of subdued hills, wide river valleys and tectonic basins – and the Himalaya Mountains. The Bhote Koshi River has cut a steep, northeast trending gorge, bordered by joint controlled cliffs that reach 3,500 m elevation.
Colluvium, talus, landslide deposits and alluvium contribute to shape the landscape and the Bhote Koshi river channel. Bedrock consists primarily of low-grade metasedimentary rock, bedded quartzites, phyllite and schists and gneiss.
The area is characterized by intense folding and faulting. The mountainous region of the Himalayas in the Sindhupalchowk District is deforming as a result of the collision of the India-Eurasian plates triggering frequent earthquakes. Frequent earthquake ground shaking coupled with rainfall during the monsoon season contributes to slope instability and mass wasting events, landslides, debris flows and rockfall throughout the region. In the basin to the north, the Poiqu (China)/Bhote Koshi drainage basin, which reaches heights of more than 6,000 m, includes numerous mountain glaciers and glacial lakes, the latter of which have been prone to glacial lake outburst floods (GLOF). These processes – seismicity, mass wasting and flood hazards – continually modify the geomorphic landscape of the Bhote Koshi/Sunkoshi watershed.
Hydropower plant operation
The Upper Bhote Koshi Hydroelectric project had been continuously operated, except for brief maintenance and shutdowns to replace a transmission tower (2002), repair generator thrust bearing supports, (2009) and replace the turbine-generators (2014). Power generation was variable based on seasonal conditions, with more than 70% of the annual production occurring during the monsoon season. Beginning in August 2014, three hazard events occurred that resulted in significant plant outage time that will continue for several years.
Strike 1 – Jure Landslide
On Aug. 2, 2014, a landslide occurred after heavy rain about 16 km downstream of the powerhouse on the Sunkoshi River, at Jure. The landslide was situated on the right bank from river level at El. 795 m to the top of the scarp at El. 1500 m and was 1.3 km long and 850 m wide at its base. The rapid slope failure of soil and rock flowed downslope and across the valley and up onto the opposite bank, damming the river. The Jure landslide had an estimated volume of 6 million m3.
The landslide dam created was about 400 m long (east-west), 105 m at the base and 30 to 35 m high, which created a lake 3 km long and about 200 m in width, with a volume of 8.6 million m3. The devastation caused by the landslide and damming of the river led to at least 156 fatalities.
Significant infrastructure was damaged, including submerging the powerhouse of the Sanima Hydropower Project and 26 homes. Two gates at the Sun Koshi Hydroelectric project were washed away, and a nearly 2-km-long section of the Arniko Highway was destroyed as well.
The transmission line for Upper Bhote Koshi traverses the right abutment at Jure. Three of the transmission line towers failed and power from Upper Bhote Koshi was interrupted for six months as a new right-of-way had to be negotiated and the towers constructed.
Strike 2 – Gorkha earthquake and aftershocks
Nine months later, on April 25, 2015, Upper Bhote Koshi experienced an unscheduled emergency shutdown due to a magnitude 7.8 earthquake about 140 km west of the site at a depth of 15 km. The large earthquake affected Nepal and parts of India and Tibet, resulting in nearly 9,000 fatalities and injuring 22,000 people in Nepal alone. Subsequent to the shutdown, the emergency diesel generator at the headworks was used to allow operators to close the tunnel intake gate and open one spillway gate. The project was largely undamaged and no injuries were sustained by plant staff. At the project site, a layer of fine-grained sediment blanketed the project site that was the product of rockfall in the valley. However, there was a disruption to power and a high risk of earthquake-induced rockfall and landslide hazards due to the ground motion and shaking associated with the aftershocks.
The Gorkha earthquake caused ground failures and significant damage to the infrastructure to the east and comparatively little damage to the west. Along the margins of the Kathmandu basin, the ground failures and structural damage were more pronounced.1 Furthermore, the ground shaking triggered numerous landslides in the more sparsely populated areas between Kathmandu and the Nepal-China border. Within the Sindhupalchowk District, based on a review of satellite imagery, the earthquake triggered more than 6,000 landslides.1
More than 330 aftershocks greater than magnitude 4 occurred over the next 60 days. The two largest aftershocks, magnitude 6.7 (April 26) and magnitude 7.3 (May 12) events, occurred about 16 to 18 km south-southeast of the Upper Bhote Koshi site at a depth of 15 km. The April 26 earthquake event caused several landslide hazards that led to significant damage to the Upper Bhote Koshi facilities, specifically to the penstock and powerhouse, residences and office buildings, and along the Bhote Koshi valley including the Arniko Highway. The extensive ground motion and shaking from the aftershock earthquakes loosened the soil and rock in the steep upper valley slopes, triggering rockfall, landslides and debris flows. Following the event, a number of boulder-sized (up to 3 m) rockfall strikes impacted the surface penstock, causing it to rupture. The rupture led to the uncontrolled and rapid dewatering of the headrace tunnel, releasing more than 50,000 m3 of water that eroded and damaged the penstock trench and flooded the below-grade powerhouse.
At the headworks, rockfall also occurred from the left abutment high above the desanding basin, landing on the adjacent perimeter road and within the desanding basin, damaging the lighting fixtures and railing system around the basin.
Condition assessment
In June 2015, MWH personnel visited the project facilities to perform a condition assessment and make recommendations on rehabilitation of the project.2
At the headworks, the gravity dam structure, stilling basin, river intake and desanding basin suffered no apparent structural damage. In addition, the spillway structure, gates, piers, beams, ogee and apron suffered no apparent damage. Furthermore, there was no structural cracking observed and there were no visible offsets or displacements that could indicate latent damage.
The 3,300-m-long headrace tunnel – with unlined, shotcrete-lined and concrete-lined sections – was also found to be in satisfactory condition. Isolated rockfall in the form of two moderate-sized blocks of rock were noted in the invert in the unlined section. The largest was a slab from the sidewall of the tunnel excavation, which may have been triggered by the stress relief due to the rapid tunnel dewatering when the penstock ruptured. The shotcrete- and concrete-lined sections (>70%) of the headrace tunnel and the surge shaft appeared to have not suffered any structural cracking of the lining systems that could indicate any latent damage or exhibit any signs of stress-induced damage due to increased ground loads.
The project suffered significant damage to the downstream structures, ground surface and equipment due to frequent rockfall, subsequent rupture of the penstock and ensuing slope erosion and debris flow that submerged the powerhouse (see Figure 2). The penstock cans, stiffeners and girder supports in penstock reaches 1 and 2 and expansion joint 1 were damaged by rock impacts. The penstock trench rubble masonry slope protection and foundation was damaged below the rupture due to the uncontrolled water discharged downslope to the powerhouse. Over the middle section of the penstock, reach 3, the penstock cans had not suffered rockfall impact damage. However, the dewatering of the conveyance system severely damaged the trench foundation, dislodging five of the penstock support footing blocks. For reach 4, most of the penstock was buried and covered by the debris that had been eroded from the slope and trench foundation upslope. Reach 5 was largely undamaged, excluding significant foundation erosion of the trench sides.
Access to the powerhouse was limited to the superstructure above the erection bay level because the substructure was still submerged at the time of the inspection. It appeared the superstructure had not suffered any serious damage. Minor damage in the form of a few wall cracks, broken windows and a few rockfall impacts from the steep mountain slope from across the river were observed. Most of the electrical control cabinets in the erection bay had been temporarily submerged to about 1.2 m above the floor level.
Figure 2 – Gorkha Earthquake Effects
During the second major event, the Gorkha earthquake, rupture of the penstock and dewatering of the tunnel led to erosion downslope and flooding of the powerhouse, submerging the equipment.
Recommendations for rehabilitation
The penstock cans in the upper reaches would require replacement and the trench foundation and rubble masonry slope protection and many of the penstock supports over reach 3 would need to be reconstructed. In addition, it was recommended that penstock reaches 1 and 2 be encased in concrete and buried to protect from future rockfall and a rockfall barrier system be installed on the valley slopes above the penstock.
Clean-up of the debris and dewatering of the powerhouse, powerhouse yard, transformers and diesel generator buildings would be required. All equipment and cabinets in the powerhouse would need to be dried out and condition assessments performed. Rehabilitation of the turbine-generator units would be required and the plant equipment and power transformers would need to be mechanically and electrically balanced.
Debris flow submerged the below-grade Upper Bhote Koshi power-house during the Gorkha earthquake, submerging the powerhouse substructure, including the powerhouse erection bay level shown here.
Strike 3 – Debris-laden flood
Work to repair the penstock, powerhouse equipment and transformers was about 60% complete on July 5, 2016, when Upper Bhote Koshi experienced a third strike. The project sustained significant damage to the headworks and powerhouse from a GLOF event that occurred when a moraine-dammed lake burst in the Zhangzangbo River basin, a primary tributary of the Poiqu/Bhote Koshi River in China, about 24 km upstream of the dam.3 At the time of the GLOF, the Bhote Koshi watershed was saturated and the river was swollen, representing high flows due to heavy rainfall over the previous week and above average rainfall for June.
At the time of the flood, the Bhote Koshi River peaked at about 3.5 m above the top of the dam and at about 1.7 m above the powerhouse yard. The river discharge may have peaked at about 2,576 m3/s based on measurements made in the river basin upstream of the headworks. However, there was no recording of the peak discharge flow.3
During the flood event, the bed load carried a preponderance of large boulders, up to 8 m diameter, which struck the dam and headworks. Damage included a complete failure of the riverside wall, two bridge support piers and the deflector wall in the desanding basin and localized damage of the base slab. Flood debris accumulated upstream of the structures, obstructing river flow and blocking the spillway and river intake. As the debris continued to pile up, river flows were dispersed and resembled a braided stream, producing a number of small shallow channels and bars. As stream erosion continued, the river moved to the right side of the channel and began to erode a new channel through the right abutment, comprised of unconsolidated material. On July 9, four days after the flood, the right abutment was breached and subsequent river flows continued to bypass the dam.
At the downstream facilities, the flood waters overtopped the parapet wall and entered the powerhouse, again submerging the equipment. In addition, river erosion associated with the high river flows destroyed about a 100-m-long section of the left bank guide wall downstream of the powerhouse, causing instability of a section of the powerhouse backslope.
The debris-laded flood occurring in July 2016 cascaded over the Upper Bhote Koshi dam and headworks after failure of the desanding basin.
Condition assessment
In September 2016, MWH visited to perform a condition assessment.4 The review focused on the dam, headworks and powerhouse.
During the inspection of the dam and headworks, observations were limited by the deposition of flood debris, a sandy gravel material with cobbles and boulders, which obscured the upstream face of the dam, intakes and spillway, and by the high flow conditions of the Bhote Koshi River.
The flood debris accumulated to a height of 2 m above the top of the dam (Elevation 1,437 m) and included many large boulders (4 m to 8 m diameter). Several of the boulders were perched on the dam crest, others were lodged against the spillway gates, and some were deposited in the upstream end of the desanding basin. The impacts of the flood debris with the large boulders caused the failure of a 60-m-long section of the desanding basin river side wall, two piers and a flow deflector, and destruction of the three flushing gates and the flushing gate control building atop the river side desander wall. There was also localized erosion of the base slab.
The spillway gates were in the full open position at the time of the flood but large boulders became lodged because of their size, obstructing river flows. Some damage occurred to the spillway gate skin plates and lower cross support structural members, gate seals, sill plate and bulkhead slots. The spillway piers and trunnion anchorages appeared intact. However, the gates will need to be operated to determine if the range of functionality is satisfactory and if they sustained damage. Erosion of the concrete surface for the ogee and apron occurred, exposing rebar, and the right guide wall downstream suffered damaged. The approach channel guide wall was buried under the debris and its condition could not be determined. However, the upstream end is likely damaged from boulder strikes.
The gravity dam was constructed in three blocks and only sustained surficial damage due to the flood. Damage occurred due to the abrasive nature of the flood debris, which eroded the crest and downstream face locally to depths of 200 mm or more. The flood debris flowing over the dam eroded the abutment slope protection and likely caused some damage due to boulder impacts to the stilling basin below.
The right dam abutment and an adjacent section of the highway were severely damaged due to river erosion. A new river channel was cut through the right abutment, bypassing the dam/headworks structures. The progressive erosion of the right abutment led to slope stability issues that impacted several homes and buildings upslope of the dam. After a short period, erosion through the breach led to the undermining of dam block 1 and destroyed the cutoff wall extension beyond the dam.
In addition, the river intake trashrake system, which sits on top of the intake, was destroyed. The tunnel intake trashrake system located at the downstream end of the desanding basin sustained only minor damage.
At the downstream facilities, the impacts of the debris-laden flood were largely associated with the flooding and submergence of the powerhouse equipment and transformers. An inspection of the powerhouse after dewatering revealed the superstructure and substructure were not impacted by the flood loads. The electrical cabinets and wiring, generators and stators will need to be cleaned, tested, refurbished and where necessary replaced. The impact of the flood also resulted in the complete loss of the control room on the erection bay level of the powerhouse. In addition, two of the power transformers will need to be replaced.
Erosion of the left bank guide wall downstream of the powerhouse created localized slope stability issues. The flood discharge had destroyed a 100-m-long section of guide wall. The resultant erosion at the toe of the slope led to localized cracking, buckling and settlement of the powerhouse backslope slope protection over a 60-m-wide zone, just below the switchyard.
The control room on the erection bay floor was completely destroyed by flooding of the powerhouse.
Recommendations for rehabilitation
The dam and headworks rehabilitation will largely involve partial reconstruction of the desanding basin and right abutment, replacement and extension of dam block 1 and the foundation cutoff wall, resurfacing of the dam crest, repairs to the gated spillway, and rehabilitation of electrical and mechanical components.
A condition assessment will need to be undertaken by the original manufacturers for the turbine-generators and general contractors for electrical and mechanical equipment to determine refurbishment and replacement requirements. We expect much of the equipment can be refurbished. Also, the transformers will require replacement, the left bank guide wall reconstructed, and the powerhouse backslope repaired. Construction activities under way at the time of the flood, repair and modifications to the penstock and construction of a tailwater box structure also need to be completed.
Summary and conclusions
The extent of damage to Upper Bhote Koshi Hydroelectric project from the three incidents is in the tens of millions of dollars since August 2014. The GLOF/flood debris event resulted in the most significant structural damage to the dam and headworks, as well as the second submergence of the powerhouse resulting in water-related damage. Furthermore, the loose rock conditions in the upper slopes of the valley may complicate matters during the coming monsoon season as the steeper slopes may still be prone to instability. During high river flows, the bed load deposited in the river may continue to migrate downstream. One promising aspect has been the condition of the headrace tunnel, which was not damaged during the last two hazard events.
Although the project has suffered three “strikes,” the project is not “out.” Efforts are under way to procure the services of an EPC contractor for the reconstruction and rehabilitation of Upper Bhote Koshi.
Notes
1Geotechnical Field Reconnaissance: Gorkha (Nepal) Earthquake of April 25, 2015 and Related Shaking Sequence, Geotechnical Extreme Event Reconnaissance Association Report No. GEER-040, 2015.
2Upper Bhote Koshi Hydroelectric Project Post-Earthquake Condition Assessment Preliminary Report, MWH, 2015.
3”Special Report on Cause of July 5th, 2016 Flash Flood-Debris Flow in the Poiqu (Bhote Kowhi) River,” Executive Summary, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources, Chengdu, Sichuan, China, 2016.
4Upper Bhote Koshi Hydroelectric Project Post-Flood Condition Assessment Preliminary Report, MWH, 2016.
Reference
Bruen, Michael, et al, “Three Strikes: Natural Hazards Impact Nepal Hydropower Project,” Proceedings of HydroVision International 2017, PennWell, Tulsa, Okla., U.S., 2017.
Acknowledgment
The authors thank Gary Hoornaert with MWH for his help in developing this article.
Michael Bruen and James Witnik are vice presidents with MWH, now part of Stantec. Bikran Sthapit is chief executive officer of Bhote Koshi Power Company Ltd.
