The Melamchi water diversion tunnel is not only Nepal’s first major water supply tunnel, but the longest
of its kind in South Asia and the 14th or 15th longest in the world. Conceived in a 1988 pre-feasibility
study aimed to develop options for relieving extreme water stress in the Kathmandu Valley, the final
design was completed in 2001 and construction initiated in 2010. During construction the project has
faced technical challenges typical to tunneling operations in the Himalayas due to weakness zones,
fragile rock mass conditions, and high overburden. At least four major fault lines have been mapped
across the tunnel alignment between the Melamchi and Kathmandu Valleys. One of the fault lines, commonly
called the Gyalthum fault, is suspected to bifurcate across the tunnel route. Infrequent but intense
seismic slippage mainly occurs along the low-angle Main Himalayan Thrust (‘MHT’), evidenced in the April
2015 Gorkha earthquake and aftershocks. The project is also within a high rainfall region, and at risk
for increased sedimentation loads and more intense flooding due to climate change-related glacial
melting and more rapid snowmelt.
The main diversion tunnel is 26km in length, with headworks located in the Melamchi Valley and outlet at
the Sundarijal water treatment plant, and the system is fully gravity fed with an average expected flow
velocity of < 0.5m/sec. The internal dimensions are relatively small (3.5m W x 4.1m H), designed to
carry 6 cumecs of water. Tunnel overburden ranges from 200m to 1500m, with 42% of the tunnel depth
between 500-1000m. The tunnel is designed to withstand a peak ground acceleration of at least 0.25g.
The tunnel has been excavated entirely using the drill and blast method passing through varied
geological conditions with nearly 72% passing through Rock Class 4 and 5. It is a is D –Shaped tunnel.
After the installation of the drill holes in a pattern, (based on the tunnel dimensions, geometry,
geological and rock mechanical conditions, possible water ingress, hole size etc.), the tunnel face were
charged with explosives and were blasted to progress the tunnel through the rock. Post excavation the
supports were installed based on the design consideration including the rock mass type and quality and
the in-situ stress conditions. The support assessment mainly followed Barton’s (1994) classification
Tunnel construction is nearly completed: the final breakthrough occurred in April 2018, and remaining
works include completion of ventilation shafts, invert lining, final supports, rock dowels, and tunnel
electrical works which are currently underway. Once completed the tunnel will be the main source of raw
water to meet the drinking water supply demand of the Kathmandu Valley and relieve the valley from the
current acute water shortage.
Considering the significant importance of the tunnel as a lifeline of drinking water for the Kathmandu
Valley, its location in an zone prone to seismic risk and flash floods, and limited availability of
alternative raw water sources for the valley, ADB is supporting MWSDB to carry out a fitness test
assessment of the tunnel and headworks from natural hazards. This study has the following three
I. Assess the level of risk and vulnerability of tunnel and headworks to two main hazards:
(i) Seismic events during operation, hydrodynamic impact on the tunnel for different degrees of
(ii) Major flash floods (including Glacial Lake Outburst Floods, if applicable), to the water retaining
structures of headworks,
II. Recommend strengthening measures including structural and non-structural measures, to mitigate and
minimize these risks during operation period and improve earthquake performance of tunnel as well as
headworks, which will help reducing physical damage and economic losses in case of any eventuality; and
III. Develop operations and maintenance procedures and emergency standard operating practices and risk
mitigation plan for the above-mentioned hazards.
Scope of Work
The entire assessment will be conducted based on the already available information, reports, data and
drawings to be provided by MWSDB, selected site visits by the experts and a forthcoming condition
assessment report to be prepared by the project supervision consultants and the contractors. The
condition assessment report will be based on the physical inspection of the entire tunnel length and
covering tunnel portals, tunnel interiors including overbreak and underbreak areas, tunnel lining,
intrados and extrados of tunnel arch, and tunnel roof section, sidewall and invert.
The team of experts of Team Leader cum Tunnel Expert (Structural Engineer), Senior Geotechnical Expert,
Water Supply Expert and Hydraulic Engineer will carry out their respective expertise in following
1. Vulnerability assessment of tunnel and headworks in case of seismic event:
• Review past and ongoing studies including available earthquake data and events, engineering deigns, as-
built drawings, ground characteristics, tunnel construction methodology, geological profile, surrounding
hydrological condition, soil/ rock overburden, support type, properties of structure/ liners, earthquake
zones, soil investigation reports, condition assessment report and other relevant reports.
• Compile database with relevant scientific data related to weather and earthquake, topography, geology,
rock profile hydro-geology, hazard, and past damage caused by extreme weather events.
• Identify additional data/information need, if required, suggest MWSDB to provide those for developing
the models and disaster risk mapping. MWSDB will make best efforts to make those available. The team
will use professional judgement to arrive at a solution in case some data/ reports are not available.
• Review impact of different past earthquake events on the hydraulic tunnels in the similar geographic
region and subsequent measures adopted.
• Carry out vulnerability screening (slope stability, fault zones, abrupt changes in structural
stiffness etc) of the tunnel for ground failure potential. Based on results, identify sections which may
have risk of poor performance during a seismic event.
• Conduct detailed structural soundness assessment (using available information and data) for identified
critical tunnel sections which might have risk of poor performance and the water retaining structure of
• Based on site inspection and expert judgement suggest sample water quality testing plan to examine
chemical composition of the surrounding water to asses possible impact resulting from
infiltration/ingress of water containing chemicals (e.g. chlorides, sulphate or dissolved methane) and
swelling of soils; and review the sampling results and recommend remedial measures.
• Analyze the data and develop hydrodynamic modelling that considers seismic event for different fill
conditions to estimate worst possible scenario (including effects of water hammer effects and de-
aeration provisions) and most likely operational conditions. Also assess scenarios of possible
combination of events and their cascading impacts.
• Conduct hydrodynamic/ computational Fluid Dynamics (CFD) modelling for the tunnel headworks area to
simulate forces on the structures due to potential wave actions resulting from a seismic event using
• Suggest options including structural and non-structural measures based on scenarios and international
best practices to improve earthquake performance of tunnel as well as headworks, which will help in
reducing physical damage and economic losses in case of any seismic event.
2. Vulnerability assessment of headworks in case of major flash flood:
• Review past data and information related to flash floods, cloud bursts or stationary rainfall, heavy
rainfall, rapid snow melt, Glacial Lake Outburst Floods (GLOFs), landslides, rock falls or debris flows
in the upstream of Melamchi headworks areas.
• Investigate the hydrometeorological situation using outputs from applicable climate models, hydraulic
conditions and damage potential/risk of the flash flood.
• The expert should use suitable hydrodynamic/ CFD modelling tools (preferably freeware like HEC-HMS/RAS
or similar) for developing the critical scenario.
• Asses structural soundness (using available information report and data) of headworks structure
considering likely impact of flash flood including undercutting of foundation, and possible risk of
collapse of riverbanks, debris flows, debris deposits, river damming by debris, channel displacement.
Also assess scenarios of possible combination of events and their cascading impacts.
• Develop solution concepts which could include additional structural protection, slope stabilization
and erosion control measures, and non-structural protection/ mitigation measures.
• Define processes/ procedures to manage, minimize, and/or mitigate remaining risks.
3. Preparation of emergency standard operating practices and mitigation plan, including identification
of preventive operating practices.
• Develop or update a composite risk map, scenario maps of disasters and threat maps and damage and loss
maps and a tentative plan for restoration (in days, months) for identified scenarios. The plan should
suggest practical actions considering the resource availability and locational challenges.
• Develop guidelines for routine operation and maintenance, including preventive technical inspection
schedule and coverage to minimize the risk.
• Prepare emergency standard operating procedures (SOPs) and mitigation plan for tunnel and headworks
for forcible natural hazards.
• Assess need for developing a decision support system for operation and emergency management of tunnel
and headworks in case of geological or hydrometeorological event and their combinations and recommend
the scope for it.
• Conduct one workshops to share the findings and one day training session for line agencies staff on
emergency SOPS guidelines for routine O&M including preventive technical inspections.
MWSDB shall make available a reasonable office space and shared office facilities for the experts in
Kathmandu and in the field.
Detailed Tasks and/or Expected Output
He/she will help the team in data and report collection, follow up with clients and coordination, share
country specific knowledge and lead in organising Worksohp and training session for line agencies staff.
The expert will provide his/her expertise to produce following reports in collaboration with other
experts. The team of experts will submit to ADB and the MWSDB:
1. Existing Situation Study:
Presenting the findings of appraisals and assessments as per scope mentioned in 1 and 2 above, including
the review of reports, secondary data collection, initial assessment and methodology.
2. Draft Final Report
Draft documents of all tasks performed as per TOR including recommendations, maps, guidelines, SOPs and
mitigation plan and presenting main findings of the draft final report; and conduct one workshops to
share the findings.
3. Final Report
Final version of the above based on comments received from ADB and MWSDB and other relevant stakeholders
based on review of the draft final report, satisfying all requirements of the TOR. A comments-response
matrix should be submitted by the consultants demonstrating how comments were adequately responded to
and incorporated into the final report. The final report should include a brief note summarizing the
methodology and key findings to serve as a stand-alone advisory note for clients. The final report
should present all required information including all necessary maps/drawings and an executive summary.
The team needs to conduct one day training session for line agencies staff on emergency SOPs guidelines
for routine O&M including preventive technical inspections.
At each stage there will be a presentation to the Ministry of Water Supply and MWSDB.
Minimum Qualification Requirements
A master’s degree in civil engineering / water supply / public health engineering or equivalent, with
minimum 10 years’ experience in water supply systems. Experience in Kathmandu Valley will be preferred.
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