Expertise: River Morphology Expert
Expertise Group: Engineering
Consultant Source: International
Objective and Purpose of the Assignment
- ADB approved Loans 1820 and 3110-NEP: Melamchi Water Supply Project (MWSP) worth $145.0 million to
the Government of Nepal (GoN) to alleviate acute water shortage in Kathmandu Valley through an inter-
basin water transfer from Melamchi river. The project is implemented by the Melamchi Water Supply
Development Board (MWSDB). The project was scheduled for completion in 2021. A major component of MWSP
is the Melamchi River Water Diversion subproject (MRWDS) which comprise (i) headworks, including an
ungated weir and intake with gravel flushing system and de-sanding basins, (ii) alternative diversion
system, and (iii) water diversion tunnel of 26 km length. The project reached advanced implementation
stage with major part of the construction of the headworks structures and installation of
hydromechanical equipment and instrumentation completed. Trial run of the tunnel was successfully
completed on 27 March 2021. Post-successful trial run of the tunnel, MWSDB retained water in the tunnel
to commission the newly constructed water treatment plants (WTPs) and transmission system under
Kathmandu Valley Water Supply Improvement Project. The dewatering of the tunnel started on 15 June 2021,
and MWSDB targeted to complete the dewatering of the tunnel, physical inspection, the remaining works at
headworks and the tunnel, including any rectification works identified during tunnel inspection, and
commission the MRWDS in totality by September 2021.
B. Flood events and initial damage assessment
- On 15 June 2021, prolonged rainfall and major landslides in the higher reaches of the Melamchi river
catchment area resulted in a catastrophic flash flood mixed with mud and rocks in the Melamchi river.
Initial assessments of the event through satellite images and aerial surveys pointed at the floodwaters
and sediment debris flows being generated by breaching of one or a series of natural dams created by
rain-induced landslides in the catchment. The flood changed the river morphology, inundated the
headworks area and buried the MRWDS headworks structures under a thick deposit of sand, gravel and small
to huge boulders and debris in the entire area, including the alternative water diversion tunnel and an
Adit further downstream. It further aggravated the existing landslide on the left riverbank immediately
upstream of the headworks; and washed away the coffer dam and permanent bridges at the headworks and the
Adit as well as the civil and hydromechanical contractors’ camps and facilities on the left riverbank
downstream of Adit. Photographs of exposed parts of the structures show severe damage to the concrete
and reinforcing bars. The condition of the civil structures, the hydromechanical installations and
instrumentation installed at the gates and in the machine rooms are submerged under the boulder deposit
and debris is unknown.
- The river experienced another large flood and debris flow a few days later. Visuals from aerial
surveys show the presence of a large sediment pile near the Melamchi headwaters which was possibly
formed by an ancient landslide dam or fresh glacial flows. This pile is collapsing rapidly with big
slumps under successive rainstorm events. The visuals also indicate heavy erosion along the river
channel and extensive collapse of the V shape valley walls. It is anticipated that heavy rainfall will
bring along further major floods and debris flows along the river. Access roads, bridges and public and
private property lying downstream of the headworks area were also damaged by the floods, and access to
the headworks site is possible only by air or on foot. Motorable access to this area can be established
only after the monsoon (September – October, 2021).
C. Initial Hypothesis of Damage
- Based on assessment of damage from similar events in Nepal and elsewhere and limited visual evidence,
some likely damage can be hypothesized. It is possible that the rolling and pounding action of large
boulders, as witnessed during the present floods, could have resulted in abrasion, cracking and
fracturing of structural concrete; and possibly exposing steel reinforcing bars to corrosion, surface
abrasion or even rupture. Deformation of the concrete structures is also a possibility. The high
velocity river flows during the flood could have scoured or eroded the foundation soils of the gravel
trap or de-sanding basins, resulting in partial loss of foundation. Damage to hydromechanical
installations of the headworks in the form of cracks, abrasion, deformation or rupture of the metal
parts of gates, their sill plates and embedded parts and their operating mechanisms is likely.
- The collapse of the sediment pile near the headwaters, natural damming of the river and the
subsequent flood flows following breach of the dam(s) are expected to have impacted the river
morphology. Prior to their breaching, the natural dam(s) would have also caused deposition of sand,
gravel and boulders on their upstream. The large volume and high velocity flows in the river following
the dam breach is likely to have eroded or accreted the river bed and banks and likely resulted in
significant sediment depositions in reaches where the flows slowed down (as observed in and downstream
of the headworks area). Thus, there is a distinct possibility that a considerable amount of aggradation
and degradation has occurred in the riverbed and banks.
- Like most Nepali river catchments, the Melamchi River catchment is characterized by the presence of
glacial lakes, fragile geology and natural and human-induced disturbances, making it susceptible to
natural damming, and sediment morphological issues (deposition and erosion). Hence, there is risk that
such events may reoccur.
D. Objective of Consulting Services
- The MRWDS is the lifeline structure for Kathmandu Valley residents, and, therefore, its long-term
sustainability against natural events like earthquakes and floods is of utmost importance. The recent
floods and debris flow in Melamchi River have demonstrated the vulnerability of the MRWDS to natural
events that had not been anticipated earlier. Therefore, identification and implementation of long-term
sustainability measures for the MRWDS is essential. Equally important is the assessment and treatment of
the damage that the recent events could have caused to the headworks structures, Ambathan adit and
alternate diversion systems so that these structures can continue to function as per their design intent.
- In consideration of this, ADB plans to engage a multi-disciplinary team of international and national
experts (individuals) to carry out the following field surveys/ studies/ assessments on the Melamchi
watershed and the MRWDS structures:
a. Identify the causative factors for the recent flood and sediment debris flows in the river.
b. Conduct detailed assessment on the impacts of the flood and debris flow events on the river
morphology, riverbed, and valley slope stability and the MRWDS structures.
c. Study the feasibility of long-term measures for remediation/stabilization of the river morphology and
recommend most appropriate solutions, which are also cost effective.
d. Identify land instability and erosion hazards in the Melamchi catchment area (upstream of the
Headworks area) which could impact the safety and functioning of headworks and study the feasibility of
measures to control them and recommend most appropriate solutions, which are also cost effective.
e. Conduct detailed damage assessment of the MRWDS headworks, including its civil structures,
hydromechanical installations and instrumentation, and develop short, medium and long-term measures for
proper hydraulic functioning and structural rehabilitation and strengthening.
f. Undertake climate change analysis based on different models and assumptions and identify its impact
on the flow pattern and flood.
g. Discuss alternative intake design options or upgrades that may be required to adapt to the upstream
catchment degradation and ensure resilience to future floods.
h. Propose at least 2 comprehensive strategic options for the medium/long term integrated
recovery/adaption of the MRWDS (mainly for intake and upstream catchment) with ranking based on cost,
resilience, technical complexity, etc.
Scope of Work
The detail scope is presented in the attached Terms of References file.
Detailed Tasks and/or Expected Output
The detailed task/expected Output is presented in the attached Terms of References file.
Minimum Qualification Requirements
The detailed qualification requirement and job responsibilities are presented int the attached Terms of
Minimum General Experience: 15 Years
Minimum Specific Experience (relevant to assignment): 15 Years