Expertise: Dam Stability
Expertise Group: Biological Sciences & Ecology
Consultant Source: International
TOR Keywords: Dam Stability
Objective and Purpose of the Assignment
Project Description
A. Rationale
- Country background. Samoa is a Pacific island country, classified as a small island developing state,
divided into two main islands (Upolu and Savai’i) and two minor outer islands, with a total population
of about 200,000 people. About 70% of the population lives on Upolu, the main island and location of the
capital, Apia, home to 50,000 or about 25% of Samoa’s population, situated at the Vaisigano River
catchment – the largest in Samoa. The river basin holds great economic importance. It is crucial to meet
the requirements of 60% of Apia’s drinking water supply. It provides clean energy from three small
hydropower plants (SHPs), with a total capacity of 4.7 MW (equivalent to around 11% of a full maximum
rated capacity in Samoa). Imported diesel generation plants still supply about 70% of the country’s
electricity. - Development Problem I: Flood Management. Samoa has experienced four strong tropical cyclones in the
past 10 years: Cyclone Evan in 2012, Cyclone Pam in 2015, Cyclone Gita in 2018, and Cyclone Zazu in - In addition to strong winds, these cyclones have brought extreme rainfall flooding the city and
neighboring villages. Climate projections for Samoa suggest that climate-related risks are already high
and could increase in the future. This can potentially undermine development progress in urban Apia,
where most of Samoa’s population and economic activity, and center of government, are located. One of
the main causes of flooding in Apia is the Vaisigano River, which flows through the urban area of Apia
and is Samoa’s largest river catchment. Major floods have occurred on more than 10 occasions, and the
most extreme flood happened due to Cyclone Evan in 2012. According to a Post Disaster Needs Assessment
report prepared by the government, Cyclone Evan caused a total estimated damage of over $200.0 million
in 2012 or about 25% of Samoa’s gross domestic product. - Development Problem II: Sustainable Water Supply. Another impact of climate change is a seasonal
variation of rainfall patterns, potentially threatening water security, especially in isolated small
island states like Samoa. The Vaisigano River flows are variable, and during an extended dry period,
there can be supply issues. Adversely, there are times when the turbidity of the water in the river at
the intakes is too high and, therefore, not suitable for the raw supply to the Alaoa water treatment
plant, which supplies 60% of Apia City’s water supply. Accepting this water has the impact of clogging
the slow sand filters within the water treatment process. - Development Problem III: Energy Security. Samoa’s energy security situation is weak due to high
reliance on imported fossil fuel to generate electricity (about 70% of the maximum demand of 28 MW [peak
demand for Upolu and 1.8 MW for Savaii] and electricity generation of about 170 gigawatt-hour). The
reliance on imported fuel is reflected in the high electricity tariff of Electric Power Corporation,
which, as of 1 February 2023, ranges from $0.18 (ST0.52) to $0.26 (ST0.67) per kilowatt-hour. To reduce
the high electricity tariff caused by high dependency on fossil fuel for power generation, the
government set a 70% renewable energy target for electricity generation through 2031. As of the end of
2022, about 30% of electricity consumption was being met by renewables. - Proposed support. To address the above-mentioned multiple development problems in an integrated
manner, the government has requested support from the Asian Development Bank (ADB) and Australian
Infrastructure Financing Facility for the Pacific (AIFFP) to finance the Alaoa multipurpose dam,
including the run-of-river SHP. The primary objective of this proposed project is to prevent Apia City
flooding associated with the rainfall on the upstream catchment of the eastern and eastern-central
branches of the Vaisigano River.* The secondary objective is to improve a seasonal water supply during
dry periods and reduce the turbidity of raw water taken into Apia’s main water treatment plant during
flood periods. The tertiary objective of the dam is to have additional hydropower generation of about
0.60 MW, which will support the government’s target of 70% renewable energy for electricity generation
by 2031. In addition, the project will enhance non-physical flood management systems and capacity and
the biodiversity conservation and management capacity associated with the proposed dam development. - Government’s policy and programs. To address the above-mentioned multiple development problems in an
integrated manner, the government issued the Pathway for the Development of Samoa, FY 2021/22–FY 2025/26
(PDS), among others, focusing on securing the environment and climate change. The government also set
the goal of 70% renewable energy use by the end of 2031 to address sustainable and affordable energy
supply. Moreover, with funding from the United Nations Development Programme and the Green Climate Fund
(GCF), the government has constructed flood banks and widened storm drains in Apia, including new wide
bridges. - Alignment of development plans. The project is a high priority for the government under PDS and will
support Samoa’s Second Nationally Determined Contribution target of achieving a 26% reduction of overall
greenhouse gas by 2030. The project supports the operational priorities (OPs) of ADB’s Strategy 2030 by
(i) addressing remaining poverty and reducing inequalities (OP1); (ii) accelerating progress in gender
equality (OP2); (iii) tackling climate change, building climate and disaster resilience, and enhancing
environmental sustainability (OP3); (iv) making cities more livable (OP4); and (v) strengthening
governance and institutional capacity (OP6). It will contribute to Sustainable Development Goal 7:
ensuring access to affordable, reliable, sustainable, and modern energy for all. The project aligns with
ADB’s 2021 Energy Policy objectives, highlighting the support for countries in building higher
resilience in the transmission and distribution subsector. The project also aligns with ADB’s Pacific
Approach, 2021–2025, as it will help improve power supply side efficiency and vulnerability to disaster
and the effect of climate change.
*Options to mitigate flooding of urban areas within the Vaisigano catchment were investigated by UNDP.
UNDP used the findings of the investigation to successfully access GCF funding to strengthen the
adaptive capacity of the reduced exposure to climate risks of vulnerable livelihoods and infrastructure
in the Vaisigano River catchment. The UNDP project supports both soft solutions (catchment management
programs, capacity building, early flood warning systems, and public awareness campaigns on flood
resilient building practices) and hard solutions (channelization of the lower reaches of the Vaisigano,
ecosystem solutions to reduce flows during extreme floods, upgrading of the Lelata Bridge to increase
discharge capacity, extension of floodwalls, and urban drainage system augmentation. The levee is
designed to retain the 1:20-year annual recurrence interval [ARI] event flood). In addition to UNDP’s
ongoing works, the hydrological design objective of the proposed project is to prevent flooding of
downstream power stations for floods of equal magnitude as observed during the Cyclone Evan flood in
- The Cyclone Evan flood was assessed through flood frequency analysis to be approximately a 1:150-
year ARI event.
B. Project Description
- The project is aligned with the following impacts: reduced impact of climate change and dependency on
imported fossil fuels for power generation as well as enhanced biodiversity conservation and management.
It will have the following outcomes: (i) increased resilience to climate change-related flooding, (ii)
increased water security, (iii) increased renewable energy for power generation, and (iv) reduced
biosecurity risks facilitating regional cooperation. The proposed project will have the following
outputs:
• Output 1: Multipurpose dam constructed for (i) flood retention, and (ii) seasonal water supply. This
multipurpose dam will be approximately 60 m high with an indicative storage volume of approximately 4.0
million m3;**
• Output 2: Small hydropower plant (SHP) generation capacity added. This will support the construction
of the run-of-river toe-of-the-dam (TOD) SHP, with an indicative installed capacity of 0.60 MW;
• Output 3: Project management capacity strengthened. This output involves (i) improving the capacity of
MWTI to undertake operations and maintenance of the dam and enhance community engagement, and (ii)
supporting MWTI on project management in line with international standards and best practices; and both
design and procurement activities, construction supervision, and development of operations and
maintenance manuals; and
• Output 4: Flood management systems and capacity and biodiversity conservation and management enhanced.
This output includes:
i. Flood management: (i) developing a comprehensive flood management plan for Apia City based on
hydro/hydraulic modelling; (ii) strengthening development and control and flood drainage management;
(iii) upgrading hydro-meteorological monitoring, forecasting, and disaster early warning systems; and
(iv) developing community-based planning, preparedness, response and evacuation and recovery actions.
ii. Biodiversity conservation, management, and monitoring: (i) implementing biodiversity management and
offsetting plans, including pilot-testing an endowment fund; (ii) implementing national guidelines for
environmental flows developed and environmental flow restoration plan implemented; (iii) reducing
biosecurity risks, including control of introduction and/or spread of invasive or alien species; (iv)
conserving natural habitat including the establishment of endemic flora nursery; and (v) conducting
capacity development for catchment protection and watershed management conducted.
**The storage allocation is as follows: (i) flood attenuation: 3.0 million m3; (ii) water supply: 0.5
million m3; (iii) dead storage: 0.5 million m3; and (iv) hydropower (run-of-river): 0.0 million m3.
***The non-structural measures will be selected from the range of measures set out in previous reports
on flood management for Apia (most recently BMT2021 and Entura 2020) and which include regulations
associated with land-use management in the most flood-prone areas of Apia and as related to the
Vaisigano catchment, early warning systems, gender sensitive evacuation routes, and evacuation centers.
Scope of Work
Act as the team leader of the independent panel of experts for ADB-funded Alaoa Multipurpose Dam Project
in Samoa.
Detailed Tasks and/or Expected Output
- Act as the team leader of the independent panel of experts for ADB-funded Alaoa Multipurpose Dam
Project in Samoa. - Review and comment on the feasibility study report and the draft bidding documents for the project.
- Verify and confirm overall general dam design, dam foundation analysis, and dam seepage analysis
considering the seismically active location of the dam. - Verify and confirm cost estimates of the dam structure.
- Suggest appropriate or relevant adaptation measures.
Minimum Qualification Requirements
- Degree in Civil Engineering (Phd/Dr.-Ing. or MSc/Dipl.-Ing.) focusing on Geotechnical Engineering.
- Minimum 20 years of relevant experience, preferably with overseas assignments and a minimum of 10
years of experience in dam design. - Previous experience in conducting an independent review of high dam projects as a member/team leader
of a panel of experts. - Profound knowledge in the field of geotechnical engineering and soil mechanics including advanced
slope stability and geotechnical foundation analysis, design of geotechnical structures, soil structure
interaction, deformation and seepage analysis for dams and foundations, codes standards, procedures,
safety and other regulatory requirements and guidelines. - Experience in evaluating geotechnical exploration and testing results, interpreting and
characterizing subsurface conditions and preparing reports, memorandums and geotechnical baseline
reports. - Skills in preparation of studies for technical due diligence, pre-feasibility and feasibility study
and design for tender and construction stages of large dams. - Knowledge of hydrology and hydraulics related to water retaining structures, incl. dam stabilization
analysis, dam instrumentation, and flow control structures. - Knowledge of international best practices, codes and regulations related to the design, construction,
analysis and maintenance of dams and other hydraulic and conveyance structures. - Deep understanding of designing large dams in seismically active regions.
- Good verbal and written English communication skills.
Minimum General Experience: 20 Years
Minimum Specific Experience (relevant to assignment): 10 Years
