Coastal reservoirs: an untapped resource?

Aerial view of Singapore's Marina Bay © Nikada/ iStock

Coastal reservoirs have the potential to harness water for coastal communities that would otherwise flow to the sea. Shu-Qing Yang and Muttucumaru Sivakumar describe the benefits.

Land-based reservoirs are used widely around the world, but many existing dams are ageing, and few new dams are being built. Research by the United Nations University reveals that: “Ageing water storage infrastructure slowly grows into a significant global development issue. Thousands of large dams built in the middle of the previous century have already or will soon exceed the age of 50 years… and many are approaching 100 years.” By 2050, most of the global population will be downstream of dams. The research states that when most existing large dams are 50 to 100 years old, access to fewer reservoirs will make it more difficult to achieve SDG6 (to deliver clean water and sanitation). The problem of ageing infrastructure is most prominent in the USA, where 80% of dams were already more than 50 years old in 2020.

Meeting demand

Historically, cities have been located on riverbanks and coastlines to harness the benefits these locations provide in terms of transport, food and ecology. Products traditionally flow into cities through their ports, followed by people migrating to these coastal areas. Eight of the 10 largest cities in the world are located by the coast. The United Nations Atlas of the Oceans shows that “44% of the world’s population lived within 150km of the coast in 1950. In 2001, over half the world’s population lived within 200 km of a coastline”.

If coastal water demand can be met by other means, then the inland water infrastructures may easily meet the inland water demand. The difficulty is that the most populous coastal areas have few dams, and the existing dams are ageing. For this reason, other water supply options, such as desalination plants and wastewater reuse, are being adopted in several coastal cities.

Taikan Oki and Shinjiro Kanae published their research in Science in 2006 and assessed that globally, rivers discharge about 45,500km3/year of freshwater into the sea, while annual domestic water consumption is only 380km3/year, and agriculture and industry use 2,660km3/year and 770km3/year, or 0.8%, 5.8% and 1.7% of the freshwater runoff lost to the sea, respectively. If only a small proportion of the freshwater runoff usually lost to sea could be stored in the sea, then the coastal area should have sufficient water to meet all its water needs.

The definition

The definition of a coastal reservoir (CR) is a type of water storage inside a body of seawater where the stored water is different from the seawater outside in its physical, chemical or biological characteristics. These can be broadly divided into two groups, i.e., freshwater and seawater coastal reservoirs. The seawater coastal reservoirs can be used for tidal power generation, cooling water supply to power plants, or fish farms, for example. While freshwater coastal reservoirs have been used in many countries, such as Zuider Zee in the Netherlands, Lake Alexandrina in Australia, Saemangeum in South Korea, the Marina Barrage in Singapore, Plover Cove in Hong Kong, Thanneermukkom Bund in India, these are using first generation technology.

The first generation CRs use a river course or estuary to store the freshwater and a barrage is installed to separate the stored freshwater and the seawater. First generation CRs cannot select the desired water for storage.

The first implementation of second generation CR was constructed on the Yangtze River estuary in Shanghai, China. Second generation CRs have the ability to select incoming water for storage. Like dams, the CRs can have many design purposes, providing water supply, seawater intrusion control, flood disaster mitigation, agricultural development, and land reclamation, including urban regeneration. The Qingcaosha coastal reservoir is the largest one, with the capacity for delivering 7500 ML/d to the city of Shanghai.

Promoting the concept

The International Association for Coastal Reservoir Research ( was launched to promote the concept of coastal reservoirs in 2017 in Kuala Lumpur, Malaysia. IACRR’s mission is to supply sufficient, good quality and affordable freshwater to coastal communities with minimum environmental and negative social impact. It is dedicated to advancing all aspects of coastal reservoirs and promoting the sustainable development and management of surface water that is otherwise lost to the sea. Its 25 founding corporate members come from universities, research institutions, consultants, financial and construction companies. Its management committee includes past presidents of IAHR (hydraulics), IAHS (hydrology), IWRA (water resources) and IWA (water).

In 2018, the world’s first coastal reservoir workshop was held at the University of Wollongong, Australia. Its theme was ‘Innovative solutions to the UN’s
Sustainable Development Goal 6’. A total of 80 international representatives from universities, water planners from government departments, and practitioners from industries, participated in the various keynote sessions, technical sessions and the lead-up to a ‘Water Leaders Forum (WLF)’. The forum concluded with a signing of the ‘Wollongong Declaration on Water’ endorsing the proven concept of coastal reservoir development worldwide.

A clear message from the WLF was that around the world, cities appear to be running out of water. But in reality, it is the water that is running out of the cities. A coastal reservoir has many advantages to inland dams, including minimal land acquisition, no land submergence, no reduction of environmental flows, and no habitat or forest submergence. The inlets and outlets for coastal reservoirs can be designed in such a way that only good quality water enters the reservoir, with very little stagnation of impounded water. Innovative designs can be adopted, such as smart sensor networks, to operate coastal reservoirs more efficiently.

In 2019, IACRR organised an ‘International Workshop for Coastal Reservoirs in Earthquake Active Areas’ in National Cheng Kung University (NCKU). SDG6 is more challenging for earthquake active areas like Chinese Taipei, which is blessed by plentiful rainfall, triple the world average. However, the shortage is not water, but storage, because the earthquake risk makes large-scale inland dams highly unlikely. This workshop provided an opportunity for water professionals to assess the feasibility of coastal reservoirs in high-risk areas.

In 2020, the Ministry of National Development, Indonesian Engineers Institute and IACRR jointly organised a workshop on coastal reservoirs. Its theme was ‘Coastal security, water availability and sustainability’. Indonesia has a long history of water shortage, land subsidence, water pollution, flood disasters, climate change and sea level rise. Consequently, the government is planning to move its capital from Jakarta to Kalimantan. After their preliminary assessment, the Minister, Dr Suharso Monoarfa, stated that a second generation coastal reservoir would be an ideal solution for the Northern Java Island.

Worldwide opportunities

Coastal reservoirs provide a paradigm shift in water resources development. Rather than storing water in mountainous dams that capture water from a small portion of the catchment, water can be stored in an estuary or sea from 100% of the catchment area. One of the most significant advantages of coastal reservoirs is that natural environmental flows can be maintained and only excess flood water is captured at the end of the catchment for mitigating flood disaster and developing water resources simultaneously. This is a nature-based water solution and also a tool to help achieve the UN’s strategy to make every drop count.

Atlas of the Human Planet 2017 shows that globally a third of people in the world are exposed to earthquakes. This has almost doubled in the past 40 years. In 2015, more than 400 million people lived near one of the 220 most dangerous volcanoes subject to possible eruptions. The Pacific Ring of Fire includes countries such as Indonesia, New Zealand, the Philippines, Japan, the west coast of America, Chile and Peru. All dams in this most powerful ring are looking like the ‘Sword of Damocles’ to the people living in downstream coastal areas, but coastal reservoirs can provide water sources without any risk to the people living in earthquake regions.

Australia is the driest inhabited continent in the world, with 70% of the country either arid or semi-arid. Its central areas receive an average rainfall of 250mm/year or less. But Australia’s coastal areas are wetter (some coastal regions received 1000mm/year or more) relative to the inland areas, thus, coastal reservoirs in Australia are more suitable than dams to develop its water resources.

Africa is the world’s second driest continent, with 60% of land surfaces classified as dry lands and deserts. Coastal reservoirs offer an ideal solution for Africa’s water resources development. To illustrate this, in 2018, Cape Town became the first large city in the world to come close to a ‘Day Zero’ water crisis, i.e., the tap water runs dry. A close look of the runoff data in Cape Town shows that the city is not running out of water, but water is running out of its rivers. By collecting 5700GL/year of runoff from Orange River, Namibia and South Africa’s west beach region could be nourished.

In Asia, most megacities are situated on the coast, such as Kuala Lumpur, Singapore, Hong Kong, Shanghai, Beijing, Hanoi, Bangkok, Dhaka, Chennai, Mumbai, Karachi. Coastal reservoirs could be the solution for them.

The message is that development of coastal reservoirs has huge potential to help achieve clean water for all.

More information

The full recording of the proceedings of the 2020 workshop can be viewed in this web link.

IACRR and Hohai University are jointly organising the First International Conference on Coastal Reservoirs and Sustainable Water Management in Nanjing, China, 6-9 November 2023 (, which will include a field visit to the Qingcaosha coastal reservoir. This conference is also supported by IAHR, IWA and CIWEM.

IACRR published its first book with Elsevier in 2020: Sustainable Water Resource Development Using Coastal Reservoirs.
IACRR published its second CR book with Elsevier in 2022, Coastal Reservoir technology and applications.

Shanghai solution

The Qingcaosha coastal reservoir has sensors to monitor the quality of incoming river water and seawater. Its intake gate is open when high quality water surrounds the coastal reservoir; otherwise it is closed. The selected water inside the coastal reservoir flows about 20km from its intake to its outlet. Sedimentation in the reservoir further purifies the raw water.