Coastal reservoirs: A paradigm shift in the development of water resources

Today, the shortage of freshwater is a worldwide problem and Sri Lanka is without exception. According to the National Water Supply and Drainage Board, 8.4% of the population still has no access to safe drinking water. The figure will further increase with the increase in population and migration of the rural population to urban areas, which is another salient demographic change. 

This is the prevailing situation of drinking water requirement alone. Agriculture is also facing a grave situation in spite of the availability of several water supply sources such as surface storage reservoirs and river systems. In addition, rainwater is directly used for agriculture mostly in the dry zone. Groundwater too is used for both drinking water supply and agriculture. Strategies like the reuse of wastewater are not common in our country yet, probably due to the lack of technology. When agricultural and other water needs are taken into account, meeting all these demands with available water sources will be far beyond reality. 

Meanwhile, the unused water flowing to the sea is estimated to be approximately 25% of the total annual runoff of the country. As such, the country needs to utilise that water sooner rather than later to meet ever-escalating domestic water demands. At the moment, many proposed reservoirs in the country are at a standstill due to land acquisition issues and probable negative environmental impacts. Further, finding new dam sites are becoming difficult owing to similar issues. 

On the contrary, many countries have embarked on developing coastal reservoirs as a solution to their water crises – a paradigm shift in water resources engineering. This novel water resource strategy would be a promising solution for Sri Lanka too, and this article discusses its appropriateness under the Sri Lankan context.

What is known as a coastal reservoir?

A coastal reservoir is built by constructing a dam in the sea close to the river mouth. The reservoir stores the freshwater otherwise carried by the river to the sea at its lower end. The dam should be so constructed as to act as an impermeable barrier between the fresh river water and the salty seawater. 

Zuider Zee in the Netherlands built in 1932 is considered the first coastal reservoir in the world. Already, there are many first-generation coastal reservoirs constructed by several countries, and among them are Saemanguem in South Korea, Marina Barrage in Singapore, Qingcaosha in China, Plover Cove in Hongkong, and Thanneermukkom Bund in India. Saemanguem reservoir, completed in 2010, has a storage capacity of 530 million cubic meters (MCM) being the largest coastal reservoir capacity-wise in the world. In fact, the capacity of the Saemanguem reservoir is little more than half of the capacity of the Senanayake Samudra reservoir in Sri Lanka. Senanayake Samudra being the largest reservoir in Sri Lanka has a capacity of 950 MCM.

Advantages of coastal reservoirs

One of the best advantages of a coastal reservoir is that it can capture every single drop of runoff water flowing in the river to the sea. Unlike inland reservoirs, water will be abundant for coastal reservoirs, as it covers the entire catchment of a river. 

Construction of an inland reservoir needs an ideal set of hydrological, topographical, and geological conditions. Further, many social and environmental implications have to be carefully addressed. In contrast, for the construction of a coastal reservoir, the land requirement will be negligible and no land acquisition is required. Similarly, damages to ecosystems can be greatly prevented. There will be no conflicts among water users when compared with inland reservoirs, as there are no downstream water users.

It is envisaged that more and more people will migrate to urban areas in the future. According to the geography of the country, many urban areas are located in the sea belt. When such migration and population growth are taken together, water requirements in the coastal areas will be massive and difficult to meet with the available sources. Therefore, coastal reservoirs would be the ideal solution for meeting this water requirement in the future. 

Disadvantages

oastal reservoirs may collect comparatively a higher amount of waste and contaminants transported with water from the catchment compared with that of an inland reservoir. In other words, the water quality of a coastal reservoir may not be that good. Therefore, a costly treatment process will be needed to make the water suitable for drinking. Or else, water will have to be used for only other purposes except drinking. In addition, for the supply of water from a coastal reservoir, pumping may be necessary as the demand sites are usually located at higher elevations. Hence, this additional cost of water pumping can be regarded as a disadvantage. Further, many inland reservoirs are capable of mitigating floods, whereas coastal reservoirs are unable to serve that purpose as they are located at the end of river catchments. 

Opportunities for Sri Lanka

As a consequence of climate change, we are experiencing a change in rainfall pattern and that further increases the amount of unused water flowing to the sea. In the meantime, we have got only a few potential locations left for the construction of surface storage reservoirs. Until the country embarked on the construction of the port city project many might not have thought that a sea area could be converted to a land area. Similarly, there can be plenty of opportunities to build coastal reservoirs on the coasts of Sri Lanka. So the best strategy for developing water resources in the future would be building coastal reservoirs where possible, given the fact that we have many rivers radially flowing through the country creating favourable conditions for such projects.

(Eng. Thushara Dissanayake can be reached at [email protected].)