Dams indispensable but not without adverse impacts

By Eng. Thushara Dissanayake

Human civilization began along rivers in the world over, as water is an essential natural resource for people. These early river valley civilizations include the Indus valley, the ancient Egypt near the Nile river, the Mesopotamia civilization along the Tigris and Euphrates rivers, and the Chinese civilization along the Yellow river. Later on, with the increase in population and technological advancements thousands of dams have been built to meet the water needs during dry periods by regulating river flows.

Today, according to estimates there are 59,071 large dams all over the world with a total storage of 16,201 Km3 according to registered data of International Commission on Large Dams (ICOLD). A dam is categorised as large if the height is greater than 15m. A dam of height 5-15 m is also considered as a large dam if its storage capacity is more than 3 MCM. Accordingly, Sri Lanka has around 80 large dams, 270 medium dams and over 10,000 small dams.

Dam types concrete, earthen, rockfill and masonry. This marvelous engineering innovation has been very useful to humanity as it provides a wide range of social, environmental and economic benefits. Providing water for domestic use, agriculture, hydropower generation, and flood mitigation are some of the main functions of dams. However, these manmade structures are not without significant impact on the environment and society. Knowing these impacts are useful in mitigating them for the ecological sustainability and social wellbeing of a country. Although the impacts of large dams are studied extensively, those of small dams are often neglected. Following are some major and quantifiable impacts of dams.

Siltation behind the dam

A dam greatly alters the morphological processes of rivers and streams crossed by the dam. River water naturally transport the soil eroded in the upper watersheds by rain as sediment materials up to sea. When a dam is built across a river this sediment transportation process is completely blocked. This makes the river section downstream to the reservoir sediment-hungry and river bed erosion is the result. Thus, deepened river bed cause the ground water table to decrease. The result is either a decrease in water availability or a complete drying up of nearby domestic or agro wells. These issues are further exacerbated by sand mining in rivers.

The formation of meandering river sections and oxbow lakes occur downstream many large dams in Sri Lanka due to lack of downstream sediment transport. In low lying agricultural areas, this change of river course has become a worrying issue for land owners. Further, bridge foundations in the river are exposed due to bed erosion threatening their stability. On the other hand, deficiency in sediment transport to the sea causes beaches to vanish and result in coastal erosion.

Increased flood risks

Most of the time reservoirs are considered flood mitigation structures. However, during continuous rains the emergency flood gates of the reservoirs are opened for their own safety. If the downstream part of the reservoir is also receiving high rainfalls the opening of flood gates would create excessive floods due to above – normal – flow capacity in the river endangering people and property in a far worse manner.

Reduction of downstream river flow

Reduced river flows increase the salinity level of river water closer to coastal areas impacting drinking water extractions. This has to be overcome by the construction of costly structures such as salinity barriers across rivers. The salinity intrusion can also contaminate ground water aquifers.

Dams regulate flood waters and as a consequence flood plain characteristics are altered. Generally, the soil eroded by the rain is the topmost soil layer of the river’s watershed and it is rich in nutrients. Hence, floodplains will be deprived of such nutrients as long as floodplains are not inundated by flood waters affecting agricultural activities carried out in flood plain areas. The lack of nutrient rich riverine sediments alters the ecologies of river deltas, estuaries, coastal wetlands and marine environments as well.

Block of passage for migratory animals

Migratory animals, especially fish travel along the river to upstream areas for breeding. On the other hand, the species in the food chain that depend on these migratory animals are negatively impacted when their passage is blocked by a dam.

Land inundation

With the construction of a dam a large area is inundated. For instance, the inundation area due to the construction of Senanayake Samudra, the largest reservoir in Sri Lanka by volume, is around 7,689 hectares. Not only bare lands, sometimes, inundation of townships, wetlands, forest and agriculture areas are also possible if permitted by proper environmental assessment processes.

Production of greenhouse gases

Today, the world is gravely concerned about climate change, which is known to be a consequence of global warming due to increasing greenhouse gases in the atmosphere. Often, carbon dioxide emissions from the combustion of fossil fuels are considered the major contributors for global warming. Yet, reservoirs also contribute to greenhouse gas emissions by releasing methane which is a high potential greenhouse gas. Methane is produced in reservoir beds from the decomposition of the plant materials primarily inundated after the reservoir construction.

Social Impacts

Dams have caused displacement of a huge number of people around the world as an adverse impact due to inundation of public properties. Most of the major dams in Sri Lanka were constructed many decades ago without facing any severe resettlement issues, whereas future dam construction works would not be so, due to urbanisation and increased population density. Further, dam construction has resulted in conflicts of water sharing specially in transboundary river basins. Being a small country, Sri Lanka is fortunate not to have such conflicts except for the Mahaweli Ganga, where minor conflicts are on the verge with ongoing developments. Loss of cultural heritage assets is another issue which could be more sensitive to people.

Dam failure risks

If a dam failed, it would be a huge disaster to people living close by and their properties. Dam failures could occur due to myriad reasons. Overtopping, piping, slope stability and settlement are the common modes of failure, though at times, exact reason is unknown.

In January 1986, Sri Lanka experienced the breach of the Kantalai dam in the Eastern Province. The disaster resulted in 127 deaths and affected 10,864 residents. The number of houses destroyed was 1,200, besides significant damage to agriculture and infrastructure in the area. Apart from that, massive floods in December 1957 caused failure of 35 large dams, 53 medium dams and 1500 small dams in the country within few days. The Banqiao dam failure of China, in 1975, is considered to be the deadliest of its kind ever. It destroyed 171,000 lives and displaced millions of people.

Final remarks

It is not wise to assume that all engineering innovations are sound and safe. The way science and technology evolved throughout the history of mankind has posed many threats to the sustainability of the world. Hence, enhancing the positive impacts of such technological creations and taking action to mitigate the negative impacts to the best possible level are the responsibilities of those who are involved in these endeavours.

(The writer is a chartered Civil Engineer. He can be reached through dmtsdissa@yahoo.com)