Environmental and social impacts of reservoirs: Issues and mitigation: A case study of the proposed drinking water reservoir in Per Aru Vavuniya

Abstract

Dams and reservoirs have played a key role in economic development serving a variety of purposes, including electricity generation, flood control, irrigation and drinking water. However, there is a growing concern that dam projects cause irreversible environment change, which are often complex, multiple and essentially negative. Among the ill effects on environment, settlement of suspended particles limiting storage capacity, limiting the flow of sediments downstream thus limiting nutrient rich sediments to floodplains and leading to scouring of the riverbed, impact on biological diversity are some of the physical and ecological impacts. On the sociological side, large dams have enormous consequences for people’s lives and livelihoods which include displacement and resettlement.

The case study in this article is the construction of a reservoir across Per Aru River in Vavuniya District, Sri Lanka for the purpose of providing drinking water by the National Water Supply and Drainage Board. The earth dam is 10.5 m high and 3 km long. The capacity of the reservoir at the full supply level of 71m is 3.85 MCM and the reservoir area is 238.74 ha. The rate of abstraction is 6700 cum/day. The land use of the affected area is dry mixed evergreen forest and scrub forest (168 ha), paddy lands (65.87 ha) and uplands (14.94 ha). There are no houses in the reservoir area.

Among the major ecological impacts, submergence of the lands including forest (riverine, dry mixed evergreen and scrub forest) amounting to losing 279 species of plants can be mentioned. Among them there are 25 endemic species, 4 vulnerable species and 3 nationally threatened species. 316 faunal species had been identified in the affected habitats including 28 endemic species, 3 proposed endemic species, 3 critically endangered species, 1 endangered species, 11 vulnerable species and 19 nearly threatened species. However, most of them are found in similar habitats in the locality. The most affected are the aquatic fish. There are 3 endemic fish species in the river (Flying barb/Bearded rasbora, Lanka labuca and Filamented barb) and a nearly threatened species (Climbing perch). Due to the reservoir loading and the reduced flow to the downstream river they will be much affected. The reservoir is to be located across a proposed jungle corridor and therefore there would be an impact on the migration of fauna especially elephants although the availability of water for most part of the year will be beneficial. With regard to the social impacts, 130 claimants will lose their cultivations (paddy and uplands). One kovil will also be submerged.

The above ecological and social impacts have been mitigated in the Project to minimize the adverse impacts. An environmental flow will be maintained which varies from 0.1 to 0.4 MCM throughout the year based on the natural flow pattern, a fish pass in the dam will facilitate fish migration downstream. A compensatory afforestation programme has been planned including planting/enrichment of 100m buffer zone from FSL which will also include plant species which will be lost due to the project. River reservations and areas in the upper catchment will also be enriched. The reservoir will not be fenced to facilitate the animals to use it too.

With regard to the mitigation of social impacts, alternative lands will be provided to the farmers who will lose the land in a location preferred by them. This will be done according to the National Involuntary Resettlement Policy. The progress of the mitigations will be monitoring closely by adhering to the Monitoring Programme.

Therefore it can be concluded that the concerns and adverse impacts of dams can be minimized or eliminated by careful planning and design that incorporate public involvement and input early in the process. The challenge for the future will be the utilization of dams and reservoirs for the wise management of the world’s water resources as part of each nation’s social and economic development goals.