A community based initiative in Sri Lanka to keep our natural environment clean

Thalangama Wetland Watch

The Thalangama Area


Intended Work Items Under Improvements to Madiwela East Division – Part 1
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The Thalangama (Talangama) tank had its original spillway (steel gate/weir) at its northern end which drains direction Parliament lake, and received a new larger spillway on the right (eastern) side with a link canal to the Averihena tank and eventually Kelani river. The purpose of this new spillway and link canal is to convey the surplus water from the Thalangama tank via the Averihena tank into the Madiwela East Diversion canal to reduce the flooding risks in the vicinity of the parliament. The Madiwela East Diversion canal starts at the tail end of the Avarihena lake and runs towards Athurugiriya Road and as from there as concrete canal up to river. The main purpose of the 2017/18 dredging and construction activities in the lake area served the rehabilitation of the existing irrigation and drainage system. An overview map showing the rehabilitation work components is presented on this page and an article by the Sunday Times with a drainage map of Colombo, showing the link between the Thalangama Lake and the Parliament is here.

Because of siltation and encroachment the original capacity of the Thalangama tank, which was 50-60 Ac.ft. (61,000-74,000m3) has decreased to 32 Ac.ft. (39,000 m3) causing a water deficit for the farmers, and limiting the area’s flood retention capacity. Although there exists different opinions about the exact numbers, the need for lake desilting and removal of excess vegetation (especially parts of the invasive Wel Atha trees) to increase the water storage and irrigation capacity of the lake was of mutual agreement by all stakeholders, considering however, that the environmental protection status of the area demanded a careful and balanced approach.

After the stop of the dredging at the end of 2017, the construction work continued along the Averihena tank. Local residents at Lake Road wonder how far the dredging has affected local biodiversity. An obvious impact has been the release of plant nutrients from the moved and removed sediments and trees into the water, resulting in a rapid spread of nutrient-loving water hyacinths on the central part of the lake. This has been supported by the dropping from large bird colonies resting overnight on the Wel Atha tree islands of the lake.

The dredging in 2017 reclaimed a significant lake area which was overgrown with trees while preserving a part of them to maintain local biodiversity and night resting places for birds.

Celebrating the end of the work and rebuilt road along the lake after the trucks and caterpillars left in April 2018.


2019: The construction work along the lake was re-launched with a new contractor. The work dates back to the original plan (see map above) to install a new flood gate next to the circular spillway. The intention is that based on rain forecast the flood gate will automatically open to release water from the tank through two large concrete tubes towards the Averihena lake and eventually the Kelani river. This will allow the Thalangama lake to absorb more water under extreme rainfall events and in this way prevent potential flooding.

2020: Regular updates on the new flood gate can be found under “News from the Lake”. The work was finished in October 2020.