How the 1920s Beira Lake scheme was meant to lower flood waters
The Beira Lake with an area of about 175 acres is meant to be a flood water outlet when the Kelani water level rises. The Kelani bund on the north side is close to the river bank, thus protecting the area behind. But the south side bund is located far away from the river bank thus the low lying areas between the river and the bund get flooded when the water level rises, as seen in the recent flooding.
The St Sebastian north canal gates on Nagalagam Street are closed when the river water level rises above the canal water level, to protect the surrounding areas along with the bund. The St. Sebastian south canal is connected to Beira Lake at St. Sebastian Hill near the Court complex but the lake water level is higher than the canal water level. The Beira water level is six feet above sea level held by three structures namely St. Sebastian locks (now replaced by a wall), McCallum locks (near Lake House on the opposite side) and the semi circular shaped Galle Face spillway whose crest is six feet above sea level (located near the Presidential Secretariat).
The pumps at St Sebastian locks pump water from the canal into the lake and the water that flows into the lake system spills over at GF spillway when the lake water level rises above six feet. The essential condition is that the canal must be clean and not polluted otherwise the lake too would get polluted. This operation would lower the water level in the surrounding areas of the Sebastian Canal.
In the past there were extensive marshy lands around Kelani river like the Kimbulawela swamp to hold large volumes of water until it drained out to the sea.
When there are no adequate retention areas then excess water would fill up roads and other developed low lying lands until it drains out.
The Wellawatta outfall, Dehiwela outfall and Mutwal tunnel outlet flowing into the sea near the flour milling complex at Mutwal are the available sea outlets which are limited in quantity and water retention areas to hold water volumes temporarily.Water retention areas are an essential flood mitigation measure that must be looked at in the future.
To prevent flooding in low lying areas along the Kelani which are located between the river bank and the south side bund either the bund has to be relocated close to the river bank or the families have to be relocated. In addition it is essential to preserve water retention areas as much as possible.
The primary function of the Beira Lake scheme developed around 1920 is not to act as a flood relief but to assist in lowering flood waters by pumping at St. Sebastian pumps. D. Godage Via email
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