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Rainwater runoffs from deforested slopes breach bunds
View(s):During two days in December, it rained so hard over Galenbindunuwewa that the water in the massive Kivulekada tank surged six feet overnight. Even the spill gates could not tackle such a swell, so irrigation officials cut away a section of the bund to let the torrents of water through.
“The water rose by the hour,” explained Divisional Irrigation Engineer Athula Wijewardena. “One day, we had 150mm of rain, followed by 100mm the next. This was one month’s rainfall!” Nobody had expected it to pour with such intensity over such a short period of time.
However, it was the second time in as many years that the bund of the Kivulekada tank gave way. “I have been a cultivator in this area for 30 years,” said H.G. Bandulasena, 54. “As farmers, we see a distinct change in the way it rains.”
“Before, it would pour fairly regularly over a month or two,” he explained. “But last year and this year, we got the same amount of rain within a day or two. It was not spread out.”
Such fierce downpours were also experienced in other parts of the country too. Deluges in Matale caused the Deduru Oya to surge. As water gushed downstream, it uprooted massive trees. These got stuck under the Korakahe Bridge on the Kurunegala main road. When the river burst its banks, the flow was around 15 feet steep in some places. The Sunday Times saw evidence of this near the Peddava village.
“It was like a tsunami,” said Gunaratne Banda. “The water was that high.”
“Normally, you get slight drizzling rain during the Maha season,” said Dr Wickramasinghe. “This year, there were two days of intense rain. The Victoria Reservoir was about 40% full before the rains. Do you know that within two days it started spilling?”
There is a change in the weather pattern, Dr. Wickramasinghe said, attributing it to climate change. When it rains with such force over a short period of time, much of the water goes waste. “It runs off without getting into the soil,” he explained.
There is change also in the pattern of land use. “Land is increasingly getting covered by either rooftops or cement,” Dr. Wickramasinghe observed. “Drainage channels are getting covered, because of road carpeting. The water doesn’t really get absorbed. It just flows over the surface, into channels and eventually to the sea.”
The better option would be for water to be absorbed into the soil. This is then gradually released during dry spells. Dr. Wickramasinghe predicted that Sri Lanka will experience similar flooding in future.
“Victoria Dam was commissioned in the 1980s,” he said. “During the first 10 years, it spilled over only a few times. Now it is spilling over more often, including this year and last year.”
Environmental scientist and chemist at the University of Peradeniya, Prof. O.A. Ilepuruma blamed flash floods and spilling irrigation tanks, and on inadequate forest cover which absorbs heavy rain like a sponge.
“For example, Elkaduwa in the Matale district got the highest rain fall of 300 mm in one day during the past rainy period,” he said. “The forests around Elkaduwa have now been cleared and the waters fill the Deduru Oya which originates in the eastern slopes of the Matale district. The result is excessive run-offs of rain water through the Deduru Oya which inundated many areas of the Kurunegala district, even up to Chilaw.”
Tanks were overflowing due to silting of reservoirs, he added. “All major reservoirs are silted to varying extents. There is reduced water holding capacities. The sluice gates have to be opened sooner than usual. This is due to the destruction of the forest cover in the catchment areas of the tanks.”
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