The massive water leak of the Aluth Diwulwewa tank has been brought under control. Huruluwewa Divisional Irrigation Engineer Drasun Thilakarathna told the Sunday Observer that there were 37 medium tanks and two major reservoirs, Huruluwewa tank (55,000 acre feet) and Manankattiya tank (4,876 acre feet) are in the Huruluwewa Irrigation Division which includes Palugaswewa, Galenbidunuwewa, Kahatagasdigiliya and the Horowpathana Divisional Secretariat divisions.
He said that the largest number of medium tanks in the country is within the Huruluwewa Divisional Irrigation jurisdictions. The Aluth Diwulwewa tank is the second largest medium tank in the Huruluwewa Division.
The Aluth Diwulwewa is one of the oldest medium tanks in the Huruluwewa irrigation division with a water capacity of 7,000 acre feet. On January 10, we had to open all four sluice gates since the water capacity of the tank increased up to 6,960 acre feet due to heavy rains. The water controller and his men having opened the four spill gates had gone to the sluice gate about 500 metres away from the spill to close it, since no more irrigation water had to be released through the sluice opening. It was then that they detected the unnatural whirling movement of tank water in the sluice area creating an emergency in the vicinity.
Irrigation officials, villagers, the police and Army personnel strengthened the collapsing tank bund at vulnerable areas of the tank to avert an impending danger where the tank bund could have burst at any moment. Nearly 5,000 sand bags and 50 loads of boulders were used to minimise the uncontrollable pressurised water flow from the tank which could have caused a major flood in the area and irreparable damage to the tank, Irrigation Engineer Dasun Thilakarathna said.
Residents evacuated
Around 70 families residing in the risky area were evacuated to the Gangasiripura Indrajothiyaramaya temple premises for temporary shelter. Nearly 300 acres of paddy fields cultivated in the Maha season with the waters of the tank were protected due to the timely intervention. The water level had dropped to 3,127 acre feet. When the disaster occurred, the height of the water level stood at 24.4 feet out of 25 feet, the maximum, holding a water capacity of 6,900 acre feet, 100 acre feet less than the maximum water capacity of the tank.
The tank bund was cut at two places to minimise the risk of increasing high water pressure damaging the bund, close to the South bank sluice and at the emergency braking area called Gampaha Sorowwa. The Irrigation Department’s Engineering Geology branch officials visited the vicinity of the site to test whether there were any more risky areas in the tank’s interior including the bund side. The Departmental design branch (North and North Central) group of engineers visited the tank for observations and re-designing.
The Chief Engineers, Vallipuram Thavakumar, D.G.K. Kamil and Prageeth Waliwita told the Sunday Observer that most of these sluice barrels had been constructed hundred years ago. The sluice tower (vertical part of the sluice) is almost six-decades old. These structures are now ageing and need to be reconstructed. The material used for the construction of sluice barrels may have decayed and weakened. In the Aluth Diwulwewa case, the sluice barrel nearly collapsed suddenly and all earthen materials around the barrel could have been washed out and as a result, a vacuum would have been created towards the upstream slope of the sluice location. The top surface of the earthen bund collapsed and a sinkhole appeared. This was the first observation by farmers in the area on January 10 early morning. They informed the Irrigation Department officers and other relevant agencies and authorities.
Water level reduction
Immediate action was taken by the Irrigation Department with the support of the Army, villagers and other agencies to seal the sinkhole cavity. While sealing it, the Irrigation Department took steps to reduce the water level by cutting the emergency sections of the bund soon after it was rectified. Later, another sinkhole emerged adjacent to the previous sinkhole, and it was also sealed.
According to the Chief Engineers, of 170 medium irrigation schemes such as Aluth Diwulwewa, 89 schemes are under the purview of the Anuradhapura District Irrigation Engineer’s Office. Many medium irrigation schemes have aged and the structures weakened. There was no medium tanks’ rehabilitation program to address these disadvantages even after four decades.
The sluice barrel section of the earthen dam is the most vulnerable for the formation of a sinkhole, due to some vibration effects due to the water flow and the time lapses is one of the main reasons for these types of severe damage. Another reason is the weak bond between the surfaces of the earth in the sluice barrel. The present sluice structure will be re-installed as soon as possible, the Engineers said.
