Why was Bengaluru ravaged by floods?

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Madiwala lake, with a watershed area of 71.74 sq km and a depth of 4.5 metres, starts swelling two hours after a rainfall of 72.3 mm per hour. In case of 161 mm per hour rainfall, the flooding time shrinks to just 90 minutes. However, the innundation is almost immediate from the direct incidence of rainfall on the tank, followed by a run-off from the catchment. The inflow is escalated due to a high run-off rate, poor percolation and loss of upstream tanks. The cascading effect will be innundation of Agara, Bellandur and Varthur lakes. And, the areas surrounding these water bodies bear the brunt of flooding. Year after year, it is almost the same localities that are suffering, going from bad to worse.

Last year, the city saw massive demolition operations — of properties built along the course of storm water drains hindering the flow of water. But sadly, the areas where the razing was carried out, were not spared of the worst flooding during the rainfall in the last two days. Encroachments, closure of natural water bodies, its wasteweir, mindless dumping in the drains, shoddy desilting/cleaning work done by civic authorities, are oft-heard reasons.

Going a step further, it is tampering of Madivala, Agara, Bellandur and Varthur lakes and its connected valleys that is also strongly contributing to the watery situation we are in. Considering that the city’s lakes are interconnected, and these four water bodies in the east and south of Bengaluru form a strong network, the closure of the catchment, embankments and valleys have had a cascading effect in the surrounding areas. A technical study carried out by environmental architect, Mohan S Rao, who did BMRDA’s structure plan for the city with assistance from civic activist V Ravichandar some months ago after the massive demolition operations, has brought out how construction of outer ring road on a narrow valley and letting of residue from the treatment plants into water bodies, are also the prime factors for Bengaluru’s watery doom.

Workable solutions
The study, titled “Urban flooding in Indian cities and natural drainage systems: The case of Bengaluru,” has dissected urban flooding and water scenario, with Bellandur and Madiwala tanks area as case studies. The study has also given workable solutions and the report is before every authority concerned, including the urban development department secretary and the BBMP commissioner.The study has sketched how the outer ring road that meanders along the areas where these water bodies are situated, is built on a narrow valley region enclosed by ridge formations that has resulted in a greater tendency for water to flow back and spread along the highway. The engineer who has dissected these lakes minutely has found that inflow has escalated, outflow is slower and the capacity of tank is lost. Take the case of Madiwala. It has a watershed area of 71.74 sq km with a depth of 4.5 metres with the original capacity being 3,542 ML. With this capacity, the lake will start flooding after two hours for a rainfall of 72.3 mm per hour. For a rainfall event of 161 mm per hour, it will start flooding after 90 minutes. The capacity of the tank is lost as it is already full with sewage discharged from adjacent areas, the report highlighted.
SOURCE: http://timesofindia.indiatimes.com/city/bengaluru/why-was-bengaluru-ravaged-by-floods/articleshow/60116145.cms
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