District authorities and Public Works Department (PWD) engineers single out breaches in rivers, tanks, lakes, ponds, and canals as the major cause of the all-round devastation. While some breaches were natural — owing to excess storage and weakened bunds — many were man-made. Many encroachments have come up all along the Cauvery and its network of tributaries. The well-designed flood plains, called the padugai, have either been widened to form part of the riverbed or have been used up for cultivation and illegal construction. So what was meant to be a natural protective barrier against flooding became the first casualty. With this barrier gone, roads and farmland quickly went under the floodwaters. Even the normally bone-dry river Coleroon carrying well over three lakh cusecs of water!
The PWD engineers say under the old, somewhat rundown irrigation system that still exists in many parts of the composite Thanjavur district, the Cauvery was linked to a network of tanks through feeder channels. During a bountiful monsoon, all the tanks were filled up and the excess water drained into the irrigation system. Over the years — because such floods are not common — encroachments came up on riverbeds, feeder canals, and the draining system. These were not just huts or houses, but sometimes even constructions by the Government and local bodies. They came in the way of water flow and redirected the floodwaters to habitations and paddy fields, causing extensive damage. "When the natural flow of water is obstructed, it finds its level in other ways," explains a retired PWD Chief Engineer.
Another flood situation made worse by man-made factors. Encroachments take the blame again as they did in Bangalore, in addition to other factors. From The Hindu: