GUWAHATI, April 11 - �Embankments are the cheapest, durable and least technical structures to prevent flood or inundation caused by rivers. The benefits of embankments are manifold, if they are maintained properly,� said noted river engineer Nomal Chandra Das. A former Secretary of the State�s Water Resources Department, Das was talking to this newspaper.
During the British rule, the two trunk roads � North and South Trunk Roads � built along both the banks of the Brahmaputra, served as the passage for surface communication as well as embankments to prevent the mighty river from flooding the countryside, he said.
However, he maintained the two trunk roads had upset the natural drainage of the State by disturbing the natural flow of the tributaries and other watercourses denying them the required passage at most of the places. The courses of most of the tributaries or the watercourses were either closed or diverted by these trunk roads. This was one of the major factors that aggravated the flood situation in Assam.
Following the 1954 flood havoc all over the country, the Rashtriya Badh Ayog was constituted and it recommended, among others, construction of embankments along the banks of the Brahmaputra and Barak rivers and their tributaries on a massive scale.
Earlier, embankments were built by the Ahom Swargadeos and the British rulers in patches. In the post-Independence period, major embankments came up in the State between 1951 and 1954 and between 1955 and 1956.
But the hydro-meteorological data and sub-surface information available with the authorities were inadequate for the purpose of constructing these structures. This is the main reason for the failure of the embankments.
These structures were built basing on the design flow of the rivers, which were calculated applying the statistical probability theory. Embankments were normally designed with 25 years� flood frequency data, while in the case of the anti-erosion structures, such data of 50 years and in the case of the dams, such data of 100 years are usually used.
At places, it is observed that the embankments were breached and this is because of not maintaining proper slopes or adequate heights. In the first case, the embankments fail due to seepage and in the second case these structures fail due to their being overtopped by floodwaters, said the river engineer.
Now that the observed flood data are adequately available since 1962, the embankments� sections, such as height, top width, side slopes, should be provided as per the present design flood, he said.