Incorporation of “Risk Factor” in Design Flood Review of Existing Dams

At present there are about 4877 completed large dams in India. More than 75% of Indian dams are at least two decades old and for these dams the original design floods call for revisions. Examination of revised design flood studies carried out for a set of 118 dams under ‘Dam Rehabilitation and Improvement Project (DRIP)’ reveal that the design flood values have increased substantially with trends indicating that percentage increases in the revised design flood values are greater for lower dams and also for older dams. Paper illustrates that the upward revision in the design flood estimate of existing dam in no way leads to amplification of the actual risk associated with that dam, and it only underlines the gap between the acceptable risk level and existing risk level. The paper shows that flood related risk mitigation of existing dams is a challenging task often facing constraints of technical and economical viabilities. With the substantial data set of design flood revisions of DRIP dams, the paper brings out the skew observed in distribution of revised design floods on account of present-day norms related to computation of design floods, as contained in the Indian Standard IS: 11223 – 1985 (reaffirmed 1995). Paper also proposes amendments in this Standard in respect of existing large dams with an alternative recourse of formulating a new Standard for revision of design floods incorporating the risk factor associated with these large dam structures.