Design flood review to help assess hydrological risk of dams

New Delhi, Aug 5 As India availed the second tranche of a loan from the World Bank for the phase II of the Dam Rehabilitation and Improvement Project (DRIP), a report about the earlier phase has claimed that the DRIP project has successfully reduced the hydrological risk of 210 reservoirs by undertaking various structural and non-structural risk-reduction measures.

Over 250 design flood reviews were carried out and associated rehabilitation measures planned for dams under DRIP. Flood routing analysis showed that 118 dam reservoirs of the 169 can safely accommodate the increase in design flood through reservoir operations.

DRIP Phase II & Phase III aim at rehabilitation of about 700 Dams spread across 19 states over next 10 years. India has 5,334 large dams in operation and there are several thousand smaller dams apart from about 411 under construction. While 973 dams are 50 to 100 years old; 2,992 are aged between 25 and 50 and hence the policy push for safety of dams was necessary, officials said.

The Dam Safety Bill 2019 was passed by the Lok Sabha on August 2, 2019, and it is currently pending in the Rajya Sabha.

The report mentions the hydrological review of the participating dams in DRIP phase I as several of the "ageing and existing dams were designed for floods based on empirical formulae, local historical records or even a limited number of measurements."

"With the technological advances and availability of more data, there have been improvements in the analysis of extreme flows and tools for evaluating the hydrological events and selection of the Inflow Design Flood (IDF). The current technology was used in DRIP and over 250 design flood reviews were carried out and associated rehabilitation measures planned/ completed," the report released in July end said.

Whenever there is high flood, excess water is let off the dam reservoir through an emergency spillway. Historical data for the area gives the hazard potential to the planners. IDF means the magnitude of the reservoir flood selected viz-a-viz design requirement of the spillway in view of the dam size and hazard potential.

After initial design flood review of 250 dams, 210 dam projects (reservoirs) were finally considered for improving their ability to safely pass the revised IDF. It was seen that inflow design flood increased in about 86 per cent of the 210 reservoirs, the report said.

After flood routing analysis in 169 reservoirs, it was found that 118 reservoirs (about 70 per cent) can safely accommodate the increase in the design flood through reservoir operations, it added.

As per Central Water Resources (CWC) records, there have been 36 reported cases of dam failures in India; the worst one, in 1979 at Machu Dam in Gujarat, that resulted in the death of over 2,000 people apart from the other damages. Most of these failures were caused by breaching due to flooding and overtopping due to inadequate spillway capacity.

The DRIP project has successfully reduced the hydrological risk of 210 reservoirs by undertaking various structural and non-structural risk-reduction measures.

These were necessary steps in view of the lack of adequate risk assessment and emergency planning. An April 2021 report from the World Bank regarding DRIP pointed out how "historical patterns of insufficient dam maintenance, a lack of emergency planning, and recent changes in dam safety standards have raised concerns about the safety of India's large dams"

The World Bank report listed a number of reasons that contribute to the dam safety concern in India: a weak regulatory framework for dam safety, institutional capacity constraints; a lack of coordination amongst the various agencies that are responsible for dam operations; ad hoc and insufficient investments in dam safety and operations and maintenance (O&M); and lack of data to guide safety measures.

Dams often lack adequate instrumentation to monitor dam health and less than 10 per cent of dams have Emergency Action Plans (EAPs), the report had pointed out, and added, "To further complicate matters, many older dams were not designed to current safety standards."

In view of the USD 250 million loan from the World Bank for the DRIP II that India signed for on Wednesday, environmentalists had another view to offer. "In some cases, rather than strengthening, it may be better from various angles, including ecological and social benefits, to decommission the project rather than strengthening and increasing the life," Shripad Dharmadhikari of Manthan Adhyayan Kendra, who tracks water and energy related developmental issues, told .