Sharad Chandra, director, flood forecast monitoring division, Central Water Commission, spoke to DTE on increasing instances of urban flooding
Urban flooding is increasingly becoming a common occurrence in India, the latest victim being Hyderabad.
The city received unusually heavy rainfall October 13-14, 2020, due to a deep depression that developed in the Bay of Bengal. Heavy damage to property, roads and human lives were reported. Experts have argued that rapid urbanisation exacerbated the process.
Down To Earth spoke with Sharad Chandra, director, flood forecast monitoring division, Central Water Commission (CWC), to understand the phenomenon and the way ahead. Edited excerpts:
Shagun Kapil: Urban flooding has become an annual event now, especially in southern and western India.
Sharad Chandra: There are two factors behind the phenomenon. Rainfall pattern has changed completely; that is, we now get intense rainfall in fewer days. Floods usually occur due to extreme rainfall, in which case run-off is generated. The infiltration capacity of the ground surface falls short, leading to the generation of more run-off.
The infiltration capacity has also decreased due to more concretisation. More pavements are coming up, and drains don’t have the capacity to carry so much run-off.
The recent Hyderabad floods showed that along with climate change, the city is also bearing the brunt of poor planning and encroachment of its lakes. Near 50 per cent of Hyderabad lakes are full; their capacity has decreased due to encroachment, siltation and dumping of solid waste.
We should have considered expanding these lakes. Water bodies are an essential source of flood moderation and act as retention or detention basin. The importance of these water bodies is not being realised.
(Retention basin just retains water during high run-off and detention basin absorbs water during floods and this water can be diverted to a stream or river itself for different purposes when flood recedes).
SK: What measures should be adopted to keep urban flooding in check?
SC: Short-term measures should be deployed to clean clogged drains and encroachments and avoid indiscriminate development on water bodies. Plastic is one of the major sources of pollution in drains.
We need engineering solutions for the long term. We need to review design criteria of urban water drains. We need to statistically update this design.
SK: What policy-level changes do you think can have an impact on mitigating flood risk in India?
SC: Globally, the first strategy adopted for flood risk mitigation is flood-proofing at micro level. We cannot control floods fully. That is why the concept of flood risk management or control exists.
In Indian context, we need to go to the block- , taluk- , or village-level to assess the flood situation there. Japan has done hazard-mapping at the level of municipality. This gives insight into the time and intensity of the flood and helps recognise high-risk zones.
In the United States, a federal management agency does hazard-zonation mapping to grant flood insurance.
SK: Floods are also induced by dams. The Kerala floods of 2018 as well as the Karnataka and Maharashtra floods of 2019 are the recent examples. This year, Gujarat’s Bharuch faced flooding due to Sardar Sarovar dam. Why is this happening more frequently now?
SC: From the point of view of water resources, dams were designed to moderate floods. But if you maintain reservoir levels high in June and July, you end up inviting trouble. There should be slow filling of reservoir during June, July and August, and by September-end, one can have aggressive filling. Many a time these rule curves are not followed.
We also need to understand that India has no dam dedicated to flood moderation. The US has nearly 2,000 dams only for this purpose. But we have adopted multipurpose reservoir system for power production or irrigation benefit.
We are still a developing country, so that’s understandable. But to serve these purposes, dams are filled up as quickly as possible.
SK: What’s the importance of the rule curve? Is it also the time to revise this for individual dams keeping in mind the change in rainfall pattern?
SC: A rule curve was agreed so that we can do flood moderation and not suffer any commercial loss as far as power generation is concerned. Today, one-third of our disasters are floods. So, one needs to compromise on a few power or irrigation benefits.
Even in a multi-purpose reservoir, you can derive the incidental benefit and flood moderation by properly designing the rule curve.
Having a rule curve is not the only necessity. Its frequent updation is required as rainfall pattern is changing. A rule curve is derived from historical long-term inflow and outflow data.
But as the rainfall pattern has changed, inflow series has been affected. So, there is a need to revise the rule curve with a frequency of five-ten years. But this initiative has to be taken by states and dam authorities, following which the CWC can help them.
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