Not just monsoon, poor data behind Delhi’s rain chaos
Till August 5, Delhi’s total monsoon rainfall this year was 32% less than the Long Period Average (LPA). While this figure can change – rainfall until 30 September is counted as monsoon rainfall – it is not very surprising.Updated: Aug 08, 2020 14:43 IST
Two men, one 56 and another 28, drowned in two separate incidents in New Delhi on 19 July. While the deaths were an unfortunate and extreme event, water logging after rains is a common phenomenon in the city. The problem has persisted despite a fall in Delhi’s total monsoon rainfall as well as heavy rain events.
Fixing this needs an overhaul of the drainage system, which, according to experts cannot be done, unless there is more data, both on rainfall and drainage infrastructure.
Till August 5, Delhi’s total monsoon rainfall this year was 32% less than the Long Period Average (LPA). While this figure can change – rainfall until 30 September is counted as monsoon rainfall – it is not very surprising. An HT analysis of India Meteorological Department’s (IMD) daily rainfall database shows that total rainfall has been declining over the past decade in Delhi. Each grid in this database is a box covering 0.25-degree latitude and longitude and Delhi is approximated by two such grids. Not only has Delhi’s total rainfall been coming down, the share of heavy rainfall in total rainfall has also come down.
See Chart 1: Delhi’s rainfall
Given this, what explains the consistent, if not worsening problem of flooding? The answer depends on a multitude of factors, including the intensity of rainfall. This useful metric measures if a given amount of rainfall took place in a small area and over a short period of time or if it was spread over a long time or a large area. Since the gridded data set, which has been used for analysing Delhi’s rainfall, provides only 24-hour data averaged over two large areas, it is difficult to know from it either the time for which it rained, or whether it rained more in certain parts of the city. Both have implications for water logging. For the same value of total rainfall, water logging can be more severe if the rain is fast and concentrated in a locality. In Delhi’s case the problem is, nobody knows how and how long it rained.
The 2018 Drainage Master Plan (DMP) report for Delhi prepared by the Indian Institute of Technology, Delhi, for example, could analyse long-term sub-hourly rainfall for only two stations: Palam and Safdarjung. The report identifies high intensity rainfall events that are likely to repeat in a given time frame. As expected, increasing the time frame raises the chances of the intensity of the rain being higher. Over a five year period, for instance, Palam is likely to encounter a 15-minute rainfall event of 95.84 mm/hour, and Safdarjung 112.22 mm/hour. These numbers were 79.5 mm/hour and 87.2 mm/hour for a two year period. Any plan to prevent water logging or flooding will have to take these numbers into account. Similar numbers will also have to be generated for more parts in the city. Currently, there is not enough information. Delhi currently has 18-20 stations.
National Disaster Management Authority (NDMA) guidelines for urban flooding mandate one rainfall measurement station every four square kilometers in Class I, II and III cities. Based on these guidelines, issued in 2010, Delhi would need 371 stations. IMD is in the process of setting up these stations in the next two years, said Mrutyunjay Mohapatra, director general of meteorology at IMD.
Granular information on rainfall is just one part of the story. “All these things give you just that this much volume of water is coming,” said AK Gosain, professor at IIT Delhi, who authored the 2018 plan. “What is more important is what happens once the rainfall occurs in an area and gets converted into run-off and how the run-off is to be evacuated from the area,” he explained. Run-off refers to the flow of water on the ground when excess rainwater cannot be absorbed by the ground. This is a function of other factors. For example, more concretisation in an area would mean less water getting absorbed by the ground despite the same intensity of rainfall. This would lead to more run-off and a higher risk of flooding. Similarly, data on drains in an area provides details of source and volume of run-off and the time this water takes to travel through the drains. “Then comes the question of how good your drain is. Is it performing its function that it is supposed to perform? Those questions pertain to the management of the drains,” Gosain said.
The IIT study accounted for these factors using simulation models to make suggestions for preventing floods in Delhi. It first calculated flood volumes at junctions in the drainage network as it exists. The report found that some drains had abrupt changes in their height or were sloping in a direction that would prevent water from being evacuated . So another scenario calculated flood volumes after correcting these flaws. A third and fourth scenario allowed water bodies and nearby parks to absorb some of the water. These successive steps – all low-cost, according to the report – themselves led to a reduction in number of junctions that got flooded. Therefore, in addition to monitoring and cleaning of drains, even simple steps such as rejuvenation of water bodies in an area can help in reducing flooding in Delhi.
See Chart 2: Number of junctions flooding under different scenarios for Najafgarh basin
There is a big hurdle in implementing these strategies though – the lack of authenticated data. DMP highlighted the massive effort required in collecting digitised data for modeling. The team received erroneous data on connectivity, flow directions, dimensions, etc. and had to make assumptions to render it usable for analysis. This is worrying because the analysis cannot be a one-time exercise. DMP itself suggests that fresh simulations should be performed after verifying the digital data used with the reality on the ground.
“More will is needed on the part of the departments to really join hands and work in an integrated manner.,” Gosain said. That will be difficult -- the jurisdiction of the storm run-off system in Delhi, according to DMP, is divided between 11 different agencies.
It will take a rainbow coalition to prevent Delhi’s rainy day blues.