MONSOON RAINFALL AND FLOODS IN DELHI
Decadal averages of Delhi's rainfall Rainfall in mm Light Moderate Heavy
800 700 600 500 400 300 200 100 0 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)
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 modelling. 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.