BMC’s flood forecast plans hit high-security roadblock

The Brihanmumbai Municipal Corporation’s grand plans to arm itself with a mathematical model that will help predict a deluge like the 944-mm deluge of July 26, 2005, just received a huge setback.

Looking to prepare a flood modelling system, the BMC had sought permissions to shoot a series of low-flying photographs of the city’s surfaces for a contour mapping exercise, but were rebuffed. Last week, the Ministry of Defence rejected the plans for ‘security reasons’.

The civic administration had contracted private firm MWH for the contour mapping exercise, but may now have to drop the plan. “Only last week, we received a letter from the Ministry of Defence saying that since there are a number of high security areas in Mumbai, low-flying aerial photography could not be allowed. We therefore cannot carry out the contour mapping now,” said Chief Engineer (Storm Water Drains) S S Korlekar.

He added that the BMC is keen to carry out the work so that the mapping can be completed in a year’s time. “We will have a meeting with senior officials to decide on the future course of action,” Korlekar said.

The proposed flood modelling system, practised with great success worldwide, involves using digitised maps of Mumbai and detailed contour maps, showing the precise carrying capacity of drains and geological factors like hill slopes, river beds, etc. Feeding current, real-time rainfall data into a complex lattice of these fixed parameters will help predict which areas could get inundated. The system usually yields effective and sustained flood forecasts.

Among Indian agencies using the system is the Andhra Pradesh State Disaster Mitigation Society also uses flood modelling.

Contour mapping is the most important aspect of the flood modelling system, since it will show points of elevation and depths, the steepness of slopes and runoff data. Contour mapping was also one of the recommendations made by the Chitale Committee set up by the state government to look into the causes of the July 26 inundation. Being a coastal city and also at a higher flood-risk owing to improper drainage, contour mapping for Mumbai was said to be a key task.

“By using the flood modelling we would know which areas face a flood risk on a certain day, and this would help us take adequate steps,” said a senior official.

Currently, the BMC has no advanced technology to predict floods, apart from weather stations and rain gauges that merely measure rainfall data.