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Mumbai rains: Why high intensity rain causes flooding?
On August 29, 2017, heavy rains for three hours created flood-like situation in Mumbai disrupting transportation and the supply of electricity in the city. Many people were left stranded at various places. As per Indian Meteorological Department's warning more downpour is expected in the coming days. So, Mumbai is likely to remain marooned with woes of the denizens unlimited.

Immediately after huge water logging, news channels started airing about problems being faced by the public due to incessant rainfall and flooding. Many channels through their debates started blasting the Brihanmumbai Municipal Corporation (BMC) and Maharashtra government citing that both had failed to serve Mumbaikars in making proper arrangements in advance so as to avoid public inconvenience during heavy rains. Channel anchors heckled political representatives questioning them how can Mumbai be flooded despite so much money allotted for drainage systems etc!

Here there are two points. First is the inefficiency of the BMC and state government who should have cleaned the drains and repaired the potholes well ahead of the approaching rainy season. Yes, this issue needs to be properly scrutinised.

However, for all those who think that only the administration, that did not do its work properly, is responsible for this flood-like situation, I would say that they are completely wrong! Even in well maintained global cities of Europe and the United States, temporary floods happen due to high intensity rains and people suffer for days. Why such floods occur? Let me explain in layman's terms.

The drainage system of a city is designed based upon the average rainfall of that particular area/city. The total rainfall per year is counted and then it is divided by rainy days of a year to find the average rainfall per day. Such data is regularly updated with various government departments including the Met Department. Whenever town planning people design drains of a particular city, they refer to the average rainfall of that area. However, they do keep some extra margin, for precautionary sake. So if average rainfall occurs exactly as per the designed capacity of the drains there would be no flooding and all the runoff area of that particular catchment will be drained out.

However, if the rainfall occurs more than the designed capacity of the drainage system then there would be a temporary ponding of water which will take some time (couple of hours to couple of days depending upon the amount of rainfall) to drain.

In Mumbai, around 86 mm of rainfall occurred just within three hours which resulted in huge water (88 mm multiplied with the entire catchment area) logging. Evidently much excess than the drains were designed for. So the water kept accumulating as the receding took around 18 hours.

The more intense is the rain, the more time it would take for the water to recede, even taking days at times since the drains can only discharge as per their designed capacity (quantity per hour).

One might argue that why aren't the drains designed according to highest rainfall per hour in recorded history. I would say that such a design is neither viable nor economical. If one starts designing drainage system to withstand highest rainfall ever recorded then the drain's width would be more than the road's width. In some ancient cities the roads were considered as storm water drains (although they too took some time to absorb the entire precipitation), but in modern days we need specified roads to cater to the present day vehicular traffic.

To understand the issue, examine the terrace of your building. There must be many pipes on the roof-top for draining the accumulated water during rain. If there is heavy downpour, one can see water logging on the roof-top/terrace despite those drain pipes being there, as it is takes some time to recede as per the capacity of those drain pipes.

Thus if there's heavy rain, which is more than the average, we must take it for granted that there would be water logging. People should be always prepared to face such problems. Those who are bemoaning Mumbai floods must also see the present condition of Houston, in the US state of Texas. Interestingly, Houston is the fourth largest city in the United States of America, but still media reports are showing cars submerged under water and people riding on boats.

Instead of panicking and bluntly blaming the administration we need to allow natural furies to have their say. During cyclones, snowfalls or excessive rains we must stay at safer places and then resume our work after the fury has subsided.

However, having said that, it certainly doesn't exonerate Mumbai's administration for not having made advance preparations. A well maintained drainage system could have checked heavy water logging, thus ensuring early resumption of normal life in the city.

At the same time, people must also ensure that drains are not choked. In Indian cities people normally through garbage and trash in to these drains which chokes them. No municipality can ever keep the drains clean if the citizens don't cooperate either ignorantly or intentionally.

(The author is a professional civil engineer with 30 years of experience.)

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