Chennai's mayor this week put a number on the city's drainage: 1,671 kilometers of stormwater drains being desilted. It is the kind of statistic that reads, on the wire, as routine municipal housekeeping. Read it a different way and it is a confession, delivered in kilometers, by the capital of the most flood-exposed coastal city in India.

The arithmetic of Chennai's flooding problem is simple and unforgiving. The city sits at roughly four meters of elevation, behind the Adyar, Cooum and Kosasthalaiyar river systems, and it has been building outward faster than it has been building down. The October 2015 catastrophe — six days of rain, 20-plus fatalities in official counts, a city paralyzed for weeks — was not an anomaly. It was the expected value of a particular kind of development, paid for in a single bad monsoon.

Since then, every government that has held the corporation has run the same program: desilt before the southwest monsoon, apologize during it, desilt again after it. The difference between administrations is rarely the engineering and more often the accounting. The 1,671-kilometer figure, announced now by the Chennai mayor, is the DMK-era claim that the current cycle is bigger than the last. The opposition's claim, which the wire does not carry but the city remembers, is that the last cycle's "record" was also a record.

There are three reasons the number matters more than the press release.

First, desilting is the cheapest variable in the entire flood equation, and therefore the most likely one to be overstated. A kilometer of drain reported as "desilted" in the municipal register may have had its silt lifted to a disposal site — or it may have been re-deposited within two weeks by an illegal drain into the very channel being cleaned. The drains of Chennai are a closed loop of municipal failure: the collector system, the disposal system, and the encroachment on both are all the same system, run by the same hands, audited by the same city. The 1,671-kilometer figure is only as good as the disposal-site receipts behind it, and those receipts have never been the part of the story that gets published.

Second, the timing is political. A drainage announcement in late September, ahead of the retreating monsoon and the pre-pollitical season, is not an engineering choice. The mayor's office knows the next flood will be blamed on the sitting government whether or not the drains are clean, because the voters' memory of 2015 is not a memory of a drainage failure. It is a memory of a government. The 1,671 kilometers is the DMK's attempt to write its name into the infrastructure record before the next monsoon overwrites it.

Third, and this is the part the municipal wire omits: Chennai's real flood problem is not the drains. It is the water body. The city's natural lakes — Red Hills, Chembarambakkam, and the dozens of smaller tanks that used to absorb the monsoon's first, worst hour — have been filled, encroached, and, in the case of Red Hills, reduced to a parking lot and a reservoir of political dispute. You can desilt every drain in the city and the flood still happens, because the drains discharge into a city that has deleted its own sponge. The 1,671 kilometers of pipe work is necessary. It is not sufficient. It was never going to be.

What the number will tell us, in the end, is what it always tells us: whether the next monsoon is met with a desilting program that was finished, or a desilting program that was announced. The city has the receipts for both. It is time to publish them.