Why does it flood in Chennai? Reasons vary from the seemingly obvious to the more subtle. This is a 4 part series on the causes of urban flooding in Chennai, as have emerged from my data analysis. Though the four sectors are inherently interlinked, I will try to provide an overview here. See Part 1 for Geography & Part 2 for Urbanisation.
Urban flooding basically means there’s too much water to be soaked into the ground, so the water runs off. From this perspective, it’s a question of the rate of rainfall. Manage the rain and make it work for you, storing it to be used throughout the year whilst preventing flood run off. This is called rain water harvesting (RWH).
On a macro scale, RWH has primarily been concerned with irrigation. Over 1,000 years ago, Tamil kings created a highly effective system of interconnected tanks called eris. These seasonal rain-fed tanks collect the rainwater, and had connections from one tank to another and eventually to the sea. Some were specifically temple tanks, such as the iconic Kapaleeswarar.
The use of the past tense is not incidental. Because of urbanisation, irrigation is no longer relevant within Chennai, along with the primacy of the eri tank system. With the rising pressure for land, tank beds and connections that only got wet once in a while became very appealing. Also, external pressures have played a role. The British pushed for filling in certain waterways, and the World Bank funded the Eri Scheme, which was to fill in “defunct” and “useless” waterbodies for infrastructure development. As a result of the general thirst for land, over 400 tanks have lost more than 90% of their waterspread, and 27 are irretrievable.
At a micro level, the tanks were used for local domestic usage, such as drinking and washing of clothes. The tank floor was never paved, thus allowed percolation to recharge the groundwater tables. Along with this, people have traditionally been dependent on shallow dug wells for domestic usage.
Today, where there used to be a village house with a 5 person family and their well, lies an apartment complex with 40 relying on the same water and no well. Deep borewells were dug to source water, though these mostly draw on non-renewable confined aquifers. As a result of persistent over-extraction combined with drastically reduced areas allowing for groundwater recharge, Chennai has severe water shortage issues, which are only compounded by socioeconomic differences in entitlements (Anand 2001).
Many flood-risk hotspots within the city are explained away as being “naturally low-lying areas”. Which is true. However, collective memories often neglect the fact as to why they are low-lying areas: they used to be tanks. No wonder they flood when it rains. At a street and household level, because the rain is not being harvested from the roads and buildings it leads to stagnation, which can be anything from small puddles to big floods. These become breeding grounds for mosquitoes, not to mention causing anxiety: whether you’re walking or driving, you don’t know how deep that puddle is. (Those who don’t complain are the auto-rickshaw drivers whose prices skyrocket!)
There are possible solutions. After several years of severe drought in the early noughties, the state government created a law in 2002 making RWH mandatory for all buildings, and only gives planning permission for new buildings with RWH structures in place. This is landmark legislation and has been widely promoted as the government combating water scarcity. The enormous media campaign has greatly increased awareness of the importance of RWH. In this sense, RWH would be collecting rooftop terrace water and bringing it through a pipe to a well, sump or storage tank in the backyard. There are many other methods which are cheap and easy, and I recommend this book if you’re looking for a how-to.
Yet with little enforcement for the millions of old buildings, non-compliance is over 3/4 of the population. Many RWH structures in place are inadequate. Not all areas of Chennai have soil which is suitable, so sometimes one needs to literally dig a little deeper to reach a sandy layer suitable for percolation. Crucially, at a planning level RWH is divorced from the drainage network, so the vast majority of run-off from the roads is not harvested but let into polluted drains and washed out to sea.
In place, groups such as the Rain Centre are advocating for a return to wells. These are vertical solutions to controlling floods, rather than the horizontal storm water drains. Finding one vertical spot to dig is a lot easier than trying to plan a well-connected network that navigates through the tangled maze of roads, electricity/telephone/internet cables, sewage systems, water connections, etc. that are hidden under the road who knows where. These also increase the functional overlap of flood-mitigating structures, which is crucial to building resilience.
Nevertheless, at a macro level the reduced land cover remain the main issue. Knocking down buildings to revive larger tanks and canals has such a high political cost it is nearly impossible. To really understand this, we need to go back to the land itself: How have the roles and values of land changed?