Think of the estuaries with their limited flood discharge capacity as a permanent traffic bottleneck on a highway. Building SWDs l is like building feeder roads leading to the highway while increasing the vehicle population on the streets.

Residents crossing a flooded Chennai street on a rickshaw pulled by an old manpti
Delve Opinion Thursday, November 18, 2021 - 14:53

The recent bout of #ChennaiRains turning to #ChennaiFloods has predictably triggered the usual hand-wringing about causes and solutions to Chennai’s predicament with water, and the usual promises of stormwater drains (SWD) as a solution to the city’s flooding woes. But SWDs are not likely to solve the city’s flooding problem.

It is common knowledge now that the city has been built on floodplains, water courses and waterbodies, and that sprawling wetlands like the Pallikaranai, and Kovalam and Ennore backwaters, have been carved up as residential, commercial and industrial real estate by successive governments. This piece will not go into the rights and wrongs of the decisions that led to such urbanisation, or about the merits of growth. Rather it will explore whether SWDs present a viable solution to keep the city dry in its current state of growth, and whether the city can continue to grow without endangering itself.

Limits to urban growth

Limits to growth in human settlements are dictated by a variety of factors — location of the settlement, climate, economic prospects, and availability of water being some of them. Some limits can be stretched, while others can be quite rigid. Local drinking water resources, for instance, can impose a stretchable or rigid limit depending on what kind of human settlement seeks to grow. For a settlement without political or economic power — say, a hamlet of adivasis— the quantum of water available within digging depth or transportable distance, can impose a rigid limit on its expansion or existence. If local sources dry up, the residents will have to migrate to find new sources of water. However, if the same settlement were a metropolis like Chennai, water from distant locations can be commandeered to the urban.

There are other limits that pay no heed to the power of the metropolis. The capacity of natural drains, particularly in coastal plains like Chennai, is one such rigid limit that, if exceeded, can only be overcome through expensive engineering interventions.

Chennai has four natural mega drains that gather and discharge rainwater run-off from various catchments into the sea. The Kosasthalaiyar river drains through the Ennore estuary; Cooum river through the estuary near Napier Bridge; Adyar river through the river mouth north of Urur Kuppam; and the Pallikaranai marshland and Kovalam backwaters through the Kovalam estuary.

The amount of floodwater that can be discharged at these exit points is finite. It cannot be enhanced. In fact, human interventions can drastically reduce the discharge capacity at the river mouth. The construction of the Kamarajar Port in Kattupalli, and the Madras Harbour north of Marina, for instance, has led to the permanent closure of the Ennore and Cooum estuaries. Even after using expensive dredgers, the river mouths are kept barely open. Bridges and roads like the Port-Maduravoyal Expressway proposed to be built along the Cooum’s river bed will further reduce the flood carrying capacity of the river and aggravate flooding.

Limits to drains

According to a Chennai Metropolitan Development Authority (CMDA) document prepared sometime between the 2005 and 2015 floods, Kosasthalaiyar’s maximum flood discharge capacity is reported as 1,10,000 cusecs (cubic feet per second). Adyar’s flood discharge capacity – i.e. the rate at which it can discharge floodwater into the sea – is 39,000 cusecs. The same document reports that flood discharge during the 2005 rains was 55,000 cusecs indicating that the river was in spate.

Artificial drains, including major or macro drains like Buckingham Canal, Otteri Nullah or Virugambakkam Canal, also have fixed capacity. If the catchment generates more rainwater run-off than the canal can handle, the water will have to wait until the canal drains into the river. If the river itself is swollen and flooded with more water than it can handle, then the canal will begin to overflow until the river is able to discharge water into the sea and free up some carrying capacity to accommodate the canal’s water. Tides can add another frustration here. For the river to discharge into the sea, the sea must be ready and willing to accept it. If the peak rains coincide with a rising tide, the river will be unable to discharge its waters.

Just as natural drains have capacities, macro and micro drains, including stormwater drains, too have design capacities. These are fixed probabilistically based on the recurrence period of flooding for that particular drain. For instance, let’s say that the PWD reports the flood discharge of natural drains for a recurrence period of 100 years. Then, when one says that Kosasthalaiyar (a natural drain) has an anticipated flood discharge of 1,25,000 cusecs, it means that there is 1/100 (1%) chance in any year that rains will cause a flood discharge of 1,25,000 cusecs in the river. It does not mean that it will only happen once every 100 years. In fact, 1/100 year floods can happen even in two consecutive years.

These calculations are made for certain land-use conditions for rainfall of a given recurrence period. So, if a 24-hour rainfall of 400 mm is taken as a 1/100 year event, then for a given land-use, Kosasthalaiyar’s flood discharge of 1,25,000 cusecs will also have a 1/100 chance of occurring on any given year. But if the land-use changes, as it does with urbanisation, the discharge of 1,25,000 cusecs can occur with even higher probability, i.e. with even lighter rains that have a 2 or 4% probability of occurring on any given year. In other words, the same quantum of rainfall can cause more frequent flooding, and the probability of extreme flooding can double (1/50 year event) or even quadruple (1/25 year event).

Urbanisation, built-up area, and rainwater run-off

The rate at which rainwater is converted to run-off that needs to be drained depends on two factors:

1) Rainfall intensity, which is the amount of rain in cm or mm that falls in a given time. For urban flooding, hourly or sub-hourly rainfall rates are important. Slow and steady rain will allow for more water to percolate. Long bouts of slow, but steady rain is good for groundwater recharge. With such rains, run-off will begin only after the earth is saturated. If the rainfall is intense, the rate of rainfall will be higher than the rate of percolation and will result in run-off that will find and stay in a waterbody or find a drain or river to end up in the sea.

2) Land-use, land cover (LULC), which refers to the use to which a land is put and the nature of the land surface. Open, unbuilt land could have sandy soil and/or clayey soil, or rocky surface of varying depths, or a combination of all of these and with varying kinds of vegetation. For a given rainfall intensity, each of these land-covers will generate a different quantum of run-off. Sandy soils, such as vegetated dunes can soak in more water, and will result in less run-off. Clayey and rocky surfaces will generate more run-off than sandy soils.

Urbanisation involves covering open, unbuilt earth with built-up surfaces of concrete, turf, debris or asphalt. Almost all the rainwater  that falls on such surfaces is converted into run-off. So for a given rainfall intensity, the run-off generated within an urban catchment will increase with increase in built up area. The same rainfall that generated run-off that could be comfortably conveyed by a drain when the proportion of built-up area to open, unbuilt spaces was low, will result in extreme run-off and flooding if the proportion increases.

Things will be worse if the urbanisation involves conversion of waterways and drains into built-up areas.

Chennai’s built up area was 47 square km in 1980; by 2010, it had increased to 402 square km, according to a study by Care Earth. Meanwhile, the capacity of major drains have remained the same, or reduced due to encroachments or siltation, and area under wetlands decreased from 176 square km to 71 square km.

A senior water resources engineer that I consulted concurs that the maximum flood discharge capacity of a natural drain will impose a limit on the extent of SWD network in the catchment. “Our macro drain capacity of 1/25 year is long broken and exceeded due to urbanisation,” the engineer noted. The Mambalam Canal, for instance, had an original design capacity to handle a 1/25 year flood event. That means the chance that the canal will flood for a given rainfall in any given year is 1/25 or 4%. According to the Detailed Project Report for restoration of the canal, the current capacity of the drain is insufficient to even handle a 1/2 year event. i.e. there is a 50% chance that the canal will flood in any given year.

Building more SWDs emptying into these broken macro drains or overwhelmed natural drains is throwing good money down the drain, literally.

Think of the estuaries with their limited flood discharge capacity as a permanent traffic bottleneck on a highway. Building SWDs and expanding the urban sprawl is like building feeder roads leading to the highway even while increasing the vehicle population on the streets. This will not reduce flooding in the catchment just as the roads won’t reduce traffic congestion given the permanent bottleneck.

Take, for instance, the Rs 3,200 crore Integrated SWD project in the Kosasthalaiyar River’s sub-basin. The Asian Development Bank-funded project claims to extend SWD coverage to the built-up areas in Manali, Thiruvottiyur, Tondiarpet, Madhavaram, Royapuram, Kolathur and Ambattur. If implemented successfully, these drains will collect rainwater run-off and convey them to the Kosasthalaiyar’s backwaters in Ennore. These drains are designed on the assumption that Kosasthalaiyar still has and will have the theoretical flood discharge capacity.

That’s a wrong assumption. Between 2015 and 2021, the Ennore backwaters have lost more than 700 acres, including around 250 acres to a plastics industrial estate and 40 acres to accommodate TANTRANSCO’s (Tamil Nadu Transmission Corporation) transmission towers. Additionally, more than 1,000 acres of the backwaters are choked with coal ash from TANGEDCO’s (Tamil Nadu Generation and Distribution Corporation Limited) leaky pipelines despite a 1996 order of the Madras High Court and a 2017 direction from the NGT prohibiting leaks and ordering cleanup.

And there’s more planned. Adani’s proposal for an expanded port and industrial complex will require the conversion of more than 3,000 acres of wetlands from sinks and safe storage for excess floodwater, into sources of floodwater. On January 13, 2021, when community opposition to the Adani Port expansion was at its peak, DMK chief and now Chief Minister MK Stalin called out the state and Union governments for their support to Adani and for selling out on Tamil Nadu’s welfare. A tweet by the leader called for cancellation of the project because it will drown 35 lakh people in Chennai and Thiruvallur. This was subsequently followed by an election-time promise that the project will be cancelled if the DMK is voted to power. Half a year has passed since the party came to power, with no declaration on the fate of this project.

Chennai’s vulnerability to floods, sea-borne shocks, and water scarcity are a result of urban and industrial growth, and the densification of built-up areas. The better-off and the wealthy with better coping capacity may say they are willing to accept the heightened risk of floods in return for the benefits of such predatory growth. But the worst-affected are the poor who lack the coping capacity and ability to build back their lives after each flood. With severe floods increasing in intensity, it may be worthwhile to ask the city’s poor if they are willing to put up with the ever-increasing risk of continued urbanisation.

Nityanand is a Chennai-based writer and social activist. Views expressed are the author’s own.

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