Chennai’s black & odorous waterways pose a challenge

Aquatic life cannot survive in river water that has high sediment loads, low dissolved oxygen, and toxics. Chennai’s focus so far has been only on treating the point sources of river pollution such as urban sewage and industrial effluents.
Chennai’s black & odorous waterways pose a challenge
Cooum river


CHENNAI: According to a recent article in National Geographic, rivers are the most degraded ecosystems in the world. Urban rivers, in particular, have been treated with disdain, desecrated in every possible way, and reduced to dumping grounds and sewers. Chennai is no exception. All its three waterways — Cooum and Adyar rivers and Buckingham Canal — are severely degraded within city limits and have turned into black and odorous gutters that are almost completely devoid of aquatic life. As of date, river restoration projects worth Rs 2,484 crore are ongoing — Cooum (Rs 647 crore), Adyar river (Rs 555 crore), and Buckingham Canal (Rs 1,282 crore).

River restoration is a long-term process that calls for sustained commitment, patience, funding, and good governance. The generally accepted test of its success is the return of thriving aquatic life to a river that was once ‘biologically dead’. Despite spending several thousand crore rupees since 1967, Chennai is nowhere near attaining this goal. On the contrary, its waterways have gone from bad to worse. Clearly, there is a need for serious course correction.

Aquatic life cannot survive in river water that has high sediment loads, low dissolved oxygen, and toxics. Chennai’s focus so far has been only on treating the point sources of river pollution such as urban sewage and industrial effluents. But the more difficult part, about which Chennai has done very little, is tackling the diffuse sources of pollution such as storm water runoffs and agricultural runoffs.

These may contribute up to 40 per cent of river pollution. Storm water runoffs, conveyed through drains or otherwise, carry sediment, garbage, oil, and other contaminants and mostly pollute the urban reaches of a river. Agricultural runoffs carry sediment, nutrients (nitrogen and phosphorus from fertilizers), and harmful pesticide residues and pollute the upper, rural reaches.

Of particular concern, but mostly neglected, are the headwater streams which may be small in size but usually represent 50-70 per cent of the total river length due to their sheer numbers. The small size of headwater streams, along with their proximity to human activities, makes them highly vulnerable to filling-in and degradation, with negative impacts on both river water quantity and quality.

Therefore, tackling river pollution calls for a comprehensive, whole river basin approach. Chennai’s piecemeal, ‘band-aid solutions’ and attempts to restore only the urban reaches of the rivers will not work.

Improving river water quality is necessary but not sufficient. Successful restoration of Chennai’s rivers and the return of thriving aquatic life require that the other underlying reasons for ecological degradation are also addressed. For this, it is essential to first reject the conventional wisdom which regards engineering solutions such as dams, weirs, embankments, floodwalls, culverts, channelisation, and dredging as inevitable. Engineering solutions tend to emphasise short-term benefits to the denial of long-term costs and consequences.

For example, dams and weirs disrupt a river’s upstream-downstream connectivity, hinder the dispersal of plant seeds, upset the life cycles of fish and other aquatic life which migrate upstream for spawning, affect the transportation of sediment by the river, and weaken the downstream flow reducing its ability to flush the pollutants away.

Similarly, embankments and floodwalls separate rivers from their floodplains, affect the floodplains’ ability to function as natural sponges to store surplus water during floods and release them gradually later, upset the life cycles of several species of amphibians and other aquatic life that breed or lay eggs only on the floodplains, cause downstream flooding and sediment deposits, and increase flood heights.

Further, unscientific dredging and sand and gravel mining destroy the all-important hyporheic zone (the transition zone between the river bed and the groundwater table) which harbours water-filtering microorganisms that purify water naturally, and other unique aquatic fauna. It plays a critical role in recharging the groundwater table not just beneath the river but also laterally across large areas on either side of the river.

In 1997, the severely degraded Becva river in the Czech Republic broke through its embankments and floodwalls during a major flood. These were not rebuilt and the river was neglected. After some years, the river was found to have substantially rejuvenated itself through entirely natural processes.

Engineering solutions weaken a river’s natural ability to heal itself besides destroying habitats. But like surgeries, they should be the last option for river restoration, not the first.

Another important reason for Chennai’s lack of success in river restoration is extensive developments in the floodplains right up to the urban riverfront (including, in several places, encroachments on the banks), leading to the loss of ecologically critical riparian vegetation and wetlands.

In its natural state, a river typically has a densely vegetated corridor or riparian buffer on either side which regulates water quality by filtering out sediments, nutrients, and pollutants. It is usually interspersed with wetlands that are nature’s sewage treatment plants and cleanse water through bioremediation, making it the best conservation tool for protecting the rural as well as urban reaches of a river from diffuse sources of pollution.

The buffer also provides several other invaluable eco-services. It slows down floodwaters and storm water runoffs, allows sediments to drop, helps build floodplains, recharges groundwater, reduces erosion, and stabilises riverbanks. It provides a diversity of habitats and contributes to in-stream woody material for fish to hide from predators and macroinvertebrates to colonise.

How wide should the riparian buffer be? In ‘Riparian Buffer Zones: Functions and Recommended Widths’ (2005), Ellen Hawes and Markelle Smith have recommended minimum widths of 30 metres for erosion/sediment control; 100 metres to protect water quality from nutrients, pesticides, and bio-contaminants; and 70 metres to protect aquatic habitats.

Therefore, Chennai needs to recreate the lost riparian buffers along the entire length of its rivers. All the encroachments on lands classified as ‘river poramboke’ should be evicted. Studies have shown that it makes economic sense to purchase or compulsorily acquire flood-prone, privately owned lands to provide enough width for an effective riparian buffer. The Transfer of Development Rights (TDR) scheme can also be used to acquire urban land for riparian buffer in exchange for a higher floor space index (FSI) for development elsewhere.

Chennai has been criticised for constructing parks on the river banks because such ‘cosmetic restoration’ does not address the underlying reasons responsible for river degradation. Riparian vegetation may not be attractive in the traditional sense but it provides critical eco-services that parks cannot. The right kinds of vegetation for the riparian buffer are the native plant communities that are extremely well-rooted (typically with root mass 2-5 times greater than the above-ground mass), can withstand frequent flooding, and need very little maintenance. Lawns, monocultures and exotic species should be avoided.

The current scientific paradigm underpinning all successful river restoration efforts around the world is that an unimpaired, free-flowing river without any artificial barriers and with an effective riparian buffer on both sides has an inherent capacity to self-heal and self-purify by natural processes. This approach seeks to take down or bypass dams and weirs, remove or notch embankments and floodwalls, and revive floodplains and riparian buffers so as to return rivers to a more natural state. Unfortunately, Chennai is still stuck with a technocentric mindset and is continuing to build more check dams, embankments, floodwalls and diversion channels. Chennai’s river restoration efforts (which are not even 40 per cent complete) may take at least 15 more years, but only if the correct scientific paradigm is adopted and earnest efforts are made.

— The writer is a former IAS officer and former Vice Chancellor of Indian Maritime University, Chennai

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