Urban Drainage: Case of Chennai floods

What are the reasons for increasing instances of floods in cities, and the Chennai floods as a case study

What are the causes of impeded urban drainage? Discuss the reasons with emphasis on Chennai floods. Suggest a not on urban drainage management.

At all major metros the natural flow of rainwater was hindered by encroachments on flood plains, constructions on open grounds meant for absorption of excess rainwater, and clogging of drains. Over the years, the rain-holding capacity of the cities was reduced putting a strain on the age-old stormwater drainage system.

Illegal encroachment

A large part of land along Mumbai’s Mithi river, the lifeline for the city’s drainage system, has been encroached upon. Adyar in Chennai faces similar fate.

Post Mumbai floods, a National Institute of Disaster Management study found the city had the lowest ratio of open ground per head of population in any Indian metropolis. In Delhi, according to MCD estimates, builders have illegally covered about 60 per cent drains in residential colonies. As a result, stormwater drains are either filled up with mud or covered with concrete slabs. In most metros drains have simply disappeared from many colonies due to extension of roads.

Lack of coordination

Road engineering and stormwater drainage system should complement each other through coordination between engineering and sanitation departments. Unfortunately, such a coordination does not exist in any Indian city.

Poor maintenance of drains and inadequate desilting before monsoon means that city roads get waterlogged within a few minutes of rain. In Delhi the MCD has consistently failed to meet the deadline to desilt its over 1,200 stormwater drains. And with multiple departments involved in the cleaning process, it is easy to pass the buck. In most cities desilting is still done manually, which is not only slow but also ineffective.

Design defects

The success of any drainage system depends on the efficacy of its design. For the water to run off properly, storm water drains need to have the correct incline. In many cities residential colony drains were not even joined with the main stormwater drains. The gradient of the internal drains is faulty which results in water logging leading to vector-borne diseases.

Urban floods: Case of Chennai floods

Chennai floods were a creation of many cumulative factors that joined together under a fortuitous circumstances No one cause was more or less important

  1. Gradual encroachment of the channel of Adyar river by waste dumping, plastics, sediments and non biodegradable products
  2. Conversion of Coume river into a ballad by sewage dumping consequently converting it into a sewage channel.
  3. Redemption of Pallikamarnyi wetlands for urban and construction expansion destroyed the buffer and prevented infiltration and absorption of surface water, allowed heavy surface runoff.
  4. Illegal construction around the mouth of the rivers and on Pallikamarnyi Wetlands prevented smooth runoff and led to inundation.
  5. Construction has covered the city like mat and prevents infiltration inducing fast runoff that is the course of floods.
  6. The situation was compounded by
  • heavy and incessant rains for over a week with increased frequency and three times more than the normal rainfall
  • The High tide coincided with
  • the impeded drainage of the city flush water, and
  • opening of the gates of the reservoir used for drinking water storage

Three coincidences- high rainfall, impeded drainage, Blocked channels, high tide and opening of gates of the reserve all cumulatively became responsible for Chennai floods.

Preventing Urban Floods

  • Reclaim urban water bodies
  • Clear the natural drainage way.
  • Identify areas of High Flood Level, and do flood plain zoning
  • Put Contingency plan on anvil.

Suggestions for further action

  • Delinking of the sewer and stormwater drainage systems and a complete ban on riverside constructions.
  • Use of machines like high pressure jetting pumps can do complete desilting cost effectively and reduce the human drudgery.
  • The basic framework of drainage needs effective planning, use of appropriate technologies, accountable administration, and community participation and, above all, unequivocal political will. A good beginning could be made by reducing the generation of waste and introducing some basic recycling at the level of residential colonies followed by treatment of industrial waste before discharging it in water bodies.

The Task Ahead

  • Ensure a single authority for construction and maintenance of drains.
  • Construct separate drains for stormwater and sewage flow.
  • Remove illegal encroachments along drains and water bodies.
  • Use jetting pumps and vacuum machines to desilt urban drains.
  • Design drains with proper gradient and width for flow of water.
  • Treat industrial effluents before disposing them into rivers and seas.
  • Make rainwater harvesting mandatory in all the metros.
  • Construct porous pavements for parking lots, driveways and sidewalks.

By K Siddhartha

Earth Scientist, Author, Mentor, Educationist & Consultant | 20+ years mentoring experience | 40+ books across humanities disciplines | Advisor to Maldivian & Sri Lankan Govt. on Education and Environmental Issues.

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