How to tackle flooding due to rain in your neighbourhoods

In 1993,  Raghunandan commissioned the visionary Ar Sheila Sri Prakash to design his home in RA Puram, Chennai.

Published: 07th July 2018 04:00 AM  |   Last Updated: 06th July 2018 10:22 PM   |  A+A-

Express News Service

In 1993,  Raghunandan commissioned the visionary Ar Sheila Sri Prakash to design his home in RA Puram, Chennai. A rainwater harvesting (RWH) arrangement was included as one of its new radical features. The details were published in a leading design magazine along with diagrams. Almost a decade later the design was copied and an amendment made to Section 215 (a) of the Tamil Nadu District Municipalities Act, 1920 and Building Rules 1973. RWH structures had to be constructed in all buildings, new and old; and Tamil Nadu became the first state in 2003 to make RWH compulsory. Subsequently all the remaining states enacted similar statutes.

Every July the southwest monsoon will inundate Mumbai. The nightmare of collapsing bridges, falling buildings and distraught residents wailing amid flooded neighbourhoods, recurs unfailingly. Elected representatives are chastised and TV anchors shout themselves hoarse in a ghoulish charade.  The waters will slowly recede and many cities will “limp back to normalcy.” Urbanites will within weeks complacently slide into water shortages and reckless water tankers will run over pedestrians and motorists. Deja vu! Chennaites now dreadfully await the turn of winds and their northeast monsoon depressions.

It is time citizens collectively acted to tackle these annual floods in neighbourhoods, streets and homes. The National Building Code urges a network of storm water drains in residential areas to harvest rainwater falling on rooftops and ground surfaces. Pebble beds are recommended to filter the water before they reach recharge wells (measuring 1m × 1m × 2m), located within the storm water drains.

Rainwater is fairly pure and easily meets drinking standards. Green building ratings reward designs that retain precipitation within the site; but do not mandate their storage in tanks. Well designed RWH systems prevent mixing of aquifers with sewage or wastewater. They must have oil and grease traps to avoid pollution. A chamber preceding the structure / well settles suspended particles by gravity and stops silt from reaching the subsoil water.

A recharge structure has slotted pipes similar to those used in borewells. These pipes are terminated at least 5 metres above the natural static subsoil water at its highest level. Incoming flows must freely pass through the natural ground conditions at those depths to cleanly augment an aquifer. Interestingly, recharge structures cannot be used to draw underground water for any purpose. A more sophisticated system would require lakes and reservoirs to collect, treat and reuse the water. According to the National Building Code, “the local authority preparing a town-planning scheme or a development plan should see that the local water bodies are preserved, and if dry, are activated by directing water-courses appropriately.”

Chennai Metropolitan Area measures 1,189 sq km and receives around 1,300 mm of annual rains.  This is equivalent to more than three years of Chennai’s yearly needs. This rainy season, let’s resolve to not waste the water falling on buildings and plots. We must channel them into tanks for reuse or recharge the ground. Water flooding onto streets and going  into the sea is a wasted bounty; and the periodic swings between scarcity and deluge must be better managed. Water is the elixir of life that nature lavishes so generously. Why throw it away uselessly?

Pavitra Sriprakash

Twitter@pavisriprakash

The writer is an architect, urban designer, dancer and chief designer at Shilpa Architects

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