Friday, 26 September 2014

From Trickle to flood

From trickle to flood

It’s been raining cats and dogs in the City. Why don’t you harvest rainwater?

Over the last few weeks, we've been talking about the need to conserve precious resources such as water. And rainwater harvesting has emerged as a viable alternative to traditional and perennial sources of water in hilly areas, in places where the level of fluoride and arsenic is above permissible limits and in cities that battle water shortage in summer and flooding during the monsoon.

Good, bad & ugly
Bangalore has over 300 major lakes. Situated 920 metres above sea level, the naturally undulating terrain of the city, with its hills and valleys, forms a unique radial drainage pattern, which is ideal for capturing and storing rainwater.
Each valley at the ridge-top gives birth to small streams that cascade to form major stream systems in the three valleys of Hebbal, Koramangala-Challaghatta, and Vrishabhavati.
Over the years, an increase in population has led to a spurt in the number of residential buildings that have replaced traditional bungalows and their large open spaces. With the demand for land use in developed areas spiralling, vertical growth has become inevitable. This has also resulted in an increase in rooftop areas, pavements and roads.

  With massive concreting and asphalting of ground areas, the soil exposed for rainwater infiltration has decreased, leading to a drastic fall in the groundwater table and disappearance of open wells.
Over the last 30 years, Bangalore has experienced five years of severe drought and three years of moderate drought. The quest for water has resulted in over exploitation of groundwater. On an average, the groundwater level has progressively declined. Consequently, the quantity of run-off water in the storm drains has increased tremendously.
Bangalore is primarily dependent on River Cauvery, which is approx 95 km. away and 500 metres below Bangalore. While there is emphasis on centralised piped supply of water to the city, surface water bodies like lakes and tanks, which were traditional sources of supply, are on a steep decline. A conservative estimate puts the number of borewells in Bangalore at around two lakh. Usually, groundwater in shallow wells is polluted. Groundwater levels are falling due to heavy extraction and the absence of rechargeable aquifers. In fact, several deep borewells have run dry. To address the issue of water shortage, rainwater harvesting is a perfect solution.

Get the basics right
Rainwater harvesting is the process of collecting and storing rainwater in a scientific and controlled manner for future use. In cities, it includes:
  * Rooftop rainwater harvesting
  * Rainwater harvesting in paved and unpaved areas (fields, parks, pavements etc)
  * Rainwater harvesting in large areas with open ponds, lakes, tanks etc.
Rainwater harvesting potential in urban areas is huge. On a site measuring 40ft x 60ft (2400 sq ft), with an annual rainfall of about 1000 mm or 39.4 inches (Bangalore receives 1000 mm of rainfall annually), about 2,23,000 lts of rainwater can be harvested.

The amount of rainwater that can be harvested from the available rainwater in the plot depends on potential rainfall, catchment area available, collection methods and its efficiency etc.
Rainwater harvesting is a community-based programme and its success largely depends on the collective participation of everyone – from individuals to government bodies to builders to architects. With some thought and planning, you can store rainwater for direct use in tanks above ground or in underground sumps. Water thus stored can be used directly to flush toilets, water plants etc. Rainwater can be used to recharge groundwater through recharge pits, dug wells, borewells, soak pits, recharge trenches, etc.
Next week, we will see how some buildings in Bangalore have already taken the RWH route to sensible living.

Water facts
• Water supply in cities is woefully inadequate to meet galloping demand
•  Surface water is in short supply, hence the dependence on groundwater
• Due to rapid urbanisation, infiltration of rainwater into the sub-soil has decreased drastically, so recharging of groundwater has diminished
•  Rainwater, which is easily available, is an immediate source to augment the existing water supply

What you can do?
Rainwater harvesting is the smartest and most sensible solution we have to our water woes because:
 * It is environment friendly
 * It is an ideal solution for water requirement in areas having inadequate water resources
 * It increases groundwater level
 * It improves groundwater quality
 * It mitigates the effects of drought
 * It reduces the run-off, which otherwise floods storm-water drains
 * It limits flooding of roads and low-lying areas
 * It reduces soil erosion
 * It is cost-effective and easy to maintain
 * It brings about a huge saving in water and electricity bills