Rainwater Harvesting in Coimbatore

Rainwater Harvesting in Coimbatore Home Guide

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Coimbatore depends on the Siruvani and Pilloor schemes for piped water and on borewells for much of the rest, and many areas see groundwater levels fall in the dry months. A working rainwater harvesting (RWH) system catches the rain that falls on your own roof and either stores it or sends it back into the ground. This guide explains the rule, the main options for a house, and how to check the work.

The Tamil Nadu Rule

In July 2003, after several drought years, Tamil Nadu amended its municipal laws to make rainwater harvesting structures mandatory for buildings, public and private. Local bodies were told to approve new building plans only with RWH provided, and new buildings could be refused water and sewer connections without it. Tamil Nadu is widely described as the first Indian state to make rooftop RWH compulsory for all buildings.

Rules on the size and type of structure are set by the local body and the building rules, and they are revised from time to time. Before you build or modify, check the current requirement with the Coimbatore Corporation or your municipality or panchayat, and ask your architect or engineer to show the RWH design in the approval drawings.

How Much Water Can a Roof Give?

A simple rule of thumb: 1 mm of rain on 1 square metre of roof gives about 1 litre of water. So a 100 sq m (about 1,075 sq ft) roof gives roughly 100 litres for every millimetre of rain, before losses. Even a moderate shower yields thousands of litres from a typical house, which is why RWH is worth doing properly.

The Main Options

1. Recharge pit or trench

Rainwater from the roof flows through pipes into a pit filled with layers of filter material, usually pebbles, gravel and coarse sand, and soaks into the ground.

Suits most independent houses.
Helps nearby open wells and borewells over time.
Needs space away from the foundation and away from any septic tank or soak pit.

2. Recharge well or borewell recharge

A deeper recharge well, or an existing borewell, receives filtered roof water. This can recharge deeper layers faster.

Needs a desilting chamber and a proper filter before the well.
Only roof water, never road runoff, drain water or water from areas where vehicles are washed.

3. Storage sump or tank

Filtered roof water is stored in a separate sump for washing, gardening and, with suitable treatment, other uses.

Useful where the ground absorbs water slowly.
Keep it covered and separate from your corporation water sump, unless an engineer designs a safe connection.
If you ever plan to drink stored rainwater, get it tested and use proper purification; see our guide on choosing a water purifier service centre.

4. Combination

Many homes store what they can use and send the overflow to a recharge pit. This is often the most practical design.

Key Parts of a Good System

Clean roof and gutters: leaves and bird droppings are the main source of dirt.
First-flush arrangement: diverts the first few minutes of rain, which washes the roof, away from the tank or pit.
Filter: a mesh screen plus a filter chamber with sand and gravel, or a ready-made filter unit.
Pipes sized for heavy rain, so water does not overflow onto walls.
Overflow path from the sump or pit to a safe outlet.
Inspection covers so you can clean each chamber.

Apartments and Larger Buildings

In apartments, RWH is usually a common facility. Ask the association:

Where the recharge pits or wells are, and whether they were cleaned this year.
Whether terrace outlets are connected to the RWH system or straight to the drain.
Who maintains it and how often.

Choosing Someone to Build or Repair It

Plumbers, civil contractors and specialist RWH firms all do this work. Ask:

For a site visit first: they should look at the roof area, soil, existing sumps and borewell before quoting.
What each part is made of and its size: pit depth and diameter, filter media, pipe size.
Where the pit will go relative to the foundation, septic tank and borewell.
For a written quote that lists every item and a short workmanship warranty.

Our guide on choosing a plumber in Coimbatore covers general points on quotes and payments.

RWH and Your Borewell

If your borewell yield drops in summer, recharge can help over the years, but it is not an instant fix and results depend on the ground below your plot. Our borewell drilling guide explains what to check before drilling a new one; recharging the existing well is worth trying first.

Roof and Terrace Care

Water that pools on the terrace ends up as leaks, not harvest. Make sure the terrace slopes to the outlets and the outlets are not blocked. If your ceiling already shows damp patches, see our guide to terrace waterproofing in Coimbatore before the rains.

Yearly Maintenance Checklist

Before each monsoon: clean the roof, gutters, mesh and first-flush.
Once a year: open the filter chamber, remove silt, wash or replace the top sand layer.
Check the recharge pit after the first heavy rain: water should drain away within a reasonable time. If it stands for days, the filter is clogged.
Keep sumps and pits covered for safety, especially with children around, and to stop mosquitoes breeding.

Red Flags

A "system" that is only a pipe dropped into the ground with no filter.
Road or drain water led into a borewell.
Pits dug right next to the foundation or the septic tank.
No written quote or no description of the materials.

Next Steps

Check your roof outlets and the existing structures, then get a site visit and written quote from two or three RWH builders. Confirm the current rules with your local body. You can find plumbers and civil contractors listed in the Kovai Yellow Pages directory.

Frequently Asked Questions (FAQ)

Is rainwater harvesting compulsory in Tamil Nadu?

Yes. In 2003 Tamil Nadu amended its municipal laws to make rainwater harvesting structures mandatory for buildings, and local bodies may refuse plan approval or water and sewer connections to new buildings without them. Check the current requirement for your plot with your local body.

What is the difference between a recharge pit and a storage sump?

A recharge pit lets rainwater soak into the ground to raise the groundwater level and help borewells. A storage sump or tank keeps filtered rainwater for later use. Many houses use both: store what you can use and recharge the overflow.

Can I send rainwater directly into my borewell?

Rooftop water can be used to recharge a borewell, but only after it passes through a proper filter and a desilting chamber, so that mud and dirt do not choke or contaminate the well. Road runoff and drain water must never go into a borewell.

How often should a rainwater harvesting system be cleaned?

Clean roof gutters and the first filter before each monsoon, and check the recharge pit and filter media at least once a year. Coimbatore gets rain in both the south-west and north-east monsoons, so a check before each season is sensible.