KMC Building Rules, 2009, rule 144(1) says a rainwater harvesting system shall form part of a new building and be included in the plan, for direct use, for recharging groundwater or for both. The rules found give no plot threshold. Kolkata averages 1,713.5 mm of rain a year, so a 100 sq m concrete roof can yield about 1.2 lakh litres.
What do KMC and NKDA require?
Both authorities ask for a harvesting system in the plan of a new building, and neither text found sets a size of storage or recharge.
KMC: rule 144(1) of the Kolkata Municipal Corporation Building Rules, 2009 reads that the system “shall form a part of the building and shall have to be included in the plan, either for direct use of the rain water or for ground water recharging or both”. It applies to a new building or any housing complex, and to expansion as per the Environmental Impact Assessment guideline of the State or Central Government. The system must comply with Central and State statutory requirements. Rule 117(2) also lets the Commissioner require rain pipes or a harvesting system in line with rule 144. The text of rule 144 found for this post gives no plot-size threshold, volume or recharge rate, so ask KMC, or your architect, for the figure that applies to your plot.
NKDA: a 2021 amendment adds sub-rule 28(4) to the New Town rules. Roof-top harvesting shall form part of the building and be included in the plan for new buildings and complexes. A 15 per cent reduction in the building plan sanction fee may be allowed if a system is added to an existing building. The same notification asks for wastewater recycling at 25,000 litres a day and above.
Groundwater law: the West Bengal Ground Water Resources (Management, Control and Regulation) Act, 2005 sets up authorities, including a Corporation Level Authority for KMC, and asks the State authority to promote water harvesting. The summaries found do not add a building-plan requirement. The Act’s full text was not read.
The rules change, so check the current text with the authority. The guides to KMC plan sanction and NKDA plan approval describe how the plan is submitted.
How much water can a roof collect?
The Central Ground Water Board gives a formula: water available in litres equals annual rainfall in mm, times roof area in sq m, times a runoff coefficient. Its runoff coefficients for roofs are:
| Roof surface | Runoff coefficient |
|---|---|
| GI sheet | 0.9 |
| Asbestos sheet | 0.8 |
| Tiled | 0.75 |
| Concrete | 0.7 |
The India Meteorological Department gives Kolkata (Alipore) an average annual rainfall of 1,713.5 mm over 106 years of records. June to September bring about 75 per cent of it, July is the rainiest month at 363.2 mm, and there are 84 rainy days a year. These are the 1951 to 2000 normals, and the 1991 to 2020 normal was not found.
For a 100 sq m concrete roof, the arithmetic is 1,713.5 x 100 x 0.7, which is about 119,945 litres, or 120 cubic metres, in a year. In July alone it is about 25,400 litres. This is arithmetic from the two published figures, and it is an upper bound: the first rain is diverted, and gutters and pipes lose some.
What does a rainwater harvesting system consist of?
The CGWB manual lists the parts of a roof-top system:
- The roof catchment.
- Gutters and drain pipes, which carry the water from the roof to a down pipe.
- A first flush pipe.
- A filter unit.
- A storage tank and a collection sump.
- A pump, if the water is to be lifted.
The manual says the storage tank and filter unit are the most expensive and critical components. It describes a semi-automatic first flush as a separate vertical pipe fixed to the down pipe, with a valve below the T junction, closed once the first rain has washed the roof. No millimetre or litre rule for first flush was found in the manual. For the collection sump it mentions a 60 x 60 x 60 cm size.
For city tank sizing the manual uses the rain in a single spell: rainfall per spell times roof area times the runoff coefficient.
How does recharge work?
Recharge sends clean rainwater into the ground through pits, trenches or shafts, so that it joins the aquifer. Chapter 6 of the CGWB manual covers recharge pits, shafts and trenches. This post gives no recharge dimensions, because the manual’s figures depend on the soil and on the water table, and Kolkata’s alluvial soil needs a site investigation. Ask the engineer for the design.
Water quality matters when groundwater is recharged or used. A 23-year review of 4,210 groundwater samples from all 141 KMC wards found that 14.2 per cent had arsenic above 10 micrograms per litre and 5.2 per cent above 50, in 77 and 37 wards respectively, with higher contamination in the south of the city. The CGWB state profile lists iron above 1.0 mg per litre and lead above 0.01 mg per litre for Kolkata. A harvesting system lets you use rainwater for non-potable needs, and drinking water must be tested.
Is there a BIS standard?
BIS publishes IS 15797 of 2008, Roof top rainwater harvesting, Guidelines, as a code of practice, with no amendment issued. The text could not be opened for this post, so it gives no first flush volume, tank sizing or distance figures from the standard. Ask your engineer to design to it.
What should an owner do?
Treat harvesting as part of the building, and put it on the plan:
- Ask your architect which authority applies and what the current rule requires on your plot.
- Mark the roof catchment, the down pipes, the filter, the tank and the recharge point on the drawings.
- Choose the use: toilet flushing and garden use are simpler than drinking.
- Keep the roof clean, and clear the gutters and the filter before each monsoon.
The studio’s building plan approval service covers submission, and its architecture service covers the design. On the damp that rain can cause, see the guide to seepage in Kolkata houses. To discuss a plot, use the contact page.
