CHAPTER - 10
RAIN WATER HARVESTING
10.1 Rainwater Harvesting principle:
The harvesting of rainwater simply involves the collection of water from surfaces on which rain falls, and subsequently storing this water for use. The rainwater collected can be stored for direct use or can be recharged into the underground aquifers. In scientific terms water harvesting (broadly) refers to collection and storage of rainwater from the rooftops. This also restricts evaporation and seepage into building foundations. All buildings having a plot size of 100 Sq.m. or more, while submitting the building plans for sanction, shall mandatorily include the complete proposal of rainwater harvesting.
A rainwater harvesting system consists of:
i. Roof catchment
iii. Down pipes
iv. Rain water/ Storm water drains v. Filter chamber
vi. Storage Tanks/ Pits/ Sumps
vii. Ground Water recharge structures like pit, trench, tube well or combination of above structure.
Rainwater Harvesting is a way to capture the rain runoff, store that water aboveground or charge the underground aquifers and use it later. This happens naturally in open rural areas. But in congested, over-paved metropolitan cities, there is a need to devise methods to capture the rain water. The rainwater that is incident on the surface/ roof top is guided to bore wells or pits or new/old/ abandoned wells through Small diameter pipes to recharge the underground water which can be used later whenever required. Rainwater can be harvested to the extent of 55,000 liters per 100Sq. m area per year from rooftops.
10.2 Rainwater harvesting techniques:
There are two main techniques of rain water harvestings.
a. Storage of rainwater on surface for future use.
b. Recharge to ground water.
10.3 Harvesting provisions in various Building categories:
All buildings in a city contribute to the rainwater runoff during the monsoon and hence such runoff can be harvested for water reuse/recharge. Rainwater harvesting in a building site includes storage or recharging into ground of rainwater falling on the terrace or on any paved or unpaved surface within the building site. The following systems may be adopted for harvesting the rainwater drawn from terrace and the paved surface.
a. Open well of a minimum of 1.00 m dia. and 6.00 m in depth into which rainwater may be channeled and allowed after filtration for removing silt and floating material. The well shall be provided with ventilating covers. The water from the open well may be used for non-potable domestic purposes such as washing, flushing and for watering the garden, etc.
b. Rainwater harvesting for recharge of ground water may be done through a bore well around which a pit of one meter width may be excavated up to a depth of at least 3.00 m and refilled with stone aggregate and sand. The filter drain water may be channeled to the refilled pit for recharging the bore well.
c. An impervious storage tank of required capacity may be constructed in the setback or other than, space and the rainwater may be channeled to the storage tank. The storage tank may be raised to a convenient height above the surface and shall always be provided with ventilating the surface and shall always be provided with ventilating covers and shall have draw off taps suitably place so that the rain water may be drawn off for domestic, washing, gardening and such other purposes. The storage tanks shall be provided with an overflow.
d. The surplus rainwater after storage may be recharged into ground through percolation pits, trenches, or combination of pits and trenches. Depending on the geomorphologic and topographical condition, the pits may be of the size of1.20 m width x 1.20 m length x 2.00 m to 2.50 m depth. The trenches can be or0.60 m width x 2.00 m to 6.00 m length x 1.50 m to 2.00 depth. Terrace water shall be channeled to pits or trenches. Such pits or trenches shall be backfilled with filter media comprising the following materials. –
i. 40 mm stone aggregate as bottom layer up to 50% of the depth;
ii. 20 mm stone aggregate as lower middle layer up to 20% of the depth;
iii. Course sand as upper middle layer up to 20% of the depth;
iv. A thin layer of fine sand as top layer;
v. Top 10% of the pits / trenches will be empty and a splash is to be provided in this portion in such a way that roof top water falls on the splash pad;
vi. Brick masonry wall is to be constructed on the exposed surface of pits /trenches and the cement mortar plastered; vii. The depth of wall below ground shall be such that the wall prevents losesoil entering into pits / trenches. The projection of the wall above ground shall at least be 15 cm;
viii. Perforated concrete slabs shall be provided on the pits / trenches.
e. If the open space surrounding the building is not paved, the top layer up to a sufficient depth shall be removed land refilled with course sand to allow percolation of rainwater into ground.
The terrace shall be connected to the open well / bore well / storage tank /recharge pit /trench by means of H.D.P.E. / P.V.C. pipes through filter media. A valve system shall be provided to enable the first washings from roof or terrace catchments, as they would contain undesirable dirt. The mouths of all pipes and opening shall be covered with mosquito (insect) proof wire net. For the efficient discharge of rainwater, there shall be at least two rain water pipes of 100 mm dia. for a roof area of 100Sq.mRainwater harvesting structures shall be sited as not to endanger the stability of building or earthwork. The structures shall be designed such that no dampness is caused in any part of the walls or foundation of the building or those of an adjacent building.
10.4 Rain Water Harvesting Provisions for Open spaces in cities
The open spaces/recreational land use generally constitute regional parks, district parks, playground and stadium, sports complex, monument zones, public parking, Plaza and other public open space. This may be as high as 30% to 50% of the city’s geographic area. All such public open spaces above the size of 500 Sq.m. shall have arrangements for complete utilization and capture of storm water with scientific rainwater harvesting arrangements. Following ideas may also be included: i. Well cum Channel cum Percolation pits. ii. Use of abandoned bore wells for recharging of ground water iii. Artificial or natural Storage of storm water runoff from larger sites.
10.5 Ground Water Recharge
Recharging of ground water should be made mandatory not only for residential buildings but for all types of buildings, including Group Housing Societies having a plot area more than 500 Sq.m. and above. The Ground Water Recharge should also be mandatory for open spaces like parks, parking, plazas and playgrounds. The harvesting and recharge structures could be constructed by the Authority with the involvement of community based organizations like Resident Welfare Associations.
10.6 Enforcement and Monitoring
a. The Authority shall constitute a Rainwater Harvesting Cell which will be responsible for enforcement and monitoring of the provisions of Rainwater Harvesting. The cell shall employ qualified persons who are well versed with the interpretation of Building Bye Laws and responsible for enforcement as well as monitoring the functioning of the Rainwater Harvesting System.
b. The Authority shall include inspection of Rainwater Harvesting Structures before issuing Completion Certificates or NOCs for service connections to the property.
c. Set an example in the city by ensuring that Rainwater is harvested in the properties assets owned by them including public buildings, markets, centers, parking spaces, roads and parks etc.
d. The Authority shall also establish a mechanism to monitor 100% of RWH provisions in all the buildings above 1000 Sq.m. with annual physical verification, while buildings less than 1000 Sq.m. can be monitored on the basis of 10% random survey by competent authority.
e. With regard to open public spaces viz., Parks, playgrounds etc. the implementation of provision rainwater harvesting may be done with the help of Residents Welfare Associations, Community Building Organization andNon- Governmental Organizations
f. The Authority shall ensure earmarking budgetary provision for the creation and maintenance of rainwater harvesting structures in public spaces owned and maintained by them, like parking spaces, parks, plazas etc.
g. The practice of incentives and penalties to promote rain water harvesting shall be formulated by the local authority based on best practices. Authority shall design its own incentive and penalty systems, considering the water level and scarcity.