RAIN WATER HARVESTING
The process of augmenting the underground water table by artificial infiltration of rainwater and surface run off is known as Rain Water Harvesting Mechanism.
Recent studies and practical experiences in the World indicate that the ground water table level is depleting alarmingly due to excessive tapping of underground water. The pace of development and the ever increasing population in urban areas lead to the over exploitation of available water resources thereby leads to the importance on artificial recharge of ground water become essential.
The main advantages are
– Level of Ground Water Table will be raised and maintaining.
– Salinity in the Water reduced and improvement in water quality.
– Deterioration of Buildings due to subsoil movement will be reduced.
Tamilnadu State Government as recently introduced a practical method for Rain Water Harvesting Mechanism and made it compulsory for all the structures located in a town/city. This method has got a set of procedures for establishing a Rain Water Harvesting Mechanism and may vary depending on the structures and locations, they are:-
1. Construction of Percolation Pit to recharge the ground water.
2. Construction of Percolation Pit with Bore to recharge the ground water.
3. Roof Top Water Collection and Recharge the ground water.
4. Recharging the defunct Open Wells, Bore Wells and Hand Pumps.
5. Rain Water Harvesting through Open Tanks and Ponds.
6. Storm Water Run off Collection and Recharging of ground water.
7. Artificial Recharge of ground water through Storm Water Drains.
Among the above methods, roof top water collection method is suggested at the individual structure within the Manapparai Municipal Limit.
RAIN WATER HARVESTING STRUCTURES CREATED IN GOVERNMENT BUILDINGS (URBAN)
| Type | Number of municipal Building | Number of municipal Buildings covered |
| 1.Municipal Building | 11Nos | 11Nos |
| 2.Open wells in use | 14 | 11 |
| 3.Failed Bore wells | 103 | |
| 4. Over Head Tanks | 306 | 76 |
| 5.Public Fountains | 14 | 14 |
| 6. Hand pump /plat Forms | 12 | 12 |
RAIN WATER HARVESTING STRUCTURES CREATED IN NON GOVERNMENT BUILDINGS (URBAN)
| Category | Total no of buildings | No.Of Buildings Covered | Balance no of Buildings |
| Private Houses | 11621 | 5087 | 6534 |
| Commercial Building | 87 | 73 | 12 |
| Industrial Buildings | 2 | 2 | Nil |
| Schools and Hospitals | 39 | 34 | 5 |
| Total | 10130 | 10130 | Nil |
How to harvest Rainwater ?
Harvesting rain water is very simple. When it rains we have to collect this water and not to allow it to run away. There are various techniques to do it. Broadly, rain water can be harvested for two purposes.
1. Stored for ready use in containers above ground or below ground.
2. Charged into the ground for withdrawal later (ground water recharging).
As indicated above rain water falling on the terrace can be collected through pipelines/drains and stored in a storage tank for direct use or diverted into a well for ground water recharge. Rain fall over the open spaces around the building may also be diverted for recharge purpose. The details of various RWH techniques are described in the Chapter “Different methods of Rainwater Harvesting”.
- What is Roof Top Harvesting?
To collect and store the rainwater which falls on the terrace of the buildings/houses. The water collected from the terrace is of good quality and it can be stored in tanks/sumps either for direct use or can be diverted to the existing bore well/open well for ground water recharge and storage.
The terrace of the building should be maintained clean. A grill/mesh has to be fixed at the entrance of the rain water pipe in the terrace to arrest large particles such as papers, leaves, etc. A filter chamber has to be provided to filter small/minute dust particles before diverting the rain water into the storage tank or open well/bore well.
- How to harvest rain water in the open spaces?
Rain water collected in the open spaces, being relatively dirty in nature, cannot be used for direct recharge of the well and therefore used for ground water recharge, using appropriate recharge methods/structures. In the absence of open well, the roof-top water may also be harvested along with open space water.A dwarf wall of (7.5 cm height) has to be constructed at the entrance/gate to avoid run off into the street/road. If man holes are present (sewerage/waste water line) the height of which has to be raised a little to avoid draining of rain water through the manholes.
- What is Roof Top Harvesting?
To collect and store the rainwater which falls on the terrace of the buildings/houses.The water collected from the terrace is of good quality and it can be stored in tanks/sumps either for direct use or can be diverted to the existing bore well/open well for ground water recharge and storage.
The terrace of the building should be maintained clean. A grill/mesh has to be fixed at the entrance of the rain water pipe in the terrace to arrest large particles such as papers, leaves, etc. A filter chamber has to be provided to filter small/minute dust particles before diverting the rain water into the storage tank or open well/bore well.
- How to harvest rain water in the open spaces?
Rain water collected in the open spaces, being relatively dirty in nature, cannot be used for direct recharge of the well and therefore used for ground water recharge, using appropriate recharge methods/structures. In the absence of open well, the roof-top water may also be harvested along with open space water.A dwarf wall of (7.5 cm height) has to be constructed at the entrance/gate to avoid run off into the street/road. If man holes are present (sewerage/waste water line) the height of which has to be raised a little to avoid draining of rain water through the manholes.
- How to harvest rain water in the open spaces?
Rain water collected in the open spaces, being relatively dirty in nature, cannot be used for direct recharge of the well and therefore used for ground water recharge, using appropriate recharge methods/structures. In the absence of open well, the roof-top water may also be harvested along with open space water.A dwarf wall of (7.5 cm height) has to be constructed at the entrance/gate to avoid run off into the street/road. If man holes are present (sewerage/waste water line) the height of which has to be raised a little to avoid draining of rain water through the manholes.
How much water can be harveste?
The following is on illustrative calculation that highlights the enormous potential for rainwater harvesting. The same procedure can be applied to get the potential for any land of or rooftop area. using rainfall data for that area.
Consider a building with a flat terrace area of 100 sq.m. The average annual rainfall in chennai is proximately 1100 mm (44 inches). In simple terms, this means that if the terrace floor is assumed to be impermeable, and all the rain that falls on it is retained without evaporation, then, in one year, there will be rainwater on the terrace floor to a height of 1100mm.
Area of plot = 100sq.m.
Height of rainfall= 1.1.(1100 mm or 44 inches)
Volume of rainfall over the plot= Area of plot x Height of rainfall
= 100 sq.m. 1.1cu.m.110 cu.m.(1.10.000 liters)
Assuming that only 60% of that total rainfall is effectively harvested.
Volume of water harvested= 66,000 liters
This volume is about four times the annual drinks water requirement of a five member family. The average daily water requirement per person is 10 liters according to IS172: Indian standard code of Basic Requirement for Water Supply, Drainage and Sanitation.