Rainwater harvesting systems from rooftops work on the following conventions. Generally roofs are flat and accessible. This means the roofs can be cleaned easily. Rainfall intensity is less than 50 mm per hour. It is a convention to build sumps for water storage and the size is usually 5000 to 6000 litres, this is because water required for construction is usually bought from private tankers and stored in these sumps. Average rainfall over the last 10 years is around 970 millimetres with about 59 rainy days spread from April to November. Rainfall is a relatively well-distributed and typically bi-modal with peak in April-May and September-October. This makes Bangalore relatively better for rainwater harvesting because of the spread.
Architects, engineers and planners need to integrate rainwater harvesting systems in building designs, landscapes and neighbourhoods. Right from small houses to the city itself, rainwater harvesting can be adopted provided inter institutional coordination and professional involvement is generated. In a high water demand scenario, it will be rainwater harvesting and water recycling which will provide for a sustainable water supply and mitigate urban flooding to an extent. For apartments, individual houses, institutions and industries, the first step could be a Rain Barrel. Depending on the performance of the Rain Barrel, the project could be scaled up to cover the entire plot or site to ensure maximum utilization of rainwater.
Residence of Pradeep and Pushpa
Located 17km north of the city in a suburb called Vidyaranyapura, adjacent to the University of Agricultural Sciences campus – GKVK – the city water lines have not yet reached the colony. Water supply is through common bore -wells being maintained by the Association. The plot area for the site is 288sqm and the roof area of the house is 121sqm. By appropriately sloping roofs, locating down-pipes and providing filters at each down-pipe, clear water is guided to a sump of capacity 8000lt. Overflow from the sump is led to a storm water drain. For the remaining plot area ground slopes are worked and water collected in a small water body. Overflow from the water body will also recharge the ground water through percolation pits. Every year it is estimated that 115,700lt of rainwater will be harvested and 32,400lt of water will be recharged. It is estimated that an additional cost of about `4000/- has been incurred mainly in the filters towards rainwater harvesting.
Residence of Prithvi and Purshottam
Located in Sahakara Nagar about 13km north of the city centre. City water lines are connected to the house. The two have an open well in the plot, which is about 6m deep. Water levels in the dry season are at about 5m below the ground and in the monsoon it comes up to about 1 metre below the ground. The flat roof of about 60sqm was gently sloped to a single point for the down water pipe, which would come down close to the sump location. A first rain separator was provided to segregate the first 2.5mm of rainwater. A drum filter designed by the owner was installed on the down pipe. After filtration the water is led into a sump of capacity 6000lt.
Overflow from the sump is led to the open well to recharge the unconfined aquifer. If by chance the well water reaches to the top, provision is made for leading it out to a storm water drain outside. Every year close to 54,000lt will be harvested and about 40,000lt recharged. An additional cost of about `2,500/- towards filter and pipeline to open well has been incurred towards rainwater harvesting. Zero rainwater runoff house. Rainwater is harvested in sump and excess water recharged to an open well.