There two methods of RWH and choosing which one to do will depend on factors such as whether it’s only roofs or also other surface runoffs, whether we aim to directly reuse the water and for what, amount of rain the place gets, soil quality for infiltration etc.

We have used passive as well as active systems in our small and medium projects with success. A passive system uses the earth as a container to collect and store water whereas an active system will use tanks, pumps and piping to store and reuse water.

When using a passive harvesting system, especially over larger areas, slowing runoffs, increasing retention and infiltration areas is at the top of the list and one sees this in the Amaidhi project where we used swales in the gardens and drainage channels on the roads to redirect water and have it move slowly across grassed surfaces before evacuating into ponds.

Acting as reservoirs, in the case of Amaidhi as well as Arati projects, the ponds are dug out taking into consideration land slopes and are interconnected such that when a pond at the higher level gets full, its overflow evacuates into the pond at the next level. Sizing of retention ponds is an important factor that would take into consideration rainfall received in that area as well as surfaces over which the runoff goes. It is ideal to have grassed swales as the grass roots behave like filters, slow down the speed with which the water would otherwise rush through and allows infiltration along the way.

When space is a constrain, such as in dense urban areas like Chennai, using an active system would not only take care of the roof runoffs, but also supplement the water available for uses within the project. A simple active system has been used in the Planter house project that uses roof rain runoff to water the planters along the facades. The active RWH system at the Planter house project, collects the rain runoffs from the roofs as well as the overflow from the planters through interconnected pipes and is stored in an underground sump below the parking area. A timed pump then irrigates all the elevated planters – the overflow from which is rerouted to the storage tank to minimize water wasted.