What, Why & How of Rain Water Harvesting (RWH)?
Let us first think of why this term has been coined? It’s obvious that due to various human interventions we have somehow managed successfully to break the ecological and hydrological cycle present in nature and thus have to find out ways to supplement it. Rain water harvesting is one of the subset to this solution. In layman terms, rain water harvesting is nothing but making right use of the rain water in direct or indirect ways. It can be done by recharging groundwater levels through injection wells, storing it in lime plastered masonry or concrete tanks for further domestic use, creating artificial reservoirs by collecting rain water from vast area channelled through efficient storm water networks.
Major benefit of implementing RWH system is that you’re not stuck with one water source and even enhances the storm water management at your door and get away from the risk of flooding.
Widely used RWH system in present days
First and foremost step for planning RWH system is conducting hydrogeological survey. It is supplemented by geophysical survey to ascertain water-bearing zones of shallow, deep aquifers and salinity levels which helps to understand subsurface geology and feasibility at various locations by identifying the zones which are favorable for planning of artificial recharge structures. Out of all geophysical surveys, electrical resistivity method is the best and reliable for ground water prospecting and is principally based on the study of resistance offered by sub-surface formation to the flow of current. There will be appreciable concentration of current in places where there are large masses of highly conductive bodies. Based on the results, the depth of the injection well can be identified. In addition to that, the hydraulic conductivity i.e. recharge capacity can be obtained by on-site aquifer pumping test conducted on test bore.
Further, boring is done up to required depth and casing pipe is lowered. Perforated casing pipe is placed in area identified with fractures i.e. zones where recharge can be possible and in rest, blank casing pipe is placed. After lowering the casing pipe, a well is created up to 3-5 meter depth with all filter media present surrounding it. Filter media consist of layers of gravels, coarse sand, charcoal & activated carbon which helps to purify water before it gets recharged. In addition to that, a dual “V-wire screen” is provided at bottom of the well surrounding the casing pipe. This screen gives inwardly widening V-shape slots. The shape does not give space for any sand particle to get stuck inside the slot and hence these screens are non-clogging.
Such injection wells can be planned in line with storm water drain network, inside the artificial water retaining structure (e.g. pond) or independently in a landscape area with slope toward the injection well.
Regulations in Gujarat for RWH:
Under the Gujarat Development Control Regulations, buildings with area between 500 and 1500 sq. meters, the owner or developer shall have to undertake Rainwater Harvesting as per the Authority Specifications. For buildings with area between 1500 to 4000 sq. meters, owner/developer has to provide percolation wells with rain water harvesting system at one percolating well for every 4000 sq. meters or part thereof of building unit. The state Roads and Buildings Department has made rainwater harvesting mandatory for all government buildings.
Fig. Pictorial presentation of Rain Water Harvesting done by Injection well system
– Keval Vejani Design Engineer