Using the metro lines – rainwater harvesting

Everybody hates a metro when it is being built , everybody loves to ride in one – Quote

As the elevated tracks come up over the city a mass transport network called the metro is being built all over the city. In phases it is likely to cover over a 100 kilometre and become one of the prime modes of transport to the city. Apart from the transport benefits it will bring to the city there is one other potential benefit that could accrue to the city due to its construction.

The metro rail track offers a tremendous potential for rainwater harvesting. A total length of about 45 kilometres and a width of about 12 metres means that the endowment of rainwater on the relatively very clean track with Bangalore’s rains of 970 mm annually is about 523 million litres. Considering a coefficient of runoff of 0.90 the harvestable rainwater is around 471 million litres annually. This could provide about 13,000 people with their annual requirement of water at 100 litres per day. A substantial sum.

Since the metro will intend to develop a small landscaped area between piers of about 25 metres and with a width of 2.50 metres, part of the rainwater could be used for this landscape purpose. The additional rainwater could be used to recharge the groundwater aquifer without causing any urban flooding.

The proposed strategy for the rainwater harvesting in the metro could include

–          A rain barrel attached to every pier to store rainwater above the ground. This will enable water to be drawn by gravity for drip irrigation of the landscaped area between the piers.

–          A sump tank to store the water which will overflow from the rain barrel. There could be a small pumping arrangement to refill the Rain Barrel when empty and draw the water for use as and when required. The sump tank would also have an arrangement for a cowl which could be used to fill water from a private water tanker in case the sump goes empty for want of rain and during long stretches of dry days.

–          A recharge well to take the final overflow from the sump tank and recharge the aquifer so that the groundwater table is made up. The recharge well can also be used as a regular well in stretches where the groundwater table is high. This will also help improve both the quantity and quality of groundwater along the stretch of the metro as a positive externality.

A landscape designed for low water use, say growing plants like bougainvillea, would demand roughly 4 litres per square metre daily which could be distributed through drip irrigation systems.

By integrating rainwater harvesting the urban flooding issues around some areas could be addressed. If done in conjunction with the BBMP storm water drain, an integrated strategy for managing storm waters could be obtained providing great benefits to the groundwater.

It is good to look at negative externalities of a project and convert them to benefits so that society at large is benefitted. In this is the path to satisfy urban water demand and become waterwise.