Urban Rainwater Harvesting

Urban rainwater harvesting captures rainfall from roofs and other relatively clean surfaces instead of allowing it to leave immediately through storm drains. After removing debris and contaminants, the collected runoff is conveyed to recharge structures, where it infiltrates into suitable soil or aquifers and supplements groundwater recharge.

Why recharge is needed in built-up areas

Buildings, roads and pavements create impervious surfaces that reduce natural infiltration and rapidly route rainfall into drains. Urban rainwater harvesting restores part of this disrupted pathway by collecting runoff close to where rain falls and directing it underground.

• Decentralised rooftop systems can operate within small urban plots where open land for infiltration is limited.
• Reducing the volume and speed of surface runoff can also ease pressure on storm-water drainage and lessen local waterlogging.

How collected water reaches groundwater

Roof runoff is carried through gutters and downpipes, with a first-flush arrangement and filters used to exclude initial dirty runoff, leaves and sediment. The treated water is then routed through a structure selected according to local soil, available space and aquifer conditions.

• Recharge pits and trenches increase the area and time available for water to percolate through permeable strata.
• Recharge shafts or wells can convey water past relatively impermeable near-surface layers towards deeper permeable formations.
• Where recharge is unsuitable, rainwater may instead be stored in tanks for non-potable uses, indirectly reducing groundwater withdrawal.

Conditions for effective and safe recharge

Recharge must be based on local hydrogeology, groundwater level, rainfall and catchment characteristics. It cannot be treated merely as disposal of urban runoff.

• Polluted runoff, sewage and industrial effluents must not be admitted because they can contaminate aquifers.
• Filters, silt traps and recharge structures require regular cleaning, especially before and during the rainy season.
• Sites with contaminated groundwater, unsuitable formations or risks of waterlogging require careful technical assessment before recharge is attempted.

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