Why RWH is necessary; methods

Why necessary

• Water scarcity and falling groundwater levels, especially in urban areas like the Kathmandu valley.
• Poor or irregular piped water supply; RWH reduces dependence on tankers and groundwater pumping.
• Reduces surface runoff, urban flooding and soil erosion.
• Rainwater is soft and nearly free of salts; it is cheap and simple to collect.
• Saves the cost of water and energy; helps groundwater recharge.

Method: rooftop harvesting with storage

Rain falling on the roof is collected and stored for use. Steps:

1. The roof (catchment) is kept clean; gutters are fixed at the eaves with a slope of about 1:100.
2. Water flows through down pipes (75-100 mm).
3. A first-flush diverter discards the initial dirty water.
4. Water passes through a filter (gravel, sand, charcoal).
5. Clean water is stored in a tank (underground or overhead; RCC, plastic or ferrocement), with overflow to a recharge pit.
6. Disinfection (chlorine) if the water is used for drinking.

  roof -> gutter -> down pipe -> first flush -> filter
        -> storage tank -> use ; overflow -> recharge pit

Water harvested per year = annual rainfall x roof area x runoff coefficient (0.8-0.9). The tank is sized from the daily demand and the length of the dry period.

Other method (groundwater recharge): filtered roof runoff is led to a recharge pit or trench (1-2 m wide, 2-3 m deep) filled with boulders, gravel and sand, or to a recharge well, so that it soaks into the aquifer.