Rooftop Rainwater Harvesting Design: Runoff, Tank and Recharge Pit
Many Indian cities, including Chennai, Bengaluru and Delhi, require rainwater harvesting for new buildings above a certain plot size. A well-designed rooftop system either stores rainwater for direct use or sends it into the ground to recharge the aquifer. The design is simple engineering arithmetic based on roof area, rainfall and demand. This article explains the components and works through runoff, pipe flow, storage tank and recharge pit calculations for a typical house.
Components of a Rooftop System
| Component | Purpose |
|---|---|
| Catchment (roof) | Collects rainfall; RCC, tiled or metal sheet roofs |
| Gutters and down pipes | Carry water from the roof to ground level |
| First flush device | Diverts the first, dirtiest runoff of each rain away from storage |
| Filter | Sand, gravel and charcoal or mesh filter to remove debris and silt |
| Storage tank (sump) | Stores filtered water for non-potable or treated use |
| Recharge structure | Pit, trench or recharge well that infiltrates excess water into the ground |
Guidelines for rooftop rainwater harvesting are given in IS 15797, and the Central Ground Water Board publishes manuals on recharge structures. Local building bye-laws specify what is mandatory in each city.
Step 1: Annual Harvestable Runoff
Not all rain that falls on a roof can be collected. Some is lost to evaporation, splashing and wetting of the surface. The runoff coefficient C accounts for this.
Harvestable runoff (m³) = A x R x C A = plan area of roof (m²) (use plan area, not sloping area) R = rainfall (m) C = runoff coefficient
| Catchment surface | Typical runoff coefficient |
|---|---|
| RCC roof, metal sheet roof | 0.80 to 0.95 |
| Clay tile roof | 0.75 to 0.90 |
| Paved area | 0.60 to 0.85 |
| Lawn or garden | 0.05 to 0.30 |
Worked example
A house has an RCC roof of plan area 120 m². Average annual rainfall at the site is 800 mm. Take C = 0.85.
Runoff = 120 x 0.800 x 0.85 = 81.6 m³ per year = 81,600 litres
That is the water available in an average year. In a dry year the figure will be lower, so storage should not be sized on the average alone.
Step 2: Peak Flow for Gutters and Down Pipes
Pipes must carry the peak flow during intense rain, not the annual total. Use the rational formula with a design rainfall intensity for short storms at the site.
Q (litres/second) = C x i x A / 3600 i = rainfall intensity (mm/hour) For A = 120 m², C = 0.9, i = 75 mm/h (illustrative): Q = 0.9 x 75 x 120 / 3600 = 2.25 L/s
With two down pipes each draining half the roof, each pipe carries about 1.1 L/s. Choose pipe diameters from a plumbing design table for that flow and slope; undersized pipes overflow at the gutter during heavy storms.
Step 3: Storage Tank Size
For direct use, the tank is sized for the demand during the dry period, limited by the water available. A simple approach is the dry-period method.
Family of 5, water for drinking and cooking = 10 L/person/day Daily demand = 5 x 10 = 50 L/day Dry period between monsoons = 120 days Storage required = 50 x 120 = 6,000 litres Check: annual runoff 81,600 L >> 6,000 L -> supply is sufficient
Provide a 6,000 litre sump, for example 2.0 m x 1.5 m x 2.0 m water depth plus freeboard. The excess runoff, about 75,600 litres a year in this case, is sent to the recharge pit. For larger demands such as flushing and gardening, prepare a month-by-month balance of inflow and demand and size the tank for the largest cumulative deficit.
Step 4: Recharge Pit
A recharge pit is filled from bottom to top with boulders, then gravel, then coarse sand, with a thin top layer that can be cleaned. Water fills the voids during a storm and slowly infiltrates into the soil. The pit is sized to hold the runoff of a design storm, allowing for the fact that only the voids store water.
Design storm (illustrative) = 25 mm in a short spell Runoff = 120 x 0.025 x 0.85 = 2.55 m³ Porosity of filter media (boulders and gravel) = about 0.40 Gross pit volume = 2.55 / 0.40 = 6.4 m³ Provide two pits of 1.5 m x 1.5 m x 1.5 m deep Volume = 2 x 3.375 = 6.75 m³ > 6.4 m³ -> OK
Infiltration during the storm adds a margin. In soils with very low permeability, such as clay, a recharge well or bore with a filter chamber may be needed to reach a permeable layer. The bottom of a recharge pit should stay well above the groundwater table so that water is filtered by the soil before reaching the aquifer.
Common Mistakes
- Connecting roof pipes directly to the sump without a first flush or filter, which fills the tank with dust and leaves.
- Mixing kitchen or toilet waste water with the rainwater system.
- Building a recharge pit close to a septic tank or soak pit, risking groundwater contamination.
- Never cleaning the filter media, so the pit clogs within a few seasons.
Summary
Annual harvestable runoff equals roof area times rainfall times runoff coefficient, about 81.6 m³ for a 120 m² roof with 800 mm of rain. Down pipes are sized from peak flow using Q = CiA/3600, storage from dry-period demand, and recharge pits from the runoff of a design storm divided by the porosity of the filter media. With a first flush, a filter and regular cleaning, such a system reliably saves water and recharges groundwater.