What is the main formula to calculate how much water can be harvested?

The volume of harvested water is calculated as: Roof Area × Rainfall × Runoff Coefficient (Clause 6.2).

What is a typical runoff coefficient for a metal roof?

The runoff coefficient for a corrugated metal sheet roof is typically between 0.80 and 0.90 (Annex A).

How much water should be allocated per person for drinking and cooking when designing the system?

A minimum of 10 litres per capita per day (lpcd) is recommended for drinking and cooking purposes (Clause 5.4).

What is the purpose of a first flush system?

To divert the initial rainwater that washes away dust, leaves, and bird droppings from the roof, thus preventing contamination of the stored water (Clause 4.5).

IS 14961

: 2001

Guidelines for Rain Water Harvesting in Hilly Areas by Roof Water Collection System

CurrentSpecializedGuidelinesBIMWater Resources · Hill Area Development Engineering

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This standard provides guidelines for planning, designing, and implementing roof water collection systems for rainwater harvesting in hilly areas. It covers aspects like estimating rainfall runoff from roofs, sizing of system components like gutters and tanks, and maintaining water quality for domestic use.

Guidelines for Rain Water Harvesting in Hilly Areas by Roof Water Collection System

Quick Reference Values

Runoff coefficient for corrugated metal sheets0.80 - 0.90

Runoff coefficient for tiles0.75 - 0.85

Recommended minimum water allowance for drinking & cooking10 litres per capita per day (lpcd)

Recommended quantity for first flush0.5 to 1.0 mm of rainfall

Minimum diameter for downpipe75 mm

Key Formulas

V = A x R x C — Volume of Runoff (V) = Roof Area (A) x Rainfall (R) x Runoff Coefficient (C)

Practical Notes

Ensure a robust 'first flush' mechanism is installed to divert the initial, more contaminated rainwater, which is crucial for maintaining water quality in the storage tank.

The structural stability of the storage tank is critical, especially on sloped terrain. A proper foundation and potential retaining structures should be designed, possibly requiring geotechnical input.

In hilly areas, local rainfall can vary significantly from regional data. It is advisable to use locally observed data or install a rain gauge for more accurate system sizing.