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Runoff from a 20 ha built-up area

Short answer

Weighted runoff coefficient is 0.44. Using the rational formula Q=CiA/360 with A=20 ha and i=50 mm/hr gives runoff = 1.222 m3/s (about 1222 L/s).

A built-up area of 20 ha consisting of 40% roof and pavements with runoff coefficient as 0.8 and 60% lawns and gardens with runoff coefficient as 0.2 has a rainfall intensity of 50 mm/hr. What will be the runoff from that area?

The runoff from the area is found by the Rational formula Q=CiA/360Q = CiA/360, with AA in hectares, ii in mm/hr and QQ in m³/s.

Weighted runoff coefficient

C=0.40(0.8)+0.60(0.2)1.00=0.32+0.12=0.44C=\frac{0.40(0.8)+0.60(0.2)}{1.00}=0.32+0.12=0.44

Runoff

Q=C i A360=0.44×50×20360=1.222 m3/sQ=\frac{C\,i\,A}{360}=\frac{0.44\times50\times20}{360}=1.222\ \text{m}^3/\text{s}

Answer: Runoff = 1.222 m³/s (about 1222 l/s).

✓ Answer checked by Claude Opus 5.5 (high effort)

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