Rainfall Runoff Calculator

What is Stormwater Runoff?

When rain falls on a catchment area, some water infiltrates into the ground, some evaporates, and the rest becomes surface runoff that flows into drains, rivers, and streams. Quantifying runoff is essential for stormwater drainage design, flood risk assessment, and water resource planning.

The Rational Method

For catchments up to ~200 ha, the Rational Method gives peak runoff rate:

Q = C × i × A / 360 (m³/s)

Or: Q = C × i × A (when Q in L/s, i in mm/hr, A in ha)

Where:

• C = runoff coefficient (0 = all infiltrates; 1 = all runs off)
• i = rainfall intensity (mm/hr) for the storm duration = time of concentration
• A = catchment area (ha or km²)

Runoff Coefficients (C Values)

| Surface | C | |---------|---| | Impervious (roofs, concrete) | 0.90–0.95 | | Paved road | 0.85 | | Lawns (sandy soil) | 0.10–0.35 | | Lawns (clay soil) | 0.25–0.50 | | Woodland | 0.10–0.20 | | Agricultural land | 0.20–0.45 |

Practical Examples

Example: Urban catchment, 5 ha, 60% impervious (C=0.85) + 40% grass (C=0.35). Weighted C = 0.6×0.85 + 0.4×0.35 = 0.65. 2-year storm intensity = 50mm/hr. Q = 0.65 × 50 × 5 = 162.5 L/s — size the drain to handle this flow.