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06·ToolsArcheve AIPNEW · Hydrology

Rational Method
peak runoff.

Q = CiA for small catchments — with composite runoff coefficient across up to three sub-areas. SI units, instant results.

Sub-areas (leave area 0 to skip)
Peak flow Q—
Peak flow—
Total area—
Composite C—
Branded A4 sheet — inputs, results & section diagram. Drop it straight into your report appendix.
Q = C·i·A / 360 with A in hectares. Rational Method applies to small catchments (typically < 80 ha) where time of concentration sets the design-storm duration. Preliminary aid — verify against project criteria.

Got your peak flow. Sizing a network or pond from it — and need the design defensible at municipal review?

Brief us →

How it works

Q = C·i·A / 360 — Q in m³/s, C the dimensionless runoff coefficient, i the design rainfall intensity (mm/hr) at a duration equal to the catchment's time of concentration, and A the area in hectares. With multiple surfaces, the composite coefficient is the area-weighted mean: Cw = ΣC·A / ΣA.

Runoff coefficient C — typical values

SurfaceC
Roofs0.85 – 0.95
Asphalt / concrete pavement0.80 – 0.95
Gravel surfaces0.40 – 0.60
Urban landscape / irrigated gardens0.15 – 0.35
Desert / sparse natural ground (Gulf)0.30 – 0.60
Steep rocky wadi slopes0.50 – 0.80

Gulf caution: arid soils crust and shed water far more aggressively than temperate lookup tables assume — municipal criteria (MOMRAH, Abu Dhabi DMT) often mandate minimum C values for desert catchments. Our note on Saudi rainfall design criteria covers selecting the intensity i defensibly.

When the Rational Method stops being enough

Beyond ~80 ha, with significant storage, or where flow paths interact — peak-only methods miss timing and volume effects that govern the design. That's hydrograph and 2D territory: stormwater modelling in HEC-HMS, HEC-RAS 2D or InfoWorks ICM. Capacity-check the receiving pipe or channel with the Manning calculator.