Rational Method (Peak Runoff)
Estimates peak stormwater discharge from a catchment.
Formula
Variables
Example
C=0.7, i=50 mm/h over 2 ha gives a large peak flow
How it works
The Rational Method estimates the highest flow a drainage area can produce during a storm by combining how hard it rains, how much of that rain becomes runoff, and how much land is draining to the outlet. The coefficient C accounts for losses such as infiltration, evaporation, surface storage, and ponding, so only a fraction of the rainfall contributes to flow. It works best as a simple first estimate for small, fairly uniform catchments where the peak runoff is mainly controlled by a single storm of roughly uniform intensity.
When to use it
Use this formula when sizing small stormwater features such as gutters, culverts, roadside drains, and inlet capacity. It is also useful for quick flood screening or preliminary design before more detailed hydrologic modelling.
Worked example, step by step
Given: C = 0.45 I = 80 mm/h A = 1.5 ha Step 1: Use the Rational Method formula Q = C i A Step 2: Substitute the values Q = 0.45 × 80 × 1.5 Step 3: Calculate Q = 54 So the peak runoff is 54 when using C, mm/h, and ha consistently in the standard Rational Method form. In common engineering units, this corresponds to about 1.50 m³/s if the area is converted and the usual unit conversion factor is applied.
Common mistakes to avoid
- Mixing units without checking whether the formula version needs a conversion factor
- Using a runoff coefficient that is too low or too high for the actual surface type
- Applying the method to a very large or highly variable catchment where a simple peak-flow estimate is unreliable
Did You Know?
Cities have high runoff coefficients (~0.9) because pavement absorbs almost no rain.
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