Rational method

HDAT uses the Rational method for catchments below the configured area threshold.

Inputs

• \(C\): runoff coefficient (dimensionless)
• \(A\): catchment area in hectares (\(ha\))
• \(I\): rainfall intensity matched to duration \(T_c\) (typically from IDF fit)

Peak discharge equation (legacy)

\[ Q = \frac{C \cdot A \cdot I}{360} \]

Where:

• \(Q\) is in \(m^3/s\)
• \(I\) is consistent with the legacy convention used in the parity spec

Engineering notes

• Ensure intensity \(I\) corresponds to the same return period as the computed peak.
• Ensure area and intensity units are consistent with the conversion constant \(360\) used in the legacy equation.
• Document the selected \(T_c\) method and the intensity segment used (IDF segment A/B/C).