Time of concentration (Tc)¶
HDAT supports multiple time-of-concentration methods and uses \(T_c\) (minutes) to select rainfall intensity from IDF curves for each return period.
Kirpich (legacy formula)¶
Inputs:
- \(L\): flow path length, \(lfp_m\) in meters
- \(\Delta H\): elevation difference, \(hDif_m\) in meters
Equation:
\[
T_c = \frac{L^{1.15}}{(\Delta H)^{0.385}\cdot 52}
\]
Bransby-Williams (legacy formula)¶
Inputs:
- \(L\): flow path length, \(lfp_{km}\) in kilometers
- \(A\): catchment area, \(ca_{km^2}\) in square kilometers
- \(S\): slope (dimensionless, \(m/m\))
Equation:
\[
T_c = 14.56 \cdot \frac{L}{A^{0.1}\cdot S^{0.2}}
\]
Method selection: Rational vs SCS¶
Legacy behavior selects the hydrology method by catchment area threshold:
- if \(A_{ha} > A_{threshold}\) then SCS
- else Rational
This preserves legacy “small catchment vs large catchment” method branching.