Formulae & Notes
$$\text{Kirpich: } T_c = 0.0663 \times L^{0.77} \times S^{-0.385}$$
$L$ in meters, $S$ is dimensionless slope, $T_c$ in minutes.
$$\text{Bransby-Williams: } T_c = 0.94 \times \frac{L^{0.56}}{\,S^{0.34}\,n^{0.11}}$$
$L$ in meters, $S$ dimensionless slope, $n$ roughness, $T_c$ in minutes.
Coefficients vary by reference.
$$\text{Izzard: } T_c = \frac{0.1 \times L}{\sqrt{S}\,\sqrt{I}}$$
$L$ in meters, $S$ dimensionless slope, $I$ rainfall intensity (mm/h),
$T_c$ in minutes.
$$\text{Kinematic Wave: } T_c = 6.94 \times n^{0.6}\, L^{0.6}\,\big/\!\big(S^{0.3}\,(P2)^{0.4}\big)$$
$n$ = Manning’s roughness, $L$ in meters, $S$ dimensionless slope,
$P2$ = 2-yr rainfall (mm/h), $T_c$ in minutes.
All formulas are approximations and may differ based on local guidelines
or references. Adjust coefficients carefully to match your
region's typical design practices.