Retention swales

Triangular channel

$L={\frac {151,400Q_{p}^{\frac {5}{8}}m^{\frac {5}{8}}S^{\frac {3}{16}}}{n^{\frac {3}{8}}\left({\sqrt {1+m^{2}}}\right)^{\frac {5}{8}}f}}$

Trapezoidal channel

$L={\frac {360,000Q_{p}}{\left\{b+2.388\left[{\frac {Q_{p}n}{\left(2{\sqrt {1+m^{2}-m}}\right)S^{\frac {1}{2}}}}\right]^{\frac {3}{8}}{\sqrt {1+m^{2}}}\right\}f}}$