# Retention swales

These sizing equations are suggested for use in calculating the capacity of swales which have a larger proportion of surface flow. i.e. grass swales, rather than bioswales.
In many cases the length of swale required will exceed the available space, so that an underground infiltration trench or a dry pond will be a preferred solution.

### Triangular channel

Sizing a triangular channel for complete volume retention:

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

### Trapezoidal channel

Sizing a trapezoidal channel for complete volume retention: ${\displaystyle L={\frac {360,000Q_{p}}{\left\{b+2.388\left[{\frac {Q_{p}n}{\left(2{\sqrt {1+m^{2}-m}}\right)S^{\frac {1}{2}}}}\right]^{\frac {3}{8}}{\sqrt {1+m^{2}}}\right\}q}}}$

Where:

• 'L' = length of swale in m
• 'Qp' = peak flow of the storm to be controlled, in m3/s
• 'm' = swale side slope (dimensionless)
• 'S' = the longitudinal slope (dimensionless)
• 'n' = Manning's coefficeint (dimensionless)
• 'b' = bottom width of trapezoidal swale, in m.