# Retention swales

These sizing equations are suggested for use in calculating the capacity of swalesA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. which have a larger proportion of surface flow. i.e. grass swalesVegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales., rather than bioswales.
In many cases the length of swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. 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$L=\frac{151,400Q_{p}^{\frac{5}{8}}m^{\frac{5}{8}}S^{\frac{3}{16}}}{n^{\frac{3}{8}}\left (\sqrt{1+m^{2}} \right )^{\frac{5}{8}}q}$

### Trapezoidal channel

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

Where:

• L = length of swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. in m
• Qp = peak flow of the storm to be controlled, in m3/s
• m = swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. side slope (m/m)
• S = the longitudinal slope (m/m)
• n = Manning's coefficient (dimensionless)
• b = bottom width of trapezoidal swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades., in m.