# Flow in a swale

From LID SWM Planning and Design Guide

Flow (*Q*) in an open channel, such as a swale, may be calculated using Manning's equation\[Q=VA=\frac{R^{\frac{2}{3}}S^{\frac{1}{2}}}{n}\]
Where\[R=\frac{A}{P}\]

Where:

*Q*= the flow in the 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. (m^{3}/s)*A*= the cross sectional area of the 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. (m^{2})*P*= the wetted perimeter of the 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. (m)*S*= the longitudinal slope (m/m)*n*= Manning's coefficient or 'Manning's n' (dimensionless)

n value range | Good condition turf grass | Other |
---|---|---|

0.002 - 0.010 | Where the average depth of flow is at ≥ 2 times the height of the vegetation | Supple tree seedlings such as willow or cottonwood growing where the average depth of flow is at ≥ 3 times the height of the vegetation |

0.010 - 0.025 | Where the average depth of flow is between 1 - 2 times the height of the vegetation | Moderately dense weeds, or tree seedlings growing where the average depth of flow is from 2 -3 times the height of the vegetation |

0.025 - 0.050 | Where the average depth of flow is about equal to the height of the vegetation | 8- to 10-year-old willow or cottonwood trees inter-grown with some weeds and brush (none of the vegetation in foliage) |

0.050 - 0.100 | Where the average depth of flow is < 0.5 times height of the vegetation | bushy willow trees about 1 year old inter-grown with weeds along side slopes (all vegetation in full foliage) |

- ↑ Arcement, G.J., Schneider, R. Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains U.S.G.S. WATER-SUPPLY PAPER 2339, 1989 http://eh.sdsu.edu/usgs_report_2339.pdf Accessed Dec 19 2017