# Flow in a swale

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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. (m3/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. (m2)
• 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 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 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 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) 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)
1. 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