Flow in a swale

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 swale (m³/s)
• A = the cross sectional area of the swale (m²)
• P = the wetted perimeter of the swale (m)
• S = the longitudinal slope (m/m)
• n = Manning's coefficient (dimensionless)

---

Manning's n of vegetated surfaces

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)

A 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.