City of Frisco Engineering Standards

Engineering Standards
Version 6/19/12
Table 4.2 Sheet Flow ‘n’ Values
Surface Description
n
Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011
Fallow (no residue) 0.05
Cultivated soils
Residue cover less than 20% 0.06
Residue cover greater than 20% 0.17
Grass:
Short Prairie Grass 0.15
Dense grasses 0.24
Range (natural) 0.13
Woods:
Light underbrush 0.40
Dense underbrush 0.80
2. Shallow Concentrated Flow
Shallow concentrated flow begins where sheet flow ends. A projected slope should be
established along the flow line for the shallow concentrated flow length. The T t in minutes
for shallow concentrated flow is determined by the following equation:
T t =
L
3600V
T t = travel time (hr)
L = flow length (ft)
V = velocity (fps)
Unpaved = 16.1345*(S)^0.5
Paved = 20.3282*(S)^0.5
3. Open Channel Flow
Open Channel Flow is where the runoff is located within a defined channel or in some cases,
closed storm systems. The T t for open channel flow is determined using the following
equation:
T t =
L
3600V
V = 1.49r2 3s 1 2
n
T t = travel time (hr)
V = average velocity (ft/s)
r = hydraulic radius (ft)
A = cross sectional flow (ft 2 )
P = wetted perimeter (ft)
s = slope of the hydraulic grade line (channel slope, ft/ft)
n = Manning’s roughness coefficient
Section 4 – Drainage Design Requirements May 2012 Page 4-4