Low Impact Development Manual for Michigan - OSEH - University ...

Figure 7.62
Example of decreasing roughness (“n” value)
with increasing flow depth
Source: Schueler and Claytor, 1996
Stormwater Functions and
Calculations
Utilize Manning’s equation to calculate the velocity
associated with the flow from the peak discharge of
the 10 year storm or local standard. Maintain velocity
of the 10 year and water quality criteria at non-erosive
rates (Table 7.19).
Manning’s Equation
Where;
Q = VA =
( )
2/3
1.49 A
n WP
Q = Flow in cfs
V = Velocity in ft/sec
A = Area in ft2 n = Manning’s roughness coefficient
WP = Wetted Perimeter in ft
S = Slope in ft/ft
S 1/2
Manning’s roughness coefficient, or ‘n’ value in the
equation, varies with the type of vegetative cover and
design flow depth. As a conservative approach, the lower
value between design depth (Figure 7.62) and vegetative
cover/swale configuration (Table 7.17) should be
used in design to determine flow velocities.
If driveways or roads cross a swale, culvert capacity
may supersede Manning’s equation for determination of
design flow depth. In these cases, use standard culvert
calculations to establish that the backwater elevation
would not exceed the banks of the swale. If the maximum
permissible velocity is exceeded at the culvert
outlet, energy dissipation measures must be implemented.
Table 7.18 provides stabilization methods and
Table 7.19 provides recommended velocities for various
swale configurations.
Volume calculations (as it relates to the
use of check dams)
The volume stored behind each check-dam (Figure 7.63)
can be approximated from the following equation:
Storage Volume = 0.5 x (Length of Swale Impoundment
Area per Check Dam) x (Depth of Check Dam)
x[(Top Width of Check Dam) + (Bottom Width of
Check Dam)] / 2
Active infiltration during the storm should also be
accounted for when appropriate according to guidance
provided in the Infiltration BMP and Chapter 9.
Figure 7.63
Storage behind check dam
Source: Northern Virginia Planning District Commission, 1992
LID Manual for Michigan – Chapter 7 Page 323