Volume 2-05, Chapter 3 - City of Wichita

Section 3.2.3 - Vegetative Filter Strip
• When a berm is used at the downstream end of the filter strip, outlets must be provided to
drain the WQ V over a 24 hour period.
• Maximum discharge loading per foot of filter strip width (perpendicular to flow path) is
found using the Manning’s equation:
0.00237
2
Equation 3-1 q = Y S
n
5
3
1
where:
q = discharge per foot of width of filter strip (cfs/ft)
Y = allowable depth of flow (inches)
S = slope of filter strip (percent)
n = Manning’s “n” roughness coefficient
(use n = 0.15 for medium grass, 0.25 for dense grass, and 0.35 for very dense Bermuda-type grass)
• The minimum width of a filter strip is:
Equation 3-2
W
fMIN
=
Q
q
wv
where:
W fMIN = minimum filter strip width perpendicular to flow (feet)
Q = peak flow for the water quality (1.2”) storm (cfs)
q = flow per unit width of filter strip (cfs/ft)
Filter without Berm
• Size filter strip (parallel to flow path) for a contact time of 5 minutes minimum
• Equation for filter length is based on the SCS TR55 sheet flow travel time (equation 4-5):
Equation 3-3
L
f
=
1.25 0.625
( T ) ( P ) ( S )
t
2−24
3.34n
where:
L f = length of filter strip parallel to flow path (ft)
T t = travel time through filter strip (5 minute minimum)
P 2-24 = 2-year, 24-hour rainfall depth = 3.5 inches (see Chapter 4)
S = slope of filter strip (percent)
n = Manning’s “n” roughness coefficient
0.5
(use n = 0.15 for medium grass, 0.25 for dense grass, and 0.35 for very dense Bermuda-type grass)
Filter Strips with Berm
• Size outlet pipes to ensure that the bermed area drains within 24 hours.
• Specify grasses resistant to frequent inundation within the shallow ponding limit.
Page 3 - 56
Volume 2, Technical Guidance