Volume 2-05, Chapter 3 - City of Wichita

Section 3.2.3 - Vegetative Filter Strip
From Figure 4-6 for a Type II storm (using the limiting values) q u
= 940 csm . Therefore,
in
the peak discharge for the WQ v (expressed as Q wq ) can be calculated using equation 4-18:
csm 0.
20ac
Qwq = qu
* A * WQv
= 940 *
* 0.
84in
= 0.
25cfs
in
640ac
2
mi
Minimum Filter Width
Using Equation 3-2:
W
Q
=
q
0.25
0.006
wq
fMIN
= = 42
Filter without Berm
feet
• 2-year, 24-hour storm = 3.5 inches (see Chapter 4)
• Use 5 minute travel (minimum contact) time
Using Equation 3-3:
() 5
1 . 25
625
( 3 5) 0 .
* . * () 4
0.
5
L f
=
= 39feet
3.
34 * 0.
25
Note: Reducing the filter strip slope to 2% and planting a denser grass (raising the Manning n
to 0.35) would reduce the filter strip length to 20 feet.
Filter with Berm
Compute the required WQ V in cubic feet:
P * Rv
* A 1.2 * 0.7 * 0.2
WQv = =
= 0.014 ac-ft or 610 ft 3
12
12
The volume of the “wedge” of water captured by the filter strip is:
Volume = W * depth*
f
L f
Where “depth” is average depth of water captured in the wedge.
For an average captured depth of 4.5 inches or 0.38 feet (a berm height of 9 inches):
Volume = 42ft
* = ft
3
* 0.
375ft
40ft
630 is > 610 ft 3 , therefore OK
Size outlet(s) to drain the stored volume of water over a 24 hour period.
Page 3 - 60
Volume 2, Technical Guidance