Volume 2-05, Chapter 3 - City of Wichita

Section 3.2.4 - Grassed Channel
3.2.4.6 Design Example
Basic Data
Small commercial lot 300 feet deep x 145 feet wide
• Drainage area (A) = 1.0 acres
• Rainfall (P) = 1.2 inches (water quality event)
• Runoff coefficient (R v ) = 0.70
Water Quality Peak Flow
Compute the Water Quality Protection Volume in inches over the drainage area:
WQv = P* Rv
= 1.2*0.70 = 0. 84inches
Compute modified CN for P = 1.2-inches and WQ v in inches for Q:
CN
=
1000
⎡
⎢10
+ 5P
+ 10Q
− 10
⎣
( ) 1 ⎤
Q
2
+ 1.25QP
2
⎥
⎦
1000
CN =
= 96.3 (Use CN=96)
⎡
1
10 + ( 5*1.2) + ( 10 * 0.84) − 10 * ( 0.84
2
+ 1.25* 0.84 *1.2)
2 ⎤
⎢
⎥
⎣
⎦
For CN = 96 and an estimated time of concentration (T c ) of 8 minutes (0.13 hours), compute
the Q wq for a 1.2-inch storm.
0.
083
From Table 4-9, I a = 0.083, therefore = = 0.
07
P 1.
2
From Figure 4-6 for a Type II storm (using the limiting values)
csm 1.
0ac
Q wq
= 940 * * 0.
84in
= 1.
23cfs
in
640ac
2
mi
I a
q u
= 940 csm and therefore:
in
Utilize Q wq to Size the Channel
The maximum flow depth for water quality treatment should be approximately the same height
of the grass. A maximum flow depth of 4 inches is allowed for water quality design. A
Page 3 - 66
Volume 2, Technical Guidance