Volume 2-05, Chapter 3 - City of Wichita

Section 3.2.4 - Grassed Channel
maximum flow velocity of 1.0 foot per second for water quality treatment is required. For
Manning’s n use 0.15 for medium grass, 0.25 for dense grass, and 0.35 for very dense
Bermuda-type grass. Site slope is 2% or 0.02 ft/ft for the example.
Input variables:
n = 0.15
S = 0.02 ft/ft
D = 4/12 = 0.33 ft
Then:
1.49 2 1
3 2
Q wq
= Q = VA = D S DW
n
where:
Q = peak flow (cfs)
V = velocity (ft/sec)
A = flow area (ft 2 ) = WD
W = channel bottom width (ft)
D = flow depth (ft)
S = slope (ft/ft)
(Note: D approximates hydraulic radius and WD approximates flow area for shallow flows)
Then for a known n, Q, D and S, a minimum width can be calculated.
nQ
0.15*1.23
W = =
= 5. 56 ft minimum
5 1
5 1
1.49D
3S
2 1.49 * 0.33 3 * 0.02 2
Q 1.23
V = =
0. 66 fps
WD 5.56 * 4 = (OK)
12
(Note: WD approximates flow area for shallow flows.)
Minimum length for 5-minute residence time:
L = V * T = 0.66 * 5* 60 = 198 ft
where:
Volume 2, Technical Guidance Page 3 - 67