FLOOD00540

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Title: Colorado Flood Hydrology Manual
• Date: 1/1/1995
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Educational/Technical/Reference Information

L "' length of the main stream channel (mi) from tile outlet
to the divide
Leo = length along tile main channel (mi) from the outlet
to a channel point nearest the watershed centroid

The peak rate and time of the unit hydrograph Can be developed using Eqs (IV-3) and (IV-
21). Alter four additional points defined by Eqs (IV-HI) and :IV-20) are plotted, tile rest of
the 5-min unit hydrograph can be fitted to provide a total area representing 1.0 incil of
direct runoff. A hand fit curve is applied to th" points. adjusting it to maintain trle proper
runoff volume.

For small watersheds (less tlan 90 acres), the time to peak is:

t p , C. 39

(P~-0.3El Pa' 0.07)
------------- - . ----
----------
(P~. 0.49 Po '0.14)

(T, - 6 Pa)

(IV-23)

._iii

-

__iBm

Example 7.3 - Determine the storm hydrograph using the Colorado Urban Hydrogr3ph
Procedure (CUHP) and the fOllowing information.

Watershed Area = 0.38 square miles (243 acres)
Watershed Length = 1.28 miles
Distance to Centroid = 0.52 miles
Impervious Area = 44%
Slope along Watershed = 0.')102 It/lt

Excess precipitation in inches for 5 minutes intervals:
0.04 0.09 0.31 0.15 0.06 0.05 0.03 0.03 0.03 0.03 0.03 0.03

Solution:

1. From Figure 1 - C, ~ 0.OD1

2. tp = C, [(L L" IS)]"'" = (0.091)[(1.28)(0.52)/0.0102]048 = 0.225 hours ort3.5 rnin

3. From Figure 1 - Peaking ~arameter P ~ 6.21
4. Cp = PC,Ao IS = (6.21 )(0.091 )(0.032-))0 '5 ~ 0.49
5. qp = 640 Cp I tp = (El40)(0.49)/0.225 = 1394 Gfs I sq. mile
6. Op = qpA = (t,l94)(0.38) = 530 cfs

Colorado Flood
Hydrology Manual 7-46

[R4/:T