FLOOD03016

• County: El Paso
• Community: Colorado Springs
• Stream Name: Arkansas River and Tributaries
• Basin: Arkansas
• Title: Areawide Urban Runoff Control Manual
• Date: 9/28/1979
• Prepared For: Pikes Peak Area Council of Governments
• Prepared By: Gilbert Meyer & Sams
• Floodplain - Doc Type: Miscellaneous

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For storm events having a return frequency greater than 10
years, the effect of infiltration and other losses will be less,
and the coefficient must be adjusted accordingly. Frequency
factors (Cf) are shown in Table 111-3.

TABLE III-3

FREQUENCY FACTORS - RATIONAL FORMULA

Design Storm
Return Frequency

Frequency
Factor, Cf

2
5
10
25
---- 50
100

1.0
1.0
1.0
1.1
1.2
1. 25

with the inclusion of the frequency factor, the Rational
Formula for runoff calculation becomes:

Q = CiACf

In no case shall the product C x Cf exceed 1.0.

The rainfall intensity shall be adjusted to the time of
concentration (Tc) for the basin. Tc is the time which runoff
from the part most remote hydraulically, not necessarily in
distance,-Bakes-to reach the point of design. Tc in urban
areas is best calculated from the hydraulic properties of the
path of flow. An average velocity of flow for each type of
section and/or conduit divided into the length of travel to the
design point will give the time of concentration. For areas
where flow is overland, rather than in a confined section, an
estimate of time of travel can be made from Figure III-I.

Given a time of concentration, determination of the average
intensity of the precipitation for the period is made from a
graph constructed for depth-duration versus time. A graph for
the Colorado Springs vicinity is shown as Figure 111-2. To
construct a similar graph for other locations, the following
procedure is used:

1. Determine one hour point rainfall for the appropriate
return frequency and location (Figures II-I, 11-2,
11-3 and/or NOAA Atlas 2).

2. Determine ratio of average intensity to one hour
rainfall from Figure 111-3 for a given Tc'

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