FLOOD09316

• County: Teller
• Community: Cripple Creek
• Stream Name: Pony Gulch, Poverty Gulch
• Basin: Arkansas
• Title: Drainage Master Plan
• Date: 4/1/1992
• Prepared For: Cripple Creek
• Prepared By: GMS
• Floodplain - Doc Type: Floodplain Report/Masterplan

The runoff <:oefficients presented in Table IV-l areapp1ieable for the initial
storm (i.e., lO-year storm, lOa-year storm). These coefficients are based on
the assumption that the design storm docs not occur when the ground surface
is frozen, covered by melting snuw or saturated by a previous storm. The
designer must exercise good judgement in estimating runoffcocffi cientsfor
those conditions as nc<:essary.

of all future drainage facility improvements. The Soil Conservation Service
analyzed historical Cripple Creek weather station precipitation data and
determined that the data was more appropriate than NOAA Atlas data and
therefore was used in their study. The SCS also selected their 24-hour Type
II rainfall distribution curve for use in their study. Based on the above
information, the la-year 24-hour point rainfall is 2.03 inches and the lOO-year
24-hour point rainfal1 is 3.93 inches. Using the 24-hour Type II rainfall
distribution curve, the one hour point rainfall for the IO-year and lOO-year
design storm events becomes 0.93 inches and 1.80 inches respectively.

3,

Frequency Factor

For storm cvents having a recurrence frequency greater than 10 years, the
effect of infiltration and other losses will be less, therefore the r ational
equation must he adjusted by a frequency factor. Freqllency factors, Cf, are
shown in Table IV-2.

TABLE IV.2

i

Rainfall intensity, i, is the average rate of rainfall in inches per hour, for a
storm of a given duration equal to the estimated runoff time of concentration.
Intensity is selected on the basis of design frequency of exceedance, a
statistical parameter established by design criteria, and the storm rainfall
duration. For the RationalMethod,thecriticalrainfallintensityis arainfalJ
having a duration equal to the estimated runoff time of concentration for the
drainage basin bcing studied.

FREQUENCY FACTORS

Design Storm
Fre(jllencv

Frequency
Factor. C(

10
25
50
1DO

1.0
1.1
1.2
1.25

One of the basic assumptions underlying the Rational Method of analysis is
that runoff is a function of the average rainfall ruteduring the time required
for water to flow from the most hydraulically remote point of the drainage
basin tu the design point under consideration. Runoff time of concentration
is usually estimated by calculating the travel time through the hasin. Overland
flow, sturm sewer and/or road gUller flow and channel fl()w are all typical
phases of flow commonly used in calculating travel time.

In no case shall the product C x 4 exceed 1.0.

4,

Rainfalllntensity

During preparation of the preliminary hydrology report, the Soil Conservation
Service developed rainfall data for use in developing their peak di scharge
rates. The rainfall data developed by the SCS has heen reviewed and used
in the development of this Drainage Master Plan and I'oill he used in design

The travel time for overland flow is the estimate in time required for flow to
lravel from the uppermost part of the drain.age hasin to a defined ch anndor
mlet of a local storm sewer system. Overland flow can he significant in small
basins because a significant portion of time of concentration is due to

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