FLOOD05940

• Designation Number: 325
• County: Teller
• Community: Cripple Creek
• Stream Name: Pony Gulch, Poverty Gulch
• Basin: Arkansas
• Title: Drainage Master Plan - Cripple Creek, Final Report
• Date: 4/1/1992
• Designation Date: 7/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

The runoffcoefficicnts presented in Table IV-I are applicable for the initial
storm (Le., lO-year storm, 100'year storm). These coefficients are based on
the assumption that the design storm does not occur when thc ground surface
is frozen, covered by melting snuw or saturated by a previous storm. The
de,igner must exercise goodjudgcmcnt in e~timating runoffcoefficicnts for
those conditions as nccessary.

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 u~ed in their study. The SCS also selected their 24-hour Type
II rainfall distribution curve for use in their ~tudy. Based on the above
information, the lO-year 24-hour point rainfall is 2.03 inches and me lOO-year
24-hour point rainfall is 3.93 inches. Using the 24-hour Type II rainfall
distribution curve. the one hour point rainfall for the IO-year and 1oo-year
design storm events becomes 0.93 inches and 1.80 inches respectively.

3.

Frequency Factor

For storm events having a recurrence frequency greater than 10 years, the
effect of infiltration and other losses will be less. therefore the r ational
equation must be 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 seleeted on the basis of design frequency of exceedance, a
statistical parameter established by design criteria, and the storm rainfall
duration. For the RationalMethod,thecriticalrainfallintensityis a rainfall
having a duration equal to the estimated runoff time of con cent tat ion for the
drainage basin bcing studied.

FREQUENCY FACTORS

Design Storm
Frel1\1encv

Frequency
Factor. C(

10
25
50
1DO

10
1.1
1.2
125

One of the basie assumptions underlying the Rational Method of analysis is
that runoff is a function uf the averuge rainfall ruteduring the time required
for water to flow from the most hydraulically remote point of the drainage
basin to the design point under consideration. Runoff time of concentration
is usually estimated by calculating the travel timc through the hasin. Overland
flow, storm sewer and/or road gUller flow and channel f1()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.

Rainfall Intensity

During preparation of the preliminary hydrology report, the Soil Con~ervution
Service developed rainfall data for use in developing their peak di scharge
rates. The rainfall data develuped by the SCS has heen reviewed and used
in the development of this Drainage Ma~ter Plan and 1'0111 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 ,ystem. Overland flow can he significant in small
basins beeause a significant portion of time of conccntration is due to

JV.9

IV-tO