FLOOD01780

• Designation Number: 445
• County: Teller
• Community: Cripple Creek
• Stream Name: Pony Gulch, Poverty Gulch
• Title: Floodplain Management Study - Cripple Creek, Colorado
• Date: 12/1/1992
• Designation Date: 3/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Time of concentration values were computed from an emperical equation
in Soil Conservation service Engineering Field Manual - Chapter 2,
intended for small rural watersheds.

Rather than repeat review further details of the hydrology analysis,
we refer to the previously mentioned drainage master plan which has
good documentation of the methodology used.

Tc_[L~O.8((1000ICN)_9)~O.7]/1140(Y^O.5)
1'c= time of concentration in hours
L= flow length in feet
eN" runoff curve number
y~ average watershed slope

Table 1

Runoff Curve Numbers (eN)

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Land Use

Alluvium
Along Streams

Hillside

Y"lOOCI!A
y= average watershed slope in percent
A- drainage area in square feet
Ia contour interval in feet
C~ total contour length in feet
It was determined by representatives of the Colorado Water
Conservation Board, Soil Conservation Service, and GMS Inc. at a
meeting on January 6, 1992 that the Rational Formula (Q . CIA) should
be used in making peak discharge determinations. This method was
selected because of its applicability to small drainage areas, local
drainage design and its simplicity of use by land development related
technicians.

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Open Space (Parks etc) " "
Residential
1/4 ac lots " "
1/2 ac lots 70 "
1 ac lots " "
Commercial & Business " "

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The TR-20 discharge values developed for this study were used for
calibratin~ the Rational Formula in regards to the runoff coefficients
(C values).
since the SCS 24 hour type II rainfall distribution was used in TR-20,
it should also be used to determine rainfall intensity values (I).
Most of the application of the rational formula will be for small
drainage areas where the time of concentration is less than 30
minutes. Therefore a rainfall duration of 30 minutes was selected to~
determining I. The greatest 30 minute intensity from the SCS 24 hour
type II rainfall distribution produces 37.8 % of the total 24 hour
rainfall depth. This 30 minute rainfall depth was determined for each
storm frequency of interest and then converted to inches per hour for
use as I, see Table 2.

Table 2

Rainfall Intensity I

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10 yr

Frequency
50 yr

100 yr

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24 tir Frecip (In)
30 Min Precip (In)

2.U3

3.25

3.93

0.77

1.23

1.49

I (In/Hr)

1.53

2.46

2.97

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The rational formula was rearranged (C=Q/IA) to compute C values using
previously determined Q and A values from TR-20 data, and I values
previously discussed. A relationship uetwe~n C values and curve
numbers (CN) fro~ TR-20 was established. CN values for the TR-20
watersheds reflect weighted average CNs for a variety of land use~ and
soils within each watershed. The CNs generally did not exceed the low
80s. Since the rational equation may be used for smaller individual
land uses and soils, such as a city block of commercial buildings, the
CNs from TR-20 may not be sufficiently site specific. Therefore for
land uses with CNs greater than the low 80s, a table (Table 14-1) from
the "Handbook of Applied Hydrology" by Ven Te Chmol was useful in
selecting C values for more site specific types of land uses.