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

overland flow. The velocity of over!<md flow can vary with surface cover and
tillage characteristics. If the slope and land use of the overland fIow reach
are knOI'."I1, travel time can be calculated using the followingequatio n:

T, '" 1.12 (1.1 - CIO) LM S~_lJ

Where:

ClO '" adjusted runoff coefficient for 10-
year flow;

L '" length of overland flow in feet;

S" slope of flow path lfi
percent;

T '" travel time in minutes or
,
time of concentration

Traveltime through a storm sewer, road gutter system or open channel is the
sum of travel times in each individual componelll of the drainage system. In
most cases. average velocities can be used without a significant loss of
aCCllracy. Times of concentration, calculated for fully developed land use,
should not be less than 5 minutes to avoid oversizing of inlets, storm sewers
and open channels to unrealistic levels.

Given a time of concentration, determination ufthe average int ensityofthe
precipitation for the period is made from a graph constructed fur
depth/duration versus time. A graph for the Cripple Creek area is shown as
Figure 2 in Appendix A, 'Time ' (ntensity - Frequency Curves.'

IV.II

5,

Peak Runoff Rates

It is realized that the Rational Method of predicting peak storm wat errunoff
rates may not be as predse a.~ other analytical methods. However, it is felt
that this method does result in reasonable estimates of peak runoff rate~.
Considering the assumptions which must be made with regard to land use.
proposed dcvelopment palterns and soil types, the use of morc sopbisticated
analytical techniques probably do not materially improve the preci~ion of the
peak storm water runoff quantities which have been developed.

Major drainage in the City of Cripple Creek occurs through numerous n atural
drainagechannelsandmanmadestructures.lnutilizingtheRationalMethod
for determining storm water runoff quantities, the City of Cripple Creek was
divided into 19 drainage basins and 99 subbasins. Design points were
established at specific points of interest to detennine accumulated peak flow
for the hydraulic analysis of existing structures and preliminary design of
propo>cd ~tructures. E>;hihit 1, "Drainage Map", olltlilles the subbasins and
indicates design point locations. Peak flow data at each design point for each
storm event (IQ-.year, lOO-year) is presented in Appendix B, Table 3,
"Hydrology: Pony Gulch. Future Development," and Table 4, "Hydrology:
Cripple Creek - Future Development:

A schematic flow diagram of the Pony Gldch and Cripple Creek watersheds
depicting basins, subbasins, design points, and existing structures is shown as
FiKure 3 in Appendix A, "Subbasin Schematic." Basic information relating to
cach subbasin, including drainage area, runoff coefficients, and time of
concentration. is provided in Tables 3 and 4 of Appendix B.

IV-12