FLOOD04641

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Proposed Hydrology for use in Hydraulic Analysis of FEMA Flood Studies and Restudies
• Date: 9/1/1989
• Prepared For: State of Colorado
• Prepared By: Resource Consultants
• Floodplain - Doc Type: Educational/Technical/Reference Information

I

I

1.2

I

I

I

I

I

I

I

I

1.3

I

I

I

I

I

I

I

I

I

Modeling Procedure

The U. S. Soil Conservation Service (SCS)-developed hydrology
model TR-20 was used for all of the hydrologic analyses

conducted. This model takes into account the contributing
basin rainfall runoff characteristics of area, slope, soil type
and land use, together with specific storm events to generate
flow discharges. Effects of channel flood routing, detention
storage routing and stream diversions can also be represented.

Standard SCS basin parameters for input to the model include
soil and land use curve number (CN), time of eoncentration
(Tc)' and runoff area; they were derived under the guidelines
of Procedures for Determlnin~ Peak Flows in Colorado (Reference
1). Reach flow characteristies are represented by x and m
parameters defined by the TR-20 manual (Reference 2) for use in
determining channel flood routing. Basin soil characteristics
were obtained from the SCS Soil Survey of El Paso Countv Area.
Colorado (Reference 3).

As specified In the FEMA guidelInes, peak flood discharges are
required for the ] 0-, 50. 100-, and 500'year events. For this
study, the 10-, 50-, and 100-year values were generated
dIrectly from computer modeling of TR-20. The 500-year peak
discharge was obtained from extrapolation of a probability
curve constructed from the 10-, 50-, and 100-year results.

Storm CrIterIa

Rainfall depth, duration and distribution for the 10-, 50- and
100-year events were obtaIned from specifications set forth
under the City of Colorado SprIngs and El Paso Countv DraInage
CrIteria Manu;U. (DCM) (Reference 4). Two possIble
distributions for the crItical design storm are presented with
the fInal choIce determined by definition as, "the storm
distributIon producing the greatest level of protection for
flood conveyance and storage facilities." One storm is for a
24-hour duration of raInfall according to the SCS Type IIA
distribution. The other is a 2-hour duration based on the 1-
hour rainfall depth with distribution similar to the Colorado
Urban Ilydrograph Procedure 2-hour storm distribution.

Rainfall In most of the Colorado Springs area does not vary
much from basin to basin. An exception to this occurs wIth
those b,3Sins located in the western part of the city up against
the foothills. These areas tend to get slightly higher
rainfall amounts as determined from the NOAA Atlas II
(Reference 5) isopluvial maps. As a result, two different sets
of rainfall depth were used in this study for the 24-hour
storm. The values for the 2-hour storm are the same for both
areas, '~hich is a result of the DCH equation used to determine
the I-hour raInfall. Slightly higher elevations for the area
tended to offset the changes in rainfall depth used in the

3

R€)OURC€ CON)UlTt\NT) INC