FLOOD10401

• County: El Paso
• Community: Colorado Springs
• Stream Name: Fountain Creek
• Basin: Arkansas
• Title: Fountain Creek Drainage Basin Planning Study - Volume 1
• Date: 7/1/1994
• Prepared For: Colorado Springs
• Prepared By: Muller Engineering Company, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Rainfall

A number of separate procedures for the determination of storm rainfall amounts
for use in hydrologic models are outlined in the City/County Drainage Criteria
Manual. The storm type used for this drainage basin study utilizes the 24-hour
SCS Type II storm distribution determined to be the critical storm in the
companion Monument Creek Study.

Point rainfall depths of 3.10 and 4.35 inches for the 10- and 100-year storms,
respectively, were extracted from the National Oceanographic and Atmospheric
Administration (NOAA) Atlas 2 precipitation-frequency maps (NOAA 1973) for the
Fountain Creek area. The point rainfall amounts along with the basin size were
used in the development of depth-area-duration relationships, then elliptical
rainfall patterns, following the procedures presented in Hydrometeorological
Reports No. 51 (NOAA 1978) and No. 52 (NOAA 1982). There has been no
modification to the rainfall values to reflect the known orographic impacts of
the foothills which include the reduction of precipitable moisture with increase
in elevation and the breakup of storm centers into multiple smaller cells over
more local areas.

Hydrometeorological Reports No. 51 (HMR 51) and 52 (HMR 52) were used in
accordance with requirements set forth by the City of Colorado Springs, City
Engineering Division, and are consistent with expressed opinions of the hydrology
review committee and the application of this procedure in the companion study.
While HMR 51 and HMR 52 were originally developed to apply to the PMS/PMF
computation, for this study the procedure was applied to the 10- and 100-year
frequency events by imputing the proper rainfall amounts.

The HMR 52, "Probable Maximum Storm Computation" computer program developed by
the U.S. Army Corps of Engineers, was used to perform the computationally intense
operations to produce basin-average precipitation isohyetal values for the
elliptical pattern previously mentioned. It utilized as input the 10- and 100-
year rainfall depth-area-durations computed by hand in accordance with HMR 51 and
a digitized description of the drainage basin boundaries.

HMR 51 and HMR 52 were applied by delineating two drainage basins within the
Fountain Creek study. An elliptical storm pattern was first developed for the
Fountain Creek drainage basin upstream of the confluence with Monument Creek as
shown on Figure 4.7-4 (Volume II). Initial point rainfall depths were
subsequently adjusted to reflect the actual basin centroid location. The revised
numbers were 3.00 inches and 4.20 inches for the 10- and 100-year storms
respectively.

The second elliptical storm pattern generated was for the combined Fountain and
Monument Creek drainage basins. Figure 4.7-5 shows the elliptical storm pattern
placement over the Fountain and Monument Creek watersheds. Figure 4.7-6 (Volume
II) shows the combined elliptical storm pattern placement over the Fountain Creek
watershed in more detail. Isohyetal values and centroidal storm placement over
the basins was used to determine sub-basin average rainfall values for each
individual basin for input into the hydrologic model.

4.0-18