FLOOD03694

• County: El Paso
• Community: El Paso County
• Stream Name: Fountain Creek
• Basin: Arkansas
• Title: Hydrology Addendum to FIS El Paso County and Incorporated Areas
• Date: 8/23/1999
• Prepared For: El Paso County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolol!:ic Analvsis

Table 1 lists all streams and stream reaches for which approximate or detailed floodplain
boundary information is contained in the August 23, 1999 PIS.

For streams studied by detailed methods in EI Paso County, discharge frequency relationships
were determined using standard hydrologic methods including the following:

. Discharge-frequency analysis of stream gage records.
. Hydrologic modeling using synthetic unit hydrograph methodologies.
. Regional regression equations,

Discharge rates for streams studied by detailed methods are presented in the PIS generally for the
10-, 50-, 100- and 500-year events.

Hydrologic analyses and discharge rates for streams studied by approximate methods are not
documented by FEMA in the 1999 PIS, As previously discussed, Colorado statutes require that
this information be provided in order for floodplain studies to be "designated" and thus made
legally usable for floodplain regulation. For the purposes of this HA, 100-year flow rates for
approximate streams in El Paso County have been determined using regional regression
equations presented in Guidelines For Determining lOO-Year Flood Flows For Approximate
Floodplains in Colorado, Version 4.0 (CWCB, January 1999). The regional regression equations
from this document applicable to EI Paso County are listed in Table 2. A list of approximate
streams and 100-year discharge rates determined for them is provided in Table 3.

There are a number of flooding sources in EI Paso county where detailed studies exist for
downstream reaches of a stream while more upstream reaches are covered only by approximate
studies. In these cases, both the detailed study 100-year discharge from Table 3 (Summary of
Discharges) of the PIS and the approximate 100-year discharge derived from the CWCB regional
regression equations is provided in Table 3 of this HA. Both discharges have been presented in
order to identify any conflicts. Significant conflicts occur only for Ruxton Creek and Williams
Canyon, both of which are tributaries to Fountain Creek near Manitou Springs.

Ruxton Creek. In the case of Ruxton Creek, the CWCB regional regression equation estimates a
discharge significantly higher than indicated by the detailed study. Flows computed by the two
approaches are presented in Table 3. It is recommended that the regional regression equation be
utilized for discharge estimates in the approximate study reach.

Williams Canyon. In the case of Williams Canyon, the 1999 PIS detailed hydrology indicates a
significantly higher discharge than the CWCB regression equations. Flows computed by the two
approaches are presented in Table 3. It is recommended that the discharge rates established for
the detailed study reach also be utilized for the short reach of the stream studied by approximate
methods.

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