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

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II condition, or the 2-hour AMC III condition as recommended in
the DCM. In order to investigate any trends, data was complied
from the studi.es for drainage areas in which detention routing
or other flow attenuation factors did not occur. Two sets of
flows were used - one for the 2-hour AMC III condition and one
for the 24-hour AMC II condition.

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In almost every case, the 2-hour AMC III condition exceeded the
24-hour condition for all ranges of flows. This is expected
since the AMC III condition utilizes a higher curve number than
the AMC II condition used in the 24-hour storm. An interesting
point to note is that the highest yield watersheds are basins
which are almost fully developed. In these cases, the flows
for the 2-hour AMC III and 24-hour AMC II are nearly the same.
This should be expected since the higher the curve number of
the basin, the less difference in runoff between the AMC II and
AMC III condition.

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Cenerally, the 2-hour AMC III condition is most closely
approximated when a small thunderstorm cell of high intensity
occurs. In this case, the rainfall occurs so quickly that
nearly instantaneous runoff will result even though the soil
may not: be totally saturated. It appears from experience in
the Colorado Springs area that these intense thunderstorm cells
are not of great areal extent, and therefore would not
generally occur on a larger basin area. An undeveloped basin
experiencing this type of storm could effeetively produce as
much runoff as a similarly developed basin under less intense
rainfall conditions.

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In addition, it appears that in fully developed conditions, the
storm criteria used for the basin is not as important since the
values obtained under both criteria are the same. In fact, the
24-hour AMC II storm would probably result in a more severe
condition for routing through detention ponds or other
structures where runoff volume is a concern.

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Several meetings were held to discuss these points with the
various agencies involved in this study and to select specific
flow values to be used in the floodplain mapping. Those
attending included the Department of Public Works for both the
City of Colorado Springs and El Paso County, the Regional
Floodplain Administrator, the Colorado Conservation Board, and
FEMA.

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Appropriate design storms for each basin were chosen depending
primarily upon basin size and level of development. A summary
of the lOO.year peak discharge for both the 24-hour AMC II and
2-hour AMC III storms is presented in Table 1.2. The
recommended values for use in this study are shown underlined
for clarity. In the remainder of the report, only those
discharge values associated with the design storm of each
respective basin are presented.

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RE:SOURCE: CONSUlTI\NTS INC

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