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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12.3

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Hydrologic Analysis

The hydrologic analysis conducted for this study utilized
information developed In previous studies. Basin boundaries
were kept the same with the exception of subbasin 11; current
drainag" patterns resulted in an area reduction of 32% for this
subbasin. The largest change was to revise the hydrology to
reflect existing basin conditions. Previous studies had used
fully developed conditions for the hydrology, which does not
reflect current condItions. Presented below is a brief
description of the basin hydrology and the assumptions that
were made as part of the study.

The basin conditions for this study assumed that the Fountain
Valley Ranch below Bradley Road was developed. Currently,
drainagH improvements, streets, and utilities are in-place and
approximately one-half of the development has houses
construeted on it. This development has occurred in the lower
portion of the basin near Security Boulevard, although the
draInage structures are In place through Bradley Road.
Structures in-place include the detention pond (Pond #5) and
the new Bradley Road culvert which detain water upstream of the
road embankment (Pond #1.). The remainder of the basin was
assumed undeveloped except for subbasin 11 which was assumed to
be 50% developed. Table 12.2 presents the assumed area, CN,
and Tc for each subbasin shown in Figure 12.1. The Tc for the
subbasins were modified from those used in the SLA and RBO
r"ports to account for the current development conditions of
the basin.

Record drawings of recent construction were reviewed and
utilized to develop stage volume-discharge data for the two
existlnl\ detention areas. ThIs Information was incorporated
into the TR-20 model for the basin.

Review of recent grading plans for the Colorado SprIngs
Municipal Airport at Peterson Field (Reference 30) revealed
seven sIzable depression areas in subbasins 1, 2, 3, and 4 as
shown in Figure 12.1. Approximate storage volume capacity for
each of these sites was calculated from the grading plans by
planimeter. Runoff calculations for the areas flowing into the
depressIons were made for the 100-year storm to determine the
impact of this retention on the basin. It was found that the
majority of runoff from subbasins 1, 2, and 4 and 32% of runoff
from subbasin 3 is effectively captured by these depressions.
These areas are shown in Figure 12.1 as noncontributing area.
They were excluded from the TR-20 model.

Table 12.3 summarizes the peak flows computed for the basin.
These flows are considerably higher than the flows developed by
SLA and RBD for fully developed condItions. Two major
dIfferences in the hydrologic assumptions account for the
changes in the peak flows. The first is that the design storm
for the RBD and SLA studies was a I-hour 2.5 inch rain. This

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