FLOOD07627

• Designation Number: 220
• County: Douglas
• Community: Unincorporated Douglas County
• Stream Name: Marcy Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Marcy Gulch
• Date: 2/1/1983
• Designation Date: 3/1/1984
• Floodplain - Doc Type: Floodplain Report/Masterplan

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the upper portion of the basin. Elevations range from approximately 5,425
feet (above mean sea level) at AT&SF Railroad to approximately 6,145 feet
at the uppermost drainage basin boundary.

III. HYDROLOGIC ANALYSIS

The Marcy Gulch drainage basin lies near the southwestern edge of the
Denver basin, an area extending from Colorado Springs north to Greeley, and
from the foothills eastward approximately 60 miles onto the pl ains. The
Denver and Dawson Formations underl ie the Marcy Gulch drainage basin, and
are characterized by sand, gravel, silt and cl ay ranging in depths of 0
feet to 60 feet, with slopes of 0 to 15 percent.

Peak discharges (Qs) were determined at various locations in the Marcy
Gulch watershed by employing the Colorado Urban Hydrograph Procedure
(CUHP). This hydrologic model is described in the UDFCD drainage manual
(Ref. B) and in the user man ua 1 for the CUHP computer program (Ref. C).
The unit hydrographs utilized in the CUHP method are determined from
empirical relationships between parameters of the unit hydrograph and
physical characteristics of the watershed. These rel ationships are based
on field data from developing watersheds in the Denver area. The re-
lationships util ized in this study were furnished by UDFCD (October 1982)
and will be described below.

Vegetation in the Marcy Gulch basin can be characterized by three major
plant communities, grassland, shrubland, and riparian woodland. The
predominant vegetation type is grassland. Shrubs and wooded areas are
located mainly along the larger drainage channels with low growing shrubs
sometimes overgrowing the channel bottoms.

The rainfall used in the CUHP method was based on a storm duration of
two hours and the distribution shown in Table 1. This duration and dis-
tribution were recommended by UDFCD (November 1982) as a typical 100-year
flood-producing storm for the Denver area. The distribution is given as a
percentage of the I-hour, IOO-year rainfall as determined from the NOAA
atl as (Ref. A). The I-hour depth was found to be 2.57 inches and thus the
corresponding 2-hour depth of 2.97 inches is distributed as shown in Table
1. The percentage of I-hour depth distribution shown in the table was
previously reviewed with .and accepted by UDFCD. The rainfall was assumed
to be uniformly distributed throughout the drainage basin. No areal
correction factors were applied since the reduction would only be a few
percent for a basin of this size (4.25 mi.2). Surface retention losses
were taken as 0.1 inch for impervious surfaces and 0.4 inches for pervious
areas. Infi ltrat i on rates were determi ned from Horton's equat i on with a
maximum rate of 3.0 in./hr., minimum rate of 0.5 in./hr., and decay rate of
0.0018 sec. -1. When these losses are applied to the total rainfall, the
resulting excess rainfall is 1.97 inches for the entire watershed.

C. CLIMATOLOGY AND FLOOD HISTORY

The climate over ~he Marcy Gulch drainage basin can be classified as
semi-arid. The average annual precipitation in the Marcy Gulch drainage
basin is 17 to 19 inches, 75 percent of which occurs during the months of
April through September. The period of heaviest precipitation is during
these spring and sUrJl1ler months. Snowmelt is not usually a contributing
factor.

Numerous thunderstorms duri ng spri ng and summer months, whi 1 e generally
limited in area, produce occasional periods of intense rainfall, and
most of the major floods in this area have resulted from intense thunder-
storm activity. The thunderstorms are generally the result of convective
activity occurring during the afternoon or early evening. These storms are
generally of short duration and moderate intensity. The most severe
thunderstorms are associated with frontal activity coupled with orographic
effects. No specific flood history is avail able for Marcy Gulch. However,
field observation indicates that the secondary road just downstream of the
Denver and Rio Grande-Western Railroad has been damaged by overtopping due
to flooding, although the return frequency of that flooding is unknown.

In the CUHP method, the time to peak and peak Q of the unit hydrograph are
found from the coefficients Cp and Ct. These parameters are corrulated to
the percent imperviousness and derived from Figures 1 and 2.

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