FLOOD10152

• County: Boulder
• County: Adams
• County: Jefferson
• Community: Broomfield, Westminster
• Stream Name: Big Dry Creek Tributaries
• Basin: South Platte
• Title: FHAD - Broomfield Area
• Date: 6/1/1979
• Prepared For: Broomfield, Westminster
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION III
HYDROLOGICAL SUMMARY

GENERAL COMMENTS

DEVELOPMENT OF FLOOD FLOWS

The extent of present flooding and the sizing and character of major drainage
works and other urban storm drainage facilities are determined by the magni-
tude of runoff, Furthermore, the design of the facilities must be based upon
expected sediment problems, including both degradation and aggradation, and
water quality problems.

Prior to this study future development flood discharges for the 2-, 10-,
and 100-year events Were calculated for the subject basins by Gingery
Associates Inc. and published in the Comprehensive Drainage Plan Amendment in
February, 1978, Peak flows were also developed for the basins by the Bureau
of Reclamation in preparation of the Flood Insurance Study for the City of
Broomfield. Due to conflicts which arose regarding differences in peak flow
values, the Urban Drainage and Flood Control District was requested (0 check
the hydrology for the basin by conducting an independent study. The UD&FCD
methodology is outlined below.

The magnitude of a flood can be judged by its maximum rate of flow, even

though the maximum rate may last for only

a flood is Judged by its return period.

the average interval of time within which

10 minutes.

The frequency of

The return period is defined as

a given event

will be equalled
of 100 years has

1. Rainfall data was developed for the 10-year and 100-year storms

using Volume I I I of the NOAA Atlas (Ref. 20),

2. A computer model of each basin was set up using the Environmental

Protection Agency Storm Water Management Model to simulate storm

events on the basis of rainfall data input and basin characterization.

or exceeded once.

Thus, a flood having a return period

a 1 percent probability of being equalled or exceeded in any given year.

Runoff flows were determined at many locations for return periods of 2, 5,

10, 50, and 100 years. This section highlights basic information; details

of the computational methods are presented in Appendix B.

The approach taken by the Denver Regional Council of Governments (DRCOG)
Urban Storm Drainage Criteria Manual in the determination of runoff flows
is:

The Storm Water Management Model yielded flow values which agreed closely
with the Gingery Associates, Inc. figures. Additional details regarding rainfal
input and typical hydrographs and a brief description of the EPA SWMM Model
may be found in Appendix B.

1. Determination of rainfall that would most 1 ikely occur during an
event with a given return period by use of isohyetal (line of equal
depth rainfall) depth-duration-frequency maps that were synthesized
during the formation of the manual. This provides a basic input for
the runoff computation.
2. Field inspection and analysis of data pertaining to stream lengths,
impervious COver, slopes, and other physical phenomena pertaining
to runoff flow.
3. Determination of runoff flows based on the data established in
and 2 above.

Figures III-I through I I 1-5 are graphical summaries of flood flows for

various frequencies for each of the drainage basins studied.

Soi 1 s

Typical area soils are made up mostly of the Nunn, Platner and Renshill

series as defined by the Soil Conservation Service (SCS), These soils

are generally well drained, nearly level to moderately sloping materials

formed in loamy alluvial material. In a typical profile, the surface layer

is grayish brown loam, 4 to 9 inches thick. The subsoil is grayish brown

loam and clay, 10 to 17 inches thick, and overlies 1 ight yellowish brown

and grayish bro,"n loam that extends from 28 to 49 inches, Slopes vary