FLOOD09702

• County: Adams
• Community: Broomfield, Westminster
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 11/1/1979
• Prepared For: Broomfield, Westminster
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION III
HYDROLDGICAL SUMMARY

SOILS
In the 3207 basin, 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 light yellowish brown and
grayish brown loam that extends from 28 to 49 inches. Slopes vary from 0 to
9 percent and the natural vegetat ion is grass, although most undeveloped
areas are cultivated. Permeability of these soils is moderate to slow.

HYDROLOGY FOR DIRECT FLOW AREA 3207
Runoff flows were determined at various locations for return periods of 2, 5,
10, and 100 years so that a determination could be made concerning the storm
frequency for which drainage facilities should be designed, This section
highlights basic information and describes the computational methods utiliz-
oed.

Pri or to thi s study future development flood di scharges for the 2, 10 and
100-year events were cal cul ated for Di rect Flow Area 3207 by Gi ngery and
Associates and publ ished in the Comprehensive Drainage Plan Amendment in
February 1978, Peak flows were also developed for the basin by the Bureau of
Recl amati on in preparati on of the Flood Insurance Study for the Ci ty of
Broomfield. Due to conflicts which arose regarding differences in peak flow
values, the Urban Drainage and Flood Control District was requested to check
the hydrology for the basin by conducting an independent studY. The UD&FCD
methodology is outlined below.

1. Rainfall data was developed for the 2, 5, 10 and 100- year storms
using Volume III of the NOAA Atlas (Ref. 20).

Time
(Minutes)

o
10
20
30
40
50
60
70
80
90
100
110
120

TABLE IlI-l
INCREMENTAL RAINFALL DATA
(All values in inches)

Incremental Rainfall for Various Frequencies
2- Year 5- Year 10- Year 100-Year

2. A computer model of the basin was set up using the Environmental Pro-
tection Agency Storm Water Management Model to simulate storm events
on the basis of rainfall data input and basin characterization.

o
0.05
0.08
0.16
0.44
0.15
0.08
0.07
0.05
0.03
0.03
0.03
0.02

o
0.06
0.10
0.23
0.58
0.19
0.10
0.08
0.05
0.05
0.03
0.03
0.02

o
0.10
0.13
0.31
0.77
0.22
0.13
0.06
0.05
0.04
0.04
0.03
0.03

o
0.16
0.18
0.48
1. 20
0.35
0.19
0.08
0.06
0.06
0.05
0.05
0.04

The Storm Water Management Model yielded flow values which agreed closely
with previously developed figures. For this study, then, the computer model
was used to develop peak flows for the 2-year and 5-year events to supplement
those flows already determined.

INFIL TRATION

The storm water management model utilizes horton's equation for prediction of
infiltration capacity into the soil as a function of time,
f = foo + (f -foo) e-at
cap 0

RAINFALL
Depth-duration-frequency curves were developed for the basin to represent the
various frequencies of design rainfall. These curves were developed from
Volume 3 of the NOAA Atlas (Ref. 20) and are shown in Figure III-I. The time
incremented rainfall is derived from these curves, rearranged to a design
rainfall for each of the various frequency events and presented in Table
III-I.

where fcap= infiltration capacity into soil, in./hr.;
foo = minimum or ultimate value of f (at t-oo ),[0.50 in./hr.];
fo = maximum or initial value of f (at t = 0), [1.80 in./hr.];
t = time from beginning of storm, hr.; and
a = decay coefficient, hr-l [.00115].

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