FLOOD08658

• County: Adams
• Community: Thornton, Adams County
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 11/1/1986
• Prepared For: Thornton, Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
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I

to 75 square miles were applied to the 15-minute most intense period of the
design storms based on Area-Depth Curves developed by NOAA. Table I shows
typical design storms used for hydrologic modeling; these storms include
the area adjustment for a watershed between 50 and 75 square miles in area.

TABLE I
SIX-HOUR DESIGN STORMS
BIG DRY CREEK
AREAllY ADJUSTED FOR WATERSHEDS OF 50 TO 75 SQUARE MILES

The drainage sub-basin parameters used in the CUHP model are tabulated in
Table II. The procedure used to determine the CUHP parameters is outlined
in the Runoff Chapter of Reference 4. A schematic of the SWMM routing
network is shown in Figure 2. The 5WMM conveyance element parameters were
estimated based upon the project mapping and field observations and are
listed in Table III. Detailed documentation of the hydrologic analysis is
available at the office of the UDFCD. The following paragraphs contai,n
specific information regarding the hydrologic analysis.

- ext
f ~ f + (f. - f )e
o , 0

Time (Min)

5
10
15
20
25
30
35
40
45
50 to 75
80
85 to 115
120 to 155
160 to 360

10-Year Design
Storm 5-min.
Rainfall Increments
(inches)

Time (Min)

100-Year Design
Storm 5-min.
Rainfall Increments
( inches)

where:

.03
.07
.14
.16
.26
.12
.09
.08
.07
.06
.04
.03
.02
.01

5
10
15
20
25
30
35
40
45
50
55 to 65
70 to 75
80 to 120
125 to 190
195 to 360

.04
.10
.14
.21
.23
.40
.27
.21
.20
.15
.13
.07
.03
.02
.01

Parameters to predict infiltration loss rates were estimated based on a
preliminary inventory of soils prepared by the U.S.D.A. Soil Conservation
Service (Reference 9). Infiltration losses were computed within the CUHP
model using Horton's Equation:

f ~
f. ~
,
fo ~
ex ~
t =

infiltration rate in inches/hour,
initial infiltration rate in inches/hour,
final infiltration rate in inches/hour,
decay coefficient in per second units, and
time in seconds.

TOTALS

2.20

3.37

Future land use in the tributary watershed was estimated usin9 planning
documents from Thornton, Broomfield, Westminster, and Adams County. This
information was then translated into future percent impervious land surface
area (I ) for the hydrology modeling, and is shown in Figure 1 and
a
summarized in Table IV. Values of I for each sub-basin are listed with
a
the other CUHP parameters in Table II.

5