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

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TABLE I II
CONVEYANCE ELEMENT PARAMETERS FOR SWMM
BIG DRY CREEK

TABLE IV
PERCENT IMPERVIOUS LAND USE
BIG DRY CREEK

Element Side Slopes
Number Width Length Slope H to V Manning's Depth
(Fig. 2) Type (ft) (ft) ( ft/ft) Lt Rt n (ft)
1 Channe 1 10 12,700 .0025 1 1 .040 5
Overflow 235 8,860 .0036 50 160 .060 20
2 D.R.
3 Channel 15 15,200 .0021 1 1 .040 5
Overflow 100 9,030 .0035 190 70 .060 20
4 D.R.
5 Channel 20 13,800 .0023 1 1 .040 5
Overfl ow 60 8,555 .0037 110 400 .060 20
6 D.R.
7 D.R.
8 Channel 25 6,600 .0012 1 1 .040 5
Overflow 100 3,380 .0024 180 35 .060 20
9 D.R.
10 Channel 185 4,980 .0054 30 45 .060 20
11 D.R.

Future Land Use

Percent Impervious (Ial
5
80
60

Open space, Greenbelt

Office Park, Commercial

Mid to High Density
Residential (6-12 DU's/ac)

Low Density Residential
(3-6 DU's/ac)

40

Reference 4 recommends that an Area Adj ustment of the des i gn storm be
incorporated into the analysi s for watershed areas exceedi ng five square
mil es. Different area adj ustment parameters are provi ded for watersheds
with areas between 30 and 50 square miles and with areas between 50 and 75
square miles. These Area Adjustments were incorporated into the hydrologic
analysi s.

Note: D.R. = Direct Routing Element. Parameters for these elements do
not affect hydrology modeling outcome, and are therefore
not listed.

Because of the relatively large si~e of the Big Dry Creek watershed and the
requirement that Area Adjustments would be applied to the design storms, it
was determined that two different nydrologic conditions should be evaluated
to determine which would produce the larger peak flows along the Study
Reach. The first condition was a design storm over the entire 65.4 square
mile uncontrolled watershed, with appropriate Area Adjustments applied as
the tributary area increased from 39.2 square miles to 65.4 square miles.
The second condition was a design storm of lesser areal extent but higher
rainfall intensity located over the 26.2 square mile portion of the
watershed di rectly tributary to the Study Reach, with appropri ate Area
Adjustments appl ied as the tributary area in the Study Reach increased
from zero to 26.2 square miles. As will be illustrated in detail, the
former condi ti on produced hi gher peak flows along the Study Reach and was
therefore determined to be the governing condition.

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