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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control improvements, or natural processes.
report for planning or design purposes, the
UDFCD, the City of Thornton, or Adams
information in this report has been amended.

Prior to utilization of this
user is advised to contact the
County to determine if the

- STUDY AREA AND BASIN DESCRIPTION -

- ACKNOWLEDGMENTS -

Big Dry Creek is a left bank tributary of the South Platte River. Its
confl uence wi th the South Pl atte Ri ver is located approximately 5 mi 1 es
north of its intersection with the Weld County line. Big Dry Creek has its
headwaters west of Standley lake in Jefferson County and drains portions of
the relatively steep foothills immediately east of the Front Range of the
Rocky Mountai ns. From its headwaters, it flows northeasterly through
Jefferson and Adams Counties, and into Weld County where it jOins the South
Platte River.

This report was prepared by Muller Engineering Company, Inc., Consulting
Engineers of lakewood, Colorado, under contract with the Urban Drainage and
Flood Control District (Agreement No. 85-02.08).

The reach of Big Dry Creek which is evaluated herein is located within
Adams County and is approximately 9.1 stream miles in length. The upstream
study 1 imi tis immedi ately east of Interstate 25. The Study Reach
terminates at 168th Avenue, which is the boundary between Adams and Weld
Counties, This channel segment of Big Dry Creek will be referred to in
this report as the Study Reach.

Topographi c mappi ng, di gi ti zed cross sections, and survey data for stream
crossings were provided by Delta Aerial Surveys, Inc., of Denver, Colorado.
Original mapping was prepared with two-foot contour intervals at a scale of
1" = 200' (Reference 2). The mapping was prepared according to the
specifications of the UDFCD.

Future 1 and use data were obtai ned through the Pl anni ng Departments of
Adams County and the Cities of Thornton, Broomfield, and Westminster.

A watershed of 61. 0 square mil es is tri butary to Bi g Dry Creek at the
upstream 1 imit of the Study Reach. Watersheds upstream of Standley lake
and Great Western Reservoir contain 16,8 and 5.0 square miles,
respectively; storm runoff from each watershed is largely controlled by the
reservoi rs. Hydrol 09i c ana lyses by the Urban Drai nage and Flood Control
District have indicated that these two reservoirs have minimal impact on
the IOO-year di scharge at the upstream study 1 imit. Consequently, a net
watershed of 39.2 square miles (downstream from the two reservoirs)
di scharges storm runoff to Bi g Dry Creek upstream of Interstate 25. An
additional watershed of 26,2 square miles (in Adams and Weld Counties) is
directly tributary to Big Dry Creek along the Study Reach, resulting in a
total uncontrolled watershed area of 65.4 square miles at 168th Avenue.

The watershed directly tributary to the Study Reach ranges in elevation
from 5460 feet along the basin divide to 5040 feet at the downstream study
1 imit. From the downstream study 1 imi t, Bi g Dry Creek conti nues another
nine stream miles to meet the South Platte River at elevation 4900.

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