FLOOD03719

• County: Adams
• Community: Thornton
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Floodplain Information Report
• Date: 4/15/1988
• Prepared For: Thornton
• Prepared By: McLaughlin Water Engineers, Ltd.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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(Greiner Engineering Sciences, Inc., Nov. 1986). For comparison purposes, the same flow
is used in this study at 1-25, but 11,190 cfs is used for the rest of the golf course site.

Federal Emergency Management Agency
A new Flood Insurance Study (FIS) was recently completed by FEMA for the City of
Westminster (FEMA, 1987). The downstream limit of that study is 1-25. The study used
3,830, 7,010, 8,839, and 16,729 cfs for the 10-, 50-, 100-, and 500-year events,
respectively. In order to satisfy FEMA's requirements, the above flows for major storm
events should be used (Liou, Feb. 1988). Therefore, a flow of 8,839 cfs was used for the
100-year event and FEMA floodway.

This study matches the 100-year water surface shown in Westminster's FIS at Huron
Street. At the downstream limit of this study, however, such a match will not be
possible. The downstream limit is in Adams County, and the Adams County FIS (FEMA,
1987) used a 100-year flow of 11,300 cfs. Normal depth calculations are used to start
the FEMA 100-year water surface profile for this study, with a flow of 8,839 cfs. It is
anticipated that as the lower reaches of Big Dry Creek are re-studied, the revised 100-
year water surface will match the starting water surface elevation (136.0 MSL at
UDFCD Section 1570) calculated in this study.

LOW FLOWS
Since the early 1900's, Big Dry Creek has been used to convey irrigation water during
the summer months. In recent years, the City of Broomfield's and City of Westminster's
wastewater treatment plants, located upstream of 1-25, have released more effluent to Big
Dry Creek as the populations within these cities expanded. Unfortunately, there have
been no historic gages installed to directly measure the flow in Big Dry Creek.

In July 1987, a stream staff gage was installed and calibrated at the 136th Avenue
bridge just west of Washington Street. The gage was monitored during the year, and a
rating curve was developed. The rating curve and gage readings are shown in Appendix
A of the "Thornton Municipal Golf Course - Utilities Engineering Report." Flows as
high as 32 cfs and as low as 10 cfs were measured. The average flow was about 20 cfs.

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