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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

downstream of 1-25.
study will not match

Therefore, the flow conditions
previous FIS studies.

at the downstream end of this

Low flows measured in the summer of 1987 are shown in Appendix A of the separate
Utilities Report. Flows ranged from about 10 to 30 cfs with an average flow of about
20 cfs.

Section ill - Floodplain Hydraulics
Existing conditions at 136th Avenue utilized roadway elevations as a result of
construction along Washington Street completed in the fall of 1987 by the Northern
Metropolitan District. Since the map datum for this study is 0.86 feet higher than the
datum used for UDFCD and FEMA studies, starting water surface elevations were
corrected from those found in previous reports.

Proposed conditions downstream of 136th Avenue were investigated to determine their
possible impacts on conditions upstream within the golf course area. Improvements
included a low-flow channel, drop structure, bridge replacement at 136th Avenue and
Washington Street, relocation of the German Ditch headgate, and raising Washington
Street from 136th Avenue north for a distance of 4,150 feet. The proposed
improvements had a negligible tail water (the downstream water surface at a particular
location) change at 136th Avenue; however, several acres northeast of the 136th
Avenue/Washington Street intersection (currently located in unincorporated Adams
County) were removed from the 100-year floodplain. The proposed design criteria for
this site was to keep the 100-year flow off 136th Avenue and Washington Street and to
have 1 foot of clearance under the bridge's low chords (the bottom elevation of the
bridge girder).

A new floodway was determined for FEMA's 100-year event downstream of 136th Avenue
due to revised hydrology. The 100-year UDFCD's floodway was not revised downstream
of 136th Avenue.

Existing conditions upstream of 136th Avenue were modeled to determine:

1. Differences in water surfaces between current and previous studies; and
2. Base conditions to evaluate proposed improvements.

1-2