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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EXISTING CONDITIONS DOWNSTREAM OF 136TH AVENUE

The roadway elevation at 136th Avenue and Washington Street has changed due to
construction in the fall of 1987. Design elevations are shown on Drawing No.1 (in
report pocket). The new road elevations were not modeled in previous UDFCD studies.
This study utilizes the new road elevations as the existing condition.

The mapping for this study is on a datum which is 0.86 feet higher than the datum used
for UDFCD and FEMA studies. Hence, 0.86 feet should be subtracted from the elevations
in this report, unless otherwise stated, to convert to UDFCD elevation datum, which is
depicted as Mean Sea Level (MSL).

The method used to analyze the flow along each stream segment in Big Dry Creek is
based upon the standard step method of determining water surface profiles from
backwater calculations. This method is characterized by dividing the channel into
reaches. The water surface profile is computed from one end of the reach to the other
by means of balancing Bernoulli's Energy Equation for open channel flow. The computer
model, developed by the U. S. Army Corps of Engineers and called HEC-2 Water SUrface
Profiles, was used for this floodplain investigation.

The HEC-2 output file for existing conditions is shown in a Technical Appendix to this
report, which is separately bound. The floodplain analysis begins at Section 157 from
Muller's FHAD (Nov. 1986) and ends at UDFCD Section 4020 from Greiner's FHAD (Nov.
1986). This last section is at the downstream side of Huron Street and the same cross
section was used for the UDFCD Westminster FHAD, and FEMA's Flood Insurance Study
(FIS) for Westminster.

The existing conditions model was used to start proposed conditions flood events
upstream (south) of 136th Avenue, because no immediate construction will occur in this
area in conjunction with the golf course. However, future improvement is planned at
the 136th A venue bridge and possible channel improvement could occur from 136th
Avenue north to Washington Street. The analysis of the proposed conditions is explained
below.

III-2