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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PROPOSED CONDITIONS AT 136TH AVENUE AND UPSTREAM

In some cases, proposed floodplain limits follow along the golf course property line.
The owners of the abutting property (Block 1, Lot 4, of the Rolling Hills Subdivision,
Amended) plan to fill to the golf course property line above the elevation of the 100-
year event. It is assumed that a 4 to 1 slope will exist at the property line in the
future, and the proposed conditions are modeled as such.

Because habitable structures are not planned in the floodplain on the course, this study
proposes to consider both the UDFCD and FEMA flood way as having the same
delineation as the floodplain. Therefore, the FEMA floodway downstream of 136th
Avenue is shown, but upstream of 136th Avenue the floodway will follow the floodplain
delineation in plan view. The plan view map illustrating the golf course flood events is
shown on Drawing No.4.

The profiles of all events are shown on Drawing Nos. 5 and 6. Two invert profiles are
also shown, one from Muller's (Aug. 1987) outfall study and the other is the invert
surveyed (using field instrument methods) for this study.

Some zones of high channel velocity will occur, similar to the existing floodplain. A
plan view map is provided (Drawing No.7) to show these zones, and recommended
treatment is also outlined in detail (Drawing No.8).

Muller's (Aug. 1987) outfall study recommends an improvement to the 1-25 bridge to
prevent overtopping. The study suggests jacking three 108-inch CMP culverts adjacent
to the existing structure. However, upstream water surface reduction could occur by
lowering the existing bridge invert and including a drop structure upstream of the
bridge. This improvement may be a more cost effective and practical approach to
prevent overtopping and is described later.

floodplain and floodway
According to Muller's (Aug. 1987) outfall study, the 136th Avenue bridge will be
replaced to pass the 100-year flood without overtopping the street. HEC-2 modeling of
these improvements showed that the 100-year water surface would be lowered to a level
0.45 feet less than the level shown in Muller's (Aug. 1987) FHAD study. However, these
improvements allow the low chord of the bridge to be submerged and subjected to the

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