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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higher than required by the present plan and the relocation of the German Ditch
headgate need not be as far upstream as planned. The German Ditch system, from the
headgate to the Parshall flume east of North Washington Street, can be lowered 2.0 feet
without affecting ditch operation.

Lowering the German Ditch in this area allows the headgate to be constructed several
hundred feet downstream of the proposed location and closer to the present headgate.
The crest wall needed for the headgate works can also be incorporated into the drop
structure called for by the outfall study.

The proposed improvements will remove several acres in the northeast corner of 136th
Avenue at North Washington Street from the 100-year floodplain. In addition, the Big
Dry Creek 100-year floodplain, which currently crosses from west to east over North
Washington Street, will be confined to the proposed bridge section and flooding of
North Washington Street will not occur.

EXISTING CONDITIONS AT 136TH AVENUE AND UPSTREAM
Stream Morphology
Big Dry Creek is a meandering stream in the reach between 136th Avenue and 1-25. The
stream generally has 4- to 8-foot banks, but in some areas the banks are 10 feet deep.
Bank failure along the outside curve of meanders is a continuing problem. Fine sand,
removed from the banks, is deposited upstream of stable channel bottoms such as at
136th Avenue and at several clay outcrops through the channel reach. Short reaches of
riffle sections occur at the clay outcrops. Generally, the reach from 136th Avenue to
1-25 is a deposition zone. The overbanks are primarily flat and open rangeland.
Overbank flooding probably occurs at least every other year.

Existing Floodplain Conditions
As described previously, the recent improvements to North Washington Street (which
resulted in a higher street profile) were included in the existing condition modeling.
These changes resulted in a 1.0-foot rise in the 100-year water surface above that
determined by Muller's (Nov. 1986) FHAD study.

Big Dry Creek flows through a corrugated metal bridge structure at 136th Avenue. The
structure is severely undersized and 136th A venue would be overtopped during the 2-

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