FLOOD01787

• County: Boulder
• Stream Name: South Boulder Creek
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

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Mr. Bill DeGroot
December 18, 1996
Page 2

wide at the present location of U.S. 36. Like any typical alluvial fan below a mountain
canyon, floods conveyed and deposited sediment in the main stream bed and across the
valley bottom, resulting in a stream that changed in location from time to time. Today, in
many reaches, the western side of the valley is lower than the eastern side of the valley
where the main South Boulder Creek currently runs. Overflows can occur upstream of
Highway 93, Highway 36, and Arapahoe and travel to the west side of the valley. Migration
of the mainstream to the west side could occur, but obviously would be and has been
resisted by man.

Floods have and will cause overflow of significant velocities among many shallow routes.
This overflow can also concentrate at certain locations in an urban setting, potentially
increasing flood hazards.

There is evidence that South Boulder Creek flooding has traveled along the path identified
by Love. U.S. Geological Survey mapping reflects that the South Boulder Creek alluvial
material extends into the existing neighborhoods north of U.S. 36 and along Foothills
Parkway. Examination of historical and current topography shows that Dry Creek Ditch No.
2 and west valley conveyances carry both local drainage and South Boulder Creek
overflows. The relatively small flood of May 1969 flooded the west side of the valley and
caused the closure of U.S. 36 for a period of time near the South Boulder Road and Foothills
interchange.

Love's study was the first to quantify the South Boulder Creek Spill to the west side of the
valley at Highway 36 and Viele (Foothills Interchange). Correction of the 100-year Federal
Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM's) and the
UDFCD FHAD floodplain for the west valley north of Highway 36 will likely result in the
addition of a number of fingers of flow and areas of shallow flooding. A revised 500-year
floodplain limit will probably result in major changes to the FIRM.

The Flatirons Levee and the construction of U.S. 36 have impacted the South Boulder Creek
floodplain, causing redirection and redistribution of flood waters. The original U.S. 36
highway construction created an obstruction across the South Boulder Valley which
redistributed flood waters. The addition of the highway divider further obstructed and
redistributed flood waters, which has worsened flood hazards to some parties. Viele
Channel improvements somewhat reduce these impacts, but have not fully mitigated the
impacts of the highway and divider.

If the existing levee around the Flatirons Pit is adequate, the 100-year floodplain would be
as described herein. The upper and middle Flatirons Levee forces a narrow floodplain on the
east side of the valley, approximately 1500 feet upstream of Highway 36. Flow spills to the
west valley largely from the open space reach of 600 feet above Highway 36. The upper
and middle Flatirons Levee reduces the spills leaving South Boulder Creek from that which

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