FLOOD09573

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

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SECTION I

SUMMARY

INTRODUCTION

In 1977 the U.S. Army Corps of Engineers (Corps) conducted a hydrology study
that included the South Boulder Creek Basin. That study provided the hydrology for
the Urban Drainage and Flood Control District (UDFCD) Flood Hazard Area
Delineation (FHA D) and the Federal Emergency Management Agency (FEMA) Flood
Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) for South Boulder
Creek,

In 1996, Love and Associates identified that South Boulder Creek could spill greater
flows than identified in the FHAD and that these spills could travel to the west side
of the valley and overtop Highway 36, largely west of Foothills and thus posed a
hazard to the neighborhood north of 36. .

Phase I effort reviewed the FHAD and Love's investigation, and identified 100 year
spills on the order of 2700 cfs from South Boulder Creek above Highway 36, The
spill was shown to commingle with local drainage from the Dry Creek Ditch No. 2
and the Viele watershed. The Phase I effort obtained a copy of the Corps'
hydrology model to identify the timing of the Viele inflows and allow simplified
simulation of its drainage network. Some general problems were noted in the
Corps' model, but our effort focused on the determination of how much of the
South Boulder Creek spill would return via Viele and how much would continue to
the West Valley spill route. Phase I determined that about 1800 cfs would
continue to the West Valley north of Highway 36.

The Urban Drainage and Flood Control District (UDFCD) authorized this Phase"
initial hydrology study to estimate the distribution of flooding in West Valley
downstream of Highway 36 to the area of the Wellman Ditch, For the purposes of
the Phase" study the Corps' hydrology remained unchanged upstream of Highway
36 other than modifications to better reflect floodplain routing downstream of the
mountains. All new basins and routing elements were inserted for the Viele Basin
and the area downstream of Highway 36.

Also, since questions regarding hydrology had been raised by the local governments
and our work, requests of the Corps were made to obtain background data, basin
mapping, and verification studies,

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