FLOOD00385

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: South Boulder Creek Interim Hydrology Study
• Date: 7/1/2000
• Prepared For: Boulder County
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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UDFCD authorized the Phase II Initial Reconnaissance 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 II study, the Corps' hydrology
was not changed upstream of Highway 36 other than modifications to better reflect
floodplain routing downstream of the mountains. All new basins and routing
elements were inserted into the hydrology model for the Viele Basin and the area
downstream of Highway 36.

Also, since questions regarding hydrology had been raised by the sponsors and by
our work, requests to the Corps were made to obtain background data, basin
mapping, and verification studies. A watershed basin map for the area near and
above Gross Reservoir and a mem.orandum discussing model parameter sensitivity
testing and rainfall were received.

OBJECTIVES OF THIS STUDY

The primary objective of this study is to prepare a new hydrologic analysis of the
South Boulder Creek Watershed.

The initial use will be for the evaluation of flood damage mitigation alternatives for
the reach of South Boulder Creek from Highway 93 to Boulder Creek.

Ultimate uses of the study will be for base hydrology for UDFCD flood hazard
delineation studies (FHAD). FEMA Flood Insurance Rate Maps (FIRM). and planning
of road and bridge improvements, channel modifications, and flood control
structures. The hydrology should meet the requirements of the "Rules and
Regulations for the Designation of Floodplains and of Storm or Flood Water Runoff
Channels in Colorado" by CWCB and FEMA's "Guidelines and Specifications for
Study Contractors."

SIMPLIFIED SCOPE

This effort is focused on:

1. Determining the design rainfall from NOAA references that is most
appropriate for the South Boulder Creek watershed, particularly on the South
Boulder watercourse from Highway 93 downstream to Boulder Creek.

2. Checking the Corps' SWMM runoff model upstream of Gross Reservoir,
especially with respect to runoff area and infiltration characteristics.

3. Reconstructing the model, especially from just upstream of Highway 93 to
Highway 36, using the City and County database (detailed topographic

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