FLOOD10391 ‎(2)‎

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Title: Proposal, South Boulder Creek Floodplain Study Hydrology and Hydraulic Analyses, RFP No 17-2003
• Date: 3/14/2003
• Prepared For: Boulder Department of Public Works Utilities Division
• Prepared By: Ayres Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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AYRES

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. Providing a basis for comparison with other representative Front Range
streams, which together have experienced a more full range of flood causing
events

The team will also work together to supplement the paleohydrologic evaluation with an
examination of the evidence and history of debris flows and how this is related to past
flooding events.

Hydrology - Regional

A customized local, regional hydrologic analysis will be completed covering Front
Range foothill streams located within the upper South Platte River basin. This will be
conducted with best available gage data and other already existing evaluations of gage
data. While individual gage analyses will not be accomplished to the same level of
detail as the Eldorado Springs gage, the level of detail will be consistent with the use of
the information within a regional context.

This local regional analysis will be used together with other established regional
analyses including the USGS 2000 analysis of magnitude and frequency of Colorado
Floods, the CWCB guidelines for 100-year approximate floodplain flood flows, and the
NRCS peak flows in Colorado for reasonableness and consistency.

TASK 4: FLOODPLAIN HYDRAULIC MODELING

Background

South Boulder Creek is a hydraulically complex channel and floodplain that requires 2-
dimensional (2-D) hydrodynamic modeling to accurately delineate floodplain
boundaries, map areas of inundation, assess risks to structures and, as stated in the
RFP, to "produce a visual picture of the floodplain for public understanding." The
hydraulic complexity is mainly caused by elevation differences between the channel
and the floodplain. The main channel flows along the eastern side of the valley at a
higher elevation than the floodplain along the western side. Further complicating the
hydraulics are split flows occurring near U.S. 36, Highway 93, and other locations.
Prior to any model development, Ayres Associates would conduct a detailed site
reconnaissance to assess channel and floodplain hydraulic roughness, channel
stability, debris potential, potential for channel avulsions, structures, embankments,
levees, ditches, and land use patterns. The reconnaissance will be conducted by
engineers and an Ayres Associates geomorphologist.

Limitations of 1-dimensional Modeling

A one-dimensional dynamic model, such as HEC-RAS, is not well suited for this project
for the following reasons. (1) In 1-dimensional modeling water surface elevations are
assumed to be equal across a cross section. Ayres Associates' experience with 2-
dimensional modeling shows that this is rarely the case, especially for areas with
significant lateral flow components. Due to high spatially varied flow along South
Boulder Creek, water surface elevation, flow depths and flow quantities would not be
accurately simulated in a 1-dimensional model. (2) Flow within a cross section is
assumed to be distributed proportionally to the conveyance distribution. This
assumption means that a low point along the western valley edge would receive

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