FLOOD10344

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: South Boulder Creek Correspondence
• Floodplain - Doc Type: Correspondence

published relations of watershed area to flood discharge for other streams in the region. The
precipitation information should be complimented by recorded radar data available from
NOAA and NCAR, and recommended improvements to rainfall analysis offered to TEA by
Henz Meteorological Services should be included.

The IRP also noted that the nature of South Boulder Creek flooding comes from two storm
populations: convective storms in the summer and early fall, and a combination of snowmelt
and storms in the spring. Both sources should be considered.

As an alternative rainfall analysis method (or a valuable supplement), a synthetic hydrograph
could be applied without the complications of synthesizing the precipitation. This approach
would need to include critical assumptions regarding the operation and storage performance
of Gross Reservoir and the behavior of inadvertent storage in the upper basin.

· Paleohydrologic Information - Paleoflood data offers information to compare with historic
records and extend major flow analysis over many years. Physical paleostage indicators will
help define the largest flood events that may have occurred along with smaller more frequent
elements. Botanical indicators such as tree scars may also provide an indicator of frequency
for events in recent times. Paleoflood evaluation is also critical in determining storm return
intervals for historic floods.

· Flood-Frequency Analyses - Flood frequency analysis with state-of-the-art techniques are
needed, such as recommended by the National Research Council their 1999 report Improving
American River floodfrequency analyses. This analysis would be done using revised gage,
historical, and paleoflood data for South Boulder Creek and other gauged streams within the
basin or in nearby basins. Regional flood frequency analyses would be needed in
conjunction with hydrologic modeling results to obtain flood frequency estimates for South
Boulder Creek.

· Rainfall-Runoff Modeling - State of the art modeling techniques and programs may offer
greater and more representative results. The use of 2-dimensional models (which incorporate
high-resolution topography, soil types, land-use, etc.) will allow greater assessment of
rainfall-runoff impacts under multiple scenarios. Such models should be pursued subject to
ultimate acceptance by FEMA.

· Stream Flow Routing - The elements of dynamic storage, dynamic routing and physical
storage vessels (such as Gross Reservoir and inadvertent storage) need to be added to the
hydrology. These elements may account for greater attenuation of peak flood flows such as
Gross Reservoir storage, storage above railroad embankments, spills to Baseline Reservoir,
online and offline floodplain storage in the valley, and flow attenuation along multiple and
dispersed flow routes.

· Floodplain Determination Model - A revised and updated floodplain model should be
produced to identify the extent of flooding in the basin to allow the most effective evaluation
of mitigation measures and to apply floodplain regulation before long-term mitigation
measures can be implemented. A 2-D model for this application can determine (or closely

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