FLOOD00306

• County: Denver
• Community: All
• Stream Name: All
• Title: Determining Flood Hazards on Alluvial Fans
• Date: 7/17/1999
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

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07/16/99

Two-dimensional models may be appropriate for determining flood hazards on an alluvial fan.
Different two-dimensional models may be particularly useful in the analysis and modeling of some
or all of the following situations: flows that contain a high amount of sediment, unconfined flows,
split-flows, mud/debris flows, and complex urban flooding. For use in defining flood-hazards for
the NFIP, all hydraulic models must meet the conditions of Section 65.6 (a) (6) of the Code of
Federal Regulations.

One-dimensional sediment transport models or the methods described in Section 6A are also useful
for the analysis of conditions on alluvial fans.

5E Geomorphic Data, Post-Flood Hazard Verification, and Historical Information

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Assumptions: relies primarily on qualitative information, post-flood verification, historical
data, and interpretive studies

Limitations: approximate method

Recommended Applications: alluvial fans with little or no urbanization

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The geomorphic approach is for active alluvial fans where deposition, erosion and unstable flow
paths are possible. Traditional cngineering methods as described in Section 5D generally are
inappropriate for areas with these hydraulic characteristics. Probabilistic methods as described in
Section 5B and contained in the FAN computer program, also contain inherent limiting assumptions
that may not adequately represent field conditions and may not be applicable to many active alluvial
fans.

In some situations the information collected during Stage 2 (identification of active and inactive
areas) may be used to delineate an approximate floodplain on an alluvial fan. In situations where
geomorphic field investigations, coupled with historical documentation, and documentation of
hydrologic and hydraulic characteristics of flood event(s) (post-flood hazard verification) is
available, an approximate flood hazard delineation is possible.

By combining quantitative data on an actual flood event, historical information and photographs of
other flood events, time sequence aerial photography documenting recent activity or inactivity, and
field investigation of the morphologic characteristics and relative ages ofthe fan, an approximate
(Zone A) flood hazard map may be warranted.

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For many alluvial fans the various flood indicators (Stage 2 information) provide limited or partial
information. Because the flood assessment of active alluvial fans is more uncertain than more
traditional flood assessment, all assumptions and limitations should be well documented and
considered in the overall evaluation.

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