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

Active alluvial fan flooding is a type of flood-hazard that occurs only on alluvial fans. It is
characterized by flow path uncertainty so great that this uncertainty cannot be set aside in realistic
assessments of flood risk or in the reliable mitigation of the hazard. An active alluvial fan flooding
hazard is indicated by three related criteria: (a) flow path uncertainty below the hydrographic apex,
(b) abrupt deposition and ensuing erosion of sediment as a stream or debris flow loses its
competence to carry material eroded from a steeper, upstream source area, and ( c) an environment
where the combination of sediment availability, slope, and topography creates an ultrahazardous
condition for which elevation on fill will not reliably mitigate the risk.

GUIDELINES FOR DETERMINING FLOOD HAZARDS ON ALLUVIAL FANS

I INTRODUCTION

Alluvial fans, and flooding on alluvial fans, show great diversity because of variations in climate,
fan history, rates and styles of tectonism, source area lithology, vegetation and land use.
Acknowledging this diversity, this appendix provides an approach that considers site-specific
conditions in the identification and mapping of flood-hazards on alluvial fans. Investigation and
analysis of the site-specific conditions may require knowledge in various disciplines such as
geomorphology, soil science, hydrology, and hydraulic engineering. Although the scope of study
may constrain the degree of site-specific consideration performed, it is essential that field inspections
of the alluvial fan be conducted.

As defined in 44 CFR 59.1, the current (1999) definition of "Alluvial Fan Flooding" means flooding
occurring on the surface olan alluvial fan or similar landform which originates at the apex and is
characterized by high-velocity flows; active processes of erosion, sediment transport. and
deposition; and, unpredictable flow paths.

The current definition under 44 CFR 59.1 will be revised to be consistent with the approach
described in this appendix and specifically to eliminate reference to "similar landforms ". The
process described in this appendix is intendedfor flooding onlv on alluvialfans as described below.

As interim guidance in the determination of "similar landform ", unless the landform under
investigation meets the three criteria under Stage 1 for composition, morphology, and location, the
landform is not considered to be "similar ".

This appendix provides guidance for the identification and mapping of flood-hazards occurring on
alluvial fans, irrespective of the level of fan forming activity. The term allUVial fan flooding
encompasses what will be described as active alluvial fan flooding and inactive alluvial fan flooding.

Inactive allUVial fan flooding is similar to traditional riverine flood-hazards, but occurs only on
alluvial fans. It is characterized by flow paths with a higher degree of certainty in realistic