FLOOD10336

• County: Statewide
• Community: State of California
• Basin: Statewide
• Title: Alluvial Fans in California - Identification, Evaluation and Classification
• Date: 5/1/2000
• Floodplain - Doc Type: Educational/Technical/Reference Information

Base Classification System

Based on the results of the three-step procedure, alluvial fans can be separated into four
types (classes) according to activity, potential for flooding, and development. These four are
Class I - Inactive fan with no flooding, Class 2 - Inactive fan with flooding, Class 3 - Active fan
with flooding and no development, and Class 4 - Active fan with flooding and development.
The classification system is shown on the flowchart on Figure 3. In some cases, however, an
alluvial fan landform may have areas identified as both active and inactive based on the three-
stage approach. For these special cases, the active and inactive portions will need to be evaluated
separately. In this regard, the four classes as shown on Figure 3, would be required to be mapped
to relate it to implementation of the land use recommendations presented in Figure 4. The base
classification system is intended to map the identified alluvial fans on quadrangle maps for State
uses. They are not intended to be mapped on FEMA Flood Insurance Rate Maps (FIRM). The
statewide mapping of the alluvial fans will be addressed in the next phase of this study.

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APPLICATION TO LAND USE

By knowing the severity and extent of areas at risk from alluvial fan flooding, local
governments in California can plan new development, manage existing development, and adopt
and enforce regulations to protect property, and lives. Figure 4 shows land use recommendations
for each class of alluvial fan.

Planning New Development

When considering allowing development in an alluvial fan area, local officials in
California should consider the type and characteristics of the fan, Each of the four classes shown
on Figure 4 has reasons for or against new development. For the inactive fan with no flooding
(Class I), development can be allowed without any restrictions related to flooding, For the
inactive fan with flooding (Class 2), development should be restricted to the floodway fringe
area with the bottom floor at or above the IOO-year flood level. The Department of Water
Resources recommends two feet above the IOO-year flood level. For active alluvial fans, all of
the fan would be considered to be subject to flooding, As a result, development on active fans
with no existing development (Class 3) should be restricted and strict zoning regulations should
be established to avoid development in areas at risk from flooding. For active fans with existing
development (Class 4), protective structural/floodproofing measures or relocation may reduce
damages, however, new development should not be allowed. Flood insurance should be
mandatory for existing development to mitigate flood losses and damages.