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Alluvial Fan Flooding and Protection

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Definition - Alluvial fans are created when flash floods move rapidly down the steep desert canyons, depositing sand and debris in a fan-shaped pattern onto the desert floor.  In subsequent years, smaller flood flows will typically move along an existing channel, or wash, on the fan until either an obstruction is encountered or the sediment builds up on that section of the fan to a level at or above the general elevation of the local fan.  When this condition is reached, floodwater can suddenly change course and move to a new wash located on the fan.  A larger  flood, which is typically too large to be contained by  the existing washes, will tend to move as sheetflow across the fan and may even establish a new wash.  Therefore, all areas on the fan are subject to flooding unless appropriate flood protection is provided.

Specific Sources of Flash Floods – The County has analyzed and mapped the following alluvial fans to assist in designing flood protection measure for them:  Box Canyon, Unnamed Canyon, Coyote Canyon, El Vado Canyon, Henderson Canyon, Borrego Palm Canyon, Fire Canyon, Hellhole Canyon, Dry Canyon, and Culp-Tubb Canyon.  These areas are shown on the San Diego County  Flood Insurance Rate Map (FIRM).

FIRMs show alluvial fan hazards as Special Flood Hazard Areas (SFHAs) designated Zone AO and provide information on flood depths and velocities.  SFHAs designated Zone AO are areas subject to inundation by 1-percent-annual-chance (100-year) sheet-type flow that is sometimes associated with high velocities.

Building in an Alluvial Fan Area - Construction within alluvial fan areas is subject to the regulations below, which were developed by the County in accordance with Chapter 44 of the Code of Federal Regulations, Part 60.3.

  • It must be demonstrated that the lowest floors and utilities of proposed structures will be elevated 1 foot above the base flood depth in accordance with County Code of Regulatory Ordinances Section 811.501(c)(2).
  • It must be demonstrated that all public roads are elevated above the base flood depth in accordance with County Public Road Standards Section 5.11(I).
  • Certain requirements apply if fill will be placed to meet the requirements noted above, and if the placement of fill will cause changes to the depths, velocities, or floodplain boundaries shown on the FIRMs or related digital data for the area.  In these cases a Conditional Letter of Map Revision (CLOMR) must be obtained from FEMA before grading begins, and a Letter of Map Revision (LOMR) must be obtained from FEMA before project completion, in accordance with County Code Regulatory Ordinances Section 811.503(b) and the NFIP regulations.
  • Applicants for projects that would require a CLOMR or LOMR should be familiar with the requirements to obtain FEMA approval of their proposed revision.  NFIP regulations require that a a CLOMR and LOMR must be obtained in situations that include (but are not limited to) the design and implementation of facilities intended to capture and safely convey the base flood flow at the fan’s apex.  Projects accepted by FEMA for this purpose include:  channelization, levee/floodwall, dam, and sedimentation basin.  For further information, see the FEMA MT-2 Form, Section 6(b)(1)). However, some of these may not be acceptable to the County.
  • FEMA will not issue a CLOMR-F (based on fill) or LOMR-F for areas of alluvial fan flooding.
  • Structures raised on piers so that the lowest horizontal structural member is at least 1 foot above the base flood depth may be allowed without requiring a CLOMR or LOMR.  In these instances:
  • Analyses may be required depending upon the size of the development and the extent of obstruction to flow.
  • Pier foundations shall be designed to account for scour caused by the base flood.
  • Applicants should be informed of flood insurance requirements.
  • The use of one- or two-dimensional hydraulic models within the designated floodplain in the Borrego Springs area will be acceptable only for alluvial fan areas determined to be inactive by an acceptable analysis in accordance with Appendix G of the Guidelines and Specifications for Flood Hazard Mapping Partners, Section G.2.2.
  • All residential development in the mapped areas of alluvial flooding needs to be performed in accordance with the requirements of ASCE-24, per the California Building Code.

Summary - Several methods of flood protection are available for flood-safe construction in Borrego Springs.  However, no one method of design is acceptable for use on every lot.  The method of flood protection chosen for a specific property must adequately address the local conditions of the land on and upstream of that property.  Lots with unique characteristics may require special engineering studies to determine associated flood hazards and flood protection details.

Before purchasing any lot or architectural plans, potential project owners or developers should obtain all available information about the local geology and possible flood hazards.  It is the property owner’s responsibility to make certain that the lot and plans together satisfy intended project goals and incorporate flood protection for the property without detriment to adjacent properties.  For additional information, please contact the Flood Control Counter at the County of San Diego Operations Center Annex at (858) 694-2112.


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If you have questions or would like additional information, 
call the County Stormwater hotline at 1(888) 846-0800
or e-mail us at watersheds@sdcounty.ca.gov

Report a Stormwater Violation On-Line
 
COUNTY OF SAN DIEGO
Department of Public Works
Flood Control
 
5510 Overland Ave., Suite 410, MS O326
San Diego,  CA  92123,  USA
(858) 495-5318           Fax: (858) 495-5263