What the Potential Sea Level Rise Maps Are Intended to Conceptually Illustrate
The “Potential Sea Level Rise” maps illustrate the concepts of Marin County’s Local Coastal
Program Policy C-EH-8 – Minimum Floor Elevations in Flood Hazard Areas. These maps were
developed for illustrative purposes and have not been adopted by the Marin County Board of
Supervisors. The policy would apply when a new or remodeled building requires a Coastal
Permit, based on actual conditions of the site. This policy requires subject building elevations
to accommodate three feet of sea level rise above and in addition to the Federal Emergency
Management Agency (FEMA) Base Flood Elevation requirements, as described below. In areas
outside FEMA Special Flood Hazard Areas that are nevertheless subject to sea level rise, the 3
foot building elevation would also be required to accommodate for future sea level rise (Base
Sea Level Rise Elevation).
The FEMA Base Flood Elevation is an existing regulatory requirement for elevating and floodproofing of structures located within Special Flood Hazard Areas based upon high-intensity
storm floodwater elevations that have a projected 1% chance to occur in any given year
(commonly referred to as the “100-year flood”). Special Flood Hazard Areas include lands that
would be impacted by floodwaters, and for West Marin consist of “VE Zones” (subject to wave
velocity) and “AE”/”AO” Zones (subject to rising waters without waves). Special Flood Hazard
Areas are delineated and Base Flood Elevations are determined based upon historical flooding
trends and do not account for future Sea Level Rise which is currently not reflected in FEMA’s
Flood Insurance Rate Maps.
The intent of the proposed County policy is to fully prepare for future sea level rise conditions
by requiring buildings to elevate 3 feet above the required Base Flood Elevation in Special Flood
Hazard Areas (VE, AO or AE as indicated on the Potential Sea Level Rise Maps) and 3 feet above
existing underlying topography in areas projected to be affected by sea level rise located
outside of Special Flood Hazard Areas (Sea Level Rise Exposure Zones as indicated on the
Potential Sea Level Rise Maps). Three feet approximately equals 100 cm, which is a midpoint
projection of Sea Level Rise for the year 2100 based upon the National Research Council’s SeaLevel Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future,
which provides state level guidance for SLR adaptation.
Policy C-EH-8 was developed through the multi-year update process of Marin County’s Local
Coastal Plan (LCP). A parallel County effort, Collaboration: Sea-level Marin Adaptation Response
Team (C-SMART), augmented the LCP update through assessing potential future sea level rise
impacts including the identification of areas exposed to rising sea levels at various future
scenarios. Robust public outreach has been included through C-SMART’s entirety, including
public workshops which unveiled the sea level rise exposure maps and a public comment period
providing an opportunity for local stakeholders to submit input on the draft Vulnerability
Assessment. In April 2016 the Marin County Board of Supervisors adopted the Local Coastal
Plan, providing that 3 feet of building elevation be added to account for sea level rise (both in
Special Flood Hazard Areas and Sea Level Rise Exposure Zones).
FEMA’s Flood Insurance Rate Maps identify Base Flood Elevations used for National Flood
Insurance Program regulatory purposes. Base Flood Elevations used to inform the “Potential
Sea Level Rise” maps are from FEMA’s 2015 Preliminary Flood Insurance Rate Maps for Marin
County. The proposed Flood Hazard Determinations were published to the Federal Register in
January 2016, with a 90 day appeal period from February 9 to May 9, 2016. The Proposed Flood
Hazard Determinations were published twice in the legal ad section of the Marin Independent
Journal on February 2 and February 9, 2016. To further introduce the maps and answer
questions, two public meetings were held in March 2016, one in Stinson Beach and the other in
Marshall.
The “Potential Sea Level Rise” maps illustrate Special Flood Hazard Areas, as well as areas
exposed to 3 feet of Sea Level Rise outside of Special Flood Hazard Areas (Sea Level Rise
Exposure Zone). Numeric values in parenthesis represent Base Flood Elevations based on an
official vertical datum related to current sea level (referred to as NAVD88) plus three additional
feet to account for future sea level rise. Required building elevations would be determined by
actual measurements on a case by case basis taking into account a building’s existing elevation
above sea level (NAVD88). In “VE” Zones, measurements are made from the sea level (NAVD88)
to the lowest horizontal structural member (e.g., floor joists). In the “A” zones, the
measurement is to the lowest finished floor. In Special Flood Hazard Areas, the values in
goldenrod represent the approximate height distance between the required BFE + 3 feet and
the average underlying topography shown on County base maps. Where the proposed County
policy would apply to lots located outside of Special Flood Hazard Areas (i.e., no Base Flood
Elevation requirement), those values represent the height distances based on 3 feet of sea level
rise alone. Most if not all existing buildings are already elevated above the underlying grade, so
the actual height distances existing buildings would have to be raised to meet the proposed
policy is less than the values shown in goldenrod (for example, if a remodel project is subject to
a required elevation of 9 NAVD88 pursuant to the proposed policy, and the existing home is
currently at elevation 4 NAVD88, the remodeled home would be elevated 5 feet to satisfy the
proposed policy). Finally, the white numbers show the elevation of the white topographic
lines.