HomeMy WebLinkAboutTC Agd Pkt 2014-07-02 (2)TOWN OF TIBURON
1505 Tiburon Boulevard
Tiburon, CA 94920
To: Mayor and Members of Town Council
From: Community Development Department
Town Council Meeting
July 2, 2014
Agenda Item:
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Subject: Presentation by Kathleen Schaefer, Federal Emergency Management Agency
(FEMA), Regarding the San Francisco Bay Coastal Study and Proposed
Revised Flood Rate Insurance Maps for the Tiburon Peninsula
Reviewed By:
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At a recent meeting, the Town Council directed staff to arrange a presentation by a representative
of the Federal Emergency Management Agency (FEMA) to address that agency's San Francisco
Bay Coastal Study and the proposed revised flood insurance rate rates produced as a result of the
study.
Kathleen Schaefer, Regional Engineer for Risk Analysis, FEMA Region IX, will make the
presentation. Ms. Schaefer is also the Project Manager for the Coastal Study.
Questions for Ms. Schaefer previously submitted to staff were forwarded for her consideration.
She has provided the Town with a recently- released FAQ sheet regarding the proposed map
updates and Coastal Study, as well as an information sheet on the 100 -year flood phenomenon.
RECOMMENDATION
Receive the presentation and direct questions to Ms. Schaefer.
EXHIBITS
1. FEMA Frequently Asked Questions (FAQ) information sheet dated June 2014.
2. U.S. Geological Survey information sheet regarding 100 -year floods (April 2010).
Prepared by: Scott Anderson, Director of Community Development Xr
TOWN OF TIBURON PAGE 1 OF 1
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SAN FRANCISCO BAY AREA COASTAL STUDYUs-,U
�//�� //'�� Frequently Asked Questions (FAQ) FE1V11-A.
The San Francisco Bay Area Coastal (BAC) Study's Effect on Your Community
Q: When will the new FIRM panels be issued?
A: The revised FIRM panels will be issued as a series of Physical Map Revisions (PMRs), one per county for the
9 coastal counties. The preliminary FIRM panels, FIS Reports, and digital databases are being released on a
county -by- county basis starting with Marin County in spring 2014. It is anticipated that the preliminary FIRMs
for all 9 San Francisco Bay Area counties will be released before the summer of 2015.
Q: Where can I see the Flood Insurance Rate Map (FIRM) panels?
A: Following the Flood Risk Review (FRR) meeting, scheduled on a county -by- county basis, revised FIRM
panels will be available as draft work maps for local officials within the Flood Risk Review Commenting
Tool (Commenting Tool) for 60 days. Following the 60 -day comment period, the draft work maps will be
available on FEMA's GeoPlatform until the FIRM panels are distributed as preliminary data on FEMA's Map
Service Center. The preliminary FIRM panels, FIS Report, and digital database will serve as best available
data until the maps become effective following FEMA's post- preliminary mapping process, including the
statutory 90 -day appeal period that follows proposed changes to Flood Hazard Determinations and the Letter
of Final Determination sent to community CEOs.
Q: How will the flood zone designations change as a result of this coastal study?
A: In general, the BFEs are increasing by approximately 1 foot throughout the San Francisco Bay Area; however,
additional changes are also evident. The previous FIRMS were based on analyses completed in the 1980s. The
methods and data sources available for the current study allow for a much more in -depth evaluation of coastal
flood and wave hazards. The BAC study results, once complete, will provide a more detailed picture of the
coastal flood risk. The new study is resulting in new Zone VE designations along the shoreline, and also the
conversion of some Zone VE designations to Zone AE along the shoreline, and each transect is associated
with an assigned calculated BFE. Some areas designated Zone D, Zone B, or Zone X (shaded) may also be
revised, depending on the results of the analyses.
Q: Will riverine flood analyses be re- evaluated using the revised downstream coastal boundary condition?
A: No. Results from the coastal analysis will be superimposed on the riverine study to show the extent of coastal
flooding upstream, and the most significant flood hazard (the higher Base Flood Elevation of either the riverine
or the coastal study) will be mapped and used for regulatory purposes. Joint probability analysis is not being
conducted at the riverine - coastal confluences at this time. FEMA will not supersede A Zones along riverine
studies because we cannot assume coastal study results will be higher or whether the coastal study propagates
up river. If this is a concern within your community, communities may choose to have the coastal and riverine
tie -in analysis studied.
Q: Will communities have an opportunity to provide comments on the revised maps?
A: FEMA actively engages with communities throughout the mapping process. A secure online tool was
developed to give community officials an opportunity to review the draft maps and provide comments within a
GIS format. Following the FRR meetings with each county, community officials have 60 days to review the
draft maps and post comments. FRR meetings are technical in focus and intended to communicate the BAC
Study process, including the coastal analyses, regional modeling and mapping process. Comments collected
from the online tool for each county will be considered as preliminary FIRM panels are developed.
Communities will also have the opportunity to provide comments and appeals on the Preliminary FIRM panels
1 Terms in bold are defined on subsequent pages of the Frequently Asked Questions sheet.
June 2014 www.r9coastal.org Page 1 of 9
EXHIBIT NO.-L
SAN FRANCISCO BAY AREA COASTAL STUDY ;! Frequently Asked Questions (FAQ) T FEMA
or
during a formal 90 -day appeal period that will be initiated by publication of the proposed changes to the Flood
Hazard Determinations (FHD) in the Federal Register.
After the proposed FHD publishes in the Federal Register, FEMA will send formal correspondence to the
Chief Executive Officer (CEO) for each of the impacted communities providing details about the proposed
FHD notice. Following this correspondence, FEMA will publish the proposed FHD twice in the legal section
of the local paper. The 90 -day appeal period will start on the date of the second newspaper publication. The
appeal process applies to areas where FEMA has formally established new BFEs, and when any regulatory
floodway or SFHA boundary or zone designations have been added or modified. During the 9.0 -day appeal
period affected communities may submit an appeal, with supporting data, to FEMA via the community's CEO.
Any appeals must be based on data that show the proposed BFEs to be scientifically or technically incorrect.
For more information on Appeals and Protests to National Flood Insurance Program maps, please visit the
FEMA library document posted at: www.fema.gov /library /viewRecord.do ?id =3250, and amendments by way
of FEMA Operating Guidance Procedure Memorandum 57, Expanded Appeals Process.
Q: How will the BAC Study help my community develop resilience and adaptation strategies?
A: Communities may choose to adopt the preliminary data as best available data when permitting and planning
future land use decisions to support adaptation strategies and resilience.
FEMA maps the flood that has a one - percent- annual -chance of occurring in any given year and communities
are encouraged to consider FEMA's regulatory data as conservative. Coastal flood maps represent "snapshots"
of flood risk for a local area at the time the study was performed. Flood maps can become obsolete as physical
conditions change, or as our understanding of local flooding and flood effects improves. Obsolete flood maps
have consequences for life safety (individuals may be unaware of their flood risk), insurance reasons (flood
insurance premiums may no longer reflect actual flood risk) and for land use and building permit reasons
(development, building design, and post -flood recovery decisions are no longer tied to the nature and severity
of flood hazards).
Q: Will communities need to adopt new floodplain ordinances as a result of the BAC Study?
A: If the ordinances in effect do not include sections for coastal high hazard areas as cited in the Code of Federal
Regulations (Title 44, Chapter 1, Section 60.3), communities will be required to amend their ordinances. The
amended ordinances will have to be adopted and in effect prior to the effective date of the PMR for the county
in which a particular community is located.
The BAC Study Supports Regional Resilience
Q: What products will communities receive from FEMA as a result of the BAC Study?
A: Communities will receive revised FIRM panels in the form of a Physical Map Revision, a revised Flood
Insurance Study Report, and a digital database as regulatory products.
Q: Is FEMA coordinating with organizations such as BCDC and the California Coastal Conservancy?
A: FEMA is working with BCDC and other agencies. FEMA has provided data and support for BCDC's
Adapting to Rising Tides project, and is working with BCDC to support the use of the FEMA modeling data to
enhance the regional understanding of coastal flood hazards and support regional resilience efforts.
The study team is also collaborating with other federal and state agencies, non - governmental organizations,
and academic institutions. A partial list of non - community based organizations includes the U.S. Geological
Survey, the U.S. Army Corps of Engineers, the NOAA, and the California Department of Water Resources.
June 2014 www.r9coastal.org Page 2 of 9
SAN FRANCISCO BAY AREA COASTAL STUDY����+' �/�
Frequently Asked Questions (FAQ) ', FEMA
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Q: Is FEMA working with other agencies and organizations that are focused on coastal flood hazard analysis?
A: FEMA is reaching out to other agencies and organizations throughout the Bay Area to enhance the regional
understanding of coastal flood hazards. FEMA has collaborated with, and provided data to the Adapting to
Rising Rides project and the extension of the Our Coast Our Future project into San Francisco Bay. FEMA has
also engaged the Bay Area Flood Protection Agencies Association and the Floodplain Management
Association and provided study updates at several local meetings throughout the study process. FEMA has
hosted a series of workshops that brought together local communities, consultants, and other state and federal
agencies to discuss coastal hazards and data sharing. The most recent workshop was hosted by the Santa Clara
Valley Water District at the commencement of FEMA's detailed coastal analysis in the South Bay counties of
San Mateo and Santa Clara. The workshop was attended by over 85 individuals, and included a series of
presentations that highlighted eight ongoing projects in the area.
Q: How can communities leverage data?
A: The 2010 and 2011 detailed topographic data collected as part of the California Coastal Mapping Program for
the entire California open coast and San Francisco Bay is publicly available as raw data. Additionally,
communities may choose to use the preliminary FIRM panels and data as best available data when permitting
and planning future land use decisions, particularly for critical structures and evacuation planning.
FEMA's Mapping of Coastal Studies
Q: Why does FEMA map coastal flood hazards?
A: Coastal flood maps represent "snapshots" of flood risk for a local area at the time the study was performed.
Flood maps can become obsolete as physical conditions change, or as our understanding of local flooding and
flood effects improves. Obsolete flood maps have consequences for life safety (individuals may be living at
risk and not know it), insurance reasons (flood insurance premiums may no longer reflect actual flood risk) and
for land use and building permit reasons (development, building design, and post -flood recovery decisions are
no longer tied to the nature and severity of flood hazards). Thus, it is in the best interest of the community and
its citizens to maintain up -to -date flood hazard maps.
New flood studies are undertaken and flood maps are updated for a number of reasons, including:
• Better topographic data;
• A longer period of record to characterize coastal flood events;
• Improved flood modeling procedures; and
• Changes in land use and land characteristics, shoreline erosion, construction of flood barriers
The January 1982 flood event resulted in over $75 million dollars in damages to infrastructure around the Bay.
During flood disasters homes are destroyed, lives are disrupted or even lost. Understanding coastal flood risks
and taking steps to make our homes and communities more resilient makes sense both on an economic and
human level.
Q: Why is FEMA conducting a study of the SF Bay shoreline?
A: This is the first comprehensive study of coastal flood risk for the San Francisco Bay shoreline. FEMA is
currently performing a detailed engineering study of the Bay as part of our nationwide Risk Mapping,
Assessment, and Planning (Risk MAP) program to determine revised Base (1% annual chance) Flood
Elevations (BFEs), the extent of the Special Flood Hazard Areas (SFHAs) for coastal communities, and
June 2014 www.r9coastal.org Page 3 of 9
SAN FRANCISCO BAY AREA COASTAL STUDY ' FEMA
Frequently Asked Questions (FAQ) ':
update the Flood Insurance Study (FIS) reports and Flood Insurance Rate Maps ( FIRMs) accordingly. The
study is called the San Francisco Bay Area Coastal (BAC) study 2.
Q: Are the old maps incorrect? Is the information from those maps still valuable?
A: The information from previous effective Flood Insurance Rate Map panels and Flood Insurance Study reports
is still valuable and will be considered and utilized as appropriate. Advances in technology and new data
collected will be used to reanalyze the coastal flood hazards and map the results to produce updated FIRM
panels. All superseded versions of FIRM panels should be archived for future reference, including flood
insurance rating and building compliance determinations. Historic maps are available from FEMA's Map
Service Center accessible through www.fema.gov.
Q: Are tsunamis considered in the BAC Study?
A: Tsunami hazards will not be included in determining the BFEs as part of the BAC Study. Although the
consequences of a major tsunami impacting a populated coastline can be devastating, the historical rarity of
tsunami events on the California coast means that they do not have any statistical influence on the 1% annual
chance flood elevations which are dominated by the more frequent swell, storm, high tide, and El Nino events.
Tsunami recurrence intervals are being investigated by FEMA in a separate pilot study /demonstration project
for the City of Crescent City, Del Norte County to assess the hazards from tsunamis. This probabilistic
tsunami hazard analysis is being conducted in collaboration with Dr. Frank Gonzales at the University of
Washington as part of FEMA's Open Pacific Coast Study. This pilot study may result in non - regulatory
informational maps; however, these products are not tied to flood insurance requirements under the National
Flood Insurance Program (NFIP).
FEMA is also collaborating with the California Geological Survey and the California Emergency Management
Agency using a different approach to assess hazards from tsunami impacts. These methods and results will be
assessed for use in non - regulatory Risk MAP products to help communities develop strategies to mitigate risks
from tsunami hazards in ports and harbors.
Q: How will FEMA account for sea level rise on FIRM panels as part of the BAC Study?
A: FEMA maps existing flood risk, therefore, sea level rise will not be included in the BAC Study. The BFEs,
which are the elevations of the 1% annual chance coastal flood above current sea level will be depicted on the
revised FIRM panels. FEMA is collaborating with regional efforts focused on sea level rise in San Francisco
Bay, such as Adapting to Rising Tides and Our Coast Our Future, and providing data on existing coastal flood
hazards that may be used to inform future conditions.
FEMA does not map predicted long -term changes onto FIRMs but rather depicts the existing conditions and
current risk of flooding from coastal hazards at the time of publication. Substantial changes to areas along the
Bay shoreline following the BAC Study may trigger a re -study and the issuance of FIRM revisions when the
effective maps no longer reflect the actual risks. Future re- studies may be local or regional, rather than
countywide. To mitigate against the effects of long term climate effects as well as
extreme flood events that could occur at any time, FEMA encourages communities to regulate development in
SFHAs to higher than the minimum standards required for compliance with the NFIP regulations. Requiring
the first floor elevations of new and substantially damaged or improved structures built one or more feet
above the BFEs depicted on the FIRMS is an example of a higher standard to mitigate flood risk.
Terms in bold are defined on subsequent pages of the Frequently Asked Questions sheet.
June 2014 www.r9coastal.org Page 4 of 9
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SAN FRANCISCO BAY AREA COASTAL STUDY !' foF FEMA
Frequently Asked Questions (FAQ) ��, J
Q: What is a response -based approach to analyzing coastal flood hazards?
A: A response -based approach to analyzing coastal flood hazards for the BAC Study refers to the approach used
to determine the 1 % annual chance total water level (TWL) that will inundate the coastal region. Unlike the
Atlantic and Gulf coasts of the U.S., where the primary wave effects accompany a discrete event such as a
hurricane or Nor'easter, the Pacific coast and San Francisco Bay is subject to coastal flooding caused by the
interrelationship of various physical processes such as distant swell events, locally generated wind - driven
waves, nearshore tidal variations, and elevated water levels due to El Nino effects.
Since a number of physical processes can occur simultaneously and the statistical interrelationships among
them are not well understood, assessing the total result or response of the shoreline to the combined processes
as they occur in nature eliminates the need to determine the complex statistical relationships among the various
processes. Instead, the response of the shoreline to each occurrence of these simultaneous physical processes
is recreated from archived wave and tide records at hourly intervals for the period of study. In this way, the
entire 31 -year time series of swell, wave, tide, and shoreline interactions is analyzed in the North Bay and the
1 %- annual- chance TWL is calculated statistically to establish the BFE and inform the SFHA mapping. In the
South Bay, the entire 54 -year time series of wave, tide, and shoreline interactions is analyzed in a similar
manner; ocean swell is neglected as ocean swell is not observed to propagate into the far South Bay.
Q: What storm event is being considered for FEMA's BAC Study?
A: FEMA maps the 1 %- annual chance (100 -year) flood event. Coastal flood hazards will be analyzed using the
The physical processes that result in flooding along the Bay
shoreline vary therefore FEMA does not have a single well -
defined event, such as a hurricane, that results in a Bay -wide 100 -
year coastal hazard. Along the Pacific coast, FEMA has to
complete statistical analysis and consider all of the processes that could impact coastal flood hazards
individually at each transect location. During El Nino years we do see flooding at the 100- year level, but not
bay -wide.
General Terms and Definitions
Q: What is the base flood, the Base Flood Elevation (BFE), and the I% Annual Chance?
A: The base flood is the flood event that has a 1% annual chance of being equaled or exceeded in any given year.
This may also be referred to as the 100 -year storm event. The BFE is the computed elevation to which
floodwater is anticipated to rise during the base flood. BFEs are shown on Flood Insurance Rate Maps
(FIRMS) and on the flood profiles. The BFE is the regulatory requirement for the elevation or floodproofing of
structures. The relationship between the BFE and a structure's elevation determines the flood insurance
June 2014 www.r9coastal.org Page 5 of 9
response -based approach presented in FEMA's Guidelines for
Coastal Flood Hazard Analysis and Mapping for the Pacific Coast
of the United States (Appendix D from FEMA's Guidelines and
., p.. • „
Specifications for Flood Hazard Mapping Partners), which can be
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found on FEMA's website at
www.fema.gov /library /viewRecord.do ?id =2188. BFEs are
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calculated statistically from the annual maxima of the calculated
Total Water Surface elevations over a 31 -year period from 1973
through 2004 in the North and Central Bays (all areas to the north
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of the Hayward-San Mateo Bridge), and a 54 year period from
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1956 — 2009 in the South Bay (all areas to the south of the
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Hayward -San Mateo Bridge).
The physical processes that result in flooding along the Bay
shoreline vary therefore FEMA does not have a single well -
defined event, such as a hurricane, that results in a Bay -wide 100 -
year coastal hazard. Along the Pacific coast, FEMA has to
complete statistical analysis and consider all of the processes that could impact coastal flood hazards
individually at each transect location. During El Nino years we do see flooding at the 100- year level, but not
bay -wide.
General Terms and Definitions
Q: What is the base flood, the Base Flood Elevation (BFE), and the I% Annual Chance?
A: The base flood is the flood event that has a 1% annual chance of being equaled or exceeded in any given year.
This may also be referred to as the 100 -year storm event. The BFE is the computed elevation to which
floodwater is anticipated to rise during the base flood. BFEs are shown on Flood Insurance Rate Maps
(FIRMS) and on the flood profiles. The BFE is the regulatory requirement for the elevation or floodproofing of
structures. The relationship between the BFE and a structure's elevation determines the flood insurance
June 2014 www.r9coastal.org Page 5 of 9
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SAN FRANCISCO BAY AREA COASTAL STUDY
Frequently Asked Questions (FAQ) �.� FEMA
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premium. The coastal BFE is calculated by adding the still water elevation and the wave effects (wave setup
and wave runup), if applicable.
Q: Can an event greater than 1% Annual Chance occur?
A: The FEMA mapping approach is a steady -state approach, and it doesn't account for how water would flow
back out off of the inundated land, just how the floodwaters would propagate inland. In an extreme storm surge
situation, the water levels could be elevated for several hours, and there could be successive extreme high tides
as well. FEMA does not consider a hydrograph approach that considers the full tidal signal. The FEMA
approach is to assume an infinite source of water with coastal flooding. This is conservative, given the
uncertainties in how flood waters could recede (e.g. through storm drain networks, pumping).
Q: What coastal flood zone designations and definitions are used in FIRMs and what do they represent?
A: The primary flood zones encountered in the coastal areas on the current FIRMs are as follows. Except as
noted, the NFIP mandatory flood insurance purchase requirements apply in flood zones depicted on FIRMS.
• Zone AE is as an area subject to inundation by the 1- percent - annual -chance flood event as determined by
detailed study methods. BFEs are shown within these areas.
• Zone VE is an area subject to inundation by the 1- percent - annual -chance flood event with additional hazards
due to storm- induced velocity action by a 3 -foot or Was envelope
higher wave. BFEs are shown within these areas. and aFEs 1ErM a>:mm
• Zone V is a coastal area subject to inundation by
the 1- percent - annual -chance flood event with
additional hazard associated with storm waves as
determined by approximate study methods and
therefore do not have BFEs (BFEs) assigned to
them.
• Zone A is an area subject to inundation by the 1-
percent- annual -chance flood event as determined
by approximate study methods and therefore do not
have BFEs assigned to them.
• Zone AO is an area subject to inundation by 1-
percent- annual - chance shallow flooding (usually
sheet flow on sloping terrain) where average flood
depths are 1 -3 feet. Average
flood depths derived from detailed hydraulic
analyses are shown within this zone.
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Zone AH is an area subject to inundation by 1- percent - annual -chance shallow flooding (usually areas of
pending) where average depths are between land 3 feet. BFEs derived from detailed hydraulic analyses are
shown in this zone. Zone D is an area subject to undetermined flood hazards. Since no flood hazard analysis
has been conducted, no BFEs are assigned to them, and flood insurance rates are commensurate with the
uncertainty of the flood risk. Mandatory flood insurance purchase requirements do not apply in Zone D areas.
At Zone X (shaded) is an area subject to inundation of a flood event between the 1- percent - annual -chance (100 -
year) flood event and the 0.2- percent- annual -chance (500 -year) flood event. It is also used to show areas that
are protected from inundation by the 1- percent - annual -chance flood event by an accredited flood protection
structures. Mandatory flood insurance purchase requirements do not apply in Zone X (shaded) areas. [These
areas were designated as Zone B on earlier versions of FIRMs.]
June 2014 www.r9coastal.org Page 6 of 9
SAN FRANCISCO BAY AREA COASTAL STUDY �
Frequently Asked Questions (FAQ) FEMA
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Q: What is the Special Flood Hazard Area?
A: The area delineated on a FIRM that is subject to inundation by the 1- percent- chance annual or base flood.
Q: What is a tidal datum?
A: In general, a datum is a base elevation used as a reference from which to calculate heights or depths. A tidal
datum is a standard elevation defined by a certain phase of the tide. Tidal datums are used as references to
measure local water levels. Some of the most commonly used tidal datums are:
• Mean higher high water (MHHW): the average (or mean) of the higher high water height of each tidal
day observed over the National Tidal Datum Epoch (NTDE)
• Mean high water (MHW): the average of all high water heights observed over the NTDE.
• Mean tide level (MTL): the arithmetic mean of mean high water and mean low water observed over
the NTDE
• Mean sea level (MSL): the arithmetic mean of hourly wave heights observed over the NTDE
• Mean low water (MLW): the average of all low water heights observed over the NTDE
• Mean lower low water (MLLW): the average of the lower low water height of each tidal day observed
over the National Tidal Datum Epoch (NTDE).
Q: What is a tidal epoch?
A: A tidal epoch is a specific 19 -year period adopted by the National Ocean Service as the official time segment
over which tide observations are used to obtain mean values (e.g., mean lower low water, mean sea level,
mean higher high water, etc.) for calculating and setting tidal datums. It is necessary to adopt a specified
period because of periodic and long -term trends in sea levels (such as sea level rise). The present National
Tidal Datum Epoch (NTDE) is 1983 through 2001. The need for updating the NTDE is reviewed every 20 -25
years based on long -term changes in sea levels.
Q: How does the NAVD88 datum relate to a tidal datum (e.g., mean high tide)?
A: Vertical datums are typically categorized as either an orthometric datum (e.g. NAVD88) or tidal datum (e.g.
mean high tide). Orthometric datums are based on gravitational surfaces (the geoid) of the Earth, with one or
more tide stations used as control points. NAVD88 was defined based on geodetic observations throughout
North America and relies on the Father Point tide gage in Quebec, Canada as the only control point. This
differs from tidal datums, which are based on averaged phases of the local tide. Mean high tide (often referred
to as mean high water) is defined as the average of all local high water heights observed over the National
Tidal Datum Epoch.
In order to integrate data referenced to these two datum types, it is first necessary to apply conversions. For
example, the figure below shows the relationship between NAVD88 and the commonly used tidal
datums at the San Francisco Presidio tide gage. For this location, mean high water is calculated as 5.29 feet
NAVD88; however, because mean high water is based on the average of local high tide heights, conversions
will vary for each tide gage and should not be extrapolated to other locations. In other words, mean high water
at a station inside San Francisco Bay would not be equal to 5.29 feet NAVD88. NOAA maintains the tide gage
network in San Francisco Bay, and publishes the tidal datum conversations to NAVD88, if available. The
NOAA Tide and Currents website maintains the latest information on tide gages, tidal water level data, and
tidal datums: htto : / /tidesandcurrents.noaa.sov/
June 2014 www.r9coastal.org Page 7 of 9
PART. {,
SAN FRANCISCO BAY AREA COASTAL STUDY ^E`
Frequently Asked Questions (FAQ) FEMA
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Q: What drives coastal hazards?
A: Flooding from coastal hazards within the San Francisco Bay may be the result of annual high tide inundation
("King Tides "), extreme high tide inundation, El Nino winter storms, and/or wind wave events. For additional
information, refer to the most current BAC Study brochure posted to the BAC Study page here:
hhtt : / /www r9map org/Pages /San- Francisco- Coastal - Bay - Studv aspxx or the Coastal Beat article titled `Coastal
Flood Processes Along the California Coast.'
BAC Study Technical Details
Q: What level of detail is FEMA applying to the BAC
Study?
A: Coastal hydraulic analysis determines how high and
how far water goes as it rushes up a beach using transects
oriented perpendicular to the coastline instead of cross
sections as are used in riverine studies. The spacing of
transects along the shore determines the level of study
detail. For undeveloped areas or areas with homogeneous
features (such as large tracts of marsh and agricultural
land in Sonoma, Napa, and Solano Counties), transects
may be widely spaced; for areas with variable topography
or increased population, transects will be more numerous
and more closely spaced. Transect -based modeling will
be conducted identically for all transects regardless of
placement.
Q: What is the extent of the mapping for the BAC Study? Does it cover entire counties or just along the
shoreline?
A: The FIRM panels along the shoreline of the 9 San Francisco Bay Area coastal counties will be revised and re-
issued based on the results of FEMA's analysis. Riverine flood
hazard data shown on the currently effective versions of these Image Courtesy of Fugro Earth Data, Inc.
panels will not be updated, unless FEMA has received and
approved a separate flood study. l t i
Q: What terrain elevation data will be used in the coastal studies?
A: 2010 and 2011 detailed topographic data was collected as part
image Source: htW: / /meted.ucar.edu
of the California Coastal
Mapping Program for the
entire California open
coast and San Francisco
Bay. The dataset
provides 1 -foot contour
resolution (a 9.25 -cm RSME vertical and 12.26 -cm RSME horizontal
specification and 1 -meter post
spacing). The data set extends to either the 10 -meter elevation contour
or 500 meters inland from the shoreline, whichever provides greater
coverage.
June 2014 www.r9coastal.org Page 8 of 9
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SAN FRANCISCO BAY AREA COASTAL STUDY
Frequently Asked Questions (FAQ) ;f FEMA
Bathymetric data was compiled from all available data from the National Oceanic and Atmospheric
Administration/National Ocean Service, including the 2005 USGS South Bay hydrographic data, and the most
recent USACE bathymetric dredging surveys. These topographic and bathymetric datasets will provide a
single, comprehensive, and consistent terrain elevation model for use in the coastal analyses and mapping
within San Francisco Bay.
Q: What will be used as the base layer for the FEMA maps?
A: FEMA mapping standards allow for both vector -based maps (street centerlines only) and raster -based maps
(aerial photographs). The most appropriate choice is selected in coordination with the community.
Q: Where can I get the data?
A: Coastal mapping LiDAR is available from the California Coastal Mapping Program at:
httn: / /www opc ca gov /2012/03/ coastal- mappinsz-lidar -data- available/
Q: What is wave runup, overtopping, overland wave propagation and stillwater elevation (SWEL)?
A: Wave runup is the dominant wave hazard along shore lines with steep terrain, including natural features such
as coastal bluffs and dunes, and engineered structures such as revetments, levees and flood walls. Overtopping
occurs when a wave crest exceeds the height of terrain or engineered structures.
Overland wave propagation occurs when elevated water levels often referred to as the stillwater elevation
(SWEL), inundate normally dry land and waves are able to propagate farther inland. Overland wave
propagation is estimated for areas where the topography is beaches, marshes, and inland developed areas.
For additional information, refer to the Coastal Beat article titled `Translating Coastal Flood Hazard Modeling
Results into Floodplain Manning.'
Q: What are surge, seas, and swell?
A: Two common wave types that impact the Bay are seas and swell. Seas are generated by local winds and are
often characterized by unorganized, choppy waves that quickly subside with calming of the breeze. They are a
common occurrence in the Bay and are an important local hazard to consider throughout the region.
Storms occurring over the open ocean can generate large waves that propagate away from the storm, traveling
for thousands of miles across ocean basins until impacting land. Large swells generated in the Pacific pass
through the Golden Gate, impacting some areas of the Bay. The largest swells occur in the Central Bay,
between the Bay Bridge and the Richmond -San Rafael Bridge.
Although not a wave, another hazard caused by wind is surge. As winds from large, local storm events push
across the water, they cause the water to pile up along shorelines. These elevated water levels exacerbate
flooding and erosion hazards associated with the storm. The largest surges occur along the South Bay shoreline
where the depths are shallower and more influenced by changes in water levels.
Stay Informed
Q: How can I stay informed?
A: Sign up for the quarterly e- bulletin, Coastal Beat, that includes schedule updates, technical articles, and
information to assist local officials better understand the coastal mapping process. Visit www.r9coastal.org for
schedules updates on a county -by- county basis.
June 2014 www.r9coastal.org Page 9 of 9
100 -Year Flood —Its All About Chance
Haven't we already had one this century?
A flood is any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream. Floods
occur for many reasons, such as long- lasting rainfall over a broad area, locally intense thunderstorm - generated rainfall,
or rapid melting of a large snow pack with or without accompanying rainfall. Because floods result from many different
circumstances, not all floods are equal in magnitude, duration, or effect. Placing floods in context allows society to
address such issues as the risk to life and property, and to study and understand the environmental benefits of floods.
Trying to place contextual framework around floods is where such terms as "100 -year flood" came into being.
In the 1960's, the United States government decided to use the
I- percent annual exceedance probability (AEP) flood as the basis
for the National Flood Insurance Program. The 1- percent AEP
flood was thought to be a fair balance between protecting the
public and overly stringent regulation. Because the 1- percent AEP
flood has a 1 in 100 chance of being equaled or exceeded in any
I year, and it has an average recurrence interval of 100 years, it
often is referred to as the "100 -year flood ".
Scientists and engineers frequently use statistical probability
(chance) to put a context to floods and their occurrence. If the
probability of a particular flood magnitude being equaled or
exceeded is known, then risk can be assessed. To determine these
probabilities all the annual peak streamflow values measured at a streamgage are examined.
on a river where the height of the water and the quantity of flow (streamflow) are recorded.
(USGS) operates more than 7,500 streamgages nationwide
(see map) that allow for assessment of the probability of
floods. Examining all the annual peak streamflow values that
occurred at a streamgage with time allows us to estimate the
AEP for various flood magnitudes. For example, we can say
there is a 1 in 100 chance that next year's flood will equal or
exceed the 1- percent AEP flood.
More recently, people talk about larger floods, such as
the "500 -year flood," as tolerance for risk is reduced and
increased protection from flooding is desired. The "500 -year
flood" corresponds to an AEP of 0.2 percent, which means a
flood of that size or greater has a 0.2- percent chance (or I in
500 chance) of occurring in a given year.
U.S. Department of the Interior
U.S. Geological Survey
A streamgage is a location
The U.S. Geological Survey
?.ITFI3I' NJ. 61
General Information Product 106
April 2010
"On the river near me, we have had two 100 -year floods in 15 years...
I really am confused about this 100 -year flood stuff... "
The "100 -year flood" is an
estimate of the long -term average
recurrence interval, which does not
mean that we really have 100 years
between each flood of greater or
equal magnitude. Floods happen
irregularly.
Consider the following: if we had
1,000 years of streamflow'data,
we would expect to see about
10 floods of equal or greater
magnitude than the "100 -year
flood." These floods would not
occur at 100 -year intervals. In one
part of the 1,000 -year record it
could be 15 or fewer years between
"100 -year floods," whereas in
other parts, it could be 150 or more
years between "100 -year floods."
U.S. Department of the Interior General Information Product 106
U.S. Geological Survey April
Incidence of the 10 -year flood for the Embarras River at Ste. Marie, IL (03345500). The variability in time
between "10 -year floods" ranges from 4 to as many as 28 years between floods.
The graph above shows how irregularly floods have occurred during the past 98 years on the Embarras River
near Ste. Marie, IL. The magnitude of the 10 -year flood has been determined through statistical analysis to be
approximately 3 1, 100 cubic feet per second (ft' /s). You can see from the graph that the actual interval between
floods greater than this magnitude ranged from 4 to 28 years, but the average of these intervals is about 10 years.
Admittedly, use of such terms as the "100 -year flood" can confuse or unintentionally mislead those unfamiliar
with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is
encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval
terminology. For example, one would discuss the "1- percent AEP flood" as opposed to the "100 -year flood."
2010
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U.S. Department of the Interior General Information Product 106
U.S. Geological Survey April
Incidence of the 10 -year flood for the Embarras River at Ste. Marie, IL (03345500). The variability in time
between "10 -year floods" ranges from 4 to as many as 28 years between floods.
The graph above shows how irregularly floods have occurred during the past 98 years on the Embarras River
near Ste. Marie, IL. The magnitude of the 10 -year flood has been determined through statistical analysis to be
approximately 3 1, 100 cubic feet per second (ft' /s). You can see from the graph that the actual interval between
floods greater than this magnitude ranged from 4 to 28 years, but the average of these intervals is about 10 years.
Admittedly, use of such terms as the "100 -year flood" can confuse or unintentionally mislead those unfamiliar
with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is
encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval
terminology. For example, one would discuss the "1- percent AEP flood" as opposed to the "100 -year flood."
2010
■
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r,I''� 1'� miff ri' it
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Incidence of the 10 -year flood for the Embarras River at Ste. Marie, IL (03345500). The variability in time
between "10 -year floods" ranges from 4 to as many as 28 years between floods.
The graph above shows how irregularly floods have occurred during the past 98 years on the Embarras River
near Ste. Marie, IL. The magnitude of the 10 -year flood has been determined through statistical analysis to be
approximately 3 1, 100 cubic feet per second (ft' /s). You can see from the graph that the actual interval between
floods greater than this magnitude ranged from 4 to 28 years, but the average of these intervals is about 10 years.
Admittedly, use of such terms as the "100 -year flood" can confuse or unintentionally mislead those unfamiliar
with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is
encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval
terminology. For example, one would discuss the "1- percent AEP flood" as opposed to the "100 -year flood."
2010
The accuracy of the 1- percent AEP flood varies depending on the amount of data available, the accuracy of those
data, land -use changes in the river drainage area, climate cycles, and how well the data fits the statistical probability
distribution. As a demonstration of the uncertainty in the estimates of flood probability, the flood probability relation
for the Big Piney River near Big Piney, MO, is plotted in the figure below as the solid black line. Above and below that
solid black line are two dashed lines that represent the 90- percent confidence intervals of this relation. These confidence
intervals simply mean that we are 90- percent confident that the true flood magnitude for a particular AEP lies between the
confidence limit lines; or, there is a 10- percent chance that the true value lies somewhere outside the confidence interval
tines. The l- percent AEP flood (" 100 -year flood ") for the Big Piney River at this location has an estimated magnitude of
44,300 cubic feet per second (ft-/s). We know that 44,300 ft' /s is an estimate, but by looking closer at the graph, we can
say that we are 90- percent confident that the true value of the 1- percent AEP flood is between 36,600 ft -/s and 56,400 ftYs.
Most policy makers and water managers often are more concerned with the height of the water in the river (river levels)
than the streamflow quantity. The uncertainty for the streamflow quantity of the 1- percent AEP flood for the Big. Piney
River can be translated into an uncertainty of the river level. A streamflow of 36,600 ft' /s corresponds to a river level
of 20.6 ft, whereas a streamflow of 56,400 ft' /s corresponds to a river level of 22.85 ft. Stated another way, the flood
probability analysis reveals that we are 90- percent sure that the river elevation will be between 20.6 and 22.85 on the Big
Piney River at Big Piney for the I- percent AEP flood.
1 00.000
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feetger second_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
84,300 cubic feet per second _
36.600 cubic feet per second
Upper band of 90- percent
confidence interval
Lower bend of 90- percent
confidence interval
98 95 90 80 70 50
Annual Exceedance Probability, Percent
Solid line through data indicates fitted frequency curve; dashed lines indicate
90- percent confidence limits for the Big Piney River near Big Piney, M0.
1 .5 .2
U.S. Department of the Interior General Information Product 106
U.S. Geological Survey April 2010
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the value pl long-term mream0ow data
Collecting more data and updating the) percent AEP (100 year) estimate provides bettermformetion for
agencies charged with managing flood prone areas and protecting life and property.
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Urban development in Champaign- Urbana, IL has increased the magnitude of
flooding of the Boneyard Creek.
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Flood control damson the Green River in Washington State have
reduced the magnitude of floods.
The 1- percent AEP flood has a 1- percent chance of occurring in any given year; however, during the span of a 30 -year
mortgage, a home in the 1- percent AEP (100 -year) floodplain has a 26- percent chance of being flooded at least once
during those 30 years! The value of 26 percent is based on probability theory that accounts for each of the 30 years having
a I- percent chance of flooding.
Current stre a inflow conditions forth a United States
are available on the World Wide Web:
waterwatch.usgs.gov
U.S. Department of the Interior
U.S. Geological Survey
Author information
Robert R. Holmes, Jr. and
Karen Dinicola
For more information contact:
Office of Surface Water
415 National Center
Reston, Virginia 29192
793 -646 -5391
General Information Product 106
April 2010