Coastal Flood Hazards
in San Francisco Bay
A Detailed Look at Variable Local Flood
Responses
Krista Conner, Michael Baker Jr., Inc.
Kathy Schaefer, FEMA Region IX
May 19, 2011
Overview
 Key Terminology
 Scope of Study
 Special Study
Considerations
 Overland Coastal Flood
Hazard Analysis
 Next Steps
 Conclusions
Source: Aslak Raanes
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Key Terminology
 Seas: Local wind-generated waves
 Swell: Long period waves originating from Pacific Ocean storms that
enter SF Bay through Golden Gate
Swell
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Key Terminology Continued
 Overland Wave Hazard Analyses: Wave Propagation (WHAFIS), Runup,
Overtopping
 Extreme Value Analysis (EVA): Statistical analysis to obtain
1%-annual-chance values
 Event-based study approach
• A 1%-annual-chance event defined to calculate 1%-annual-chance flood
• Typically, forcing from single source (e.g., hurricane)
• Typically, elevated water levels and wave conditions coincident
 Response-based study approach
• Many events evaluated for resulting flood “response” (overland waves and
runup/overtopping)
• Forcing from various sources (e.g., seas and swell)
• Elevated water levels and wave conditions NOT coincident
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Scope of Study
 Study Team Members
• FEMA Region IX
• FSG (Nolte and DHI)
• BakerAECOM
• NHC
 Study Area
 Scope
•
Regional MIKE 21 hydrodynamic and wave
modeling
• Extreme Value Analysis
• Overland Coastal Flood Hazard Analyses
• Mapping
From Regional Coastal Hazard Modeling Study for North and Central
San Francisco Bay (DHI, 2010)
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Special Study Considerations
 Sheltered water body
• Complex geometry
Vertical
Wall
• Variable shoreline characteristics
• Dense development and population
Natural
Shoreline
 Seas and swell waves
Revetment
• Born of different forcing
• Temporal variability
• Spatial variability
• Directional variability
 Non-coincident peaks in water levels and waves
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Overland Coastal Hazard Analyses
 Data sources
 Extreme Value Analysis year
definition
 Runup and overtopping
 Overland wave propagation
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Data Sources
 Topography and Bathymetry
• Same as DHI study
• FEMA LiDAR (Collected by Merrick
2004)
• USACE 2003 Dredging Surveys
• NOAA NOS GEODAS Bathymetric
Survey
 Wind Speeds
• OAK
• SFO
• Travis Air Force Base
From Regional Coastal Hazard Modeling Study for North and Central San Francisco Bay (DHI, 2010)
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Data Sources Continued
 Water Level and Wave Starting
Conditions
• DHI hydrodynamic and wave
study results
• 31-year time series extracted at
points along the coast
 Land Use Information
• Field reconnaissance
• Aerial imagery
From Regional Coastal Hazard Modeling Study for North and Central San Francisco Bay (DHI, 2010)
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Extreme Value Analysis
 Seasonality in swell waves
 Calendar year does not
ensure event independence
 “Storm year” definition:
July 1- June 30
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Runup and Overtopping
 Runup Methods
• DIM—natural coast with slope < 1/8
• TAW—natural coast with slope ≥ 1/8; revetments
• SPM—vertical walls
 Overtopping Method: Cox-Machemehl
Overtopping
Wave Runup
Total Stillwater
Wave Setup
Surge
Normal Bay-Level Shoreline
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Runup and Overtopping
 Response-Based Approach
 Runup calculated each hourly time step for 31-year time series
 Waves filtered for “onshore” directionality
 Runup from seas, swell, and seas+swell evaluated
 1% total water level (TWL) calculated by EVA of TWL annual
maxima
bluff or coastal
structure
limit of swash
Runup Height
Total
Runup
Wave Setup
Surge
0ft, NAVD
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TWL
Runup and Overtopping:
Overtopping
 Overtopping evaluated for single 1%-annual-chance “event”
 C-M Method required input parameters:
• TWL
Extended slope
• Dynamic water level (DWL)
Potential runup
• Wave period
 Event defined as:
TWL
Overtopping
bore
Dynamic water level
• 1%-annual-chance TWL
• Maximum DWL of 31 TWL
annual maxima
• Average wave period from
TWL annual maxima
Overtopping
Depth
Overtopping
Limit
Static + dynamic setup
SWEL
α
0ft,
NAVD
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Overland Wave Propagation
 WHAFIS 4 Model
 Results from WHAFIS model used in flood hazard mapping:
• Inland extent of inundation—SFHA boundary
• Wave heights along transect—flood zone designations (VE and AE Zones)
• Wave crest elevations along transect—BFEs
Zone VE
Zone AE
Zone X
SFHA
Boundary
Wave Crest
Profile
SWEL
H = 3 ft
Normal Bay Level
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Overland Wave Propagation cont.
 Dual-event approach
• Scenario 1: 1%-annual-chance water level w/appropriate wave
• Scenario 2: 1%-annual-chance wave height w/appropriate water level
 Scenarios constrained by 1%-annual-chance wave crest elevation
 Seas and swell modeled separately
Scenario 1
Scenario 2
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Overland Wave Propagation cont.
 Wind speeds
 Marsh Grass
•
• Scenario 1: maximum wind
Non-default vegetation types
• Parameters defined from field survey
• Spatial extent based on field survey and
elevation (Goals Project, 2000)*
associated with 31 SWEL annual
maxima events
• Scenario 2: maximum wind
associated with 31 wave height
annual maxima events
• Only onshore vector component of
Marsh
Habitat
Tidal
Range
Vegetation
Type
Salt Marsh Low
MSLMHW
Cord Grass
Salt Marsh Mid
MHWMHHW
Pickleweed
Salt Marsh High
>MHHW
Bulrush
wind speed used
From North San Francisco FEMA Mapping Vegetation Parameters for
WHAFIS Modeling (NHC, 2010)
*GOALS Project (2000). Baylands Ecosystem Species and Community Profiles: Life histories and environmental requirements of key plants, fish and wildlife. Prepared by the San
Francisco Bay Area Wetlands Ecosystem Goals Project. P.R. Olofson, editor. San Francisco Bay Regional Water Quality Control Board, Oakland, CA.
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Next Steps
 Overland wave hazard analyses for remaining counties
 Develop work maps
 Community meetings
 DFIRM production
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Conclusions
 Various unique challenges
• Presence of seas and swell
• Decoupled water levels and waves
• Extreme variability in shoreline characteristics
 Innovative study approaches developed to address challenges
• Combined seas and swell with directional consideration
• Definition of 1% overtopping event
• Dual-event WHAFIS approach
• Wind speed values for WHAFIS analysis
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