Green infrastructure for the global warming era
THE HORIZONTAL LEVEE
NATURE’S LOW-COST DEFENSE AGAINST SEA LEVEL RISE
Rising Tides in San Francisco Bay 1
A Closer Look: Sea Level Rise and Shoreline Flooding 4
San Francisco Bay’s Natural Flood Defense 6
A Closer Look: Tidal Marsh—The Bay’s Front-Line
Defense Against Sea Level Rise
The Horizontal Levee 10
Conclusions and Recommendations 12
RISING TIDES IN SAN FRANCISCO BAY
Beneath the Golden Gate Bridge the oldest tide As in New Orleans and other heavily urbanized
gauge on the west coast recorded a rise in seal level coastal cities, tens of billions of dollars have been
of almost eight inches between 1900 and 2000. The invested in development, both commercial and
increase has caused recent storms to inflict greater flood residential, along the shoreline of San Francisco Bay.
and erosion damage on low-lying coastal communities. These developments are threatened by increasingly
Current forecasts project that mean sea level will rise fierce storms as sea level continues to rise.
another 14 inches by 2050. By 2100, scientists estimate San Francisco Bay possesses a very large inventory
an increase of 55 inches. Some amount of sea level of restorable marshes that could help to significantly
rise is inevitable, and major public facilities such as reduce the damage caused by future storms. Wetland
highways, railroads and airports are threatened with restoration efforts have been underway in San
flooding. Francisco Bay for over thirty years. Starting with
Scientists have known for decades that coastal small, experimental projects in the late 1970s, they
wetlands protect oceanfront cities from the powerful have evolved into a region-wide program with a goal
destructive forces of storms. Hurricane Katrina to restore over 100,000 acres of bay marshes. However,
slammed into New Orleans with much greater force that program has only lately come to incorporate sea
because of the massive loss of Louisiana’s coastal level rise projections into restoration design. Scientists
wetlands. Now even politicians are calling for now recognize that, toward the end of the century,
accelerated restoration of Gulf Coast wetlands to help many of the Bay’s restored wetlands will be at risk of
buffer the impact of future storms. being drowned by rising seas.
37,000 acres of tidal marshes were destroyed to create solar salt evaporation ponds. 3,000 years old tidal marsh channels survive in salt ponds.
The Lost Marshlands of San Francisco Bay spaces that lie between the open waters of the bay and the developed shoreline.
Scientists refer to these diked tidal marshes as the San Francisco “baylands.”
One hundred ninety-six thousand acres of tidal marshlands thrived in San The San Francisco baylands have subsided relative to sea level as a result of
Francisco Bay’s shallows before 1850. Along with serving as nursery grounds having been disconnected from the tidal waters of the bay. Though the original
for estuary fisheries, the marshlands functioned as barrier islands that protected marsh plains once existed at an elevation well above mean sea level, their surface
the shore from erosive storm surges. During the last century and a half, 180,000 elevation has subsided to roughly five to ten feet below sea level. An extensive
acres of these marshes were filled, diked or drained. Some of the drained tidal network of earthen levees prevents bay tidal waters from inundating these sunken
wetlands were intensively developed, such as San Francisco’s Financial and baylands. The levees are in relatively poor condition in most locations, though
Marina districts, Foster City, and San Francisco International Airport, but most some levees that protect more intensively developed areas are maintained to a
of these diked wetlands were converted to solar salt evaporation ponds and higher standard.
agriculture lands. They remain today as salt ponds, hay farms and other open
Historic and existing baylands habitats throughout San Francisco Bay. Most tidal marshes in the North Bay were converted to agricultural fields, while marshes in the South
Bay were largely converted to commercial salt ponds and other industrial uses. Of the original 196,000-acre tidal marsh complex, 180,000 acres were destroyed. From the
Baylands Habitat Goals Project 1999.
A Closer Look
San Francisco Bay
Conservation and Development Commission San Francisco Bay Area
Area potentially exposed to an approximate Shoreline Areas
16-inch sea level rise
San Francisco Bay
Potentially Exposed San
Sea Level Rise and
Conservation and Development Commission
Area potentially exposed to an
55-inch sea level rise To Sea Level Rise
Area potentially exposed to an approximate
16-inch sea level rise
Area potentially exposed to an approximate
POTENTIALLY EXPOSED do not account for shoreline protection orlevel rise These maps
DISCLAIMER: The inundation data used in these maps
are for informational purposes only. Users agree to hold harmless and blameless the State of California and its representatives
SEA LEVEL RISE
TOhazards,insurance requirements, or property values or be used in lieu DISCLAIMER: The Rate Maps data used in these maps do not account for shore
and its agents for any liability associated with the use of the maps. The maps and data shall not be used to assess actual
coastal of Flood Insurance inundation issued by the
Federal Emergency Management Agency (FEMA). are for informational purposes only. Users agree to hold harmless and blameless
and its agents for any liability associated with the use of the maps. The maps and
coastal hazards,insurance requirements, or property values or be used in lieu of F
Federal Emergency Management Agency (FEMA).
San Francisco Bay’s existing system of shoreline flood protection structures
is haphazardly designed and maintained. Levees are overtopped regularly in key
locations, resulting in flooding of vital public facilities, especially heavily used
roads and highways. The flood management system has relied for decades on an Vallejo
aging network of earthen dikes that are continually sinking into soft bay mud. Bay Vallejo
Rising sea level is making the network obsolete. San Pablo
Martinez Pittsburg Bay
Sea level rose in San Francisco Bay by more than seven inches between 1900 Rafael
and 2000 as a result of global warming. The California Ocean Protection Council Richmond Rafael
estimates that sea level will rise an additional 14 inches by 2050 and up to 55
inches by 2100. The greatest threat to the developed shoreline in the near term is
not posed by flooding caused by increased mean sea levels on calm days, but by Oakland
flooding caused by increasingly frequent storms that occur in combination with
higher tides. San
This study examines strategies to use San Francisco Bay’s recovering tidal Pacific
marsh ecosystem as an opportunity to help shoreline communities manage the San Francisco
impacts of sea level rise. Specifically, it considers the flood protection functions Ocean San Francisco
that tidal marshes perform naturally and evaluates the merits of integrating
marshes into a multi-purpose shoreline management regime to reduce flood Fremont
risk. It examines the current functions of Bay tidal marshes as well as of current
flood risk management practices. It considers how environmental conditions NORTH
are likely to change in the era of global warming and how to best adapt flood
risk management practices in response to those changes. The study’s intended 0 2 4 8 MILES
audience is planners, politicians, regulators and other stakeholders with the 0 2 4 8 MILES
Sea level rise data provided by:
authority to make or affect decisions that influence the configuration and use of Potential inundation with 16 and 55 inches of sea level rise. (From San Francisco Bay
SOURCE: Knowles, N. 2008. Siegel, S.W. and P. A. M. Bachand, 2002.
the San Francisco Bay shoreline. Conservation and Development Commission.)
SOURCE: Knowles, N. 2008. Siegel, S.W. and P. A. M. Bachand, 2002.
A Change in the Weather
Storm surges occurring atop higher sea levels already are causing increased
flooding within the San Francisco baylands, as well as within low-lying developed
areas near the bay. Major roads along the shoreline are regularly flooded during
winter storms and high tides. Residential and commercial areas within bay area
cities similarly are experiencing increased flooding. The aging network of bayland
levees is increasingly overburdened and will prove thoroughly inadequate as sea
level continues to rise during the coming decades.
The National Academy of Sciences predicts that, “the incidence of extreme
high water events (1.4 m above historical mean sea level) in the San Francisco
Bay area will increase substantially with sea level rise, from less than 10 hours per
decade today to a few hundred hours per decade by 2050 and to several thousand
hours per decade by 2100.” (NAS, 2012.) The NAS notes the important role that
coastal wetlands can play in reducing the destructive force of these storms.
Adapting to Higher Tides
Rising sea levels also threaten to submerge large areas of tidal marshes,
including many areas that have been restored over the past 35 years. These
projects are part of a regional marsh restoration initiative that has quietly grown
into the nation’s largest coastal wetland restoration program. The purpose of the
program is to reverse the historic trend of wetland destruction in order to recover
the significant benefits provided by tidal marshes and associated wetlands. Those
benefits include providing habitat for numerous wildlife species and performing
vital flood protection functions.
The decades ahead offer a window of opportunity to restore San Francisco Bay
tidal wetlands. Restored marshes would not only provide greatly expanded areas
of habitat for declining wildlife populations, but would also provide tangible flood
protection benefits, buying time to develop long-term solutions to the problem
of sea level rise. A restored tidal wetland buffer would reduce the frequency and
magnitude of shoreline flooding, and thereby reduce the significant costs of Valuable development built atop Bay tidal marshes is at risk. Redwood City shoreline.
defending and rebuilding valuable development.
SAN FRANCISCO BAY’S
NATURAL FLOOD DEFENSE
The Bay Institute’s analysis determined that tidal marshes
can significantly reduce the destructive power of storm surge.
This finding suggests that shoreline flood protection is improved
significantly when areas of tidal marsh exist between the
developed shoreline and the open waters of the bay. Further, it
indicates that by using tidal marsh in combination with a levee
constructed at the landward edge of the marsh, the size of the
levee can be reduced significantly while still providing the same
level of flood protection as would be provided by a larger levee
that was not fronted by tidal marsh.
The Bay Institute’s study describes and evaluates the costs
and benefits of employing marsh restoration as an adaptation
strategy to rising sea levels in San Francisco Bay. Although the
study’s particulars pertain to San Francisco Bay, they can be
extrapolated to many similar coastal areas around the nation
and the globe. From San Francisco Bay to the Gulf Coast, from
Holland to the Rhone River Delta in France, and from Tokyo
to London, hundreds of coastal urban centers are beginning
to lament having destroyed valuable tidal marshes, and are
considering ways to recapture the substantial free benefits that Surviving ancient tidal marsh near Petaluma.
Reduction in Wave Height Over Marsh for Different Water Levels
1% Annual Chance
50% Annual Chance
Relative to Offshore Wave Height
Wave Height Over Marsh
0 200 400 600 800 1000
Width of Marsh, ft
Our analysis concluded that tidal marsh can reduce wave
Tidal Marshes Reduce Shoreline Flooding energy in extreme storm events by over 50%, and that a hybrid flood
The Bay Institute examined two strategies to prevent or reduce protection system comprising a landward levee and an adjacent
the impact of shoreline flooding in San Francisco Bay caused by tidal marsh provides an equivalent level of flood protection to
sea level rise. We compared the traditional approach that relies on that of a much larger landward levee alone. Further, the cost of
construction of engineered earthen dikes to a hybrid approach that the hybrid system is almost half that of the traditional levee alone.
combines tidal marsh restoration with construction of dikes. First, These results suggest that it would be far more cost effective to
we analyzed the capacity of tidal marshes to reduce wave run-up build a hybrid flood protection system than it would to build a
and overtopping and, thereby, reduce the need to build larger dikes conventional earthen levee.
in the absence of buffering tidal marsh. Second, we compared the
costs of the two approaches.
Tidal Marshes Save Levee Cost/Mile (In Millions) Over 50 Years
Hundreds of Millions of Dollars $14M
Sea level rise caused by global warming is already inflicting $12.5M
damage to developed areas along the San Francisco Bay shoreline. $12M Tidal Marsh
That damage and its associated costs will increase as sea level rise
accelerates. The current flood management system is inadequate $10M
to maintain sufficient levels of protection. Important public
Cost per Mile
infrastructure, including highways, bridges, roads, rail lines, utilities $8M
and airports, will experience increased damage from flooding in $6.29M $6.69M
the coming decades. In the near term, between now and the second $6M
half of the 21st century, the bulk of that damage will be inflicted by
storms arriving on higher tides. $4M
The conventional and, until now, presumed least costly approach
to addressing flood risk has been to increase the height and bulk of $2M
levees. (Other more costly alternatives include construction of sea
walls and tidal barriers.) Although it has been recognized for many 0
Traditional Levee Tidal Marsh
years that tidal marshes and associated wetlands provide tangible Earthen Levee
flood protection benefits, those benefits generally have been Earthen Levee
dismissed during planning and construction of flood protection Shoreline Type
networks. restoration can be used as an effective flood protection method that
An effort is currently underway across the entire San Francisco is far more cost effective than traditional approaches. Second, a new
Bay to restore vast tracts of tidal marshes and associated wetlands. marsh restoration paradigm can facilitate marsh survival during
However, design of these restoration projects has generally not the current era of sea level rise, thereby protecting important marsh
incorporated provisions for long-term sea level rise. In order to functions.
fully realize the benefits of the marsh restoration program, new The study clearly finds that nature’s capital is quite tangible and
designs must be developed and implemented that can accommodate can be put to much greater benefit than is currently the case. It
increasing sea levels. further demonstrates that to continue to rely on old solutions is far
This study identifies two strategies that can be employed to more costly to society. It points the way to a powerful tool in the sea
accomplish two critical public policy objectives. First, tidal marsh level rise adaptation toolbox.
A Closer Look
The Bay’s Front-Line Defense
Against Sea Level Rise
An unofficial program of tidal marsh restoration has been underway in San
Francisco Bay for over 30 years. During this time, this program has grown to become
the largest coastal wetland restoration project in the United States. Its goal is to restore
as much of the vast tidal marsh ecosystem that existed before the California Gold
Rush as possible. Restoration advocates hope that the complex functions of the Bay’s
intertidal network of meandering sloughs and pickleweed isles can be revitalized, and
that something approaching the variety and abundance of plant and animal species
Tidal marshes provide flood protection for shoreline development.
that once thrived there will rebound and flourish.
A fundamental premise of tidal marsh restoration is that these habitats will restore Current forecasts indicate that San Francisco Bay’s original zone of intertidal
themselves naturally if proper conditions are created initially. Wetland restoration habitat—a vast area comprising almost 200,000 acres—will experience much greater
scientists have learned that natural tidal marsh restoration processes can take years or flood risk in the future, threatening large areas of essential shoreline development in
even decades to establish self-sustaining marshes. addition to wildlife habitat. Consequently, wetland managers are considering whether
Two basic presumptions—that the primary purpose of marsh restoration is to it is possible to modify current restoration strategies to accomplish two additional
recover depleted habitat for wildlife, and that the process of restoration should be objectives: enable restored marshes to keep pace with sea level rise and improve flood
allowed to happen on nature’s timescale—have been called into question by forecasts protection of developed shoreline areas.
of increased rates of sea level rise caused by global warming. Wetland scientists now This study considers whether it is possible to accomplish these two objectives
believe that these forecasts suggest that many restored and restorable marshes will be by employing a multi-purpose, integrated approach to restoring and managing San
submerged if no action is taken. Francisco Bay’s original intertidal zone.
THE HORIZONTAL LEVEE
Nature’s Low-Cost Defense Against Sea Level Rise
The Horizontal Levee is a new concept in coastal flood protection growth of the marsh plain in order to keep pace with sea level rise.
that can be applied during the current era of sea level rise. It uses Similar brackish, back-marsh networks existed historically as part
the natural flood protection benefits of coastal tidal marshes to of the shoreline wetland complex, but were destroyed to make way
reduce the destructive forces of storms. In San Francisco Bay we for development.
are well on our way to restoring the massive tidal marsh complex This new marsh restoration paradigm would use sediment
that existed here prior to European colonization. By modifying the dredged from nearby flood control channels as construction
design and accelerating implementation, the restored tidal marsh material for the brackish marsh substrate. That material currently
network can play a key role in protecting communities and essential is excavated from those channels and disposed of in landfills.
infrastructure around the Bay’s shoreline for several decades. Reclaimed wastewater from existing public treatment plants would
The horizontal levee incorporates a brackish marsh at the be used to irrigate the marsh. Water treatment plants currently
landward edge of typical tidal marsh restoration designs. The spend considerable sums to pipe, pump and discharge wastewater
brackish marsh would function as a self-maintaining levee, building at distant locations in the bay.
in elevation as plant root systems expand. It accelerates vertical
Implementation of this new approach will require that the foundation that draws from the combined expertise of multiple
current informal San Francisco Bay wetland restoration program federal, state and local agencies, as well as from nongovernmental
be integrated into a broader shoreline management program that organizations and private enterprise. Creation of an EPA geographic
incorporates flood management and water treatment infrastructure, program for San Francisco Bay will accelerate completion of the
and that it be adequately funded. The best model for this integrated new green shoreline infrastructure in response to the immediate
approach is the U. S. Environmental Protection Agency’s geographic threat of sea level rise, while saving hundreds of millions of dollars
program. The EPA geographic program is built on an interagency compared to current practices.
The Horizontal Levee Tida
• The greatest threat to developed areas along the shoreline of San Francisco Bay prior to the latter part of the century is from flooding
caused by storms occurring during periods of high tides, not from elevated sea levels alone.
• Prior to the latter half of the century it is possible to adapt to sea level rise and protect existing land uses by employing strategic
modifications of shoreline management systems.
• Tidal marshes can provide significant flood protection benefits by reducing wave energy during storms.
• Flood protection costs could be reduced by almost 50% by integrating marsh restoration into a new multi-purpose system.
• A “Horizontal Levee,” a hybrid tidal marsh-flood protection system, can be constructed to keep pace with sea level rise for several
decades in critical locations if construction begins immediately.
• If construction of the Horizontal Levee system is delayed for too long, it will be unable to keep pace with expected sea level increases
and will fail to provide the desired flood protection and habitat benefits.
• The Horizontal Levee should be adopted regionally as a key element in a cost-effective, multi-benefit shoreline management strategy.
• Public agencies should establish partnerships to accelerate design and implementation of the regional Horizontal Levee green
infrastructure. Flood management districts, water and sanitation agencies, and wildlife agencies should be core, managing partners.
• Congress should establish a San Francisco Bay Geographic Program within the Environmental Protection Agency and authorize
funding of at least $1 billion for the purpose of coordinating and implementing a regional Horizontal Levee adaptation program. The
Program should also investigate other cost-effective green infrastructure strategies that hold promise to adapt to sea level rise and
other climate change impacts.
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