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OCR for page 117
8
Decisions for the Future
STRATEGIC DECISIONS
An accelerated rise in relative sea level would force people
who live on the coasts to face a number of important decisions.
In the past, keeping the coastal infrastructure above the historical
slow rise In sea level for the most part has been achieved through
normal maintenance or abandonment of facilities. The effects
from sea level rise scenarios adopted In this report imply that
a more considered and planned approach to the preservation or
abandonment of coastal facilities and communities is needed.
As discussed In Chapters 3-7, the types of responses to sea
level rise include retreat from the shoreline or the use of struc-
tures to prevent flooding and shoreline recession. The choice of
response strategy will depend on several factors. A clear un-
derstanding of the natural processes-underlying shoreline erosion
and knowledge of the efficacy of coastal structures are important
to both strategies. Additional site-specific considerations are the
economics involved as well as the social and environmental costs.
Limited information exists concerning these issues.
For well-developed coastal communities, with a high density
of buildings and expensive shoreline facilities such as harbors and
resorts, the strategy of choice in all likelihood will be to protect
117
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118
RESPONDING TO CHANGES IN SEA LEVEL
the existing infrastructure. For eroding shorelines that are less de-
veloped the decision becomes more difficult. Because the costs and
benefits of protection must be weighed against those of retreat-
ing from the shoreline, consideration must be given to economic,
social, environmental, and geological and geographical factors.
Economic Factors
Long-Term Costs
The initial cost to stabilize a shoreline is expensive, whether it
is through beach fill, groins, or sea walls. Continual maintenance
of the structures represents an ongoing cost to the community;
the magnitude of the cost will vary with time, depending on fu-
ture sea levels. However, if these costs are outweighed by the
benefits of maintaining the coastal infrastructure and beach, then
stabilization is the rational choice.
Stabilization in the low wave-energy environments of most
coastal bays and wetlands wiD be much less expensive than in open-
coast areas. Social ant] environmental factors must be considered
when making decisions about wetlands.
The decision to retreat may seem attractive on first consid-
eration because the construction cost for engineering structures
is zero. However, retreat involves foregoing the use of land and
perhaps buildings, which represents a large short- to medium-term
loss, which knight be outweighted by the long-term cost-saving of
doing nothing in an engineering context.
Design Life Versus Remedial Measures
~ planning for sea level rise, it Is necessary to consider whether
a stabilization project needs to be made at the outset, or whether
remedial measures can be taken periodically during the life of the
structure or facility. Two examples illustrate the difference. A
highway is to be built across low-lying land to an estuary; its
useful life might be on the order of 60 years. For every foot of
increased elevation (to allow for the sea level rise) the cost may
increase by millions of dollars. On the other hand, with no absolute
assurances of the rate of future sea level rise, and since repaving
and raising the row to counteract the expected local subsidence
will be undertaken anyway, the logical decision is probably to deal
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DECISIONS FOR THE FUTURE
119
with sea level as part of ongoing maintenance rather than building
for it initially.
A second example concerns the construction of a beach-front
park and pavilion with expensive shops. A rise in sea level of 1
It may jeopardize the entire investment, which may also have a
Midyear design life. ~ this case, the prudent design might include
an extra foot of elevation and a horizontal setback of X feet to
safeguard against expensive reconstruction.
Social Factors
Degree of Risk from Sea Level Rise
Along with other factors, the consequences of damage from a
sea level rise in relation to the degree of risk should be evaluated.
If risk to human life is a factor, as in the case of a beach-front
hotel, then a higher factor of safety is justified, requiring planning
for sea level rise in the initial design.
An example of a structure with a high risk to life would be a
levee protecting a major population center. The consequences of
levee overtopping, failure, and flooding to a large urban area would
require careful and thorough analysis and a conservative allowance
for sea level rise, including the wave action accompanying major
storms.
Maintenance Capability
Waterfront facilities are owned by many individuals and agen-
cies who have varying attitudes and capabilities with regard to
funding, monitoring, and maintaining their properties. Facilities
that are likely to remain unattended for long periods, or those
whose owners are unable to modify or maintain them, may need
to be built with a conservative allowance for sea level rise. In
this category would be beach-front hotels, graving docks, and
high-level bridges. On the other hand, owners of levees, roadways,
breakwaters, and similar facilities are likely to monitor their struc-
tures, and are In a position to adjust them to withstand the erects
associated with rising sea levels.
Further, once the decision is made to stabilize, it becomes
more clifficult in the future to change the decision. With the
retreat option or a decision to do nothing, the cost to reverse the
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RESPONDING TO CHANGES IN SEA LEVEL
decision if the economucs become favorable for stabilization Is far
less.
ENVIRON1~:NTAI FACTORS
The environmental effects of coastal structures or retreat from
the shoreline vary with the site. It is clear that the use of coastal
structures may safeguard breeding areas or specialized habitats,
or result in their loss. The same is true of the retreat alternative.
For coastal wetlands located in relatively protected regions,
the costs of stabilizing shoreline positions would probably not be
high, due to the less energetic wave cInnate In bays and lagoons.
If there is a retreat along bay shorelines, or a ban on shoreline
stabilization, marshlands will be allowed to retreat with the sea
level rise.
Geological/Geographical Factors
The location of the site of interest is extremely important in
the decision-making process. High-energy shorelines are more ex-
pensive to maintain than low-energy shorelines. Coastlines with
an abundance of sand in the littoral stream are more easily main-
tained than those that exist on shorelines with little or no sand
supply.
Local factors can be the predominant cause of shoreline reces-
sion, overwhelming present day erosion rates attributable to sea
leered rise. For example, tidal inlets trap sand from the littoral
transport, and the beaches on the downdrift side of mIets are of-
ten in a state of erosion. The most effective solution for affected
communities is to solve first the more massive and immediate local
erosion problem, by bypassing sand around the inlet, and then to
address the long-term sea level rise problem.
The concept of sand rights should be involved in the decision-
making process. The impacts of the actions of one community
on neighboring communities that share the same sand supply sys-
tem must be evaluated. For example, if eroding bluffs along the
shoreline are known to be the source of sand for downdrift commu-
nities, then decisions to stabilize the bluffs will have far-reaching
consequences. In general, sources of sand should be protected
to allow the sand to move in the littoral system, while sinks of
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DECISIONS FOR THE FUTURE
121
sand are locations where stabilization would be extremely effec-
tive. If structural means are employed that reduce the natural
sand supply, consideration should be given to requiring mitigation
measures, such as supplying sand from an alternative source. The
ideal location for communities that need to stabilize the beaches
would be at the terminus of the sand supply for a given coastal
reach.
NATIONAL POLICY DECISIONS
Responding to changing sea level will take on greater urgency
and relevance in the future as several apparently irreversible trends
combine to render the coastal zone more hazardous as a site for
major investment. These trends include
~ increasing human settlement, services, and installation in
coastal areas susceptible to inundation, erosion, or destruction due
to increases in relative sea level, storm surges, and tidal crescendos;
~ land subsidence, which affects the U.S. East Coast from
Long Island to Georgia, the Gulf Coast from Texas to I`ou~siana,
and local areas on the Pacific Coast; and
. the steady increase of atmospheric greenhouse gases, which
is theorized to enhance global warming, thus contributing to glacial
melting and eustatic sea level rise.
Coping with these trends will require
0 research, data acquisition, and analysis of the specific ef-
fects of sea level rise in relation to other environmental changes
and the response of specific coastal works to rise; and
_ ~ ~ · ~ · ~ . · ~ ~ · ~ ——
.
curlew ana cooramatlon ot national and regional policy
concerning the coastline of the United States and its dependencies,
especially to take account of new knowledge.
Prediction of climatic, oceanographic, and geologic processes
that are potentially hazardous to coastal structures within 10 to
50 years and the ability to warn the public of hazards are also
needed. The expertise for developing mechanisms to cope with
long-term sea level rise is available from specialists in these areas:
coastal surveying, monitoring, and preservation; coastal and har-
bor engineering; tidal measurement and prediction; meteorology;
climatology; geodesy; geology, oceanography; ecology; and coastal
management.
Representative terms from entire chapter:
level rise
