National Academies Press: OpenBook

Responding to Changes in Sea Level: Engineering Implications (1987)

Chapter: 8 Decisions for the Future

« Previous: 7 Assessment of Response Strategies for Specific Facilities and Systems
Suggested Citation:"8 Decisions for the Future." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
Page 117
Suggested Citation:"8 Decisions for the Future." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
Page 118
Suggested Citation:"8 Decisions for the Future." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
Page 119
Suggested Citation:"8 Decisions for the Future." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
Page 120
Suggested Citation:"8 Decisions for the Future." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
Page 121

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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

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

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

120 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

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.

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Over the last 100 years, sea level has risen approximately 12 centimeters and is expected to continue rising at an even faster rate. This situation has serious implications for human activity along our coasts. In this book, geological and coastal engineering experts examine recent sea level trends and project changes over the next 100 years, anticipating shoreline response to changing sea level and the consequences for coastal development and uses. Scenarios for future sea level rise and several case studies are presented.


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