National Academies Press: OpenBook

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

Chapter: 3 Relative Sea Level Rise Effects and Responses

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Suggested Citation:"3 Relative Sea Level Rise Effects and Responses." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
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Suggested Citation:"3 Relative Sea Level Rise Effects and Responses." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
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Page 32
Suggested Citation:"3 Relative Sea Level Rise Effects and Responses." National Research Council. 1987. Responding to Changes in Sea Level: Engineering Implications. Washington, DC: The National Academies Press. doi: 10.17226/1006.
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Page 33

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3 Relative Sea Level Rise Effects and Responses As sea level rises, the shoreline may respond by flooding or erosion. The magnitude of these effects wiD depend on a number of factors, including whether the shoreline is on the open ocean or in a protected bay, the effect of any stabilizing structures, and population density and activities. The related shoreline changes will have economic and envi- ronment~ impacts that will require decisions to define a proper response. For example, for sandy shorelines, the possible reme- dies to erosion range from shoreline stabilization (through beach restoration or protective structures) to retreat from the shoreline. Each is technically feasible, but the appropriate response will usu- ally be dictated by ~ combination of economic and environmental concerns. ~ the case of retreat, planning and timing are important ele- ments of the process. Given that a definite proper choice exists on a site-by-site basis, it follows that an inappropriate choice could be very expensive. For example, continued shoreline stabilization of an area of high erosional tendency and low economic base could be too costly. On the other hand, if the natural system is only slightly out of balance and a large tax base exists, a decision to retreat could be inappropriate and unduly costly. This latter con- dition would certainly hold for port cities, which have considerable investments in infrastructure. 3

32 RESPONDING TO CHANGES IN SEA LEVEL Thus, there is a need for development and application of methodologies for estimating the expected erosion impact and attendant storm damage. With such a framework available and the conveyance to the public of plans for responding to a major problem (rebuilding or retreating), a much more rational response could be developed for implementation. This would clearly be of value to those responsible for allocating future financial resources and directing responses in times of high emotions, losses of per- sonal property, and possible losses of life. The decision to retreat or rebuild following a major storm would be a matter of the gravest concern. Many situations can be envisioned in which the temptation would be to make the wrong choice. For example, suppose that a storm of 50~year-return frequency were to strike an area, causing widespread damage and major change. Without the knowledge that this was indeed a very rare storm, the enormity of the damage and emotional impact could cause decision makers to be swayed toward retreat. On the other hand, if a relatively frequent storm (say, with a return period of 40 years) causes major damage due to the relatively high vulnerability of an area, retreat may be the appropriate response. The development of a rational decision-making framework re- quires an understandin g of both complex physical coastal processes and the economics of the area. Chapters 4 - of this report review some of the necessary knowledge of coastal processes and the in- fluence of sea level rise. Historically, U.S. efforts to cope with relative sea level rise have been limited in scope and modest compared to those in some European countries. Until 1929, when the U.S. Army Chief of Engineers appointed the Board on Sand Movement and Beach Erosion to carry out field studies of shore processes, there was no organized effort to study the engineering problems of ocean shores. The board was appointed to study the processes at work, not to solve particular erosion problems. The U.S. Army Corps of Engineers had responsibility for improving and maintaining navigation works of all kinds, including the tidal entrances to coastal harbors. In 1930, the U.S. Congress authorized a successor and permanent organization, the Beach Erosion Board, now the Coastal Engineering Research Center. From 1929 to the beginning of World War Il. the Beach Ero- sion Board and a few universities worked on coastal processes and engineering problems. Support was limited and progress was slow. Beginning about 1940, and continuing after the war, amphibious

RELATIVE SEA LEVEL RISE EFFECTS AND RESPONSES 33 operations, offshore petroleum production, coastal siting of electric power stations, the growth of coastal communities, environmental and ecological interest, and other developments caused a tremen- dous growth in the support for coastal studies and projects and university prograrrm. The number of professional coastal experts has increased proportionately with the growth of these programs. The engineering of works on the ocean coasts deals prunarily with the characteristics and effects of wind-generated, ocean sur- face waves. Expertise in this area is the essential qualification of the coastal engineer. Because of this, categorization of engineering problems should be made on the basis of "waves" or "no waves, and this differentiation should extend to the effects of a sea level rise; those involving wave action differ fundamentally from those that do not. Before discussing the effect of rising sea level on specific types of coastal projects, there are some principles that qualified coastal engineers generally accept as valid: 1. Structures are expensive and the ocean is a relentless ad- versary. A development ~set-back" line may be the only action justified for undeveloped or lightly developed shores. 2. Sandy coastal shores are made of natural units, such as the length between inlets or a beach terminated by headlands, and must be treated as such. The effect of any structure anywhere in one of these units on the remainder of the shoreline must be analyzed before construction, and the plan should provide for mitigation of adverse effects, if any. 3. Sand tends to collect in sheltered areas around coastal structures such as groins or jetties. Plans should provide for pre- dict~ng the location and capacity of such areas and for filling them initially with sand from other than the active shoreline, so as not to deplete other portions of the sand system. 4. The direction and magnitude of littoral transport is the most uncertain feature in the plans for a coastal project. Site- specific data are difficult and expensive to obtain, but it is ex- tremely risky to proceed without them. 5. The choice of coastal structure for erosion mitigation will depend on site-specific factors. Structures that work satisfactorily in one location may prove to be totally inadequate (and perhaps detrimental) in another application.

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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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