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Executive Summary Recently, several studies have projected increases in the rate of eustatic sea level rise, which some climatologists and oceanog- raphers believe may accelerate due to a future warming of the atmosphere associated with the "greenhouse effects produced by human-induced loading of the atmosphere with carbon dioxide and other gases. This interdisciplinary study of the engineering implications of relative mean sea level change examines recent sea level trends; projections of continuing relative change (over the next 100 years); shoreline response; consequences for engineering works and built facilities; methods for protecting structures from erosion and flood- ing, as well as adapting to shoreline retreat; and the need for new technologies for mitigation. . To provide a useful basis for sensitivity design calculations and policy decisions that must take sea level rise into account, the committee adopted three plausible variations in eustatic sea level rise to the year 2100, aD displaying a greater rate of rise in the distant future than in the next decade and all with an increased rate of rise relative to the present: 50, 100, and 150 cm. To ensure 1
2 RESPONDING TO CHANGES IN SEA LEVEL efficient planning, it is important that these projections be kept up to date (see "Recommendations herein). Over the past century, worldwide sea level has risen about 12 cm. In many places along the U.S. shoreline, subsidence exceeds the eustatic component by a factor of 2, and in Louisiana by a factor of 10. In higher latitudes, glacial rebound is much greater than the eustatic component of sea level rise and, in locations like Hudson Bay, Canada, has resulted in a relative Towering of sea level in excess of 130 cm/century. These substantial differences must be considered in developing responses to a relative Change. Early In this study the committee questioned whether sea level change trends based on tide gauges located inside bays and estuaries are representative of open-coast trends. A special study was commissioned by the Marine Board to address this question (Mehta and Philip, 1986~. The study concluded that (1) gauges located either inside or outside bays are subject to different influences that tend to degrade the quality of the data; (2) with more distant locations inside the bay, gauges contain a greater quantity of "noises that is not representative of the outside mean sea level; and (3) in the short term (over several decades), inside gauges will underestimate rise taking place on the open coast. Certainly, the number of long- term, open-coast tide gauges should be increased with a special emphasis on the Southern Hemisphere, where present coverage is poor. Over the next 25 years, the highest rate of sea level rise recommended for consideration in this report would produce a 1~ cm rise. Although this may seem a relatively small increase, there are three situations in which the effects are greatly magnified: 1. Sandy beaches on the outer coast exposed to ocean waves where natural processes may cause beaches to erode 1 m or more for a 1-cm rise in sea level (Bruun rule). 2. The wedge of saline water through estuaries and tidal rivers may advance as much as 1 km for a l(>cm rise in mean sea level. This will be of special concern for drinking water supplies and coastal ecosystems during droughts. (Louisiana loses 100 km2 (1 percent] of its wetlands for a I-cm rise.) 3. Salinity intrusion in coastal aquifers where the landward displacement of the salt- and freshwater interface is a large mul- tiplier of the sea level rise. Current problems of salinity intrusion
EXECUTIVE SUMMARY 3 into groundwater supplies will be increased with only relatively small rmes in sea level. A significant increase in sea level could cause widespread shoreline erosion and inundation. The two general response oh tions available are to: 1. stabilize the shoreline, either through beach nourishment or by new or augmented coastal armoring; or 2. retreat from the shoreline, maintaining a more-or-less equal elevation above local sea level. Whether to defend or to retreat depends on several factors includ- ing the future sea level rise rate and the cost of retreat. The former is poorly known and the latter will vary from place to place. Potential problems associated with sea level change can be categorized into two classes: those of the open coast where both water level and wave action are concerns, and those of inland tidal waters where wave action is usually much less severe. Wave action effects are so complete and potentially so devastating that they require special expertise for structural design. As a result of erosion along the open coast, structures not designed for such forces may become exposed to wave action. Design procedures for coastal structures should include a re- view of data on past water levels, including the maximum level, and should then provide some margin of safety to cover uncertain- ties. In some cases (e.g., docks), structures conservatively designed with expected lives of 50 years or less should not be significantly affected by sea level rise even if a rise Is not considered specifically in the design. Other structures, such as sea walls and hotels on the open coast, would be vulnerable to even a small rise. Where possible, considerations of sea level changes should be incorporated into coastal land-use planning. Areas designated primarily for industrial use may not be significantly affected by required coastal protection. However, designated uses contingent on the continuation of existing environmental features, such as shoreline conditions, may lirn~t shoreline response options. For example, coastal armoring of a formerly sandy beach may reduce the environmental desirability of the area. Construction of almost any conceivable protection against sea level rise can be carried out in a relatively short period of time. Therefore, if a substantial increase should occur, there will be time to implement protective measures. However, in areas where such
4 RESPONDING TO CHANGES IN SEA LEVEL protection would not be justified, a cost-effective abandonment of facilities would require decades to implement. Throughout the world, works exist that are applicable for protection against the effects of a rise In relative mean sea level. These options should be reviewed by engineers, planners, and policymal~ers. The committee concluded that the most appropriate present engineering strategy is not to adopt one particular sea level rise scenario, but instead to be aware of the probability of increas- ing sea level and to keep Al response options open. In many engineering projects, it may be desirable to carry out sensitivity calculations, using specific sea level rise scenarios. If a particu- lar structure is ill-suited for retrofitting, it will undoubtedly be appropriate to allow for an acceleration of sea level rise in the initial design. Sea ferret change during a structure's design service life should be considered along with other factors, but a change does not present such essentially new problems as to require new techniques of analysis. The committee's recommendations highlight the need for con- tinuing and increased scientific study of the rates and causes of sea level change and the development of a sound basis for fore- casting these changes. Efforts to understand coastal processes and the effects of sea level rise on engineering projects should also be expancled considerably. A concise listing of the committee's specific conclusions and recommendations follows. CONCLUSIONS AND RECOMMENDATIONS Conclusions 1. Relative mean sea level, on statistical average, is rising at the majority of tide gauge stations situated on continental coasts around the world. Relative mean sea leered is generally falling near geological plate boundaries and in formerly glaciated areas such as Alaska, Canada, Scandinavia, and Scotland. Relative mean sea level ~ not rising in limited areas of the continental United States, including portions of the Pacific Coast. 2. The contrasting signals concerning relative mean sea level behavior in different parts of the United States (and the world in general) are interpreted as due to differing rates of vertical motion
EXECUTI YE SUMMARY 5 of the land surfaces. Subsidence and glacial rebound are significant contributors to vertical land displacements. 3. Large, short-term (2-7 year) fluctuations worldwide are related to meteorological phenomena, notably shifts in the mean jet-stream path and the E! N~no-Southern Oscillation mechanisms, which lead to atmospheric pressure anomalies and temperature changes that may cause rise or fall of mean sea level by 15-30 cm over a few years. 4. Studies of a very small number of tide gauge records dating more than 100 years (the oldest being Amsterdam, started In 1682) show that after removal of the subsidence factor where known, mean sea level has been fluctuating through a range of not more than 4~150 cm fin long-term fluctuations) for at least 300 years. 5. The geological record over the last 6,000 years or so indi- cates that there has been a general, long-term rise with shor~term fluctuations probably not exceeding 200 cm during the last 1,500 years. 6. Monitoring of relative mean sea level behavior is at present inadequate for measuring the possible global result of future cli- mate warming due to rising greenhouse gases. The most serious gaps in present tide gauge coverage are in three areas: (a) high po- lar latitudes, (b) Oceanic locations, and (c) the entire Southern Hemisphere. 7. Because of localization of many extreme subsidence pro- cesses, especially those connected with anthropogenic extraction of fluids such as groundwater and hydrocarbons, tide gauges are needed at every major coastal city to gather data to assist in evaluating the long-term regional trend of relative mean sea level. 8. The risk of accelerated mean sea level rise is sufficiently established to warrant consideration in the planning and design Of coastal facilities. Although there is substantial local variability and statistical uncertainty, average relative sea level over the past century appears to have risen about 30 cm relative to the East Coast of the United States and 11 cm along the West Coast, excluding Alaska, where glacial rebound has resulted in a lowering of relative sea level. Rates of relative sea level rise along the Gulf Coast are highly variable, ranging from a high of more than 100 cm/century in parts of the Mississippi delta plain to a low of less than 20 cm/century along Florida's west coast. 9. Accelerated sea level rise would clearly contribute toward
6 RESPONDING TO CHANGES IN SEA LEVEL a tendency for exacerbated beach erosion. However, in some ar- eas, anthropogenic effects, particularly in the form of poor sand management practices at channel entrances, constructed or modi- fied for navigational purposes, have resulted in augmented erosion rates that are clearly much greater than would naturally occur. Thus, for some years into the future, sea level rise may play a secondary role in these areas. 10. As noted previously, the two response options to sea level rise are stabilization and retreat. Retreat is most appropriate in areas with a low degree of development. Given that a "proper choice exists for each location, selecting an incorrect response alternative could be unduly expensive. 11. There does not now appear to be reason for emergency action regarding engineering structures to mitigate the effects of anticipated increases in future eustatic sea level rise. Sea level change during the design service life should be considered along with other factors, but it does not present such essentially new problems as to require new techniques of analysis. The ejects of- sea level rise can be accommodated during maintenance periods or upon redesign and replacement of most existing structures and facilities. There are very limited geographic areas where current subsidence rates may require near-term action as has been the case in Japan and Terminal Island, California. 12. When not restrained by funding, availability of materials, or work force, construction of almost any conceivable protection against sea level rise can be carried out in a very short time; short, that is, relative to the rate of sea level rise. 13. Defensive or mitigative strategies are site specific and cannot be developed nationwide on the bash of a blanket general- ization or comprehensive legislation. Recommendations 1. The prognosis for sea level rise should not be a cause for alarm or complacency. Present decisions should not be based on a particular sea level rise scenario. Rather, those charged with planning or design responsibilities in the coastal zone should be aware of and sensitized to the probabilities of and quantitative un- certainties related to future sea level rise. Options should be kept open to enable the most appropriate response to future changes
EXECUTIVE SUMMARY 7 in the rate of sea level rise. Long-term planning and policy devel- opment should explicitly consider the high probability of future increased rates of sea level rme. 2. The three previously described scenarios of sea level rise used in this study (see Figure 2-2) provide a useful range of possible future sea level changes for design calculations. The general shape of these curves is concave upward with greater rates of rise in the distant future than those in the next decade or so. The confidence that these scenarios wiD encompass the actual levels decreases with increasing time, and significant deviations outside the range of these scenarios are possible, including an amelioration in the rate of rise. Thus, the committee recommends that these projections be updated approximately every decade to incorporate additional data and to provide an improved basis for planning and response to the rise. 3. Practitioners can more readily incorporate the implications of sea level rise if probabilities reflecting uncertainties are attached to the projections. Thus, it is recommended that appropriate sta- tistical techniques be applied to develop a probability distribution associated with sea level rise through the year 2100 and that all updated projections include such information. 4. Feasibility studies for coastal projects (e.g., shore protection projects of the U.S. Army Corps of Engineers and storm surge studies of the Federal Emergency Management Agency) should consider the high probability of accelerated sea level rise. It may be some time before precise estimates of future sea level rise are possible. In the meantime, the risks associated with a substantial rise should not be disregarded. Instead, feasibility studies should consider which designs are most appropriate for a range of possible future rates of rise. Strategies that would be appropriate for the entire range of uncertainty should receive preference over those that would be optional for a particular rate of rise but unsuccessful for other possible outcomes. 5. The federal government should acquire long-term reliable accurate data from a water-level measuring system for open-ocean stations at scientifically important locations throughout the world. Critical stations should include documentation of vertical ground motion and the temporal salinity and temperatures of the water column. Tide gauges should be installed at every major coastal city. 6. The ~rnportant decision for maintaining or abandoning
8 RESPONDING TO CHANGES IN SEA LEVEL coastal facilities In the face of rising sea level should be well documented by scientific knowledge. Agencies that fund coastal research, such as the U.S. Navy, U.S. Army, National Science Foundation, National Oceanic and Atmospheric Administration, U.S. Geological Survey, and the Environmental Protection Agency, should increase their funding for coastal processes research. The federal research funding effort should focus on studies directed to- ward understanding nature's response to relative sea level rise and developing appropriate engineering responses. A substantial por- tion of this research should be conducted at universities and other laboratories and centers throughout the coastal United States to ensure the development of requisite engineering capability in re- gions of the country where it will be most helpful.