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Go Back Part I:  Physical Environment
Chapter 3: Ocean CoastsGo Forward

Coastal Elevations

By Dean B Gesch, Benjamin T. Gutierrez, and Stephen.K. Gill
Coastal Elevations (PDF, 18pp, 4.4 MB ) was originally published as Chapter 2 of the US Climate Change Science Program's Synthesis and Assessment Product 4.1. A reasonable way of citing this paper would be: Gesch, D.B., B.T. Gutierrez, and S.K. Gill, 2009. Coastal elevations. In: J.G. Titus (coordinating lead author), K.E. Anderson, D.R. Cahoon, D.B. Gesch, S.K. Gill, B.T. Gutierrez, E.R. Thieler, and S.J. Williams (lead authors). Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region. U.S. Environmental Protection Agency, Washington DC, pp. 25-42.

The chapter's Key Findings and Conclusions section are available below online.

This chapter is written for mapping specialists, research directors, and other people interested in the details of elevation mapping. Those with a general interest in how much land is close to sea level may be interested in the elevation maps elsewhere on this web site, the public review version of this chapter, or the public review version of the state and local chapters of this report.

For additional reports focused on the implications of rising sea level, go to More Sea Level Rise Reports.

Key Findings

Coastal changes are driven by complex and interrelated processes.

Summary, Conclusions, and Future Directions

The topic of coastal elevations is most relevant to the first SAP 4.1 prospectus question: "Which lands are currently at an elevation that could lead them to be inundated by the tides without shore protection measures?" The difficulty in directly answering this question for the mid-Atlantic region with a high degree of confidence was recognized. Collectively, the available previous studies do not provide the full answer for this region with the degree of confidence that is optimal for local decision making. Fortunately, new elevation data, especially lidar, are becoming available and are being integrated into the USGS NED as well as being used in sea-level rise applications . Also, research is progressing on how to take advantage of the increased spatial resolution and vertical accuracy of new data ).

Using national geospatial standards for accuracy assessment and reporting, the currently best available elevation data for the entire mid-Atlantic region do not support an assessment using a sea-level rise increment of 1 m or less, which is slightly above the range of current estimates for the remainder of this century and the high scenario used in this Product. Where lidar data meeting current industry standards for accuracy are available, the land area below the 1-m contour (simulating a 1-m sea-level rise) can be estimated for those sites along the coast at which inundation will be the primary response. The current USGS holdings of the best available elevation data include lidar for North Carolina, parts of Maryland, and parts of New Jersey (Figure 2.3). Lidar data for portions of Delaware and more of New Jersey and Maryland will be integrated into the NED in 2009. However, it may be some time before the full extent of the mid-Atlantic region has sufficient coverage of elevation data that are suitable for detailed assessments of sub-meter increments of sea-level rise and development of spatially explicit local planning maps.

Given the current status of the NED for the mid-Atlantic region (Figure 2.3), the finest increment of sea-level rise that is supported by the underlying elevation data varies across the area (Table 2.4 and Figure 2.4). At a minimum, a sea-level rise increment used for inundation modeling should not be smaller than the range of statistical uncertainty of the elevation data. For instance, if an elevation dataset has a vertical accuracy of 1 m at 95-percent confidence, the smallest sea-level rise increment that should be considered is 1 m. Even then, the reliability of the vulnerable area delineation would not be high because the modeled sea-level rise increment is the same as the inherent vertical uncertainty of the elevation data. Thus, the reliability of a delineation of a given sea-level rise scenario will be better if the inherent vertical uncertainty of the elevation data is much less than the modeled water level rise For example, a sea-level rise of 0.5 m is reliably modeled with elevation data having a vertical accuracy of 0.25 m at 95-percent confidence. This guideline, with the elevation data being at least twice as accurate as the modeled sea-level rise, was applied to derive the numbers in Table 2.4.

High-quality lidar elevation data, such as that which could be collected in a national lidar survey, would be necessary for the entire coastal zone to complete a comprehensive assessment of sea-level rise vulnerability in the mid-Atlantic region. Lidar remote sensing has been recognized as a means to provide highly detailed and accurate data for numerous applications, and there is significant interest from the geospatial community in developing an initiative for a national lidar collection for the United States . If such an initiative is successful, then a truly national assessment of potential sea-level rise impacts could be realized. A U.S. national lidar dataset would facilitate consistent assessment of vulnerability across state or jurisdictional boundaries, an approach for which coastal states have voiced strong advocacy (Coastal States Organization, 2007). Even with the current investment in lidar by several states, there is a clear federal role in the development of a national lidar program .

Use of recent, high-accuracy lidar elevation data, especially with full consideration of elevation uncertainty as described in Section 2.4, will result in a new class of vulnerability maps and statistical summaries of impacts. These new assessment products will include a specific level of confidence, with ranges of variables reported. The level of statistical confidence could even be user selectable if assessment reports publish results at several confidence levels. It is clear that improved elevation data and analysis techniques will lead to better sea-level rise impact assessments. However, new assessments must include recognition that inundation, defined as submergence of the uplands, is the primary response to rising seas in only some areas. In other areas, the response may be dominated by more complex responses such as those involving shoreline erosion, wetland accretion, or barrier island migration. These assessments should first consider the geological setting and the dominant local physical processes at work to determine where inundation might be the primary response. Analysis of lidar elevation data, as outlined above, should then be conducted in those areas.

Investigators conducting sea-level rise impact studies should strive to use approaches that generally follow the guidelines above so that results can be consistent across larger areas and subsequent use of the maps and data can reference a common baseline. Assessment results, ideally with spatially explicit vulnerability maps and summary statistics having all the qualities described in Section 2.4, should be published in peer-reviewed journals so that decision makers can be confident of a sound scientific base for their decisions made on the basis of the findings. If necessary, assessment results can be reformatted into products that are more easily used by local planners and decision makers, but the scientific validity of the information remains.

Go Back Part I:  Physical Environment
Chapter 3: Ocean CoastsGo Forward

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