Pilot Methodologies for Estimating the Impacts of Sea-level Rise
Since each of the FHWA pilot study areas included coastal areas, all of the pilots considered vulnerability to sea-level rise to be a key climate change risk. Each of the pilots assessed this vulnerability differently, but they experienced common challenges and faced similar types of decisions. The purpose of this Appendix is to describe the pilots' approaches and to detail the relevant data sources, methodologies, challenges and barriers involved in conducting local-scale vulnerability assessments of the impacts of sea-level rise.
Spatial analysis is a key part of assessing the vulnerability of assets to sea-level rise. Without information on location and elevation, it is impossible to determine which assets will be exposed to sea level rise, or for that matter storm surge and wave impacts associated with tropical storms or other coastal storms. In order to characterize exposure, most studies of sea-level rise risk rely on inundation mapping. This method uses a Geographic Information System (GIS) to map inundated areas by analyzing areas of land that fall below increased water levels under different scenarios of sea-level rise (also called the "bathtub model").
As described in NOAA (2009), inundation mapping involves the following key steps:
The utility and accuracy of a sea-level rise assessment depends on the resolution of the underlying elevation data. One standard source of elevation data, the USGS National Elevation Dataset, supplies elevation data with a horizontal resolution of 30m and 10m for the entire United States, which may be considered relatively coarse where coastlines are highly developed. Vertical resolution can vary, based on the source of the elevation data utilized by NED, but the stated accuracy of available data from the NED is around +/-2.4m. However, since global projected sea-level rise changes only reach 2m by the end of the 21st century (if at all), maps based on the NED will generally not provide accurate predictions of exposure of specific assets. In order to obtain more useful elevation information, local assessments will likely need to rely on digital elevation models derived from high resolution LiDAR (Light Detection and Ranging) data. These data are not available in all locations. In addition, projects must ensure that the LiDAR data have been properly processed, including adjustments to the vertical datum before use.
In addition to the horizontal and vertical resolution requirements for elevation data, there are other challenges for analyzing sea-level rise vulnerability at a local scale. As described in CCSP (2009), inundation mapping can be misleading because elevation is not the only determinant of coastal vulnerability. Mapping may not take into account the uplift or subsidence of the land surface. In areas where land is sinking, the apparent rate of sea-level rise (often referred to as the "local" or "relative" rate of sea-level rise) will be greater than the rate associated with changes in the global mean sea level.
Sea-level rise will likely occur slowly over a period of time and will manifest differently in different areas due to ongoing coastal processes, such as changes in tidal flow and sediment volume. In some places, land will become permanently flooded, while in other areas it will erode. Barrier islands and wetlands may migrate or disappear, and storms, waves, and currents will continually modify the landscape as the sea-level rises. Although the simple bath tub approach may indicate the relative risk among areas, it may not serve as a "prediction" for how the future landscape will appear. Typically, such limitations are not critical for identifying areas at risk at broad spatial scales (e.g., regionally or nationally) or communicating these risks to the general public. However, they may be important to keep in mind when such maps are used for local land use planning.
Useful Guidance on Analyzing Exposure to Sea-Level Rise
Army Corps of Engineers (2009). Water Resources Policies and Authorities Incorporating Sea-Level Change Considerations in Civil Works Programs. http://www.dbw.ca.gov/csmw/pdf/EC_Sea_Level_Change.pdf
CCSP (2009). Coastal Sensitivity to Sea-Level Rise: A Focus on the Mid-Atlantic Region. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. http://www.climatescience.gov/Library/sap/sap4-1/final-report/
NOAA, Coastal Services Center (2009). Coastal Inundation Mapping Guidebook. Charleston, South Carolina. http://www.csc.noaa.gov/digitalcoast/_/pdf/guidebook.pdf
NOAA (2010). Technical Considerations for Use of Geospatial Data in Sea Level Change Mapping and Assessment. http://www.csc.noaa.gov/digitalcoast/_/pdf/SLC_Technical_Considerations_Document.pdf
Useful Data Sources and Models for Inundation Mapping or Sea-Level Rise Risk Assessment
NOAA. SLOSH (Sea, Lake, and Overland Surges from Hurricanes Model http://www.nhc.noaa.gov/prepare/hazards.php
Thieler, R., J. Williams, and E. Hammar-Klose. National Assessment of Coastal Vulnerability to Sea-Level Rise. Woods Hole Field Center, Woods Hole, MA. USGS. http://woodshole.er.usgs.gov/project-pages/cvi/
Examples of Regional or Local Inundation Mapping
Keim, B.D., T.W. Doyle, V.R. Burkett, I. Van Heerden, S.A. Binselam, M.F. Wehner, C. Tebaldi, T.G. Houston, and D.M. Beagan (2008). How is the Gulf Coast Climate Changing? In: Impacts of Climate Change and Variability on Transportation Systems and Infrastructure: Gulf Coast Study, Phase I. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. http://www.climatescience.gov/Library/sap/sap4-7/final-report/