2014/07/16 Council Agenda Packet

Tacoma 2050: Medium Sea Level Rise Scenario, High Sea Level Rise Scenario Tacoma 2100: Medium Sea Level Rise Scenario, High Sea Level Rise Scenario Summary of Methodology Mapping the sea level rise scenarios required using the command line window in an ArcMap project to analyze LiDAR elevation data. Several commands were executed in order to select the areas of land that are below the threshold height and above the ordinary high water, and are also hydrologically connected to the Puget Sound. This approach does not take into account local morphodynamic conditions, which would be necessary to analyze the vulnerability of a specific site to sea level rise. Constraints The sea level rise map products in this report resulting from the UWCIG/Ecology sea level rise estimates and the LiDAR data should not be used to evaluate whether specific sites or properties will experience impacts or inundation. The purpose of these maps is to give a broad picture of what sea level rise may potentially look like in the areas most likely to be affected by inundation. One way to do a more detailed analysis of the impacts of sea level rise on a facility of high importance would be to install a global positioning system (GPS) unit to accurately measure the vertical land movement on that particular site. In addition to tectonic forces, local vertical land movement can also be caused by sediment compaction, which could also be monitored by a GPS unit. Additionally, a site- and use -specific survey would be necessary to determine precisely accurate elevation data. The following sections explain the constraints inherent in the mapping methodology, the sea level rise estimates and the LiDAR data. The purpose of these sections is to transparently identify the limitations of the resulting maps and to emphasize that the maps should be interpreted with these limitations in mind. Mapping Methodology: Much of the Puget Sound shoreline terrain is comprised of steep slopes and bluffs. The sea level rise maps do not depict potential impacts to these types of areas. The mapping methodology purely portrays the increased elevation of the ordinary high water sea level (long-term inundation) and, in the case of Seattle, episodic storm surge; it does not illustrate the dynamic changes that will be associated with different types of topographies, such as "erosion of beaches and bluffs, landward migration (translation) of barrier beaches ... and loss of many marshes and wetlands" (Johannessen and MacLennan 2007). Steeply sloped areas that do not appear to be impacted by sea level rise on the maps may be affected by these other processes. Additionally, the maps that depict possible sea level rise coupled with extreme event storm surge do not take into account the inland impact from sea spray. Sea Level Rise Estimates: The January 2008 UWCIG/Ecology report, "Sea Level Rise in the Coastal Waters of Washington State," emphasizes that: "(1) these calculations have not formally quantified the probabilities, (2) SLR [sea level rise] cannot be estimated accurately at specific locations, and (3) these numbers are for advisory purposes and are not actual predictions." Therefore the estimates are not appropriate for site specific analysis and should be interpreted with caution. LiDAR: Considerable care has been taken to ensure that these data are as accurate as possible. We believe most of the data is adequate for determination of flood hazards, for hydrologic modeling, for determination of slope angles, and similar uses with a level of detail appropriate to a horizontal scale of 1:12,000 (1 inch = 1,000 feet) or smaller and vertical accuracy on the order of a foot. Locally, the data is of considerably poorer quality. Users should carefully determine the place -to -place accuracy and fitness of these data for your particular purposes. For many purposes a site- and use -specific field survey will be necessary. For more complete metadata, see Appendix C or visit the Puget Sound LiDAR Consortium website at pugetsoundlidar.ess.washington.edu. Mapped Scenarios Summary As illustrated in Exhibits 1-10, the waterfront, river delta and tideflat areas in the region experience some level of inundation under these sea level rise scenarios. There are numerous planning implications to these results, L-17 Appendix L: Climate Change Background