Oregon Coast 2050 Flooding and Impacted Assets

The Estuarine Sea Level Rise Exposure Inventory project is inventorying assets within six scenarios developed to represent future flooding along Oregon’s estuaries. The goal is to determine the assets and geographies most likely to be affected by sea level rise in 21 of Oregon’s estuaries, and prioritize areas to focus future resources and further study. The project area includes 21 estuaries and the surrounding low-lying shorelands (less than 25 feet in elevation). The sea-level rise scenario polygons were created by using sea-level rise projections coupled with coastal flood event water levels. The sea-level rise projections are from the National Research Council of the National Academies "Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future" (2012, pg 96). The report gave a range of sea-level rise projections for 2030, 2050, and 2100. We used the high value of the range for each year projected, from a 2012 baseline, to provide a large range of possible water levels. The 2030, 2050, and 2100 sea-level rise estimate are 0.75 feet, 1.5 feet, and 4.6 feet, respectively. The flood event water levels were determined from NOAA's extreme water level calculations at the Crescent City, Charleston, South Beach, and Astoria tide stations. The flood event levels used were the 1% and 50% annual exceedance probability values. For example, the 50% exceedance elevation is 0.8 m above Mean Higher High Water (MHHW) at South Beach, Oregon. This means there is a 50% chance that the tide will exceed 0.8 m above MHHW in a given year, or on average, once every two year period. The water surface models are based on the combined flood event water level and sea-level rise estimates. The water surface was created using the National Oceanic and Atmospheric Administration (NOAA) VDatum tool. The land surface model is based on lidar elevation measurements supplied by the Oregon Lidar Consortium (OLC) from 2008 and 2009. The extent of the scenario areas are the intersection of the modeled water surface and the lidar digital elevation model. It is important to note that the flooding model does not account for artificial (man-made) hydrologic barriers such as dikes, flood gates, restrictive culverts, road and railroad embankments, etc. Areas behind these barriers at an elevation lower than the modeled water level are displayed at flooded. This was an intentional choice to be conservative in our approach because we cannot guarantee the effectiveness of hydrologic barriers in the future. Dikes and tide gates will certainly change over time; for example, increasing storm intensity and peak flows generally lead to dike erosion and dike breaches. Maintenance of dikes and tide gates is dependent on the economic value of the land uses behind them; and that value is affected by drainage system functionality, which can decrease with sea-level rise. Lastly, all flood modeling has uncertainties. These scenarios are intended to be used as planning‐level tools that illustrate the potential for flooding under future sea level rise and flood events. Although this information is appropriate for conducting vulnerability and risk assessments, more detailed modeling is needed for engineering design. The maps depict possible future flooding that could occur if nothing is done to adapt or prepare for sea-level rise over the next century. The scenario modeling relied on a 1‐m digital elevation model created from lidar data collected in 2008 and 2009. If development and earthwork has occurred along the estuaries after 2009 (i.e., if a project was completed that raised or modified ground elevations), these changes are not captured. In addition, the scenarios are based on ‘bath-tub’ model outputs and do not account for all of the complex and dynamic estuarine and riverine processes, or future conditions such as erosion, subsidence, future construction or shoreline protection upgrades, and other changes to the region that may occur in response to sea level rise. More detailed methodology can be found in the exposure inventory report available at http://coastalatlas.net/index.php/tools/planners/68-slr.