DrainageBackflow_flood_depth_at_MHHW_plus_4ft_SLR en Charles "Chip" Fletcher University of Hawai'i Coastal Geology Group (CGG) fletcher@soest.hawaii.edu http://www.soest.hawaii.edu/coasts/ information pointOfContact 2019-08-30 ISO 19115 ISO19115:2003/Cor 1 2006 EPSG:26904 Drainage Backflow Flooding at Average Daily High Tide (MHHW): Honolulu Primary Urban Center: 4 ft Sea Level Rise 2019-07-31 creation Charles "Chip" Fletcher University of Hawai'i Coastal Geology Group (CGG) fletcher@soest.hawaii.edu http://www.soest.hawaii.edu/coasts/ pointOfContact Tiffany Anderson University of Hawai'i Coastal Geology Group (CGG) tranders@hawaii.edu originator This raster dataset shows simulated drainage backflow flood depths (in feet), vertically referenced to the local mean sea level (LMSL) tidal datum, during an average daily high tide, in the presence of 4.0 ft additional sea level rise. Drainage backflow flooding occurs when seawater flows from the ocean through storm drains and discharges into urban areas. This raster dataset is part of a larger study (see below) to improve understanding of the various types of flooding that will be exacerbated by the confluence of high tidal events and ongoing sea level rise (SLR) in the Primary Urban Center (PUC) on O`ahu. ***** To determine flooding due to drainage backflow, a tidal surface is created as a horizontal plain whose elevation is the mean higher high water (MHHW) tidal datum at the Honolulu tide gauge (1.08 ft above local mean sea level). Future tidal surfaces are then created by increasing the water level above the MHHW surface by the projected amount of SLR. Flood depths were calculated as the elevation difference between the simulated tidal surface and a digital elevation model (DEM) that represents the current topography. Areas flooded due to drainage backflow are identified as those flooded pixels showing surficial water flow connection to storm drains. The DEM used in this study was obtained from the U.S. Army Corps of Engineers, which was created from LiDAR data of the Honolulu area collected in 2013. This "bare earth" DEM (vegetation and structures removed) was used to represent the current topography of the study area for the urban corridor stretching from Hickam Field and the Daniel K. Inouye International Airport to Waikiki and Diamond Head along the south shore of O`ahu. ***** This raster dataset is part of a larger study titled “Modeling Sea Level Rise-Related Flood Components and Impacts on Infrastructure in the Primary Urban Center, O`ahu, Hawai`i. Further details describing the study can be found in the manuscript submitted for publication: Habel, S., Fletcher, C.H., Anderson, T.R., and Thompson, P. (In Review). Compound Flooding Related to Sea-Level Rise and Urban Infrastructure, Nature Scientific Reports. ***** Overview of the Study: The study objectives include simulation of three types of flooding that will occur over the coming decades: 1) Direct marine, flooding that occurs as ocean water flows unimpeded over the land surface; 2) Storm drain backflow, flooding that occurs as marine waters travel from the ocean and up through drainage infrastructure, leading to flooding where backflow discharges into urban areas; and 3) Groundwater inundation, flooding that occurs as the water table is lifted above the ground elevation. Each of these flood components will require a separate set of adaptation strategies to mitigate impacts. ***** Sea level rise values modeled: A total of five higher mean sea levels were modeled to simulate future sea level rise. The lowest three – 1.1, 2.0, and 3.2 feet above mean higher high water (MHHW) – correspond to values used in the Hawai`i Sea Level Rise Vulnerability and Adaptation Report (2017). Two greater sea level elevations – 4.0 and 5.0 feet above MHHW – were added to reflect physically plausible SLR scenarios for use when considering high-risk infrastructure (e.g., a power plant). ***** Timing and frequency of each sea level-related flood event: The federal agency NOAA released a set of six global mean sea level (GMSL) scenarios that range from a low scenario (rising 0.3 m [1.0 ft] by year 2100) to an extreme scenario (rising 2.5 m [8.2 ft] by 2100). The current trajectory of global emissions leads to a GMSL that roughly follows the NOAA Intermediate scenario (rising 1.0 m [3.3 ft] by 2100). However, extreme tidal fluctuations lead to episodes of higher sea level, in excess of the mean sea level. Thus, a location will experience episodes of flooding due to high tide before more permanent inundation due to global mean sea level rise. These tidal flooding events can be responsible for significant impacts. ***** For guidance on when, and how often, we might expect tidal flooding in the PUC on O‘ahu, please refer to the tables in the document titled ‘BWS_Flood_Tables_Thompson.pdf’ available at ‘ftp://soest.hawaii.edu/coastal/BWS_Flood_Tables/’. The tables show projected arrival times and frequencies of flooding over the 21st century. These projections are shown for two NOAA SLR scenarios: Intermediate (3.3 ft GMSL by 2100) and Intermediate-High (4.9 ft GMSL by 2100). Following the current trajectory of global emissions, the NOAA Intermediate GMSL scenario is most likely to occur; however, increased emissions are expected over the 21st century, leading to higher levels of GMSL that would tend toward the Intermediate-High scenario. For higher risk projects, the higher SLR scenario is recommended. Values in the table are from Thompson et al.’s (2019) statistical method that uses the local tide gauge to estimate the number of local flood days, given a particular sea level rise threshold. The table does not include estimates for 4-ft and 5-ft of SLR because they were not yet available from the study; however, extrapolating from the available data, flooding due to these higher SLR values appear plausible around and after mid-century. Reference: Thompson, P.R., Widlansky, M.J., Merrifield, M.A., Becker, J.M., & Marra, J.J. (2019). A statistical model for frequency of coastal flooding in Honolulu, Hawaii, during the 21st century. Journal of Geophysical Research: Oceans, 124, 2787-2802. https://doi.org/10.1029/2018JC014741 This raster dataset is part of a study, whose purpose is to improve understanding of the various types of flooding that will be exacerbated by the combination of high tidal events and ongoing sea level rise (SLR) in the Primary Urban Center (PUC) on O`ahu. Charles "Chip" Fletcher University of Hawai'i Coastal Geology Group (CGG) fletcher@soest.hawaii.edu http://www.soest.hawaii.edu/coasts/ pointOfContact flooding sea level rise storm drain backflow theme Hawaii Honolulu place The authors shall not be held liable for improper or incorrect use of the data described and/or contained herein. These data are not legal documents and are not intended to be used as such. The information contained in these data is dynamic and may change over time. The data are not better than the original sources from which they were derived. It is the responsibility of the data user to use the data appropriately and consistent within the limitations of geospatial data in general and these data in particular. Although these data have been processed successfully on a computer system at SOEST, no warranty expressed or implied is made regarding the utility of the data on another system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. This disclaimer applies both to individual use of the data and aggregate use with other data. 1 eng