Flood depth grids, 100-year flood; Columbia corridor drainage districts in Columbia and Multnomah Counties, Oregon

Oregon Department of Geology and Mineral Industries 2018 Flood depth grids, 100-year flood; Columbia corridor drainage districts in Columbia and Multnomah Counties, Oregon raster digital data Oregon Department of Geology and Mineral Industries Special Paper SP-50 Portland, Oregon, USA Oregon Department of Geology and Mineral Industries http://www.oregongeology.org Oregon Department of Geology and Mineral Industries 2018 Flood risk assessment for the Columbia corridor drainage districts in Multnomah County, Oregon raster digital data Oregon Department of Geology and Mineral Industries Special Paper SP-50 Portland, Oregon, USA Oregon Department of Geology and Mineral Industries http://www.oregongeology.org/ This dataset contains a flood depth grid within the Columbia Corridor Drainage Districts and Sauvie Island Drainage Districts in Columbia and Multnomah County, Oregon. The dataset was created by DOGAMI staff through the subtraction of the lidar-derived, bare earth, ground elevation from a 100-year flood water surface elevation. A uniform water surface elevations was used for each drainage district. These water surface elevations were provided by Multnomah County Drainage District and were consistent with the Columbia River 100-year water surface elevations in the FEMA Flood Insurance Study (FIS) for Multnomah County and incorporated areas (2016). The flood depth values are measured in feet. This dataset was created for a Hazus-based flood loss estimation for the Columbia corridor drainage districts' flood risk assessment. Data use considerations: 1) Given that all flood depths are based on a single, uniform water surface elevation within each drainage district, the flood depths will not account for the slight change in water surface elevation within each drainage district that could occur dependent on the location of the levee breach. 2) Due to the inability of lidar to measure ground surface elevations within an enclosed structure, the areas within buildings in the lidar-derived topography, and the subsequent flood depths, are linearly interpolated based on the lidar-measured ground elevations directly outside the perimeter of the building. As a result, the flood depth grid values within large buildings may show a wide range of flood depths. It is important to carefully review variations in flood depths before selecting a given value to represent a building and careful attention must be paid to the first floor height of the given structure. 3) No hydraulic analysis was performed by DOGAMI to create this depth grid. 4) This depth grid is non-regulatory; it is for planning and loss estimate purposes only. 2018 Complete None planned -122.868236 -122.382858 45.742624 45.533141 ISO 19115 Topic Category geoscientificinformation Oregon Geospatial Enterprise Office Metadata Keyword Thesaurus flood levee depth drainage district catastrophic flood Oregon Geospatial Enterprise Office Metadata Keyword Thesaurus Multnomah Oregon United States of America Oregon Geospatial Enterprise Office Metadata Keyword Thesaurus Oregon Geospatial Enterprise Office Metadata Keyword Thesaurus flood stage recent None REPORT DATA: This product is for informational purposes and may not have been prepared for or be suitable for legal, engineering, or surveying purposes. Users of this information should review or consult the primary data and information sources to ascertain the usability of the information. This publication cannot substitute for site-specific investigations by qualified practitioners. Site-specific data may give results that differ from the results shown in the publication. See the accompanying text report for more details on the limitations of the methods and data used to prepare this publication. Oregon Department of Geology and Mineral Industries mailing and physical address

800 NE Oregon Street Suite 965

Portland Oregon 97232 USA 1 971 673 1555 1 971 673 1562 dogami-info@oregon.gov When contacting DOGAMI about this data set, please reference the data set title or other identifying information in your correspondence. DOGAMI publications can be downloaded from http://www.oregongeology.org Christina Appleby Oregon Department of Geology and Mineral Industries Geohazards Analyst Depth_Grids_100.jpg This graphic shows the depth of flooding for a 100-year flood within the Columbia Corridor Drainage Districts. S:\Technical_Review_Committee\Submitted for Technical Review\Appleby_Flood_Risk_Assessment_Columbia_Corridor_20180514\GIS and Spreadsheet\Thumbnail_images\Depth_Grids_100.jpg The Columbia Corridor Drainage Districts' Flood Risk Assessment was funded by the FEMA Region X RiskMap program. Unclassified Lidar vertical accuracy Root Mean Square Error for the Metro 2014 lidar dataset was 0.03 m (0.11 ft), and 0.029 m (0.10 ft) for the USACE Columbia River 2010 dataset. We must caution that this accuracy is established on open, hard-packed surfaces, and that depth-in-structure may be at significant variance with the flood depth grid information. As part of the standard lidar point cloud data processing, buildings are excised by the lidar vendor for bare earth elevation derivation, but the resultant surface for the excised building is not necessarily flat, thus creating significant variability across the building footprint. Further, a structure may be elevated above the bare earth, and not exposed to flooding. Raster data; not applicable. All areas behind the levees in SIDIC, PEN1, PEN2, MCDD, and SDIC. Our boundaries for the northwest corner of PEN1 district did not recognize the temporary levee barrier system that can be erected during flood emergencies on N. Marine Drive, thus the flood depth grid may include portions of several buildings that are not assumed to be levee-protected. The flood depth grids extend beyond the formal diking district boundaries on the southern edge of Columbia Slough. This extension sometimes includes ponding areas and wetlands, but it also includes development in the southeast corner of MCDD-E, to the east of NE 223rd Avenue. That area uses the MCDD-E water surface elevation values, as the source of flooding would come from MCDD-E and not SIDIC. For both lidar data collections, processes were designed by the lidar data vendor to achieve a horizontal accuracy for unobscured raw point cloud data of less than 30 cm. Lidar vertical accuracy Root Mean Square Error for the Metro 2014 lidar dataset was 0.03 m (0.11 ft), and 0.029 m (0.10 ft) for the USACE Columbia River 2010 dataset. We must caution that this accuracy is established on open, hard-packed surfaces, and that depth-in-structure may be at significant variance with the flood depth grid information. As part of the standard lidar point cloud data processing, buildings are excised by the lidar vendor for bare earth elevation derivation, but the resultant surface for the excised building is not necessarily flat, thus creating significant variability across the building footprint. Further, a structure may be elevated above the bare earth, and not exposed to flooding. Oregon Lidar Consortium 2014 OLC Metro raster digital data Oregon Department of Geology and Mineral Industries Lidar Imagery DOGAMI Lidar Viewer Oregon Lidar Consortium https://gis.dogami.oregon.gov/maps/lidarviewer/ 2014 United States Army Corps of Engineers 2010 Columbia River USACE, Delivery 1 raster digital data Oregon Department of Geology and Mineral Industries Lidar Imagery DOGAMI Lidar Viewer Oregon Lidar Consortium https://gis.dogami.oregon.gov/maps/lidarviewer/ 2009-2010 Federal Emergency Management Agency 2016 Flood insurance study: Multnomah County, Oregon and incorporated areas Table Flood Insurance Study 41051CV000B Washington D.C. Federal Emergency Management Agency https://msc.fema.gov/portal Drainage district boundaries for MCDD were obtained from Multnomah County Drainage District (personal communications, 2017). The boundaries were slightly modified to include the levee crest and exclude areas beyond the levees. The southern boundaries of the MCDD districts were expanded to include ponding areas and wetlands that would flood in the event of a levee breach. The boundaries are not to be interpreted as official diking district boundaries. "Lidar bare-earth data for the study area was obtained from the Oregon Lidar Consortium Metro 2014 and the USACE 2010 datasets. Within areas of overlapping lidar data, we used the more recent, 2014 dataset. The bare-earth elevation grid data were retained in the original 3-foot resolution. The flood depth grid was then a simple subtraction: the specified Water Surface Elevation for a given diking district, minus the bare earth mosaic. Negative values were set to NoData. All elevations use the NAVD88 datum. Flood depth data were purposely restricted to areas behind the levees. There are several points at which the 500-year flood level would overtop the levees. " Oregon Department of Geology and Mineral Industries mailing and physical address

800 NE Oregon Street Suite 965