City Of Gresham Flood Insurance Study (OR)
A FEMA Flood Insurance Study is being conducted along approximately 30 stream miles of 18 streams in the City of Gresham, in Multnomah County, OR. The study will include new detailed AE Zones, new approximate A Zones, and refinement of existing AE Zones using new topography. New hydrology, using HEC-HMS models, has been developed for the Johnson Creek, Kelley Creek and Fairview Creek watersheds. Ground surveys for approximately 200 cross sections and 90 structures have been completed using a combination of the latest RTK GPS and Optical Total Station equipment. This allowed for all of the survey data to be quickly and easily geo-referenced to existing National Geodetic Survey (NGS) monuments, resulting in easy integration with recently acquired digital 2-foot contour mapping and aerial ortho-photography.

HEC-RAS was used to perform the hydraulic analysis. Water surface profiles for the 10-, 50-, 100-, and 500-year flood events as well as the 100-year floodway were determined. All geometry data within the hydraulic model is geo-referenced, so output data was post-processed using automated mapping and display techniques in ArcGIS. After thorough in-house review, the completed products (including H&H models, flood profiles, floodway data tables, work maps, DFIRMs, FIS text updates, FBS audit results, and all backup data) were submitted to FEMA as a Physical Map Revision (PMR) for incorporation into the Flood Insurance Study for Multnomah County and Incorporated Areas.