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Open-File Report O-21-15, Flood depth and channel migration zone maps, Benton, Marion, Morrow, And Washington Counties, Oregon

by Christina A. Appleby, Matt C. Williams, Lowell H. Anthony, and Ian P. Madin; 72 p. report, 225 p. appendix, four Esri® geodatabase with internal metadata, external metadata in .xml format, nineteen spreadsheets

What's in this report?
This report describes the methods and results of flood depth and channel migration zone mapping for Benton, Marion, Morrow, and Washington Counties, Oregon. This information can help communities plan and prepare for natural disasters.

 

Presentations

DOGAMI staff recorded virtual presentations for each of the counties that describes the work from this publication and the accompanying landslide, coseismic liquefaction susceptibility, and coseismic soil amplification class maps in Open-File Report O-21-14 (Hairston-Porter, R. and others, 2021). These presentations are available at the following links:

 

Benton County Presentation

Marion County Presentation

Morrow County Presentation

Washington County Presentation

Report downloads:

·        Report (72. p, 4.93 MB file)

·        Appendix (225. p, 483 MB file)

·        GIS data bundle: 4 geodatabases with metadata (4.35 GB, zip file)

·        Spreadsheet bundle: 19 spreadsheets (906 MB, zip file)

·        Complete publication bundle: report, appendix, geodatabases, metadata, and spreadsheets (4.83 MB, zip file)

 

·        Benton County bundle: report, appendix, geodatabase, metadata, and spreadsheets (2.6 GB, zip file)

·        Marion County bundle: report, appendix, geodatabase, metadata, and spreadsheets (1.87 GB, zip file)

·        Morrow County bundle: report, appendix, geodatabase, metadata, and spreadsheets (104 MB, zip file)

·        Washington County bundle: report, appendix, geodatabase, metadata, and spreadsheets (269 MB, zip file)

EXECUTIVE SUMMARY
This study provides Oregon communities with new information about the natural hazards from floods and channel migration. During 2018 to 2021, the Oregon Department of Geology and Mineral Industries (DOGAMI) produced flood depth maps for rivers in Benton, Marion, and Morrow Counties and based on Federal Emergency Management Agency (FEMA) flood insurance studies. Also developed are channel migration zone maps for major rivers within Benton, Marion, Morrow, and Washington Counties. These maps are the first of their kind published for these study areas.

We produced flood depth maps for the 10-, 50-, 100- and 500-year flood zones using the adopted FIS maps for each individual stream within the study area most current data (as of December 2020). The maps were created by interpolating each flood model’s cross section elevation data into a continuous water surface elevation (WSEL) surface, and then subtracting the corresponding lidar-based ground surface digital elevation model (DEM). Although this approach is simple, great care was needed to prepare the data for this process in order to create accurate and detailed representations of the modeled flood depths.

Channel migration zone (CMZ) maps define the area within which a given stream is likely to move laterally during the next 30- and 100-years. In this study, we mapped CMZs along a total of 464 river miles, across 20 rivers in four counties. The components that comprise these CMZ maps are the active channel, historical migration area, 30-year and 100-year erosion hazard area, avulsion hazard area, and flagged stream banks. The method we used was primarily based on the interpretation of historical aerial photographs, high-resolution lidar topography, geologic and flood inundation maps.

These maps were designed to be used in risk assessments to identify people, places, buildings, and infrastructure most vulnerable to floods and channel migration at a neighborhood-scale. The maps in this study do not replace a site analysis by a land surveyor, geologist, or engineer. The hazard maps and risk assessment results can be used by local and state emergency managers, planners, community leaders, residents, and other stakeholders to make informed decisions about flood hazards including their mitigation, land use, and environmental management. These hazard maps will provide a timely and valuable resource for the county and community planning efforts including when developing Natural Hazard Mitigation Plan (NHMP) updates.

GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA
Geodatabase is Esri® version 10.7 format.
Metadata is embedded in the geodatabase and is also provided as separate .xml formatted files.

Each dataset listed below has an associated, standalone .xml file containing metadata in the Federal Geographic Data Committee Content Standard for Digital Geospatial Metadata format.

Flood GIS data

Dataset name

Description

Depth_10yr

This raster dataset depicts the 10-year flood depth (in feet) based on the effective FEMA Flood Insurance Studies.

Depth_50yr

This raster dataset depicts the 50-year flood depth (in feet) based on the effective FEMA Flood Insurance Studies.

Depth_100yr

This raster dataset depicts the 100-year flood depth (in feet) based on the effective FEMA Flood Insurance Studies.

Depth_500yr

This raster dataset depicts the 500-year flood depth (in feet) based on the effective FEMA Flood Insurance Studies.

CMZ GIS Data

Dataset type

Description

Channel Migration Zone (CMZ)

This polygon represents the area within which a channel is likely to move laterally during the next 100-years.

Active Channel (AC)

This polygon represents the area within the floodplain that regularly conveys water including the exposed, unvegetated sediment deposits both within and adjacent to the river.

Historical Migration Area (HMA)

This polygon represents the channel has occupied in the historical photographic record, including the active channel.

30-Year Erosion Hazard Area (EHA)

This polygon represents the areas at risk from lateral migration and channel widening over across 30-years based on historical erosion rates.

100-Year Erosion Hazard Area (EHA)

This polygon represents the areas at risk from lateral migration and channel widening over across 100-years based on historical erosion rates.

Avulsion Hazard Area (AHA)

This polygon represents the areas at greatest risk of rapid channel diversion and occupation.

Flag

This polyline represents the channel banks which have been identified for further geotechnical inspection due to signs of recent migration or location directly adjacent to landslide deposits

Study Area

This polygon represents the study area extent.

 

SPREADSHEETS
The spreadsheets provide a summary of the channel description and notes for all rivers within this study by river segment. These descriptions include physical channel characteristics (e.g., river segment length, slope, sinuosity, etc.), geomorphic and geologic conditions, erosion rates, and observed avulsions, and more.

APPENDIX
The appendix PDFs contain maps showing the channel migration zones for all rivers within this study for Benton, Marion, Morrow, and Washington Counties. The figure below shows an example of the style of these maps. Each map shows the channel migration zone, active channel, historical migration area, 30- and 100-year erosion hazard area, avulsion hazard area, and flagged stream banks.

Diagram

Description automatically generated