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Background

Project Purpose
Limitations
Source Data
Development
Funding

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Project Purpose

This web tool is part of a project to update flood hazard layers for Coos County and place them in context within a larger framework of natural hazards. The Federal Emergency Management Agency (FEMA) supported the project to improve depiction of potential flood elevations using high-quality lidar data acquired by DOGAMI in 2008. Lidar is now available for most of the county, including complete coverage of all population centers. Previous mapping of the flood hazard was performed in the mid-1980s using best available elevation data that was limited in most cases to USGS topographic maps – often with elevation contour intervals no better than 40 feet. Even with very accurate hydrologic and hydraulic information, reliably locating which structures are prone to flooding is impossible with such coarse elevation data. Lidar acquired in Coos County provides vastly improved precision, allowing for the creation of 1-foot contour intervals. Using this high-quality elevation data, DOGAMI re-mapped the flood hazard with such detail that individual structures can be readily identified as "in or out" of the flood hazard zone.

Lidar also greatly improves the accuracy of mapping for other natural hazards. For this project, DOGAMI prepared lidar-based maps of tsunami inundation, earthquake hazards (including liquefaction, ground motion amplification, absolute shaking, and landslide susceptibility), river channel migration zones, and active and historic landslides.

The intended outcome of this project is to provide local communities access to the best available hazard information, allowing them to make planning decisions informed by the latest science. The web tool is intentionally designed to allow users to identify individual structures and properties, so that everyone can easily see what hazards are present at sites of interest.

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Limitations

See Limitations page.

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Source Data

Parcels
The tax lot layer shows the boundaries of real properties in Coos County, and was provided by the Coos County Assessor's Office in 2009. The boundaries shown on the map should not be considered authoritative representations of the precise location of property lines.

FEMA 100-Year Flood Zone (Prelim)
Developed by DOGAMI in 2010. With flood elevations derived from various hydrologic and hydraulic models, flood zones were mapped using high-quality lidar elevation data acquired in 2008. Detailed riverine flood elevations were developed by CH2M Hill in the mid-1980's within the city limits of Lakeside, Coquille, and Myrtle Point and the unincorporated villages of Riverton and Allegany. The location of floodways was also calculated where detailed riverine flood elevations were developed. Detailed estuarine flood elevations were developed by CH2M Hill in the mid-1980s within the city limits of Bandon, North Bend, and Coos Bay. Detailed coastal flood elevations were developed by DOGAMI in 2010 within the city limits of Bandon and for Bastendorff Beach County Park, Yoakam Point State Park, and Sunset Bay State Park. Approximate riverine flood elevations were developed by DOGAMI in 2010 for all riverine flood zones not modeled by detailed methods. Approximate coastal flood elevations were developed by DOGAMI in 2010 for all coastal flood zones not modeled by detailed methods. All flood zones meet specifications set forth by FEMA's National Flood Insurance Program. See FEMA Flood Insurance Study report for Coos County and Unincorporated Areas for more information about the detailed and approximate methods used define flood elevations. This data layer is a preliminary version of Coos County's new digital flood insurance rate map (DFIRM). The new DFIRM is projected to become effective in 2012. The existing DFIRM (effective date: 9/25/2009) is to be used for flood insurance purposes until the new DFIRM is adopted. These data are for planning purposes only.

Change from Previous FEMA 100-Year Flood Zone (Draft)
Developed by DOGAMI in 2010. This layer shows differences between most recent 100-year flood zone, developed by DOGAMI for FEMA and the previous 100-year flood zone. The layer highlights improvements from high-quality lidar elevation data.

FEMA DFIRM Panels
This layer displays the map panel scheme for printed FEMA flood insurance rate maps (FIRMs). Learn more about Flood Insurance Rate Maps (FIRMs) on FEMA's website. View the Coos County FIRM map index as a PDF.

Tsunami – 2010 Detailed Modeling (Bandon Only)
DOGAMI is updating its earlier tsunami inundation maps with a new series that takes advantage of a better understanding of the fault movements likely in a Cascadia subduction earthquake, better numerical models to propagate the tsunami wave to the shore and high resolution lidar data to accurately depict the extent of tsunami flooding. A further refinement is the modeling of numerous likely sizes of earthquake to show the range of likely tsunamis that could flood the coast.These data are currently only available for the south half of Coos County. Two layers are shown, one represents tsunami flooding from the largest plausible earthquake, the other represents flooding expected from the most likely subduction earthquake.

Tsunami – 1995 Regulatory Line (SB 379 line)
The 1995 Oregon legislature passed Senate Bill 379 which limits construction of new essential and special occupancy facilities in tsunami inundation zones. DOGAMI prepared maps showing the location of the landward limit of the regulatory tsunami inundation zone to implement SB 379. These maps were made using a best-available geologic model of the most likely locally generated tsunami, along with a numerical model of the propagation of the tsunami wave. The location of the SB 379 line on the ground was mapped by estimating the location of the numerically modeled tsunami elevation on the coast using existing topographic maps. Along much of the coast these maps have low resolution and accuracy, so the SB 379 line has many of the same accuracy problems as the 1985 FEMA flood zones described above. However, the SB 379 line, as depicted by the 1995 DOGAMI maps (http://www.oregongeology.org/tsuclearinghouse/pubs-evacbro.htm) is the only officially recognized regulatory boundary.

Existing Landslides
division of landslide mappingLandslides pose a significant hazard in the study area and can take many different shapes and forms. Landslides can be initiated in marginally stable slopes by a number of natural and human disturbances. Processes and conditions that can trigger slope failure include earthquake shaking, deforestation, deforestation-related activities such as road building, and rapid snow melt. Two of the most common triggering events in the Pacific Northwest are intense rainfall and manmade changes to land.

To create this layer, we extracted existing landslide polygons from the Statewide Landslide Information Database of Oregon (SLIDO r1). This GIS compilation of landslide areas (landslide inventory) was derived from published geologic reports and existing geologic hazard studies. We then used lidar-derived images to correct this database in three main ways: we 1) removed the incorrectly mapped slides, 2) redrew the extents of the previously mapped slides, and 3) added new landslides. Due to time constraints, we performed this new mapping at a maximum scale of 1:10,000. The scale limitation restricted our mapping to only the relatively large deep-seated landslides in this area.

Earthquake

Earthquake – 500-Year Absolute Earthquake Shaking
The amount of damage to a structure during a given earthquake will depend on the size of the earthquake and its distance from the structure, the contribution of liquefaction or ground motion amplification from the soils beneth the structure, and the construction of the building itself. The likely shaking and damage layer shows an estimate of the level of earthquake shaking and building damage that has a 10% chance of occurring in the next 50 years. The map starts with the 2008 USGS National Hazard Map for Peak Ground Acceleration (http://earthquake.usgs.gov/hazards/). These values are combined with the ground motion amplification layer to produce a map of amplified peak ground acceleration at every site. This map is then converted into a layer showing strength of shaking and damage potential using the relationships defined for the USGS SHAKEMAP publications (http://pubs.usgs.gov/tm/2005/12A01/).

Earthquake - Landslide Susceptibility
Slopes that have failed in the past often remain in a weakened state, and many landslide areas tend to fail repeatedly over time. In some cases, areas that have previously failed assume rather subtle geometries, and these areas may or may not be obvious on relative hazard maps that emphasize slope. Previously failed areas are nonetheless particularly important to identify, as they may pose a substantial hazard for future instability. They are also one of the key inputs for creating susceptibility maps or maps that show areas of relative low to high potential for future landslides.

We prepared the generalized landslide susceptibility map using an approach similar to the Wilson and Keefer methodology (Wilson, R.C., and Keefer, K.D. 1985, Predicting areal limits of earthquake-induced landsliding, in Ziony, J.I., Ed., Evaluating earthquake hazards in Los Angeles Region, U.S. Geological Survey, Professional Paper 1360, p. 317-345) employed within HAZUS. The method combines two important factors relating to landslide susceptibility: slope and geologic material strength. In regional applications such as this study, slope gradient is derived from digital elevation models (in this case, a 10-m DEM). We estimated material strength by grouping geologic units into three classes based on the unit characteristics identified in the geologic reports and our comparisons with other landslide hazard information. The resulting landslide susceptibility map has three categories: Low, Moderate, and High.

Earthquake – Ground Shaking Amplification
Geologic deposits of soft rock or sediment can significantly increase the strength of shaking during a moderate to strong earthquake. This process, called Ground Motion Amplification, can cause increased damage in structures that are founded on the soft geologic materials. To make the ground motion amplification layer for this web map, we first redrew the existing digital geologic map (Oregon Geologic Data Compilation version 5; http://www.oregongeology.org/sub/ogdc/index.htm) using the lidar data as a guide to the correct locations of the boundaries between geologic deposits. We then assigned each geologic unit to a ground motion amplification category (NEHRP site class; http://www.fema.gov/media-library-data/20130726-1532-20490-4965/fema_450_1_provisions.pdf), based on measurements of the shear wave velocities for various geologic units from around Oregon, or from the geotechnical literature. The resulting hazard layer shows areas with high, moderate and low ground motion amplification hazard.

Earthquake – Liquefaction
Geologic deposits of loose sand and silt that are saturated with water typically become fluid when shaken during a strong earthquake. This process, called liquefaction, dramatically reduces the strength of the soil, and allows structures to tilt or topple, underground tanks to rise, and can cause dramatic earth movements even on flat land. In most major earthquakes, much of the severe damage is due to liquefaction. To make the liquefaction layer for this web map, we first redrew the existing digital geologic map (Oregon Geologic Data Compilation version 5; http://www.oregongeology.org/sub/ogdc/index.htm) using the lidar data as a guide to the correct locations of the boundaries between geologic deposits. We then assigned each geologic unit to a liquefaction susceptibility category, based on tables from the geotechnical literature. The resulting hazard layer shows areas with high, moderate, low and no liquefaction hazard.

Channel Migration Zones
Conventional flood hazard maps like the FEMA 100-year flood zone layer only show the hazard posed by standing floodwaters on a given floodplain. Damage from bank erosion as river channels naturally migrate may be far greater locally and can occur even in the absence of major flooding. The channel migration hazard layer shows areas susceptible to future channel movement and erosion. Following published methodology (Rapp, C., and Abbe, T., 2003, A framework for delineating channel migration zones, Washington Department of Ecology, Ecology Publication 03-06-027, 139 p.; http://www.ecy.wa.gov/pubs/0306027.pdf), we examined lidar-derived digital elevation models and historical and current aerial photographs to map the position of the channel over time. From this basic data set we then calculated local erosion rates and identified areas of potential avulsion (river jumping to a new channel), where infrastructure may prevent erosion, and where infrastructure is at risk from erosion.


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Development

Contact team members.

May 2011: Redeployed using Esri ArcGIS Server and ArcGIS Viewer for Flex version 2.2.
December 2010: Deployed using Esri ArcGIS Server.

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Funding

This project was funded by FEMA's NFIP Map Modernization program under contract EMS-2008-GR-0013.