Special Paper 50, Flood risk assessment for the Columbia Corridor drainage districts in Multnomah County, Oregon, by Christina A. Appleby and John M. Bauer.[Go back] | Publications Center | DOGAMI Home
Special Paper 50, Flood risk assessment for the Columbia Corridor drainage districts in Multnomah County, Oregon, by Christina A. Appleby and John M. Bauer.
PUBLICATION CONTENTS
Report (109 p., 19 MB PDF)
Damage-to-Parked-Vehicles Spreadsheet (two sheets: Damage Calculation and Depth-Damage Relationship; 20 KB Excel file)
Metadata bundle (5 .xml files, 22 KB zipped file; see below for individual xml files)
Geodatabase + metadata (447 MB zipped file; see below for geodatabase description)
Publication bundle (report, spreadsheet, geodatabase, xml metadata; 461 MB zipped file)
REPORT SUMMARY
On Memorial Day in 1948, Vanport, one of Oregon's largest cities, was destroyed when Columbia River flood waters broke through the existing levee system and flooded the city. In the years since this historic event, increasing demand for land within the greater Portland area has driven the development of thousands of new buildings within the area's drainage districts. Despite the region's history of flooding and proximity to the Columbia and Willamette Rivers, little is known about the chronic flood risk posed to people and assets located behind the levees should the levees breach again.
The objective of this study was to quantify the impact of a levee breach on each of the Columbia corridor drainage districts in Multnomah County during a major (100-year or 500-year, i.e., 1-percent or 0.2-percent annual chance) flood event (Figure 1). We considered the (1) damage to buildings, (2) displaced population, (3) employment-related economic loss, (4) damage to above-ground key infrastructure, (5) exposure of hazardous materials, (6) exposure of community facilities, and (7) exposure of transportation networks and damage to parked vehicles. For the purposes of this study, we did not model a specific cause or location of levee failure but instead assumed that should a portion of the modern levee system be structurally undermined during a flood, water would be able to flow freely into the affected drainage district. To assess the impact of flooding, we used the Federal Emergency Management Agency (FEMA) Hazus-MH methodology to estimate damage to assets, and we used Geographic Information Systems (GIS) to assess flooding exposure.
The findings of this study indicate that a major flood in conjunction with a levee breach would have a catastrophic impact on any of the drainage districts (Table 1). Depending on the affected district, between 51% and 95% of buildings in the district would be exposed to a 100-year flood. Such an event would result in millions of dollars in building, content, and inventory damage in Sauvie Island Drainage Improvement Company (SIDIC), Peninsula Drainage District 1 (PEN 1), Peninsula Drainage District 2 (PEN 2), or Sandy Drainage Improvement Company (SDIC) and billions of dollars in damage in Multnomah County Drainage District – West (MCDD W) or Multnomah County Drainage District – East (MCDD E). Nearly three-fourths of the more than 8,000 residents living behind the levees are located in areas vulnerable to a 100-year flood without effective levee protection. Our assessment indicates that more than 2,200 residents would be displaced from PEN 2 and more than 1,500 residents would be displaced from MCDD W and from MCDD E. In addition, most businesses would initially close after a major flood and it could take months or years for some of those businesses to repair and reopen. In MCDD W, there would be more than 1,300 businesses closed and 35,000 employees unable to return to work, which would result in a substantial loss in wages and employment throughout the region.
Given the potential severe impacts of such a flood, it is critical that local, state, and federal governmental agencies as well as businesses, residents, and community-based organizations act to minimize the risk of flooding and plan for a potential levee failure. We support maintaining or upgrading the geotechnical strength of existing levees to meet federal requirements including FEMA accreditation and U.S. Army Corps of Engineers (USACE) PL84-99 standards. Developing and maintaining evacuation and emergency response plans at household, business, drainage district, city, and county levels will also increase local resilience. In addition, we recommend continuing public outreach and education campaigns to raise awareness of the risk posed by a major 100- or 500-year flood and levee failure.
Our assessment also indicates that the majority of buildings storing hazardous material, community assets, and key infrastructure are located in areas that would be highly vulnerable to flooding without effective levee protection. Given the importance of these sites and the potential risk they pose, we recommend that inspection be performed at each site to identify opportunities to increase flood resilience.
Figure 1. Location of Columbia Corridor drainage districts in Multnomah County, Oregon.
Table 1. Summary of results for a levee breach and 100-year flood in the Columbia corridor drainage districts.
| Sauvie Island Drainage Improvement | Company Peninsula Drainage District 1 | Peninsula Drainage District 2 | Multnomah County Drainage District–West | Multnomah County Drainage District–East | Sandy Drainage Improvement Company | |
| Initially displaced residents | 381 | 13 | 2,270 | 1,799 | 1,521 | — |
| Number of exposed buildings | 486 | 42 | 1,075 | 1,115 | 740 | 91 |
| Total repair cost (building, content, and inventory) ($ millions)1 | $133.3 to 150.0 M | $33.2 to 39.8 M | $672.6 to 760.2 M | $3,588.5 to 4,746.0 M | $1,068.8 to 1,395.0 M | $256.3 to 345.5 M |
| Number of businesses initially closed due to flooding | 29 | 11 | 237 | 1,310 | 259 | 93 |
| Number of employees initially unable to return to work | 170 | 902 | 4,259 | 35,275 | 7,554 | 4,453 |
GEOGRAPHIC INFORMATION SYSTEM (GIS) DATA
Geodatabase is Esri® version 10.1 format.
Metadata is embedded in the geodatabase and is also provided as separate .xml formatted files.
| Columbia_Corridor_Flood_Risk_Assessment.gdb (data bundle; metadata only bundle) | ||
| rasters | ||
| Flood_depth_100yr | 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. | .xml |
| Flood_depth_500yr | 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 500-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 500-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. | .xml |
| feature dataset - Results_Summarized_By_Drainage_District | ||
| Building_Loss_100yr | This dataset was created by DOGAMI staff and includes polygons representing Sauvie Island Drainage Improvement Company (SIDIC), Peninsula Drainage District 1 (PEN 1), Peninsula Drainage District 2 (PEN 2), Multnomah County Drainage District 1 (MCDD), and Sandy Drainage Improvement Company (SDIC). Each polygon has a list of associated attributes including the total replacement value of buildings, contents, and inventory and the cost to replace or repair buildings, contents, and inventory due to a 100-year, standard and long duration (> 3 days of inundation) flood. Additional attributes include the debris produced during a 100-year flood and the average depth of flooding for flooded structures. All values are summed by drainage district. DOGAMI staff produced building, content, and inventory damage estimates following FEMA's Hazus methodology given the 100-year flood depth grid. | .xml |
| Building_Loss_500yr | This dataset was created by DOGAMI staff and includes polygons representing Sauvie Island Drainage Improvement Company (SIDIC), Peninsula Drainage District 1 (PEN 1), Peninsula Drainage District 2 (PEN 2), Multnomah County Drainage District 1 (MCDD), and Sandy Drainage Improvement Company (SDIC). Each polygon has a list of associated attributes including the total replacement value of buildings, contents, and inventory and the cost to replace or repair buildings, contents, and inventory due to a 500-year, standard and long duration (> 3 days of inundation) flood. Additional attributes include the debris produced during a 500-year flood and the average depth of flooding for flooded structures. All values are summed by drainage district. DOGAMI staff produced building, content, and inventory damage estimates following FEMA's Hazus methodology given the 500-year flood depth grid. | .xml |
| Impacts_on_Population_ Employment_and_Additional_Assets |
This dataset was created by DOGAMI staff and includes polygons representing Sauvie Island Drainage Improvement Company (SIDIC), Peninsula Drainage District 1 (PEN 1), Peninsula Drainage District 2 (PEN 2), Multnomah County Drainage District 1 (MCDD), and Sandy Drainage Improvement Company (SDIC). Each polygon has a list of associated attributes including the displaced population, number of businesses able to reopen and employees able to return to work immediately after flood waters recede, employee wages during the first year after flooding, and number of key infrastructure, hazardous materials, and community assets exposed to flooding during a levee breach and either 100-year or 500-year flood. Note that many of these attributes may be expressed either as a loss (e.g. initially displaced population) or as a positive a value (e.g. population able to return immediately to residences); these values sum to the total value provided (e.g. total population). All values are summed by drainage district. DOGAMI staff calculated post-flood annual wages based on business recovery times provided by FEMA's Hazus methodology given the 100-year flood depth grid. | .xml |