FLOOD RISK ASSESSMENT WITHIN THE COLUMBIA CORRIDOR DRAINAGE DISTRICTS, MULTNOMAH COUNTY, OREGON

On Memorial Day in 1948, one of Oregon’s largest cities was destroyed when Columbia River flood waters broke through the existing levee system and flooded Vanport, Oregon. 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, adjacent to the former site of Vanport. 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 a levee breach occur again.

The objective of this study was to quantify the impact of a levee breach on each of the five Columbia corridor drainage districts in Multnomah County during a 1-percent or 0.2-percent annual chance flood event. 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. To assess the impact of flooding, we used the Federal Emergency Management Agency’s Hazus methodology to estimate damage to assets, and we used Geographic Information Systems to assess flooding exposure.

Our study indicates that a major flood in conjunction with a levee breach would have a catastrophic impact on any of the five drainage districts. Depending on the affected district, between 51% and 95% of buildings in the district would be exposed in a 1-percent annual chance flood, resulting in millions if not billions of dollars in building, content, and inventory damage. In total, nearly 6,000 residents and more than 50,000 employees live and work in areas behind levees and vulnerable to a 1-percent annual chance flood. The majority of community assets, such as the Portland International Airport, and numerous sites storing hazardous materials are also at risk. 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 make plans for a potential levee failure.