Flood risk assessment for the Columbia Corridor drainage districts in Multnomah County, Oregon

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.