Conducted jointly by the US Geological Survey (USGS) and US Army Corps of Engineers (USACE), as part of a Planning Assistance to States partnership agreement with the Levee Ready Columbia partnership, a study entitled Assessment of Columbia and Willamette River Flood Stage on the Columbia Corridor Levee System at Portland, Oregon, in a Future Climate is published.
This study will help the Levee Ready Columbia partnership understand and plan for the ways climate change will impact conditions on the lower Columbia River.
The study used two internationally vetted river models, along with extensive climate science data, to simulate the effects of extreme but plausible high-water events on the Columbia over three periods of thirty years up to 2059. Three future hydrologic scenarios were used to create a high, medium, and low version of an “extreme but plausible” flood event on the Columbia.
Although many people think about rising sea levels when they hear ‘climate change,’ this study indicates that rising sea levels may only impact the Columbia as far inland as Rainier. What’s more likely to have a direct impact on our local levee system, is the anticipated increase in precipitation in the Cascades and Willamette Valley, which will create higher wintertime flows. The study also indicates that we should be prepared for earlier snowmelt and more wintertime rain-on-snow events, quickly melting the snowpack, which also leads to higher river flows.
The assessment indicates that these warmer, wetter conditions will be more frequent in the future and could raise the river levels during major flood events by as much as 40-percent over the next half-century. The higher flows will also last longer, leaving the levees saturated for a longer period than they were originally designed. Additional research is needed to determine how these longer periods of saturation will impact levee operations.
The primary purpose of the study is to model what river conditions would look like during extreme but plausible flood events. The modeling found that under major flood conditions, the Columbia River could rise as much as 4.5 feet higher than similar events in the past akin to the 1996 floods on the Columbia and Willamette Rivers. For context, the Columbia River crested at 32.5 ft (NAVD 88) in February 1996. At the projected elevation during similar wintertime rain-on-snow event in the future, floodwaters would likely overtop portions of the Columbia River levee in Peninsula Drainage District #1 (PEN 1) on the far western end of the system where the levees are the lowest.
The most important take away from this study is that we are likely to see higher levels of precipitation during the winter and earlier snowmelt and earlier peak river flows, which are not as effectively controlled by upstream dams as the springtime freshet (e.g., when the snowpack in the mountains melts into the river and is carried downstream through the watershed to the sea during the late spring).