Friday, April 15, 2016
Snake River Basin snowpack, inflows, post-wildfire conditions create potential flood risks
An April 1 near-average snowpack and current seasonal runoff forecasts throughout the Snake River Basin prompted regional water managers to U.S. Army Corps of Engineers-Walla Walla District rate the 2016 spring flood potential at normal to slightly above normal.
“Although most areas typically at-risk of seasonal flooding haven’t experienced any major issues yet this year, the amount of snowpack remaining in the mountains still poses the chance that even regulated flows may approach flood stage, especially if significant precipitation or unexpectedly warm temperatures occur,” said Steve Hall, Walla Walla District’s water-management program manager.
The Walla Walla District’s area of operations includes about 107,000 square miles, primarily encompassing the Snake River Basin, in parts of six states: Washington, Oregon, Idaho, Wyoming, Utah and Nevada.
Cumulative precipitation amounts within the Walla Walla District varied from 117-174 percent of average for the lower-Snake River area, and from 98-139 percent of average for the middle- and upper-Snake River areas, from October 2016 to March 2016. The Clearwater River Basin is at 144 percent of normal.
Basin precipitation data was obtained from the National Weather Service (NWS) River Forecast Center in Portland, OR.
Unregulated streamflows for major basins in the Walla Walla District were 92 percent of average for the Snake River near Heise; 108 percent of average for the Boise River at Lucky Peak Dam, near Boise; 118 percent of average for the North Fork Clearwater River at Dworshak Dam, near Orofino; and 92 percent for Lower Granite Lake inflows on the lower-Snake River near Pomeroy, Washington.
Snowpack measurements reported by the Natural Resource Conservation Service (NRCS) on April 1, 2016, varied between 102-114 percent of average in the lower-Snake River area, 67-115 percent of average in the middle-Snake River area, and 96-110 percent of average in the upper-Snake River area. Individual sub-basin snowpack reports are available on the NRCS website http://www.wcc.nrcs.usda.gov/basin.html.
The April 1 forecasts of spring runoff varied between 39-129 percent of average throughout the District. In general, the forecasts for most sub-basins within the Snake River Basin are just below average to just over average.
The April through July runoff volume forecast for the Snake River at Lower Granite Dam is 19.5 million acre-feet (AF) or 98 percent of average.
As of April 1 storage for major reservoirs within the Walla Walla District is normal and varies between 28-99 percent full. Most major reservoirs with flood-risk-management responsibility have adequate space available based on the current volume forecasts and ten-day weather forecasts.
Dworshak Reservoir is currently transitioning to refill operations while releasing spring augmentation flows per NOAA’s Federal Columbia River System biological opinion to benefit juvenile salmon and steelhead outmigration. Spring snowmelt in the Clearwater sub-basin appears to be occurring earlier than normal.
Additional reservoir storage summaries are available on the NRCS website http://www.wcc.nrcs.usda.gov/basin.html.
“By the first of April, most of what we’re going to see in snowpack accumulation has peaked, making it somewhat easier to predict what affect precipitation and temperatures will have on reservoir inflows. Flood potential within the district is reevaluated daily throughout the month of April and May,” Hall explained. “Those living in areas where wildfires occurred last year should be especially alert to potential flooding conditions.”
Wildfires in Washington, Idaho and Oregon during 2015 resulted in large, exposed, burned areas, highly prone to flash flooding and erosion. Walla Walla District provided technical assistance to assess post-wildfire conditions and associated flood risks in Clearwater and Idaho counties, Idaho because of the elevated threat to the local communities.
For this spring, the greatest threat of flooding in these areas would be caused by rain-on-snow events in April and May. The district is monitoring conditions and coordinating with state and local jurisdictions in the event additional requests for assistance or flood support are needed.
Corps emergency management staff communicate with local officials to obtain on-site observations from communities in which flooding frequently occurs.
The Corps works with states, counties and other public entities to provide necessary resources and information. The Corps does not have authority to provide disaster assistance directly to individuals.
The first responsibility for protecting life, homes and property from flood damage rests with the individual. Local governments and agencies, such as flood control districts, may share in this responsibility, and together form a community's first line of defense in preventing flood damages.
Occasionally, however, local resources are not able to minimize the effects of flooding. The Corps’ flood assistance program is intended to supplement state and local governments and special-purpose districts when more help is needed.
Walla Walla District is prepared to assist states and municipalities with flood-management support. That assistance could include technical expertise, supplies and materials, equipment or contracts for emergency flood-fighting work. District flood support teams and technical experts are ready to deploy should local emergency managers request Corps assistance.
State and local agencies needing disaster assistance from the Corps should contact the Walla Walla District Emergency Management Office at 509-527-7146, or 509-380-4538.
Individuals and business owners are encouraged to contact local emergency management agencies to ensure they understand how to prepare, respond and recover from a flood.