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Warm winters increase risk of rain-on-snow flash floods, researchers say

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Warm winters increase risk of rain-on-snow flash floods, researchers say

A new scientific paper is warning that the threat of damaging flash floods in portions of western North American river basins is on the rise and could create new flood management challenges on rivers and reservoirs upstream of hydropower projects.

Researchers at the University of Colorado Boulder and the National Center for Atmospheric Research said the risk is increasing that warmer winters and corresponding heavy rainfalls tied to climate change will melt snow that has accumulated high in mountain ranges and send destructive flash floods down nearby rivers and waterways.

The increasing threat of floods could present new reservoir management challenges on rivers that provide the water for hydropower projects, including in the Pacific Northwest. Hydropower is a key source of cheap, carbon-free renewable electricity and supplies on average more than half of the Pacific Northwest's annual energy, according to the Northwest Power and Conservation Council.

Rapid snowmelt in February 2017 led to the spillways failing at the Oroville dam in the Sacramento River basin in California, about 190,000 people evacuating, and the nearby Edward C Hyatt hydropower plant closing for weeks. A similar rain-on-snow flood in June 2013 in Alberta was the costliest natural disaster in Canadian history, according to the paper published Aug. 6 in the scientific journal Nature Climate Change.

Keith Musselman, one of the authors of the paper "Projected increases and shifts in rain-on-snow flood risk over western North America," said in an interview that many factors go into flood management, which federal agencies regulate at public dams. For instance, managers must address several competing interests, including the need to supply water to downstream interests, generate hydropower energy and spill the water in a way that preserves fish and other wildlife populations. Moreover, "the decisions are sensitive to changes in climate and in flood risk," he said.

The paper said reservoir management rules may need to be revisited more frequently as conditions and weather patterns change. For example, more space may need to be left in reservoirs to accommodate flood waters, and the timing of water availability and spring refill of the reservoirs may have to change.

Most scientists expect that flood patterns in historically snow-dominated mountainous regions will shift from spring snowmelt-driven events that provide much of the water for hydropower in the spring and summer months toward more frequent rain-dominated winter floods, the paper said. The paper takes a deeper dive into exactly what those changes could mean when it comes to rain-on-snow flooding.

Researchers found that as winter temperatures continue to rise, the risk of rain melting snowpack and creating major floods in western North American river basins increases the flood risk by as much as 200% for central and southern Sierra Nevada basins and the Colorado River headwaters.

The risk of runoff and flooding increased by 20% to as much as 100% for the Cascade Mountains, the northern Sierra Nevada, interior British Columbia and the Canadian Rockies in warmer temperatures, the paper said. The Sacramento River basin, where the Oroville dam is located, could also see up to a 20% increase in risk.

The degree of risk from rain-on-snow floods is largely determined by the amount of snow accumulation, commonly referred to as snowpack, on the mountains, which in turn is dependent on whether temperatures are cold enough to produce snow instead of rain. As a result, lower elevations on the mountains where warmer weather will prevent as much snowpack from accruing may see a decreased risk of rain-on-snow floods.

This means lower elevation basins such as along the Pacific Coast, the U.S. Southwest and some inland basins could see from little change to large declines in the average runoff from rain-on-snow melt. The paper does not delve into the impacts of rainfall on snow-free ground.