CASCADING HAZARDSHurricane Helene Set Up Future Disasters, from Landslides to Flooding -- Cascading Hazards Like These Are Now Upending Risk Models

By Brian J. Yanites

Hurricane Helene lasted only a few days in September 2024, but it altered the landscape of the Southeastern U.S. in profound ways that will affect the hazards local residents face far into the future.

Mudslides buried roads and reshaped river channels. Uprooted trees left soil on hillslopes exposed to the elements. Sediment that washed into rivers changed how water flows through the landscape, leaving some areas more prone to flooding and erosion.

Helene was a powerful reminder that natural hazards don’t disappear when the skies clear – they evolve.

These transformations are part of what scientists call cascading hazards. They occur when one natural event alters the landscape in ways that lead to future hazards. A landslide triggered by a storm might clog a river, leading to downstream flooding months or years later. A wildfire can alter the soil and vegetation, setting the stage for debris flows with the next rainstorm.

I study these disasters as a geomorphologist. In a new paper in the journal Science, I and a team of scientists from 18 universities and the U.S. Geological Survey explain why hazard models – used to help communities prepare for disasters – can’t just rely on the past. Instead, they need to be nimble enough to forecast how hazards evolve in real time.

The Science Behind Cascading Hazards
Cascading hazards aren’t random. They emerge from physical processes that operate continuously across the landscape – sediment movement, weathering, erosion. Together, the atmosphere, biosphere and the earth are constantly reshaping the conditions that cause natural disasters.

For instance, earthquakes fracture rock and shake loose soil. Even if landslides don’t occur during the quake itself, the ground may be weakened, leaving it primed for failure during later rainstorms.

That’s exactly what happened after the 2008 earthquake in Sichuan Province, China, which led to a surge in debris flows long after the initial seismic event.

Earth’s surface retains a “memory” of these events. Sediment disturbed in an earthquake, wildfire or severe storm will move downslope over years or even decades, reshaping the landscape as it goes.

The 1950 Assam earthquake in India is a striking example: It triggered thousands of landslides. The sediment from these landslides gradually moved through the river system, eventually causing flooding and changing river channels in Bangladesh some 20 years later.