Results show a need to revise existing methods for calculating flood risk.
Flood frequency analysis is a technique used to estimate flood risk, providing statistics such as “100-year flood” and “500-year flood” forecasts that are critical to infrastructure design, dam safety analysis and flood mapping in flood-prone areas. But the method used to calculate these flood frequencies is due for an update, according to a new study by scientists at the Desert Research Institute, University of Wisconsin-Madison and Colorado State University.
Floods, even in a single watershed, are known to be caused by a variety of sources, including rainfall, snowmelt or rain-on-snow events in which rain falls on the existing snowpack. However, flood frequencies have traditionally been estimated under the assumption that these flood “drivers”, or root causes, were unimportant.
In a US National Science Foundation (NSF)-support research published in Geophysical Research Letters, a team led by Yu Guo of the Desert Research Institute examined the most common drivers – rainfall, snowmelt and rain-on-snow events – of historic floods for 308 watersheds in the western US and investigated the impact of different flood types on the resulting flood frequencies.
Laura Lautz, programme director of NSF’s division of earth sciences, noted: “This study shows that rainfall and rain-on-snow events have greater potential to cause rare but very large floods than current 100-year flood frequency forecasts predict. As we’ve been seeing in Yellowstone, where heavy rains and snowmelt are driving floods far larger than any observed in the historical record, these events can cause widespread damage and devastation.”
The findings showed that 64% of the watersheds frequently experienced two or three flood types throughout the study period and that rainfall-driven floods, including rain-on-snow, tended to be substantially larger than snowmelt floods across watershed sizes.
Further analysis showed that by neglecting the roles of each flood type, conventional methods for generating flood frequency estimates tended to result in underestimation of flood frequency at more than half of the sites, especially at the 100-year flood level and beyond.
“In practice, the role of different mechanisms has often been ignored in deriving the flood frequencies,” said Yu. “We showed that neglecting such information can result in uncertainties in estimated flood frequencies which are critical for infrastructure.”
The findings have implications for estimating flood frequencies into the future, as climate change pushes conditions in snowmelt-dominated watersheds towards increased rainfall.
Daniel Wright, assistant professor of civil and environmental engineering at the University of Wisconsin-Madison, concluded: “How the 100-year flood will evolve due to climate change is one of the most important unanswered questions in water resources management. We need to focus on the fundamental science of how the water cycle, including extreme rainstorms and snow dynamics, are and will continue to change in a warming climate.”