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Volume 32, Issue 4

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Tuesday, 24 January 2017 21:54

Fires Lead to Floods: Lessons Learned in the West

Written by  LINDA POTTER

This summer, fires raged through the West: In California, more than 4,000 firefighters were stretched to their limits battling nine wildfires, calling in assistance from some 650 firefighters from Arizona, and tapping additional resources from New Mexico.

Fires Create Optimal Conditions for Future Floods
One thing that most people haven’t thought about yet is the pending likelihood of additional damage from flooding. It turns out damage from fires leaves afflicted areas more vulnerable to the possibility of flash flooding than they were before. Hillside fires scorch the ground, destroy vegetation, and leave nothing behind to absorb rainfall, slow floodwaters, or stop soil from sliding downhill in mudslides. Soil seared by fires also becomes more water repellant, and thus less likely to absorb rainfall.

The nature of the seasons increases the effect. In Arizona, the beginning of the summer is hot and dry, but come mid-to late-summer, the weather patterns change as tropical moisture moves in from the south. This is called the monsoon season, which typically lasts from June until September. It subjects the state to impressive amounts of lightning, relentless downpours, damaging winds, and towering walls of dust. In fact, many parts of the state receive about half their annual rainfall during these few months.

Weather patterns combine with destroyed vegetation and water repellent soil to create a perfect storm for flood damage, that usually leaves nearby communities little time after a fire disaster to prepare for potential flooding emergencies.

The good news is, smart planning can help state authorities mobilize and construct infrastructure to help mitigate post-burn flooding. Using data about topography, sediment accumulation and nearby flood control structures, experts can develop predictive models for Arizona communities that forecast likely post-burn flood inundation areas as well as probable water depth and velocity. Forewarned, Arizona planners can mobilize and construct infrastructure to help mitigate post-burn flooding.

After the Doce and Yarnell Hill fires in 2013, which tragically claimed the lives of 19 firefighters, local communities worked to provide post-burn floodplain mapping and risk identification for Yavapai County. Within 36 hours of the fire’s containment, models were developed using timesaving GIS-based automation in conjunction with hydrologic and hydraulic software to present predictive, post-burn flood inundation areas, depths, water surface elevations, and velocities. These models provided flood inundation limits to help the community get a clearer picture of what they may face in the future while the burn areas recovered.

Fires again erupted in Yavapai County in June of 2016. Fortunately, they were contained before reaching nearby urban areas, resulting in fewer damages than 2013. The county again turned to experts to use models to predict potential increases in flooding and develop strategies to minimize that risk. The counties then built infrastructure to block or reroute water to mitigate post-burn flooding.

Increasing Community Resilience
Lessons learned in Arizona from these recent wildfires can be applied anywhere. Communities can increase their resilience against post-burn flooding and other types of disasters by collecting data ahead of time, before the disaster occurs. For example, topography is an essential input into the modelling process, and lead times to obtain topography may be several months. Yavapai County could respond quickly because it had proactively committed to gathering data in advance, and so when the flooding appeared, it already had an extensive amount of topographic information readily available for our use. Moreover, it had worked to create a variety of existing condition (pre-burn) models, which allowed it to quickly characterize changed conditions.

Increased flood risk occurs for many reasons besides wildfires. For example, if an upstream flood control structure is compromised, it will be critical to know the downstream areas that are most at risk in a very short amount of time. Some counties that have been historically threatened by fires and floods are following Yavapai County’s good example by: choosing to be forward-looking, putting in-place programs that help fund proactive data collection, and providing post-disaster services. These items help communities recover and avoid additional destruction.

Potter LindaLinda Potter is a senior project manager for Atkins in North America with more than 20 years of experience in floodplain management, hydrology, hydraulics, grading, and drainage. Her experience in Arizona includes more than 100 design concept projects, floodplain projects resulting in flood insurance rate map revisions, and floodplain infrastructure design projects. She is an Association of State Floodplain Managers (ASFPM) certified floodplain manager, and is currently on the board of directors for the Arizona Floodplain Management Association (AFMA).