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22 Aug 2016
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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.
This pattern of flux between hot and dry and severe storms often results in severe wildfires followed by flash flooding. Hillside fires scorch the ground, destroy vegetation, and leave nothing behind to absorb rainfall and slow floodwaters. And with the type of fire damage we frequently see in Arizona, flooding probability increases due to hydrophobic (water-repellent) soil, which is a temporary by-product of its exposure to high temperatures. The soil also tends to slide downhill because there is little vegetation to hold it in place, contributing to mudslides.
The combination of these weather patterns often leaves little time after a fire disaster to prepare for potential flooding emergencies in nearby communities.
After the Doce and Yarnell Hill fires in 2013, which tragically claimed the lives of 19 firefighters, Atkins was mobilized to provide post-burn floodplain mapping and risk identification for Yavapai County. Within 36 hours of the fire’s containment, we developed models using timesaving GIS-based automation in conjunction with hydrologic and hydraulic software to develop 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 the 2013 event. We once again used our models to predict potential increases in flooding risk and worked with the county to develop strategies that would minimize their risk.
The county was able to immediately mobilize and construct infrastructure to help mitigate post burn flooding. This included upstream collection channels and sediment collection basins. On August 2, 2016, the area that burned only a few weeks earlier near Yarnell received over two inches of rain in less than one hour. Thankfully, damage was limited due to the quick work of the county. Post-storm estimates indicated that a 135-acre drainage area produced an estimated 1,200 cubic yards of sediment in one hour.
Sediment accumulation one month after wildfire (August 2016). In only one hour, more than six feet of sediment accumulated in a preventative sediment collection basin. (Photo courtesy of Yavapai County)
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 modeling process, and lead times to obtain topography may be several months. We were able to respond quickly in Yavapai County due to the extensive amount of topographic information readily available for our use. Additionally, the creation of existing condition (pre-burn) models allow for quick characterization of 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. There are even programs that can help fund pro-active data collection.
Providing post-disaster services has been a fast-paced, rewarding aspect of my career as a water resources engineer. My experience in Yavapai County allowed me to directly provide information that helped a community recover and avoid additional destruction. And while many times it can be difficult to witness the devastation that Mother Nature is capable of delivering, it helps to know that we can work together to minimize damage through foresight and resilient engineering practices.
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