2020 Issue

30 Wildfires and Civil Engineering Ben Willardson, PE, Ph.D. — CWE and Mike Rau — CUWCD F ire plays an important role in most wildland ecosys- tems. Vegetation often depends on fire to create a period of rebirth by removing dead materials and re- leasing nutrients back into the environment (Ainsworth and Doss, 1995). Across the United States, wildfires burn more than 4 million acres annually, costing Federal agencies above $768 million a year (1994-2002) in suppression alone (Butry et al., 2008). Some of the most well-known fires have burned large sections of famous national parks such as Yellowstone and Yosemite. Utah was impacted by several fires in 2018, including the Dollar Ridge, Bear Trap, Coal Hollow, Pole Creek, Notch, and Willow Creek Fires. Estimates of the burned watershed areas are over 200,000 acres, or over 300 square miles of forest lands. These fires have impacted many watersheds that provide water supplies for cities and towns throughout Utah. These fires set the stage for impacts to highways, drinking water systems, and dwellings. The impacts to the Duchesne Valley Water Treatment Plant operated by the Central Utah Water Conservancy District is provided as an example of the impacts faced by civil engineers after a fire. See Figure 1. Figure 1 - Dollar Ridge Fire the annual acreage consumed by wildfires in the lower 48 states dropped from 40 to 50 million acres a year (Laverty, 2001). Across the Western United States, the aggressive fire-suppression policies appeared to be successful. How - ever, these policies have set the stage for the intense fires experienced over the last few decades. Many fires are caused by lightning. Others are human-made. Full fire suppression allowed forests and wildlands to grow without the effects of fire, disrupting ecological cycles and changing the structure and makeup of the forests (Laverty, 2001; Pierson, Jr. et al., 2003). Other vegetation that had been regularly eliminated from forests by periodic, low-in- tensity fires, became a dominant part of the forest. This vegetation became susceptible to insects and disease, which left dead trees, mixed brush, and downed material to fill the forest floor. The accumulation of materials, when dried by extended periods of drought, creates the fuels that allow extremely large fires to burn across large areas of forest and wildland (Laverty, 2001). Changes to Vegetation and Soils During Fires Fire in forested areas is an important natural disturbance mechanism that plays a role of variable significance depending on climate, fire frequency, and geomorphic conditions. The role of fire is particularly important in regions where frequent fires, steep terrain, vegetation, and post-fire seasonal precipitation interact to produce dramatic impacts (USDA, 2005). The amount of vegeta- tion consumed by a fire depends on the fire regime and fire severity (USDA, 2005). The USDA (2005) provides an in-depth discussion of fire regimes and severities. Low- severity fires rarely produce adverse effects on watershed hydrologic conditions, while high-severity fires generally result in higher runoff and erosion. Wildfires can leave large areas devoid of vegetation and vulnerable to producing large volumes of runoff, leading to flash floods, floods, or mudslides (NOAA, 2004). The high rate of runoff following brush fires may result from the combined effects of denudation and formation of a water-repellent soil layer beneath the ground surface (Nas- seri, 1988). The type of vegetative cover on soil changes the infiltration rates. The change is due to the effects of Between the 1930s and 1970s, firefighting tactics and equipment became increasingly more sophisticated, effec- tive fire suppression efforts increased dramatically, and