As wildfires wreak havoc through the western states – California and Colorado, in particular – residents are hoping for rain to help limit the immediate threat of fire damage. Unfortunately, the damages from wildfires can continue long after the flames stop burning. In addition to the destruction of man-made facilities and private property, including trails, campsites and even homes, wildfires can result in significant increases in runoff and erosion, which can negatively impact downstream users.
Fires can completely change the functionality of a watershed, leading to increased stormwater runoff, large amounts of sediment and debris causing erosion and strain on existing water quality control systems, and flooding that can cause extensive damages to infrastructure.
Wildfires create increased stormwater runoff in several different ways by transforming the natural landscape. Fires are generally more prevalent in rural forested areas that have a dense tree population to fuel the flames. Severe long-lasting fires can heat the temperature of the soil up to 2,200 degrees Fahrenheit, which can cause soils to condense and form a water-repellent layer. This reduces the ground saturation and the watershed’s ability to retain water and dispel it through plant evapotranspiration, creating increased stormwater runoff.
The loss of vegetation also decreases surface roughness, causing the elevated levels of stormwater to reach collection points quicker, resulting in higher runoff rates during a post-wildfire storm event. Vegetation also helps hold the soil intact, so as the vegetation is destroyed there is an increase in the amount of sediment and debris that is transported downstream. These increases are magnified in areas with steep terrain, as found in the Rocky Mountain Region.
Post-wildfire flooding damage can be as minor as flooded parking lots, but it can also lead to significant damage to property, infrastructure and can even put lives in danger. Stormwater collection and conveyance systems such as reservoirs, ponds, channels, culverts, bridges, inlets and pipes were established to handle the pre-fire stormwater runoff. With the additional runoff associated with post-fire conditions, stormwater facilities now become undersized and function less efficiently.
With strained facilities, stormwater begins to overtop ponds and channel banks, and overwhelm reservoirs, ponds, swales, pipes and other infrastructure. Additionally, due to the increased amount of sediment and debris, the stormwater causes erosion in some areas, degrades water quality, and affects flood locations that were previously unaffected by storm events – including residential areas. Downstream flooding can also cause sewer line backups, disrupt water purification and pick up ground-level contaminants (such as chemicals from cars and lawns) that were previously stored above the water line.
While stormwater systems are designed to withstand major flooding events, wildfires can cause such an excessive increase in stormwater runoff that they still overwhelm the systems in place. While changing the design to accommodate such excess would be largely infeasible and costly, there are measures that we can take to help limit the impacts once the fire has burned out.
Wildfires can affect an entire community’s water resources. Stormwater engineers can help community officials and the general public understand the impacts of wildfires that can be expected and help identify potential problem areas, such as areas that can expect increased flooding and decreased water quality. Engineers can do this by providing articles, webinars, and presentations to help inform the public, and by working closely with government agencies and municipalities on how to mitigate effects for cities and counties as a whole.
The United States Department of Agriculture (USDA) Burn Area Emergency Response (BAER) program provides high-level assessments of the affected burn areas and recommends emergency treatments following wildfires. There are state and federal funding sources that can help support the cost of more in-depth water resource engineering analyses. The in-depth engineering analyses can help communities identify problem areas, suggest mitigation techniques, and assist with prioritization of the recommended improvements. Proper prioritization will help ensure the efficient use of damage mitigation and repair funding, leading to a response that limits long-lasting effects effectively.
Improvements to the downstream drainage system can be designed and implemented to help mitigate potential flood and sediment damage. These improvements can include additional flood storage, sediment traps, channel regrading and armoring, check dams, culvert and pipe replacement, and a variety of other methods to protect the watershed and the community. This can also tie into the burn area analysis, as taking a close look at the post-fire hydrologic recovery of the watershed can influence the design of future measures that can control excessive flows of stormwater and debris.
Though wildfires can have a significant impact on the watershed, not all of these changes are permanent. The watershed is at its most vulnerable immediately following the fire, making it important to take action early on erosion and sediment control measures. However, as time goes on, the vegetation begins to rebound, soils begin to return to their pre-fire condition and the watershed slowly transforms back to its natural state – although some areas will face irreversible changes due to the severity of the burn. The amount of time it takes to return to a pre-fire state varies based on the location, rainfall, and severity of the fire, but eventually the watershed will return to a healthier state.
Please reach out to the Galloway Water Resources Team if you have any questions or would like any additional information. Our team has experience evaluating post-fire hydrologic and hydraulic conditions, and designing improvements to protect both the watershed and public from abnormal conditions resulting from wildfires.
Click HERE to visit our contact form, or you can email Galloway Sr. Water Resources Project Engineer Adam Lacey directly.