Updated September 2021
As wildfires wreak havoc across the western states of Oregon, California, and Colorado this summer, residents are hoping for rain to help limit the immediate threat of fire damage. Unfortunately, additional damages from wildfires can continue long after the flames stop burning. In addition to burning of structures and property including trails, campsites, and homes, wildfires can cause significant increases in runoff and erosion, which can also impact downstream users and facilities, including those located outside the burn area. Fires change the functionality of a watershed, leading to increased stormwater runoff and large amounts of sediment/debris causing erosion, water quality issues, and flooding.
Wildfires transform the natural landscape creating increased stormwater runoff in several different ways. The most intense and transforming wildfires generally occur in old growth forested areas that have a dense tree population to fuel the flames. Severe long-lasting fires can heat the soil to temperatures in excess of 2,000 degrees Fahrenheit. This intense heat can cause soils and the overlying leaves, needles, and sap to condense and form a water-repellent (hydrophobic) layer on or near the ground surface. This impermeable layer reduces water infiltration and the soil’s ability to retain water and dispel it through plant evapotranspiration.
Due to canopy and surface vegetation loss after a fire, more rainfall reaches the ground without being trapped. Unimpeded rainfall hits the ground harder, resulting in impact erosion and increased removal of topsoil that is vital for post fire rehabilitation. Hydrophilic soils and erosion tend to decrease the surface roughness, causing the elevated amounts of surface runoff to reach collection points quicker, resulting in higher discharge rates in streams during post-wildfire storm events. The loss of vegetation along streams can lead to channel and bank erosion, increasing the amount of sediment and debris transported downstream. These increases are magnified in areas with steep terrain such as the western United States and specifically the Rocky Mountain Region.
Post-wildfire flooding can lead to significant property damage, infrastructure damage, and the endangerment of lives. Post-wildfire flooding can be as minor as inundating parking lots or as major as closing primary transportation infrastructure such as I-70 in Glenwood Canyon, Colorado, during most of the summer of 2021. Stormwater collection and conveyance systems such as streams, reservoirs, ponds, channels, culverts, bridges, inlets, and pipes were typically designed to handle the pre-fire stormwater runoff. With the additional runoff associated with post-fire conditions, stormwater facilities can become undersized and function less efficiently.
With strained facilities, stormwater begins to overtop ponds and stream banks, promoting damage to roads, crossing facilities, and property. 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.
Additionally, due to the increased amount of sediment and debris in the water, post-wildfire flooding can cause erosion in some areas and sediment deposition in other areas, resulting in flooding of locations that were previously unaffected by large flood events. Large debris can get caught and jammed in the stream creating debris dams. When these debris dams break during a flood event, downstream inhabitants can experience a “wall” of water and debris in the flood flows. Most concerning is the potential life and death situation created for unaware downstream inhabitants. Often located outside of the burn area, they can be suddenly impacted by large amounts of water and debris. These situations are difficult to predict but have the potential to occur over the next few years in and near the post-fire burn areas in the western United States.
There are many government and private sector efforts and programs in place to assist with post disaster recovery. Each disaster presents specific challenges and solutions.
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 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 efficient use of damage mitigation and repair funding, leading to a response that effectively limits and minimizes the potential for short-term disasters and long-lasting impacts.
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, debris basins, 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 stormwater and debris.
Wildfires can affect an entire community’s water resources. Water resource engineers can help the community understand potential wildfire impacts such as areas that can expect increased flooding and decreased water quality. This can be accomplished by providing articles, webinars, and presentations to help inform the public and by working closely with government agencies on mitigation solutions to protect public and private infrastructure and property. The importance of post-fire inspections and assessments to identify hazardous conditions cannot be emphasized enough during education opportunities.
Though wildfires can have an immediate impact on the watershed, not all these changes are permanent. The watershed is at its most vulnerable immediately following the fire, making it important to act early on erosion and sediment control measures. 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. However, due to the severity of some burn areas, irreversible changes will need assistance to return to a closer natural state. The amount of time it takes to return to a pre-fire state varies based on location, rainfall, and severity of the fire, but eventually the watershed will return to its 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 assessing post disaster conditions and damage potential. We can evaluate hydrologic and hydraulic conditions and design improvements to protect watersheds and the public from abnormal conditions resulting from disasters, including wildfires.
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