Once the flames have been extinguished, forest fires have a second opportunity to devastate landscapes through flooding and water pollution.
This week, large areas of Southern California were again subject to evacuation orders. Some residents in Ventura, Los Angeles, and Santa Barbara counties that were recently evacuated due to the Thomas, La Tuna, and Creek fires were again asked to leave…but this time because of the potential for flooding and mudslides.
That’s because land that has been burned has much higher rates of stormwater runoff when it rains. This is due in part to the fact that soils that have been subject to fire are typically less able to absorb rainfall.
Last year’s flash flooding in Southern California had catastrophic consequences, resulting in the loss of life, property damage, large power outages, and freeway and rail closures. Yesterday’s storm has similarly resulted in the loss of life and property damage, as mudflows in fire-ravaged areas trapped people, caused traffic accidents, and swept houses from their foundations.
On top of all this, runoff in post-fire areas is likely to carry higher amounts of pollution. It pollutes waterbodies and can have disastrous effects for watersheds and drinking water supplies.
Post-fire runoff occurs during subsequent rain or snow events, and picks up pollutants on its way into local waterways. This increased pollutant loading is often obvious, with people describing rivers and streams in post-fire landscapes as looking like “chocolate milk.” While sediments and nutrients are primary pollutant concerns, post-fire runoff can also contain other harmful pollutants such as carbon, organic compounds like PAHs, trace metals, and the very chemicals used to suppress the fires. These effects are often compounded by the increased runoff that occurs after a fire. While increased pollutant loads may be short-lived, with metals and PAH levels typically returning to pre-fire levels within a year, the concentration and mass of pollutants released during these events is often 10 times greater than pre-fire levels.
In California, this is particularly troublesome because sediment is already a major pollutant found in the state’s waterways, and it often ends up trapped in reservoirs, thereby reducing their overall capacity.
And California is not alone.
Albuquerque, New Mexico has already had difficulty with its water allocations due in part to ash-laden water from increased forest fire activity.
Green infrastructure for stormwater management can help alleviate the problems found in post-fire landscapes by reducing stormwater runoff and protecting floodplains. Green infrastructure that enhances infiltration can help protect communities from flooding that occurs after a fire. And infrastructure such as vegetated stream buffers and constructed wetlands have been proven to reduce pollutants such as metals, sediment, and nitrogen. When planned properly, these multi-benefit green infrastructure projects can also act as emergency migration corridors, serving as an oasis for displaced creatures fleeing fire-ravaged landscapes, and contribute to biodiversity.
In fact, green infrastructure may be better suited for reducing the impacts of stormwater-related flooding compared to traditional, “gray” infrastructure.
For example, 30 days after the Waldo Canyon fire, two inches of rain caused destructive floods, with several clogged drainages resulting in the scouring of roads and culverts that were overwhelmed with runoff, rendering them ineffective. In these instances, well-designed green infrastructure can be more effective for managing stormwater runoff, and the need for such infrastructure will only grow as climate change increases the frequency of both forest fires and heavy rainfall events. Unfortunately, Southern California can expect to experience such disasters more frequently; six of the ten U.S. cities with the highest disaster risk are found in this area.
While research on stormwater issues and solutions in post-fire landscapes is still in its nascent phase, it is clear that green infrastructure can play a vital role in the prevention and mitigation of post-fire flooding, mudslides, and pollutant loading of waterways caused by stormwater.