After two weeks of wildfires, weekend flooding in New Mexico caused severe explosive flooding and debris flows near Ruidoso.
Footage posted to social media showed dark floodwaters, blackened by soot and ash from the South Fork and Salt fires, flowing down mountain canyons and into town, turning Highway 70 into a river and overturning fuel tankers. Homes and businesses were damaged, and emergency services reported 77 water rescues.
“It’s going to be a long road to recovery,” said Kerry Gladden, communications director for the town of Ruidoso. Monsoon season typically begins around July 4, and this year, the wildfires have been burning for two weeks, significantly increasing the risk of flooding. “This is going to continue to happen every time we get heavy rain,” Gladden said.
Last month, the South Fork and Salt fires killed two people and burned more than 25,000 acres, but the burn marks could pose a greater risk to residents than the fires themselves.
Climate change, primarily driven by fossil fuel burning, continues to cause severe fires that kill plants and dry out the soil, and an increase in extreme rainfall events that deliver more rainfall in a short period of time. The combination of dry soil and heavy rainfall increases the potential for hazards such as flash floods and debris flows, which are dangerous mixes of water, mud, rocks, and trees after a fire.
“It’s a mistake to think of a blast flood or debris flow as a footnote to the fire itself, rather than a larger part of it,” said Don Falke, a professor of natural resources and fire ecology at the University of Arizona. “They can be more destructive and cause more casualties than the fire itself.”
On Saturday afternoon, 34-year-old Brittany Smith was helping her parents return to their cabin after officials announced the wildfires were under control when suddenly a new emergency alert rang on her phone: a heavy rain warning and an evacuation order.
That afternoon, 6 1/2 feet of dark water surged through the steep-sided Upper Canyon neighborhood. As families tried to return on Sunday, the town of Ruidoso issued its third evacuation warning. “Go now!” was the order.
Three factors increase the likelihood and risk of flooding and debris flows after a fire: how badly the soil has burned, how intense the rainfall is, and how steep the terrain is.
The canopy of trees and plants on the forest floor generally acts like a sponge, trapping rainfall. This is especially important during the intense monsoons that occur in the southwestern summer.
But that sponge effect is destroyed by high-temperature fires. When it rains, the dead soil moves quickly, making steep slopes unstable.
The effects can last for years. “The fact that fire severity has increased over the past few decades is driving increased post-fire risk,” said Luke McGuire, an assistant professor of geomorphology at the University of Arizona.
. According to the South Fork Fire map released Monday afternoon, most of the fire had moderate to severe burn severities. Debris flow maps released at the same time showed that many burn areas of the South Fork Fire had up to a 100 percent debris flow potential under certain rain conditions.
“The maps show that the flood and debris flow risk in this watershed has increased significantly since before the fire,” said Karen Miranda Gleason, communications director for the Burned Area Emergency Response Team.
Over the past 150 years, land management practices have generally been directed toward minimizing natural fires and intentional fires. Intentional fires are the practice of intentionally setting small, controlled fires as a preventive measure.
TJ Clifford, a team leader with the Interior Department’s BAER, said the New Mexico fires might not have burned the soil as badly if land management practices, such as thinning forests or directing fires, had been used to maintain the area. But that might not be popular.
“Prescribed fire is smoke in the air, and the public doesn’t like smoke in the air,” he said. “It’s very hard to get support.”
While floodwaters have already hit the area, debris flows are still an imminent threat. Clifford said floodwaters are like pulling a silk robe through a channel, while debris flows, a type of landslide, are like rubbing sandpaper along a channel, washing away everything it hits.
“Debris runoff from fires is a different beast than flooding,” Dr. McGuire said. It can cause a variety of problems, often having a more severe impact on people and infrastructure than flooding, and hitting areas outside the normal floodplain.
Dr. McGuire and his colleagues published a study in Nature Reviews Earth & Environment in May showing that debris flows are becoming more frequent: 68% of the areas around the world that have already experienced a debris flow are at risk of another one in the future.
Smith and her parents' home have so far escaped damage, but their neighbors have not been so lucky. Burning trees line the washed-out driveway, while across the street a chimney made of river rock towers over a home that collapsed in the fire. “We're in a state of total chaos,” Smith said Sunday. “The Upper Canyon looks like it's been devastated.”
The official cause of the fire is still under investigation. The FBI is offering a $10,000 reward for information leading to the arrest of the person who started the fire.