After two weeks of wildfires, a deluge in New Mexico caused severe flooding and debris flows near Ruidoso over the weekend.
Dark floodwaters, blackened with soot and ash from the South Fork and Salt fires, rushed from mountain canyons and into the city, turning Highway 70 into a river and pushing over a tanker, according to videos posted on social media. 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, public information officer for the Village of Ruidoso. Monsoon season usually starts around July 4, and this year, it coincided with two weeks of wildfires, greatly increasing the risk of flooding. “This will continue to happen whenever we have heavy rainfall,” Ms Gladden said.
While the South Fork and Salt wildfires killed two people and burned more than 25,000 acres last month, the burn marks they left behind could put residents at even greater risk than the fires themselves.
Climate change, driven primarily by the burning of fossil fuels, continues to cause increases in both high intensity fires which kill vegetation and dry out soils, and extreme rainfall events which deliver more rainfall in a shorter period of time. The combination of dry ground and heavy rain increases the potential for hazards such as flash floods and debris flows – a dangerous mix of water, mud, boulders and trees after a fire.
“It’s a mistake to think of flash flooding or debris flow after the event as a footnote, rather than a large part of the fire itself,” said Don Falk, a professor of natural resources and fire ecology at the University of Arizona. “It can be more destructive and cause more loss of life from fire.”
On Saturday afternoon, Brittany Smith, 34, was helping her parents back to their cabin after officials said the fires were contained. Then their phones suddenly lit up with a new emergency alert: a flood warning and urgent evacuation order.
That afternoon, a six and a half feet a wall of dark water rushed into their neighborhood in the Upper Canyon, a canyon with steep slopes. On Sunday, as the family tried to return, the village of Ruidoso started third evacuation warning: “GO now!” said the order.
Three factors increase the likelihood and risk of flooding and debris flow after a fire: how badly the land has burned, how heavy the rainfall is, and how steep the landscape is.
The canopy of trees and vegetation in the forest would normally act as a sponge, absorbing the rainfall. This is especially important during the heavy monsoons that occur in the southwest summers.
However, this sponge effect is destroyed by extremely hot fires. When the rains come, the dead soil moves quickly, which destabilizes the steep slopes.
The effect can last for years. “The fact that in recent decades the severity of fire has increased,” said Luke McGuire, associate professor of geomorphology at the University of Arizona, “that leads to an increase in these post-fire hazards.”
. ONE Map of the South Fork fire, released Monday afternoon, showed the majority of the fire had moderate to severe burn severity. ONE debris flow map released at the same time showed that much of the burned area in the South Fork fire had up to a 100 percent chance of debris flow under certain rain conditions.
“Our maps tell us that the risk of flooding and debris flow in these watersheds has increased significantly before the fires,” said Karen Miranda Gleason, the public information officer for the Burned Area emergency response team.
For the past 150 years, land management practices have minimized both natural and prescribed burning, which is the practice of intentionally setting smaller, controlled fires as a preventative measure.
TJ Clifford, head of the Interior Department’s BAER team, said the New Mexico wildfires would not have burned the land so badly if the area had been preserved using land management practices such as forest thinning or prescribed fire. But this may be unpopular.
“Prescribed fire is smoke in the air and the public doesn’t like smoke in the air,” he said. “It’s very difficult to have support.”
While flooding has already hit the area, debris flows are still a looming threat. While flooding is like pulling a silk dress through a canal, Mr. Clifford said, a debris flow, a type of landslide, is like sandpapering a canal, channeling whatever gets hit.
“Post-fire debris flows are different beasts than floods,” Dr McGuire said. They can create different problems, often affecting people and infrastructure more severely than a flood and affecting areas outside a typical flood plain.
Dr. McGuire and colleagues published one study in May in Nature Reviews Earth & Environment that showed post-debris flows were increasingly recurring. In 68 percent of global locations where debris flows had already occurred, another was likely to occur in the future.
While Ms Smith and her parents’ home has so far been spared, neighbors have not been so lucky. Charred trees line their washed-out street, but just across the street, river-rock chimneys tower over fire-levelled houses. “Our emotions are all over the place,” Ms Smith said on Sunday. “Upper Canyon looks disastrous.”
The official cause of the fire is still under investigation. The FBI is offering a $10,000 reward for any information that could lead to the arrest of those responsible for starting the fire.