WASHINGTON – The intensity of the Monument Fire and the steeply sloped area where it occurred make mudslides there very likely during Arizona’s monsoon season, according to the U.S. Geological Survey.
In a report released Thursday, the USGS said 1 to 1.4 inches of rain in a half-hour would cause at least an 80 percent probability of mudslides in 13 of the 18 drainage basins studied in the burn area. During any given year, there’s a 50 percent chance a storm will bring that much rain.
The USGS report came as the Small Business Administration announced it would include damage from mudslides and monsoonal flooding in its disaster declaration for the area, making loans available to help businesses and individuals repair or replace damaged property.
USGS ran computer models of three differently sized storms and their impacts on thousands of acres scorched by the Monument Fire. More-intense storms increased the likelihood of mudslides in more areas: Storms dropping 1.3 to 1.8 inches of rain in 30 minutes raised the number of basins with an 80 percent probability of mudslides to 16; it rose to 17 basins for rainfall of 1.5 to 2 inches in half an hour.
The computer models also predicted the volume of the mudslides that might result from such storms, varying from enough to fill an Olympic-size swimming pool to enough to fill 86 of them. But just the volume of the floods — which are the consistency of a wet concrete slurry — doesn’t reflect how dangerous they are.
“It doesn’t take much debris for it to cause a lot of destruction,” said Barbara Ruddy, a USGS hydrologist and geographic information systems specialist and the report’s lead author.
National Weather Service data shows that Sierra Vista — a town close to the burn area — averaged 8.2 inches of rain over the last 21 monsoon seasons. The most rain in that time period was 13.2 inches in 2006, and last year monsoons dropped 11.1 inches of rain.
Of course, weather doesn’t always heed computer models.
“What the model is predicting has already happened,” said Ruddy, referring to a mudslide in Hereford, Ariz., that happened while she was working on the report.
A rain gauge near Beatty’s Guest Ranch recorded 1.64 inches of rain in a little more than an hour the afternoon of July 10. During the most intense part of the storm, 0.72 inches of rain fell in 10 minutes, almost three times as much geologists have determined will cause mudslides, according to a report by Ann Youberg, a research geologist with the Arizona Geological Survey.
Ranch owner Tom Beatty Sr. said flows of debris up to 200 yards wide crossed his property in that storm. The mudslides filled in a quarter-acre pond, pushed in two walls of a 100-year-old cabin, and dumped nearly six feet of sand in another rental property.
They also ruined Beatty’s parking area and blocked roads. The damage ended the ranch’s operating season early.
“People have been canceling because they need to know when they can get out of here, and I can’t tell them,” Beatty said.
Beatty said he has no plans to apply for the government loans that are available. He said he will rely on a home equity loan and help from volunteers and friends to start cleaning up, but he may wait a year to do so.
In the meantime, state geologists are taking the opportunity to study what can happen after intense fires and improve their prediction models.
“We haven’t had fires like this for several years, so this is a good chance,” said Youberg.