In the wake of the Castle Rock Fire, dire predictions about the potential for serious flooding are causing local government officials to sit up and take notice.
Of grave concern, at least in the next few years, is the chance that slopes burned by the fire may release a torrent of floodwaters during heavy rains.
Immediately at risk from such catastrophic flooding would be people and property in the Warm Springs Creek and Greenhorn Gulch drainages, U.S. Forest Service officials told the Blaine County Commission on Tuesday morning. The officials are members of a Burned Area Emergency Response (BAER) team working to assess risk and the need for post-fire recovery actions in the burn area.
Charles Luce, part of the interagency team, said the Forest Service only has the authority to carry out recovery actions on federal ground.
"There are problems that happen downstream that we are not authorized to work on," he said.
A rainstorm that passed over the Warm Springs Creek drainage on Sept. 5 provided an excellent example of what can happen when a quick burst of rain is dumped on burned hillsides denuded of vegetation. The short storm, which measuring stations indicated dropped less than a half-inch of rain, caused significant sheet flooding accompanied by a series of moderate landslides comprised of dirt, rock and woody debris.
The landslides ran over Warm Springs Road and in several places into Warm Springs Creek.
Quite tellingly, Luce said, National Weather Service officials consider the Sept. 5 storm a two-year rain event, meaning such squalls happen on average every two years. He said a much rarer rain event—such as a 25-year or even a 100-year episode—could be disastrous for Warm Springs and Greenhorn Gulch homes in the path of a torrent of floodwater and debris.
Fortunately, the chance for severe flooding in the fire area should noticeably decline in as little as a few years, Luce said.
Until then, local residents need to be aware of the risk, Commissioner Sarah Michael said.
"There are a lot of people who are vulnerable," Michael said.
Beyond the loss of vegetation on burned hillsides, another cause for concern is the fact that some soils in the burn area have become hydrophobic, meaning they can't absorb water.
Luce said that in places where wildfire intensity is severe, soils can lose most of their ability to absorb water. In such cases, water won't enter the soil and beads up like moisture on Gore-Tex. He said officials are analyzing the levels of water repellency that exist across the landscape, which
depend on the severity of the local burning.
He said where the intensity is considered severe, water repellency of the soil can range from 70 percent to 100 percent, with the latter meaning no ability to absorb water.
To gauge the potential for high water repellency, hydrologists often look at how intensely burned a forest is, Luce said. If all of the trees in a forest have lost their needles and the forest floor is a barren expanse of ash, then chances are the soil there will be highly water repellent, he said.
Luce said that as water repellency decreases so, too, does the danger from more normal rainstorms.
"Once you get down to 50 percent it takes a very rare, very severe rain event to cause serious flooding," he said.
As part of their work, the BAER team will consider ways to lessen the flood potential. Among the actions they can take are emergency stabilization measures like placing perpendicular log barriers, straw and other water-control checks on burned slopes. But Luce said the fire area is too large for flood-control measures to eliminate or even greatly reduce the danger.
"With anything bigger than a two- to three-year event, those things aren't going to do much," he said.
Forest Service and local government officials have apparently decided that a better use of financial resources would be establish early-warning systems for the problem areas.
To that end, the BAER team is working with the National Weather Service to install rain gauges that will provide a warning if floods are predicted. Officials say the gauges could be in place next week.
Beyond the establishment of an early-warning system, BAER team leader John Chatel said that people need to look out for themselves. He said homes in burned areas that are below a drainage or gully should be considered more at risk from flooding.
"We can't mitigate all of the hazards out there," Chatel said.
