Flash Floods and Debris Flows: Not If, But When
Incident: Carpenter 1 Wildfire
Just like wildfire, heavy rains are inevitable in the Spring Mountains. Even without fire, the drenching rains of monsoon season often cause flooding in southern Nevada. Burned soils of the Carpenter 1 fire should increase flooding for two reasons. First, severely burned dirt becomes “hydrophobic.” It gets a glasslike sheen and sheds water rather than absorbing it. Second, the physical effect of fire tends to remove vegetation that holds soil in place, and even loosens rocks and boulders on slopes as the soil bakes.
In this area, flooding after a fire tends to bring black and muddy water rushing downstream. Ash floats upon water until it soaks through. Vast quantities of ash may sweep off hillsides and come to rest several feet deep in creek bottoms where deposited by moving water.
The Spring Mountains canyon bottoms are shaped by water. A geologist or hydrologist looking in Kyle and Trout Canyons recognizes that the valley bottom shows “water features” made by moving water, at several scales. Moving water has carved both the dry creek beds and soil ledges many feet higher in the valleys. About an inch of rain above Harris Canyon on July 11, 2013, made backcountry roads impassable, but also created a high-water mark more than six feet high in one canyon bottleneck.
If we get storms this weekend or later, Forest Service analysts say that flooding could bring debris flows out of any canyon that receives rain. Quick floods may easily bring mud onto the Trout, Lovell, or Kyle Canyon roads, down Harris Canyon to U.S. 95, or beyond toward Las Vegas. Nevada Department of Transportation has staged graders, loaders and other heavy equipment to respond to the inevitable dark floods that follow rain on a burned area.
Both flash floods and debris flows often occur far downstream from any storm or clouds. Water appears in a creek without warning, without rain.
Do not enter flooded areas in your vehicle or afoot. One foot of water can sweep a car or truck off the road. National Weather Service advice when issuing any flood advisory is “Turn around. Don’t drown.” Because floods begin and end quickly, use patience in postponing travel during a flood warning.
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