Arizona road hazard has surprising source

Sand and sand-sized clumps of silt and clay blowing across sediment fans (like in the foreground) formed by ephemeral mountain streams are the major debris sources for dust storms that cause brownout conditions along Interstate 10 (background) in Arizona. Credit: Eric McDonald. Sand and sand-sized clumps of silt and clay blowing across sediment fans (like in the foreground) formed by ephemeral mountain streams are the major debris sources for dust storms that cause brownout conditions along Interstate 10 (background) in Arizona. Credit: Eric McDonald.

Blowing dust is one of Arizona’s deadliest weather-related hazards. Between 1955 and 2011, brownout conditions created by dust storms caused more than 1,500 motor vehicle accidents across the state, resulting in 157 fatalities and more than 1,300 injuries, according to a 2016 NOAA Technical Memorandum.

Most of these casualties occurred along interstate highways in the southern half of the state, where the combination of arid climate, high speed limits and heavy traffic makes drivers more susceptible to rapid reductions in visibility caused by dust storms. The state’s deadliest corridor is a 145-kilometer-long stretch of Interstate 10 between Phoenix and Tucson, where 176 dust-related accidents took place between 1955 and 2011, including a severe multicar crash near Casa Grande in 1970 that killed 12 people.

Along another dangerous stretch of I-10 between Tucson and the New Mexico border, brownouts occur so frequently that state officials have asked landowners to stabilize denuded desert in recent years by watering cleared lands and spraying disturbed soils with a binding chemical. The success of such mitigation measures, however, depends on treating the actual sources of the dust. Abandoned farmland along the interstate is typically blamed as the main source, but there has been little research done — particularly by people with geomorphic or soil science expertise — to confirm this, says Eric McDonald, a geomorphologist at the Desert Research Institute in Reno, Nev.

When McDonald examined Google Earth images of the Casa Grande region, he realized that much of the abandoned cropland is covered with preserved furrows, many of which were plowed prior to 1994. “If abandoned cropland was really a source of the dust, those furrows should be gone” due to wind erosion, McDonald says. “There’s also a rather thick physical crust on those cropland soils that’s very hard to erode with surface winds.”

Figuring there must be other sources of Arizona’s troublesome dust, McDonald, along with Mark Sweeney, a sedimentologist at the University of South Dakota, systematically characterized the soils along I-10 near Casa Grande. After identifying the most common types of soil in the area, the researchers studied the soils’ properties and measured their potential as dust emitters using a Portable In-situ Wind Erosion Lab (PI-SWERL). This instrument — basically a scaled-down wind tunnel — allowed the researchers to quantify the wind speeds required to start moving particles across the land surface and to estimate the amounts of dust that a particular soil can emit.

McDonald and Sweeney identified three landforms that could serve as significant dust sources: abandoned croplands, fine-grained loam soils in debris fans formed by ephemeral mountain streams, and small coppice dunes that form locally around vegetation. But to their surprise, none of these exhibited high dust emissions.

Instead, the PI-SWERL measurements showed that most dust came from patches of loose, medium-to-coarse-grained sand and sand-sized clumps of silt and clay migrating across the fine-grained debris fan soils. When blown by the wind, these particles not only create dust by disturbing the surface, McDonald says, but the silt and clay clumps also break apart as they bounce along, which adds even more silt and clay to dust clouds. McDonald presented the team’s results at the annual meeting of the Geological Society of America in October in Seattle.

The team’s data indicated that high wind speeds of about 20 to 30 meters per second are necessary to mobilize these loose surface sands. These velocities overlap with the range of wind speeds reported during strong Pacific storm systems, which are most common in spring, as well as during summer haboobs, dense walls of blowing dust produced by strong thunderstorm gusts. This observation explains why Arizona’s dust-related accidents peak in April and in July, McDonald says.

The takeaway message, according to both researchers, is that the main dust source along I-10 appears to be a natural, rather than human-altered, sediment reservoir. “Everyone has been going after cropland to minimize dust emissions,” Sweeney says, but “there are these other potential sources of dust in the desert.”

It thus appears that spraying down croplands and other disturbed soils isn’t going to stop the dust storms, so the researchers say they hope their work will help transportation officials instead deploy wind-speed sensors as part of an early warning system that gives drivers enough time to slow down and safely get off the highway. “Warnings are more important than mitigation measures, which won’t be able to stop a large-scale dust storm that’s coming,” Sweeney says.
 

Terri Cook

Terri Cook

Based in Boulder, Colo., and trained as a geologist, Cook is a freelance writer whose career has focused on exploring and explaining the history of our amazing planet, including as a roving correspondent for EARTH. Follow her travels at www.down2earthscience.com. Follow her @GeoTravelTerri.

Thursday, January 25, 2018 - 06:00

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