Collision Course:
When Flash Floods Hit the Desert

100 year storm. These vehicles were swept into a wash near the intersection of Boulder Highway and Sahara Avenue in Las Vegas during the "100 year storm" flood event on July 8, 1999, so called because of the one percent chance that an event this large would occur in any year. The storm just missed qualifying as one of the top ten storms for precipitation in a 24-hour period in Las Vegas since 1937 when official record keeping began.- Photo by Jim Laurie, courtesy of the Las Vegas Review Journal.

When it rains, it pours.
There may be no better use for that old saying than describing flash flood season in the desert southwest. Usually occurring during the hot, dry summer months, flash floods are driven by monsoon systems and can be devastating to anything—or anyone—in the way. While flash floods can happen anywhere in the world, Nevada, Arizona, and Southern California are particularly susceptible. That is because of their proximity to the Gulf of California and its peculiar summer weather pattern.

In July and August, monsoon systems form as warm current flowing from southerly tropical areas heats the Gulf of California's waters. As the sea surface temperature rises, moisture lifts into the air above it. And monsoons are triggered by the smallest temperature changes in sea surface. Once the threshold—29.5º degrees C—is reached, heat- and moisture-laden thunderstorms blow inland. What happens next is the stuff of legend, and the topic of much study.

Flash flooding comes unexpectedly. As monsoon-driven thunderstorms hit parched desert areas, the bone-dry soil may not absorb several inches of rainfall in just a few hours. The resulting runoff of the peak flow follows the path of least resistance—downhill.

In large desert communities, like Las Vegas, flooding may start within an hour of the storm. That's just what happened in July 1999, when four inches of rain fell in three hours. Several areas of Las Vegas were swept with fast moving, chocolate-colored water that washed away cars, rocks, and anything else in its path. Two people were killed, many more injured, and property losses totaled millions of dollars.

Although we cannot control the forces of nature that create flash floods, we may be able to minimize their destructive capacity.

Curious about flash floods. Dr. Richard French's curiousity about flash floods developed into an international reputation for his research on the behavior of intermittent flow events. He's interested in how they are influenced by environmental factors and by engineered control efforts.

According to Dr. Richard French, who has studied flash flooding for more than 20 years, while we can't eliminate flash flooding, we may be able to mitigate it. French, a civil engineer and research professor with the Desert Research Institute, has devoted much of his professional life to studying desert flash flooding and its effects in unpopulated areas as well as fast-growing urban centers like Las Vegas.

"More than 20 years ago, we began site characterization on the Nevada Test Site's (NTS) low-level radioactive waste area," French recalls. The U.S. Department of Energy's Waste Management Division contracted French's team to determine the flooding risk at its NTS waste storage facility. "Based on available data and modeling, our hypothesis anticipated that the existing flood control dike would protect the waste site in a long-duration rain." But computer models showed the channel along the dike nearly full. That scenario did not bode well for the test site's waste containment system. Fortunately, the models proved inaccurate.

Instead of filling up and overflowing the dike during long-duration rains—those lasting 24 hours or more—the channel held only a few inches of water. The next question the researchers asked was where did the water go?

"Sixty to 70 percent of the water disappeared before it got to the downstream catch basin," explains French. "That means huge transmission losses." Transmission loss is the amount of water absorbed into the soil, or evaporated into dry air, before it arrives at a flood mitigation structure. In a long-duration rain, evaporation is not a factor because the moisture content of the air is high, so the transmission loss is attributed predominantly to absorption.

When rainfall won't stay put. DRI surface water hydrologist Julianne Miller stands next to a flume used to observe the behavior of flash flood water during an experiment at the Nevada Test Site. She's seeking to understand what geological factors influence how much water runs off across the surface, and how much will soak into the ground. - Photo by Dennis Gustafson, Bechtel Nevada

Evidence from the NTS transmission loss studies led researchers to conclude that the channel and dike protecting the waste site from flooding are more effective than once thought. Further study also shows that transmission loss happens because all desert soils are not created equal. According to Julianne Miller, a DRI research hydrologist who oversees the studies at the NTS, a few million years make a big difference in how much water runs off of a surface. Older surfaces are harder, less absorbent, and produce more runoff. Younger soil holds, and absorbs, water.

"During a first rainfall event, the older, more set surfaces - what we call desert pavement - hold fluid in the upper level just below the surface," Miller reports. ";In subsequent rainfall, the older surface sheds water in runoff."; She explains that, while geologically younger, softer soils hold water from a first rainfall like their older neighbors. During second rainfall events, however, they react differently. "We hypothesized that the younger soils seem to suck water through the moist upper layers into the soil beneath."; This is one explanation for the disappearing floodwater, and it has proven useful in developing experiments.

However, there are other uses for the ongoing study. "Our research provides a better understanding of how best to develop growing desert areas like Las Vegas," comments French. Development can cause flash flood problems downstream, because development covers the desert with impermeable surfaces—like roads, parking lots, and roofs. On impermeable surfaces, floodwaters move faster, are not absorbed through transmission loss, and are potentially more dangerous to life and property. And because Las Vegas is in a bowl-shaped valley, flash floods flow from the increasingly paved upper slopes and through low-lying, heavily populated areas of town toward Lake Mead. However, bigger flood control structures may not be the answer.

One possible result of the data from the NTS research lies in decreasing the size of flood control basins and channels. According to French, any detention basin built with federal money must be built to withstand a maximum flow event.

As it stands, Las Vegas Valley plans to spend more than $1.5 billion in flood protection structures in the next 25 years. The plan includes detention basins in the hills outside of town, smaller overflow basins-used as parks and playgrounds in dry weather — with built-in energy dissipation structures to slow the flow of floodwater, new bridges in low-lying areas, and runoff canals in new developments. Many of these structures are in place.

Yet, Miller muses over the results of the NTS research: "If we are able to show there is not as much water during a maximum rainfall event as we thought," she says, "the possibility of building smaller detention basins presents itself."
Only time and additional flash flood research results will tell.

- Lynn Taylor