Monday, July 26, 1999 | 11:26 a.m.
Many residents caught by surprise in the July 8 flood never thought they would see that much water in the sunny desert of Southern Nevada.
Seasoned desert dwellers, however, know that the floods and the mud are a way of life from July through September as hot, moist air streams north from Mexico, bringing with it what weather forecasters call monsoons.
And Las Vegans can expect more storm water in the valley capable of wreaking damage despite local attempts at flood control, weather experts say.
The Regional Flood Control District began an ambitious plan in 1989 to make the valley flood-resistant within 40 years. At the time the memory of a series summer floods in 1984 that turned streets into streams for almost six weeks was still fresh.
The 1987 Legislature approved a quarter-cent sales tax to fund the flood control projects.
The plan the regional agency came up with has two forces working against it. First growth in the Las Vegas Valley and the paving for parking and homes that goes with it work at cross-purposes with the flood-control plan. Second the plan itself works against nature by sending water in straight lines rather than its natural meandering path, damaging marshes that protect the valley's water quality.
Danger of growth
The growth in the valley, say federal scientists who have been studying the area for more than 40 years, could turn formerly uneventful rains into deadly flash floods.
About 20 flood control projects have been completed since 1989 at a cost of $300 million. They were a big help earlier this month.
"If we hadn't had the existing detention basins, we probably would have had a lot more flooding" on July 8, said U.S. Geological Survey hydrologist Richard Kane, who is in charge of measuring water flow in the Las Vegas Wash.
But the flood once fully measured was only a 15- to 20-year event, not a 100-year storm as originally reported. A 100-year storm has a 1 percent chance of happening in any given year.
"This was a major storm, a major flood no doubt," said retired U.S. Geological Survey scientist Pat Glancy, who was brought back to the USGS after the flood to help with measurements. But he added, "I'm not sure we've had the big one yet."
The effect of growth and paved surfaces on the flooding was clear, Glancy said. Peak flows from several monitors in the wash showed that what used to take 24 hours to run off the valley traveled from the western part to the eastern part of the valley in six hours or less.
Ironically the Regional Flood Control projects contribute to the speed of the runoff, because many flood channels are lined with concrete, allowing the runoff to gain momentum across the valley, he said.
The swifter runoff has helped change the Las Vegas Wash from a creek to something resembling a small canyon, USGS hydrologist Jon Wilson said.
Development in the northwestern valley has cemented the desert's surface so floodwaters gain momentum as they travel east to the wash, he said.
This cementing causes flows to run faster, heavier and higher, Wilson said. Flooding then cuts wider channel banks and moves more sediment into the wash.
"A 10-year storm can look like a 100-year storm today because the valley changes dramatically as we add more roads and more parking lots," Wilson said.
The growth is having another unintended effect on the valley's flood control. The system to control floodwaters is being built backward to protect the fast-growing west side of the valley, Col. John Carrol of the Army Corps of Engineers said.
Instead of starting downstream in the eastern part of the valley, as engineering standards would dictate, the corps has been moving dirt along the upper Flamingo and Tropicana washes on the western side to protect new homes and highway development, Carrol said. Some of that work was damaged in the recent storm, he said.
The corps' projects are "roughly half a step ahead" of current development approved by the county, Carrol said.
"It's an issue of finishing the system," he said. "When we get done with the whole system, it will prevent what happened."
Working against nature
The county's projects may protect homeowners, roads and other public structures but critics say they will only do more harm to the Las Vegas Wash, and eventually to the valley's drinking water.
Flooding has decimated the wash's wetlands in the past two decades. Those wetlands act as a biological cleanser for water flowing from the Las Vegas Valley into Lake Mead, the valley's primary source of drinking water.
Before 1984 2,000 acres of wetlands snaked across eastern Las Vegas Valley. After the 1984 flood, barely 200 acres of marshland survived, washed downstream by muddy water raging through the wash.
Norma Cox, who has served on committees to protect the Las Vegas Wash for almost 30 years, has urged officials to protect the wetlands that act as the final funnel for runoff from the Spring, Sheep and McCullough mountain ranges before dumping it into the lake.
Cox, a retired research analyst for the Public Health Service, recommended 15 years ago that the county use retention basins, capturing floodwaters and reusing them for irrigation or recharging the ground water, rather than sending them down the wash. That would allow the smaller amount of water that went into the washes to move at a slower rate and find its own way.
County officials decided instead to build detention basins on the edges of the valley and at key places along the washes. The detention basins are designed to gather excess water and slow it down before letting it into the wash to complete its trip to the lake.
A detention basin is generally empty three to five days after a flood. A retention basin would hold water for several weeks until it could be reused. The benefit of a retention basin, Cox argued, was that damaging floodwaters would never get to the Las Vegas Wash and would not bring with it pollutants picked up along the way.
The Regional Flood Control District Board, consisting of elected officials and community representatives, decided not to pursue retention basins, because they require more land. The board decided it was too expensive to pursue at the time, according to the board's chairman, County Commissioner Bruce Woodbury.
Local water officials had told flood control experts at the time that recharging ground water with storm runoff was not worthwhile and too expensive.
The Regional Flood Control District had contacted other Southwestern cities and found no one else was trying to capture floods within a valley and use storm waters for recharging ground water.
Flood Control District Director Gale Fraser defends the practice of moving the floodwaters out of the valley as fast as possible as a way of protecting valley residents and visitors from street flooding that can float away cars and people.
Another $200 million of projects is scheduled for the next year.
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