An Overview of the Dangers of Flash Floods in the Las Vegas Valley

Torrential rains produced severe and unprecedented flash flooding across the Las Vegas Valley on July 8, 1999. Much of the Las Vegas valley experienced from 35%-70% of its annual rainfall. 13 inches) over 60-90 minutes. The resulting runoff from these rains caused widespread street flooding and record flows in normally dry washes and flood control detention basins, The floods caused over $20,000,000 in property damage and took 2 lives.

Flash floods are not unusual in the Las Vegas Valley; however, this event was extreme in its scope and intensity.

The purpose of this paper is to review the meteorological conditions that preceded this event and discuss how the urbanization of the Las Vegas valley contributed to the flash-flood threat.

For a flash flood to occur, heavy precipitation must fallin a region that has appropriate hydrological ingredients in place. The hydrological ingredients include antecedent precipitation, topographic role, and land use. For heavy precipitation to occur, high rainfall rates must be sustained. High rainfall rates, in turn, result from the rapid ascent of moist air.

The long duration of high rainfall rates results from the slow movement of the rainfall-producing system.

The Las Vegas valley is located in the Basin and Range Physiographic Province. This region is characterized by a series of generally north-south trending mountain ranges and intervening valleys filled with eroded sediments. The Las Vegas valley is very prone to flooding due to its geologic and orographic. composition. The Spring Mountains are located on the west side of the valley, while the Sheep Range borders the valley and the north.

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Smaller mountains are on the east and southeast sides of the valley. The Spring Mountains are primarily composed of limestone rock. These factors, along with the amount of urban development in the area can often increase the severity of flash flooding

The atmosphere over much of southern Nevada had experienced a tremendous increase in moisture levels before the flooding. The relative humidity was high enough that convective cold pools would not be strong enough to undercut the impinging storm-relative flow to new updrafts. Also, extremely light winds throughout the depth of the troposphere, with a deep moisture supply and deep layer of convective instability, provided the ingredients for very efficient ran rates.

Catalyst is bringing the moisture into the region was a westward-moving inverted trough in the middle levels of the atmosphere, The mid and upper-level support provided by the inverted trough was strong enough to produce persistent nocturnal showers and thunderstorms over portions of northwest Arizona overnight and into the early morning. The persistent development immediately signaled that abundant daytime heating would not be necessary for thunderstorms and showers to develop over extreme southern Nevada,

These conditions created a large amount of flash flooding, Flooding of this magnitude across such a large area of the Las Vegas valley was unprecedented in the eyes of many observers. This event falls between the 50 and 100-year flood criteria along portions of the Las Vegas Wash according to the US, Geological Survey.

Given the unique geology and orography of the Las Vegas valley, it will remain vulnerable to flash flooding in the future. More and more people will become susceptible to these foods as the Las Vegas valley continued to grow at a rate of $000-6000 people per month. Although no amount of prevention can spare everyone from the effects of such floods. these effects can be reduced through the continued work and intervention of the Clark County Regional Flood Control District and The National Weather Service.