County debates strength of flood

Tuesday, Aug. 12, 1997 | 9:59 a.m.

The storms that inundated Henderson and Boulder City exceeded in some areas the 100-year flood rainfall level, Clark County officials said today.

"It was a very rare occurrence," Kevin Eubanks, assistant general manager of the Clark County Regional Flood Control, said of Sunday's storm that dropped nearly three inches in a few hours. "In general, it exceeded the 100-year rainfall at some of our gauges."

A 100-year flood is an occurrence that has a 1 percent chance of being equaled or exceeded in any given year. By comparison, a 500-year flood is an occurrence that has a 0.2 percent chance of being equaled or exceeded in any given year.

Although Sunday's storm exceeded the 100-year rainfall occurrence, it did not necessarily exceed the 100-year runoff, the second factor in determining a 100-year flood.

Data on runoff have not yet been fully analyzed, Eubanks said, indicating there is a chance there was 100-year runoff in "a few places." However, he noted that two key detention basins were just 20 percent filled to capacity.

In Southern Nevada, flood facilities are designed to withstand 2.77 inches of rain in six hours. On Sunday, 2.8 inches fell during that period, with 2.5 inches falling in a single hour.

For a one-hour storm, Southern Nevada flood facilities are designed to handle 2.06 inches of rain, Eubanks said.

Similar major storms occurred in Las Vegas in 1984 and in 1976.

Weather service officials said while the amounts of rain that fell Saturday and Sunday were high, the phenomena that caused them were not uncommon.

"A microburst (Saturday's storm) is like air rushing from a deflating balloon," said local National Weather Service spokesman Charlie Schlott. "It is a downrush of air that occurs in the decaying stages of a thunderstorm."

During the formation of a thunderstorm, air rises, Schlott said, noting that during a microburst, winds can exceed 60-80 mph.

Sunday's storm was caused by "a combination of a moist, unstable air mass over Southern Nevada being punched by a jetstream from Southern California," Schlott explained. "This caused air to rise and form numerous thunderstorms."

Golf ball-size hail fell in some areas during both storms. This, Schlott said, was caused by water droplets rising to higher altitudes and freezing. When they got too heavy, the ice balls fell to earth before they could dissolve into rain.