Flash Floods Are A Very Real Threat

[Iceaxe]

Flash Floods Are A Very Real Threat
The Spectrum
By PATRICE ST. GERMAIN

HURRICANE - The best way to stay safe during the flash flood season while hiking or in the backcountry is to make good decisions before you go.

Flash flood danger for Washington County is low this week but more storms are on the way and the weather pattern that creates the sudden and heavy downpours resulting in floods remains a part of the weather picture until October.

Because of the wide areas of sandstone, the water during these sudden storms doesn't infiltrate the ground, instead running off the sandstone, which sometimes creates spectacular waterfalls but also creates potentially dangerous situations.

"When you have three inches of rain over a four-square-mile area, you are talking a lot of water," National Weather Service hydrologist Brian McInerney said. "The water runs through slot canyons or natural river channels and picks up speed as it adds volume."
McInerney said during the summer months, the direct heating of the Earth's crust causes a low pressure system to form. The air is less dense and rises, creating a void. That air is replaced by air from the ocean that moves northward from Mexico, which condenses into clouds and thunderstorm activity results from this system; heavy thunderstorms, such as last week when three inches of rain fell in an hour in some areas.

Slot canyons are really dangerous, McInerney said, but any water running off from flash floods should be avoided.

Tom Haraden, assistant chief of interpretation and visitor services at Zion National Park, said hikers into the backcountry need to make good decisions before entering the slot canyons.

"One question people always ask is 'When is it safe?' and the answer is it is never safe," Haraden said.

Haraden said park personnel will close a canyon if the weather service announces a flash flood is imminent.

But by making a decision to close or not close a slot canyon, Haraden said it leaves the impression that a canyon is safe on one day but not on another. Visitors, Haraden said, need to make a reasonable assessment of the risk.

Looking at the sky is not a good indicator of weather, he said.

Haraden said last year, he was to take a resident and group of friends hiking in the Subway but cancelled the trip after listening to the weather report and checking satellite images for the area.

Although the weather was fine in the canyon, Haraden made the call to cancel the trip and two hours later, it poured.

"I got a call telling me I made a good call," Haraden said. "But you make a good call if it doesn't rain and you are still alive. You still made a good call."

One factor of flash flooding that is often forgotten is the shear force of the water.

When the river or streams quickly rise and become a rolling mass of mud, water and debris, it's easy to see the power, but people must realize that even two feet of water can carry away a vehicle.

A gallon of water weighs 8.3 pounds and a cubic foot of water is 7.48 gallons of water weighing 62.3 pounds.

The velocity of the water quickly increases with the volume creating a deadly force residents have seen take down banks, homes and wash away vehicles.

Fortunately, there have been no fatalities in recent floods and even in Zion National Park there have been no recent water-related deaths.

The last water fatality was in 2001 when a young boy was swept away after running in a waterfall following a brief rainstorm.

Two hikers in the Narrows were swept to their deaths during a flash flood in July 1998.

Haraden said hikers in other areas of the park besides slot canyons, need to watch for waterfalls and water running over trails.

This year, the first flash flood was on July 24 on the Virgin River. Since then there have been 19 flash floods in Southern Utah.

While flash flooding is normal for the area until October, McInerney said there is a different mechanism for flash flooding in September and October.

The sun isn't warming the ground as much but still with warming temperatures, big, vertically stacked thunderclouds form, which are moisture laden from Pacific low intensity pressure systems that overwhelm the hydrologic system.

McInerney said with the radar and associated software now available, the weather service has a better mechanism and timeframe for warning people about flash floods, which may save some lives.

[James_B_Wads2000]

"When you have three inches of rain over a four-square-mile area, you are talking a lot of water," National Weather Service hydrologist Brian McInerney said.

27,878,400 cubic feet by my calculations.

"One question people always ask is 'When is it safe?' and the answer is it is never safe," Haraden said.

Haraden said. "But you make a good call if it doesn't rain and you are still alive."

I think those are very wise words to live by.

The last water fatality was in 2001 when a young boy was swept away after running in a waterfall following a brief rainstorm.

What about those two little kids swept away in the truck last year outside of Price?


James