Flash Flood Awareness

Weather Talk

POSTED: Tuesday, June 17, 2014 - 3:10pm

UPDATED: Monday, June 23, 2014 - 9:17am

Flash flooding is the #1 Thunderstorm related killer

I was on vacation (still staying cool in the mountains) for the first day of “Monsoon Awareness Week” last week and did not cover the topic for that day “Flash Floods”.  So we shall cover the topic and be completely up to speed and prepared for our "Monsoon
Season”! The Monsoon kicked off this past Sunday, June 15th and last through September 30th.

The #1 Thunderstorm related killer is Flash Floods. Most of deaths related to flash floods occur in vehicles and that is why the slogan “Turn Around Don't Drown™” was created. In the dry deserts many people have the misconception that we are safe from the type of flooding that occurs in the rest of the country because we do not have many big rivers or we are not near the ocean and experience hurricanes. Those of us that were here during the "2006 El Paso Floods", where northeast El Paso and West El Paso saw over 10” of rainfall in just over 24 hours, know that is not true. Same goes for residents in the Lower and Mission Valleys last September in 2013. Over 1” to 2” of rain fell. In both events many people were stacking sandbag and desperately trying to keep water out of their homes, roadways became rivers and ponds were shut down. Some areas motorists were rescued from the vehicles stalled in 3 to 5 feet of water. deep! In today’s “Weather Talk” I will talk about how flash flooding happens, the dangers and give you some National Weather Service safety tips.

Many of us have heard moving water only 2 feet deep will carry away most vehicles. We also know that most flash flooding occurs when there is too much rainfall falling over a certain area, sometimes all at once or over a longer period of time.

According to the National Oceanic and Atmospheric Agency (NOAA) here is the definition of a flash flood;

“Flash floods are short term events occurring within 6 hours of a causative event (such as heavy rain, dam break, levee failure, snow melt and ice jams) and often within 2 hours start of high intensity rainfall. A flash flood is characterized by a rapid stream rise with depths of water that can reach above the banks of the creek. Flash flood damage and most fatalities tend to occur in the areas immediately adjacent to a stream or arroyo. Additionally, heavy rain falling on steep terrain can weaken soil and cause mud slides, damaging homes, roads and property. Flash floods can be produced when slow moving or multiple thunderstorms occur over the same area. When storms move faster, flash flooding is less likely since the rain is distributed over a broader area”

Since many flash floods happen close to streams or here in the desert, arroyos, know you proximity to streams and arroyos. Flooding can be caused by heavy rainfall many miles upstream and then this excess water gains momentum and increases in depth and size as it rushes downstream.

Urban areas because of the buildings, roadways, driveways and parking lots increase runoff by reducing the amount of rainfall the ground can absorb. This runoff increases the flash flood potential. Sometimes and in some areas cities the water is designed to go into ditches, holding ponds or underground storm drains. These ditches, ponds or drains can get clogged or overwhelmed. This causes ditches, low lying areas including roadways to become dangerous and occasionally deadly.

Embankments, known as levees are built along side streams, rivers and some arroyos to help prevent the water from flooding the surrounding areas. These levees can fail causing sometimes massive flooding in a matter of minutes! Here in El Paso County this happens quite often around the Socorro and San Elizario areas.

I did not realize that there are over 76,000 dams in the United States and about 80% of them are earthen dams. Dam failures have caused catastrophic flooding historically.
Earthen dams are more easily compromised by heavy rainfall than cement made structures. Water flowing over an earthen dam can cause the dam to quickly weaken and eventually fail sending a destructive wall of water downstream.

Many people like to hang out or plan activities around a stream, river or even a dry arroyo. A stream only 6 inches deep can swell to a 10 foot deep raging river in less than 30 minutes if thunderstorm deluges an area with intense rainfall. Be aware of rapidly changing weather conditions if you’re near a stream, river or arroyo. The river or stream may become muddy or rise quickly, if you notice this move to higher ground right away. There may be a roaring sound upstream; if you hear this a wall of water may be headed your way! Walls of water have been known to reach some 10 to 30 feet high! Even higher if the channel is in a canyon!

An arroyo or wash is a channel where flash flooding has occurred so naturally there is a good probability flash flooding will occur there a again. An arroyo is a channel, funneling all of the water into one place. .Because of this, the arroyo rapidly fills with water, which appears in such a high volume that it cannot be absorbed by the ground. This fast moving water rises, and ultimately turns into a river which can create a flash flood, violently crashing through the arroyo. This water can uproot trees, dislodge rocks and sweep up anything including people and animals that are caught in the way.

Turn Around Don't Drown™ Safety Tips

  • Driving around barricades is illegal and dangerous.
  • Do not let children play near storm drains or washes after a heavy rain.
  • Avoid low-water crossings.
  • Avoid camping in a wash (an arroyo) or in the bottom of a canyon with steep side slopes.
  • Be especially cautious at night. Flood dangers are much more difficult to see in the dark.
  • Even a less serious urban flood can be dangerous. Driving too fast through standing water can cause a car to hydroplane. The best defense is to slow down or pull well off the road (with the lights off) for a few minutes to wait out heavy rains.
  • Avoid areas already flooded, especially if the water is flowing fast.
  • Do not attempt to cross flowing streams.
  • Do not camp or park a vehicle along streams and washes, particularly during threatening conditions.
  • If flooding occurs, get to higher ground. Get out of areas subject to flooding. This includes dips, low spots, canyons, washes, etc.
  • Roadbeds may be washed out under floodwaters. Never drive through flooded roadways.
  • If your vehicle is suddenly caught in rising water, leave it immediately and seek higher ground.
  • If a traffic signal is out, treat the intersection as a 4-way stop.
  • As little as ten inches of water can float average-sized cars, mini-vans, SUVs and trucks. Strength of the flow is the critical force.
  • When in doubt, wait it out, or find a safer route.

Even here in the dry desert flash flooding can cause damage and even loss of life. Now we know flash flooding can occur anywhere. Urban areas actually increase the risk of flash floods because of all the roadways, drive ways and parking lots. We have talked about with all the monsoon awareness topics how important it is for you to stay current with the weather forecasts and the current changing weather conditions. Of course you can stay up to date with NewsChannel 9 on the air, at www.ktsm.com/weather ,  Facebook; www.facebook.com/NewsChannel9ElPaso,  twitter:  https://twitter.com/NC9. Twitter is more up to the minute in severe weather situations compared to Facebook with it's new delayed algorithm. Keeping yourself informed will keep you safe.
 

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
cdebroder@ktsm.com
www.facebook.com/pages/Charles-DeBroder/
https://twitter.com/wxchuckNC9

Comments News Comments

Another safety tip:
When you have to drive through a small amount of water, keep from making a big wave with your wheels that can splash onto other cars' windshields. This can blind them temporarily and cause them to do irrational things (like slam on their brakes).

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