Emergency Management

What is it?

A flash flood is a rapid rise in water (a few minutes to a few hours depending on rainfall) in streams, creeks, and storm drains that can pose a threat to life and property. Flash floods are usually caused by excessive rainfall but ice jams suddenly breaking up, dam failures, or levee breaches can also cause flash flooding. Due to the rapidly changing weather patterns that cause flash floods, predicting them is very difficult to say the least.

Our most recent flash flood in Teton County occured on July 26th, 2007 in Jensen Canyon up Fish Creek Road in Wilson,WY. It washed out a private road and overturned two vehicles, but no one was injured. You can read more about it here.

Clickable Map of Jensen Canyon Flash Flood
View Larger Map      Right Click and "Save As" For Google Earth File

Probably the most infamous flash flood in Teton County's recorded history is the Kelly Flood. On June 23, 1925, part of the northern face of Sheep Mountain became unstable after weeks of heavy rain and slid into the Gros Ventre River. The 50 million cubic yards of sedimentary rock formed a natural dam 200 feet high and 400 yards wide that created Lower Slide Lake. On May 18th, 1927, a portion of the natural dam broke causing a flash flood to rush down the Gros Ventre river. The flood was at least 6 feet deep for at least 25 miles downstream, and wiped out the town of Kelly which was 6 miles downstream. Six people died, and many others lost everything they owned.

Fifteen miles downstream in Wilson,WY, 6 feet of water innundated the town and hundreds of farm animals died. In narrower sections of the Snake River near Hoback, flood waters rose as much as 50 feet. The next day the waters reached Idaho Falls,ID and covered lowland sections there. When all was said and done, damages totaled $500,000 which would be over 27 million in today's dollars. You can still see the bald rock on the north face of Sheep Mountain (Sleeping Indian) as a reminder of this tragic event (Windows Into the Earth; Smith and Siegel, 2000).

Clickable Map of Kelly Flash Flood
View Larger Map     Right-Click and "Save As" for Google Earth File

Remember, it doesn't have to be raining where you are for a flash flood to occur; there can be events upstream that can affect you directly. An example of this would be a failure of the Jackson Lake Dam. The dam was rebuilt in 1989 to help withstand an earthquake along the Teton Fault, and is monitored by Bureau of Reclamation staff. Failure of the dam in the absence of a catastrophic event such as an earthquake is highly unlikely, but it is better to be prepared for the unlikely than to be caught unaware.

Under normal circumstances (average yearly rainfall, average capacity of Jackson Lake, proper operation of the dam spillways, etc.), here are the estimated times for dam failure flood waters to reach several locations along the Snake River:

Location	Distance Downstream from Dam (Miles)	Travel Time of Failure Floodwaters (Hours)
Buffalo Ranger Station	4.4	0.5
Bar BC Ranch	21.6	4.5
Park Headquarters at Moose	24.5	5.0
Town of Wilson	38.8	8.5
S Highway 89 at South Park Bridge	51.0	12.5
Hoback Junction	56.1	13.5
Astoria Hot Springs	59.2	14.0
Head of Palisades Reservoir	79.0	16.5

  

Clickable Map of Estimated Dam Failure Flood Travel Times
View Larger Map     Right-Click and "Save As" for Google Earth File

     

Remember, these are only estimates. Luckily we have never had to check these calculations against a real dam failure. If there are reports of dam failure, tune into your NOAA All-Hazards Weather Radio or your local media for instruction from Emergency Management. To see if your home or workplace are within areas that are estimated to be affected by a dam failure, check out our inundation flood maps.

What are the risk factors?
Here are a few situations that can increase the chances of a flash flood occurring:

• High rates of rainfall.
  Rainfall rates of .5 to 1 inch per hour can easily be more than the ground can absorb.

• Stationary storm fronts.
  Stalled storms can cause extended durations of rain in one location, increasing the chance for rapid flooding downstream.

• Extended periods of drought.
  Dry, impermeable soil will force water downhill as opposed to it being absorbed into the ground.

• Fully saturated soil.
  Similar to dry, impermeable soil, when soil has been fully saturated the water from new rain can't be absorbed. This forces water downstream at more rapid rates.

• Recent wildfire in the area.
  Fire can affect the soil and give it a hard crust that is difficult for water to penetrate.

• Construction or development in the area.
  Increasing amounts of pavement and roadways without proper planning for drainage can decrease the amount of rainfall that can be absorbed into the ground, hence increasing the risk for flash flooding.

What should I do?
Here are a few things to look out for or do in regards to flash flooding:

• As for any disaster, have a plan and preparedness kit ready for your family.
  If you don't have a family emergency plan and a 72 hour kit, click here to get started.

• Before a flood ever strikes, get flood insurance.
  Many homeowners are shocked to discover that their homeowner's (or renter's) insurance policies do not cover flooding. Check with your insurance agent to see what your coverage would be in the event of a flood. You can also check here for information on the National Flood Insurance Program.

  Remember, barring a few exceptions, flood insurance takes 30 days from payment of the first premium to take effect, so don't wait until the middle of flood season to get insured!

• Secure outside propane or other fuel tanks.
  During the flood of Fort Collins, CO in 1997, one major issue that emergency responders faced were outside propane and heating oil tanks floating away. As the tanks became free of their moorings, the fuel lines ruptured and caused massive fires during the flood. Having your tanks properly secured before a flood reduces this risk. Also be sure that your underground tanks are properly secured and are at the proper depth. It is possible for poorly set underground tanks to become buoyant during a flood and come to the surface.

• If you suspect a flash flood, immediately head to higher ground.
  Keeping your safety in mind, warn those around you and get to the highest point possible. As little as 6 inches of rushing water can sweep you downstream, so do everything you can to stay away from running flood waters.

• During periods of intense rainfall, stay tuned to your NOAA Weather Radio, local television, or local radio.
  What you are listening for are Flash Flood Watches or Flash Flood Warnings. A watch means that flash floods are possible, and a warning means that flash floods are occurring or imminent. If you are in an area where a flash flood warning is issued, immediately get to higher ground.

• Listen for distant sounds like thunder or a train, especially up canyons.
  Many people describe the sound of an oncoming flash flood as a train, rumble, thunder, or loud wind. Canyon walls can amplify this sound as water comes rushing down stream. If you hear this, immediately head to higher ground.

• Watch for rapidly rising water.
  If you see streams that are normally dry or nearly dry that suddenly have rushing water in them, this can be a sign of an approaching flash flood. Head for higher ground.

• Do not approach flood waters
  Flood waters can be contaminated with automobile fuel, waste, or other hazardous materials. Additionally, as little as 6 inches of fast moving water can sweep you off of your feet. Never attempt to cross flowing water in a flood situation.

• Never attempt to cross flood waters in your vehicle.
  One foot of water can float most passenger vehicles, and two feet of water is enough to float large trucks and other vehicles. You may find this hard to believe, but think of how massive aircraft carriers are, and they float just fine! It isn't the size of the vehicle, but the buoyancy.

  Additionally, you can't tell if that "puddle" you are about to drive through is actually a pond due to the road being washed out. Nearly half of all flash flood fatalities are auto related. Remember the National Weather Service's saying: Turn Around, Don't Drown®. Watch the following video to see for yourself why you shouldn't drive through flood waters. Especially take note of how deep the water is when the driver finally exits the vehicle and stands up:

  Launch in external player

  Having trouble viewing the video? Click here

• Stay tuned to EAS broadcasts for instruction before, during, and following the disaster.
  Listen to your NOAA All-Hazards Weather Radio or another Emergency Alert System (EAS) broadcaster for instructions from emergency services before, during, and following a disaster.

• Educate yourself.
  Check out our Teton County Flood Maps to see if your home or workplace is in a floodplain or at risk for flash flooding. These maps only take into consideration major flooding or catastrophic failure of the Jackson Lake Dam. Remember if the conditions are right, flash floods can occur almost anywhere!

  Another great resource is FEMA's flood page, where you'll find more information on what to do before, during, and after a flood. Also check out Ready.gov's section on floods for more information.

  For a quick reference guide, you can check out this pamphlet on flood preparedness by the National Weather Service and the Red Cross (Adobe Reader required).

What are the impacts?
Flash floods generally dissipate as quickly as they begin. However, the flood waters aren't always the only issue:

• Death or serious injury
  The very nature of flash floods makes them fast and very difficult to predict. Since they can occur without warning, it is no surprise that people can be seriously injured or killed by these natural disasters. Also, many flash floods occur at night while people are sleeping adding to the risk. Flash floods are the #1 weather-related killer in the U.S. according to the National Weather Service.

• Immediate property damage.
  In addition to the force of the water, flash floods can carry large debris such as boulders. This combination can cause heavy structural damage to homes making them uninhabitable and can carry away large pieces of property such as vehicles.

• Long-term property damage.
  Rooms, vehicles, and other property that came in contact with flood waters may not be salvageable. Homes that were flooded can be susceptible to harmful molds. Vehicles that were submerged usually have irreparable water damage to the engine and other critical components.

• Loss of critical infrastructure.
  Large debris and floodwaters can cause structural damage to bridges and roadways, making travel impossible. Power, telephone, and cable lines can be taken out by flash floods as well. Flood waters can disrupt or contaminate ground water, making tap water unfit for consumption.

• Deposited sediment and silt.
  Flood waters carry along with them sand, silt, and other debris as they travel. When the velocity of the flash flood slows, it begins depositing this debris. Floods can leave behind large amounts of silt and other debris that can make travel difficult and can be costly to remove.

• Uninsured or underinsured homeowners.
  Being infrequent and unpredictable, many homeowners do not consider flash floods when insuring their homes. Your regular homeowner's or renter's insurance does not cover flood damage; for that you need flood insurance through the National Flood Insurance Program (NFIP). Check this link for information on flood insurance, calculating your risk for floods, and interactive demonstrations of potential flood damage.

• Economic losses.
  Depending on the damage caused, it may prevent local businesses from opening or keep customers from getting to those businesses.

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