What is it?
A flood is defined as a rising body of water and its overflowing onto normally dry land. This is typically slower in both onset and dissipation than a flash flood. In areas where this is to be expected annually, such as farmland, this isn't an issue. But when we put people and property in the way of rising waters, we need to attempt to mitigate floods through planning, code enforcement, and physical structures.
Although the Bureau of Reclamation has constructed the Jackson Lake Dam, and a joint effort between the Teton County Road & Levee department and the US Army Corps of Engineers maintains levees along the Snake and Gros Ventre Rivers, there is always the possibility of flood even without dam or levee failure.
What are the risk factors?
Here are some of the risk factors for flooding:
Living within a floodplain.
If you live in an area that naturally floods during the spring when runoff is high, or during the end of the winter when ice jams form, you are at a higher risk for flooding. To see what your risk for flooding is, check out FEMA's Map Service Center or Teton County GIS for more information. If you would prefer to look at the most currently available maps in person, you can contact your local floodplain administrator.
If you live in the Town of Jackson you can contact the Engineering Department (your floodplain administrator) to see FEMA Flood Insurance Rate Maps (FIRM's), which calculate your risk of flooding.
If you live out in the County, Teton County Engineer's Office would be your floodplain administrator. You can call them or stop by their office to look at FIRM's to see what your risk of flooding might be.
Stalled storm systems with heavy rain.
Just as with flash floods, a storm system that is slow moving with high precipitation rates (over 0.5"/hour) can easily cause flooding. In areas such as narrow canyons this can result in flash flooding. In open areas along rivers, lakes, or streams this can cause flooding that starts slowly but also takes a long time to recede.
Sudden extreme temperature drops near moving bodies of water.
When the surfaces of creeks and rivers begin to lose heat at high rates (like when a sudden cold front moves through), this can cause water to become supercooled. The turbulence from the water flow then causes this supercooled water to mix throughout the entire body of the water's depth. This creates what is known as "frazil ice". Frazil ice tends to not be as buoyant as regular ice, so it begins to accumulate on the upstream side of rocks on the streambed. This freezing from the bottom-up of the stream or river reduces the volume available for water to flow, causing it to top the banks. This frazil ice can also catch pieces of surface ice causing ice jams.
Ice jams are essentially dams made up of frazil ice and pieces of surface ice. They can occur any time during the winter, but we usually see them in late winter or very early spring when surface ice begins breaking up and gets caught up on frazil ice as it flows downstream. These ice jams can be dangerous for two reasons:
Locally, we typically get ice jams on areas of Flat Creek around N Highway 89 (the Dairy Queen) and also further downstream near the Southern end of South Park Loop Rd in the Melody Ranch area. It isn't beyond the realm of possibility for ice jams to form on the Snake River, but in the last 30 years there have only been small ice floes and light frazil ice formation on the Snake. Needless to say, an ice jam on the Snake River would have much more dire consequences than on Flat Creek.
First, the ice jams can cause flooding upstream as they prevent water from flowing downstream.
Second, if the ice jam suddenly breaks, it can release all of the water it was holding back, causing flash flooding downstream.
A winter with high snowfall amounts followed by a rapid spring thaw.
A rapid spring thaw can cause increased rates of snowmelt resulting in flooding. The degree of springtime flooding potential is determined by the amount of remaining snowpack, the spring temperature trend, and the capacity of streams, rivers, and lakes to handle the snowmelt. You can check the National Weather Service web site for current hydrologic data for our area.
A "rain-on-snow" event.
Snow can act like a tarp, preventing rain water from reaching our loose, gravelly soils to be absorbed before it runs into streams and creeks. When we have early spring rains that fall on snow, it can accelerate the rise in creeks, streams, and rivers more than if that same amount of precipitation fell on exposed soil.
What should I do?
You react to a regular flood much like you do to a flash flood. Here are a few other items of interest:
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!
If given enough advance notice of a flood, and your safety is not at risk, erect sandbag dikes.
If you have enough warning and your safety isn't compromised, you can begin building sandbag dikes along your property. The US Army Corps of Engineers has a great document on how to properly use sandbags in a flood situation. You'll need Adobe Reader to view the PDF file.
Remember, you want to have at least 8ft between your home's foundation and the sandbag dike. This ensures you have walking space to maintain the dike and area to expand the dike if necessary. Additionally, with this space, the pressure of the floodwaters up against the dike won't press against your home's foundation causing indirect damage.
Teton County Emergency Management has a limited number of sandbags available to local residents and businesses in the event of flooding. Sandbags will be issued when flood potential is high (such as during National Weather Service flood watches and warnings) on a first-come, first-serve basis. Recipients of sandbags are responsible for obtaining their own sandbag fill and building their sandbag dikes. If bags are still in a usuable condition, they should be returned to Emergency Management following the flood event. Contact Emergency Management at 733-9572 for more information.
And no matter how vigorously you are defending your property and how many people you have helping you, some situations are not defensible. If ordered to evacuate by local emergency services, you must do so for your safety and that of your family, friends, and neighbors.
Install mitigation devices to reduce the formation of frazil ice.
This is beyond the scope of most homeowners, but the Town of Jackson and Teton County have addressed this in many areas. Here are some of the ice jam mitigation projects:
"Thaw wells" on Flat Creek.
The Town of Jackson Engineering Department maintains three wells that can pump ground water into Flat Creek at Crabtree Ln near the bike path, Karns Meadow, and the High School Rd bridge. The water from the wells isn't heated, but since the ground water is warmer than the supercooled water in the creeks, this can help to prevent the formation of frazil ice.
Heat tubes under bridges and in culverts.
Teton County Road & Levee uses heat tubes to prevent ice formation in culverts in the Buffalo Valley area that in the past would fill with ice and cause flooding. These tubes run on electric current and keep the temperature in the culverts high enough to prevent ice accumulation.
Stream management on Flat Creek.
The Town of Jackson Engineering Department has completed a stream modification and enhancement project on Flat Creek from N Highway 89 (Dairy Queen) downstream to W Broadway (Staples). The enhancement includes digging deeper channels that not only allows for more water flow to prevent frazil ice, but also improves fish habitat. The plan is to continue this project further downstream in coming years.
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.
FEMA has a very informative site on flooding. If you are a homeowner who is interested, you can take the FEMA independent study course "Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures" here. Another informative FEMA independent study course is "Anticipating Hazardous Weather & Community Risk" which is available here. All of FEMA's independent study courses are free of charge.
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?
The impacts for a flood are very similar to those for a flash flood, so click here to review those. The main thing to keep in mind is that just as the onset of a flood is slow, so is the recession of the waters. This means that the recovery process can take much longer than that of a flash flood, which usually dissipates quickly.