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.
Flash Floods from Dam Failure
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.), following are the estimated times for dam failure flood waters to reach several locations along the Snake River.
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.
|Location||Miles Downstream from Dam
||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|
|Town of Wilson||38.8||8.5|
|South Highway 89 at South Park Bridge||51||12.5|
|Astoria Hot Springs||59.2||14|
|Head of Palisades Reservoir||79||16.5|
Following 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.