As snows melt, waters rise. Flooding is a coast to coast threat to the United States and its territories nearly every day of the year.
If you know what to do before, during, and after a flood you can increase your chances of survival.
Flooding typically occurs when prolonged rain falls over several days, when intense rain falls over a short period of time, or when an ice or debris jam causes a river or stream to overflow onto the surrounding area. Flooding can also result from the failure of a water control structure, such as a levee or dam. The most common cause of flooding is water due to rain and/or snowmelt that accumulates faster than soils can absorb it or rivers can carry it away.
With Spring heading our way, think about your potential risk from snowmelt and ice jam flooding - these impact most areas far from any snow, in the low lands where the water heads.
Snowmelt flooding occurs when the major source of water involved in a flood is caused by melting snow. The northern tier states and mountainous areas of the U.S. are particularly susceptible to snowmelt flooding. Unlike rainfall that can reach the soil almost immediately, the snowpack can store the water for an extended amount of time until temperatures rise above freezing and the snow melts. This frozen storage delays the arrival of water to the soil for days, weeks, or even months. Once it begins to melt and does reach the soil, water from snowmelt behaves much as it would if it had come from rain instead of snow by either infiltrating into the soil, running off, or both. Flooding can occur when there is more water than the soil can absorb or can be contained in storage capacities in the soil, rivers, lakes and reservoirs.
High soil moisture conditions prior to snowmelt can contribute to snowmelt flooding. Rainfall during the late fall is particularly important because there is less evapotranspiration and less time for the soil to drain and dry before it freezes. Ground frost or frozen soil is another contributor. Deep, hard ground frost prevents snowmelt from infiltrating into the soil. Cold temperatures prior to heavy snowfall and normal or above normal soil moisture contribute to this.
Deep snow cover can worsen snowmelt flooding since there is more water stored and available for snowmelt. Also, when snow cover is widespread, it usually keeps air temperatures cooler and delays spring warming, which increases the potential for more rapid snowmelt. Rain falling while snow is still on the ground contributes more water for flooding and helps to melt the snowpack, thus rain-on-snow events are watched carefully.
Most often, snowmelt is a relatively slow phenomenon. Snowmelt rates are usually comparable to light or moderate rainfall.Â Important exceptions to this can occur, especially during unusually warm periods with high dew point temperatures, and when nighttime temperatures remain above freezing. Snowmelt rates can be much higher than normal under these conditions, which can increase the risk of snowmelt flooding.
Ice jams are common during the winter and spring along rivers, streams and creeks in the higher latitudes of the continental U.S. as well as in Alaska. Many of the record flood events along major rivers in Alaska are the result of ice jams Debris jams can occur at any time of year and have the same implications as an ice jam. As ice or debris moves downstream, it may get caught on any sort of obstruction to the water flow. When this occurs, water can be held back, causing upstream flooding. When the jam finally breaks, flash flooding can occur downstream.
Typically, an ice jam is resolved when the ice melts. With debris jams, the options are to take measures to remove the jam or wait for the debris to break free. In addition to causing flooding, these jams may also have economic and ecological implications. They might delay or suspend navigation along a waterway, affect hydropower operations, or cause damage to vessels. Jams can cause riverbank erosion, impede migration of aquatic creatures and adversely impact wildlife habitats. Loss of life has also been attributed to flooding caused by ice and debris jams.
Snowmelt and the breakup of river ice often occur at about the same time. Ice jams often form as a result of the sudden push exerted on the ice by a surge of runoff into the river associated with snowmelt. Ice jams can act as dams on the river that result in flooding behind the dam until the ice melts or the jam weakens to the point that the ice releases and moves downstream. A serious ice jam will threaten areas upstream and downstream of its location. Six inch thick ice can destroy large trees and knock houses off their foundations. Once an ice jam gives way, a location may experience a flash flood as all the water and debris that was trapped, rushes downstream.
Approximately seventy-five percent of all Presidential disaster declarations are associated with flooding. Below are the most common flood hazards to impact the United States.
- Flood Risk Mapping Guidelines and Standards
- Flooding & Extreme Precipitation
- Flood Safety
- TURN AROUND, DON’T DROWN