Flooding is a coast-to-coast threat to the United States and its territories in all months of the year. 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. 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.
River flooding occurs when river levels rise and overflow their banks or the edges of their main channel and inundate areas that are normally dry. River flooding can be caused by heavy rainfall, dam failures, rapid snowmelt and ice jams. The National Weather Service issues Flood Warnings for designated River Forecast Points where a flood stage has been established.When people think of tropical storms and hurricanes they typically think of strong winds, yet the highest percentage of all tropical cyclone deaths are due to flooding. Coastal flooding generally occurs with a land-falling or near-land system such as a Tropical Storm or Hurricane. Storm surge and large waves produced by hurricanes pose the greatest threat to life and property along the coast. The destructive power of storm surge and large battering waves can result in loss of life; destruction of buildings; erosion of beaches and dunes; and damage to roads and bridges along the coast. Storm surges undermine building foundations by constant agitation of the water piled high by the tropical cyclone. The end result can be a complete demolition of homes and businesses.
Tropical cyclones can cause flooding in the U.S. each spring through fall. While the official hurricane Season runs from June to November in the Atlantic and May to November in the Pacific, tropical storms have been known to occur outside of this timeframe. Tropical cyclones can bring copious amounts of precipitation onshore. The majority of the heaviest rain occurs to the right of the center of the storm; however, it should be noted that rain bands on both sides of the system can produce heavy rain.
Tropical systems are not the only type of storms that can cause coastal inundation and storm surge. At all times of the year, storms can impact the U.S. coastal regions. The Northwestern U.S. is often approached by Pacific storm systems that bring large amounts of precipitation to the area. During the cold season, large storm systems, called Nor'easters, bring heavy precipitation to the Northeastern U.S. as well. These storm systems can also be accompanied by strong winds which cause storm surge and additional flooding along the coastal areas.
Storm Surge is a particular problem directly along the coast. Entire coastlines can be altered by the sheer magnitude of the water battering the shores. Storm surge can also travel several miles inland causing additional flooding and destruction. The addition of intense rainfall along the coast can add to the flooding potential. It is important to note that surge related fatalities have been greatly reduced due to improved evacuation practices in coastal areas; however, since the 1970s, inland flooding has been responsible for more than half of the deaths associated with tropical cyclones in the United States. Unfortunately, the inland flood threat is often under emphasized during a tropical event. Typically, greater rainfall amounts and flooding are associated with tropical cyclones that have a slow forward speed or stall over an area. Any remnants of a tropical system can cause flooding if conditions are right. Both river flooding and flash flooding are common with tropical cyclones. Water from heavy rainfall quickly gathers in low lying areas. It may surround homes and inundate roadways. To stay safe during a tropical cyclone, it is important to stay aware of the evolving situation by listening to the local news and heeding any calls to action from law enforcement.
Tropical systems and coastal storms can affect more than just the states lining the coasts. Storms that strike the coast of the Gulf of Mexico often track northward into the eastern half of the U.S. before being caught by the easterly flow and pushed off into the Atlantic. As a storm moves inland, away from its primary moisture source (the oceans or Gulf), precipitation amounts will begin to diminish; however, depending on the strength of the storm, the amount of moisture being carried with it can take heavy rainfall well into the interior states.
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In areas where wildfires have occurred, vegetation may have been burned away and soil properties may have been altered, leaving behind bare ground that tends to repel water. This is called a burn scar. When rain falls over a burn scar, the ground is unable to absorb the moisture, leaving the water to collect or run across the surface of the ground towards the lowest point. Wildfires are common in the western U.S.; however, wildfires occur in all 50 states, so this type of flooding is possible anywhere.
Without vegetation to hold the soil in place, flooding can produce mud and debris flows. When normally dry soil becomes overly saturated, it can reach a point where it turns to a liquid state and flows downhill, essentially becoming a river of mud. Mud and debris flows can destroy homes, wash out bridges and roadways, and knock down trees. They can also deposit large amounts of mud and other debris on previously clear surfaces, damaging or burying everything in their path. Areas where ground cover has recently changed dramatically, such as an area impacted by a wildfire, can be at a higher risk for mudflows.
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.
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. 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.
In dry areas of the U.S. significant rainfall can quickly cause flooding. For example, much of the year the desert southwest is very dry. However, each summer, the weather pattern changes, bringing moisture and thunderstorms into the area. Because of the heat and arid climate, the ground is quite hard and unable to absorb much of the precipitation that does fall. The water from these storms rushes to low-lying areas, often into a canyon or dried up river bed. It can be dangerous for hikers to walk along these canyons or dried river beds even when it is not raining. A thunderstorm could take place several miles away, but eventually that water will arrive downstream. If a hiker is not monitoring the situation, they may be caught by surprise, and potentially swept away by the flood waters.
According to FEMA and the National Inventory of Dams (2007), there are more than 80,000 dams in the United States. Approximately one third of these pose a "high" or "significant" hazard to life and property if failure occurs. Dam failure or levee breaches can occur with little warning. Intense storms may produce a flash flood in a few minutes or hours while other failures and breaches can take much longer to occur, from days to weeks.
Causes of dam failure vary from natural causes such as prolonged rainfall, landslides, earthquakes, or erosion to human causes such as improper maintenance and design, negligent operation, or sabotage and terrorism. Dam failures are categorized into three groups: overtopping, in which the water level exceeds the top of the dam; excessive seepage, in which water seeps through the ground; and structural failure, where part of the dam doesn't complete its job sufficiently.
In some cases, flooding may even occur well away from where heavy rain initially fell. This is especially common in the western United States where low lying areas may be very dry one minute, and filled with rushing water from upstream the next.