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Flood Related Hazards -Things to Know about Flooding

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. 
Flood Related Hazards -Things to Know about Flooding

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. 
River flooding is classified as  Minor, Moderate, or Major based on water height and impacts along the river. Minor river flooding means that low-lying  areas adjacent to the stream or river, mainly rural areas and farmland and  secondary roadways near the river flood. Moderate flooding means water levels  rise high enough to impact homes and businesses near the river and some  evacuations may be needed. Larger roads and highways may also be impacted. Major flooding means that extensive rural  and/or urban flooding is expected. Towns may become isolated and major traffic  routes may be flooded. Evacuation of numerous homes and business may be  required.
Snowmelt flooding occurs when the major source of water involved in a flood is caused by melting snow. 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.

Areas Most Likely to Flood During Heavy Rains

While any area can flood with enough rainfall, some areas are more prone to flooding. It should come as little surprise that all of these areas are either near bodies or water or are on lower ground than the surrounding area.
  • Floodplains – The areas surrounding ocean, lakes, rivers, and streams are most prone to flooding during heavy rains.
  • Valleys and Lowlands – Any area that slopes downward, like a valley or gully, is more likely to flood during heavy rains. This includes areas that are at or below sea level.
  • Near Mountains – Even if the area is flat, water from heavy rains will flow down mountains or hills.

What areas are at risk from flash floods?

Densely populated areas are at a high risk for flash floods. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential.

Sometimes, streams through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become death traps.

Areas near rivers are at risk from flash floods. Embankments, known as levees, are often built along rivers and are used to prevent high water from flooding bordering land.

Dam failures can send a sudden destructive wall of water downstream.