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.