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FLOODS
Floods are the most common disaster with in the United States as well as in Idaho
Floods have been the most serious, devastating, and costly natural hazard to affect Idaho. Most Idaho residents live near rivers which are subject to periodic flooding. Floods in Idaho frequently damage roads, farmlands, and structures, often disrupt lives and businesses, and occasionally cause loss of lives. A
few streams in Idaho are subject to almost annual flooding, but in most areas damaging floods are much less frequent.
Historically, the greatest impact has been to the northern and north central parts
of the state where communities are vulnerable to flooding from the many rivers, lakes and creeks in the area. The steep, mountainous terrain creates a flood-prone environment
and development is often confined to areas adjacent to stream channels. Significant events and disasters have occurred throughout the state, though, and few areas are
truly flood-free. Irrigation systems and small streams that flow through communities can bring in floodwater to areas that would normally be free
flood risk.
Flood events may be classified under three general categories:
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Riverine Flooding: a rise in the volume of a
stream until that stream exceeds its normal channel and spills unto adjacent lands.
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Flash Flooding: higher velocity in a smaller
area and may recede relatively quickly.
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Ice/Debris Jam Flooding: floating debris or
ice accumulates at a natural or man-made obstruction and restricts the flow of water.
Floods vary greatly in frequency and magnitude. Small flood events occur much more frequently than large, devastating events.
The most commonly reported flood magnitude measure is the “base flood.” This is the magnitude of flood having a one-percent
chance of being equaled or exceeded in any given year. Although unlikely, “base floods” can occur in any year, even successive ones. This
magnitude is also referred to as the “100-year Flood” or “Regulatory Flood” by State government.
The areas adjacent to the channel that normally carries water is referred to as the floodplain. In practical terms, the floodplain is the area that is inundated by flood
waters.
In regulatory terms, the floodplain is the area that is under the control of floodplain regulations and programs (such as the
National Flood Insurance Program).
Idaho State Code defines the floodplain as:
That land that has been or may be
covered by floodwaters, or is surrounded by floodwater and inaccessible, during the occurrence of the regulatory flood.
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