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Annual flooding

Flooding is the temporary covering of the soil surface by flowing water from any source, such as streams overflowing their banks, runoff from adjacent or surrounding slopes, inflow from high tides, or any combination of sources.  Shallow water standing or flowing that is not concentrated as local runoff during or shortly after rain or snow melt is excluded from the definition of flooding. Standing water (ponding) or water that forms a permanent covering is also excluded from the definition.

Annual Flooding Duration – The average duration of inundation per flood occurrence.

  • Very brief– Less than 2 days.

  • Brief– 2 to 7 days.

  • Long– 7 to 30 days.

  • Very long– More than 30 days.

Annual Flooding Frequency – Number of times flooding occurs over a period of time.

  • None – No reasonable possibility of flooding; near 0 percent chance of flooding in any year or less than 1 time in 500 years.

  • Rare – Flooding unlikely but possible under unusual weather conditions; 0 to 5 percent chance of flooding in any year or nearly 0 to 5 times in 100 years

  • Occasional – Flooding is expected infrequently under usual weather conditions; 5 to 50 percent chance of flooding in any year or 5 to 50 times in 100 years.

  • Common – Flooding is expected more frequently than Occasional and less than Frequent.

  • Frequent – Flooding is likely to occur often under usual weather conditions; more than a 50 percent chance of flooding in any year or more than 50 times in 100 years.

Yearly flooding frequency classes are assigned to months to indicate the months of occurrence and not the frequency of the flooding during the month, except for the very frequent class.  The time period expressed includes two-thirds to three-fourths of the occurrences.  Time period and duration of the flood are the most critical factors that determine the growth and survival of a given plant species. Flooding during the dormant season has few if any harmful effects on plant growth or mortality and may improve the growth of some species. If inundation from flood water occurs for long periods during the growing season, the soil becomes oxygen deficient and plants may be damaged or killed.

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