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Soil Survey Manual - Table 3-4

Frequency and duration of inundation classes

Class Criteria
Frequency  
None (N) No reasonable possibility
Rare (R) 1 to 5 times in 100 years
Occasional (O) 5 to 50 times in 100 years
Frequent (F) > 50 times in 100 years
Common (C) Occasional and frequent can be grouped for certain purposes and called common
Duration  
Extremely Brief (BE) < 4 hours (flooding only)
Very Brief (BV) 4 - 48 hours
Brief (B) 2 - 7 days
Long (L) 7 days to 1 month
Very Long (LV) > 1 month