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Picture of flooding on the Duck River, Tennessee

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Flood Magnitude and Frequency

Flood-frequency analysis provides information about the magnitude and frequency of selected flood discharges. Frequency commonly is expressed in terms of exceedance probability (a dimensionless number ranging from 0 to 1.0) or as a recurrence interval (the reciprocal of exceedance probability) in years (table 1). Flood-frequency information is important for defining flood-hazard areas; for managing floodplains; and for designing bridges, culverts, dams, levees, and other structures.

Table 1. Exceedance probabilities and associated recurrence intervals

Annual exceedance probability Recurrence interval (n-year flood)
0.6667 1.50
0.50 2.00
0.4292 2.33
0.20 5.00
0.10 10.00
0.04 25.00
0.02 50.00
0.01 100.00
0.005 200.00
0.002 500.00


  •  OFR 95-417: Flood magnitude and frequency of Hollow Brook at the culvert on New Jersey route 35, at Neptune Township, New Jersey
  •  OFR 95-323: Flood magnitude and frequency of Eightmile Branch tributary at the culvert on New Jersey route 72, at Stafford Township, New Jersey
  •  OFR 94-500: Flood magnitude and frequency of Chandlers Run at two sites, at Pennsauken Township, New Jersey
  •  OFR 94-499: Flood magnitude and frequency of Pochack Creek at two sites, at Pennsauken Township, New Jersey
  •  OFR 93-662: Flood magnitude and frequency of Delaware River tributary at the culvert on New Jersey Route 29, at Lambertville, New Jersey
  •  OFR 71-113: Extent and frequency of floods on Crosswicks Creek from New Egypt to Bordentown, New Jersey
  •  HA 104: Floods on Raritan and Millstone Rivers in Somerset County, New Jersey

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