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Flood frequency Analysis is used to identify the risk associated with a particular storm. The risk is expressed in terms of the exceedance probability and more commonly as it's inverse - 'Recurrence interval', for example, 100 year storm which has annual exceedence probability of 0.01. PeakFQ software allows this analysis based on records of annual maximum flood discharges based on the procedures prescribed in Bulletin 17B. More information on bulletin 17B can be found on USGS website by clicking here. PeakFQ estimates the recurrence interval and associated flood magnitude for the following ten events:

Recurrence Interval - Year	1.50	2	2.33	5	10	25	50	100	200	500
Annual Exceedance Probability	0.667	0.50	0.43	0.20	0.1	0.04	0.02	0.01	0.005	0.002

The surface water data for the gages located throughout United States can be found on the USGS website. Click Here for the data. Make sure you download the data in watstore format and not the tab separated format.