Return Period

Return period is the average interval between events that equal or exceed a given magnitude, the reciprocal of the annual exceedance probability. A 1-in-100-year storm tide has a 0.01 chance of being equaled or exceeded in any year, not one occurrence each century, and carries a 63 percent chance of happening at least once over a 100-year design life. Coastal return levels come from extreme-value analysis (a generalized extreme value or peaks-over-threshold fit) of tide-gauge and surge records. Sea-level rise shortens the return period of any fixed level, which is why the future sea-level allowance is defined to hold the return period constant.