Future Sea Level Allowance

A future sea-level allowance is the height a coastal structure or planning level is raised so that its present-day flood return period is preserved under projected sea-level rise. It is computed by combining the central rise projection with the statistics of extreme sea levels, so the allowance accounts for both the mean shift and the change in extreme-event frequency, not just the projected rise alone. IPCC AR6 frames it through the joint distribution of sea-level rise and storm-tide return periods. A defense designed to the 1-in-100-year level today needs an allowance that keeps it at 1-in-100 for its full design life.