  Sea-level rise (SLR) causes estimates of flood risk made under the assumption
of stationary mean sea level to be biased low. However, adjustments to flood
return levels made assuming fixed increases of sea level are also inaccurate
when applied to sea level that is rising over time at an uncertain rate. To
accommodate both the temporal dynamics of SLR and their uncertainty, we develop
an Average Annual Design Life Level (AADLL) metric and associated Design Life
SLR (DL-SLR) allowances. The AADLL is the flood level corresponding to a
time-integrated annual expected probability of occurrence (AEP) under
uncertainty over the design life of an asset; DL-SLR allowances are the
adjustment from 2000 levels that maintain current average probability over the
design life. Given non-stationary and uncertain sea-level rise, AADLL flood
levels and DL-SLR allowances provide estimates of flood protection heights and
offsets for different planning horizons and different levels of confidence in
SLR projections in coastal areas. Here we employ probabilistic sea-level rise
projections to illustrate the calculation of AADLL flood levels and DL-SLR
allowances for a set of long-duration tide gauges along U.S. coastlines.
