Allowances for evolving coastal flood risk under uncertain local sea-level rise

TL;DR

This paper introduces a new metric, AADLL, and allowances, DL-SLR, to better estimate and adjust flood risk levels in coastal areas considering uncertain and evolving sea-level rise over time.

Contribution

It develops a novel probabilistic framework and metrics to account for non-stationary and uncertain sea-level rise in coastal flood risk management.

Findings

AADLL flood levels vary with planning horizons and confidence levels.

DL-SLR allowances effectively maintain current flood probabilities over asset lifetimes.

Application to U.S. tide gauges demonstrates practical calculation methods.

Abstract

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.