How important is extreme sea level variability for coastal flood risk assessments?

Abstract:

Long-term mean sea level rise is often considered as the only driver in coastal flood risk assessments, ignoring other important contributors such as interannual to multi-decadal variability of mean sea level and storm surges, and long period tides (i.e. 4.4-year perigean and 18.6-year nodal cycles). In an earlier study, we quantified the aggregated variability from these variables at seven coherent regions along the U.S. coasts, termed as aggregated extreme sea level (AESL) indicators. Here, we examine the potential contributions of AESL variability to coastal flood risks, expressed as average annual losses (AAL) for 17 large U.S. coastal cities. We employ the same model as Hallegate et al. (2013) to estimate AAL from modified extreme sea level distributions obtained by adding ranges of AESL variability. The range of AESL variability is the historical absolute difference of the lowest and highest values of AESL indicator at a tide gauge. Results show that AAL increases, particularly for cities along the U.S. Atlantic and Gulf of Mexico coasts when AESL variability is considered. Additionally, we examine how AESL variability modifies the present-day 100-year water levels over the historical period in terms of amplification factors. Results reveal that amplification factors vary by regions and are as high as 20, meaning that the AESL variability can turn a 100-year event into a 5-year event over a certain period of time. We also translate historical ranges of AESL variability into equivalent regional sea level rises in the future under different scenarios. Depending on the location and considered sea level rise scenarios, AESL variability is equivalent to sea level rise that is expected over the next few years up to several decades (in some cases up to 2100). This study signifies the consequences of AESL variability in regional coastal flood risk and importance to consider this variability on top of the log-term mean sea level rise in risk assessments, infrastructure design, and decision-making.