Abstract
A regional frequency analysis (RFA) of tide gauge (TG) data fit with a Generalized Pareto Distribution (GPD) is used to estimate contemporary extreme sea level (ESL) probabilities and the risk of a damaging flood along Pacific Basin coastlines. Methods to localize and spatially granulate the regional ESL (sub-annual to 500-yr) probabilities and their uncertainties are presented to help planners of often-remote Pacific Basin communities assess (ocean) flood risk of various threshold severities under current and future sea levels. Downscaling methods include use of local TG observations of various record lengths (e.g., 1-19+ years), and if no in-situ data exist, tide range information. Low-probability RFA ESLs localized at TG locations are higher than other recent assessments and generally more precise (narrower confidence intervals). This is due to increased rare-event sampling as measured by numerous TGs regionally. For example, the 100-yr ESLs (1% annual chance event) are 0.15 m and 0.25 higher (median at-site difference) than a single-TG based analysis that is closely aligned to those supporting recent Intergovernmental Panel on Climate Change (IPCC) assessments and a third-generation global tide and surge model, respectively. Height thresholds for damaging flood levels along Pacific Basin coastlines are proposed. These floods vary between about 0.6-1.2 m or more above the average highest tide and are associated with warning levels of the U.S. National Oceanic and Atmospheric Administration (NOAA). The risk of a damaging flood assessed by the RFA ESL probabilities under contemporary sea levels have about a (median) 20 to 25-yr return interval (4-5% annual chance) for TG locations along Pacific coastlines. Considering localized sea level rise projections of the IPCC associated with a global rise of about 0.5 m by 2100 under a reduced emissions scenario, damaging floods are projected to occur annually by 2055 and >10 times/year by 2100 at the majority of TG locations.
Highlights
Coastal flood risk is on the rise along many coastlines
In light of rising seas, there is a need for data and mapped (e.g., Figure 1) information regarding current and future LESL probabilities along all populated coastlines—not just where tide gauges (TGs) exist—for planning and preparedness purposes
Even where TGs exist, measurements often are not long enough to capture historically significant events at a particular location or miss those completely that are of regional significance
Summary
Coastal flood risk is on the rise along many coastlines. Across the Pacific Basin and its vast coastlines, decision makers are facing mounting impacts from rising seas and they need flood risk information to plan and implement welltimed adaptation solutions (Keener et al, 2018). With a global sea level rise of 0.5 m (or more) projected by 2100 under a reduced emissions scenario (Church et al, 2013; Oppenheimer et al, 2019) and impacts and expenditures mounting (Moftakhari et al, 2017), the need for contemporary and future projected estimates of coastal flood risk by decision makers here and elsewhere is growing (Le Cozannet et al, 2017; Hall et al, 2019; Kopp et al, 2019)
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