New method for quantifying ocean hazards for Hawaiian coastal highways

The Hawaiian Islands, in the North Pacific Ocean, are vulnerable to ocean hazards such as waves, sea level rise and coastal erosion. Here, we study and develop a set of criteria, the Ocean Hazard Classification Scheme (OHCS), to analyze and rate the ocean hazard to coastal roads. Our study focuses on State of Hawaii Department of Transportation (HDOT) coastal routes, at 302 mileposts identified as “susceptible”, due to the condition of or proximity of the road to the ocean. The OHCS presents five main ocean hazard variables, which are sea level rise, maximum annually recurring waves, shoreline change, tsunami, and storm surge. For sea level rise, we apply the data, which consider both the historical and projected scenarios, during 1905-2050. For maximum annually recurring wave, we consider the significant wave height during 2010-2018, which is modeled in the wind-driven Simulating WAves Nearshore wave model. For the shoreline change, we apply the mean projected shoreline change rate during 2008-2100. These three variables are ranked into five classes, 1 (very low) to 5 (very high), according to the percentile of the observed maximum values, respectively. For tsunami, we consider both the historical and hypothetical flow depth. We classify the mileposts into three categories according to the values of historical and hypothetical flow depths. The mileposts are ranked into three classes, 3 (moderate), 4 (high), and 5 (very high), in Category 1, two classes, 1 (very low) and 2 (low), in Category 2, and one class, 1 (very low), in Category 3 according to the percentile of the observed maximum values, respectively. For storm surge, we consider the inundation height caused by Category 1 to 4 storms. Storm inundation heights are ranked into five classes from 1 (very low) to 5 (very high), according to the percentile of the average inundation heights for each category storm.