At the local level, for City and County of Honolulu, the vulnerability in terms of accessibility reduction is unevenly distributed spatially. The North part of the island near Kahuku, the east part near Hawaii Kai, and the central part near Honolulu Harbor may experience more impacts from tidal flooding than others. These communities also have a low level of accessibility even without coastal flooding.

In Mapunapuna industrial district of Honolulu, heavy rains during monthly highest tides have submerged the road a waist deep because storm drains were backed up with high ocean water (SOEST, n.d.).

City and county of Honolulu is selected as a case study given its special vulnerability to sea level rise. In Hawaii, a 0.15 meter increase of sea level has been observed in the past century with an expected increase of 0.9 meter by 2100. Sea level rise may also cause substantial groundwater inundation. estimate that “0.6m of potential sea-level rise causes substantial flooding, and 1m sea-level rise inundates 10% of a 1-km wide heavily urbanized coastal zone” in Honolulu. The impacts of sea level rise have already revealed in recent years. Data from NOAA tide stations around Hawai’i show that observed water levels have been 3-6 inches above predicted tidal heights since early 2016. In late April, levels peaked at more than 9 inches above predicted tides at the Honolulu Harbor tide gauge, resulting in the highest daily mean water level ever observed over the 112-year record. Despite the severity of the problem, there is a lack of understanding of sea level rise’s potential system impacts on Hawaii’s transportation roadway network, a lack of empirical study about social sensitivity to such impacts, and a need to identify the most vulnerable communities for adaptation prioritization.

Table 3-1 shows that the number of tidal flooding events and durations increases in recent years, especially in terms of annual flooding events. The duration and frequency of transportation network disruption increases with such trend as well.

Table 3-1 Duration and Frequency of Coastal Flooding on Oahu 2012-2018

Annual Exceedance Probability Levels (2017) Water Level (MHHW datum) Duration of Events (hours)
    2012 2013 2014 2015 2016 2017 2018
1% 1.41 ft 0 0 0 0 0 0 0
10% 1.18 ft 0 0 0 0 0 10 0
50% 0.89 ft 0 0 1 0 26 63 11
99% 0.49 ft 33 45 93 48 229 423 46

Table 5-1 summarizes the number and length of affected roadways, and Appendix B shows the flooding maps and affected roadways under each scenario. The maps and table show that there are no significant differences among the extent and impacts of different tidal flooding levels (1%, 10%, 50%, 99% annual excellence probability levels) on transportation road network on Oahu. Therefore, the 1% annual excellence extreme scenario is selected to perform detailed vulnerability analyses.

Table 5-1 Summary of Affected Roadways

Annual Flood Exceedance Probability Number of Road Segments Affected Length of Affected Roads (miles)
1% 161 33.11
10% 160 32.2
50% 154 31.54
99% 154 30.32

Appendix B : Flooding Maps and Affected RoadsFigure 5-3 Accessibility Reduction by TAZ on the Island of Oahu. 

Transportation Vulnerability Assessment

TAZs experiencing the highest amount of accessibility reduction locate in the North part of the island near Kahuku, the east part near Hawaii Kai, and the central part near Honolulu Harbor. In general, these places are also TAZs have low original accessibility to all kinds of opportunities.

Figure 5-3 Accessibility Reduction by TAZ on the Island of Oahu. 

Community Mapping Analysis

In total we got 48 participants from community mapping activities, 25 from Sunset and 23 from Kaneohe. The 48 participants identified a total of 165 vulnerable transportation infrastructure they either currently experienced flooding or where they were concerned about flooding in the future (Figure 6-2). Of 48 responses, over 50% respondents chose storm surge and coastal erosion as number one most frequently experience coastal hazard conditions, and followed by king tide and inland groundwater inundation contains 6.2% response each (see figure 6-3). The type of impacts associated with these hazards conditions vary, but the most prevalent are flooding and inundation of the origin/ destination and along the trip route, and detour due to road closure (Figure 6-4). Other coastal flooding, on the other hand, caused the least impact. Almost all participants could drive a car and had at least one car per household, although there were also numerous complaints of experiencing costal erosion and storm surge over bike paths and pedestrian walkways.