Impact and Recommendations

The following section briefly describes the impacts at Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP. This section describes conditions that may require attention from resource managers.

It is estimated that sea-level on the Big Island of Hawai‘i will be approximately 0.15-0.41 m above present by 2050, and 0.32-1.55 m by 2100 (Table 2). This will create a number of coastal impacts to Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP ranging from passive flooding of low-lying areas, to increased coastal overtopping and erosion. These impacts will threaten the existence of low-lying coastal landmarks, historic sites, and coastal habitat of the flora and fauna.

Kaloko-Honokōhau NHP

Coastal overtopping at Kaloko Seawall will increase significantly under projected sea-level rise. The number of large swell events which fully overtop the seawall will increase by a factor of 3-4 by 2050 and by a factor of greater than 10 by 2100. This overtopping will continue to cause significant undermining of the seawall. The seawall is currently being rehabilitated and maintained after decades of disrepair. The northern portion of the wall is currently undergoing rehabilitation, the southern portion was completed in 2004. If maintenance of the seawall does not actively continue into the future, the wall will begin to fall into disrepair as overtopping increases.

Coastal erosion will continue along the beach fronting ‘Aimakapā Fishpond. The beach is approximately 11-22 ft wide and the average erosion rate is between 0.2-0.5 ft per year. If these rates continue the beach will erode to the point where the likelihood of a breach of the fishpond is possible by 2050. If the beach experiences erosional events to the point where a breach is possible, then mitigation measures should be considered. One measure is salinity testing to determine the amount of mixing occurring between seawater and the groundwater-fed fishpond. Also, monitoring high tides and large swell events can be done (e.g., using photography). This monitoring can help determine if active measures (e.g., sandbagging) need to be taken to prevent a breach.

Sea-level rise will passively flood the wetlands of the middle portion of Kaloko-Honokōhau NHP between Kaloko Seawall and the beach fronting ‘Aimakapā Fishpond. Impacts from this flooding include hampering park access along coastal trails and, habitat changes, especially to vegetation that is not salt-tolerant. Because sea-level rise is a subtle process, flooding will increase gradually. Long-term monitoring of wave damage should be conducted in this section. Monitoring can include beach profiles for sections where beaches exist or photographs of wave action on the coast before, during, and after the maximum annual high tide and should include land that will be inundated in 2050 (Figure 14).

The Heiau on the southern portion at ‘Ai‘ōpio Fishtrap will be impinged by sea-level rise, although it is not likely to experience failure based on 2050 projected sea-level rise.

Sea-level rise will submerge the ‘Ai‘ōpio Fishtrap during low tide by 2050, and constantly submerge the fish trap by 2100. Long-term monitoring of the fishtrap during high tide and large swell events is recommended. If the fishtrap is in danger of submersion, we recommend working with local cultural agencies to determine whether to prevent or allow submersion of the cultural site.

Potential tsunami hazards based on the 1946 tsunami appear to be minimal. Measured runup from the 1946 tsunami is higher than the modeled runup and should be taken into account during decision-making processes. Tsunamis originating form other directions were not analyzed for this report, but could be a potential topic for future work.

Pu‘ukoholā Heiau NHS

Projected sea levels will passively flood Pelekane Beach on the northern portion of Pu‘ukoholā Heiau NHS. This will result in increased erosion and loss of trees backing the beach. By 2100 the entire beach will likely be submerged at high tide. The archeological sites behind Pelekane Beach will also be flooded under large swell by 2100. We recommend monitoring this section of beach. Beach profiling (either biannually or annually) can be used to document any changes in the beach which could threatened archeological sites. Differential GPS can be used to monitor these archeological sites. Also, long-term monitoring of wave action due to high tide and large swell events will document any impacts and changes that occur due to sea level. If archeological sites show deterioration due to wave action, we recommend consulting with local cultural agencies to discuss appropriate action for these sites.

Sea-level rise will passively flood sections of the Ala Kahakai NHT by 2100. However, impacts other than the loss of Pelekane Beach appear minimal. The main Heiau of Pu‘ukoholā Heiau NHS (Pu‘ukoholā Heiau and Mailekini Heiau) will not be threatened by coastal hazards in the foreseeable future. We recommend continuous monitoring of the coastal trail.

Potential tsunami hazards based on the 1946 tsunami appear to be minimal. Measured runup from the 1946 tsunami is higher than the modeled runup and should be taken into account during decision-making processes. Tsunamis originating form other directions were not analyzed for this report, but could be a potential topic for future work.