This report accompanies the digital hazard posters for Pu‘ukoholā Heiau National Historic Site (Pu‘ukoholā Heiau NHS or PUHE) and Kaloko-Honokōhau National Historical Park (Kaloko-Honokōhau NHP or KAHO) on the island of Hawai‘i. It contains information relevant to resource management and scientific research.
The Hawaiian Islands are seldom directly hit by hurricanes. However, high waves from low-pressure systems and other storms affect the coastline on a regular basis. As sea level rises, these high wave events will have further impacts on the coast, which will threaten the ecology and cultural resources found in this region. For planning and emergency response purposes, the National Park Service (NPS) requested that the University of Hawaii Coastal Geology Group and USGS complete an assessment of coastal vulnerability to wave overtopping, sea-level rise, and flooding for two national park units in the Pacific Islands Network (PACN). By identifying vulnerable sections within these parks, NPS managers can identify and document cultural structures that might be threatened and plan safety protocols when storms approach. Storm vulnerability assessments are completed for Pu‘ukoholā Heiau National Historic Site (Pu‘ukohol NHS or PUHE) and Kaloko-Honokōhau National Historical Park (Kaloko-Honokōhau NHP or KAHO) on the Island of Hawai‘i.
This report includes assessment of the current shoreline morphology and several coastal hazards at the National Parks. The addressed hazards are coastal inundation and overtopping from large swells, sea-level rise, tsunamis, and coastal erosion. This report includes an interpretation of the results and provides explanations regarding coastal hazards. Products also include maps of historical shoreline change showing coastal erosion areas for each park and a paleotsunami evaluation.
One goal of this project is to generate maps revealing areas vulnerable to storms, extreme wave events, and sea-level rise. Other objectives include viewing the shoreline morphology through digital elevation models and producing posters that depict rates of shoreline change for each park. The approach taken for each hazard assessment in this report is listed below:
- Coastal Inundation Maps (wave over-topping, sea-level rise and tsunami)
A time series of overtopping frequencies are provided using various published sea level scenarios (e.g., IPCC, 2007; Rahmstorf, 2007, and others). Inundation by tsunami is numerically modeled using the 1946 Gulf of Alaska source event as a base scenario. Overtopping and tsunami inundation hazard areas are defined on maps for each park.
- Shoreline Morphology
Digital elevation models of shoreline morphology including photogrammetry and interpretive layers will be provided for the parks.
- Historical Shoreline Change and Coastal Erosion
A 50-year projection of shoreline change is produced in the form of a poster. Deliverables consist of maps for each park showing modern change rates spaced 20 m along the shore and a 2050 erosion hazard zone at 95% probability.
- Paleotsunami assessment
Evaluation of paleotsunami history is included within the tsunami hazard section.
Regions of the parks vulnerable to many or specific coastal hazards are detailed in this report. A description of specific areas that are vulnerable to coastal hazards is provided along with recommended mitigation steps. For both parks, potential tsunami hazards appear to be minimal. However, impacts to Pelekane Beach at Pu‘ukoholā Heiau NHS have the greatest threat of damage to historical and cultural sections of the park. Both parks will also see an increase in the frequency of wave overtopping due to sea-level rise.
Sea-level rise is a subtle process and there may not be a clear trigger for adaptive management. Most likely, seasonal wave damage will increase and this will be the clearest indication. In order to prevent a state of disrepair and preserve key coastal features and points of interest, park personnel should monitor regions vulnerable to coastal impacts detailed in this report. Ideally, monitoring should last 25-50 years. For areas that will be flooded or submerged in the future due to sea-level rise, continuous monitoring of the wave action should start now. Begin monitoring the vulnerable areas during maximum annual high tide events (monitoring should start a few days before the high tide event) and large swell events. Use photography to record the wave wash and any damage associated with it, beach profiles to record the beach shape (if a beach exists), and differential GPS on archeological sites. As sea level rises with time, increase the frequency of monitoring. If enough erosion or inundation and overtopping events threaten the archeological sites, coordinate with local agencies to determine if the historic sites should be preserved in the short- (or long-) term. If preservation of the sites is the goal, consider enacting active measures (e.g. sandbagging).
The Ala Kahakai National Historic Trail (NHT) along the Pu‘ukoholā Heiau NHS coast will see a higher frequency in flooding due to sea-level rise. The trail at Pelekane currently experiences wave spray and overwash on a yearly basis, with visible erosion and root exposure seaward of the trail. The southern end of the trail may experience wave spray and overwash at sea-level conditions of 0.5 – 1 m and greater.
At Pu‘ukoholā Heiau NHS, Pelekane Bay and the remaining archeological sites are threatened by coastal hazards. Wave runup at sea-level scenarios of 0.25 – 0.5 m will extend inland under the trees at Pelekane beach. This 2 NPS Geologic Resources Division will cause extensive erosion and tree loss. Erosion and sea-level rise above 0.5 m will cause the beach to be mostly submerged at high tide. As sea level rises to 1m, the beach at its present state will be constantly submerged. The archeological sites at Pelekane will be exposed to wave overwash and spray at sea-level scenarios of 0.5 – 1 m. At sea-level rise greater than 1 m, the sites may be submerged at high tide.
The kuapā (seawall) at Kaloko Fishpond, the beach seaward of ‘Aimakapā Fishpond, and the archeological sites at the southern portion of Kaloko-Honokōhau NHP including the ‘Ai‘ōpio Fishtrap and Pu`uoina Heiau are the historic sites and features that are at greatest risk to deterioration from coastal hazards. Kaloko seawall and the northern section of Honokōhau beach incur the largest risk of overtopping and deterioration from wave impacts because they are at the park's lowest elevation. Currently, Kaloko seawall experiences overwash and wave spray during small to moderate swell and high tides. Continual maintenance of Kaloko Seawall is recommended to prevent loss.
The beach fronting ‘Aimakapā Fishpond is eroding at average rates of 0.08 – 0.15 m (0.25 – 0.5 ft) per year. If these rates continue and if sea level continues to rise, the fishpond will be breached by 2050, affecting the coastal trail and the ecology of the wetland habitat. The beach separates the fishpond from the ocean. Presently, areas of this beach are partially overtopped more than once a year. As sea level rises, the frequency of overwash will increase. At sea-level scenarios of 0.5 – 1 m, the entire beach will be fully overtopped several times a year, which may cause significant erosion and breaching of the sand barrier. Breaching will result in mixing seawater with groundwater, causing the salinity and nutrient levels of the estuary to be similar to seawater. The change in salinity and nutrient levels may pose a serious hazard to the park by impacting the flora and fauna of the estuary, including two federally endangered waterbirds. Monitoring salinity because of increased overtopping is recommended.
For ‘Ai‘ōpio Fishtrap, significant impacts such as flooding will occur during projected sea-level conditions. Since 1950, sand has been migrating north along Honokōhau Beach and out to ‘Aimakapā Fishpond , exposing several cultural sites to minor wave action. ‘Ai‘ōpio Fishtrap will be entirely submerged by 2100. The Heiau at ‘Ai‘ōpio Fishtrap will feel the effects of higher sea levels, but should not experience failure based on 2050 sea-level conditions.