This section describes the paleotsunami evaluation and the modeling of inundation by tsunami at Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP.
Tsunamis are a series of waves of very long wavelength (100's km) and period (10's minutes – 1 hour or more) that can travel up to 1,000 km/hr in the open ocean. They are caused by disturbances that displace large volumes of water and are usually generated by seafloor displacement during earthquakes, but they can also be caused by volcanic eruptions, submarine landslides, and oceanic bolide impacts. Tsunamis can impact coasts on either ocean-wide, regional (~ 1,000 km) or local (~100 km) scales. In the open ocean the tsunami wave height may be only a meter or two, but as the wave approaches shallow water it slows down and begins shoaling resulting in dramatic increases in wave height. Damage from a tsunami is caused by inundation (flooding of the land surface), wave impact, and sediment erosion and deposition. In general the larger the tsunami the greater the impact. However, tsunami runup height (elevation at the limit of inundation) and inundation from an individual tsunami typically vary greatly over short distances due to complex interactions between the wave and land surface.
Historic tsunamis are events that have either been observed or measured and are documented in some type of written or oral record. Historic tsunamis in Hawai‘i have either been caused by ocean-wide events derived from distant earthquakes, or locally-derived phenomena. Located in the middle of the Pacific Ocean, Hawai‘i may receive tsunamis from a number of seismic sources including the Aleutian Islands, Japan, Chile, Kamchatka, and South Pacific islands. Walker (1994) noted 22 Pacific basin tsunamis with runup greater than 1 m have been observed in Hawai‘i since 1812. The highest Hawai‘i tsunami runup elevation reported by Lander & Lockridge (1989) was 16.4 m at Waikolu Valley, Molokai as a result of the 1946 Aleutian Islands earthquake event. Tsunami runup on the island of Hawai‘i from the 1946 tsunami ranged from 2 m at Honaunau to 12 m at Waipio Valley (Lander & Lockridge, 1989). The last large tsunami of distant origin to affect the Hawaiian Islands was generated by a great (magnitude 9.5) earthquake in Chile in 1960 that caused extensive damage in the Hilo area (Dudley & Lee, 1998). Since that last occurrence there has been widespread and intensive human development along the Hawaiian shoreline. Recently installed monitoring systems in the Pacific Ocean are designed to give early warning of impending ocean-wide tsunamis.
In addition to ocean-wide events, the Hawaiian Islands are subject to locally generated tsunamis. Twenty-three tsunamis with measurable runup and a local source have been recorded for Hawaii since 1840 as documented in the NOAA World Data Center (WDC) Historical Tsunami Database at the National Geophysical Data Center (NGDC; available on-line at: http://www.ngdc.noaa.gov/hazard/tsu_db.shtml , last accessed on 11/21/08). Maximum runup height from the NGDC data base is 14.3 m at Keauhou Landing (SE Hawaii) as a result of the locally generated tsunami created by the 1975 Kalapana earthquake (M 7.2) characterized by rapid coastal subsidence and associated submarine slump (Day et al. 2005). This was the largest locally generated tsunami to impactHawai‘i in the 20th century and it produced deposits as much as 320 m inland and up to 10 m above sea level (Goff et al. 2006). A similar locally generated tsunami was caused by magnitude 7.9 shock of 1868 located on the south flank of Mauna Loa. Locally generated tsunamis arrive very soon after the generating event, therefore the generating event, such as an earthquake or volcanic eruption, should serve as a warning to evacuate from the coast.
Because of its coastal setting, Pu‘ukoholā Heiau NHS is vulnerable to increased ocean-inundation potential from tsunamis (Figure 28). An event similar to the tsunami generated by the 1946 Aleutian Islands would most likely severely damage the beach and park infrastructure at Pelekane Beach, while causing less damage to the rocky shoreline of the park.
Figure 28. Coastal hazards for Kawaihae Bay, Hawai‘i (from Fletcher et al. 2002). Pu‘ukoholā Heiau NHS is part of Kawaihae. The map shows 7 natural hazards, including tsunami hazards (http://pubs.usgs.gov/imap/i2761/).
Kaloko-Honokōhau NHP is also vulnerable to hazards that increase ocean inundation potential such as tsunamis, storms, and sea-level rise (Figure 29). Lander & Lockridge (1989) identified historical tsunamis that have struck the coast near Kailua-Kona since 1896. The tsunami runup ranged in height from 0.6 m to 3.4 m with the largest runup originating from a 1946 earthquake in the Aleutian Islands. A tsunami of similar magnitude occurring today would most-likely cause damage to the beaches and park infrastructure and historical sites near the coast. The basalt rock areas are relatively stable and would likely undergo little change.
Figure 29. Coastal hazards for Keahole (A) and Kailua-Kona (B), Hawai‘i (from Fletcher et al. 2002). Kaloko-Honokōhau NHP is part of Keahole and Kailua-Kona. The map shows 7 natural hazards, including tsunami hazards (http://pubs.usgs.gov/imap/i2761/).
Tsunamis in which there is no historical record are termed paleotsunamis, and their occurrence and distribution is based primarily on the identification, dating, and mapping of sedimentary deposits formed by a tsunami. Identification of tsunami deposits can be used to delineate areas impacted by tsunamis and provide clues to the magnitude of the event. Multiple deposits can provide information on the recurrence interval and extend the record of tsunami impacts back through time. Tsunami deposits are created during the erosion and deposition of sediment that occurs during the passage of the tsunami waves. They have similar characteristics to other wave formed deposits such as those formed by storm waves, but there are a number of criteria that aid in their identification, such as:
- marine debris, such as skeletal material from marine organisms deposited well inland (100's m) and at high elevations (up to 10+ m)
- sheet-like deposits that gradually thin inland
- deposits that infill topographic lows and thin on topographic highs
- large blocks ( > 2m) transported 100's m inland
- sharp erosional basal contact with underlying material
- normally graded (fining upward) sand layer(s)
In general, the morphology of tsunami deposits tend to be sheet-like and extend farther inland than storm deposits that tend to form shore-parallel ridges of limited inland extent. Perched beach ridges are prominent features at Kaloko-Honokōhau NHP and are most-likely the products of seasonal storms that strike the coast. No paleotsunami deposits have been described from either Kaloko-Honokōhau NHP or Pu‘ukoholā Heiau NHS, although these areas lack detailed geologic studies using recent advancements in paleotsunami identification.
Elsewhere on the Island ofHawai‘i there is paleotsunami evidence in the geologic record indicating there have been extremely rare, but locally severe, mega-tsunamis (McMurtry et al. 2004). Fossiliferous marine conglomerates along the northwest coast of Kohala Volcano have been interpreted as mega-tsunami deposits generated by a flank-failure submarine landslide on western Mauna Loa. According to McMurtry et al. (2004), that landslide and tsunami occurred about 110,000 years ago; the tsunami had an estimated runup more than 400 m high and an inundation greater than 6 km inland on the flanks of Kohala Volcano. Catastrophic flank failures are extremely rare geologic events but are an important process in volcanic island evolution. These flank failures influence the island shape and the morphology of the coastal zone.
We have constructed a tsunami model of the national parks based on the April 1, 1946 tsunami that originated from a magnitude 7.5 earthquake in the Aleutian Islands. This event is regarded as one of the most devastating tsunamis in Hawai‘i and is thus a candidate to evaluate tsunami risk assessment. The tsunami was recorded in the Honolulu tide gauge, and reported to have a period of 15 minutes (Green 1946). Mader (2004) conducted extensive tsunami modeling studies of Hawai‘i including the 1946 tsunami, which used tsunami water-level boundary conditions of 1 m wave heights with 1000 sec period as boundary conditions for a model of the Hawaiian Island. We employed this approach in our assessment of the national parks. High-resolution models of the national parks were nested in the regional model of the Hawaiian Islands to estimate the extent of inundation. The modeling was performed with the Delft3D modeling system, which is a non-linear shallow water equations model capable of simulating tsunami propagation and inundation (see Appendix A for model settings). The model output of the 1946 tsunami scenario is shown in Figure 30. While the tsunami wave heights were only slightly larger than 1 m in the open waters north of the Hawaiian Islands, the nearshore water levels on north facing shores of the islands were significantly larger than 1 m due to shoaling (Figure 31).
Figure 30 . 1946 Alaska Aleutian tsunami scenario modeled water-levels for the Hawaiian Island regional grid at Pu‘ukoholā Heiau NHS and Kaloko-Honokōhau NHP using Delft3D.
Figure 31 . The maximum water levels of the 1946 tsunami model.
As seen in both Figure 30 and Figure 31, the maximum water levels at the national parks sites were very small. The tsunami loses energy as it refracts around the islands, and the national parks sites are in such a location that energy loss is significant. Kaloko-Honokōhau NHP is particularly in a tsunami shadow zone, while Pu‘ukoholā Heiau NHS does receive tsunami energy that refracts between the Alenuihaha Channel between Maui and the Big Island.
The maximum water levels modeled in high-resolution grids near at the national parks sites are reported in Figure 32 and Figure 33. Predicted water levels, or runup, is greater at Pu‘ukoholā Heiau NHS than Kaloko-Honokōhau NHP due to the larger offshore tsunami wave heights seen in Figure 30, and resonance inside the partially enclosed basin of the southern portion of the Kawaihae breakwater, Pelekane Beach, and the coastline of the park. Maximum water levels inside this region reach 1.8 m. Such elevated water levels would extend far inland into the marsh area backing Pelekane Beach. Additionally, the archeological sites at Pelekane would be threatened by flooding and wave impacts.
The majority of Kaloko-Honokōhau NHP experiences slightly elevated tsunami water levels of 0.2 m, which should not cause any major impacts. The beach fronting ‘Aimakapā Fishpond experiences the largest tsunami water levels (0.4 m) at the park, due to shoaling over the reef. However, these water levels are still much smaller than the high swell runup levels reported in Table 6. Historical records of the 1946 tsunami show that Kawaihae had a runup value of 4.3 m and Kailua-Kona had a runup value of 3.4 m (Lander & Lockridge 1989). These values exceed our model predictions.
The flooding model of the parks shown in Figure 32 and Figure 33 was performed on grids that are not as high resolution as the digital elevation model grids, thus we use GIS software to map the tsunami inundation extent. The final inundation contours for the 1946 tsunami scenario are given in Figure 34 and Figure 35. Based on these maps, impacts to Pelekane Beach at Pu‘ukoholā Heiau NHS seem to be the greatest threat of damage to culturally and historically significant regions of the parks posed by tsunami hazards. The risk posed by Aleutian tsunamis to Kaloko-Honokōhau NHP seems minimal primarily due to the shadowing effect of the islands to the northwest.
Figure 32 . Maximum 1946 tsunami scenario water levels at Pu‘ukoholā Heiau NHS modeled by Delft3D.
Figure 33 . Maximum 1946 tsunami scenario water levels at Kaloko-Honokōhau NHP modeled by Delft3D.
Figure 34 . 1946 tsunami scenario maximum inundation contours at Pu‘ukoholā Heiau NHS.
Figure 35 . 1946 tsunami scenario maximum inundation contours at Kaloko-Honokōhau NHP.