geosciences

Article Intermittent but Rapid Changes to Coastal Landscapes: The Tsunami and El Niño Wave-Formed Sea Arch at Laie Point, , , U.S.A.

Benjamin R. Jordan

Faculty of Science, -Hawaii, BYUH #1967, 55-220 Kulanui Street, Laie, HI 96762, USA; [email protected]

Abstract: Kukuiho’olua Island is an islet that lies 164 m due north of Laie Point, a peninsula of cemented, coastal, Pleistocene and Holocene sand dunes. Kukuiho’olua Island consists of the same dune deposits as Laie Point and is cut by a sea arch, which, documented here for first time, may have formed during the 1 April 1946 “April Fools’s Day Tsunami.” The tsunami-source of formation is supported by previous modeling by other authors, which indicated that the geometry of overhanging sea cliffs can greatly strengthen and focus the force of tsunami waves. Additional changes occurred to the island and arch during the 2015–2016 El Niño event, which was one of the strongest on record. During the event, anomalous wave heights and reversed wind directions occurred across the Pacific. On the night of 24–25 February 2016, large storm waves, resulting from the unique El Niño conditions washed out a large boulder that had lain within the arch since its initial formation, significantly   increasing the open beneath the arch. Large waves also rose high enough for seawater to flow over the peninsula at Laie Point, causing significant erosion of its upper surface. These changes at Citation: Jordan, B.R. Intermittent but Rapid Changes to Coastal Laie Point and Kukuio’olua Island serve as examples of long-term, intermittent change to a coastline— Landscapes: The Tsunami and El changes that, although infrequent, can occur quickly and dramatically, potentially making them Niño Wave-Formed Sea Arch at Laie geologic hazards. Point, Oahu, Hawaii, U.S.A. Geosciences 2021, 11, 147. https:// Keywords: Kukuiho’olua Island; Laie; Hawaii; ENSO; tsunami; sea arch; coastal erosion; El Niño doi.org/10.3390/geosciences11030147

Academic Editors: Jesus Martinez-Frias and 1. Introduction Elena Marrocchino 1.1. Overview The documentation of coastal change is important at a time of rising eustatic sea Received: 12 February 2021 levels [1,2]. Rising sea levels can lead to greater inundation and erosion of shorelines [3] Accepted: 9 March 2021 Published: 23 March 2021 and thus greater impacts from storm and tsunami waves, which can be significant hazards to coastal communities. Understanding the history of coastal change at a given location is important for correctly interpreting future hazards at the location and can serve as a proxy Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in for understanding similar hazards and change at other locations. Such documentation is published maps and institutional affil- also critical for correctly interpreting the geologic history of islands and coasts [4]. This iations. paper documents the evolution and change of a sea arch in a small island off of the northeast coast of the larger Hawaiian Island of Oahu, as well as other changes to the nearby coastal peninsula—changes caused by short-duration, but high energy events related to storms and tsunamis.

Copyright: © 2021 by the author. 1.2. Geographical and Geological Setting Licensee MDPI, Basel, Switzerland. This article is an open access article Kukuiho’olua Island is a small, islet that lies 164 m due north of Laie Point (tradi- distributed under the terms and tionally called Laniloa Point), a peninsula of cemented, coastal, Pleistocene and Holocene conditions of the Creative Commons sand dunes [5] on the northeastern coast of the Hawaiian Island of Oahu (Figure1). Attribution (CC BY) license (https:// Kukuiho’olua Island consists of the same dune deposits as Laie Point. The island is creativecommons.org/licenses/by/ 173.75 m in length at its longest, with its eastern area having a width of 47 m, which is the 4.0/). widest part, and a width of 27.5 m on its western side. The island runs due east–west, with

Geosciences 2021, 11, 147. https://doi.org/10.3390/geosciences11030147 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 13

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in length at its longest, with its eastern area having a width of 47 m, which is the widest part, and a width of 27.5 m on its western side. The island runs due east–west, with a a narrow, eroded, center that forms a sea arch [6]. The opening of the arch is perpendicular narrow, eroded, center that forms a sea arch [6]. The opening of the arch is perpendicular to the length of the island. The width of this central area that forms the arch is 8.6 m. In to the length of the island. The width of this central area that forms the arch is 8.6 m. In addition to Kukuho’olua, there are four other small islands in the vicinity of Laie Point. To addition to Kukuho’olua, there are four other small islands in the vicinity of Laie Point. the north, the northernmost island is Kihewamoku, with Mokuaia Island (locally known To the north, the northernmost island is Kihewamoku, with Mokuaia Island (locally as “Goat Island”) and Pulemoku Island lying between Kihewamoku and Kukuiho’olua. known as “Goat Island”) and Pulemoku Island lying between Kihewamoku and Ku- To the southeast of Kukuiho’olua and directly east of Laie Point is another islet named kuiho’olua. To the southeast of Kukuiho’olua and directly east of Laie Point is another Mokualai Island. The bathymetry around Laie Point, Mokualai Island, and Kukuiho’olua islet named Mokualai Island. The bathymetry around Laie Point, Mokualai Island, and has not been definitively mapped due to dangerous wave and current regimes that are Kukuiho’olua has not been definitively mapped due to dangerous wave and current re- formed when swells generated by the easterly Trade Winds interact with the relatively gimes that are formed when swells generated by the easterly Trade Winds interact with shallow sea bottom of the area. It is these nearly continuous easterly waves that are the the relatively shallow sea bottom of the area. It is these nearly continuous easterly waves source for the overall erosion of the coastline in this area. Despite the lack of high-quality that are the source for the overall erosion of the coastline in this area. Despite the lack of bathymetric data, it is estimated from satellite data that the area between Laie Point and the high-quality bathymetric data, it is estimated from satellite data that the area between Laie nearest islands ranges in depth from 1 m to 3 m. On the outer, open-ocean side of the point Point and the nearest islands ranges in depth from 1 m to 3 m. On the outer, open-ocean and islands, running north and south, the depth increases fairly consistently to depths of side of the point and islands, running north and south, the depth increases fairly consist- 10ently m at to 300 depths m from of 10 the m at land. 300 Tom from the east, the land. the depths To the increaseeast, the graduallydepths increase to 20 mgradually at 200 m fromto 20 land,m at 200 then m thefrom depth land, increases then the depth dramatically increases as dramatically the sides of as the the island sides slopeof the intoisland the deepslope ocean into the (Figure deep1 ocean). (Figure 1).

FigureFigure 1.1. LocationLocation of Kukuiho’olua Island, Island, off off of of the the northeaster northeasternn coast coast of ofOahu, Oahu, U.S.A. U.S.A. Base Base map map from Google Earth. from Google Earth.

Today,Today, althoughalthough littlelittle known compared to to other other attractions attractions on on the the Island Island of of Oahu, Oahu, LaieLaie PointPoint isis visitedvisited byby thousandsthousands of tourists tourists each each year year and and is is considered considered by by some some to to be be amongamong thethe most picturesque picturesque locations locations on on Oahu Oahu—this—this is mostly is mostly due dueto the to sea the arch sea of arch Ku- of Kukuiho’oluakuiho’olua Island Island just just to the to thenorth north of the of Point. the Point. Prior Prior to this to work, this work, there therehas been has no been sci- no scientificallyentifically documented documented account account of ofthe the changes changes that that have have occurred occurred due due to to coastal coastal erosion erosion atat Kukuiho’oluaKukuiho’olua IslandIsland andand Laie Point Point,, or even even the the fact fact that that the the sea sea arch arch exists exists.. The The coastal coastal erosionerosion atat Kukuiho’oluaKukuiho’olua Island also represents represents an an example example of of the the dramatic dramatic coastal coastal changes changes thatthat cancan occur,occur, eveneven inin lithifiedlithified sediments, during during tsunami tsunami and and large large storm storm surge surge events, events, whichwhich oftenoften resultresult fromfrom ElEl NiñoNiño Southern Oscillation (ENSO) (ENSO) conditions. conditions. TheThe HawaiianHawaiian Islands are the product product of of the the moving moving Pacific Pacific Plate Plate over over a astationary stationary mantlemantle heatheat source,source, nownow called the Hawaiian Hawaiian Hot Hot Spot Spot [7 [7],], which which has has produced produced numerous numerous volcanoes,volcanoes, each of of which which extend extend from from the the seafloor seafloor to its to surface its surface [8–11]. [8 The–11]. island The of island Oahu of Oahuconsists consists of three, of three,large, shield large, shieldvolcanoes: volcanoes: The newly The discovered newly discovered Kaena Volcano KaenaVolcano [12], along [12 ], along with the Wainae and Ko’olau volcanoes. Kaena, the oldest, is estimated to be 3.5–4.9 Ma [13]. The Wainae Volcano was active between 3.08 and 3.93 Ma [14]. The Ko’olau Volcano is the youngest and was active between at least 2.1 and 3.3 Ma [15]. Geosciences 2021, 11, x FOR PEER REVIEW 3 of 13

Geosciences 2021, 11, 147 3 of 13 with the Wainae and Ko’olau volcanoes. Kaena, the oldest, is estimated to be 3.5–4.9 Ma [13]. The Wainae Volcano was active between 3.08 and 3.93 Ma [14]. The Ko’olau Volcano is the youngest and was active between at least 2.1 and 3.3 Ma [15]. OahuOahu isis surroundedsurrounded byby modernmodern and fossil fringing reefs that that likely likely began began forming forming shortlyshortly afterafter thethe onsetonset ofof thethe erosionalerosional stage of volcanic island evolution [ [66]].. ItIt hashas long been been noted noted that that local local sea sea levels levels have have varied varied widely widely over over the geologic the geologic his- historytory of ofthe the Hawaiian Hawaiian Islands Islands [16– [1619–].19 As]. each As each island island formed formed during during its time its timeover overthe Ha- the Hawaiianwaiian Hotspot, Hotspot, litho lithosphericspheric loading loading due due to to the the eruption eruption and and solidification solidification of of flows flows causedcaused significant significant subsidence subsidence and and local local sea sea level level rise. rise. Subsequent Subsequent erosion erosion and and mass mass wasting wast- reduceding reduced the mass the mass of the of islands the islands and led and to led isostatic to isostatic readjustment readjustment and uplift and asuplift the variousas the islandsvarious moved islands away moved from away the from hot spot the hot [20 –spot22]. [ This20–22 uplift]. This has uplift led tohas the led subaerial to the subaerial exposer ofexposer the reefs. of the In addition,reefs. In addition, over the over entire the time entire of its time formation of its formation and erosion, and theerosion, island the of Oahuisland has of Oahu been impactedhas been impacted by eustatic by seaeustatic level sea changes level changes brought brought on by intermittent on by intermittent glacial andglacial interglacial and interglacial periods thatperiods have that been have occurring been occurring over the lastover few the Ma, last sincefew Ma, the beginningsince the ofbeginning the Pleistocene of the Pleistocene [19,23]. [19,23]. TheThe fossilfossil reefs, reefs, and and carbonate carbonate sediments sediments that that subsequently subsequently formed formed from from them, them, under- un- liederlie and and form form the the bulk bulk rock rock of of Laie Laie Point Point and and Kukuiho’olua Kukuiho’olua Island.Island. As mentioned mentioned above, above, thethe majoritymajority ofof the rock rock in in the the subaeria subaeriallylly exposed exposed part part of ofthe the Point Point and and the theisland island consist con- sistof lithified of lithified Holocene Holocene and andPleistocene Pleistocene beach beach dunes, dunes, all of all which of which are considered are considered to be toless be lessthan than 150 150ka. These ka. These dunes dunes are thi arenly thinly bedded bedded and are and thought are thought to have to resulted have resulted from sand from sandbeing being blown blown and concentrated and concentrated from off from of offthe ofreef the flat reef during flat during a glacial a glaciallow stand low in stand sea inlevel sea [2 level4,25] [.24 The,25 ].smaller The smaller islands islandsof Kihewamoku, of Kihewamoku, Pulemoku, Pulemoku, and Mokualai and Mokualai are formed are formedfrom the from same the dune same deposits. dune deposits. The largest The island, largest Mokuaia, island, Mokuaia, instead consists instead of consists late Pleisto- of late Pleistocenecene to recent to recentreef, 11 reef, ka or 11 younger, ka or younger, and is andcapped is capped by even by more even recen moretly recently formed formed sand sand(Figure (Figure 2) [5,2)[6]. 5It, 26is ].thought It is thought that all of that the all islands of the are islands sea stacks are seathat stacks were once that connected were once connectedto headland tos. headlands. Kihewamoku Kihewamoku was connected was connectedto a point tosouth a point of the south present of the-day present-day Kahuku KahukuGolf Course, Golf in Course, an area in known an area as known Adams as Field, Adams while Field, Mokuaia while and Mokuaia Pulemoku and Pulemokuwere con- werenected connected to a point to at a the point Malaekahana at the Malaekahana State Recreation State Recreation Area. Kukuiho’olua Area. Kukuiho’olua and Mokualai and Mokualaiwere connected were connected to Laie Point to Laie (Figure Point 2). (Figure 2).

FigureFigure 2.2.Geologic Geologic mapmap ofof thethe Laie,Laie, HawaiiHawaii area,area, includingincluding Kukuiho’oluaKukuiho’olua Island (after Sherrod et al., 2007).al., 2007). Compare Compare to Figureto Figure1. Insert1. Insert ( A ()A illustrates) illustrates the the cross-bedded cross-bedded naturenature of the Pleistocene Pleistocene-age-age sandstone that makes up Laie Point and Kukuiho’olua Island. Insert (B) is a close-up view of the sandstone that makes up Laie Point and Kukuiho’olua Island. Insert (B) is a close-up view of the sandstone. sandstone.

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1.3. Kukuiho’olua Island in Human History In Hawaiian mythology, Laie Point and the nearby islands formed when a great warrior, named Kana, set out to kill a mo’o or giant lizard, which had killed many people in the area. Kana easily defeated the mo’o and chopped it into five pieces and tossed them into the sea [27]. Kukuiho’olua Island as considered the head because it had large sea caves on the north and south sides that resembled eye sockets. In 1865, the area of Lai’e was purchased by the Church of Jesus Christ of Latter-day Saints (LDS) as a place for new Mormon converts to gather and form a community [28]. In 1868 the area of Lai’e, in order to become self-sufficient, was turned into a large sugar plantation with a sugar mill [28]. However, although Laie Point is mentioned from time to time, there is little to no descriptive record of the offshore islets in the plantation records. Even early geologists such as J. B. Pollock and H.T. Stearns say almost nothing about the islands at Laie Point. Stearns [18] only mentions the Point as a type area for one of the glacial-interglacial sea level stands and gives a very brief description of the rock-type.

2. Materials and Methods Analysis was conducted by examination of archival images of Kukuiho’olua Island relative to its current state, with particular focus on images taken just prior to and just after the events of 24–25 February 2016. Images were collected the morning of 25 February using a Canon Powershot SX500 IS digital camera on auto settings. Archival images were taken from the Brigham Young University-Hawaii Archives, which did not include camera information. Basic field measurements of erosion were taken using a standard field tape and photo scale. Google Earth, with its built-in ruler tool, and aerial images taken with a Mavic 2 Pro UAS were used to make an estimate of the size and volume of the boulder dislodged from the sea arch. The mass was calculated by extrapolating the density of a sample of Laie Point sandstone to the volume of the boulder. The timing of the 2016 event was determined by comparing NOAA tide data [29] and the U.S. National Weather Service sunrise/sunset calculator [30]. ENSO data used came from the U.S. National Weather Service Climate Prediction Center.

3. Results 3.1. 1946 Tsunami On 1 April 1946, a large tsunami was generated when a Ms = 7.4 earthquake, and likely (but controversial) subsequent, underwater landslide occurred in the Aleutian Trench south of Alaska [31–38]. This earthquake and tsunami have come to be known as the “April Fools Earthquake and Tsunami.” The April Fools Tsunami of 1946 reached Laie sometime between 5:55 am local time, which is when its arrival was recorded at the island of , and 6:30 am, when is when it was recorded at , on the south side of Oahu (Figure1)[39].

3.2. 2015–2016 El Niño-Generated Waves During the evening of 24–25 February 2016, a low-pressure system in the North Pacific generated large swells that approached Oahu from the northwest and reached the North Shore and Laie beginning around 22:00 local time on the 24th, peaking in the late morning, but continuing through the next two days [40]. These waves coincided with high tide in Laie around 04:00 (Table1)[ 29]. Breaking wave heights along the North Shore during this period were as much as 20 m in height (Figure3)[41]. Geosciences 2021, 11, x FOR PEER REVIEW 5 of 13

Geosciences 2021, 11, 147 5 of 13 Table 1. Wave height data for the North Shore of Oahu (NOAA Buoy #51202) and local tide data at Laie Point (NOAA Station #1612480) for 24–25 February 2016. High tide and extreme wave heights coincided around 04:00 local time, which is the most likely time for Table 1. extremeWave erosion height at dataLaie forPoint the and North Kukuiho’olua Shore of Oahu Island (NOAA (shaded Buoy #51202) rows). and LST local = Local tide dataStand- at Laieard PointTime. (NOAA MLS = StationMean Sea #1612480) Level. for 24–25 February 2016. High tide and extreme wave heights coincided around 04:00 local time, which is the most likely time for extreme erosion at Laie Point and Date (2016) Time (LST) Wave Height (m) Estimated Tide (MSL) Kukuiho’olua Island (shaded rows). LST = Local Standard Time. MLS = Mean Sea Level. 24 February 21:00 3.66 −0.23 24Date February (2016) Time22:00 (LST) Wave3.49 Height (m) Estimated−0.24 Tide (MSL) 2424 February February 23:00 21:00 3.86 3.66 −00.23.18 2524 February February 00:00 22:00 4.06 3.49 −00.24.08 2524 February February 01:00 23:00 4.36 3.86 −0.040.18 25 February 00:00 4.06 −0.08 25 February 02:00 4.66 0.16 25 February 01:00 4.36 0.04 2525 February February 03:00 02:00 5.49 4.66 0.25 0.16 2525 February February * 04:00 03:00 5.57 5.49 0.28 0.25 2525 February February *05:00 04:00 5.12 5.57 0.25 0.28 25 February 05:00 5.12 0.25 2525 February February 06:00 06:00 6.12 6.12 0.17 0.17 2525 February February 07:00 07:00 5.77 5.77 0.05 0.05 2525 February February 08:00 08:00 5.52 5.52 −00.07.07 2525 February February 09:00 09:00 6.37 6.37 −00.16.16 ** High High tide tide occurred occurred at 04:01, at 04:01, but sunrisebut sunrise was at was 06:56. at 06:56.

FigureFigure 3.3. Example of large amplitude amplitude coastal coastal waves waves that that occurred occurred on on 25 25 February February 2016, 2016, due due to toa a strongstrong ENSO-inducedENSO-inducedlow-pressure low-pressure system. system.

Observations Observations laterlater inin thethe dayday providedprovided evidenceevidence forfor significantsignificant removalremoval ofof rockrock andand sandsand fromfrom LaieLaie PointPoint (Figure(Figure4 ).4). In In addition, addition, there there was was significant significant evidence evidence that that the the largest largest waveswaves maymay havehave overtoppedovertopped thethe middle-partmiddle-part ofof thethe PointPoint andand thatthat waterwater likelylikely flowedflowed acrossacross thethe peninsula.peninsula. This isis evidencedevidenced fromfrom erosionerosion pitspits lyinglying alongalong aa pathpath ofof freshlyfreshly erodederoded sandsand andand rockrock runningrunning fromfrom thethe northnorth sideside ofof thethe peninsulapeninsula toto itsits southernsouthern edgeedge (Figure(Figure5 ).5). In In addition addition to to the the pits pits and and fresh fresh surfaces, surfaces, there there were were multiple multiple standing standing pools pools of seaof sea water water on theon the upper upper platform platform of Laieof Laie Point. Point.

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FigureFigure 4. 4.Examples Examples of of fresh fresh sandstone sandstone surfaces surfaces (light-colored) (light-colored) exposed exposed by by removal removal of of rock rock by by extreme, ex- Figure 4. Examples of fresh sandstone surfaces (light-colored) exposed by removal of rock by ex- ENSO-inducedtreme, ENSO-induced waves atwaves Laie at Point Laie onPoint 24–25 on 24 February–25 February 2016. 2016. View View of center of center image image is looking is look- east. treme,ing east. ENSO Compare-induced to Figure waves 5. at Laie Point on 24–25 February 2016. View of center image is look- ingCompare east. Compare to Figure to5. Figure 5.

Figure 5. Evidence for waves over-topping the peninsula at Laie Point, Oahu. Examples include oxidized surface removal (C) and erosion pits (A and B). View in C is looking north. Arrow indi- Figure 5. EvidenceEvidence for for waves waves over over-topping-topping the the peninsula peninsula at atLaie Laie Point, Point, Oahu. Oahu. Examples Examples include include cates likely flow path of seawater. Base map from Google Earth. oxidized surface surface removal removal ( (CC)) and and erosion erosion pits pits (A (A and,B). B View). View in Cin is C looking is looking north. north. Arrow Arrow indicates indi- cates likely flow path of seawater. Base map from Google Earth. likely flow path of seawater. Base map from Google Earth. 4. Discussion 4. Discussion Discussion Although Kukuiho’olua Island lies near the mainland of Oahu, it still lies on the out- side Although edge of Oahu’s Kukuiho’olua Kukuiho’olua surrounding, Island Island fringing lies lies near near reef the the(Figure mainland mainland 1), which of Oahu, of Oahu,means it still itthat stilllies there lieson the would on out- the sideoutsidehave edge likely edge of beenOahu’s of Oahu’slittle surrounding, to no surrounding, dampenin fringingg fringingeffect reef on (Figure reefthe size (Figure 1), and which 1energy), which means of the means that tsunami there that wouldwhen there havewouldit arrived likely have at been likely the littleisland been to during littleno dampenin to the no dampening1946g effecttsunami. on effect theDampening onsize the and size energywas and observed energy of the tsunami ofat theother tsunami whenloca- itwhentions arrived itlying arrived at withinthe atisland the theisland duringreefs during[42 the]. T1946he the run tsunami. 1946-up tsunami.on Dampeningthe north Dampening side was of observed Laie was observedPoint, at otherwhere at otherloca- the tionslocationsisland lying lies, lying withinwas within 8.2 the m, the reefswhich reefs [42 was [].42 T ].higherhe The run run-up than-up on in on anythe the northother north areaside side alongof of Laie Laie the Point, Point, 20 km where where of coast the islandislandrunning lies, lies, north was was and8.2 8.2 m, south which from was the higher Point. than In fact, in any this other height area was along one of the the 20 highest km of coastrun- runningups on the northnorth whole andand of southsouth Oahu from from [32] the . the Point. Point. In In fact, fact, this this height height was was one one of theof the highest highest run-ups run- upson the on As wholethe described whole of Oahu of above, Oahu [32]. local[32]. wi tnesses state that prior to the 1946 tsunami, no sea arch existed As describedat Kukuiho’olua above, Island. local wi Instead,tnesses two state large, that but prior shallow, to the sea1946 caves tsunami, existed no on sea either arch existedside of at the Kukuiho’olua island. They Island. all state Instead, that two the large, arch formed but shallow, during sea the caves 1946 existed tsunami. on either The side of the island. They all state that the arch formed during the 1946 tsunami. The

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As described above, local witnesses state that prior to the 1946 tsunami, no sea arch existed at Kukuiho’olua Island. Instead, two large, but shallow, sea caves existed on either side of the island. They all state that the arch formed during the 1946 tsunami. The traditional way of describing sea arch formation almost always refers to gradual erosion over time as incoming, wind-generated waves refract around some coastal point or headland, concentrating their energy in an area behind the most seaward point of the land, which leads to the formation of opposing, semi-symmetrical sea caves. Continued erosion at the back of the caves leads to collapse and a joining of the caves to form a sea arch. The process has been specifically mentioned in conjunction with the sea arch at Kukuiho’olua Island [6]. The initial sea caves on either side of Kukuiho’olua Island likely formed in this way. Although the rocky shorelines of Hawaii have been poorly studied [43] and no sci- entific record of the tsunami impacts to Kukuiho’olua Island exist, some models support the possibility that the final formation of the arch resulted from the 1946 tsunami. In models designed and run by Zhao et al. [39], the force of a tsunami wave becomes more concentrated at sea cliffs which have over hanging (>90◦) slopes. They determined that a “pressure peak,” of concentrated force is very frequent in coastal areas with undercut sea cliffs. This is especially true if the submarine slope leading up to the cliff is short. All such conditions exist at Laie Point and Kukuiho’olua Island. Their models support the idea that when the 1946 tsunami reached Kukio’olua Island, given the overhanging nature of the sea cave and the abrupt shallowing of the ocean at the foot of the island, the pressure force generated by the wave hitting the island would have been concentrated within the cave. This intense force is what would have caused the breakthrough and collapse at the back of the cave, leading to the formation of the present-day arch. The work by Zhao et al. [44] and the accounts of eyewitnesses referred to above, suggest that tsunami waves, while not necessarily significant over time when compared with wind-generated waves, can play a larger role in coastal evolution and sea arch formation than previously considered, even in lithified sediments. In addition to the example above, others have demonstrated that erosion of coastal cliffs is often intermittent and can be very localized in nature and that short-term erosion rates can be very different from long-term ones [45,46]. Such rates of erosion and changes to coastal topography are also often heavily influenced by storm-generated waves [45]. The El Niño that occurred during the 2015–2016 northern hemisphere winter was one of the strongest El Niño events in the last 65 years [47–49], and the largest since the 1997–1998 El Niño [50]. This El Niño led to large, slow, and steady-moving storms, which generated large, consistent wave trains that moved outward from winter storm systems in the North Pacific, where they impacted coastal areas thousands of kilometers from the source area. These storms resulted from the baroclinic instabilities created by the convergence of strong, El Niño-generated, horizontal, temperature gradients and strong pressure gradients, especially in the Equatorial Pacific [50]. There was a very strong reversal in sea surface pressure gradient (Figure6) that resulted in a change from the usual, easterly Trade Winds to form a new wind regime that came from the west and northwest (Figure7)[ 43,44]. The waves from these storm systems approached the North Shore of Oahu from the west and northwest throughout the 2015–2016 El Niño event. Geosciences 2021, 11, x FOR PEER REVIEW 8 of 13 Geosciences 2021, 11, 147 8 of 13

Figure 6. Variation in the Southern Oscillation Index (SOI) during the 2015–2016 ENSO. The Figure 6. Variation in the Southern Oscillation Index (SOI) during the 2015–2016 ENSO. The 2015– 2015–2016 ENSO period is bracketed by the red dotted lines. SOI is a measurement of the intensity of 2016 ENSO period is bracketed by the red dotted lines. SOI is a measurement of the intensity of thethe ENSO. ENSO. Plots Plots courtesy courtesy of of the the U.S. U.S. National National Weather ServiceService Climate Climate Prediction Prediction Center. Center. Sunrise in Laie that morning was at 06:56 local time, nearly three hours after the highest tide, which is likely why there were no direct witnesses to the changes that occurred at the Kukuiho’olua sea arch and across Laie Point. Sunrise viewers and other visitors to Laie Point that morning noted that the large boulder, consisting of the same cemented dune material as the Kukuiho’olua Island sea arch and lying beneath it had been moved from beneath the arch and had fallen into the sea (Figure8). Aerial surveys by a Mavic 2 Pro sUAV were used to approximate the size of the boulder as being roughly 25 m3 in volume (4.4 m × 2.4 m × 2.4 m). The aerial images combined with a density analysis of sandstone collected from Laie Point, suggests that the boulder had an approximate mass of 135,000 kg. Given the tides and the wave heights, it is most likely that that the boulder

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was moved sometime around 04:00, when the power of the waves concentrated around the point would likely have been greatest. The large erosion pits and pools of standing water on Laie Point itself support the likelihood that the wave heights were significant. These observations are important because Geosciences 2021, 11, x FOR PEER REVIEWLaie Point, as mentioned, is a popular location for many locals and tourists. It is especially9 of 13

popular as a night fishing location. Unwary individuals could face an unexpected wave hazard and the possibility of being swept off and into the sea if a similar combination of waves and tides occurs in the future.

FigureFigure 7. Variation 7. Variation in inthe the surface surface wind wind direction direction and and tropospheric tropospheric temperaturestemperatures of of the the central central Pacific Pa- cificOcean Ocean during during the the 2015–2016 2015–2016 ENSO. ENSO. The The 2015–2016 2015–2016 ENSO ENSO period period is bracketedis bracketed by by the the black black dotted dottedlines. lines. The The 200-hPa 200-hPa zonal zonal winds winds are are winds winds at theat the sea sea surface. surface. The The 500—hPa 500—hPa temperature temperature is is the the temperature of the Troposphere. The strong negative anomaly in the wind indicates a strong temperature of the Troposphere. The strong negative anomaly in the wind indicates a strong westerly westerly wind direction. Plots courtesy of the U.S. National Weather Service Climate Prediction wind direction. Plots courtesy of the U.S. National Weather Service Climate Prediction Center. Center. On a day to day scale, little change is apparent at Laie Point and Kukuio’olua Island. However,Sunrise ain comparison Laie that morning of images was over at 06:56 time, local coupled time, with nearly the significantthree hours changes after the in high- 1946 estand tide, 2016 which illustrated is likely that why although there were changes no direct may bewitnesses intermittent to the (Figure changes9), when that occurred changes at thedo happen,Kukuiho’olua they can sea occur arch quickly and across and dramatically.Laie Point. Sunrise This is viewers true even and when other the visitors erosion to is Laienot Point associated that morning with extreme, noted local that weatherthe large events, boulder such, consisting as a hurricane, of the but same rather cemented because duneof distantmaterial earthquakes as the Kukuiho’olua or waves generated Island sea by arch the overall and lying climate beneath system it withinhad been the moved Pacific. from beneath the arch and had fallen into the sea (Figure 8). Aerial surveys by a Mavic 2 Pro sUAV were used to approximate the size of the boulder as being roughly 25 m3 in volume (4.4 m × 2.4 m × 2.4 m). The aerial images combined with a density analysis of sandstone collected from Laie Point, suggests that the boulder had an approximate mass of 135,000 kg. Given the tides and the wave heights, it is most likely that that the boulder was moved sometime around 04:00, when the power of the waves concentrated around the point would likely have been greatest.

Geosciences 2021, 11, 147 10 of 13 Geosciences 2021, 11, x FOR PEER REVIEW 10 of 13

FigureFigure 8.8. HistoricalHistorical imagesimages showingshowing Kukuio’olua Island both before ( A)) and after after ( (BB)) the the for- formation mation of its modern sea arch. Formation of the arch occurred sometime between 1919 and 1959. of its modern sea arch. Formation of the arch occurred sometime between 1919 and 1959. The bottom The bottom image (C), shows the arch after its more recent change. image (C), shows the arch after its more recent change. The large erosion pits and pools of standing water on Laie Point itself support the likelihood that the wave heights were significant. These observations are important be- cause Laie Point, as mentioned, is a popular location for many locals and tourists. It is especially popular as a night fishing location. Unwary individuals could face an

Geosciences 2021, 11, x FOR PEER REVIEW 11 of 13

unexpected wave hazard and the possibility of being swept off and into the sea if a similar combination of waves and tides occurs in the future. On a day to day scale, little change is apparent at Laie Point and Kukuio’olua Island. However, a comparison of images over time, coupled with the significant changes in 1946 and 2016 illustrated that although changes may be intermittent (Figure 9), when changes

Geosciences 2021, 11, 147 do happen, they can occur quickly and dramatically. This is true even when the erosion11 of 13is not associated with extreme, local weather events, such as a hurricane, but rather because of distant earthquakes or waves generated by the overall climate system within the Pacific.

Figure 9. Sequence of of images images showing showing evolution evolution of of the the sea sea arch arch on on Kukuiho’olua Kukuiho’olua Island Island,, near near Laie Laie Point, Hawaii. Images taken from approximately thethe samesame location.location. ImageImage ((AA)) isis thethe earliestearliest known knownimage of image the island, of the showingisland, showing no arch. no Image arch. (B Image) is the (B second) is the earliest-known second earliest image-known of theimage island of the and islandthe first and to showthe first the to arch, show indicating the arch, thatindicating the arch that formed the arch sometime formed between sometime 1919 between and 1959. 1919 Images and 1959.(C–E) Images show the (C arch), (D in), and a fairly (E) show steady-state the arch for in several a fairly decades. steady-state Image for (F several) shows decades. the dramatic Image change (F) shows the dramatic change after the large waves of 2016. after the large waves of 2016.

5. Conclusions Conclusions While being oneone smallsmall locationlocation inin the the Pacific Pacific Ocean, Ocean, the the changes changes that that have have occurred occurred at atLaie Laie Point Point and and Kukuiho’olua Kukuiho’olua Island, Island during, during both both the the 1946 1946 “April “April Fools’ Fools’ Tsunami” Tsunami” and and the the2015–2016 2015–2016 El Niño, El Niño serve, asserve dramatic as dramatic examples example of the rapiditys of the at rapidity which coastal at which changes coastal can choccuranges after can long occur periods after long of seeming periods stasis.of seeming The recentstasis. impactsThe recent of theimpacts waves of atthe Laie waves Point at Laiewere Point significant were andsignificant dramatic. and By dramatic. eroding aBy 25 eroding m3-sized a boulder25 m3-sized from boulder within the from arch, within they thechanged arch, t ahey well-known changed a and well popular-known landmark and popular (some landmark would say(some for would the better) say for in athe single bet- ter)night in ora single morning. night These or morning. changes These suggest changes that suggest additional that events additional of similar events magnitude of similar magnitudeproduced during produced tsunamis during or tsunamis El Niño or conditions, El Niño conditions, and possibly and coinciding possibly coinciding with high with tide, highcan leadtide, tocan rapid, lead to unexpected, rapid, unexpected, and possibly and possibly hazardous hazardous changes changes to areas to thatareas are that often are oftentaken taken for granted for granted to be relativelyto be relatively unchanging unchanging on human on human timescales. timescales Coastal. Coastal monitoring, moni- toring,especially especially during during extreme extreme El Niño El years, Niño should years, beshould of ahigh be of priority a high priority for local for communities local com- munitiesas much asas itmuch is for as larger it is for regions. larger regions.

Author Contributions:Contributions: All work related to this research was done by B.R.J.B.R.J. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All data is found within the text of the article. Data Availability Statement: All data is found within the text of the article. Acknowledgments: The author is especially grateful to Wei Shi, meteorologist at the Climate Predic- tion Center (NCEP/NWS/NOAA) for his help with the ENSO condition plots, and Dale Hammond, emeritus professor at Brigham Young University-Hawaii, for his 1959 photograph. The archives at BYU-Hawaii were also valuable in locating other, historical photographs. Conflicts of Interest: The author declares no conflict of interest. Geosciences 2021, 11, 147 12 of 13

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