Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society

Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society Chapter 11 – extreme waves causes, characteristics, and impact on coastal environments and society

Extreme Waves: Causes,

Characteristics, and Impact on Coastal Environments and

Society

Jim D Hansom1,2, Adam D Switzer3,4and Jeremy Pile3

1 School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK, 2 Department

of Geography, University of Canterbury, Christchurch, New Zealand,3Earth Observatory of Singapore, Nanyang Technological University, Singapore, 4 Division of Earth Sciences, Nanyang Technological University, Singapore

ABSTRACT

The existence of extreme waves, as observed by seafarers, has been confirmed by datarecording and modeling to be more common than previously assumed Extreme wavesmainly occur during major storms at sea by means of constructive interference of wavetrains or by nonlinear wave interaction, but extreme waves may also be associated withtsunami or meteotsunami events If they arrive at the coast, most extreme waves havethe potential to cause extensive remodeling and repositioning of the shoreline envi-ronment and landforms as well as causing significant damage to human infrastructureand threat to life The impact of extreme waves on both sedimentary and rocky coastscan be substantial with sediments or rocky boulders eroded from the coastal edge beingtransported and deposited some distance inland This characteristic provides clues tothe nature of the extreme event and, if recorded within the recent sedimentary record,information about the periodicity of similar events The impact of extreme waves oncoastal communities and environment has prompted a range of mitigation and adap-tation strategies to cope with these hazards These include more robust coast defences,better modeling, prediction and warning systems, improved interagency liaison,improved technical assistance, and storm impact management information for thegeneral public, as well as provision of clear evacuation routes during a wave-relatedemergency However, since climate change seems likely to result in increased rates

of both sea-level rise and storm-related impacts, there is an emerging consensus thatadaptive management of the coastal zone may prove to be a more sustainable strategythan the alternatives

Coastal and Marine Hazards, Risks, and Disasters http://dx.doi.org/10.1016/B978-0-12-396483-0.00011-X

© 2015 Elsevier Inc All rights reserved. 307

11.1 WHAT ARE “EXTREME” WAVES?

Even when the sea surface is not particularly stormy, interactions betweenwaves may result in locally higher wave heights that can lead to fatalities atsea and at the coast and thus may become labeled in the media as extreme,giant, freak, or rogue waves Heavy sea states and severe weather conditionscaused the loss of more than 200 large cargo vessels between 1981 and 2000,with over 30 percent of the casualties due to severe weather and an additional

25 percent unexplained (Rosenthal and Lehner, 2008) In many cases single

“rogue waves,” as well as groups of extreme storm waves, have been reported

by the crew members of such ships The existence of rogue waves had beenquestioned for a long time, but remote sensing and continuous observation atoil platforms, for example in the North Sea, has demonstrated their occurrence.Rogue waves are defined as a maximum wave height of more than two timesthe significant wave height or Hs, which is statistically defined as the average

of the highest third of all waves over a 20 min period Using satellite radar and

in situ sensors to investigate ship and platform accidents at sea, the Europeanproject MaxWave showed that extreme storm waves with heights of 25 m werenot uncommon during storms (Rosenthal and Lehner, 2008) In shallow-watercoastal settings recent work has clarified the conditions under which excep-tionally large and steep waves may form (e.g., Didenkulova and Anderson,2006; Didenkulova, 2011) Using data of regular deaths in Taiwan resultingfrom such “mad-dog” waves, Tsai et al (2004) have shown that they occurmostly along cliff coasts, or at breakwaters fronting waters 110 m deep, withsteep offshore slopes

Although extreme waves can be produced in several ways, they are mostcommonly generated during major storms at sea and may then propagate to-ward land with only limited attenuation as a result of diffraction or refraction

A second category of extreme wave is a product of constructive interferenceproduced when different wave fields come together during storms to producewaves that may be several times the height of the original waves, a conditionthat also occurs in areas where storm waves propagate against a strongopposing tidal or marine current

Extreme waves can also be produced by Earth movements resulting fromtectonic or volcanic activity at or near the coast, or by landslide activity into orbeneath the ocean or by extraterrestrial bolide impact Any of these mecha-nisms can generate tsunamis (Switzer, 2014) Formerly called tidal waves orseismic sea waves, tsunamis are not tidally produced Extreme waves can also

be produced by meteorological tsunamis or meteotsunamis Propagating in thesame way and with similar coastal dynamics as tsunamis (Monserrat et al.,

2006), a meteotsunami is effectively an atmospherically generated largeamplitude oscillation or seiche caused by moving air pressure disturbancesthat can result in waves up to 6 m (Vilibic and Sepic, 2009) Meteotsunamisoccur worldwide and can be locally destructive (Bryant, 2001)

11.2 TYPES OF EXTREME WAVES AND THE CONDITIONS

THAT PRODUCE THEM

11.2.1 Storm Waves

Extreme waves are commonly produced by very strong winds blowing forlengthy periods over long ocean fetches However, when does a normal stormbecome an extreme storm likely to produce extreme waves capable of sub-stantial damage? There are clear criteria for the classification of hurricanes,cyclones, and typhoons and several scales exist to assess the winds generated

by these systems, the best known of which is the SaffireSimpson HurricaneScale for the North Atlantic and Northeastern Pacific Oceans Occurring both

in the open ocean and along coastlines, the recorded or estimated heightreached by extreme waves generated by storms varies considerably with localeffects and sampling interval For example, in a short duration 3-h storm themost probable highest individual wave (Hmpm) is statistically about 1.86 Hs.However, on January 1, 1995, at Statoil’s Draupner gas platform (16/11-E) inthe North Sea, the Hmpm of the so-called “January” or “New Year Wave” wasmeasured by laser to be 25.6 m (2.4 Hs) (Figure 11.1) This event clearlydemonstrated the existence of giant individual waves well in excess of the10.8 m Hs at that time (Guedes-Soares et al., 2003)

Also in 1995, the BP Amoco platform Schiehallion, sited in deep water inthe Northeast Atlantic 160 km west of Scotland, was struck by an extremewave that ruptured the superstructure 18 m above the waterline (Lawton,

2001) Analysis of extreme waves at Schiehallion produced a 1-year maximum

FIGURE 11.1 The “New Year Wave” of January 1, 1995 as measured at Draupner in the North Sea, with time in seconds on the X-axis against height in meters on the Y-axis Dysthe et al.,

2005

individual wave height (Hmax) of 24.3 m (Hansom et al., 2008) Wave buoyK7 (60 420 N, 4 300 W) nearby, recorded the highest individual waves

(Hmax) reaching 28 and 21 m during winter storms in 1992 and 1993,respectively (Hansom et al., 2008) Such waves were of the same order ofmagnitude as those recorded in 2004 in the Gulf of Mexico under HurricaneIvan, where an individual Hmax reached 27.7 m (Wang et al., 2005) and inFebruary 2000 near Rockall, 250 km west of Scotland, where an individualHmax reached 29.1 m and is the highest individual wave ever instrumentallyrecorded (Holliday et al., 2006)

In the North Atlantic,Hansom and Hall (2009)suggest that the maximumheight of extreme waves may have been increasing over recent decades TheWaves and Storms in the North Atlantic project reported increases in annualsignificant wave height (Hs) of 2.5e7.5 mma1over the period 1955e1994(Gunther et al., 1998) This trend may be supported by the observational data

of Gulev and Hasse (1999), indicating a 1e3 mma1 increase in annualsignificant wave height (Hs) in the North Atlantic over the period1964e1993, a trend that may be linked to intensification of the North AtlanticOscillation (NAO) (Woolf et al., 2002) Komar and Allan (2008) reportsimilar increases in both annual and winter Hs between 1976 and 2006 in theNorthwest Atlantic.Keim et al (2004) note North Atlantic winter sea con-ditions to have become rougher over the past 50e100 years, along with anincrease in the frequency of very powerful storms However, geographicalvariability exists in the Northeast Atlantic with higher rates of increase off theBritish Isles but less significant changes off Scandinavia (Wang and Swail,

2002) IndeedFeng et al (2012)detected no increase in Hs in the NorwegianSea between 2000 and 2009 but found an increase in the annual mean ofextreme wave height that correlated with the winter NAO as well as an in-crease in the number of extreme storm waves (Hmax reaching 25.6 m inNovember 2001)

11.2.2 Giant or “Rogue” Waves

It is evident that very large waves occur regularly at sea during storms Itfollows that any interaction or focusing of wave energy that allows these waves

to grow larger than their neighbors will result in waves of extreme height.Seafarers commonly describe giant waves during major storms to have asteeper forward face preceded by a deep trough or “hole in the sea,” asdescribed byMallory (1974) There are three known possible mechanisms, thefirst two of which are described by linear theory and have been understood forsome time: (1) time-space focusing; (2) current focusing; and (3) nonlinearfocusing

1 Time-space focusing is the well-known product of longer ocean wavestraveling faster than shorter waves to create constructive interference

where the crests coincide Similar effects occur when wave trains fromdifferent directions cross in the ocean or lee of an island The satelliteimages of the 100  100 km area around Draupner during the January 1storm in 1995 showed two peaks in the directional spectra, indicating thatcrossing seas were generating extreme waves (Rosenthal and Lehner,

2008) As a result of MaxWave, Meteo France now uses a Cross Sea Indexthat uses crossing wave trains as a daily warning criterion of extreme waveconditions in the Mediterranean (Toffoli et al., 2003) A similar effectoccurs at coastlines where reflected waves may constructively interactwith incident waves to produce abnormally high waves, which may thencause beach and dune erosion, overtopping, and flooding as occurredalong the coast of Northeast Scotland in the exceptional storms ofDecember 2012 (Figure 11.2)

2 Current focusing occurs where waves traverse an area of variable currentsand, acting like an optical lens, the currents focus wave action into acaustic region to produce freak, rogue, or giant waves (Figure 11.3) (Whiteand Fornberg, 1998) Such focusing of wave energy increases the proba-bility of encountering large waves in these areas For example, off the eastcoast of South Africa extreme waves are produced by the strong south-going Agulhas current where it interacts with north-going wavesemanating from storms in the Southern Ocean Many large ships havefoundered in this notorious area (Lavrenov, 1998)

FIGURE 11.2 Large peaked wave (solid arrow) at Golspie, Northern Scotland, December15,

2012, likely produced by the interaction of incident high energy storm waves (dashed arrow) with reflected waves from the abrupt end of rock armor to the north of the photograph, although infragravity wave interaction cannot be ruled out The waves removed the dune face and crest and flooded houses and mobile homes Photo: Neil Cameron.

3 Nonlinear focusing may occur after the waves are first generated, sincethey tend to separate into groups, which then become more prominent asthey propagate Linear theory suggests these waves should remain uniformand periodic but, in nature, some waves “grow” at the expense of adjacentwaves Modifications of the nonlinear Schro¨dinger equation capable ofexplaining this behavior (Dysthe, 1979; Dysthe and Trulsen, 1999; Liu

et al., 2005) have been used to explain freak waves Called a “breather,”over time the wave develops a strong focusing of wave energy so that asmall part of the wave train extracts energy from its neighbors and

“breathes” itself up to reach enhanced dimensions at their expense(Figure 11.4) A second and distinct type of nonlinear effect, demonstrated

to occur on the rocky Atlantic coast of Banneg Island, France, involveslarge infragravity waves of low-frequency (300 s period) that becometrapped against the shore during storms (Sheremet et al., 2014) Such low-frequency infragravity effects produce standing waves that may interactwith incident waves during storms, contribute to the generation of abnor-mally high wave run-up close to the shore, and enhance erosion andinundation levels

FIGURE 11.3 Modeled wave trajectories (solid dark lines) through an area of variable current (lighter solid and dashed lines) X- and Y-axes represent units of distance from the origin The parallel wave directions entering from the Y-axis are subject to deflection by the background current to produce areas of increased wave height examples which are shown by the arrows Adapted from White and Fornberg (1998)

11.2.3 Tsunami

Tsunamis in the deep ocean typically have very long wavelengths but smallwave heights and travel at speeds of more than 700 km h1 As they slow inshallow water they rapidly gain height and can be more than 30 m high, asoccurred in the Indian Ocean in 2004 (Synolakis and Kong, 2006) and in Japan

in 2011 (Mori et al., 2011) The highest extreme wave that has ever beenreliably observed was produced by a tsunami on July 9, 1958 It was caused by

an earthquake-triggered rockslide which released 30.6 million m3 of rock toplunge from a height of 914 m into the Alaskan fjord of Gilbert Inlet(Figure 11.5) The ensuing tsunami wave swept over the 524 m promontorybetween Gilbert Inlet and Lituya Bay, uprooting millions of trees that were thenswept into the Gulf of Alaska along with three fishing boats, two of whichsurvived (Miller, 1960) Whilst it is clear that the Lituya Bay tsunami causedsignificant environmental impacts to a remote and largely uninhabited coast-line, had it occurred close to a lowland coastal plain with a large residentpopulation and villages, towns, and cities, its impact would have been greatlymagnified Unfortunately, the historical and recent record is littered with tsu-namis that have impacted on many such low and densely populated coastlinesand have caused widespread destruction of lives, homes, and infrastructure

11.2.4 Meteotsunami

Meteotsunamis are atmospherically induced ocean waves that are within thetsunami frequency band (Bryant, 2001) Caused by steep pressure jumps alongfronts and squalls in the open ocean, these barotropic or pressure-dependentocean waves are amplified near the coast, particularly within enclosed baysand harbors Meteotsunami may undergo generation over several hours as thewaves move with the storm, in contrast to the instantaneous generation of a

FIGURE 11.4 Development of a “breather” wave in three different stages: in the upper diagram the central waves grow by progres- sively extracting energy from its neighbors Dysthe et al (2005)

tsunami Their occurrence has been acknowledged for some time and they areknown in the Mediterranean as rissaga, marubbio, or stigazzi, as Seeba¨r in theBaltic and as abiki or yota in Japan Their destructive effects on coasts andgreat lakes are now recognized across the world (Monserrat et al., 2006) InJuly 1992 and October 2008, respectively, Daytona Beach, Florida, andBoothbay Harbor, Maine in the U.S., were hit by waves several meters high thatappeared without warning during calm conditions Similarly, on a sunny day inJune 2011, the Yealm River mouth in southwest England reported waves up to0.8 m high and thought to be linked to a storm 482 km away in the Bay ofBiscay (Haslett et al., 2009) As a result of such events and to form the basis of

a meteotsunami warning system, the U.S National Oceanographic and mospheric Administration (NOAA) funded the TMEWS project (Towards aMEteotsunami Warning System) along the U.S coastline to document pastpotential meteotsunami events and match them with the source, generation, anddynamics of atmospheric disturbances (Sepic et al., 2009;Vilibic et al., 2012)

At-11.3 IMPACT OF EXTREME WAVES ON THE COASTAL

by widespread erosion and flooding events (Figure 11.6) Extreme wavescommonly cause localized or regional flooding and these can substantiallyaffect the socioeconomics of coastal communities In the short term coastalflooding can devastate crops, destroy infrastructure, and take the lives ofhumans and livestock In the long term there may be erosional loss of land andloss of agricultural production due to salt water flooding Recent technologicaladvances have allowed better understanding of the dynamics of coastalflooding from extreme waves Video footage of extreme wave events includingfootage of rogue waves, cyclone-induced storm surges, tsunamis, andmeteotsunamis now provide a substantial dataset from which researchers caninvestigate the nature and impact of coastal flooding from extreme waveevents Recent examples include the observations of tsunamis in Indonesia(Fritz et al., 2006) and Japan (Fritz et al., 2012) and meteotsunamis in Korea(Ha et al., 2014).

11.3.1 Extreme Wave Impacts on Sedimentary Coasts

(Sand/Gravel/Mud)

When extreme waves strike a coast composed of unconsolidated sedimentsthey commonly cause substantial erosional reconfiguration of the coastal edgeand may overwash water and sediment landward of the beach crest that doesnot return directly to the ocean (Figure 11.7) Anthony (2009) presents acomprehensive review of shore processes and deposits associated with

FIGURE 11.6 The remains of a fishing village near Tacloban, Phillipines after a 5 m þ typhoon-produced storm surge in November 2013 killed more than 6,000 people and destroyed more than 100,000 dwellings Much of the damage at the coast was done by extreme waves and the storm surge See also Figure 11.17 Photo: Switzer.

exceptional events Overwash begins when the run-up level of extreme wavesexceeds the local beach or dune crest height inundating the area behind thebeach or dune (Switzer, 2014) Although severe overwash is commonlyassociated with storms or cyclones, it is also caused by tsunami events It isparticularly common on barrier island coasts and is also referred to as marineinundation, marine flooding, or catastrophic saltwater inundation events (Goff

et al., 2001) The sediment and coastal flooding deposits that are depositedinland of a beach crest by overwash flow are manifest in a series of landformsincluding overwash sandsheets, storm fans, and tsunami deposits (Anthony,

2009) The inundation, landward sediment transport, and erosion as a result of

FIGURE 11.7 The Chandeleur Island chain on the Louisiana and Mississippi Coast of the Gulf

of Mexico before (top) and after (bottom) the storm surge and large waves from Hurricane Katrina (landfall on August 29, 2005) submerged the islands and eroded large sections of marsh Credit: USGS.

overwash can affect coastal management, primarily through the loss or age of property as well as damage to, or removal of, infrastructure such asrailtrack, roads, recreational areas, car parks, and amenities Some locationsare more susceptible than others to the occurrence of overwash, and themanagement of any coastal communities affected needs to be aware of therecurrence interval of past events to assess the level of risk from future events.Barrier island coasts are particularly susceptible to the impact of severe stormsand associated extreme waves and water levels yet many such low sedimentarycoasts are the first line of defence against storm erosion and inundation(Figure 11.7).

dam-In order to achieve a nationally consistent assessment of the hazard ciated with storms and hurricanes and improve its capability to assess thevulnerability of the U.S coast to extreme storm waves, the United StatesGeological Service (USGS) devised the Storm-Impact Scale model shown inFigure 11.8 The USGS model is based on Sallenger (2000)and comprisesfour wave energy conditions or regimes of increasing hazard of swash,collision, overwash, and inundation, the impact of each depending on thetide þ wave þ run-up þ storm setup heights (R) relative to the height of thedune toe and crest (D) (Figure 11.8) These four wave-driven regimes rangefrom no net change to the system during normal wave conditions when wavessimply reorganize the intertidal sediments (Figure 11.9(a)), through steps ofincreasing wave impact and erosional change (Figure 11.9(b) and (c)), tomajor net change involving inundation and erosion of the fronting beach and

asso-dune base

dune crest beach system

tide + runup + setup = Rhightide + surge + setup = Rlow

Dhigh

Dlow

FIGURE 11.8 Diagram showing R high R low , D high , and D low where the dashed line represents the swash excursion deviation from wave setup (solid line) R high is maximum water-level elevation expected during a hurricane and R low is the effective still-water level during a storm, D high is the elevation of the dune crest and D low is the elevation of the dune toe Credit: USGS.

dune system leading to wholesale landward migration of the system under theimpact of severe storms and associated extreme waves (Figure 11.9(d)) Themodel is useful in providing an estimation of the magnitude and nature ofchange that can occur when typhoon or hurricane waves make their landfall.The approach compares the known heights of coastal landforms (e.g., derivedfrom LiDAR survey) with modeled elevations of storm-induced water levels(e.g., forecast from probabilistic surge and wave models) in order to definecoastal change regimes These regimes describe the dominant interactionsbetween beach morphology and storm processes along beaches and dunes thatserve as the “first line of defence” for many coasts that are exposed to tropicalstorms and hurricanes, such as the east coast of the U.S and the land borderingthe Gulf of Mexico (Figure 11.7).

Nevertheless, for many such coasts the low frequency of extreme waveevents makes it extremely difficult to adequately assess both their recurrenceinterval and impact, an issue shared in common with all high-magnitude andlow-frequency natural events Both the Tohoku tsunami (2011) and Haiyan

Typhoon (2013) poignantly showed that catastrophic extreme wave events may

be too infrequent for their hazard to be characterized by historical recordsalone and so the geological and geomorphological records may be used toaddress this gap The characteristics and distribution of sediments transportedand deposited by extreme waves can help clarify the key processes associatedwith particular events, including estimates of the generation and track ofstorms, the source locations of tsunamigenic events, the water depths andvelocities of incident waves, and the mapping of past inundation distances Forexample,Figure 11.10(a)shows the extent of overwash deposition in south-eastern India following Cyclone Thane in 2011.Figure 11.10(b)shows post-event data gathering by field surveyors that will provide geomorphologicalinformation for use to better estimate the likely inundation extent and impact

of future events

Wave-derived overwash deposits are also preserved in the geological cord and can be used to infer the extent and establish the frequency of pastextreme wave events For example, where stacked sequences of overwashdeposits are found, chronologies can be developed via radiocarbon, opticallystimulated luminescence, and other dating techniques to help constrain therecurrence interval of past events (Anthony, 2009) Such information may beused to guide adaptation and mitigation efforts aimed at reducing the impact offuture extreme wave events For example, on the coast of Hokkaido, Japan,Nanayama et al (2003)show large tsunamis to have inundated the coast onaverage every 500 years between 2,000 and 7,000 years ago, with extremewaves depositing sand sheets (some extending several kms inland) Wherepreserved and accessible, such records of extreme wave events can be used tohelp guide land-use management and planning controls to help mitigate theextent and impact of future extreme wave events

FIGURE 11.10 (a) Cyclone Thane in 2011 produced overwash deposits on the southeast Indian coast near Cuddalore (Image from Google Earth) (b) Students surveying and mapping the recently deposited overwash near Cuddalore Photo: Switzer.

To correctly interpret the local history of extreme waves, investigatorsoften attempt to distinguish between the type of overwash deposit (tsunami orstorm) found in the geological record Comparative studies of both types ofdeposit reveal some differences in sedimentology, stratigraphy, faunalcomposition, and inland height and extent Although researchers have pro-posed criteria to distinguish these two types of deposits, it is apparent that eachmust be carefully considered in the context of its regional setting A variety ofphysical, sedimentological, and geochemical techniques are available, which,when carefully considered in a local geomorphic and stratigraphic context,may allow a positive discrimination between a storm or a tsumanigenicprovenance Generally, overwash deposits from extreme waves cause adecrease in the total organic matter within coastal plain sediments Othernotable changes include changes in salinity indicators and salts (Chague-Goff

et al., 2012), and in the assemblage composition of ostracods, diatoms, minifera (often derived from deeper water offshore), pollen, and aquaticplants The internal sedimentology (e.g., bedding), height above sea level,landward extent, and changes in thickness and regional continuity can alsoassist in determining the provenance of overwash deposits In some cases, theexistence of deep water heavy mineral and microfaunal assemblages now sited

fora-in a terrestrial environment may fora-indicate transport via a tsunami as opposed to

a storm (e.g., Switzer et al., 2005) It is clear that overwash deposits fromextreme waves can provide an opportunity to evaluate the recurrence of stormsand tsunami that are large enough to leave lasting sedimentary signatures.However, at present there exists no single analytical technique that canunambiguously differentiate between tsunami and storm deposits in thegeological record

11.3.2 Extreme Wave Impacts on Rock Coasts

On rocky coasts (herein including reefs and carbonate coasts), coarse clastdeposits (boulders) emplaced by extreme waves provide graphic evidence ofextreme wave events and can produce a valuable archive of their frequency andmagnitude (see Anthony, 2009) However, like the clastic deposits of sand,gravel, and mud mentioned in Section 11.3.1, difficulty often surrounds theinterpretation of whether extreme storm or tsunami waves have beenresponsible for the deposition of coarser sediments (e.g.,Switzer and Burston,

2010), and this has led to misinterpretation of some deposits and generateddebate in the literature (e.g., Scheffers et al., 2009, 2010; Hall et al., 2010).That said, some general characteristics can be used as broad indicators ofeither storm or tsunami deposition Storm deposited clasts close to sea levelcommonly form ridges, ridge complexes, and clusters composed of a range ofsizes (Morton et al., 2007; Etienne and Paris, 2010) This tendency is mirrored

at altitude on cliff-top sites where the cliffs are fronted by deep water Thiscontext allows storm waves to gain access to the coast with minimal