Chapter 5 – palaeostorm surges and inundations

Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations Chapter 5 – palaeostorm surges and inundations

Palaeostorm Surges and

in-Storm surges and associated marine inundations can be generated by all stormsapproaching the coast from the sea A storm surge is an elevation of the seasurface ahead of, and during the passage of, a storm toward the coast It is along-gravity wave with a wavelength comparable to the diameter of thegenerating storm and can be likened to a raised dome of water that inundatesthe coast Variations in the height of surges are caused by the lowered atmo-spheric pressure associated with the storm, the forward or translational speed

of the storm, the radius of maximum winds, the angle of approach of the stormtrack to the coast, the offshore bathymetry, and the shape of the coastline.Surge heights can exceed 7 m under optimal conditions The surge forms part

of the total marine inundation associated with the storm Other components ofthe inundation include the tide, wave setup (addition to the height of the water

by breaking waves), wave action (waves on top of the surge), and wave run-up(uprush of waves against or over an object or sloping shoreline, respectively).Coastal and Marine Hazards, Risks, and Disasters http://dx.doi.org/10.1016/B978-0-12-396483-0.00005-4

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Storm tide refers to the combined inundation level resulting from the stormsurge and tide Storm surges can be generated by both temperate low-pressuresystems and tropical cyclones (TCs) The latter tend to generate the largestsurges and inundations because they achieve lower central air pressures andcan generate stronger onshore winds This chapter describes the types of ev-idence used to reconstruct past or palaeomarine surges and inundationsgenerated by TCs These long-term records help to reduce risk from thishazard because they allow more robust estimates of magnitude/frequency re-lationships and return intervals to be made compared to those that can bederived from short historical records alone.

Palaeostorm inundations are recorded principally by sediments depositedinland and above the normal limit of wave-deposited sediments along a coast.The prehistoric record of storm inundations is largely restricted to the latterhalf of the Holocene, which is approximately the last 5,000e6,000 years, orsince termination of the Holocene marine transgression Although TCs wouldhave formed during the period of lower sea level between the present and lastinterglacial period, any sediments deposited by these events would likely havebeen reworked during the Holocene sea level rise The last interglacial stormdeposits are preserved in some locations, but to date, no such deposits havebeen positively identified

Sediments deposited during TCs take the form of ridges of coral rubble,cheniers, sand beach ridges, shell ridges, gravel ridges, pumice ridges, sandsplays, and washover deposits, being layers of sand within otherwise muddy ororganic sediments in back barrier lagoons All these sediments are deposited

by surge and/or waves Substantial quantities of sediment, mainly sand andrarely isolated shells, can be transported inland and deposited by the high-velocity winds These deposits are difficult to recognize because they occur

in environments where normal aeolian processes have deposited similar sized particles Storm-wind-blown deposits have also not been recognized asforming distinct sedimentary layers or units in the back beach environment,unlike aeolian deposits blown by lower velocity winds (Nott, 2014) Thefollowing sections describe the sedimentary and morphological characteristics

sand-of each sand-of these types sand-of palaeomarine inundation deposits

5.1 CORAL RUBBLE RIDGES

Coral rubble ridges occur in locations where coral reefs occur close to shore.During a storm, coral fragments are eroded from near-shore reefs by waveaction and transported either onshore or offshore (Baines et al., 1974; Davies,1983; Hughes, 1999; Rasser and Riegl, 2002) Fragments can also be trans-ported from existing accumulations of offshore coral rubble The offshoreaccumulations result from a number of erosional processes, such as biodeg-radation and wave action, during storms and fair weather conditions (Hughes,1999; Rasser and Riegl, 2002) It has been suggested that the angle of the

offshore reef slope plays a role in whether the eroded fragments are ported predominantly offshore or onshore Steep reef foreslopes favor offshoretransport of fragments, commonly to depths of greater than 50 m, which is toodeep for the fragments to be reworked and transported by storm waves.Shallow and wide reef fronts favor transport onshore and the formation ofcoral rubble ridges However, some sites, such as Curacao Island in the centralGreat Barrier Reef, Australia, (Figures 5.1e5.3) that are fronted by narrow,steep reef slopes, have extensive coral rubble ridge development on land(Hayne and Chappell, 2001; Nott and Hayne, 2001) The sites with minimalaccumulation of coral rubble in the shallow waters of the reef and maximumaccumulation of rubble in the deeper offshore waters below wave base (depth

trans-to which waves will entrain and transport sediment on ocean floor) suggest thatthe onshore ridges could have formed from predominantly live coral fragmentsbroken off during the storm At other sites, however, little doubt exists thatonshore ridges were formed from the reworking of existing accumulations ofrubble in the shallow waters offshore It is difficult to know whether the rubbleridge is deposited gradually during the storm, or as one, or as a series ofsediment units moving landward from existing offshore accumulations.Scoffin (1993)has described coral rubble ridge building as ridges that “havebeen transported and deposited like large asymmetric waves of sediment;material picked up on the seaward side is rolled up the ridge and dropped downthe advancing slope.” This suggests that an entire, or substantial part, of anoffshore accumulation of rubble is moved onshore as a single unit during thestorm Alternatively, if the ridges accumulate gradually, then it could beassumed that the ridge will increase in height over time during the storm Inthis instance, wave run-up may influence their formation

FIGURE 5.1 Map of global late Holocene tropical cyclone sedimentary records 1, Texas

Wallace and Anderson (2010) ; 2, Alabama/NW Florida Liu and Fearn (1993, 2000) ; 3, NW Florida

2 Lane et al (2011) ; 4, New York City Scileppi and Donnelly (2007) ; 5, Puerto Rico Woodruff

et al (2008) ; 6, Belize McCloskey and Keller (2009) ; 7, Japan Woodruff et al (2009) ; 8, Western Australia Nott (2011) ; 9, Gulf of Carpentaria Rhodes et al (1980) ; 10, Wonga Beach Forsyth et al (2012); 11, Cowley Beach Nott et al (2009) ; 12, Rockingham Bay Forsyth et al (2010)

It is likely that the height of an onshore coral rubble ridge is a function ofthe combined storm surge, tide, and wave setup and wave run-up Determining

to what extent wave run-up is responsible for the height of the resulting ridge

is important, as run-up can equal or exceed the height of the storm surge,depending upon various conditions Wave run-up is a function of significantwave height and wave period/length, wave refraction/diffraction, bathymetry,beach slope angle, and roughness and permeability of beach material (Neilsenand Hanslow, 1991) With very rough, coarse-grained, permeable substrates,Losada and Gimenez-Curto (1981)wave run-up can be 0.30e0.75 times that

of run-up on a sandy, largely impermeable beach under the same storm ditions Observations of historical surge-emplaced rubble ridges suggest thatwave run-up may, in some circumstances, play an insignificant role Forexample, a 3.5-m-high ridge was deposited on Funafuti Atoll during TC Bebe

con-in 1971 The surge accompanycon-ing the storm was 5 m above the level of the

FIGURE 5.2 Stratigraphy and chronology (radiocarbon mean calibrated and reservoir-corrected ages) of coral shingle ridges at Curacao Island, Northeast Australia.

FIGURE 5.3 Oblique aerial photo of Curacao Island showing transect of cross-section shown in

Figure 5.2 Photo taken by D Hopley.

reef flat, or mean low tide level, and 1.5 m higher than the elevation of theresulting ridge crest (Maragos et al., 1973) A similar situation occurred atMission Beach, south of Cairns, North Queensland, Australia, where anintense TC struck in March 1918 There, a 4.5- to 5.1-m-high ridge of pumicewas deposited by a surge as the cyclone crossed the coast Eyewitnessobservations, results from numerical storm surge and wave models of theevent, and knowledge of the tide level at the time show that together thecombined storm tide (surge plus tide) and wave setup amounted to an inun-dation level of 4.7e4.9 m This suggests that wave run-up could have onlycontributed 0.2e0.4 m of the ridge height at its highest elevation Elsewhere,where the ridge is only 4.5 m high, wave run-up does not appear to havecontributed to the formation of the pumice ridge.

Coral rubble ridges contain a number of distinct sedimentary facies or units

of sediment These include storm beach face, berm, crest, and washover facies(Hayne and Chappell, 2001) Beach face and berm facies include porous, clast-supported, coarse biogenic shingle deposits that rarely dip seaward but aregenerally structureless Crest facies are horizontally bedded and are finergrained than beach face deposits Washover facies are bedded, dip landward up

to 15, and sometimes contain imbricated clasts (imbrication is a sedimentaryfeature where particles are arranged in an overlapping shingle-like patterndipping in one direction) Each of these facies or units combines to make astorm deposit Storm deposits are commonly separated by “ground surfaces”being lenses of pumice pebbles and a weak sooty or earthy palaeosol (ancientsoil) These ground surfaces are really former ground surfaces, or the surface

of the feature that was exposed for sufficient time between individual cycloneevents so that some soil development was able to take place Ridges oftencontain only one storm deposit, but it is possible for two or more storm de-posits to occur in one ridge Careful excavation of the ridge is necessary inorder to determine the number of storm deposits comprising the ridge Sam-ples collected for geological dating from only one storm deposit, when morethan one storm deposit is present, may bias the age determination of that ridge.Coral rubble ridges often accumulate on the sheltered side of islands,presumably because particles are constantly removed by the largest or mostintense TCs on the exposed side of the island The sheltered sides willexperience lower wave energy, but perhaps the full effects of the surge.Because the wave energy is reduced, the likelihood of the ridge being removedduring subsequent cyclones is lessened Where the preservation potential forridges is high, a number of ridges are sometimes able to accumulate over time.Curacao Island on the Great Barrier Reef has 22 consecutive coral rubbleridges paralleling the shore on its northwestern (NW) or sheltered side(Figures 5.2 and 5.3) Individual ridges extend for over 100 m along the shoreand rise to over 5 m above the midtide level (the tidal range is approximately

3 m) The ridges were deposited by successive cyclones so that new ridges aredeposited seaward of the previously emplaced ridge

The age of the ridges increases progressively with the distance inland.Curacao Island is typical of many sites that preserve coral rubble ridges;however, not all sites retain as many ridges Elsewhere, such as on FitzroyIsland in North Queensland (Nott, 2003), only one or two ridges parallelingthe shore are preserved This commonly results from site’s exposure and thecyclone frequency The time interval between cyclones will determine theextent to which compaction and lithification, or induration of the ridge canproceed Over time, individual coral clasts within a ridge will weather or breakdown and will provide carbonate that will progressively cement clasts together.Cementation characteristically begins within the core of the ridge It is com-mon for loose fragments to remain on the crest and sides of the ridge severalcenturies after deposition Throughout time, the ridge becomes resistant tofurther wave attack, although looser fragments on the ridge crest can beremoved and replaced by younger fragments if subsequent storm surges arehigh enough to overtop that ridge Also, in this fashion, more recent stormdeposits can be superimposed on older ones.

Prior to stabilization, and depending upon the geomorphic setting, ridgescan migrate inland by waves washing over the ridges and transporting clasts tothe landward side of the ridge The 19-km-long, 3.5-m-high, and 35-m-widecoral rubble ridge deposited by TC Bebe (in 1971) on Funafuti Atoll continued

to move inland and along shore for many years (Baines and McLean, 1976).Indeed, in some instances on coral atolls, the ridges will move inland acrossthe reef flat and abut or lap onto existing ridges through normal, noncyclonicwave action Individual storm or cyclone deposits and ridges will be moredifficult to recognize at these locations compared to sites such as CuracaoIsland where individual ridges often remain distinct

5.2 CHENIERS AND SHELLY BEACH RIDGES

Cheniers are ridges composed partly, or entirely, of marine shells that havebeen deposited onto a mud substrate They are also separated from each other

by the substrate (Chappell and Grindrod, 1984) Beach ridges can becomposed of sand or sand and shell, and sometimes isolated coral fragments.Unlike cheniers, beach ridges are separated by sand swales The ridge andswale topography forms a distinct and continuous sand unit that may bedeposited onto any type of substrate This substrate is commonly, but notalways, composed of estuarine muds deposited during a lower sea level stand.Cheniers and beach ridges are not restricted to tropical regions, and hence canform independently of TCs

It is likely that all cheniers are deposited by storm waves; if the cheniersare in the tropics, these waves are likely to be due to TCs On the other hand,beach ridges have been recognized to form by a number of processes,including deposition by swash during low- or high-wave energy conditions, oraggradation above the mean sea-level by an offshore sand bar (Taylor and

Stone, 1996) Although all these processes, and the associated beach ridges,can occur independently of TCs, the ridges that occur well above mean sealevel, and contain layers and/or beds of shell within tropical regions, are likely

to have been deposited during cyclones

Excellent examples of these ridges occur along the shores of the Gulf ofCarpentaria, Australia (Figure 5.1) (Rhodes et al., 1980) Up to 80 individualridges paralleling the shore form a beach ridge plain that extends inland forover 3 km in places These ridges, along the eastern and southern shores of thegulf, contain shell-rich layers up to 2 m thick, interspersed within medium- tocoarse-grained sand (Rhodes et al., 1980) The ridges rise up to 6 m abovemean sea level (tidal range of approximately 2 m) and extend along the coastfor up to 10 km A number of factors suggest that these ridges were deposited

by storm surge and waves including: the height of these ridges above sea-level,the presence of abundant shell layers in the ridge stratigraphy, that sea levelshave not varied by more than 2 m in the region during the Holocene, and thatthe Gulf of Carpentaria is especially prone to the development of intense TCsbecause of its warm, shallow waters Radiocarbon dating of the ridges byRhodes et al (1980)showed that they increase in age with distance inland.5.3 SAND BEACH RIDGE PLAINS

Beach ridges are triangular to convex, swash aligned, swash- and built ridges formed in the backshore, at or above the normal spring high tidelevel They are composed of principally or purely marine deposits (Hesp,2006) Foredunes are convex shore-parallel features that form through aeoliansand deposition within backshore vegetation (Hesp, 2006)

storm-wave-A number of studies have been conducted to extract records of actual TCmarine inundations over the past 6,000 years from beach ridges along the coast

of North Queensland, Australia (Nott and Hayne, 2001; Nott, 2003; Nott et al.,2009; Forsyth et al., 2010) and Western Australia (Nott, 2011) Nott andHayne (2001), Hayne and Chappell (2001), and Nott (2003) suggested thatentire coral shingle ridges could be deposited during a single storm event.Rhodes et al (1980)suggested the same for the sand/shell beach ridges alongthe western coast of the Gulf of Carpentaria, Queensland However, this isunlikely the case for sand ridges within the coarse-grained beach ridge plains

of the wet tropics of Northeast (NE) Australia (Figure 5.1) Rather, it is morelikely that units of sand between 0.1 and 1.5 m thick are deposited onto thecrests of a ridge during marine inundations; a ridge will grow in height pro-gressively with each marine inundation event For example, TC Larry (2006)deposited a relatively thin unit of sand (0.02e0.1 m tapering landward) ontothe incipient ridge at the back of Cowley Beach, approximately 150 km south

of Cairns, Australia, in 2006 (Figure 5.4)

Sand units were deposited on top of the seaward beach ridge along Cairns’northern beaches during several TC-induced inundations between 1996 and

2001 One of these units can be seen inFigure 5.5, which shows depositionduring an inundation generated by TC Justin in 1997 The unit varies inthickness from 0.05 to 0.4 m and extends for approximately 100 m along thecrest of an existing sand beach ridge that stands 2 m above Australian HeightDatum (AHD) TC Justin was a category 2 system (Australian category sys-tem) when it made landfall, and the inundation generated by this storm wasable to overtop the lower lying section of the beach ridge Elsewhere, thebeach ridge rises to 3 m AHD and the marine inundation did not overtop theridge in these locations As a consequence, no sand unit was deposited.These events and others that have deposited similar sand units provide aglimpse of how these beach ridges develop over the longer term When amarine inundation is sufficiently large enough to overtop or reach the crest of

an existing ridge, the inundation results in the deposition of a unit of sand,causing the ridge to grow in height with each successive event Differentmagnitude inundations can be responsible for depositing a ridge until the ridgeapproaches a height that is attainable by wave run-up generated by only very

FIGURE 5.4 Beach ridge cross-sections and chronologies (Shark Bay is dated using radiocarbon and remainder using OSL) OSL, optically stimulated luminescence From Nott and Forsyth (2012)

intense TCs Hence, the most extreme inundations will deposit the final units

of sediment on the ridge The initial units of sediment constituting a ridge(those lowest in the ridge stratigraphy) could have been deposited by a range

of inundations starting from noncyclonically induced inundations, such as veryhigh tides and strong trade-wind-generated wave conditions, to the mostintense TC-generated inundations (Figure 5.6) Progressively higher run-up isrequired to deposit sand onto the ridge crest as it grows in height An elevationpoint will be reached where the vast majority of inundations can no longerreach the ridge crest and the ridge will cease to grow The timing of thisterminal stage may also be influenced by the growth rate of the next seawardridge This ridge was likely initiated before the ridge to its landward sidereached its maximum height Hence, the rate and volume of sediment delivery

to the coastal system will play a role in the height that ridges can finally attain.Periods when sediment delivery rates and volumes to the coastal system arehigh may result in a ridge not attaining its maximum height as a function of theintensity of TCs alone This is because the ridge on its seaward side hasattained sufficient height to diminish the ability of wave run-up to reach thisnext inland ridge Hence, most of the sedimentation will occur on the mostseaward, likely slightly lower elevation, ridge, and the next inland one willbecome starved of new sediment Ridges may have the opportunity to reachtheir maximum possible height, which is determined by the maximum inun-dation height for that region, during periods of relatively low sediment de-livery rates (and volumes) to the coastal system In this fashion, there will beinterplay between the processes operating to attain the maximum ridge

FIGURE 5.5 Photograph of sand unit deposited onto the first beach ridge at Clifton Beach (Cairns, North Queensland, Australia) during TC Justin, 1997 The photograph shows the sand unit extending down the rear flank of the beach ridge and extending into the swale behind the ridge This sand deposit was coarse grained and was over 30 cm thick in places From Nott (2010)

heightdthese processes being the rate of sediment delivery to the near-shoreenvironment and the maximum height of the inundations able to be generated

by TCs In the case of the ridge plains along the wet tropical coast of NEAustralia, for example, the final inundations responsible for depositing theuppermost units of sediment on the beach ridges were generated by extremeintensity TCs (Nott et al., 2009; Nott, 2010; Forsyth et al., 2010) Hence, usingthe heights of these ridges and methods developed byNott (2003)for calcu-lating the intensity of the TCs responsible suggests a 5,000- to 6,000-yearrecord of intense TCs

Beach ridges constructed by surge and waves display a key sedimentarysignature They have a sudden textural coarsening and coarse-skewed distri-bution that occurs at the base of each sedimentary unit deposited during anevent Unlike coral shingle ridges, which can be deposited entirely during asingle storm event, sand beach ridges appear to accrete progressively overtime Another ridge develops seaward; it progressively increases in height untilinundations can no longer reach its crest and a beach ridge plain develops.Each layer deposited during a storm can be identified by the sedimentary

FIGURE 5.6 Schematic sequence of accretion of sand beach ridges in Northeast Queensland The number and thickness of individual units will vary between ridges In this sequence, unit 1 is the oldest and unit 9 is the most recently deposited Lower units can be deposited by low- magnitude inundations but increasingly higher inundations (hence higher magnitude storms) are required to deposit uppermost units on a ridge From Nott et al (2013)

characteristics mentioned Coarse-skewed trends with minimal change inmean grain size characterize the upper levels of these deposits when thesedimentary unit is deposited at a location within the zone of maximumonshore winds These same trends are not apparent in sediments deposited atlocations that experience predominantly offshore winds during the TC event,which in the case of Eastern Australia is north of the eye-crossing location.Hence, it may be possible to identify the relative landfall locations of past TCsusing this diagnostic sedimentary signature (Nott et al., 2013) This coarse-skewed sedimentary trend, along with the initial textural coarsening of eachevent unit, may be useful for identifying TC sedimentary units in older beachridges where advanced pedogenesis has obscured the visual stratigraphicmarkers that are evident in more recent storm-built beach ridges Results fromthis research have also shown that geochemical signatures and microfossil data(diatoms and foraminifera) can be used to identify palaeocyclone deposits aswell as the limit of inundation of older cyclones and thus help distinguish themfrom other deposits associated with fair weather conditions (Nott et al., 2013).5.4 SHELL RIDGES

Unlike cheniers, shell ridges do not form as separate features on a mud strate Rather, they are much more akin to sand beach ridges, but are composedentirely of marine shells They are not common around the coast of NorthernAustralia, although they are extensively preserved at Shark Bay, WesternAustralia Here, the shells of the species Fragum erugatum, otherwise known asthe cardiid cockle, flourish in these hypersaline waters (Figure 5.4) Limitedcirculation of waters occurs between the open ocean and the deeply indentedbays here, particularly Hamlin Pool, because of the formation of the Faure Sill, asand bar that developed during the mid to late Holocene and stretches across themouth of Hamlin Pool The limited number of predators of F erugatum meansthat this species thrives in the near-shore environment and provides an abundantsupply of shells of remarkably uniform size (5e8 mm diameter) for potentialtransport onshore during marine inundations induced by TCs The resultingridges are composed entirely of this shell species Sand occurs only in the mostshoreward ridges This sand appears to be removed from the ridges with timebecause the second and third rows of ridges inland are devoid of sand

sub-Approximately 40 ridges parallel the coast at Hamlin Pool The ridgesincrease in age with distance inland The majority of the ridges were depositedfrom 5,500 years before present (BP) to the present (Nott, 2011) Pleistoceneridges are also preserved landward of the Holocene sequence The Pleistoceneridges differ from the Holocene ones, as the former are composed of a muchwider array of shell species, which are also larger in size compared to thesingle species of uniform size in the Holocene ridges The larger Pleistoceneshells are open ocean species and suggest greater water circulation betweenthe open ocean and the indented bays during the Pleistocene compared with

that at present The shell ridges extend in height up to 8 m AHD Such ridgescould only, within this very protected and calm water environment, have beenformed by TC-induced waves and surge.

5.5 GRAVEL RIDGES

Ridges composed of gravel and rare coral fragments are common along theKimberley Coast of NW Western Australia These ridge sequences form gravelbarriers and commonly impound back barrier lagoons in embayments alongsections of coast dominated by steep rock cliffs They are particularly commonalong the western side of Cambridge Gulf north of Wyndham in the EastKimberley region The gravel ranges in size up to 1.6 m diameter (A-axis) and1.4 m (B-axis) and have been deposited into sequences of up to nine ridgesparalleling the shore At La Crosse Island (Figure 5.7) offshore from the mouth

of the Ord River, gravel ridges have been deposited in every embayment andform a discontinuous sequence that surrounds the island The ridges at twosites on opposite sides of the island extend up to 5 m AHD Radiocarbonsamples on coral fragments embedded with the core of ridges from each ofseven ridges show that they were deposited from approximately 5,000 years

BP until recently (Nott, 2000) The radiocarbon samples do not show a gressive increase in age with distance inland, suggesting that the ridges areregularly overtopped and reworked by marine inundations Interestingly, themost landward ridges within many of the ridges have concave, circulardepressions up to several meters across and 1 m deep The origin of thesefeatures is intriguing because they do not appear to have formed by tree falland disturbance of the gravels by the uprooting of the tree, as this could beexpected to form an asymmetric-shaped depression Also, no evidence existsthat trees ever grew on the ridges Although large saltwater crocodiles are

pro-FIGURE 5.7 Gravel ridges on La Crosse Island, NW Australia NW, northwestern.