Coastal Boulder Deposits Research Articles

Imbricated Coastal Boulder Deposits are Formed by Storm Waves, and Can Preserve a Long-Term Storminess Record

Coastal boulder deposits (CBD) are archives of extreme wave events. They are emplaced well above high tide, and may include megagravel clasts weighing tens or even hundreds of tonnes. But do they represent storms or tsunami? Many are interpreted as tsunami deposits based simply on clast size and inferences about transport, despite the fact that there are no direct observations documenting formation of these inbricated boulder clusters and ridges. In this study, we use force-balanced, dynamically scaled wave-tank experiments to model storm wave interactions with boulders, and show that storm waves can produce all the features of imbricated CBD. This means that CBD, even when containing megagravel, cannot be used as de facto tsunami indicators. On the contrary, CBD should be evaluated for inclusion in long-term storminess analysis.

Tsunami and storm sediments in Oman: Characterizing extreme wave deposits using terrestrial laser scanning

Accurate determination of geometric parameters is key to a holistic understanding of storm and tsunami deposits and for modeling wave magnitudes responsible for the displacement of large boulders. We present a new approach in acquiring high-resolution geometric data on coastal boulder deposits related to extreme wave events. The reconstruction of boulder movements along coastlines contributes to a better understanding of storm and tsunami dynamics. Critical parameters for both determining their origin of the event, and providing more accurate modeling parameters, include boulder size, shape, weight, age and lithology. We used terrestrial laser scanning (TLS) on two sites with 327 boulders along the Oman coastline in order to prove the method’s validity. TLS results in very accurate and detailed three dimensional reconstructions of the boulders and can be used to reconstruct the origin of the boulders based on shape and texture. The method also provides refined size, volume and mass estimates for the boulders. According to the results at least 3 large-scale inundation events were recorded on the northeastern Oman coastline during the late Holocene. Dating results on displaced beach rock boulders suggest severe events around 7540 ± 120 cal yr. BP, 1175 ± 115 cal yr. BP and 265 ± 155 cal yr. BP, which each left a clear and distinctive coastal boulder ridge. The largest displaced boulder has a length of 7.36 m, a calculated mass of 120.5 t, and experienced a vertical uplift of 1.3 m during an inundation event. The results suggest a tsunamigenic origin of the coastal boulder trains, and highlight a potential of strong tsunami events along the Omani coastline.

Very large boulders were moved by storm waves on the west coast of Ireland in winter 2013–2014

Erdmann and co-authors (2018) present interesting observations of coastal boulder deposits along Ireland's western seaboard, Unfortunately, some of their interpretations are undermined by inaccurate information; and two of their main conclusions—that boulders greater >50 t were not moved during the recent strong storms, and that boulders 20–30 t were not moved significantly against gravity—are incorrect. Field records from before and after the storm events clearly show that boulders weighing 10 s to 100 s of tonnes were moved inland and uphill.

How does wave impact generate large boulders? Modelling hydraulic fracture of cliffs and shore platforms

Boulder quarrying by large waves along steep, high-energy coastlines contributes to erosion both by causing inland migration of cliff faces and by vertical lowering of coastal platform surfaces. It also leads to the formation of coastal boulder deposits (CBD) above and inland of the high water mark. We describe mechanisms by which hydraulic fracture creates boulders from cliffs and bedrock platforms. Intense fluid pressures are induced by wave impact on the base of overhanging ledges, and also along interior surfaces as cracks fill with water during wave runup and overtopping. These processes impose large loads on the overlying rock, and bending stresses thus induced can create or propagate microcracks in the rock. Repeated loading leads to complete fracture and detachment of bedrock slabs, which are liberated as boulders.We consider these processes using the Aran Islands (Ireland) as a type locality, both because CBD there are well documented and because flat-lying strata and orthogonal joint patterns yield tabular bedrock geometry, which lends itself to modelling. The loaded rock is modelled as a beam with a microcrack that is fractured by a bending stress from an applied hydrodynamic load. This simple approach allows us to examine general scalings and dependencies. We determine a condition for fracture, relating the fluid pressure, loaded area, fracture stress, microcrack position and geometry. In some situations, the fluid pressure need only be a fraction of the fracture stress. Fracture is most likely to occur during extreme wave conditions and when the exposed rock has a large eroded area for loading. The balance of forces suggests that larger beams break more easily, facilitating the creation of large boulders.

Quantitative roundness analysis of coastal boulder deposits

The sedimentology of coastal boulder deposits (CBD) is little known. These deposits, and especially the largest clasts within them—which commonly weigh many 10s of tonnes—are of increasing interest because they attest to extreme energy wave events acting well above the high-tide mark (in some cases >40m above high water and >100m inland). Recent observations have made clear that storm waves (rather than tsunami) are important in both creating and activating these deposits; but beyond that we know little about their hydrodynamics, or the relative frequency of boulder movement, and how this might vary with topographic setting.Roundness is a first-order characteristic that preserves a record of deposit reworking. It can tell us whether clasts move around much after they have been created from bedrock, or whether they are emplaced into CBD with little or no subsequent transport. But because roundness has historically been difficult to measure, most roundness estimates in the literature are based on qualitative visual assessment, and are therefore of limited analytical value. Fully reproducible roundness measurements are possible, however, and in the case of boulders they can be acquired quite easily using the Kirkbride (2005) device in combination with a trigonometric formula presented here.CBD at locations in Ireland and Scotland provide a test case, demonstrating the utility of quantitative boulder roundness measurements. The 567 clasts we measured are generally angular, but a substantial proportion shows some degree of rounding. Most importantly, there are demonstrable, consistent patterns relating topography and roundness. There is a strong and unsurprising inverse correlation between clast roundness and deposit elevation. There is no first-order relationship between roundness and inland distance, but there is a power-law inverse relationship between rounding and steepness (ratio of elevation to distance inland). The greatest degree of rounding is found in CBD located at the back of gentle coastal platforms with gradual slopes. These relationships provide new insights into CBD sedimentology, which we hope will be probed and tested in future studies.

Multiple dating approach (14C, 230Th/U and 36Cl) of tsunami-transported reef-top boulders on Bonaire (Leeward Antilles) – Current achievements and challenges

Dating the transport/deposition time of supratidal coarse-clast deposits is difficult, limiting their value for inferring frequency-magnitude patterns of high-energy wave events. On Bonaire (Leeward Antilles, Caribbean), these deposits form prominent landforms, and transport by one or several Holocene tsunamis is assumed at least for the largest clasts. Although a large dataset of 14C and electron spin resonance (ESR) ages is available for major coral rubble ridges and ramparts, it is still debated whether these data reflect the timing of major events, and how these datasets are biased by the reworking of coral fragments. As an attempt to overcome the current challenges for dating the dislocation of singular boulders, three distinct dating methods are implemented and compared: (i) 14C dating of boring bivalves attached to the boulders; (ii) 230Th/U dating of post-depositional, secondary calcite flowstone and subaerial microbialites at the underside of the boulders; and (iii) surface exposure dating of overturned boulders via 36Cl concentration measurements in corals. Approaches (ii) and (iii) have never been applied to coastal boulder deposits so far. The three 14C age estimates are older than 40ka, i.e. most probably beyond the applicability of the method, which is attributed to post-depositional diagenetic processes, shedding doubt on the usefulness of this method in the local context. The remarkably convergent 230Th/U ages, all pointing to the Late Holocene period (1.0–1.6ka), are minimum ages for the transport event(s). The microbialite sample yields an age of 1.23±0.23ka and both flowstone samples are in stratigraphic order: the older (onset of carbonate precipitation) and younger flowstone layers yield ages of 1.59±0.03 and 1.23±0.03ka, respectively. Four coral samples collected from the topside of overturned boulders yielded similar 36Cl concentration measurements. However, the computed ages are affected by large uncertainties, mostly due to the high natural chlorine concentration. After correction for the inherited component and chemical denudation since platform emergence (inducing additional uncertainty), the calculated 36Cl ages cluster between 2.5±1.3 and 3.0±1.4ka for three of four boulders whilst the fourth one yields an age of 6.1±1.8ka, probably related to a higher inheritance. These 230Th/U and 36Cl age estimates are coherent with a suggested tsunami age of <3.3ka obtained from the investigation of allochthonous shell horizons in sediment cores of northwestern Bonaire. Whilst 230Th/U dating of post-depositional calcite flowstone appears to be the most robust and/or accurate approach, these results illustrate the potential and current limitations of the applied methods for dating the dislocation of supralittoral boulders in carbonate-reef settings.

Coastal boulder deposits attesting to large wave impacts on the Mediterranean coast of Egypt

Coastal boulder accumulations are documented along the Mediterranean coast of Egypt between Alexandria and Marsa Matrouh at four distinct sites. The spatial distribution and dimensions of 116 medium to large boulders were documented along four representative coastal profiles. Boulders weigh up to 23 metric tons and are located up to 40 m from the shoreline. Geomorphologic features, morphometric properties and the presence of attached marine organisms attest that the boulders are detached and transported from original subtidal or intertidal settings by the impact of unusually large waves. Adapted hydrodynamic models were applied to evaluate the height of the transporting waves. Our result shows that largest boulders could be transported by tsunami waves of 2.6 m or storm wave of about 10 m in height. Radiocarbon dating was performed on fixed marine gastropod (Vermetidae and Dendropoma) shells found on four representative large boulders. A calibrated age from the easternmost site is roughly coincident to the well-known tsunami of 1303 AD in the eastern Mediterranean. Three other calibrated ages correspond to a period ranging from the eighteenth century AD to present. A large tsunami like the event of 1303 AD would have been able to transport all of the studied boulders. However, radiocarbon ages and morphological properties such as freshly broken edges and surfaces suggest younger ages for the majority of boulders. Since there have been no large, post-1303 AD tsunamis reported, we suggest that the majority, if not all, of the boulders were most likely deposited by multiple intense storms. According to the wave height model, storms with wave heights exceeding 9 m at their breaking point probably occur about once every 100 years. A relationship between the boulder deposits and the high storm frequency that characterized the little ice age in the Mediterranean Sea is plausible. This study emphasizes the potential hazard of large waves on this part of the Mediterranean coast of Egypt.

Ancient high-energy storm boulder deposits on Ko Samui, Thailand, and their significance for identifying coastal hazard risk

Coastal geomorphic processes associated with high-energy storm events are difficult to estimate over recent geological history, though their frequency and magnitude are important to assess in order to understand present-day coastal vulnerability. Studying ancient coastal boulder deposits can shed light on the previous physical conditions necessary for their deposition. In this study, we estimated the physical processes required to move reef-derived coral boulders on the east coast of Ko Samui, a rapidly developing tourist island off eastern peninsular Thailand. The position and dimensions of 97 coral boulders (weight: mean 2.9t, max. 12.7t; transport distance: max. 125m) were measured at two sites and dated using uranium/thorium methods. Flow velocities of 2.3–8.6m/s were required to transport the measured boulders, with individuals deposited up to 4.7m above mean sea level. Age-dating suggests that events capable of the highest flow velocities occurred around AD 1600 and AD 1750. These were probably driven by tropical cyclones (typhoons). Boulder transport by events of similar magnitude has not been detected within the last 250years. The non-occurrence of similar events in living memory has implications for hazard perceptions at this important tourist destination. However, there is also evidence of substantial Holocene sea-level changes in the Gulf of Thailand, as observed at nearby Ko Phaluai. This potentially offers a challenge for the interpretation of older boulders dating from the mid-Holocene, as sea level may have been more than 2m higher than present. Thus, studies using coral boulders as a proxy for past storm-wave conditions must consider the broader sea-level history, and are probably best limited to the period post-2000BP in the Gulf of Thailand.

Geomorphological control on boulder transport and coastal erosion before, during and after an extreme extra‐tropical cyclone

AbstractExtreme wave events in coastal zones are principal drivers of geomorphic change. Evidence of boulder entrainment and erosional impact during storms is increasing. However, there is currently poor time coupling between pre‐ and post‐storm measurements of coastal boulder deposits. Importantly there are no data reporting shore platform erosion, boulder entrainment and/or boulder transport during storm events – rock coast dynamics during storm events are currently unexplored. Here, we use high‐resolution (daily) field data to measure and characterize coastal boulder transport before, during and after the extreme Northeast Atlantic extra‐tropical cyclone Johanna in March 2008. Forty‐eight limestone fine‐medium boulders (n = 46) and coarse cobbles (n = 2) were tracked daily over a 0.1 km2 intertidal area during this multi‐day storm. Boulders were repeatedly entrained, transported and deposited, and in some cases broken down (n = 1) or quarried (n = 3), during the most intense days of the storm. Eighty‐one percent (n = 39) of boulders were located at both the start and end of the storm. Of these, 92% were entrained where entrainment patterns were closely aligned to wave parameters. These data firmly demonstrate rock coasts are dynamic and vulnerable under storm conditions. No statistically significant relationship was found between boulder size (mass) and net transport distance. Graphical analyses suggest that boulder size limits the maximum longshore transport distance but that for the majority of boulders lying under this threshold, other factors influence transport distance. Paired analysis of 20 similar sized and shaped boulders in different morphogenic zones demonstrates that geomorphological control affects entrainment and transport distance – where net transport distances were up to 39 times less where geomorphological control was greatest. These results have important implications for understanding and for accurately measuring and modelling boulder entrainment and transport. Coastal managers require these data for assessing erosion risk. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Stable imbrication and delicate/unstable settings in coastal boulder deposits: Indicators for tsunami dislocation?

Medium sized and coarse debris (pebbles and boulders) can exhibit patterns of setting, particularly if they are elongated or tabular. These patterns, such as imbrication, indicate certain transport processes, mostly as to direction and character. Many years of observation and measuring of coarse coastal deposits, are the basis for the conclusions presented here on environmental transport processes, their character, and their energy. Beside imbrication, special emphasis is given to unstable or delicate settings of large boulders that evidently have remained in these positions from hundreds to thousands of years. This deserves an explanation to how they arrived at these positions, and why they have not changed into a more stable position, given an environment of constant energy input along coastlines. The discussion lays emphasis on whether both settings of (mega-) boulders can be indicative of tsunami transport processes.

Localized tsunamigenic earthquakes inferred from preferential distribution of coastal boulders on the Ryukyu Islands, Japan

The occurrence of large earthquakes and tsunamis along the Ryukyu Trench is a subject of continuing interest, the key to which is the long-term geological record. Here we describe the clast size and spatial distributions of ∼2900 boulders on the reefs of the Ryukyu Islands, Japan, as markers of paleotsunamis and causative tsunamigenic earthquakes. Boulders of tsunami origin were observed only at a specific island group at the southern end, suggesting the local occurrence of tsunamigenic earthquakes there. In contrast, in the central to northern Ryukyu Islands, no evidence exists of tsunamis larger than those at the southern end of the Ryukyu Islands during the past 2000–3000 yr. These islands have numerous boulders deposited by storm waves during the past 2300 yr or earlier. Their spatial distribution has not been disturbed by large tsunamis. This suggests that large tsunamis did not strike this area during that period; nevertheless, these regions are seismically active. Our study shows that coastal boulder deposits present great potential to not only ascertain the histories and effects of paleotsunamis but also to constrain fault models of the causative earthquakes.

High-energy events, boulder deposits and the use of very high resolution remote sensing in coral reef environments

Collin, A., Etienne, S. and Planes, S., 2013. High-energy events, coastal boulder deposits and the use of very high resolution remote sensing.In recent years, coastal boulders have become a trendy proxy in studying high-energy marine inundation events. From their morphology and spatial distribution, authors are able to characterize a singular event in terms of intensity (wave height, flow velocity). Recent post-catastroph studies (e.g. Indian Ocean 2004 and Japan 2011 tsunamis) have demonstrated also the interest of boulder deposits in reconstructing the event kinematics through a multi-proxy approach. But as boulder studies require a statistically robust dataset they are field-time consuming and sometimes fieldwork takes place in remote areas with low facilities. The use of very high resolution remote sensing could overcome some of these limits. In this paper, we evaluate the possibility to identify meter-size coral boulder thrown on a reef flat during the hit of a tropical cyclone. Image analysis allows for the discrimination of major geographical object encountered on coralline islands: submerged coral boulder, emerged coral boulder, perched reef, sand beach. Within emerged boulder population, specific bands available with WorldView-2 images (i.e. red and NIR2 band) allow the spectral discrimination and mapping of fresh and weathered elements.

Comment on ‘Reassessment of hydrodynamic equations: Minimum flow velocity to initiate boulder transport by high energy events (storms, tsunamis), by N.A.K. Nandasena, R. Paris and N. Tanaka [Marine Geology 281, 70–84

Studies of coastal boulder deposits have dramatically increased in number during the last decade. These have been accompanied by modeling efforts to understand the initiation of transport and the transport itself. A recent article by Nandasena et al. (2011) has attempted to improve equations developed by Nott (1993, 2007) for the movement initiation of boulders, but we argue that these equations are not appropriate for the tsunami data (at least in North Sumatra) used to reassess Nott's set of equations. Indeed, the field work on which the discussed article is partially based is inconsistent with the findings.The sand-beach context and the distribution of these boulder deposits may invalidate the modeling approach of the authors. It is very unlikely that water alone was able to initiate the transport of the limestone clasts. The role of the smaller fractions must be assessed as these were an important component of the 2004 Boxing Day tsunami waves. Moreover, the spatial distribution of the boulder deposits, which part of the discussed article relies on and that has been described in a previous paper, is not consistent with the inferences drawn in the present article. The deposit has been described as a mix of boulders of various sizes and weight concentrated in space, without landward fining. This suggests an ‘en masse’ deposit rather than a deposit related to a progressive reduction of water velocity. Such ‘en masse’ deposits have been related in different environments to ‘debris-flows’.Hence, we suggest that this modeling needs to take into account smaller sediments and eventually move towards a Bingham flow concept for the case of tsunami waves.

Boulder Ridges on the Aran Islands (Ireland): Recent Movements Caused by Storm Waves, Not Tsunamis

Ireland’s Aran Islands are an excellent place to test whether coastal boulder deposits—including individual rocks weighing several tens of tonnes near sea level and clasts weighing several tonnes transported at tens of meters above sea level—require a tsunami for emplacement or whether storm waves can do this work. Elongate deposits of cobbles, boulders, and megagravel are strung along the Atlantic coasts of the Aran Islands. No tsunamis have affected this region in recent centuries, so if these deposits are forming or migrating at the present time, they must be storm activated. We find a diverse range of evidence for recent ridge activity. First, shells of Hiatella arctica (subtidal rock-boring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, recent motion is attested to by eyewitness accounts that pin the movement of several individual 40–80-t blocks to a specific 1991 storm and by repeat photography over the last few field seasons (2006–2011) that captures the movement of boulders (masses up to ≈10.5 t) even in years without exceptionally large storms. Finally, geographic information system comparison of nineteenth-century Ordnance Survey maps with twenty-first-century orthophotos shows that in several areas the boulder ridges have advanced tens of meters inland since the mid-nineteenth century, overrunning old field walls. These advancing ridges contain boulders with masses up to 78 t at 11 m above high-water mark, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Thus, there is abundant evidence for ridge activity since the 1839 mapping, and as there have been no tsunamis in the northeastern Atlantic during that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.

Coastal boulders as evidence for high-energy waves on the Iranian coast of Makran

Coastal boulder deposits attesting to large waves are found along the rocky coast of Makran (Iran) from Chabahar to Lipar. Boulders are either scattered on the rocky coastal platform or accumulated in imbricated clusters. The boulders are mostly rectangular and composed of biogenic calcarenite deriving from the present coastal platform. Significant morphological features observed on the boulders include supratidal karstic pools, sharp broken edges and fractures. Some boulders contain boreholes and shells of marine bivalves, suggesting detachment and transportation from the subtidal zone. The dimensions, elevation and distance from the coastline of 58 representative boulders are documented to estimate their volume, weight and inland displacement. The boulders, weighing up to 18 t, are found up to 6 m above present mean sea level and up to 40 m from the present shoreline. We applied hydrological models to estimate and compare the wave height and inundation distance required to transport the boulders inland. Our results demonstrate that no known or probable storm event on the Makran coast is capable of detaching and transporting the boulders. In contrast, tsunami wave height of 4 m is enough to detach all the boulders from the rocky coast and transport them inland. We conclude that a tsunamigenic origin for boulder deposits is most plausible. Our results imply that the western part of the Makran coast has archived evidence of paleotsunami events, probably generated by large earthquakes at the Makran subduction zone.

Wave-Emplaced Coarse Debris and Megaclasts in Ireland and Scotland: Boulder Transport in a High-Energy Littoral Environment: A Discussion

Scheffers et al. (2009) describe a reconnaissance survey of boulder accumulations along parts of the Atlantic coasts of the British Isles. A substantial part of their report concerns the characteristics of wave-transported boulder deposits found high above sea level on cliffs. These boulder deposits reach perhaps their ultimate development along the coasts of the Aran Islands, Galway Bay, western Ireland, and Eshaness, Shetland, northern Scotland, where evidence of wave action reaches elevations of 50 m above sea level (a.s.l.). We have previously investigated in detail these and numerous other similar localities and concluded that the boulder deposits are cliff-top storm deposits (CTSDs), part of an assemblage of features found on certain cliffs on these very high-energy coasts that relate to the impacts of extreme waves during major storms (Williams and Hall 2004; Hall et al. 2006, 2008; Hansom et al. 2008; Hansom and Hall 2009). However, Scheffers et al. (2009, p. 571) claim that at these and other sites “no evidence ... exists for storm wave dislocation of ... boulders found at altitudes of 120 m a.s.l.” Indeed, “no calculations of possible storm wave heights, storm wave physics, or ages of the deposits and no descriptions of extraordinary storms are sufficient to explain these displacements” (p. 568), and “alternative explanations, such as tsunamis, should be considered” (p. 571). Here we rebut these claims by briefly restat-

Wave-Emplaced Coarse Debris and Megaclasts in Ireland and Scotland: Boulder Transport in a High-Energy Littoral Environment: A Reply

Previous articleNext article No AccessWave-Emplaced Coarse Debris and Megaclasts in Ireland and Scotland: Boulder Transport in a High-Energy Littoral Environment: A ReplyAnja Scheffers, Dieter Kelletat, and Sander ScheffersAnja ScheffersSouthern Cross GeoScience, Southern Cross University, P.O. Box 157, Lismore, New South Wales 2480, Australia(e-mail: [email protected]) Search for more articles by this author , Dieter Kelletat1 University of Cologne, Institute of Geography, Albert-Magnus Platz, 50923 Cologne, Germany. Search for more articles by this author , and Sander Scheffers2 School of Environmental Science and Management, Southern Cross University, P.O. Box 157, Lismore, New South Wales 2480, Australia. Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 118, Number 6November 2010 Article DOIhttps://doi.org/10.1086/656356 Views: 48Total views on this site Citations: 17Citations are reported from Crossref HistoryReceived June 07, 2010 © 2010 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Anja Scheffers, Wibke Erdmann Progress in Tsunami Science: Toward an Improved Integration of Hydrodynamical Modeling and Geomorphic Field Evidence, Geosciences 12, no.55 (May 2022): 209.https://doi.org/10.3390/geosciences12050209Rónadh Cox, Louise O’Boyle, Jacob Cytrynbaum Imbricated Coastal Boulder Deposits are Formed by Storm Waves, and Can Preserve a Long-Term Storminess Record, Scientific Reports 9, no.11 (Jul 2019).https://doi.org/10.1038/s41598-019-47254-wDmitry Ruban, Alena Ponedelnik, Natalia Yashalova Megaclasts: Term Use and Relevant Biases, Geosciences 9, no.11 (Dec 2018): 14.https://doi.org/10.3390/geosciences9010014Ronan Autret, Guillaume Dodet, Serge Suanez, Gildas Roudaut, Bernard Fichaut Long–term variability of supratidal coastal boulder activation in Brittany (France), Geomorphology 304 (Mar 2018): 184–200.https://doi.org/10.1016/j.geomorph.2017.12.028John F. Dewey, Paul D. Ryan Storm, rogue wave, or tsunami origin for megaclast deposits in western Ireland and North Island, New Zealand?, Proceedings of the National Academy of Sciences 114, no.5050 (Nov 2017).https://doi.org/10.1073/pnas.1713233114Wibke Erdmann, Dieter Kelletat, Miriam Kuckuck Boulder Ridges and Washover Features in Galway Bay, Western Ireland, Journal of Coastal Research 335 (Sep 2017): 997–1021.https://doi.org/10.2112/JCOASTRES-D-16-00184.1Jim D. Hansom, Adam D. Switzer, Jeremy Pile Extreme Waves, (Jan 2015): 307–334.https://doi.org/10.1016/B978-0-12-396483-0.00011-XAnja M. Scheffers Paleotsunami Research—Current Debate and Controversies, (Jan 2015): 59–92.https://doi.org/10.1016/B978-0-12-396483-0.00003-0Wibke Erdmann, Dieter Kelletat, Anja Scheffers, Simon K. Haslett Study Sites, Methods and Aim, (Apr 2015): 13–17.https://doi.org/10.1007/978-3-319-16333-8_2Wibke Erdmann, Dieter Kelletat, Anja Scheffers, Simon K. Haslett Results from Field Work, (Apr 2015): 19–68.https://doi.org/10.1007/978-3-319-16333-8_3Wibke Erdmann, Dieter Kelletat, Anja Scheffers, Simon K. Haslett Introduction, (Apr 2015): 1–12.https://doi.org/10.1007/978-3-319-16333-8_1C.L. Kain, C. Gomez, A.E. Moghaddam Comment on ‘Reassessment of hydrodynamic equations: Minimum flow velocity to initiate boulder transport by high energy events (storms, tsunamis), by N.A.K. Nandasena, R. Paris and N. Tanaka [Marine Geology 281, 70–84], Marine Geology 319-322 (Aug 2012): 75–76.https://doi.org/10.1016/j.margeo.2011.08.008Rónadh Cox, Danielle B. Zentner, Brian J. Kirchner, and Mea S. Cook Boulder Ridges on the Aran Islands (Ireland): Recent Movements Caused by Storm Waves, Not Tsunamis, The Journal of Geology 120, no.33 (Jul 2015): 249–272.https://doi.org/10.1086/664787Kazuhisa Goto Current progress and perspectives of the research on tsunami boulders, Journal of the Sedimentological Society of Japan 71, no.22 (Jan 2012): 129–139.https://doi.org/10.4096/jssj.71.129Samuel Etienne, Mark Buckley, Raphaël Paris, Aruna K. Nandasena, Kate Clark, Luke Strotz, Catherine Chagué-Goff, James Goff, Bruce Richmond The use of boulders for characterising past tsunamis: Lessons from the 2004 Indian Ocean and 2009 South Pacific tsunamis, Earth-Science Reviews 107, no.1-21-2 (Jul 2011): 76–90.https://doi.org/10.1016/j.earscirev.2010.12.006Raphaël Paris, Larissa A. Naylor, Wayne J. Stephenson Boulders as a signature of storms on rock coasts, Marine Geology 283, no.1-41-4 (May 2011): 1–11.https://doi.org/10.1016/j.margeo.2011.03.016N.A.K. Nandasena, Raphaël Paris, Norio Tanaka Reassessment of hydrodynamic equations: Minimum flow velocity to initiate boulder transport by high energy events (storms, tsunamis), Marine Geology 281, no.1-41-4 (Mar 2011): 70–84.https://doi.org/10.1016/j.margeo.2011.02.005

Coastal boulder deposits in Galway Bay and the Aran Islands, western Ireland

Coastal boulder deposits in Galway Bay and the Aran Islands, western Ireland

The role of porosity in discriminating between tsunami and hurricane emplacement of boulders — A case study from the Lesser Antilles, southern Caribbean

Coastal boulder deposits are a consequence of high-energy wave impacts, such as storms, hurricanes or tsunami. Parameters useful for distinguishing between hurricane and tsunami origins include distance of a deposit from the coast, boulder weight and inferred wave height. In order to investigate the role of porosity on boulder transport and elucidate the distinction between tsunami and hurricane impacts, we performed Archimedean and optical 3D-profilometry measurements for the determination of accurate physical parameters for porous reef and coral limestone boulders from the islands of Aruba, Bonaire and Curaçao (ABC Islands, Netherlands Antilles, Leeward Islands). Subsets of different coral species and lithotypes constituting the boulders were sampled, the physical parameters of boulders were analyzed, and each boulder component was attributed to a certain range of porosity and density. Lowest porosities were observed in calcarenite (5–8%), whereas highest porosities were measured for serpulid reef rock (47–68%). Porous serpulid reef rock (0.8–1.2 g/cm 3) and the coral Diploria sp. (0.6–1.0 g/cm 3) possess the lowest bulk densities, while less porous calcarenite (2.0–2.7 g/cm 3) and the coral Montastrea cavernosa yield the highest bulk density values (1.6–2.7 g/cm 3). The obtained physical parameters were used to calculate boulder weights and both hurricane and tsunami wave heights necessary to initiate transport of these boulders. Boulders are up to 5.6 times lighter than given in previously published data, and hence required minimum hurricane or tsunami waves are lower than hitherto assumed. The calculated wave heights, the high frequency of tropical storms and hurricanes in the southern Caribbean and the occurrence of boulders exclusively on the windward sides of the islands, implicate that for boulders on the ABC Islands a hurricane origin is more likely than a tsunami origin.

Waves, coastal boulder deposits and the importance of the pre-transport setting

The pre-transport environment of a coastal boulder along with its shape, size and density determines the height of wave required for it to be transported. Different forces act on sub-aerial boulders as opposed to submerged boulders when struck by a wave. Boulders derived from joint bounded blocks on shore platforms predominantly experience lift force and require a wave of greater height to be transported than boulders in other environments. No one equation is applicable to determine the height of palaeo-waves responsible for depositing a field or ridge of imbricated coastal boulders. A range of equations and their derivation is presented here which can be applied to the respective pre-transport environment of a boulder. Such an approach is necessary when attempting to reconstruct the frequency and magnitude of past coastal wave hazards and for differentiating between tsunami and storm wave deposited boulder fields.