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Journal of Coastal Conservation

, Volume 23, Issue 4, pp 801–815 | Cite as

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

  • Bastian SchneiderEmail author
  • Gösta Hoffmann
  • Michaela Falkenroth
  • Jan Grade
Article

Abstract

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.

Keywords

Coastal hazards Tsunami Storm Roundness Roughness Boulders 

Introduction

Throughout history, humans have chosen coastlines as preferred areas of settlement (Small and Nicholls 2003). In modern times, an accelerated growth of population and infrastructure in the coastal zone has been observed globally - especially in developing countries (McGranahan et al. 2016). Consequently, both population and infrastructure are at risk, as they are exposed to coastal hazards such as tsunamis and tropical cyclones. Understanding potential magnitudes and recurrence intervals of those hazards is key to adopted urban planning and disaster risk mitigation (Macintosh 2013).

Our study area is the northeastern coastline of Oman, which has experienced numerous extreme wave events in the past. Recent events were a tsunami resulting from a Mw 8.1 earthquake in the Makran Subduction Zone (MSZ) in 1945 (e.g. Anonymous 1945; Beer and Stagg 1946; Heidarzadeh et al. 2008; Rastogi and Jaiswal 2006) and Saffir-Simpson category 5 cyclone Gonu in 2007 (e.g. Dibajnia et al. 2010; Fritz et al. 2010). These events are archived in the geological record within Holocene coastal sediments (Donato et al. 2009; Fritz et al. 2010; Hoffmann et al. 2013a; Okal et al. 2006; Pilarczyk and Reinhardt 2012). Extreme wave event deposits can be preserved as coastal boulder fields (Dawson and Stewart 2007; Nott 1997), and are valuable archives in obtaining information on the storm and tsunami history, including their frequency and magnitude (Luque et al. 2001).

Coastal boulder deposits typically occur on high-energy coastlines and are regarded as the least studied coastal clastic sediments (Cox et al. 2017). Finding sedimentological criteria to reconstruct the event character - storm or tsunami - still remains an unsolved issue in geoscience (Switzer and Burston 2010). Because of this, boulder deposits are often attributed to 'extreme wave deposits', as the bar for definitively stating whether the trigger process was a large storm or tsunami is difficult to reach. The characterization of the process responsible for boulder dislocation is challenging and requires accurate information on factors such as boulder quarry sites, transportation distance, uplift processes and the resulting orientation of the detached clasts (Etienne et al. 2011; Scheffers and Kelletat 2003). Detailed geometric measurements and spatial mapping of boulders are therefore of primary importance to decipher extreme wave deposits (Cox et al. 2017). Digital and high-precision 3D methods, such as TLS, offer great possibilities, supplementing traditional ground-based methods and surveys. The advantage of TLS data is the high-resolution and precision of point clouds. A sub-millimeter spatial resolution can be achieved with modern scanner systems (Buckley et al. 2008).

Hoffmann et al. (2013a) presented a first terrestrial laser scanning (TLS) based study on single coastal boulders in Oman. Other studies describing coastal boulder geometry using TLS were carried out in Italy (Mastronuzzi and Pignatelli 2011) and Greece (Hoffmeister et al. 2014). Armesto et al. (2009) discussed methods of determining boulder geometry using TLS in Spain.

In this study, TLS scanning was applied to a set of coastal boulders in order to determine if it could produce a precise and reproducible coastal boulder mapping dataset and whether it is faster and more objective than previous methods. The effort concentrated on boulder surface roughness as a means to discriminate between different lithologies. Precise measurements of the boulders were used to determine parameters such as volume and mass, length of axes, and orientation. Ultimately the study aims to determine the usefulness of TLS for characterizing extreme wave event deposits.

Study area and regional context

Oman is located in the north-eastern Arabian Peninsula on the shores of the Arabian Sea, which is a marginal sea of the Indian Ocean (Fig. 1). The current coastline between Muscat and Sur has a mesotidal regime with a tidal range of up to 2.5 m (UNESCO/IOC 2017). High-energy beaches with well-rounded limestone pebbles, steep limestone cliffs and long stretches of sandy beaches characterize the coastline. Oman has a hot and arid desert climate, where precipitation is low and usually occurs in local downpours along the mountain ranges (Darke 2013).
Fig. 1

Tectonic setting of the study area and cyclone tracks of Gonu and Phet. The Makran Subduction Zone poses the main tsunami threat for the coastlines of the Arabian Sea. Gonu and Phet are the strongest cyclone on record in the Northern Indian Ocean. Size of circle represents wind intensities. Inset maps show the global setting and the most important locations along the coastline under study. Modified after Schneider et al. (2016)

The northeastern coastline of Oman has experienced several extreme wave events in the historical past, caused by both tsunamis and tropical cyclones. The Makran Subduction Zone (MSZ) is the main tectonic structure responsible for the majority of tsunami events in the Arabian Sea. Earthquakes related to the MSZ triggered numerous tsunamis in the northern Indian Ocean in the late Holocene (e.g. Heidarzadeh and Kijko 2011; Rastogi and Jaiswal 2006). Submarine landslides along the Makran margin also contribute to tsunamigenic potential (Heidarzadeh and Satake 2014; Hoffmann et al. 2014).

The largest instrumentally recorded tsunami in the region occurred in 1945, when a Mw 8.1 earthquake triggered a tsunami which impacted the shorelines of the Northern Indian Ocean. Tsunamigenic earthquakes of this magnitude in the MSZ are modelled to cause waves of about 2 m on Oman's coastlines (Heidarzadeh et al. 2009a; Hoffmann et al. 2013b; Schneider et al. 2016). The modeling results are backed by geological (Donato et al. 2009; Koster et al. 2014; Pilarczyk and Reinhardt 2012; Prizomwala et al. 2015), archeological (Hoffmann et al. 2015), and historical (Kakar et al. 2014) evidence along the coastlines of Oman, India and Pakistan. Tsunamis with characteristics similar to the 1945 event are proposed to have return periods in the range of 100 to 250 years (Byrne et al. 1992; Heidarzadeh et al. 2008; Page et al. 1979).

Worst case scenarios of earthquakes with a Mw of over 9 along the MSZ are suggested by Heidarzadeh et al. (2009b) and Smith et al. (2013), resulting in modeled tsunami waves up to 15 m in northeastern Oman (Heidarzadeh et al. 2009a). Various studies (Heidarzadeh et al. 2008; Jordan 2008) suggest a tsunami event in 1008 AD in the western MSZ, which would have resulted in a considerably bigger impact on Oman’s coast than the 1945 tsunami. Characteristics and return periods of tsunamis larger than the 1945 event in the areas are enigmatic and under discussion. Newest multi-proxy studies by Hoffmann et al. (in review) suggest return periods of about 1000 years for tsunamis significantly stronger than the 1945 event.

Tropical cyclones are a second trigger for extreme wave events along Oman’s coasts is. The two most recent and significant events were Cyclone Gonu (Saffir-Simpson category 5) in 2007 (Dibajnia et al. 2010; Fritz et al. 2010), and Cyclone Phet (Saffir-Simpson category 4) in 2010 (Haggag and Badry 2012). Those are the strongest storms ever to be recorded in the Arabian Sea (Hoffmann and Reicherter 2014). Cyclone Gonu reached wind speeds up to 250 km/h and caused massive storm surges with waves locally exceeding 8 m in our study area along the coastline of Oman (Dibajnia et al. 2010). Hoffmann and Reicherter (2014) documented a 2 m high and 900 m inland flooding based on storm surge wrack lines in Ras Al Hadd after Gonu. However, only a few smaller boulders were dislocated on land along the eastern coast of Oman (Hoffmann et al. 2013a). Generally, tropical cyclones are capable of quarrying and transporting large clasts with masses exceeding 100 t, as documented by May et al. (2015) in the Philippines, by Cox et al. (2017) in Ireland, and Goto et al. (2009) in Japan.

Two study sites were selected which represented different coastal settings along the same stretch of coastline (see Fig. 1). The first site presented in this study is located between the villages of Fins and Shab (see Fig. 1). This study location is characterized by high (9 m elevation), vertical limestone cliffs that undergo regular high-energy wave impact. The site is part of a larger system of uplifted marine terraces (see Fig. 2a), which can be traced along the coastline from Quriyat to Qalhat (Kusky et al. 2005; Mattern et al. 2018; Yuan et al. 2016). The cliff is the most distal part of an uplifted wave-cut platform, with heights between 8.7 m and 11 m above mean high water (MHW) (see Fig. 2a). A series of boulder trains with clasts ranging from fine boulders (25.6 – 51.2 cm) to fine block size (4.1 – 8.2 m) (Blair and McPherson 1999) is deposited on top of the platform along the cliff (see Fig. 2b). A patchy layer of beachrock is deposited on top of the limestone platform (see Fig 2b). We use the definition of beachrock as a cemented, often conglomeratic coastal material, which was formed somewhere between the berm of the beach and the upper subtidal area (Falkenroth et al. 2018, in review; Kelly et al. 2014; Mauz et al. 2015).
Fig. 2

Field images from the Shab (A-C) and Tiwi (D) study sites. Inset A: Single boulders and accumulated boulder train resting on a steep, high-energy cliff near Shab. Note the elevated marine terraces in the background. View towards the south. Inset B: Imbricated clast structures in the boulder train. View towards the south. Inset C: Contact of the Eocene limestone unit with the overlaying conglomeratic beachrock beds. Inset D: Scanning of Tiwi Graveyard, view towards South. The white ball-shaped targets are used to register the scanned point clouds. Note the imbrication and seaward dipping of the boulders

The second study area is located on an ancient graveyard south of Tiwi, and is characterized by a lower energy wave impact. It is located about 80 - 100 m inland on a slightly seaward-inclined alluvial fan. Typical surface deposits are marine sand and gravel and recent scree, in elevations between 3 and 6 m above MHW. The ancient graveyard in Tiwi is located a few hundred meters south of the estuary of Wadi Tiwi, a deeply eroded and huge open karst system cutting through marly Eocene limestone. Furthermore, the wadi cuts into an ophiolite unit, which underlies the Eocene limestone (Wyns et al. 1991). Wadi Tiwi is a perennial wadi system with constant water discharge. The area is sparsely vegetated. Individual acacia trees dominate over a patchy shrub and grass vegetation. A boulder train of imbricated predominately beachrock boulders was deposited on the ancient graveyard (see Fig. 2d).

Methods

TLS is a stationary, ground-based method which generates 3D point clouds with a laser range detector (Telling et al. 2017). We used the Faro Focus3D X330 with a laser wavelength of 1550 nm, a range of up to 330 m and an included RGB camera system. Minimum point spacing is about 0.8 mm in a 5 m distance while the ranging error primarily depends on distance and is declared to as low as 0.15 mm in 25 m distance (Faro 2015). This allows the acquisition of complex high-density and high-precision point clouds of large areas in a short time. The field campaign at both field sites was carried out in March 2016.

Field mapping and sample collection

To better define and ground truth the TLS data sets, we mapped lithological and stratigraphic information on the sites and the boulders. A special focus was put on boulders with a beachrock-limestone contact in Shab. Orientation and elevation of this contact on dislocated clasts is an indicator on wave-induced vertical uplift and overtopping. Five beachrock and four limestone boulder samples were collected for density measurement in the sedimentological lab at Bonn University, using the Archimedes principle. We obtained a mean average density for the limestone of 2.20 g/cm3 and 2.25 g/cm3 for the beachrock conglomerate.

We collected lithophaga and oyster shells attached to dislocated boulders of marine origin for 14C dating. Three oyster samples from three different boulders in Shab and two Lithophaga samples from one boulder in Tiwi were obtained. Latest possible time of death for these marine mollusks is the moment when the boulder was washed onshore due to wave impact. Therefore, our radiocarbon ages represent minimum dates for boulder dislocation. The IntCal13 calibration curve was applied to the radiocarbon dates on marine carbonate samples (Reimer et al. 2013). Additionally, a local Delta+ value of 217 years was applied (von Rad et al. 1999; Southon et al. 2002).

TLS survey and point cloud processing

We conducted 31 individual scans in Shab and 8 individual scans in Tiwi from different positions and heights. The complex setting required scan positions with a maximum spacing of 10 m, and scanner heights between 50 cm and 2 m. This helped eliminate scan shadows and maximize geometric information on the boulders. We used white, ball-shaped targets with a diameter of 14.5 cm (see Fig. 2d) and checkerboards to create detectable features, which helped register the point clouds to high precision. Scan registration was done in Faro Scene 6.2.4.30. The area of interest was then extracted to minimize file size and speed up the post-processing. The average error of the resulting point cloud throughout the extracted study area was 1.28 mm in Shab and 1.19 mm in Tiwi according to Faro Scene.

The general workflow and the process parameters used in this study is illustrated in Fig. 3.
Fig. 3

Flowchart of input data, used software and tools, processes and parameters. Orange boxes are software packages, green boxes is input data, blue boxes are processes and parameters, yellow ellipses is final output data

CloudCompare 2.8 (Girardeau-Montaut 2015) was used to georeference the point cloud with six ground control points obtained by differential GPS (d-GPS) across the scene. Vegetation often covered ground and boulders and was hampering the point cloud analysis. Removal of the vegetation is inevitable for a comprehensive LIDAR analysis in many cases. We applied the CANUPO classifier, a supervised classification plugin presented by Brodu and Lague (2012) to identify and remove vegetation. To remove stray points in the point cloud, we used the CloudCompare 2.8 noise filter (radius 0.01). The remaining point cloud was subsampled with a spacing of 1 mm to create a homogeneous distribution within the point cloud. Creating high-resolution models of each boulder was an essential base for all analyses in this study. All relevant clasts were extracted manually, resulting in 19 individual point cloud sets in Shab and 92 individual point cloud sets in Tiwi. To calculate the geometry of the boulders, a watertight polygon mesh was required. We applied the Poisson Surface Reconstruction filter with default parameters in Meshlab (Cignoni et al. 2008) following the method presented by Kazhdan and Hoppe (2013). Afterwards, volume and general axes of each boulder were calculated, using the “Compute Geometric Measures” tool with default parameters in Meshlab.

The lower surfaces of stacked and imbricated boulders could only be partially scanned because of a lack of line-of-sight. As the TLS does not allow to scan covered parts and potential cavities underneath boulders, the calculated geometric parameters are considered as maximal parameters. We defined the longest vector possible within of each mesh as the a-axis. The b-axis is the longest possible elongation within the mesh orthogonal to the a-axis, while the c-axis is defined as the maximum elongation perpendicular to a-axis and b-axis in the mesh. Due to the limitations to perfectly scan each boulder and the generalized density measurements, all calculated geometric data and weights are best-possible estimations.

Surface roughness

Surface roughness of boulders is considered as a function of clast size and clast dominance within the conglomeratic beachrock boulders (Burton et al. 2011; Fang et al. 2015; Lichti 2005). The uppermost beachrock layer in Tiwi consists predominantly of pebbles and cobbles (1 cm – 15 cm) and only a small amount of fine silty to sandy matrix. This typically leads to a rougher and more irregular surface than a sand-dominated beachrock or a polished limestone. We took advantage of the variation of clast sizes within the beachrock stratigraphy to assign boulders to the stratigraphic layer that most likely represents where the boulders originated before dislocation by wave impact. The resolution of the TLS is high enough to depict grain sizes such as small pebbles. We used the roughness tool of CloudCompare (Girardeau-Montaut 2015) using a kernel size of 2 cm, which corresponds to the typical clast size within the beachrock in Tiwi. The roughness tool calculates the distance between each point of the cloud to the best fitting plane computed on its nearest neighbors within the point cloud, in our case within 2 cm. The tool calculates a roughness distribution of each boulder, resulting in a characteristic fingerprint distribution graph. Boulders with a smooth surface show a large amount of low-roughness surfaces, while rugged boulders show a low share of low-roughness surfaces but a larger share of mid and high-roughness surfaces sections. Typical roughness graphs of various boulders are illustrated in Fig. 4.
Fig. 4

Examples for typical surface roughness distributions curves for different lithologies within the Tiwi dataset. Classes range from smooth boulders surfaces (Class 1) to rough boulder surfaces (Class 5)

A multi-variable analysis and clustering to identify boulders with significantly similar roughness classes (RC) distributions, and therefore similar lithological surface characteristics, was applied on all boulders. We used the k-means clustering algorithm (Hartigan and Wong 1979) implemented in the stats package of R. We chose five clusters for the k-means algorithm, corresponding to the five beachrock layers on site. The k-means clustering algorithm then generates five classes with a maximum difference between the roughness class distributions (see Fig. 4), which model the different lithologies found on site. Fig. 5 illustrates generic examples of boulders of each RC and their corresponding point cloud.
Fig. 5

Typical boulders for each roughness class (RC) as colored TLS point cloud and field image

Results

Shab

Site characteristics

The cliff near Shab is made up of a pale colored, marly Eocene limestone (Wyns et al. 1991). The limestone shows a regular joint pattern and a fine fissure network with a general orientation of 30°N as structural weak zones and potential detachment area. The surface formed as a wave-cut platform by abrasion, bioerosion and, later, karstification. The limestone bedrock shows a ramp-like, slight inclination towards the sea (see Fig. 6). The first few meters between the cliff margin and the first boulder train is flat, clean of vegetation and shows no sediment cover. A beach ridge, constisting of sand, small boulders, and marine debris (Koster et al. 2014), has accumulated. Vegetation cover on the ridge itself is sparse and limited to single acacia trees, a few shrubs, and grass patches.
Fig. 6

Principle sketch of the Shab site with 0.5 m contour lines. Lithologies, 14C dating results (in cal yr. BP), post-dislocation rotation of selected boulders based on fissures are indicated. Labels B1-B19 indicate boulders with a limestone - beachrock contact, GB1 is the largest block (120.5 t). Two north-south oriented main boulder ridges and a landward fining of clasts can be observed. Inset illustrates the lithology of the limestone- beachrock contact and beachrock characteristics. Elevations in meters above MHW. The cross-section of the Shab boulder train illustrates terrain and beach ridge accumulation

Lithology

A patchy beachrock layer can be observed as an erosional remnant on top of the limestone platform, clearly separated from the limestone below (see Fig. 2, C and Fig. 6, inset). These beachrock patches reach a maximal thickness of 60 cm. The lithology present is a moderately sorted, clast-supported conglomerate with clast size ranging between granules and large cobbles in a coarse to very coarse sand matrix. Limestone (75%), marine fosills (20%), and mafic rocks (5%) make up the components of the polymictic conglomerate.

Several hundred blocks and boulders rest on top of the limestone and beachrock sequence, which Hoffmann et al. (2013a) suggested were of tsunamigenic origin. The clasts are organized in coast-parallel ridges with an alongshore extent of over 1200 m and an inland extent of up to 50 m. The block and boulders often show indications of imbrication, rotation, or overturning. The largest clasts are not embedded in the boulder trains but rest individually close to the cliff face. About 55 % of the clasts are comprised of limestone, while about 45 % are beachrock clasts. Some limestone boulders show signs of bioerosion, karstification, and weathering. Nineteen boulders show the lithological contact between the limestone and the beachrock facies. As there is only one contact plane of limestone and beachrock found in situ, we used this contact to determine the vertical uplift of the dislocated boulders. Furthermore, some limestone clasts are resting on top of the in situ beachrock, which is also a clear indicator for lateral und vertical transportation of the clasts. We regard the Shab site as a good example for the application of TLS data on the determination of the exact vertical uplift of boulders, the reconstruction of quarrying locations and movement vectors.

TLS results

We mapped 235 individual boulders in Shab: 131 were beachrock and 104 were limestone, of which 19 showed the limestone - beachrock contact plane. The clasts in Shab are loosely organized in two coast-parallel major boulder-ridges in elevations between 8.9 and 12.9 m above MHW (see Fig. 6). The seaward ridge is situated on the bedrock while the landward ridge is located on an accumulated and elevated beach ridge. The three largest blocks are located close to the cliff, clast size decreases with distance from the cliff edge and elevation (see Fig. 6 and Table 1). The limestone clasts are much larger than the beachrock boulders.
Table 1

Geometric results of limestone - beachrock contact boulders, based on TLS measurements

Boulder ID

Volume (m3)

Mass (t)

Elevation (m MHW)

Inland distance (m)

A-Axis (m)

B-Axis (m)

C-Axis (m)

Uplift (m)

GB1

44,65

120,56

9,84

6,95

7,36

6,22

1,35

1,31

Boulder 4

6,08

16,42

11,14

30,81

4,63

2,47

1,16

1,91

Boulder 5

3,12

8,43

11,18

29,09

4,03

2,12

0,81

2,43

Boulder 6

2,98

8,05

10,91

27,89

3,45

2,18

0,43

1,27

Boulder 7

0,29

0,78

10,80

28,66

1,26

1,01

0,42

1,09

Boulder 8

1,34

3,63

10,91

27,52

2,41

1,63

0,52

2,11

Boulder 9

11,01

29,72

11,13

34,32

6,42

2,48

1,11

2,81

Boulder 10

3,57

9,65

10,42

28,92

3,26

2,02

0,71

1,46

Boulder 11

1,03

2,78

10,49

33,92

2,27

1,56

0,56

1,48

Boulder 12

0,30

0,81

11,93

39.71

1,49

0,94

0,27

2,41

Boulder 13

0,12

0,33

11,08

34,63

1,14

0,68

0,31

1,20

Boulder 14

0,16

0,42

12,92

38,32

1,11

0,69

0,22

3,32

Boulder 15

1,88

5,07

10,67

29,02

2,21

1,48

0,82

1,31

Boulder 16

1,40

3,77

11,14

22,85

3,36

1,08

0,51

1,58

Boulder 17

0,43

1,15

11,69

20,42

1,67

1,30

0,23

2,07

Boulder 18

0,33

0,89

12,05

24,28

1,52

0,82

0,34

2,67

Boulder 19

0,09

0,24

12,02

27,06

0,78

0,46

0,31

2,25

Boulder 20

2,68

7,24

10,57

16,10

2,64

1,56

0,74

1,48

Boulder 21

3,33

8,99

10,55

17,63

3,96

1,97

0,52

1,72

Boulder 43

5,09

13,74

10,65

21,32

3,34

2,65

1,03

0,82

The sharp contact of limestone bedrock and beachrock allows a precise mapping of vertical uplift of the clasts with both lithologies. We mapped the contact plane between the limestone bedrock and the overlying beachrock between 8.78 and 9.15 m above MHW, with an average elevation of 8.99 m above MHW. We then extracted the highest limestone-beachrock contact we found on each boulder to calculate the maximum vertical uplift (see Table 1).

The largest documented vertical uplift of 3.32 m was assigned to a boulder of ~420 kg, 38.3 m away from the cliff edge, while the lowest documented vertical uplift of 0.82 m was found on a boulder of ~1374 kg, 21.3 m inlands. The median of the documented wave-induced vertical uplift was 1.65 m with a median mass of 4.42 t. We could not find significant coherences between mass and vertical uplift for Shab, just a minor tendency for more uplift of lighter boulders, uplift was similar for all boulder masses (see Fig. 7). The limestone boulders are mostly of similar shape, expressed in low variations of axes ratios (see Fig. 9).
Fig. 7

Mass vs. vertical uplift of limestone boulders in Shab (Three largest boulders excluded) and limestone and beachrock boulders in Tiwi

The largest block (GB1) - also the largest block in the entire study area - with a mass of ~120.5 t was vertically uplifted 1.41 m and rotated by 85°. We measured the fissures on the three largest blocks to draw conclusions on post-dislocation rotation (see Fig. 6). Other major blocks (GB2 and GB3) were rotated only 15° and 20° respectively. Information on rotation and vertical uplift allowed us to trace the quarry site of GB1 which clearly indicated, that it was quarried on the cliff edge about 17.6 m north-east of its resting point. A cavity in the cliff, which fits the boulder, can still be seen here.

Furthermore, we documented two overturned boulders (B 7 and B 17 in Fig. 6), where the limestone-beachrock contact plane is found on the basal part of the clast. The boulders have masses of ~9.6 t and ~13.7 t respectively. Both boulders are located in the first row of the boulder train.

Some rounded limestone boulders show Lithophaga drillings with in-situ Lithophaga shells still in their boreholes (see Fig. 5, roughness class 2), and remains of other mollusk shells like oysters. These organisms are found in the intertidal zone (Kázmér and Taborosi 2012). This indicates a marine origin of the clasts.

Tiwi graveyard

Site characteristics

The site is covered by beachrock, which shows two different lithologies: pebbly sandstones and conglomerates. Five beds with thicknesses ranging from 10 cm to 20 cm are distinguished. The limestone bedrock is not exposed here. A stratigraphic log (see Fig. 8, inset) reveals an interbedded succession of conglomerates and sandstones. The clasts within the beachrock are mostly rounded limestones. The sandy matrix in both, conglomerates and pebbly sandstones, consists of coarse to very coarse sand. Our study is based on the variation and distribution of clast sizes within each beachrock layer.
Fig. 8

Principle sketch of the Tiwi site. Boulder colors represent boulder roughness classes corresponding to the beachrock bed layers shown in the inset. The cross section is depicted in the bottom as a TLS point cloud. Seaward downslope movement of the large slab in the center-right is indicated with an arrow. Results of 14C dating on lithophaga shells in a limestone boulder is labelled (in yr. cal BP)

Dislocated beachrock slabs form a single, coast-parallel boulder train (see Fig. 2, d) with an extent of about 500 m. Individual clasts are scattered around the main depositional cluster (see Fig. 8). Grain sizes range from fine boulders (0.35 m length) to very coarse boulders (over 3 m length). Beachrock bedding planes, which are the weakest spots of the sequence, are exposed as staircase-like scarps of a few decimeter height. The internal sorting of the boulder trains is superficially chaotic, with a landward fining tendency. Typically, slabs are piled up, stacked and often imbricated, with a predominant seaward dipping. Individual boulders are scattered around the imbricated boulder trains. Boulder uplift in Tiwi ranges from a few decimetres to 2.6 m for the beachrock clasts and between 4.75 m and 5.55 m for the limestone boulders. A negative correlation between mass and uplift is documented (see Fig. 7).

The conglomeratic beachrock deposits are mostly flat in shape, as their detachment usually followed the bedding planes of the beachrock. This is in accordance to other areas, as slab-like shapes are typical for dislocated beachrock boulders (Lau et al. 2015). Other beachrock clasts are broken in pieces, as the slab-like shape is prone to failure. Apart from the dominant beachrock boulders, eight rounded limestone boulders are documented. Those limestone boulders are smaller in size and most of them are not incorporated in the imbricated boulder trains. The site is regarded as a good example for applying roughness and shape studies on coastal boulders due to the existence of in situ beachrock beds with different clast size and a huge variety of boulder shapes.

We sampled a total of 92 boulders in Tiwi. We recorded two main lithologies - beachrock and limestone - while over 90 % of the analyzed clasts are beachrocks. Beachrock slabs are typically embedded in the boulder train and imbricated with a seaward dip (compare cross section in Fig. 8).

TLS and surface roughness

The 92 surveyed boulders are found in elevations ranging from 3.2 m to 6.0 m above MHW. The boulders with the highest elevation above MHW are piled up on top of other boulders in the boulder train, not on the surface. We note a large variation in roundness of the clasts. Roundness is a direct indication of the amount of reworking - either by continous rounding in the surf zone or by repeated movement during extreme wave events (Cox et al. 2017). Notable is that none of the boulders is entirely made up of the upper sandy beachrock layer (see Fig. 8, inset). This layer is brittle and hence prone to erosion and shattering during wave impact.Five representative surface RCs of limestone and beachrock boulders in Tiwi are identified, based on the k-means clustering. The different lithologies are expressed in a distinct and typical roughness distribution curve (see Fig. 4). A low roughness indicates smooth surfaces, while a high roughness indicates rough surfaces. Class 1 boulders have a high amount of smooth surface parts (polished limestone boulders), while class 5 boulders are characterized by a low amount of smooth - but a high amount of rough surfaces - resulting from coarse-grained clasts within the beachrock.

The roughness clustering results were visually and sedimentologically crosschecked with field observations. In most cases, the computed result corresponded to the field observations. Not all limestone boulders are identified and allocated correctly. Smoothly polished boulders are all identified, but some limestone boulders show strong marks of bioerosion. The surface of those limestone boulders is perforated, resulting from lithophaga boreholes (Kázmér and Taborosi 2012). The surface roughness classification therefore assigns them into a second class, although the lithology is the same.

Only 9 out of 92 boulders are made of limestone, all are rounded to well-rounded. Typical limestone boulders are rather regular and ovoid boulders, showing low B-A-axis and high C-A-axis ratios. The largest limestone boulder reaches a maximum length (a-axis) of 0.93 m and a mass of 320 kg. The rounded and smooth appearance and the intense bioerosion of the limestone boulders reveal a marine origin. They are usually not embedded in the boulder train, but are resting isolated on the surface.

In contrast to the limestone boulders, the conglomeratic beachrock clasts are flat and slab-shaped. Most of those slabs have a thickness of around 20 cm, which correlates well to the average bedding thicknesses of the beachrock, while some other boulders are compound of multiple beachrock beds. The maximum measured boulder thickness (c-axis) is 59 cm. Typical slabs show high B-A-axis ratios (A-axis and B-axis are similar) but low C-A-axis ratios (A-axis is much longer than C-axis). No clear differentiation between the RCs can be made, there is no obvious correlations between RC and boulder shape (see Fig. 9). A correlation between the boulder RC and boulder mass is observed (see Fig. 10). Lightest boulders are found in class 1 (mean average 0.12 t), while class 5 has the highest mean average mass (1.84 t). The higher the RC, the heavier are the boulders.
Fig. 9

B-A-axis and C-A-axis ratio of the boulders in Shab and boulders of different RCs in Tiwi. A high B-A-axis and low C-A-axis ratio is charactreistic for dominantely flat beachrock slabs. Low B-A-axis and high C-A-axis ratios are typical for ovoid limestone boulders

Fig. 10

Relation of boulder mass and roughness classes in Tiwi

Dating results

Our 14C dating on intertidal oysters attached to the outer surface of boulders from Shab suggest at least two past events (see Table 2) which were capable of lifting boulders from the intertidal zone on top of the cliff, corresponds to a vertical uplift of up to 11 m. One event was dated to 1175 ± 115 cal yr BP (all values ±2σ) while the two dating results revealed ages of 250 ± 160 cal yr BP and 280 ± 150 cal yr BP respectively. In Tiwi, two 14C samples collected from Lithophaga shells of a limestone boulder were dated to 7475 ± 115 cal yr BP and 7605 ± 125 cal yr BP (see Fig. 8).
Table 2

14C Dating results on marine boulders within the boulder ridges from Shab and Tiwi

Boulder

Lab ID

North

East

Dated material

Conv. age (yr BP)

13C/12C (‰)

Max age (yr BP)

Min age (yr BP)

Dev. (yr)

Age cal BP (IntCal13) (yr cal BP)

Tiwi

 B31

Beta 348,985

2,525,243

732,316

Lithophaga spec.

7190 ± 40

2.4

7590

7360

115

7475 ± 115

 B31

Beta 348,986

2,525,243

732,316

Lithophaga spec.

7350 ± 40

1.2

7730

7480

125

7605 ± 125

Shab

 B79

Beta 348,982

2,530,537

729,195

Ostreidae

1850 ± 30

1.9

1290

1060

115

1175 ± 115

 B84

Beta 348,981

2,530,536

729,204

Ostreidae

860 ± 30

2

430

130

150

280 ± 150

 B86

Beta 348,980

2,530,534

729,203

Ostreidae

830 ± 30

0.3

410

90

160

250 ± 160

Discussion

By recording the boulder trains in high resolution, extracting the boulders and reconstructing dimensions, mass and surface roughness in a systematic manner, we were able to better describe and understand the characteristics of the large boulder trains along the northeastern coast of Oman.

The boulder deposits investigated in Shab and Tiwi show variations, reflecting the different settings and lithologies of each area. The boulders of Shab are generally much larger (median A-axis 2.53 m) and heavier (median 4.42 t) than the ones in Tiwi (median A-axis 1.36 m, median mass 0.52 t). This is attributed to the difference in lithology, as the majority of boulders in Shab consist of limestone, while the majority of boulders in Tiwi is made up of layered beachrock. The limestone fabric is more solid, and less prone to further erosion due to repeated wave action and weathering. Clast size in Tiwi is furthermore limited due to the layering of the local beachrock sequence. While we can find a slight native correlation between mass and vertical uplift for beachrock boulders in Tiwi, no definite correlation is possible for Shab. The limestone boulders in Tiwi are of marine origin. We can therefore conclude an inland inundation of at least 80 m and an inundation height of at least 5m for the event which was dated to 7500 yr cal BP.

Our analysis in combination with the 14C dating results from Shab and Tiwi, indicates that the area experienced multiple powerful large magnitude wave events in the past. The older event in Shab, dated to 1175± 115 cal yr BP, falls within the range of the major tsunami which may be correlated to the 1008 AD event that was reported from various locations along the coastline of the Northern Indian Ocean (Heidarzadeh et al. 2008). The younger event in Shab falls within the range of 250 - 280 ± 155 cal yr BP. The possible age range is so large that, especially in the context of difficulties with quantifying the marine reservoir effect (Lindauer et al. 2017), no definite correlation between this dated age and any historical event can be made. No major inundation events are reported from the coastlines of Oman within the possible time span from 95 – 435 yr BP. The only documented large inundation event happened in 1945. Taking uncertainties in 14C dating - as the local marine reservoir effect which is still under discussion (Lindauer et al. 2017) - into account, the dated sediments might be interpreted as sediments dislocated by the 1945 Makran tsunami. More dating control points would help to pinpoint down the accurate timing of dislocation events. The assumption, that at least two inundation events hit Shab is furthermore supported by the occurrence of two distinct beach ridges.

The events recorded in the boulders documented in Shab and Tiwi, must have been powerful and with a strong impact on the Omani coastline, as they were capable of quarrying, dislocating, and uplifting boulders of up to 120 t and overturning boulders with a mass of over 13.7 t on a 9 m high, and vertical cliff in Shab. The event documented in Tiwi was not powerful enough to lift a 4.5 t slab which is located 3.5 m above MHW and 80 m inlands, while a slab of 3.65 t was uplifted by 0.9 m. The marine limestone boulders documented on the Tiwi graveyard witnessed an inland transportation of at least 80 m from the shore and an uplift of up to 6.5 m (see Fig. 7) during a large wave event. Our results and the orientation of the boulder trains suggest wave impact directions from north-east. This would hint on the MSZ as the most likely source area and support the presumably tsunamigenic origin of the boulder ridges.

However, based on our results, tropical cyclones as a wave trigger cannot be ruled out entirely. With respect to field observations after cyclones Gonu and Phet (Dibajnia et al. 2010; Fritz et al. 2010; Hoffmann and Reicherter 2014), which did not show a dislocation of large boulders, it seems unlikely that even Saffir-Simpson category 5 cyclones along the Omani coastline are capable of mining and dislocating boulders with a mass of several tons. The wave impact and boulder depositional patterns show strong variations along the coastline even in adjacent areas, depending on available sediment and coastal configuration.

Currently, it is not possible to reliably differentiate between tsunami and storm deposits (Barbano et al. 2010; Kortekaas and Dawson 2007; Switzer and Burston 2010; Williams and Hall 2004). Several multi-proxy studies including historical data, interviews, geological survey, laboratory analyses and numerical modeling have been carried out, trying to solve this problem on coasts where both phenomena occur (e.g. Chagué-Goff et al. 2011; Ramírez-Herrera et al. 2012). However, at coastlines where extreme wave events occur infrequently, and with limited and fragmentary historical archives, most methods are not applicable. In recent time, numerical modeling expanded quickly, including modeling possible wave heights (e.g. Barbano et al. 2010; Nott 2003; Oetjen et al. 2017; Weiss et al. 2009), which can be an indicator to answer the question “storm or tsunami?”. These methods require an highly accurate model of boulder bodies, including the shape, dimensions and mass (Oetjen et al. 2017). Simplified boulder body estimation as axis estimation or simplified GPS point clouds can result in a significant overestimation of volumes and masses (Engel and May 2012; Hoffmann et al. 2013a) and do not result in sufficiently accurate boulder models, which are required for state of the art numerical wave models (Oetjen et al. 2017). Photogrammetric methods can produce results of a similar accuracy as TLS (Daneshmand et al. 2018), but are more time consuming in large, chaotic settings, where a high resolution is necessary.

Our roughness analysis showed that TLS datasets have the potential to classify rocks based on surface parameters. In our study, we focused on surface roughness, which in the first place is a function of clast size within the boulder. Other studies (Burton et al. 2011) utilized differences in surface reflectance to differentiate between rock lithologies. A combination of both approaches might increase reliability in future studies. We were able to group beachrock boulders based on the surface roughness and connect them to the beachrock layer, which they were assumingly quarried from. When comparing shape and mass of the different RCs, no striking connection between RC and shape could be drawn. Exceptions are class 1 boulders, which correspond to the rounded and spherical marine limestones. In contrast, we found a strong correlation between RC and mass. Smooth boulders of low RC show a small mass while rough boulders of high RCs are much heavier. This can be a result of variations in susceptibility to weathering and erosion and cementation between the beachrock layers.

We documented a large variety in boulder roundness expressed in different axis ratios in Tiwi, which can be attributed to a differential amount of reworking of boulders. Erosion and weathering can significantly superimpose boulder surface roughness; in our case, on the limestones due to marine bioerosion. Automated allocation to the correct surface roughness based on TLS point cloud data class failed in those cases. Consequently, the need for supplementary lithological data or a cross-check in the TLS point cloud is highlighted.

The method presented in this study is best applied on sites with a variety of clast sizes and lithologies. It is targeting on variations of the surface roughness in a millimeter to centimeter scale. Lithologies with a finer matrix, as claystones or sandstones, might be difficult to differentiate with the presented set of methods. An adjustment of scan resolution and a smaller kernel size for the roughness calculation in CloudCompare might produce better results for fine-grained lithologies. We consider the method particularly good on boulder coastlines with conglomeratic beachrock sediments of varying clast sizes, and few rock types of fine matrix.

TLS has also some limitations. The methods we present in this work rely on an extensive and detailed 3D-model of each individual boulder. Boulder extraction can be a highly subjective process, and the final mass estimate of the boulder can strongly depend on how that is done. The creation of accurate 3D boulder models in densely packed settings is hampered by shading and shielding effects. This requires more interpolation and therefore results are less accurate compared to a more open settings. Our boulder volumes are therefore considered as maximum volumes. The roughness analysis is also based on a dense point cloud. We calculated the roughness class (RC) distribution over the entire boulder point cloud. Some parts, especially the exposed faces of boulders, might therefore be overrepresented in the analysis, while other parts, often the bottom parts, are underrepresented or missing. Taking only well-depicted faces of boulders into account can increase accuracy of the method and improve clustering results. Further improvements of the method could include taking surface curvature or reflectance intensity into account to establish a more precise allocation.

Conclusions

Our study demonstrates that TLS-based point clouds analysis has a great potential for coastal boulder analysis. High-resolution point cloud data addresses questions regarding the processes responsible for extreme wave events dislocation of boulders. This includes identification of the pre-event boulder setting, the precise determination of uplift and dragging boulders experienced during one or multiple events, but also the discrimination between lithologies within the boulders and bedrock. We were able to better understand the superficially sorting, size and mass distribution of the boulder ridges in Shab and Tiwi.

We present new dating results on large dislocated boulders along the northeastern coastline of Oman, highlighting the potential for large-scale inundation events in this area. Understanding the characteristics of ocean hazards is important for local stakeholders, as the Omani coastline experienced a large growth of population and assets in the past decades.

Future studies on the application of high-resolution TLS data in coastal hazard assessment and boulder analysis will profit from improving algorithms, which will reduce the time necessary for post-processing and creation of high-resolution boulder models. We see a huge potential of TLS-derived high-resolution point cloud data in the future, especially with the technology and post-processing algorithms being improved. More regional studies of coastal boulder deposits along the shorelines of the Arabian Sea, in combination with extensive dating of extreme wave event deposits, are necessary to achieve a better understanding of regional coastal hazard history.

Notes

Acknowledgements

Financial support by The Research Council Oman (TRC-grant ORG GUtech EBR 10 013; ORG-EBR-10-006) is gratefully acknowledged. The study was also funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - HO 2550/11-1. The study is a contribution to the IGCP Project 639 “Sea Level Change - From Minutes to Millennia”. We would like to express gratitude to Philipp Marr and Marcus Rudolf for helpful and valuable comments in preparation of this work and Jacques Palami and Meriam Otarra for their English reviewing.

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Copyright information

© Springer Nature B.V. 2018

Authors and Affiliations

  1. 1.Steinmann Institute of Geology, Mineralogy and PaleontologyUniversity of BonnBonnGermany
  2. 2.Interfaculty Department of Geoinformatics - ZIGSParis Lodron University of SalzburgSalzburgAustria

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