Fields of dislodged boulders and blocks record catastrophic coastal flooding
during strong storms or tsunamis and play a pivotal role in coastal hazard
assessment. Along the rocky carbonate coast of Eastern Samar (Philippines)
we documented longshore transport of a block of 180 t and boulders (up to
23.5 t) shifted upslope to elevations of up to 10 m above mean lower low
water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion
approaches indicate that boulder dislocation occurred with flow velocities
of 8.9–9.6 m s
Boulder axes, volume, and weight of very large storm-transported
clasts from the literature documented by eyewitnesses or remote sensing.
Uncorrected and tentatively corrected volumes (
Study area and boulder field at site ESA.
Fields of dislocated boulders and blocks are among the most impressive sedimentary evidence of catastrophic coastal flooding (Williams and Hall, 2004; Scicchitano et al., 2007; Goto et al., 2010, 2011; Etienne et al., 2011; Nandasena et al., 2011; Richmond et al., 2011; Engel and May, 2012; Terry et al., 2013) and are widely used to infer the most extreme magnitudes of marine flooding events (tsunamis, storm surges) over large timescales (Etienne et al., 2011; Engel and May, 2012; Terry et al., 2013). Criteria to distinguish between tsunamis and storms include exponential landward fining of boulder fields or the generation of ridges due to strong storms, as well as more random scattering of boulders through tsunamis (Goto et al., 2010; Richmond et al., 2011). For some boulder deposits, storm transport was ruled out based on their large size, elevation and distance from the coast, and local extreme storm wave conditions (Scicchitano et al., 2007; Engel and May, 2012), while the long wave period of tsunamis has been associated with a higher transport competence (Lorang, 2011). However, the topic is still a matter of considerable debate (Goto et al., 2010; Lorang, 2011); until recently, only few studies provided unambiguous evidence for the transport of very large clasts during storms (Goto et al., 2011), e.g. based on direct observations, reliable historical documentation, or satellite data (Table 1). However, dimensions and transport distances of these clasts are often significantly smaller than those of palaeo-deposits for which the mode of transport is unknown.
We present evidence for onshore block and boulder dislocation at the carbonate coast of Eastern Samar (Philippines; Fig. 1) during Supertyphoon Haiyan (local name: Yolanda), one of the strongest tropical cyclones (TCs) on record. Using sedimentary parameters of the clasts (spatial distribution, size, orientation, etc.), bi-temporal satellite images, characteristics of the storm surge and waves inferred from local numerical models, and inverse modelling of minimum flow velocities required to initiate boulder movement, we provide insights into the hydrodynamic and sedimentary processes during a TC. These insights have important implications for the boulder-related “storm vs. tsunami” debate.
Situated directly north of Leyte, the island of Samar is part of the Eastern Visayas (Philippines) (Fig. 1a). It faces the Philippine Sea with the Philippine Trench and subduction zone to the east and the Philippine Fault to the west, the latter comprising a 1200 km long system of strike-slip faults crossing Leyte in a NW–SE direction (Barrier et al., 1991; Ramos and Tsutsumi, 2010). The inner part of Samar consists of Cretaceous to Oligocene igneous rocks, surrounded by mostly carbonate rocks of Mio-Pliocene age showing typical karst morphology (Traveglia et al., 1978).
Along the coast of Eastern Samar, Holocene fringing coral reefs are up to several hundred metres wide (Fig. 1b). Offshore, the bathymetry immediately drops down towards the Philippine Trench. Similar to other coastal areas in the Philippines, elevated reef terraces of last interglacial age (e.g. Omura et al., 2004) are present along the Eastern Samar coast as well, e.g. near the municipality of Hernani, where this study was conducted. Based on our DGPS measurements, the tidal range at site ESA on 17 February 2014 amounted to 1.5 m, which is similar to the tidal range of Laong (N Samar) given in Maeda et al. (2004).
Originating from a tropical depression which formed on 3 November
2013 over the northwestern Pacific, Supertyphoon Haiyan turned into a
tropical storm on 4 November and gained typhoon status on 5 November. On 6 November Haiyan reached the status of a category 5
cyclone on the Saffir–Simpson hurricane scale and made landfall near Guiuan
(Eastern Samar) at 04:40 on 8 November (IRIDeS, 2014; Lagmay et
al., 2015). Haiyan crossed the entire archipelago in a westward direction
without falling below category 5 on the Saffir–Simpson hurricane scale (Figs. 1a, 2). Further landfalls occurred at 07:00 on northern Leyte (close to
Dulag), and later on northern Cebu, Panay, and Palawan. Due to sustained wind
speeds of up to 314 km h
Coastal flooding rapidly reached peak levels that lasted for approximately 2 h. It was locally characterized by multiple pulses of inflowing waves with periods of several seconds (Mas et al., 2015); eyewitnesses on Leyte and Samar reported a threefold withdrawal of the sea followed by distinct flooding pulses, and waves superimposing the storm surge reached up to 4 m in Tacloban. In Eastern Samar, wave periods of 10–20 s were reported (Mas et al., 2015). High flow velocities of uprushing currents were inferred from a survivor video at Hernani, which approached for more than 1 min before receding (Gensis, 2013; Bricker and Roeber, 2015; Mas et al., 2015). Post-typhoon interviews with residents suggest that, similar to how TC Nargis had impacted Myanmar's Ayeyarwady Delta in 2008 (Fritz et al., 2009), the coastal population of Leyte and Eastern Samar lacked a proper understanding of the dimensions and devastating effects potentially connected with the term “storm surge”. This lack of awareness is typically linked to the low frequency of such highest-magnitude events (Fritz et al., 2009), a relationship best described by inverse power-law functions (Corral et al., 2010). Personal experience and adaptation is commonly restricted to events of much smaller magnitude. This classical relationship emphasizes the pivotal role of geological records of extreme-wave events for coastal hazard assessment as they may provide information on local to regional frequency–magnitude patterns over millennial timescales and can also be implemented in education and raising awareness among residents (Weiss and Bourgeois, 2012).
The exposed coast of Eastern Samar is characterized by a large fetch and a steep offshore bathymetry. Hence, it experienced maximum wind speeds with the highest model-predicted waves of up to 19 m, but only a limited wind-driven surge during Haiyan (Bricker et al. 2014). Field indicators document inundation levels of up to 6 m onshore flow depth, nearly 11 m run-ups above local event tide level, and inundation distances of up to 800 m depending on onshore topography (Quinto et al., 2014; Tajima et al. 2014; Shimozono et al., 2015).
Eastern Samar has repeatedly been impacted by severe typhoons in the historical past although they are generally less frequent compared to coastal areas further north. Catastrophic typhoons with tracks similar to the one of Haiyan occurred on 12–13 October 1897 (Soria et al., 2015), on 24–25 November 1912 (Philippine Weather Bureau, 1912), and on 4 November 1984 (Typhoon Undang/Agnes, category 4 on SSHS) (JTWC, 1985). However, on historical timescales, Supertyphoon Haiyan is believed to be the strongest typhoon to have hit Eastern Samar (Lapidez et al., 2015).
Previous typhoons and tropical storms. Location of study area,
track of Supertyphoon Haiyan, and tracks of three further storm systems
which occurred within an area of ca. 250 km N, S, and E of Eastern Samar
between 4 May and 11 November 2013 (JTWC, 2014). Tropical storm 30W (3–6 November 2013) and Typhoon Rumbia (27 June–2 July 2013) only reached
moderate wind speeds of
Pre-Haiyan satellite images available for comparison with images captured
after Haiyan date to 4 May 2013. Post-Haiyan images were captured 3 days
after the typhoon on 11 November 2013, thereby excluding subsequent typhoons
such as Basyang (31 January 2014; NDRRMC, 2014b) for coarse-clast transport.
Between 4 May and 8 November 2013, three storm systems occurred within
an area of ca. 250 km N, S, and E of Eastern Samar, according to the typhoon
database of the Joint Typhoon Warning Center (JTWC, 2014). The tracks of two
of these storm systems crossed an area of 120 km surrounding the study area,
extending from the northern tip of Mindanao to Northern Samar (Fig. 2).
Tropical storm 30W (3–6 November 2013) only reached moderate wind speeds of
Panchromatic satellite images of World View 1 (WV1, ID 1020010021141100, 4
May 2013) and WV2 (ID 10300100294524000, 11 November 2013) were used for mapping
of the pre- and post-Haiyan position of wave-transported large clasts
between Hernani and the study site. The original georeferenced images were
aligned based on unaltered coastal structures such as cliff edges using ESRI
ArcGIS software, resulting in a positional accuracy of
In the field, the elevation of all dislocated clasts was measured using a
Topcon HiPer Pro differential global positioning system (DGPS) with an
altimetric accuracy of
Two different approaches for calculating the clasts' volumes (
Based on this and a previous study (Engel and May, 2012), best estimates of
volume and weight of limestone boulders along tropical coasts are
By using Delft3D and Delft Dashboard software, a high-resolution storm surge model for the boulder site was created and nested into a coarser one, which provides the initial conditions at the open boundaries of the high-resolution model. While the built-in GEBCO (bathymetry) and SRTM (topography) data from Delft Dashboard was used for the coarse model (1 km spatial resolution), IFSAR data (topography) as well as a combination of nautical chart (near-shore bathymetry) and GEBCO (offshore bathymetry) data were used in the nested model. The different data sets were interpolated to the computational grid, resulting in a spatial resolution of 50 m. Further steps in model creation and details on boundary conditions can be found in Cuadra et al. (2014).
Wind forcing in Delft3D was based on a wind enhancement scheme (WES) following Holland's model to generate the tropical cyclone wind field (Holland, 1980). A spiderweb file was generated using the JTWC best track data of Typhoon Haiyan, which includes data about typhoon track, maximum sustained wind speed, and pressure field.
Tides may either reduce or add to the storm surge in the area. For relatively small coastal models such as the nested one presented here, the treatment of tidal forcing along the open boundaries is sufficient in generating the appropriate tidal motion. Tidal forcing in Delft3D was based on the TPXO 7.2 Global Inverse Tide Model to acquire the phases and amplitudes for cells in the model.
In order to derive estimates of minimum flow velocities required to move the
dislocated boulders, we applied the equations (initiation-of-motion
criteria) of Nandasena et al. (2011). Equations differ based on transport
modes. Values for input parameters include boulder axes (derived from field
measurements), inclination of original boulder position (
The comparison of pre- and post-Haiyan satellite images (WV1 and 2)
illustrates changes in the position of large inter- to supratidal clasts at
our study site (Fig. 3) and at several further sections of the adjacent
coastline (Fig. 4). At site ESA, the largest transported clasts are found in
the intertidal zone along the landward margin of the 150 m wide Holocene
lagoon (Figs. 1b, 3a, b). ESA 9 was shifted shore-parallel by
To the north and south of site ESA, several further clasts were shifted according to the pre- and post-Haiyan satellite images (Fig. 4a–d). Dislocation of large block-sized clasts was spotted some 500 m north of ESA, where two triangle-shaped blocks (with longest axis > 4 and > 5 m, respectively) were shifted on top of the Holocene reef platform; a distance of > 240 m is inferred for the smaller one (Fig. 4b). In the omega-shaped bay south of ESA (Pook Cove), just north of Tugnug Point, and in Nagaha Bay, numerous large clasts of pre-existing boulder fields changed position as well (Fig. 4c, d).
Large clasts transported by Haiyan at site ESA.
Further evidence of block and boulder transport during Haiyan.
Photos of largest Haiyan-transported clasts at ESA.
Boulder axes, volume, and weight of most important clasts at site
ESA, Eastern Samar.
Indicators of boulder movement during Supertyphoon Haiyan.
Flow velocities calculated for transport of largest clasts at site
ESA. For each clast, flow velocities were calculated with different
coefficients taken from the literature (cases 1–4 on
Results from the (phase-averaged) wave and storm surge model using
Delft3D and Delft Dashboard software.
Out of the 59 clasts (longest axes > 1 m) documented at site ESA (Fig. 1b), 30 clasts showed clear signs of recent transport, and 13 clasts must have been transported by a previous event based on their mature vegetation cover. For 16 clasts, dislocation during Haiyan remains ambiguous.
At ESA, the largest clast found to be dislocated amounts to
Some 50 m to the north, numerous slab-shaped boulders were dislocated on top
of the gently inclined Pleistocene carbonate platform. A boulder of
Blocks and boulders may be moved by fluid forces in the form of sliding,
rolling, or saltation (e.g. Nandasena et al. 2011), depending on flow
velocity, bottom friction, and the clasts' shape and weight. Based on
the pioneering contributions of Nott (1997, 2003), Nandasena et al. (2011)
presented improved hydrodynamic equations for calculating estimates of
minimum flow velocities (
On top of the carbonate platform, flow velocities of 6.7 m s
While previously published models with a spatial resolution of 2.5 km
resulted in maximum significant wave heights of > 15 m during
Haiyan in deep water off Eastern Samar (Bricker et al., 2014; Fig. 3),
maximum significant wave heights of
Combining pressure- and wind-driven surge as well as wave setup, our coupled
hydrodynamic and wave model results in slightly elevated maximum water
levels (< 1 m above mean sea level, a.s.l.), and maximum flow
velocities below 1.5 m s
Based on field evidence, the interpretation of satellite images, and the
intensity of previous storms, the documented coarse clast transport can be unambiguously attributed to marine flooding during Haiyan. The size of
individual clasts and in particular the dimensions of block ESA 9
(9.0
The largest transported clasts on the intertidal platform (ESA 7 and 9) show
a shore-perpendicular orientation of their longest axis (Fig. 3). Their
transport direction, as can be traced by impact marks on the carbonate
platform and bitemporal satellite image analysis, documents SE–NW-directed
water currents (Fig. 3), coinciding with the modelled flow vectors in the direct
vicinity of site ESA (Fig. 8). In contrast, for the rather flat boulders on
top of the upper carbonate platform, the orientation of their longest axis
is oblique to shore-parallel (Fig. 3), suggesting alignment to approaching
superimposed storm waves and/or deflection of water currents on top of the
reef platform by the
Compared to previously published low-resolution models (Bricker et al.,
2014; Fig. 3) and to wave heights generally expected during catastrophic
typhoons such as Haiyan, our model apparently results in underestimated
maximum significant wave heights offshore of Eastern Samar. These
discrepancies may particularly be explained by the different typhoon track
data used in this study, with 1 min sustained winds implemented in the JTWC
track. However, the timing of maximum significant wave heights in our model
is generally in agreement with the timing of the catastrophic flooding at
Hernani, which was video-captured at
However, despite the discrepancies and similarities mentioned above, flow
velocities modelled with Delft3D in this study and in all previous studies
are insufficient to account for the transport of the documented clasts (Fig. 8;
see also Bricker et al., 2014; Bricker and Roeber, 2015). For the rather
spherical boulder ESA 7, a rolling transport mode was inferred from the
field observations requiring at least 8.9 m s
While these flow velocities are based on boulder-specific
Consequently, the flow velocities calculated for case 1 represent
conservative (i.e. minimum) values, and flow speeds at this study site most
probably exceeded 8.9 m s
Based on the applied formula, quarrying of ESA 5 from the cliff edge, as
documented by the field survey, requires flow velocities of at least
A very high velocity of the typhoon over the NW Pacific (32 km h
In contrast, wind- and pressure-driven storm surge along the SE Samar coast
is believed to not have exceeded
To explain the surprisingly high inundation levels in SE Samar (Tajima et al., 2014) and the bore-like coastal flooding captured at
Hernani (Mas et al., 2015), Bricker et al. (2014) hypothesized that
infragravity waves (such as surf beat) (Munk, 1950) were caused by
non-linear wave interactions with the reef, which are not resolved by the
existing Delft3D and SWAN models. A Haiyan-related meteo-tsunami can be
excluded due to a lack of bathymetric conditions with suitable resonance
properties. Most recently, based on models simulating wave transformation
over shallow fringing reefs using Boussinesq-type equations, Shimozono et
al. (2015) and Roeber and Bricker (2015) confirmed that the extreme run-ups
and the bore-like flooding pattern in Eastern Samar must be explained by
strong coupling of sea swells and infragravity waves with periods of
> 1 min, which may have experienced excitation by resonances
with the fringing reef (cf. Péquignet et al., 2009). These models inferred
flow speeds
Surf beat resulting in pulses of elevated water depths and flow velocities
is thus assumed to be the driving process for the transport of the investigated
boulders some 4 km north of the Hernani video site. Although the NW-directed
longshore currents, as documented by the shore-parallel trajectories of ESA
7 and 9, agree with the modelled flow vectors of wind- and pressure-driven
storm surge, surf-beat-generated currents deviating from a shore-normal
direction, and/or the deflection of these currents along the cliffs, must be
assumed for the study area. However, regardless of the mechanisms
responsible for the exceptional coastal flooding pattern, the sedimentary
findings presented here give striking evidence of very high run-up and
strong wave- and surge-accompanying sustained currents along the coast of SE
Samar during Supertyphoon Haiyan. They were capable of transporting block-sized
clasts over horizontal distances of up to
Based on their SE–NW trajectory and a surge-perpendicular orientation of
their longest axis (Fig. 3), we conclude that the exceptional flooding
pattern, caused by wave setup and infragravity waves, induced the transport
of the largest clasts rather than the high breaking waves alone. This is in
contrast to many previous observations and descriptions of storm-moved
boulders, which are defined to be “wave-transported” (Table 1). However,
the shore-parallel orientation of the slab-shaped boulders on top of the
carbonate platform may suggest that superimposed waves, having reached
heights of more than 5 m (Bricker et al., 2014), contributed to their
trajectory as well. The remarkable flooding pattern video-captured at
Hernani thus affected a wider coastal section, i.e.
Supported by post-typhoon survey reports (Bricker et al., 2014; Tajima et al., 2014), recent wave models (Roeber and Bricker, 2015; Shimozono et al., 2015), eyewitness accounts, and video footage (Mas et al., 2015; Bricker and Roeber, 2015), our findings suggest that a variety of hydrodynamic processes related to TC landfall must be considered when interpreting boulder deposits along coasts. These processes may include, in addition to incident (potentially very high) gravity waves and (or on top of) pressure- and wind-driven storm surge, meteo-tsunamis or seiches (Mori et al., 2014), as well as infragravity waves with periods of up to several minutes. The sustained high-velocity coastal flooding resulting from these infragravity waves, in combination with inundation depths of several metres, is capable of transporting clasts similar to palaeo-deposits commonly related to tsunamis. This is in agreement with theory-based conclusions of Weiss (2012) that both tsunamis and storms may shift clasts of comparable sizes. Our conclusions have important implications for the interpretation of coastal block and boulder deposits and numerical simulations of their transport in similar settings. Where storms have previously been ruled out as being the cause of the dislocation and transport of very large clasts based on their dimensions, the geological legacy of Haiyan prompts the need for a careful reconsideration of possible storm-related transport.
S. M. May, D. Brill, M. Engel, M. Reyes, and H. Brückner contributed to field and lab work. S. M. May, D. Brill, M. Engel, and H. Brückner designed the study and interpreted the data. Modelling was done by C. Cuadra, A. M. F. Lagmay, J. Santiago, and J. K. Suarez. Finally, S. M. May, D. Brill, and M. Engel wrote the manuscript.
Financial support for the research is granted by the Faculty of Mathematics and Natural Sciences, University of Cologne (UoC), and a UoC postdoc grant. Invaluable logistic support was provided by Karen Tiopes and Verna Vargas (Department of Tourism, Leyte Branch). Kirstin Jacobson is acknowledged for language editing, and Jan Oetjen (RWTH Aachen) for the evaluation of the storm surge model. Mark A. C. Bahala, Lia A. L. Gonzalo (both Project NOAH), Eva Quix (UoC), and Bastian Schneider (German International Cooperation (GIZ)) kindly supported boulder mapping. We are very appreciative of the great hospitality considering the situation left by the disaster and of first-hand insights by local interviewees throughout the Visayas archipelago. Edited by: J. Turowski