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196 F. Pepe et al. / Geomorphology 303 (2018) 191–209
For joint bounded boulders, the initial transportation only occurs by younger strata of the foreshore complex system outcrops, and can be
saltation/lifting, and the equation is: correlated with the younger foreset strata exposed in the SW sector of
the Punta Faraglione coastal area.
2
u ≥2 ρs=ρwð½ Þ−1Š g C cosθ þ μ sinθð s Þ=c l A total of 81 boulders and 49 sockets were selected among those laid
in the proximity to the shorelines of Punta Faraglione and Punta Fanfalo
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where u is the minimum flow velocity to initiate boulder transporta- (Fig. 1A) and analyzed. For each boulder, we first describe the main
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tion, ρw is the density of water (1.03 g/cm ), ρs is the bulk density of characteristics (e.g. size, orientation, lithology etc.), and then apply the
boulder, c d the coefficient of drag (typically 2), c l the coefficient of lift hydrodynamic equations in order to determine both the minimum
(typically 0.178), B and C are the B- and C-axes of the boulders, θ is flow velocity and the tsunami or storm wave height that satisfy the re-
the angle of the bed slope at the pre-transport location (1° for joint quirements to initiate its transport.
bounded boulders; 1.7° and 2.5°, respectively, for the submerged boul-
ders of Punta Faraglione and Punta Fanfalo), μ s is the coefficient of static
friction (equal to 0.7, Etienne and Paris, 2010; Noormets et al., 2004), 4.1.1. Punta Faraglione
and g is the acceleration of gravity. The Punta Faraglione coastal area is an approximately 1.5 km long,
The wave height required to transport the boulders is calculated in rock shore platform exposed to the NW (Fig. 1A). It is inclined ~3° sea-
accordance with Nott (2003) using the following equation: ward and interrupted by steep sea cliffs. The submerged zone is charac-
terized by a gentle platform inclined approximately 1.5°–1.7° (transects
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H≥u = g δð Þ a, b and c in Fig. 1B).
Here boulders were detected as isolated blocks or in small groups
where δ is a constant (equal to 1 for the storm wave, and equal to 4 for that, overall, form a discontinuous berm that extended parallel to the
the tsunami wave) describing the wave typology (Fukui et al., 1963), u 2 coastline for ~950 m. This berm is characterized by a seaward imbricate
is the minimum flow velocity necessary to initiate the transportation of structure (Fig. 2b), and has an average width of ~12 m (A-sector in
boulder by considering different transportation modes (sliding, rolling/ Fig. 2c). The distance from the inner and outer edges of the berm (IE
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overturning, saltation/lifting). Values of u areobtainedbyusing and OE in Fig. 2c) to the coastline is variable in the range from ~2 to
Nandasena's equations (see above). ~17 m and from ~23 to ~44 m, respectively. The surfaces of some boul-
ders display biogenic encrustations of vermetids (Fig. 2d). The latter live
3.7. Hindcast data of wave numerical simulation in the mid-infralittoral zone, and then suggest a submerged pre-
transport location of the block. Several boulders show sharp edges and
In order to perform an evaluation of extreme wave height along the striae on their surfaces (Fig. 2e, f). Pieces of wood were also observed
Punta Faraglione and Punta Fanfalo coastal zones, we use wave data (i.e. embedded in these deposits.
offshore wave heights, their directions (Dir) and wavelengths (L m ) Underwater surveys also highlight the presence of free boulders
related to the points 8507 and 8678; see www.dicca.unige.it/ with sharp edges and sockets carved out in the rocky platform
meteocean/hindcast.html for location) of the wave energy resource as- (Fig. 2g, h) and mostly colonized by turf algae. This is in agreement
sessment in the Mediterranean Sea (see Besio et al., 2016 for details), for with the hypothesis considering the submerged pre-transport scenario
the period spanning from 1979 to 2015. The whole dataset was obtain- for some of boulders that are presently accumulated in the proximity to
ed through hindcast numerical simulations using the Wavewatch III the shorelines.
v3.14 model (Tolman, 2009). The model resolution is almost 0.1° in lon- Joint-defined blocks not yet detached from the shore platform (pre-
gitude and latitude and one hour in time. dicted boulders sensu Stephenson and Naylor, 2011, Fig. 2i) as well as
Five offshore wave direction ranges were selected from the analyzed seaward-facing and joint-defined sockets were also locally observed at
hindcast dataset on the base of A-axis orientations by considering that the edge of the shore platform (Fig. 2l). Sockets are carved out in the
the directions of the A-axis and the wave responsible for the boulder same lithologies that constitute the boulders (Fig. 2i, m). Many sockets
transport are approximately parallel. In particular, the wave direction have fresh and uneroded surfaces.
ranges are N290–N330, N330–N360, and N20–N30 for Punta Faraglione, The values of the A, B and C axes measured on 48 boulders and 28
N270–N300 and N220–N260 for Punta Fanfalo. The maximum wave sockets are shown, respectively, in Tables A1 and A2. Axesvaluesofav-
height (H 0 ) related to each direction range was chosen for each year. erage size boulders are ~1.95 × 1.40 × 0.40 m while the biggest is ~3.70
The wave heights at the breaking point (H b ) of the coastal area were × 2.30 × 0.70 m, with a resulting volume and mass, respectively, of
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calculated by using the equation proposed by Sunamura and Horikawa ~6 m and ~12.5 t.
(1975): About 98% of boulders exhibit a tabular or bladed shape (Fig. 4a) and
are characterized by slightly smoothed or angular edges, forming angles
0:2 −0:25
ð
H b =H o ¼ tanβð Þ H o =L m Þ in the range between 60 and 90° (Fig. 2n). The histogram of boulders'
thicknesses (C axis) shows a bimodal distribution marked by two fre-
where β is the slope of sea bottom in the coastal area. quency peaks at 25–30 cm and 35–40 cm (Fig. 5). Only 20 boulders
have the length of the C-axis lower than 20–25 cm or N45–50 cm. The
4. Results minimum and maximum class values of the C-axis measured in this lo-
cality are, respectively, 15–20 cm and 80–85 cm.
4.1. Boulder and socket setting and measurements The prevailing directions of the A-axis are in the range between
N120 and N150, which is almost perpendicular to the direction of
Lower Pleistocene grainstones are well exposed in both the Punta the coastline (Table A1 and Fig. 6a), while a group of 12 boulders
Faraglione and Punta Fanfalo coastal areas, and form a prograding shows directions of their A-axis in the range of N160 and N180,
beach/near foreshore complex system, with foreset beds dipping to- which form angles with the coastline between 40° and 70°. Only
wards the southwest (see Section 4.2 for the lithofacies description). for few blocks the A-axis directions are almost parallel to the
The thickness of strata ranges between ~15 and 85 cm at Punta coastline.
Faraglione, while it reaches 100 cm in the Punta Fanfalo coastal area Axes measurements of average size of sockets are ~1.60 × 1.20 ×
(Fig. 3). The dip direction of the beds is roughly perpendicular to the 0.32 m, while the biggest is ~2.50 × 1.15 × 0.50 m (Table A2). Sockets
coastline at Punta Faraglione, while it is almost parallel to the coastline are mainly characterized by a tabular or bladed shape and only 7% dis-
at Punta Fanfalo. As a consequence, in the latter locality only the play a rod shape (Fig. 4b).