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31
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2015-01.O-03
C
HIARA
F. SCHIAFFINO
(*)
, C
LAUDIA
DESSY
(**)
, N
ICOLA
CORRADI
(*)
,
G
IULIANO
FIERRO
(*)
& M
ARCO
FERRARI
(*)
(*)
DipTeRis, University of Genoa- Corso Europa, 26 - 16132 Genoa, Italy - E-mail address: chiara.schiaffi no@unige.it
(**)
Agenzia conservatoria delle coste della Sardegna - via Mameli, 96 - 09123 Cagliari, Italy
MORPHODYNAMICS OF A GRAVEL BEACH PROTECTED
BY A DETACHED LOW-CRESTED BREAKWATER.
THE CASE OF LEVANTO (EASTERN LIGURIAN SEA, ITALY)
EXTENTED ABSTRACT
Nel presente studio viene analizzata la pocket beach in ghiaia di Levanto (La Spezia, Italia) e la sua evoluzione morfologica in
relazione alla presenza di una barriera sommersa posta a protezione del tratto di costa.
E’ noto che la presenza di barriere sommerse determina accumuli di sedimenti nella zona protetta e la conseguente formazione di
morfologie cuspidate, la cui evoluzione, posta in relazione con l’intensità del moto ondoso, permette di fornire una valutazione relati-
va all’effi cacia della struttura stessa. Numerosi sono gli studi relativi a tali interazioni in spiagge in sabbia, ma ancora ridotte sono le
esperienze riportate in spiagge in ghiaia.
La spiaggia analizzata in questo studio è protetta da due strutture trasversali (pennelli) che suddividono il litorale in tre celle. Uni-
camente nella cella centrale è stata installata una barriera sommersa. La spiaggia è stata oggetto di un intervento di ripascimento che ha
previsto l’immissione di circa 16.000 m
3
di materiale di cava opportunamente frantumato e trattato. Lo studio ha interessato le due celle
di ponente, che risentono maggiormente dei fenomeni erosivi.
Il programma di monitoraggio della spiaggia è stato condotto con l’uso di una webcam. L’utilizzo di sensori remoti per il monitorag-
gio delle coste è una delle tecniche ad oggi più all’avanguardia in quanto permette di ottenere in modo continuativo ed in tempo reale
serie di immagini dei litorale con qualsiasi condizione meteorologica. Questa tecnica è adatta non solo alla defi nizione della posizione
della linea di riva ed alle sue variazioni nel tempo, ma, più in generale, è atta a defi nire l’assetto morfo-dinamico delle spiagge e com-
prenderne la loro tendenza evolutiva.
In particolare, lo studio ha previsto l’acquisizione di immagini 1280x960 pixels, per circa un anno.
Il sistema acquisiva fotografi e della spiaggia per tre volte al giorno, alle ore 8, alle ore 12 ed alle ore 16 con una frequenza di 1 mi-
nuto per 8 minuti consecutivi. Tutte le fotografi e acquisite sono quindi state georeferenziate, rettifi cate ed elaborate usando il software
Beachkeeper plus (B
RIGNONE
et alii, 2008; B
RIGNONE
et alii, 2012). Le immagini Timex e Variance derivanti dall’elaborazione delle fo-
tografi e scattate, hanno permesso di visualizzare in modo più accurato e con maggior precisione la posizione assunta dalla linea di riva
permettendo anche di calcolare i valori di Run up in corrispondenza dei 3 transetti che sono stati considerati a suddivisione di ogni cella.
Le informazioni ottenute attraverso le immagini, sono state poste in relazione con i dati meteomarini registrati dalla boa R.O.N. di
La Spezia, allo scopo di valutare l’evoluzione della morfologia costiera in relazione alle agitazioni ondose. I dati relativi alle giornate
in cui si sono verifi cate le principali mareggiate, hanno permesso di ricostruire l’evoluzione della morfologia cuspidata presente sulla
spiaggia a ridosso della barriera sommersa e di valutare l’effi cacia della struttura. In particolare, si assiste a tre fasi evolutive della
spiaggia, relazionabili con l’intensità dell’agitazione ondosa:

H
0
< 0.5 m: l’opera di difesa interagisce con il moto ondoso determinando fenomeni di diffrazione che favoriscono l’accumulo
di sedimento e la formazione della cuspide. Si assiste ad una sostanziale stabilità della linea di riva;

0.5 m < H
0
< 1 m: si iniziano a registrare fenomeni di asportazione del sedimento costituente la cuspide;

H
0
> 1 m: la cuspide viene completamente distrutta in quanto la barriera sommersa non è più in grado di contrastare l’azione
delle onde ed il sedimento precedentemente accumulato è disperso nella spiaggia sottomarina ad opera di rip currents.
Secondo le formule proposte per le spiagge sabbiose da A
HRENS
& C
OX
(1990) e P
OPE
& D
EAN
(1986), con le caratteristiche struttura-
li della barriera sommersa della spiaggia di Levanto, si sarebbe dovuta formare una cuspide permanente o un tombolo. Tale osservazione
mette in evidenza la diversa risposta tra spiagge in ghiaia ed in sabbia alla presenza di tali strutture. Infatti, a seguito delle caratteristiche
idrodinamiche del sedimento ghiaioso, nella spiaggia di Levanto si ha unicamente la formazione di una cuspide non permanente.
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C.F. SCHIAFFINO, C. DESSY, N. CORRADI, G. FIERRO & M. FERRARI
32
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
ABSTRACT
During the last decades many researches were carried out to
highlight interactions between detached low-crested breakwaters
and beach morphodynamics. However, up to now, the infl uence
of grain size on beach morphodynamic response to a breakwater
has been scantily considered. This study focused on Levanto
gravel beach, partially protected by a low-crested breakwater: the
beach was observed through a video monitoring system, with the
aim of underlining its morphological variations in connection to
wave characteristics.
According to collected Run up values, Levanto breakwater
effectively protects the beach during mild wave perturbations
(Hs < 0.5).
As to the beach’s morphological response to the barrier,
according to A
HRENS
& C
OX
(1990) and P
OPE
& D
EAN
(1986)
formulae, a periodic tombolo or a permanent salient should
form. Conversely, obtained results highlighted the formation
of a periodic salient whose evolutionary phases were strictly
dependent on wave height.
K
EYWORDS
: low-crested breakwater, gravel beach, webcam,
salient, Ligurian sea
INTRODUCTION
Detached low-crested breakwaters, usually named low-
crested structures (LCSs), are commonly used in shoreline
protection practice to shelter the coast from incoming waves,
alone or in combination with nourishment. In the last three
decades such coastal defenses have been widely used in different
parts of the world, like USA (D
ALLY
& P
OPE
, 1986; D
EAN
et
alii, 1997), Japan (R
ANASINGHE
& T
URNER
, 2006; T
HOMALLA
&
V
INCENT
, 2003) and along the Mediterranean coasts (I
SKANDER
et
alii, 2006; L
AMBERTI
& Z
ANUTTIGH
, 2005). The main function of
breakwaters is to mitigate incoming wave energy thus protecting
the beach. The beach’s morphological response to the placement
of a breakwater has to be considered during the planning process
and in particular, the possible modifi cations concerning the beach
face should be highlighted. Many models and experiments were
suggested to explain the relation between submerged breakwaters
and waves (B
UCCINO
& C
ALABRESE
, 2007; H
UR
& M
IZUTAMI
, 2003;
J
ENG
et alii, 2001; L
OSADA
et alii, 2005; R
ANASINGHE
& T
URNER
,
2006) and to identify and analyze the beach’s morphological
response to these barriers (B
ROWDER
et alii, 1996; D
EAN
et alii.
1997; H
ANSON
& K
RAUS
, 1991; H
SU
& S
ILVESTER
, 1990; T
URNER
et alii, 2000; Z
YSERMAN
& J
OHNSON
, 2002). Interactions between
incoming waves and beach hydrodynamic processes can give rise
to a tombolo extending from the shore to the structure or a salient
extending towards the structure. In some cases a null response
reaction is obtained (W
AMSLEY
et alii, 2003).
Thanks to these studies, conceptual models and numerical
predictive expertise for the design of LCSs were created. In
particular, many authors identifi ed a few parameters controlling
beach response, i.e. the length of the breakwater, the gap distance
between adjacent structures, the distance of the structure from
the original shoreline and the depth at breakwater structure below
mean water level (A
HRENS
& C
OX
, 1990; P
OPE
& D
EAN
, 1986).
Besides these studies, the infl uence of grain size on salient or
tombolo formation has been scantily considered and no studies
have focused yet their attention on gravel beache responses to
submerged breakwaters. Higher hydraulic characteristics of
gravel, signifi cant infi ltration during swash causing uprush and
backwash asymmetric motions with a perceptible reduction of
backwash transport capacity (B
USCOMBE
& M
ASSELINK
, 2006;
C
LARKE
et alii, 2004; K
ULKARNI
et alii, 2004; L
EE
et alii, 2007;
N
OLAN
et alii, 1999; O
SBORNE
, 2005; P
EDROZO
-A
CUÑA
et alii, 2006;
P
EDROZO
-A
CUÑA
et alii, 2007), offshore sediment movement
very limited due to the low effi ciency of backwash fl ow and the
predominance of longshore sediment transport are all hydraulic
and morphodynamic differences that could be responsible for and
infl uence a gravel beach response to a LCS.
In this paper a gravel beach partially protected by a LCS
was studied. The aim is to outline the interactions between the
barrier and the beachface under different wave conditions and to
highlight differences in beach behavior between protected and
unprotected sectors of the same beach.
STUDY AREA
Levanto beach is located in eastern Liguria region (north-
western Mediterranean sea). The beach, oriented NNW-SSE,
is originated by the alluvional fl at of the Ghiararo stream.
It is located in a small bay geographically delimited by two
promontories, Punta Gone to the West and Punta Picetto to the
East. Therefore it can be defi ned as a pocket beach (S
ILVESTER
et
alii, 1980) (Fig. 1).
The coastline extends for approximately 800 m and it is
divided into three sectors by two groins; the western sector (later
described as “unprotected sector”) (150 m long, 35 m wide) and
the central sector (later described as “protected sector”) (240 m
long, 35 m wide) underwent a slight erosion, while the eastern
one (400 m long, 43 m wide) was stable. The western and
eastern groins are respectively 40 m and 45 m long. Implemented
engineering projects include not only groins but also a detached
low-crested structure (LCS). The breakwater, built in the central
sector, is approximately 65 m far from the shore and it extends for
almost 100 m alongshore. The structure crest is 7 m wide with an
elevation of almost 2 m below the low tide. There are two 75 m
wide gaps between the breakwater and the lateral groins.
Together with these structures, beach nourishments began to
be carried out more than forty years ago. The last replenishment
was completed in this site during Spring 2005: the distribution
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MORPHODYNAMICS OF A GRAVEL BEACH PROTECTED BY A DETACHED LOW-CRESTED BREAKWATER.
THE CASE OF LEVANTO (EASTERN LIGURIAN SEA, ITALY)
33
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
of 16.000 m
3
of gravel with grain size of 5 Φ (about 30 mm) was
undertaken. The sediment was distributed in the unprotected and
protected sectors that had retreated most in the past.
According to J
ENNINGS
& S
HULMEISTER
’ scheme (2002)
these two sectors can be classifi ed as gravel beach due to their
sediment characteristics and their morphology. The unprotected
and protected sectors have a shore face steep of 18% and 20%
respectively. Here there is almost 30% of sand fraction and 70%
of gravel fraction. The range of mean sediment grain size in the
swash zone is 1.5÷-3.5 Φ (0.35÷11.31 mm) decreasing from the
West to the East and towards offshore up to 3.5 Φ (0.08 mm)
(B
RIGNONE
et alii, 2008; B
RIGNONE
et alii, 2012). In particular, a
salient formed by a gravel percentage higher than 70% is often
found along the protected sector.
Unlike other sectors, the eastern one is very wide, it has a
shore face steep of 10% and a percentage of sand and gravel
sediments denoting a trend that is opposite respect to the other
two sectors. This study focused only on the unstable sectors, both
protected and unprotected, in order to study their response to hard
structures and storm events.
MATERIALS AND METHODS
Wave data
From June 2005 to June 2006, wave data were recorded by
the buoy installed by the Hydro-Marine National Service in La
Spezia (43° 55’ 41.99’’N; 09° 49’ 36.01’’E) at a water depth
of 90 m. Wave parameters collected every 30 minutes included
signifi cant wave height Hs, spectral wave peak period Tp and
mean wave direction (www.idromare.it).
As the buoy lies 32 km to the SE of the study area, wave
parameters were transposed to the Levanto coast.
In order to depict sea state in Levanto beach, buoy data were
entered with a one-hour intervals in a graphic chart, where wave
height variations were related to wave direction in order to single
out the most important storm events in the analyzed period.
Furthermore, wave height and wave period were analyzed
daily to correlate beach morphological variation to sea conditions.
During this stage, duration of wave conditions was also
considered, in order to highlight a possible correlation between
timing in wave variation and beach morphological changes.
Image database
In this study, the morphological behavior of the beach face
was analyzed through a video monitoring system (A
ARNINKHOF
et alii, 2005; H
OLLAND
et alii, 1997; J
IMENEZ
et alii, 2007;
T
URNER
et alii, 2004). The image management software used is
Beachkeeper, a user-friendly program downloadable from the site
http://www.beachmed.eu (B
RIGNONE
et alii, 2008).
The webcam was installed in June 2005 at the top of a
building near the eastern part of the beach approximately 16 m
above sea level. The camera was pointed toward the western part
of the beach and afforded detailed images of unprotected and
protected sectors.
From June 2005 to June 2006, images were collected three
times a day at 8 a.m., at 12 a.m. and at 4 p.m., every two minutes
during a period of eight minutes.
All collected images were elaborated daily through image
processing techniques (A
ARNINKHOF
& R
OELVINK
, 1999;
A
LEXANDER
& H
OLMAN
, 2004; D
AVIDSON
et alii, 2004; H
OLMAN
et alii, 1993; H
OLMAN
et alii, 2003). Acquired images were
georeferenced and rectifi ed by a Beachkeeper tool converting
XYZ real world coordinates in UV image coordinates (A
BDEL
-
A
ZIZ
& K
ARARA
, 1971; H
OLLAND
et alii, 1997; M
ONTI
et alii, 1999).
Almost 4300 photos were checked and a selection was
analyzed to assess the evolution of the gravel beach. Shoreline
detection from images was carried out according to the approach
suggested by A
ARNINKHOF
et alii (2003), P
LANT
& H
OLMAN
(1997), O
JEDA
& G
UILLÉN
(2006) and L
IPPMANN
& H
OLMAN
(1989). This technique identifi es shoreline as the contact area
between still water level and beach face. Furthermore, with
the aim of minimizing errors in shoreline detection due to sea
level variations, timex averaged images, and not single images
(snapshots), were analyzed. Wave run up was also measured
on averaged images in order to eliminate variability caused
by single waves (B
OGLE
et alii, 2001; B
RYAN
& S
WALES
, 2003;
C
OCO
et alii, 2005).
Fig. 1 - Study area
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C.F. SCHIAFFINO, C. DESSY, N. CORRADI, G. FIERRO & M. FERRARI
34
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Data processing
Unprotected and protected sectors video derived data were
complemented with wave conditions in order to interrelate beach
variations with wave height, wave period and direction, and also
to juxtapose morphological modifi cations underlying differences
or similarities between the response to storm surges of protected
and unprotected sectors.
Levanto beach evolution was observed during the most
signifi cant storm events through run up analysis and short time
shoreline migration. A transect partitioning of the two different
sectors, was performed to analyze shoreline displacements in
detail. Sectors were divided by means of transepts evenly spaced
out by 5 meters and perpendicular to the shoreline. In this study
three especially representative transects are analyzed (the 3
rd
, 7
th
and 11
th
transects), lying respectively in the western, central and
eastern part of the sectors.
Shorelines were manually digitized on Timex rectifi ed images
and their intersection with fi xed transects was measured.
Run up was estimated during storm cycle events. The
parameter was calculated along the 3 transects in order to assess
the functionality of the LCS in dissipating incoming wave energy
and its infl uence on beach morphodynamics. An analysis of
detected shorelines morphology was also performed to study its
variation in relation to wave height.
In this study tidal correction was deemed unnecessary: in fact
Levanto is a micro-tidal area with a maximum tidal excursion of
about 30-40 cm (I
STITUTO
I
DROGRAFICO
DELLA
M
ARINA
, 2005).
RESULTS
Wave data
Collected La Spezia Buoy RON data related to annual
wave condition are showed in Fig. 2 Upon a total of 380 days of
observation, the frequency of waves coming from SW was 66,5%.
In particular, the most frequent wave direction was 240° (23.6%).
The most frequent wave condition, with a 50% rate, is calm
water (Hs < 0.5 m). Wave height measures between 0.5 m and 1 m
with a 17% rate with a minimum period of 1 sec and a maximum
period of 6.5 sec.
Signifi cant wave height higher than 1 m appears on 33% of
all cases with a minimum period value of 1.5 sec and a maximum
value of 7.5 sec.
Between June 2005 and June 2006, a total of 37 storm events
was identifi ed. 18 storms occurred in autumn and winter while
19 in spring and summer. Among recorded events, this study
considered the most representative storms for the three main
wave directions, namely August 15, 2005, December 3, 2005 and
May 9, 2006. In particular, during the August 2005 sea storm,
main wave direction was WSW with a signifi cant maximum wave
height of 2.2 m, a period of 4.7 sec and a duration of 22.5 hours.
December 2005 storm had SSW wave direction with an Hs of 4
m, a period of 6.5 sec and a duration of 144 hours. As for May
2006, the storm had SW wave direction, signifi cant wave height
of 2.5 m, a period of 5 sec and it lasted for 22 hours.
Only these storms were considered because of their wave
direction and because they supply a complete view of beach
response to storm events.
Shoreline evolution
Observing shoreline displacements recorded during the three
aforementioned storm events (Tab. 1), the maximum shoreline
displacement associated with run up values clearly varied between
2.5 m in the unprotected sector and 3 m in the protected sector
when the strongest storm coming from SSW reached its peaks.
Generally, in the unprotected sector, with WSW and SW
storm wave directions, Rup values obtained from graphical
image treatment are uniform for the whole sector and are similar
to calculated values. On the contrary, graphic Run up values for
the sector protected by a LCS were higher than calculated values.
Furthermore, Run up was higher in the protected sector than in
the unprotected sector.
In particular, considering the events of August 2005 and May
2006, with null or very low wave angle of incidence, Run up
values were uniform in the unprotected sector. During the same
events, in the protected sector graphic Run up was higher than
calculated values and than unprotected sector values, reaching its
maximum values in the central and eastern part of the sector.
During the December 2005 storm, Run up values trend was
slightly different. In particular, in the unprotected sector Run
up increased westward and it was clearly higher than during
the other events, due to greater wave height. This occurred also
for the sector protected by the LCS, where graphic Run up was
higher than theoretical run up and increased westward as well,
concordant with wave direction.
Tab. 1 - Shoreline displacements for protected and unprotected sectors.
H
s max
is the maximum signifi cant wave height measured during
considered intervals, R
up min
and R
up max
correspond to the mini-
mum and maximum Run up obtained from images; Ru
p calculated
is
obtained from the formula proposed by M
ASE
(1989)
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MORPHODYNAMICS OF A GRAVEL BEACH PROTECTED BY A DETACHED LOW-CRESTED BREAKWATER.
THE CASE OF LEVANTO (EASTERN LIGURIAN SEA, ITALY)
35
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 2 - Time series of a) offshore signifi cant wave height Hs, b) peak wave period and c) wave direction during the study period. Observed storm events
and cusp appearance considered are indicated by the dotted vertical lines
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C.F. SCHIAFFINO, C. DESSY, N. CORRADI, G. FIERRO & M. FERRARI
36
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Finally, morphological beach face variations were analyzed
for the sector protected by the LCS. Images analysis revealed
a salient, whose presence and form seem closely tied to wave
height values.
In particular, the salient appears in 82% of calm sea
conditions (Hs < 0.5 m). When wave height is greater than 0.5
m the structure evolves and its sediments are partially removed;
when Hs is among 0.5 and 1 m, the salient can indeed be seen in
17% of observations. When Hs > 1 the salient never appears. Its
maximum recorded width is 19 m with NW wave direction, Hs
0.05 m and Ts 3.5 seconds.
In general, a relation between wave direction and salient
formation was not evident. The only recorded connection between
salient and wave direction concerns longshore displacement. The
salient undergoes a longshore transfer eastward with WSW wave
direction. In particular, during the August storm (Fig. 3a), the
greatest recorded transfer was 25 m. Lesser transfers take place
for SW direction. During the May 2006 storm, a transfer of about
14 m was registered (Fig. 3b). As for SSW storms, no transfer
takes place as can be seen for the December storm. (Fig. 3c).
Recorded data also highlight beach rotation taking place
during sea storms. In the unprotected sector the rotation remains
clockwise and is comparatively high during May and August
storms, reaching its peak with WSW waves (almost 15°). With
SSW wave direction, rotation is instead minimal (almost 3°) and
anticlockwise. On the other hand, in the sector protected by a
LCS, mild clockwise rotations (almost 3°) occur with WSW and
SW wave direction, while, with SSW perturbations, rotation is
anticlockwise and greater (around 5°).
DISCUSSION
This study analyzed morphological and sedimentary behavior
of a gravel beach partially protected by a LCS.
In Levanto protected sector, a periodic salient formation
is observed. On the basis of defense work features and wave
conditions, according to A
HRENS
& C
OX
(1990) and P
OPE
& D
EAN
(1986) formulae for sand beaches, a permanent salient or periodic
tombolo should form. This therefore highlights a substantial
difference between gravel and sand beaches response to the setting
up of a LCS. Moreover, in this beach salient presence and/or
absence is noteworthy closely related to Hs wave values, and it
does not depend on wave direction as assumed by R
ANASINGHE
&
T
URNER
(2006). Wave direction determines instead salient position:
in fact, the higher the wave angle of incidence, the greater is its
transfer. In particular, with WSW or SW wave directions, the
salient moves towards ESE or SE.
Obtained data were compared with literature data (C
ALABRESE
et alii, 2008; R
UOL
et alii, 2003), and a few similarities with sand
beaches were noticed.
In Levanto gravel beach a pilling up can be observed during
sea storms in the sector protected by a LCS, and it is caused
by the structure itself (C
ALABRESE
et alii, 2008). In its turn, this
triggers a rise in run up, which is high not only by the structure,
as indicated by C
ALABRESE
et alii (2008) and R
UOL
et alii (2003),
but also and primarily in the beach section near its side openings.
This phenomenon is clearly tied to wave conditions, as when
wave height increases, Rup increases as well.
Run up values higher than calculated values can also be
noticed in the eastern part of the unprotected sector, especially
when there are waves with a lower angle of incidence. Therefore,
this phenomenon could be correlated to grain size as well as to
breakwaters. The predominance of longshore sediment transport
and a reduction in cross-shore movements are determined by
the following factors: a steeper beach slope, determining more
oblique wave breaking points (A
USTIN
& M
ASSELINK
, 2006);
higher sediment permeability, causing less return transport
(P
EDROZO
-A
CUÑA
et alii, 2006); lower gravel mobility due to its
size (W
ILCOCK
& K
ENWORTHY
, 2002).
The presence of longshore movements is confi rmed by high
rotations recorded for the unprotected sector’s shoreline. Such
rotations are observed in the sector protected by the LCS only
during the strongest storms, while during milder storms lesser
rotations take place. As a matter of facts, in these cases a LCS
partially interferes with waves, reducing wave energy and altering
wave direction. This mitigates longshore currents, the main cause
of gravel sediment transport, and thus minimizes shoreline rotation.
Longshore movements also trigger cusp dismantling during
high-intensity events, i.e. when the structure makes very little
contact with waves. Moreover, during the most frequent wave
movements from WSW and SW, water collecting in the eastward
sector area generates overwash, a consequent rise in run up and
thus sediment loss. The sediment is displaced beyond the groin
upstream the foot of the structure, nourishing the eastern sector,
that is indeed stable.
On the contrary, when storms are ceasing (1 m < Hs < 0.5
m), favorable conditions for salient formation are created, such
as higher wave energy dissipation and diffraction phenomena
near the structure. Salient size recorded for this beach is moderate
when compared to similar formations arisen on sand beaches
(A
HRENS
& C
OX
, 1990; P
OPE
& D
EAN
, 1986), due to a higher
sediment permeability reducing cross-shore transport.
When the sea is calm, waves cannot transport gravel sediment,
and therefore the salient is stable: it cannot increase in size but it
cannot be destroyed either.
CONCLUSION
In order to evaluate the morphodynamic reaction of a gravel
beach partially protected by a LCS, 13 months of littoral images
were acquired from a coastal video-monitoring webcam. The
beach’s morphological and sedimentary evolution was analyzed
background image
MORPHODYNAMICS OF A GRAVEL BEACH PROTECTED BY A DETACHED LOW-CRESTED BREAKWATER.
THE CASE OF LEVANTO (EASTERN LIGURIAN SEA, ITALY)
37
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 3 - Cusp trend in the protected sector. The time-space diagram represents protected sector’s shorelines at 12 a.m., with a translation of 10 m from one
another a) on August 12-19, 2005 (from bottom to top), b) on May 7-12, 2006 (from bottom to top), c) on December 2-12, 2005 (from bottom to top)
background image
C.F. SCHIAFFINO, C. DESSY, N. CORRADI, G. FIERRO & M. FERRARI
38
Italian Journal of Engineering Geology and Environment, 1 (2015)
© Sapienza Università Editrice
www.ijege.uniroma1.it
under different weather and sea conditions.
This research highlighted morphological and sedimentary
reactions of an artifi cial gravel beach protected by a LCS.
In accordance with literature data concerning sand beaches
protected by LCSs, a pilling up is recorded in the sector protected
by the breakwater, and not in the adjoining unprotected sector.
The LCS also infl uences a periodic salient formation whose
evolution is tightly related not to wave direction but to wave
height. Moreover, it was observed that the salient never reaches
its theoretical estimated size, therefore never becoming a
permanent salient and never causing the formation of a tombolo.
This feature must be related to sediment grain size (Mz < -2Ф)
causing a limited cross-shore mobility.
Generally, a Low-Crested Structure effectively protects the
beach during mild wave perturbations (Hs < 1 m). However,
during storms it is not only useless but also detrimental, due to
the fact that in such situations the rise in Rup, related also to
overwash, enables sediment transport to adjacent sectors.
ACKNOWLEDGEMENTS
The authors would like to thank Dr Francesca Baggio for
English revision and translation.
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MORPHODYNAMICS OF A GRAVEL BEACH PROTECTED BY A DETACHED LOW-CRESTED BREAKWATER.
THE CASE OF LEVANTO (EASTERN LIGURIAN SEA, ITALY)
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Received May 2014 - Accepted March 2015
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