Shadowing Effects on Beach Morphodynamics during Storm - e-Geo

Journal of Coastal Research
SI 56
73 - 77
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Shadowing Effects on Beach Morphodynamics during Storm Events on
Tróia-Sines Embayed Coast, Southwest Portugal
J. Jacob†, C. Gama‡, R. Salgado∞, J. T. Liu§ and A. Silva***
† FCMA/CIMA
Universidade do Algarve
Campus de Gambelas
8005-139 Faro, Portugal
[email protected]
‡ C. G. E
Universidade de Évora
Colégio Luís Verney,
Rua Romão Ramalho, 59
7002-554 Évora, Portugal
[email protected]
∞ C. G. E
Universidade de Évora
Colégio Luís Verney,
Rua Romão Ramalho, 59
7002-554 Évora, Portugal
[email protected]
§ Institute of Marine
Geology & Chemistry
National Sun Yat-sen
University
Kaohsiung, Taiwan
80424, ROC
[email protected]
*** C. G. E
Universidade de Évora
Colégio Luís Verney,
Rua Romão Ramalho, 59
7002-554 Évora, Portugal
[email protected]
ABSTRACT
JACOB, J., GAMA, C., SALGADO, R., LIU, J. and SILVA, A., 2009. Shadowing effects on beach morphodynamics
during storm events on Tróia-Sines embayed coast, southwest Portugal. Journal of Coastal Research, SI 56
(Proceedings of the 10th International Coastal Symposium), 73 – 77. Lisbon, Portugal, ISSN 0749-0258.
The prediction of storm effects on coastline is essential to coastal management and represents a critical issue for
the scientific community discussion. In this paper we used SWAN and Méso-NH models to simulate the wave
pattern along a ten’s kilometre-scale sandy coastline (TSEC, Tróia-Sines Embayed Coast). The obtained results
indicate a shelter effect of the Espichel Cape in the northern half of the TSEC for wave climate from the NWWNW. Under these conditions, the wave height increases in the southward direction and the Setúbal submarine
canyon locally influences entire TSEC. The influence of the Espichel Cape and the Setúbal Canyon decreases
with the rotation of the wave direction from the NW to W direction. The alongshore wave energy gradient
disappears with wave climate from SW and the TSEC is directly exposed to incident waves during storms. The
results of modelling seem to explain volumetric changes observed in the TSEC (Tróia, Comporta, Aberta Nova,
Santo André and Ribeira de Moinhos), under the effect of seven surveyed storms.
ADITIONAL INDEX WORDS: SWAN, Meso-NH, wave propagation, storm, sandy coastline, beach
volumetric changes.
INTRODUCTION
Variations in the frequency and magnitude of storm events and
the ability of the littoral system to respond are important features
to proper evaluate future effects of climate changes on the littoral
erosion (e.g. LOZANO et al., 2004; RANASINGHE et al.; 2004, LEE et
al., 1988; HILL et al., 2004). In sandy coastlines, the
understanding of the effect of storm incidence in beach
morphodynamics is therefore crucial for coastal management.
In this study, the SWAN and Méso-NH models were coupled to
estimate the wave parameters, along an embayed sandy coastline,
under the effect of storms events.
The Tróia-Sines embayed coast (TSEC) corresponds to a sandy
coastline that extends for 65 km, between the Sado Estuary and
the Sines Cape on the southwest Portuguese coast. The wave
climate data obtained in the Sines wave rider buoy (37º 55’ 16’’
N, 8º 55’ 44’’ W; water depth = 98 m) show for the offshore
waves a mean significant wave height (Hs) and peak period (Tp)
of 1.7 m and 10.8 s, respectively. Hs values ranging from 1 m up
to 2 m represent 49% of the occurrences whereas, 10% of the
obtained results correspond to Hs values higher than 3 m. For the
peak period, 60% of the Tp values range between 9 and 13 s. The
wave direction during the peak period is dominated by the NW
sector (77.3%), followed by the W (20%), SW (2.4%) and S
(0.2%) sectors (COSTA et al., 2001). This shoreline is located in
the lee side of the Setúbal peninsula that includes the Espichel
Cape (Figure 1).
This natural protection prevents the direct arrival of large
amounts of wave energy from the dominant NW direction to the
shore. This protection effect decreases towards southwest and is
responsible for wave energy gradients that control alongshore and
offshore sediment transport and dispersal. However, during storms
and/or swell from W and SW the TSEC remains totally exposed
(QUEVAUVILLER, 1987; ABECASIS, 1987; GAMA, 2005).
The aim of this work is to investigate the storm waves and wind
patterns of the most common storm events that occur in the TSEC,
considering the volumetric changes occurred in the subaerial
beaches. In order to simulate waves induced by storms,
considering the local wind, the phase averaging spectral wave
model SWAN (Simulating WAves Nearshore), was used in a
stationary mode, coupled to the Méso-NH (Mesoscale NonHydrostatic) atmospheric model. The hindcast of seven storms
events between December 1998 and February 2001 were
performed.
The results were compared with the cross-shore volumetric
changes surveyed in five beaches along the TSEC (Tróia,
Comporta, Aberta Nova, Santo André and Ribeira de Moinhos
beaches).
Journal of Coastal Research, Special Issue 56, 2009
73
Beach Processes
Figure 1. A- Location of the study area in the western Atlantic
Coast of Portugal (TSEC). B- Map of the Tróia-Sines embayed
coast showing location of Tróia, Comporta, Aberta Nova, Santo
André and Ribeira de Moinhos beaches, Sines buoy and ADCP
(Acoustic Doppler Current Profiler).
METHODS
In order to characterize the nearshore wave conditions along the
TSEC the nearshore wave field was simulated using the thirdgeneration spectral wave model SWAN (BOOIJ, N.; RIS. R.C. and
HOLTHUIJSEN, L.H., 1999; SWAN TEAM, 2008) coupled to the
atmospheric model Méso-NH (LAFORE et al., 1998). The SWAN
model is a third generation model, suitable for the simulation of
wind generated waves in coastal regions. It is based on a eulerian
formulation of the discrete spectral wave action balance equation
and includes all the relevant physical processes of the propagation
of wind waves in shallow waters, namely, shoaling and refraction
due to bottom variations, dissipation by depth-induced wave
breaking and by bottom friction, wind input and dissipation by
whitecapping (PIRES SILVA et al., 2002). Data from the Sines
wave-rider buoy, located offshore the Sines Cape were used as
seaward boundary conditions to run SWAN under storm
conditions.
The computational grid of the SWAN domain used has 46
kilometers in the West-East direction, from 9.3º W to 8.76º W and
78 kilometers in the South- North direction, from 37.8º N to 38.5º
N, with a spatial resolution of 250 m in both directions. A
directional resolution of 5º covering 360º was used and spectral
range, from 0.04 Hz up to 1.0 Hz, consisted of 34 frequencies
logarithmically distributed.
The GEBCO (General Bathymetric Chart of the Oceans) 1’ grid
bathymetry was used as input bathymetric data for the SWAN
model. The spatial resolution of GEBCO is not the most
appropriate for most of the studies in coastal areas but due to the
large extent of the TSEC and the spatial scale of wave height
alongshore variations it considered to be adequate for the present
qualitative study. In the TSEC region the bathymetry presents,
contours parallel to the coastline except in the Sado ebb-delta and
in the Setubal canyon region (Figure 1).
The SWAN simulations were forced by wind field produced by
the three-dimensional (3-D) non-hydrostatic mesoscale model
Méso-NH. In the present study, one Méso-NH simulation was
performed for each storm event, using the same 3-D domain. In
the horizontal, the domain covers a 150 km (West-East) × 180 km
(South-North) area with a 5-km resolution, centred in 38.15 ºN
and 9.05 ºW. The initial and the lateral boundary conditions were
given by large-scale operational analyses from the European
Centre for Medium-Range Weather Forecasts (ECMWF). For
each considered time the 10 m wind fields simulated by Méso-NH
were used as input to the SWAN model.
The tide level considered in each simulation was obtained from
local tidal solutions obtained with the Oregon State University
Tidal Inversion model (EGBERT et al., 1994) run for western North
Atlantic.
The spectral parameters used in this study were the significant
wave height (Hs), the peak wave period (Tp) and the peak wave
direction ().
On the west coast of Portugal when a storm occurs, Hs is
greater than 5 m (PITA and SANTOS, 1989). Using this criterion,
seven main storms were identified and selected for the period
between December 1998 and February 2001: 27 December 1998
to 1 January 1999; 21 to 23 October 1999; 4 to 7 December 2000;
1 to 2 January 2001; 23 January 2001; 24 to 25 January 2001; 6 to
7 February 2001. Due to a significant retreat occurred in the front
dune in the Comporta beach, the storm of January 1996 was also
considered. For each storm, the maximum significant wave height
and the corresponding peak period and wave direction, recorded at
the Sines buoy, were selected to be used as offshore boundary
conditions. For the storm of December 2000, due to the variation
of the incident wave direction from the SW to the WNW, different
wave directions were simulated.
The relative importance of the wind waves and swell along the
TSEC was evaluated through the analysis of the 1D and 2D
variance density spectra provided by SWAN.
The beach envelope, the pre and post-storm volumetric changes
at the five beaches considered along the TSEC (Tróia, Comporta,
Aberta Nova, Santo André and Ribeira de Moinhos) were
described by GAMA (2005), using topographic profiles that crossed
the subaerial beach, along a shore-normal transect from a point of
reference sited landward of the beach (close to or at the dune/seacliff) until the surf zone. The subaerial beach profile above the
Mean Sea Level (MSL, 2m ZH) was used to calculate the
volumetric changes.
RESULTS
In order to evaluate the accuracy of the SWAN solutions in the
nearshore region, the predictions obtained were compared to
ADCP (Acoustic Doppler Current Profiler) measurements taken at
Raposa beach (17 m depth) and to the results of the SWAN
applications of PIRES SILVA et al. (2001), for the 25 January 2001
and 7 February 2001 storms.
As can be seen in Table 1, the results show a good estimation of
the significant wave height and wave direction. Nevertheless, the
computation of the wave period shows a difference of almost 3 s.
In order to study the alongshore variation of the wave
characteristics and its influence on the beaches volumetric
changes, wave parameters were extracted, from the SWAN runs,
at 12 m isobath consisting of thirteen points distributed along the
TSEC, including the study beaches. The 12 m isobath was chosen
to ensure the characterization of wave parameters before breaking.
Journal of Coastal Research, Special Issue 56, 2009
74
Jacob et al.
The results obtained for the significant wave height as a
function of the alongshore distance are plotted in Figure 2. This
plot allows the identification of a main pattern of the wave energy
distribution along the TSEC.
convergence and divergence of wave energy between Comporta
and Raposa beaches (Figure 3A). This influence disappears when
the waves take the SW direction (Figure 3B).
Table 1: Comparison between the field wave parameters (Hs,Tp
and ) recorded by the ADCP (Raposa beach, 17 m depth) and
described in PIRES SILVA et al. (2001), the results of the SWAN
runs from these authors and the results of the present study.
25-01-2001, 15:00
ADCP
PIRES SILVA et al.
(2001)
SWAN
07-02-2001, 17:00
ADCP
PIRES SILVA et al.
(2001)
SWAN
Hs (m)
3.55
Tp (s)
14.2
 (º)
273
3.54
16.7
280
3.49
16.6
266
4.05
14.2
271
3.96
16.7
275
3.55
16.4
267
Figure 3. Map of the significant wave height contours and
direction (vector) computed by SWAN. A
30 December 1998: Hs=7.95 m; Tp=18.2 s; =290º; B
7 December 2000: Hs=6.01 m; Tp=10 s; =232º.
Figure 2. Alongshore variation of the simulated significant wave
height at 12 m isobath along the TSEC. Each simulation is
described considering the day, month, maximum significant wave
height, peak period and wave direction.
The shadow effect of the Espichel Cape is clearly identified
affecting the wave height until the Aberta Nova beach. Southward
from this beach the coast is exposed to the NW main direction
(Figure 3A). The shadow effect of the Espichel Cape disappears
when the waves come from SW (Figure 3B) and W directions. A
gradual decrease of the significant wave height characterizes the
coast in the northward direction of the Aberta Nova beach,
reaching a minimum at the Tróia beach. This tendency is only
perturbed by the refraction over the Setúbal Canyon that induces
Table 2 shows the estimations of beach envelopes and
volumetric changes of pre and post-storm beach profiles of the
Tróia, Comporta, Aberta Nova, Santo André and Ribeira de
Moinhos beaches. These values were previously calculated by
GAMA (2005) for the period between November 1998 and
February 2001. During 1 January 1996 storm, the Comporta beach
profile suffered a significant retreat of 8m measured in the frontal
dune of the Comporta beach (GAMA, 1996).
According to the obtained results, storm incidences were
responsible for high rates of active sediment remobilization in the
Aberta Nova, Santo André and Ribeira de Moinhos beaches.
Lower remobilization rates characterized the Tróia beach and
highly variable rates were obtained for the Comporta beach.
The increasing tendency of the remobilization rates towards the
south seems to follows the increase of the wave heights (Figure 2).
The Comporta beach is the only exception to this tendency.
Journal of Coastal Research, Special Issue 56, 2009
75
Beach Processes
Located close to the TSEC northern tip this beach indicated
sediment net gain during the storm of January of 1999 and the
highest remobilization rate for the storm of January of 2001.
Table 2: Volumetric changes induced by the storm incidence
expressed by the average percent of the profile envelope (%),
between the pre-storm and the post-storm field survey. (-) sign
points to sediment net lost of the profile envelope. (S) Survey
after the storm incidence. (*) frontal dune retreat of 3m.
Modified from GAMA (2005).
Tróia
Comporta
Ab.Nova
Sto.André
Rib.Moinhos
Nov983rd Jan99S
-5
13
-71
-73
-26
7thOct9925thOct99S
-15
-27
-82
-67
-64
Oct 200010th Feb01S
3
-83(*)
-54
-80
-63
The qualitative analysis of the 1D and 2D wave variance
density spectra for three points offshore the TSEC, offshore the
Tróia beach (98 m), Raposa beach (17 m; ADCP, Figure 1) and
Sines wave buoy (97 m, Figure 1), indicates that the wave climate
at the nearshore of TSEC is controlled by the swell, as expected.
The sea is more intense in Tróia and Comporta beaches but its
energy is very small and can be neglected. The influence of the
sea on wave climate seems to increase northwards, with a
decreasing of the wave height in the same direction. This suggests
an increased effect of the natural protection of the Espichel cape.
Moreover, the wave energy exhibits a small increase when the
dominant wind direction rotates from the NW to S-SW direction
and when the wind intensity increases.
DISCUSSION
This study confirms the important role of the Espichel Cape in
protecting the TSEC from the dominant NW-WNW wave
directions. The northern half of these sixty kilometre-long sandy
coastline between Tróia and north of Aberta Nova beach,
represent a shadow zone of the Espichel Cape for waves
approaching from NW and WNW sectors (Figure 3). In these
situations, the wave energy reaches the shadow zone indirectly by
refraction and, in a less degree by diffraction. Both effects are
responsible for the divergence and dispersion of the wave energy
over that entire zone.
The shelter effect of the Espichel Cape decreases with the
rotation of the offshore wave directions, from NW to W direction.
When the waves come from SW, the shelter effect no longer exists
and the value of Hs is uniform along the TSEC.
From Aberta Nova beach towards the north, the Hs values
decrease, reaching a relative minimum in the Pego – Carvalhal
beaches, and increase again with a relative maximum in the
Comporta beach (Figure 3A). This behaviour is probably due to
the influence of the Setúbal Submarine Canyon (Figure 1). This
deep submarine canyon is responsible for an important refraction
pattern affecting the spatial distribution of wave energy between
the Comporta and Raposa beaches. The location of the zone of
Setúbal Canyon influence (lee of the canyon head), moves slightly
from northward as offshore wave direction rotates from NW to W
and disappears when the waves come from SW.
Beach morphodynamics of the TSEC is characterized by an
increase in the southward direction of the volume of active
sediment (beach envelope of subaerial beach), of the beach width
and of the berm height. The morphological and volumetric
variability of this sandy coastline is controlled by the alongshore
wave energy gradient (QUEVAUVILLER, 1987; ABECASIS, 1987;
GAMA, 2005) and by the winter-storms occurrences (GAMA,
2005). The distribution pattern of the Hs values during storms
obtained with SWAN model seems to explain qualitatively these
volumetric variations.
The wave energy concentrates in the Comporta beach due to
refraction induced by the Setúbal canyon under some storm
conditions. This causes the variability of the volumetric changes
variations and episodes of dune retreat.
The low volumetric variations in the Tróia beach can be
explained by its location in the shadow zone of the Espichel cape Setúbal peninsula, sheltered from the dominant WNW-NW wave
incidence and outside de zone of influence of the Setúbal canyon.
The wave climate in Tróia is also influenced by the refraction
induced by the Sado ebb delta and by the wind sea generations.
The qualitative analysis of the 1D and 2D wave variance density
spectra obtained in three points offshore and along the TSEC
shows that the relative importance of the wind waves increases at
the TSEC northern tip due to the decrease of swell energy. For
storm conditions swell is clearly predominant although wind
waves can also have some importance in the volumetric variations
in the northern half of the TSEC, between Tróia and Comporta
beaches.
CONCLUSION
In this study, the SWAN model coupled with a mesoscale
atmospheric model (Méso-NH) was applied to the study of storm
incidence over a long sandy embayed coast in the southwest of
Portugal (TSEC). Wave rider type buoy data were used as forcing
offshore boundary condition to the SWAN model. Numerical
wave simulations based on the significant wave height (Hs), the
peak wave period (Tp) and the peak wave direction () provided
insights to better distinguish the alongshore wave energy
distribution and dispersal along the TSEC during storms.
In this region, the complex topography near the coast may
induce horizontal heterogeneities in wind field that cannot be
accessed either by local observations at the synoptic
meteorological network, or by large-scale atmospheric forecast
models. The use of a mesoscale atmospheric model seems to be
the best way to access realistic high-resolution wind fields and
quantify the relative importance of the wind waves and its
influence on the beach changes on the TSEC northern tip.
The SWAN simulation results obtained in the present study
were compared to the ADCP measurements taken in shallow
water (17 m). The results obtained show a good accuracy of the
SWAN output in the nearshore region, despite of the low
resolution of the nearshore bathymetry.
For storms coming from the WNW sector is clear the increase
of the wave height in the southward direction until reach the
Aberta Nova beach, as a result of the decrease of shelter effect
provided by the Espichel Cape. Under these offshore wave
conditions, the effect of the Setúbal Canyon is maintained,
inducing a convergence-divergent pattern of the wave energy
expressed by the variation of the wave height between Comporta
and Raposa beach. The exact location of the convergence and
divergence zones in the lee of the Setúbal canyon head seems to
be function of the incident wave direction.
The shelter effect of the Espichel cape, as well as the influence
of the Setúbal canyon, decreases with the rotation of the offshore
wave incidence from NW to W direction. When the storm waves
come from SW these effects are no longer active and,
consequently, the north-south alongshore wave energy gradient
vanishes and the TSEC becomes totally exposed to directly
incident waves.
Journal of Coastal Research, Special Issue 56, 2009
76
Jacob et al.
Although the SWAN runs were made in a stationary mode and
with a low- resolution nearshore bathymetry, the obtained results
allow the qualitative correlation between the significant wave
height and the increase of volumetric changes at the subaerial
beach along the TSEC. A quantitative study considering a more
detailed description of the effects of the Setúbal Submarine
Canyon, of the Espichel Cape and of the sea at the TSEC northern
tip is now in progress. A better characterization of the storm effect
in this dynamic littoral system can be achieved by using a more
detailed bathymetry and by increasing the number of SWAN
simulations. The results will allow a better identification of coastal
stretch of areas more sensitive to storm-related erosion.
To summarize, beach morphodynamics in TSEC as a whole is
influenced by the direction of incident storm waves and the
presence of a submarine canyon on a localized scale.
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