2. Journal of Coastal Research, Special Issue No. 65, 2013
Extreme winter storm versus Summer storm: morphological impact on a sandy beach 649
and 1.8 m during neap tides. The longshore drift is mostly from
North to South. The Biscarrosse beach shows a double bar system
with a single intertidal bar and a subtidal bar (Almar et al., 2009)
similar to other well-documented sites on this part of the coast
(e.g. Castelle et al., 2007). Based on three years of daily video
images, Peron and Sénéchal (2011) observed that despite high
energetic conditions, the inner bar exhibited mostly complex 3D
patterns, TBR and LTT (following Wright et al., 1984) states
being the most frequently observed states. They also discussed the
possibility that the presence of a subtidal bar probably explained
the persistence of TBR states (mean residence time was about 24
days with maximum up to 103 days), even during high energetic
conditions as report in other similar environments (Almar et al.,
2010; Castelle et al., 2010). In contrast to other studies (e.g.
Ranasinghe et al., 2004), up-state transitions were also found to
occur mostly sequentially and no ‘jump’ to the highest beach state
was observed. There observations indicated that both transitions
(up and down-state) were dependent on the previous beach state.
Shoreline detection, bar extraction and
morphologic data
The beach morphology has been monitored through a
CamEra video system developed by the NIWA. The video station
is composed of five color cameras fixed on the top of the
foredune, at 26 m high above the mean water level. The system
provides 4 images per hour (Almar et al., 2009). The beach
morphology was defined using rectified and merged average
images caught by a 10-minute time-exposure video. The
alongshore distance covered by the system is nearly 2 km
longshore and 1 km cross-shore. Because of technical failure, the
longhsore distance over the studied periods was only 1 km. Low
tide images were used to characterized the beach morphologic
states by showing the intertidal morphology (intertidal bar, trough
and rip channels). Those states have been classified following the
Wright and Short classification (1984).
A commonly used proxy for shoreline position is generally the
High Water Line (HWL), determinated from visual observations
(Boak and Turner, 2005). In contrast, datum-based shorelines
derived from video surveys become more common, at least at
short spatial scales (e.g. Quartel et al., 2008; Davidson et al.,
2010). Datum-based shorelines consist in extraction of the cross-
shore position of an elevation contour, for example the Mean High
Water (MHW). Topographic surveys have been acquired once per
month between February 2006 and May 2007, previous to the
video system deployment, at low tide during spring tides, using a
DGPS. The 2.6 m contour was found to be best correlated to the
supratidal beach volume and was thus considered as a suitable
proxy. This contour also allowed extracting daily shorelines from
video images over the studied period as it is positioned to the
mean neap tide HWL. Bars location was extracted from time-
exposure images at a specific water level. On these images, white
bands are present and reflect the location of predominant wave
breaking.
Both proxies gave the shoreline and inner bar position and
allowed to describe their dynamic before, during and after the
summer and winter storms. In the present study, because of the
wave conditions, errors from the images rectification and wave
set-up, generated a total error up to 12m and 20m for respectively
each alongshore averaged shoreline and bar positions. The
standard deviations of the shoreline and the bar positions allowed
to estimate alonghsore uniformities for each proxy and were
represented by error bars.
Hydrodynamic data
The hydrodynamic data have been provided through the Cap
Ferret offshore buoy located 15kms in about 54m water depth.
Those data allowed us to define two different periods of study:
one is the summer storm of the 26th
and 27th
of August 2011 and
the other is the winter storm Joachim that lasts from the 13th
to the
18th
of December 2011. For both studied events, it has been
chosen to study the beach behavior and the hydrodynamic seven
days before and after each storm. The beach states have been
showed through different parameters such as the Gourlay
parameter Ω, defined as:
Ω=Hb/(WsT) (1)
where Hb is the breaker height, T is the peak breaker period and
Ws is the mean fall velocity of the beach sand. A weighted mean
value of Ω can be computed for the several days preceding the day
for which prediction is sough (e.g. Wright et al., 1984)t:
Ω mean = (2)
where i=1 on the day preceding the beach state observations and
i=D on D days prior to the observation. The ϕ parameter depends
on the rate of memory decay. At ϕ days prior to the observation,
the weighting factor as decreased to 10%. For the dynamic of the
Biscarrosse beach, the values ϕ=10 days and D=30 were used,
following the observations of Péron and Sénéchal (2011). The
third parameter computed was the longshore energy flux,
describing the direction of the movements along the shore:
Plong =( .)Hm0.²T.sin 𝜃.cos 𝜃 (3)
Where ρ=1027 kg/m
3
is the density of water, g=9.81 m/s² the
gravity acceleration, Hm0 the significant waves height, T the peak
period of the waves and θ the incident waves angle. A positive
value of Plong means a longshore drift from North to South and a
negative value a longshore drift from South to North.
The hydrodynamic data were also used to compute the storm
power index of Dolan and Davis (1992) based on a value taking
into consideration both the duration time and the maximum
significant wave height.
Figure 1. Field area location on the French Atlantic coastline.
3. Journal of Coastal Research, Special Issue No. 65, 2013
650 Ba and Sénéchal
RESULTS
Hydrodynamic conditions
The hydrodynamic conditions observed during both periods of
study are presented in Figure 2. Because the focus of the present
study was to analyze the impact of potentially erosive events, each
event has been identified using a threshold based on an averaged
wave height estimated over 30 days prior to the event. This
threshold corresponds to 2 times this averaged height. Thus the
‘summer’ threshold has been set to 3m (this is nearly 2 times the
mean annual wave height) and the ‘winter’ threshold to 4m.
Concerning the summer storm, the apex lasted for 17h between
august 26th
and august 27th
. The mean wave’s height was about
0.9 m before the storm and reached 3.6 m during the storm, with a
maximum height of 4.5 m. The week following the storm the
mean Hs dropped to 0.8 m. Figure 2 also shows that the wave’s
period remained nearly the same with a mean value of about 9.5.
The computation of the longshore energy flux shows that the
longshore drift is mainly from the North but during the storm, the
drift is from the South, showing a negative value of Plong. During
the apex of the summer storm, the tide level shows a transition
from neap tide to spring tide conditions with mean water level
values ranging from 2.9 m on august 25th
to 3.7 m on 28th
at high
tide. The computation of the storm power index of Dolan and
Davis (1992) gives a value of 225 when for the Biscarrosse beach
such value characterizes an moderate storm. This storm can be
considered as a characteristic summer storm, short and intense
regarding to the seasonal conditions.
The apex of the winter storm Joachim lasted for 5 days, between
Decembers 13th
and 18th
2011. Figure 2 shows a mean Hs of 2.6 m
before the storm, 5.9 m during the apex (maximum Hs reached 8.5
m) and 2.3 m during the post-storm period. The peak period is
relatively long with an averaged value of about 12.5 s, typical of
long energetic winter swells. The longshore energy flux is positive
along the winter period of study. The storm Joachim took place in
a transitional period from spring to neap tide with mean water
level values ranging between 3 and 3.9 m. In the case of this
storm, the storm power index has a value of 4202 which means
that this winter storm is severe.
Morphologic response of the beach
Figure 3 shows the evolution of the shoreline and the inner bar
for both events. The outer bar dynamic has not been studied
because the bar position was not enough observable on the
images. Before the summer storm, the beach showed an accretive
behavior with a stable shoreline (cross-shore position variations
are below the error of the method) and an onshore inner bar
migration followed by a stabilization period. At the apex of the
storm we observe a weak retreat of the shoreline, suggesting only
moderate erosion as observed on the Truc Vert Beach by Capo et
al. (2009) during the first half of the storm. The recovery period is
very short as the shoreline was back to its initial position only 2
days after the apex of the storm. Along those fifteen days, the
shoreline shape did not change significantly, with standard
deviations close to the one in the beginning of the period under
study, except for the day of august 27th
, at the end of the storm.
Concerning the inner bar dynamic, Figure 3 shows that the bar is
relatively stable, its cross-shore position varying between 280m
and 306m. We do observe a weak offshore migration during the
storm but similar to the shoreline, only 2days after the apex, it
recovers its initial position. Such movement being in the error gap,
one can consider that the bar did not move significantly. Error bars
show that the bar kept its 3-D behavior over the storm period
Figure 2: Hydrodynamic data: Offshore significant waves height Hs (m); Waves peak period T (s); longshore energy flux Plong (J/m/s);
Theoritical tide level (m). The storms are highlighted in the rectangles. Left panels correspond to the summer storm and right panels
correspond to the winter storm
4. Journal of Coastal Research, Special Issue No. 65, 2013
Extreme winter storm versus Summer storm: morphological impact on a sandy beach 651
despite a maximum Gourlay parameter close to 9 at the storm
apex. Averaged Gourlay parameter over the period is around 2.7.
Time-exposure rectified images (Figure 4) indicate that the bar
shape did not change significantly over the storm and was
particularly complex with presence of possible TBR features. This
is consistent with the previous observations of Péron and Sénéchal
(2011) and Almar et al. (2010) that showed that TBR can persist
under storm conditions.
In contrast to the summer storm, before the winter storm, the
shoreline was slightly in retreat with an average position of about
130 m reaching 110 m at the apex. The retreat rate seemed not to
be increased during the winter storm (notice that because of
possible under-estimation of the set-up, onshore migration of the
shoreline position might be increased) and on the contrary of the
summer storm, the beach recovery was initiated before the falling
storm. The standard deviation showed that the shoreline shape
stayed the same during the time of study and relatively alongshore
uniform. During all the study period, the bar position is stable
around a position of 300 m offshore. Figure 3 and Figure 5
indicate that the inner bar experienced an up-state transition
during the storm from RBB to LBT state. Averaged Gourlay
parameter for this period is around 5.5.
DISCUSSION
Forecasting the impact of a storm becomes key to successful
coastal planning and management. Thus the need in establishing
storm thresholds becomes more and more important. Those
thresholds are generally computed from the incoming wave’s
parameters (height, period, direction duration) but the data
presented here indicate that these kinds of thresholds are at the
same time not entirely satisfactory. Indeed, here we presented data
obtained during two contrasting storms: one very short (12 h)
storm event associated with energetic short swells and one very
long (5 consecutive days) storm associated with extremely
energetic long swells. Despite a ratio close to 20 between the two
Dolan and Davies (1992) computed storm factors, the beach
shows a very similar behavior: shoreline retreat and recovery
being of the same order and a relatively stable inner bar. Shoreline
retreat in the winter case was initiated before the storm and may
be linked to spring tidal conditions.
Storm intensity relative to the period
Analysis of wave conditions observed previously to the storm
event indicates that, relative to them, the two storms were similar
in energy. Figures 6 and 7 represent the wave’s height
distributions computed over two weeks preceding respectively the
summer and the winter storm events and the wave’s height
distribution during the storm events. If one considers the ratio of
maximum energy during each storm over the mean energy during
each preceding storm (estimated from the wave height
distribution), the two storms are nearly similar with a mean ratio
of about 2.5. Nevertheless the winter storm is remarkable because
of its duration. The duration of it may explain why an up-state
transition has been observed during it while it has not been
observed during the summer storm. Gourlay parameters are
consistent with the beach classification proposed for this area by
Péron and Sénéchal (2011). If one focuses on the maximum
instantaneous Gourlay parameter during each storm, summer
storm is associated with 9 and winter storm with 12.5 but
averaged Gourlay parameters are respectively 2.7 and 5.5 for the
summer and winter storms.
Figure 3: Shoreline and inner bar evolution during the summer (left) and winter storms (right). The storms are highlighted in the
rectangles.
5. Journal of Coastal Research, Special Issue No. 65, 2013
652 Ba and Sénéchal
Tide influence
The tide is another parameter that can drive the morphodynamic
response of the a beach (e.g. Masselink and Short, 1993).
Masselink et al (2006) showed that the tide associated to the
offshore waves energy and the beach morphology, are determinant
for the type, intensity and duration of the waves processes on the
cross-shore profile. Recently Almar et al. (2010) showed that
inner-bar behaviour depended on the tide range rather than on
storm characteristics. Their results indicated that inner-bar
changes maxima occurred when the tide range changed from
spring to neap tide. Indeed, they found their morphological index,
a combination of the alongshore-averaged cross-shore migration
rate and the absolute variation rate of the cross-shore amplitude, to
be maximum during transitions from a persisting high-tidal range
regime to a small tidal regime. In the present study, the summer
storm occurred during a transition period from neap-tide to spring-
tide conditions while the winter storm occurred during a transition
period from spring to neap tide. This, together with the long
duration of the winter storm, may explain why no up-state
transitions was observed during the summer storm while an up-
state transition was observed during the winter storm.
Morphological interactions
Equilibrium shoreline models, such the ones developped by
Yates et al., (2009) relate the rate of cross-shore shoreline
displacemet to the, typically hourly, wave energy E and the wave
energy disequilibrium between E and the equilibrium wave energy
Eeq that would cause no change to the present shoreline location.
Recent works showed that the influence of the offshore bar(s) (e.g.
Castelle et al., 2010; Almar et al., 2010), events of sandbar
welding to the shore (e.g. Grasso et al., 2009) are key processes to
be considered. Almar et al. (2009) showed the presence of an
outer bar system at about 250 m offshore from the inner bar
system that may protect the inner bar system from exposure to
extreme wave conditions, thus inshore significant wave heights
are generally less than 2.5 m. While one can hypothetize that the
outer bar may have offer an effective protection during the short
summer storm, its efficiency during the long highly energetic
winter storm is more challenging. Recent observations (Almar et
al., 2009) reported these outer bar systems to experience an up-
state transition, from crescentic to alongshore-uniform under such
conditions as the one experienced during the winter storm. Grasso
et al. (2009) also suggested that erosion and accretion depend not
only on wave conditions but also on the initial profile and how
“distant’ it is from the equilibrium profile and thus that for a given
wave climate, knowledge of the initial and target equilibrium
profiles certainly determines how dynamic the morphological
variations are. In our present data set, the beach morpholoy prior
to the winter storm was relatively ‘close’ to the equilibrium profile
for these conditions: shoreline was experiencing retreat and the
inner bar exhibited a RBB state. Furthermore, the laboraty studies
of Grasso et al. (2009) have shown that when the waves climate
changes during a storm, the inner bar stays steady, does not move
and can be turn into a relic. Our results showed a bar quiet
immobile along the study compared to previous studies what
reported daily cross-shore migration of 10m/day for such energetic
events (Almar et al., 2010).
CONCLUDING REMARKS
In the present study the morphological response of a meso-
macrotidal sandy barred beach to two storms has been studied.
The first storm was short and intense and occured during summer
period under seasonal accretive pattern and the second one
extremely energetic and long occured during winter under
seasonal retreat pattern. Data indicate that beach responses were
similar with a similar shoreline recovery time and a stable inner
bar position. These data suggest that the conditions experienced
during the storm (Hs, duration…), the generally so-called storm
thresholds are not sufficient to predict shoreline and beach
evolution. In particular, data suggest that the wave climate
preceding the event, as well as the tide and possible morphological
interactions are key parameters. A more provided collection of
3D data by DGPS might be helpful to describe in more details the
evolution of the system. It is also necessary to point out that only
the evolution of the lower part of the supratidal beach is here
analysed and no informations from the upper beach face and
dunefoot have been collected.
ACKNOWLEDGEMENT
Biscarrosse beach system was founded by Aquitanian Region
Council and OASU. We thank V. Marieu, H.Wennekes and J.M
Figure 4. Biscarrosse beach before (upper panel) and after
(lower panel) the Summer storm.
Figure 5. Biscarrosse beach before (upper panel) and after
(lower panel) the Winter storm.
6. Journal of Coastal Research, Special Issue No. 65, 2013
Extreme winter storm versus Summer storm: morphological impact on a sandy beach 653
Escalier who contributed to the production and maintenance of the
video survey at Biscarrosse Beach.
Figure 6. Wave height distribution before and during the Summer
storm.
Figure 7. Wave height distribution before and during the Winter
storm (watch the shift in the values compared to Figure 6).
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