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Along the southwestern coast of the North Sea a large proportion of the Flemish coastal plain consists of densely populated reclaimed land, much of which lying below mean sea level. This sandy coast is exposed to fetch-limited, relatively low-energy waves punctuated by storm activity and experiences tidal range of 5.6 m at spring tides. A number of recent studies suggested medium term (10 years) gross stability of the beach-dune system. This stability was related to moderate wind regime and efficient dune management practices. In March 2007, a storm event resulted in major foredune retreat. This episodic erosive event was induced by moderate direct onshore winds blowing during more than 48 hours associated with a spring tide. Our results show that along this macrotidal coast, erosive events are not necessarily associated with strong winds. Wind direction and duration combined with a spring tide appear to hold the key to understanding the relative importance of processes controlling medium term foredune evolution.
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Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 356 - 360 ICS2009 (Proceedings) Portugal ISSN 0749-0258
Identifying Forcing Conditions Responsible for Foredune Erosion on the
Northern Coast of France
M-H. Ruz
A. Héquette
and A. Maspataud
† Laboratoire d'Océanologie et de Géosciences (UMR CNRS 8187)
Université du Littoral, Côte d’Opale, Wimereux,
62930 France
A., 2009. Identifying forcing conditions responsible for foredune
erosion on the northern coast of France. Journal of Coastal Research, SI 56 (Proceedings of the 10th International
Coastal Symposium), 356 – 360. Lisbon, Portugal, ISSN 0749-0258
Along the southwestern coast of the North Sea a large proportion of the Flemish coastal plain consists of densely
populated reclaimed land, much of which lying below mean sea level. This sandy coast is exposed to fetch-
limited, relatively low-energy waves punctuated by storm activity and experiences tidal range of 5.6 m at spring
tides. A number of recent studies suggested medium term (10 years) gross stability of the beach-dune system.
This stability was related to moderate wind regime and efficient dune management practices. In March 2007, a
storm event resulted in major foredune retreat. This episodic erosive event was induced by moderate direct
onshore winds blowing during more than 48 hours associated with a spring tide. Our results show that along this
macrotidal coast, erosive events are not necessarily associated with strong winds. Wind direction and duration
combined with a spring tide appear to hold the key to understanding the relative importance of processes
controlling medium term foredune evolution.
ADITIONAL INDEX WORDS: coastal dunes, coastal erosion, southern North Sea
Along sandy beaches, foredunes are usually defined as shore-
parallel dune ridges formed on the backshore by aeolian sand
deposition within vegetation (H
, 2002). The importance of
foredunes is well recognised (C
, 1988; A
et al., 2001).
They can delay coastal retreat and protect low-lying backshore
areas against marine invasion as they act as a buffer to extreme
waves and wind (P
, 1988; P
, 1991; S
and B
1993). This is particularly the case along the southwestern coast of
the North Sea where a large proportion of the Flemish coastal
plain consists of densely populated reclaimed land, much of which
lying below mean sea level. The extreme northern coast of France
(Fig. 1) is a macrotidal coast (tidal range of 5.6 m during mean
spring tides) characterised by a 300 to 600 m-wide beach/surfzone
consisting of parallel bars and troughs (M
and A
2001; R
and A
, 2002). This is a moderate mixed
energy coast, influenced by tides and waves. Mean significant
wave height at the coast is generally below 1 m and mean wave
period is in the order of 5 to 6 s. Strong tidal currents are canalized
by shore parallel sand banks, with a predominance of the flood.
From Dunkirk to the Belgium border (Fig. 1), inland parabolic
dunes fronted by a foredune ridge form a 7 km long well
developed coastal dune system, 5 to 25 m high and 700 to 1100 m
wide (C
et al., 2000). The established foredune is 50 m to
150 m wide and 10 m to 15 m high. This coastline is dominantly
exposed to shore-parallel moderate winds from a south to
southwesterly window (Fig. 1). Northerly onshore winds, the most
efficient in terms of potential dune accretion, are less frequent, but
they occur in winter and can induce storm surges responsible for
upper beach/dune erosion (V
and H
, 2000; R
and A
, 2008). This macrotidal moderate energy coast
presently functions under conditions of rather limited sand supply
from the shoreface in spite of the abundant stocks of sand locked
up in the shoreface tidal ridges and banks (A
, 2007).
A number of recent studies suggested medium term (10 years)
gross stability of the beach-foredune system (R
et al., 2005;
et al., 2006; 2007), mild dune scarping in winter being
Figure 1. Location map and wind conditions at the Dunkirk
meteorological station. W and T refer to the Westhinder and
Trapegeer buoys respectively.
Journal of Coastal Research, Special Issue 56, 2009
Forcing Conditions of Foredune Erosion
usually followed by limited sand accumulation at the dune toe in
spring and summer. In March 2007, however, the foredune
underwent dramatic erosion in response to a single storm event.
The aim of this contribution is a first attempt to define forcing
conditions responsible for foredune evolution along this coast.
In the central part of the beach, between Dunkirk and the
Belgium border (Fig. 1), a topographic profile perpendicular to the
foredune and the beach was monitored using a high resolution
laser electronic station along a representative coastal sector that
was characterized by relative stability prior to the March 2007
storm. Along most of the monitored site the established foredune
is 6 to 10 m high with a steep stoss slope partly vegetated. The
junction between the dune toe and the foreshore is a narrow (< 20
m wide) upper beach not reached by the mean highest tides. This
site was monitored prior and after the March 2007 erosive event.
Forcing conditions during this storm event were analysed using
meteorological and hydrographic data. Hourly mean wind speed
and direction were obtained from Dunkirk meteorological weather
station located 7 km from the study area and from a Belgium buoy
(Westhinder) located 36 km offshore (Fig. 1). Predicted and
observed hourly tidal levels at Dunkirk were obtained from the
SHOM (Service Hydrographique et Océanographique de la
Marine). In this study water levels as well as beach-dune profile
elevations refer to metres above Hydrographic Datum (HD, the
French Hydrographic Datum corresponds approximately to the
lowest astronomical tide level). Significant wave heights (H
) and
wave periods were recorded at the offshore Westhinder buoy and
at the nearshore (3 m water depth) Trapegeer buoy located 11 km
northeast of the study area (Fig. 1). In order to define potentially
erosive event occurrence over longer time periods, observed
hourly water levels recorded at Dunkirk from 1956 to 2006 were
also analysed.
Recent foredune evolution
Shoreline evolution in this area during much of the 20
was dominated by retreat (C
et al., 2000), related to both
human pressures and natural erosional processes. Coastal dunes in
this sector have been massively transformed by urban and port
development. Coastal dunes were also badly damaged during
World War II (R
et al., 2005). From 1971 to 1994 the mean
retreat rates were on the order of 0.5-1.7 m/year. The foredune
was affected by breaches and blowouts, mainly due to human
disturbance and by erosional scarping during storms. In the mid
1990s, measures to prevent degradation of the dunes and reduce
the threat of marine erosion were implemented by the
Departmental Authority of the North (Conseil Général du Nord) in
charge of the management of these coastal dunes. Wooden and
brushwood fences were erected in order to encourage sand
accumulation in the most sensitive areas. In order to promote the
recovery of natural habitats, these rehabilitation measures have
involved, since 1994, manual collection of detritus and debris
accumulating at the high tide lines. Such measures were very
successful and have contributed to coastal dune rehabilitation and
foredune stabilisation (R
et al., 2005; A
et al., 2007).
The foredune seaward slope remained however relatively steep
and partly vegetated. At the dune toe, episodic wave attack was
responsible for basal undercutting. On the other hand, the refilling
of dune blowouts and the development of a vegetation cover
suggested a relatively balanced sand budget (C
et al.,
2000). Along most of this coast the foredune ridge was described
in a state of meso-scale (decadal) stability (R
et al., 2005). This
relative stability was attributed in part to human intervention and
decrease in storminess. The analysis of strong winds ( 16 m/s)
frequency from three-hourly wind data recorded at Dunkirk (Fig.
2) showed that the period 1970-1980 was characterised by a high
frequency of strong winds (C
et al., 2000) while the last
two decades were periods of decreasing storminess (C
al., 2008).
Foredune response to a major erosive event
A major erosive event occurred in March 2007. Between March
and March 22
, the hourly mean wind speed recorded at
Dunkirk varied from 7 to 13 m/s with a mean wind speed of 10.5
m/s and maximum mean hourly wind velocity of 15.3 m/s
recorded on the 20
(Fig. 3). From March 17
18:00 to the 22
6:00 the wind blew at 10 m/s and more during 73 hours over this
110 hours period. Offshore, at the Westhinder station, the wind
speed was higher, with a mean wind speed of 14.5 m/s and
maximum mean hourly wind speed of 23 m/s recorded on the 18
(Fig. 3). Offshore, wind speed remained above 12 m/s during 88
hours. At Dunkirk, dominant wind direction was west to
southwest on March 17
and 18
, then veered to the northwest on
the 19
and maintained to the north on the 20
, 21
and 22
. The
same wind directions were recorded offshore (Fig. 3). In response
to these moderate but constant winds, significant wave heights
) recorded offshore and in the shallow subtidal zone increased
significantly. On March 18
, high velocity (>20 m/s offshore)
southwestern winds induced a rapid increase in offshore wave
heights that exceeded 3 m (Fig. 3). At the coast, this response was
less obvious, with maximum wave heights reaching only 1.5 m.
Between March 20
9:00 and 21
, 12:00, with direct onshore
winds up to 15 m/s at Dunkirk and up to about 19 m/s offshore, H
of 4 m were recorded offshore and H
above 3 m were recorded
near the coast. It is noticeable that at the coast wave height is
modulated by the vertical tidal fluctuations as maximum wave
heights were recorded at high tide. Wave periods recorded
offshore and nearshore reached maximum values of 6.7 and 6.4 s
respectively. Direct and persistent strong northern winds probably
induced wind and wave set up at the coast. This stormy event was
combined with a spring tide. At Dunkirk, predicted tidal range
increased from 5.36 m on March 18
to 5.48 on the 19
, reaching
a maximum of 6.10 m on March 21
and then decreasing to 5.9 m
on the 22
. With increasing tidal range, observed water level at
Dunkirk reached a maximum of 6.8 m above HD at 0:00 on March
, a level well above the highest predicted astronomical tides
Figure 2. Frequency of winds 16 m/s in Dunkirk for the period
1956-2000. From C
et al.(2008).
Journal of Coastal Research, Special Issue 56, 2009
Ruz et al.
(6.48 m). This observed level was 1.06 m above the predicted high
tide of the day.
This surge, also observed at low tide (Fig. 3), was not
exceptional in this area where surges of 1 m have a return period
of 0.1 year (T
and P
, 2008). On March 20
, the spring tide was combined with a surge of 0.5 m and water
level was above 6.7 m HD. These high water levels were the result
of the conjunction of spring tides, strong northerly winds and high
waves at the coast. High water levels reached an elevation above
the dune toe on several occasions between March 18
and March
(Fig. 3). Combined with waves they were responsible for
significant foredune erosion along the coast. In response to this
event, the upper beach was flattened and lowered and the foredune
front retreated about 4 m (Fig. 4). In response to intense wave
action associated with extra surge levels, the foredune was
uniformly cut into a steep scarp (Fig. 4) along 5 km of shoreline.
In the western part of the study area, sand fences erected in 2004
at the dune toe were vanished by storm waves (R
, 2008).
The analysis of high water levels recorded at Dunkirk for the
period 1956-2006 reveals a low occurrence of water levels above
the dune toe (Fig. 5). From 1996 to 2006, a period characterized
by foredune stabilization, such high water levels only occurred 4
times and over the complete 1956-2006 period, potentially erosive
high water levels occurred 27 times, with a high frequency in early
80 s (Fig. 5).
For individual dune systems, coastal sediment budgets, aeolian
sand transport and destructive marine events are key factors to
understand the relative importance of controlling processes. Under
erosive conditions with a high influx of wave energy and a
dissipative nearshore, the dunes often erode (K
et al., 2007).
Along the macrotidal coast of northern France, the patterns of
short-term to decadal-scale morphological evolution appear
determined by episodic erosive events. After almost 10 years of
relative meso-scale stability, the foredune retreated by about 3 to 5
m in response to a single event. As outlined by S
(1993), meso-scale variations of foredune response may
incorporate the annual to decadal sequences of morphological
attenuation and recovery. In this area, foredune growth observed
over the last decade reflected the absence of major storm events
combined with large spring tides during which the upper beach
and foredune can be exposed to surge conditions. This evolution
must, therefore, be viewed within a favorable context of sand
supply combined with the absence of significant erosive storm
Figure 3. Wind, waves and tide conditions during the March 2007 storm event.
Journal of Coastal Research, Special Issue 56, 2009
Forcing Conditions of Foredune Erosion
Our study suggests that major erosive events along this
macrotidal coast are not necessarily related to strongest winds.
Tidal elevation, wind duration and direction appear to be major
factors controlling coastal dune erosion. Strong winds occurring at
low tide have no impact along these beaches as well as offshore
blowing winds. As also noted by C
et al. (2004), when tidal
range is relatively large, the probability of wave set-up during
high wave conditions causing water levels exceeding normal high
tide levels is reduced. Our analyses of offshore and nearshore
wave heights during the March 2007 storm event show that higher
wave heights were not generated by the strongest offshore winds
(>20 m/s), but by more moderate winds (about 15-16 m/s)
persistently blowing onshore during more than 48 hours (Fig. 3).
Wind direction and duration appear therefore to be major forcing
parameters responsible for an increase in wave heights, especially
near the coast (Trapegeer station, Fig. 3) where waves can have
major impacts on coastal dunes. These results show that along this
coast a major erosive event can occur when moderate to strong
direct onshore winds, blowing during more than 2 days, induce a
positive surge combined with a high water level associated with a
mean spring tide.
Along this fetch-limited macrotidal coast, strong winds are
therefore not necessarily a criterion for explaining foredune
evolution. Coastal dune evolution appears to be primarily
dependent on high water level frequency. The analysis of past
uppermost water levels reaching the dune toe gives an overview of
potential erosive events (Fig. 5). It is obvious that periods of high
frequency of strong winds (Fig. 2) do not correspond with periods
of very high water levels (Fig. 5). Therefore, coastal dune erosion
is not necessarily a response to periods of increasing frequency of
strong winds as previously assumed (C
et al., 2000;
et al., 2005). Along this coast it seems that the greatest
morphological impacts at the shoreline result from locally
generated short period waves associated with coastal proximal
storms and spring tides.
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This study was partly funded by the French “Agence Nationale
pour la Recherche” (ANR-06-VMC-009) through the project
VULSACO (VULnerability of SAndy COast systems to climatic
and anthropic changes) and by European funds (FEDER) through
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figures and to Vincent Sipka for technical assistance in the field.
... Les dunes littorales du nord de la France représentent un important moyen de défense contre les inondations marines, car de vastes zones du littoral de la région sont constituées de polders dont l'altitude moyenne est sous le niveau des plus hautes mers (Maspataud et al., 2012). Étant donné que l'érosion des dunes et les inondations marines risquent d'augmenter dans un proche avenir avec l'élévation du niveau de la mer (Nicholls et Cazenave, 2010), le comportement morphologique et l'évolution des dunes littorales du nord de la France, notamment en réponse aux tempêtes, a attiré une attention croissante au cours des dernières années Ruz et al., 2009 ;Idier, 2013 ;Crapoulet et al., 2017 ;Héquette et al., 2019). L'objectif de cette étude est d'analyser l'impact des tempêtes de l'hiver 2013/2014 sur l'évolution d'un secteur dunaire du nord de la France ainsi que l'évolution pluri-annuelle (de 2014 à 2018) de ce secteur dunaire consécutivement à ces tempêtes. ...
... L'analyse de l'évolution à moyen terme (2012-2018) des secteurs dunaires étudiés montre que le cordon dunaire a été sévèrement érodé lors de la tempête Xaver fin 2013, puis dans une moindre mesure suite aux tempêtes Egon et Eleanor en 2017 et 2018. Le long des côtes macrotidales, l'impact des tempêtes sur l'érosion des dunes est particulièrement dépendant de la conjonction de surcote et de marées de fort coefficient et des vagues de forte énergie (Ruz et Meur-Ferec, 2004 ;Pye et Blott, 2008 ;Ruz et al., 2009 ;Esteves et al., 2012 ;Masselink et al., 2016). D'après nos analyses, il apparaît que l'énergie des vagues à marée haute n'est pas nécessairement le principal paramètre contrôlant l'érosion des versants dunaires, la hauteur du niveau d'eau atteint pendant la tempête qui permet, ou non, aux vagues d'atteindre le pied de dune, est également un élément important à considérer. ...
... La morphologie de l'avant côte au-devant de notre zone d'étude est caractérisée par la présence d'un banc subtidal et d'un chenal de navigation ( fig. 1) qui peuvent avoir une influence sur l'évolution du cordon dunaire. En effet, d'après Héquette et al. (2009Héquette et al. ( , 2019, ce banc ne constituerait pas forcément une barrière naturelle diminuant l'attaque des vagues lors des conditions de mer agitée, mais qu'au contraire, celles-ci se reformeraient entre le banc et la côte, notamment en face de la dune Dewulf où le chenal de navigation est plus proche et plus profond. Ce chenal contribuerait aussi au renforcement des courants longitudinaux et par conséquent à la mobilisation et le transport des sédiments vers l'est. ...
Full-text available
The coastal dunes of northern France, which constitute a natural barrier against marine flooding, were severely impacted during the series of storms in the winter of 2013/2014. Thanks to multiple sources of data, mainly from airborne LiDAR surveys, in-situ topographic profiles measurements, combined with the analysis of offshore waves, wind and water levels at the coast, we were able to characterize not only the impact of these storms, but also the post-storm evolution of these coastal dunes. In the current context of climate change, it is indeed important to know the potential impact of storms, which may coincide with increasingly high water levels, and also to analyze the capacity of coastal dunes to recover and the rates at which this could occur, particularly in macrotidal environments. Marine and weather conditions analysis from 2012 to 2018, shows the occurrence of several stormy events inducing high water levels, which are responsible to coastal dune erosion and significant shoreline retreat. The first and most important one was the storm Xaver, on 5 and 6 December 2013. This major event, associated with a water level of 7.3 m exceeding the 100-year return period, resulted in a significant retreat of the foredune east of Dunkirk. Indeed, a comparison of the of 2012 and 2014 LiDAR surveys shows erosion up to -40 m3.m-1, with a retreat of nearly 8 m of the dune foot. Following the Xaver storm, five topographic profiles were monitored. From the beginning of 2014 to the end of 2015, no dune recovery was observed. Foredune remained stable despite calm conditions, which may favor sand accumulation on dunes. From 2016 to 2018 two storms (Egon in January 2017 and Eleanor in January 2018) occurred and were responsible for a shoreline retreat of nearly 4 and sediment losses of -5 to -15 m3.m-1. Again, dune recovery between Egon and Eleanor storms was negligible (no more than +1 m3.m-1). Overall, the cumulative effect of storms Xaver, Egon and Eleanor resulted in a significant sediment lost all along this coastal area, but volume changes varied from -16 m3.m-1 to -34 m3.m-1 on dune Marchand and at dune Dewulf, erosion was about -31 m3.m-1 to less than -10 m3.m-1 although these two dune areas are located close apart. Our results show that this difference in response to storm impact from one profile to another would be related to the beach morphology prior to the storm. Indeed, although these differences initially appear to be small (no more than 0.8 m for dune toe elevations and 8 m for upper beach widths), they are significant in the response of each dune profile to storm impact. Finally, contrasting to other coastlines impacted by the series of storms of winter 2013/2014 where partial or total dune recovery was observed during the first years following this series of storms, the foredune east of Dunlirk remained in erosion and did not experience any regeneration. A critical threshold, in terms of high water levels, was therefore reached during storm Xaver. This suggests that as the relative sea level rises, the coastal dunes are likely to experience more frequent episodes of high water levels, thus altering their ability to recover.
... Current measurements conducted in previous studies show that the speeds of flood currents exceed those of the ebb, resulting in a net flood-dominated asymmetry in the coastal zone [19,43]. The prevailing winds in the region are from west to southwest, with a secondary wind direction from north to northeast [44]. As a result, the dominant wave directions are from southwest to west, originating from the English Channel, followed by waves from the northeast to north, generated in the North Sea ( Figure 1B). ...
... However, waves undergo significant shoaling and energy dissipation over the massive inner shelf sand banks (Figure 1A) and the gentle beach and nearshore slopes that characterize the coast of Northern France, resulting in significantly lower wave heights at the coast [37]. The prevailing winds in the region are from west to southwest, with a secondary wind direction from north to northeast [44]. As a result, the dominant wave directions are from southwest to west, originating from the English Channel, followed by waves from the northeast to north, generated in the North Sea ( Figure 1B). ...
Full-text available
Barred macrotidal beaches are affected by continuous horizontal displacements of different hydrodynamic zones associated with wave transformation (shoaling, breaker and surf zones) due to significant tide-induced water level changes. A series of wave and current meters, complemented by a video imagery system, were deployed on a barred beach of northern France during a 6-day experiment in order to characterize the spatial and temporal variability of wave-induced processes across the beach. Wave and current spectral analyses and analyses of cross-shore current direction and asymmetry resulted in the identification of distinct hydrodynamic processes, including the development of infragravity waves and offshore-directed flows in the breaker and surf zones. Our results revealed a high spatial variability in the hydrodynamic processes across the beach, related to the bar-trough topography, as well as significant variations in the directions and intensity of cross-shore currents at fixed locations due to the horizontal translation of the different hydrodynamic zones resulting from continuous changes in water level due to tides.
... The significance and importance of coastal dune systems are well known (Arens et al., 2001;Hanley et al., 2014;Ruz et al., 2009). Coastal sand dunes, and particularly foredunes, act as a protective barrier on many of the world's coastlines (Carter, 1991;Doody, 2012;Maximiliano-Cordova et al., 2019;Simm et al., 1996) and, hence, are an important natural defence against inundation by seawater of economically and environmentally significant land, infrastructure, and activities (Fernández-Montblanc et al., 2020;Hanley et al., 2014;Maximiliano-Cordova et al., 2019). ...
... High-magnitude, low-frequency storm events are able to displace large volumes of sediment through scarping, impacting the morphology of the foredune, and altering the coastline . Scarping can also occur due to high spring tides (Ruz and Meur-Ferec, 2004;Ruz et al., 2009;Shepard, 1950), elevated water levels, as, for example, on lakes due to meteorological effects, and by creeks and washouts on occasion (Da Silva et al., 2003;Figueiredo and Calliari, 2006;Fuller, 2002). Here, we focus on coastal foredune scarping as it occurs during high-water-level conditions. ...
The controls that affect the degree of spatio-temporal change to foredunes following scarping are reviewed herein. As sea-levels rises and climate changes, dune scarping will become more common. Thus, it is critical to understand what factors contribute to the magnitude of scarping, and what effect this has on dune systems to better manage coastal erosion into the future. Scarping occurs when foredunes are partially eroded by waves, generally during periods of high water level. The controls on the degree and magnitude of scarping examined include water level, foredune vegetation cover and species present, plant root mass, height and volume of the foredune, the original foredune morphology, surfzone–beach type, and compaction of sediment. Water-level height and duration of high water is the most significant control as it determines the elevation at which wave action can erode the dune and, therefore, the extent of scarping and dune volumetric loss. Higher plant density, greater rooting depth, high root mass, and greater compaction aid in reducing the degree of scaping. The presence of large woody debris and wrack may also influence the degree of scarping. The effects scarping has on the morphology of a foredune after the initial erosion event can range from small changes (e.g. minor, small scarps and slight slumping), to moderate changes such as the foredune translating landwards, to large change such as the transition of an entire dune system into a new transgressive dunefield phase. A new model summarising the key controls and their relationship/significance to the magnitude and extent of scarping is presented.
... The offshore wave regime is dominated by waves from the southwest to the west, originating from the English Channel, followed by waves from the northwest to northeast, generated in the North Sea ( Figure 2c) [49,50]. The modal offshore significant wave height is approximately 0.6 m, with wave periods typically ranging from 4 to 8 s, but the maximum wave height may episodically exceed 4 m, with periods of 9 to 10 s during major storms [51]. Wave heights are significantly lower at the coast, due to significant shoaling and energy dissipation over the shallow offshore sand banks, resulting in wave heights that hardly exceed 1 m in the intertidal zone, even during storms [52,53]. ...
... One of the mechanisms responsible for sand bank-landward migration is storm waves. In the region, storm waves mainly come from the southwest, originating from the English Channel, or from the north-northeast associated with strong winds blowing over the North Sea [21,51]. In the nearshore zone, however, storm wave directions become more normal to the coast [52], and therefore, more perpendicular to shore-parallel sand banks. ...
Dans les environnements côtiers, les interactions entre les processus morphodynamiques, océanographiques et anthropiques (agissant sur différentes échelles de temps) contrôlent l'évolution des systèmes littoraux. Cette thèse est axée sur une échelle de temps séculaire afin de déterminer l'influence non seulement des activités humaines, mais aussi l'impact du changement climatique. Le long de la côte nord de la France, des mesures de niveau d'eau ont été effectuées depuis le début du 19ème siècle et conservées dans les archives du Service Hydrographique et Océanographique de la Marine (Shom). Parrallèlement à ces mesures tidales, des levés hydrographiques ont été effectués pour cartographier les fonds marins de cette zone côtière, caractérisée par la présence de nombreux bancs sableux, formant des corps sédimentaires massifs parallèles à subparallèles au rivage. Pour déterminer les tendances à long terme, la numérisation et l'analyse des documents historiques ont été réalisées. Ceci a permis de reconstituer les variations du niveau de la mer à Dunkerque et Calais, et d'évaluer les changements de la morphologie et de la position des bancs sableux. A partir de ces séries inédites, l'étude de l'évolution des niveaux marins moyens et des composantes de marée a révélé des changements significatifs. De plus, l'évolution bathymétrique a montré des variations morphologiques importantes depuis le 19ème siècle, qui sont en grande partie dues à la mobilité des bancs sableux. Afin d'étudier l'influence des changements morphologiques sur les processus hydrodynamiques, une modélisation numérique de la propagation de la houle et de la circulation tidale a été réalisée à l'aide de module de la chaîne de calcul TELEMAC. Dans un contexte d'érosion sédimentaire, une accélération des courants et une augmentation de la hauteur des vagues le long du rivage sont détectées. Inversement, dans un contexte d'accumulation sédimentaire, nos résultats mettent en évidence une diminution de la vitesse des courants et une plus grande dissipation de l'énergie des vagues dans les petits-fonds. Cette étude souligne le rôle des rétroactions morphologiques entre l'hydrodynamique côtière et la morphologie du littoral. L'identification de ces mécanismes à une échelle de temps séculaire est essentielle pour évaluer les facteurs potentiels des changements côtiers.
... Under the influence of natural processes (such changes in climate as well as increased frequency of extreme events and associated abrasion processes) and adverse anthropogenic processes, there are ongoing changes in the coastal zone that often disrupt coastal dune development (McLachlan et al. 2018;Chen et al. 2019). Over the past century, there have been trends of deterioration in coastal dune systems in Europe, whose protective role against erosion is well known (Arens et al. 2011;Ruz et al. 2009;Carter 2013;Garcia-Lozano et al. 2018). ...
... 6 Ces deux sites côtiers sont exposés à des vagues du sud-ouest venant de la Manche ainsi qu'à des vagues de nord-est formées en mer du Nord. Les vagues sont généralement peu élevées (hauteur modale < 1 m) et même lors des tempêtes leur hauteur au large ne dépasse qu'exceptionnellement 4 m (Ruz et al., 2009 ;Héquette et al., 2019). La hauteur des vagues est cependant encore moindre à la côte en raison de la forte dissipation de leur énergie sur les bancs sableux qui parsèment les fonds de la mer du Nord et de la Manche orientale (Beck et al., 1991 ;Héquette et Aernouts, 2010). ...
Because climate change will likely have major impacts in low-lying coastal areas, our ability for evaluating predictable consequences of coastal hazards in the future already represents a key issue in coastal risk reduction. This paper presents examples of the application of a method aimed at assessing the future position of the shoreline and at mapping areas that will be at risk of flooding by 2065 with future sea-level rise at two coastal localities of northern France where properties are already threatened by erosion or flooding. Our results show that erosion is the main coastal hazard at one of the two study sites where a large part of a housing estate may disappear by 2065 if no action is taken for limiting coastal erosion. Although marine flooding represents the primary risk at the second study site, coastal erosion will increase the risk of flooding of properties in the future due to the decrease in width of the protective coastal dunes. Comparison of our results with the coastal hazard maps published by the French government shows significant differences that can be explained by the fact that the risk of erosion was not considered in the Coastal Risk Protection Plans (PPRL) of the two municipalities considered in this study, which raises the question of the reliability of areas at risk defined without taking into account future shoreline evolution trends.
... There are few works that evaluate and compare total foredune volume with aeolian drift potential or marine dynamics. Most of them evaluate deposition and foredune growth as a function of the aeolian drift (Davidson-Arnott and Law, 1996;Christiansen and Davidson-Arnott, 2004;de Vries et al., 2012) or the erosion produced by wave attack during extreme events (Stockdon et al., 2007;Ruz et al., 2009;Houser, 2013;Long et al., 2014;Mull and Ruggiero, 2014). One of the studies that evaluate the relationship between foredune morphology and aeolian and marine dynamics was conducted at Moçambique beach (Brazil). ...
Foredunes are highly dynamic coastal features that are intrinsically connected with their adjacent beaches. Correlations among parameters representative of aeolian and marine dynamics and the size and location of foredunes were carried out. The results indicate that foredune steady-state location depends mainly on marine dynamics, given that foredune toe level was positively correlated with the total water level of 10-year return period, which guarantees that the foredune is only eroded by waves occasionally and therefore will have time to recover and return to its long-term morphological state. Foredune steady-state size was characterized by its volume, which was positively correlated with aeolian sediment drift potential and exponentially related to the total water level with a 10-year return period. Aeolian sediment drift potential is indicative of the total amount of sand that potentially reaches the foredune, and therefore the foredune size is directly related to it. Higher total water level with a 10-year return period leads to a foredune located further from the shoreline and at a higher altitude (higher toe level as mentioned above). Consequently the beach is wider and the presence of moisture is expected to be lower. This wider and dryer beach allows the aeolian sediment transport to approximate to its maximum value, increasing its potential to build a bigger foredune. Data confirmed that the total water level with a 10-year return period and the foredune volume are exponentially related. The correlations found show that foredune steady-state size is related to both aeolian sediment drift potential and marine dynamics. Based on these results, a long-term conceptual model is proposed to explain the different morphological characteristics of the steady-state shape of foredunes as a function of the joint action of marine and wind dynamics.
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Crapoulet, A., Héquette, A., Levoy, F. and Bretel, P., 2016. Using LiDAR topographic data for identifying coastal areas of northern France vulnerable to sea-level rise. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R. A major portion of the coast of northern France consists of wide macrotidal beaches and coastal dunes protecting low-lying backshore areas (mostly reclaimed lands) from marine flooding. Although the shoreline was stable or even prograded seaward in places during the last decades, several coastal areas underwent severe erosion during the same period, while flooding sporadically occurred locally during major storms. A study of the potential impacts of sea-level rise on the coast of northern France was conducted based on airborne LiDAR topographic data collected from 2008 to 2014. Mapping of areas at risk of erosion and flooding during storm-induced events associated with high water level with a 100-year return period by 2050 was carried out using high water level statistics derived from tide gauge measurements, offshore wave climate statistics, and a sea-level rise projection based on RCP6.0 scenario (IPCC, 2013). Wave run-up was computed based on the Cariolet and Suanez (2013) equation developed for macrotidal beaches and using beach topographic profiles extracted from the LiDAR data. Results show that marine flooding would still be limited in 2050 even with a higher sea-level, but reveal that coastal dune erosion will most likely be widespread (Fig. 1). However, our study also shows that if coastal retreat proceeds during the next decades at the same or higher rates as today, several coastal dune systems will be entirely eroded in the near future, which would result in extensive storm-induced marine flooding in several coastal areas.
Conference Paper
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Meteorological data at Dunkirk between 1956 and 2001, show two periods of higher storminess between 1956 and 1962 and between 1972 to 1977. Tide-gauge records from Dunkirk harbor show similar results, the frequency of high magnitude storm surges (≥m) being higher during the 1960's and the second half of the 1970's. The assessment of the impacts of storm surges on coastal erosion was carried out through the use of a storm erosion susceptibility index (SESI) which integrates the magnitude of the storm surge, duration of the event, and wave height. At a medium time-scale (decades), no direct relationship was observed between the SESI and coastline evolution because potential impacts of storm surges highly depend on the upper beach morphology at the time of the event. At a short time scale, however, the use of the SESI appears to be useful for evaluating the potential effects of storms on shorelines.
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The comparison of aerial photographs of eroding coastal dunes located between Dunkerque (Northern France) and the Belgium border revealed that the retreat rate of the dune front increased between 1957 and 1988. Analyses of hourly water levels from the Dunkerque Harbour tide gauge showed an increase in the frequency of high water levels associated with storm surges during the same period. Significant wave heights that could be generated during these high water level events were computed according to a wave hindcast model and using wind data collected at Dunkerque. These analyses show an increase in storm magnitude and frequency during the last two decades of the study period, and suggest a strong relationship between dune front erosion and Frequency of storm surge conditions. Since relative sea-level is rising in the southern North Sea, coastal dunes will probably be more frequently reached by storm waves in the future. Consequently, more severe coastal dune erosion may take place during the next decades, increasing the risk of flooding of coastal lowlands.
Field observations following a major storm surge on 26 February 1990 on the Sefton Coast in Northwest England showed that the rate of aeolian sand transport and backshore dune construction varied significantly alongshore in response to variations in beach morphology and sand wetness. These variations, in turn, were determined by longshore variations in the character of marine sediment transport processes and the distribution of subsurface silt and peat formations which outcrop locally on the foreshore. The grain size of the beach sands was found to be very uniform and was not a factor influencing spatial variations in sand transport rate. Since the beach sediments consisted of well-sorted and very well-sorted fine sands, partly reworked from the dunes during the storm surge, deflation was not grain-size selective during the period of post-surge dune recovery.
The morphology of three macrotidal ridge and runnel beaches in northern France was analysed from over 200 profiles in order to identify intertidal spatial and short-term (weeks) profile variability, at both the inter-site and intra-site levels. The beaches, essentially composed of medium to fine sand, are exposed to fetch-limited waves variably dissipated by nearshore sand banks. They have spring tidal ranges of 5.6 to 7.2 m, and sediment budgets ranging from equilibrium to deficient or surplus. The results show that spatial and temporal morphological variability is controlled by: (1) variations in exposure to wave action that depend on the proximity of nearshore sand banks, as well as on protection offered by artificial structures; and (2) by the state of the beach sediment budget. Where equilibrium sediment budget conditions prevail, as in the Dunkerque-Est sector, the beach exhibits a regular alternation of ridges and runnels that represent a cross-shore alternation of fluid-bed interaction domains involving surf/swash activity and channel flow conditions. Energy dissipation at the bed is spent in the construction and destruction of wave and tidal micro- and meso-scale bedforms, leaving little scope for macro-scale ridge migration or change in form, except under exceptionally high wave energy conditions. Chronic sediment losses, as in Wissant Bay, or gains, as in Calais-Hoverport, are recycled respectively alongshore and to embryo dunes and are not necessarily translated in terms of significant meso-scale (years) beach volumetric changes. The short-term beach sediment budget changes however favour active bed readjustments that explain distortion of the regular ridge and runnel form and marked profile mobility, even under low to moderate wave energy conditions.
Résumé L'évolution morphologique des dunes littorales de l'est de Dunkerque est marquée par une migration du bourrelet bordier vers l'intérieur des terres. Malgré le recul du front de cette dune et la présence de formes érosives liées aux facteurs naturels (houles de tempête, vent) et une forte fréquentation humaine, son élévation et son élargissement, ainsi que le comblement d'anciennes et importantes zones de déflation, combinés au développement du couvert végétal, conduisent à un bilan sédimentaire équilibré au cours des dernières décennies. L'évolution des trois bourrelets dunaires étudiés est variable au cours du temps : les vitesses de recul varient ou même s'inversent en quelques décennies, sans doute en liaison avec les modifications subies par le système complexe de bancs et de chenaux tidaux et subtidaux qui borde le littoral.
The probability of extreme sea levels along the coasts has always been statistically estimated from the series of local observations. The inference is never conclusive, and an attempt is given here to improve the methods already used with reference to the area of the English Channel. The joint probability method (JPM) is the starting point: In most cases it underestimates the return times (or overestimates extreme levels at a fixed time). The proposed Extension is based on a more careful use of observed extremes by fitting a coefficient Cc deduced from the data set, which requires that the maximum record height be in agreement with the return period of the record length. This correction calibrates the whole series of extreme estimations to the observed maximum. Likewise an attempt to roughly explain this correction is given that explores the tide-surge interaction and seasonal dependence. The parameters are specifically computed for 15 tide-gauge stations, and the comparison is extended to other known methods, like the Gumbel one (in most cases overestimating the levels) and GEV simulations (which appear much better). Finally extreme levels with estimated return times of 10, 50, and 100 years, respectively, are proposed for each site, and a test for validity was performed by splitting certain long records into small samples, thus checking the spread of the results.
A 400m2 experimental plot, covering the transitional zone from the upper beach to the dune in Leffrinckoucke, on the southern North Sea coast of France, was monitored over 24 months via 9 high-resolution terrain surveys in order to determine the role of cost-effective and ecological brushwood fences (1.2m high, 4m long and spaced 3.5–6.7m apart) in sand accumulation on a fragile dune front subject to severe past erosion. The dunes on this coast protect densely populated former tidal wetlands that form the Flemish coastal plain, much of which lies below mean sea level. The wind regime consists of a fairly balanced mix of moderate (80% of winds are below 8ms−1) onshore, offshore and shore-parallel winds. The experimental plot showed moderate beach-dune mobility and accretion. The pattern of dune front accretion throughout the surveys showed a close relationship with wind directions. The fences induced rapid accretion on the dune front in just 10 weeks that accounted for 20% of the overall plot sand gain and 41% of the total gain over the 2-year study by this dune front unit. This rapid accretion also created hummocks of sand between which deflation corridors, oblique to the dune front, developed, in response to oblique onshore and offshore winds. However, the subsequent prevalence of variable wind approaches embracing longshore, offshore and onshore directions finally favouring the sealing of these corridors, thus illustrating the efficiency of this fence design in inducing foredune accretion. This efficiency has been favoured by mild sand supply from the nearshore zone combined with the absence of significant erosive storm events. Longer-term foredune management will need to take into account the future potential effects of storm damage due to climate change.
Beaches and coastal dunes are dynamic geomorphic systems that respond to process forcing over a broad spectrum of spatial and temporal scales. At all scales, there are potential suites of interaction between system forms and processes, and the mechanisms of interaction are stressed in this article. At micro-scales, this interaction is formalized through the concept of morphodynamics, and deterministic and probabilistic approaches are used to model sediment transport and landform development over time scales of hours to months and space scales of metres to kilometres. Meso-scale interactions are conceptualized using a sediment-budget approach. Nine characteristic environments comprise the beach and dune sediment-budget ensemble, representing a classification scheme for reciprocating coastal systems. -Authors
Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast.