Identifying Forcing Conditions Responsible for Foredune Erosion on the Northern Coast of France

Abstract

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.

Full text

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
ruz@univ-littoral.fr
ABSTRACT
R
UZ
,
M-H.,
H
EQUETTE
,
A.
and
A
ASPATAUD
,
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
INTRODUCTION
Along sandy beaches, foredunes are usually defined as shore-
parallel dune ridges formed on the backshore by aeolian sand
deposition within vegetation (H
ESP
, 2002). The importance of
foredunes is well recognised (C
ARTER
, 1988; A
RENS
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
SUTY
, 1988; P
YE
, 1991; S
HERMAN
and B
AUER
,
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
ASSELINK
and A
NTHONY
,
2001; R
EICHMÜTH
and A
NTHONY
, 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
LABAUT
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
ASSEUR
and H
ÉQUETTE
, 2000; R
UZ
and A
NTHONY
, 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
NTHONY
and
H
ÉQUETTE
, 2007).
A number of recent studies suggested medium term (10 years)
gross stability of the beach-foredune system (R
UZ
et al., 2005;
A
NTHONY
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.
356

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.
METHODS
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
s
) 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.
RESULTS
Recent foredune evolution
Shoreline evolution in this area during much of the 20
th
century
was dominated by retreat (C
LABAUT
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
UZ
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
UZ
et al., 2005; A
NTHONY
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
LABAUT
et al.,
2000). Along most of this coast the foredune ridge was described
in a state of meso-scale (decadal) stability (R
UZ
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
LABAUT
et al., 2000) while the last
two decades were periods of decreasing storminess (C
HAVEROT
et
al., 2008).
Foredune response to a major erosive event
A major erosive event occurred in March 2007. Between March
17
th
and March 22
nd
, 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
th
(Fig. 3). From March 17
th
18:00 to the 22
nd
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
th
(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
th
and 18
th
, then veered to the northwest on
the 19
th
and maintained to the north on the 20
th
, 21
st
and 22
nd
. The
same wind directions were recorded offshore (Fig. 3). In response
to these moderate but constant winds, significant wave heights
(H
s
) recorded offshore and in the shallow subtidal zone increased
significantly. On March 18
th
, 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
th
9:00 and 21
st
, 12:00, with direct onshore
winds up to 15 m/s at Dunkirk and up to about 19 m/s offshore, H
s
of 4 m were recorded offshore and H
s
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
th
to 5.48 on the 19
th
, reaching
a maximum of 6.10 m on March 21
st
and then decreasing to 5.9 m
on the 22
nd
. With increasing tidal range, observed water level at
Dunkirk reached a maximum of 6.8 m above HD at 0:00 on March
19
th
, 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
HAVEROT
et al.(2008).
357

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
OMASIN
and P
IRAZZOLI
, 2008). On March 20
th
and
21
st
, 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
th
and March
21
st
(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
UZ
and
A
NTHONY
, 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).
DISCUSSION AND CONCLUSION
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
ROON
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
HERMAN
and
B
AUER
(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
event
.
Figure 3. Wind, waves and tide conditions during the March 2007 storm event.
358

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
OOPER
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
LABAUT
et al., 2000;
C
HAVEROT
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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ACKNOWLEDGEMENTS
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
the INTERREG IIIA project Beaches At Risk. The authors would
like to thank Mr. Hans Pope of the Belgian “Agency for Maritimes
Services and Coast – Division COAST” for providing the offshore
wave data. Thanks are also due to Denis Marin for drafting of the
figures and to Vincent Sipka for technical assistance in the field.
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