Spatial Variability in Post-Storm Beach Recovery along a Macrotidal Barred Beach, Southern North Sea

Abstract

Field studies carried out on pre-and post-storm beach response have showed that large volumes of sand can be eroded in beaches and dunes during a single storm event, which is commonly followed by phases of beach recovery. No more than a few hours are required to induce significant beach erosion during a high energy event, but post-storm beach and dune recovery occurs at longer time scales, a period of several years being generally necessary for the re-establishment of the former morphology. This study documents phases of beach recovery that followed two stormy events on a macrotidal sandy beach of northern France characterized by complex intertidal bar-trough topography. The study area extends along a 1 km long coastal stretch between Dunkirk and the Belgium border, facing the fetch-limited southern North Sea. High-resolution beach profiles were surveyed at three different sites before and after storm events that resulted in upper beach and foredune erosion in March 2008. The comparison of series' of beach profiles obtained several months after these storms revealed distinct beach responses at each site, with two profiles tending to return to their pre-storm position, whilst the other profile did not recover. This variability over such a limited distance may be due to temporal and spatial variations in bar-trough beach morphology that may be responsible for the observed differences in short-term beach recovery behaviour.

Full text

Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 88 – 92 ICS2009 (Proceedings) Portugal ISSN
Spatial variability in post-storm beach recovery along a macrotidal
barred beach, southern North Sea
A. Maspataud, M-H. Ruz and A. Hequette
Laboratoire d’Océanologie et de Géosciences (UMR CNRS 8187)
Université du Littoral Côte d’Opale, Wimereux, 62930 France
e-mail : aurelie.maspataud@univ-littoral.fr
ABSTRACT
M
ASPATAUD
A.,
R
UZ
M-H. and H
EQUETTE
A., 2009. Spatial variability in post-storm beach recovery along a
macrotidal barred beach, southern North Sea. Journal of Coastal Research, SI 56 (Proceedings of the 10th
International Coastal Symposium), 88 – 92. Lisbon, Portugal, ISBN
Field studies carried out on pre- and post-storm beach response have showed that large volumes of sand can be
eroded in beaches and dunes during a single storm event, which is commonly followed by phases of beach
recovery. No more than a few hours are required to induce significant beach erosion during a high energy event,
but post-storm beach and dune recovery occurs at longer time scales, a period of several years being generally
necessary for the re-establishment of the former morphology. This study documents phases of beach recovery
that followed two stormy events on a macrotidal sandy beach of northern France characterized by complex
intertidal bar-trough topography. The study area extends along a 1 km long coastal stretch between Dunkirk and
the Belgium border, facing the fetch-limited southern North Sea. High-resolution beach profiles were surveyed at
three different sites before and after storm events that resulted in upper beach and foredune erosion in March
2008. The comparison of series’ of beach profiles obtained several months after these storms revealed distinct
beach responses at each site, with two profiles tending to return to their pre-storm position, whilst the other
profile did not recover. This variability over such a limited distance may be due to temporal and spatial
variations in bar-trough beach morphology that may be responsible for the observed differences in short-term
beach recovery behaviour.
ADITIONAL INDEX WORDS:
Storm impact, beach-dune evolution, intertidal bars and troughs.
INTRODUCTION
Storm events represent a major factor controlling short- to
medium-term morphological evolution of many sandy shorelines
(S
TONE AND
O
RFORD
, 2004). In the event of possible changing
storm regimes associated with climate change (Z
HANG
et al., 2004;
IPCC, 2007), it is important to understand the potential effects of
storms on beaches and dunes and how they recover after these
high-energy events. A number of studies have been dedicated to
assessing the impacts of storms on beaches and dunes and on their
post-storm morphological readjustments, but most of them were
conducted along microtidal and storm-dominated coastlines
(M
ORTON
et al.,
1994;
M
ORTON
et al.,
1995;
Z
HANG
et al, 2002;
S
TONE
and O
RFORD
, 2004). Although some authors investigated
the morphological response of beaches and dunes to storms on
macrotidal coasts (C
OOPER
et al., 2004; R
UZ
and M
EUR
-F
ÉREC
,
2004), very few studies were carried out on post-storm recovery in
large tidal range coastal environments. The macrotidal coast of
northern France is characterized by barred sandy beaches
(M
ASSELINK
and
A
NTHONY
, 2001; R
EICHMÜTH
and
A
NTHONY
,
2002) backed by coastal dunes that are eroding in places during
episodic storm events (B
ATTIAU
-Q
UENEY
et al., 2000). A large
spatial variability and seasonal variations in bar-trough
morphology have been observed on these beaches (R
EICHMÜTH
and A
NTHONY
, 2008). Due to the paucity of studies conducted on
post-storm recovery of beaches and dunes along this coast, our
understanding of post-storm morphological readjustments is still
limited. In this paper, we present preliminary results on storm
impacts and short-term (4 months) post-storm recovery on a
macrotidal barred sand beach located in the fetch-limited
environment of the southern North Sea coast.
STUDY AREA AND METHODS
The coast of northern France, facing the Southern North Sea and
Dover Strait, is exposed to relatively low-energy waves that are
refracted by numerous offshore sand banks. The study site consists
of a wide (300 to 600 m) dissipative beach composed of well- to
very well-sorted fine sand. The study area is affected by semi-
diurnal tides with a mean spring tide range of 5.6 m at Dunkirk
and a range of 3.5 m at neap tides. The beach presents complex
intertidal bar-trough topography (ridge-and-runnel) intersected by
drainage channels and associated with eroding coastal dunes in the
backshore (S
IPKA
and
A
NTHONY
, 1999; R
EICHMÜTH
and
A
NTHONY
,
2007). The field site extends over a 1 km long section along the
only stretch of preserved dune barrier, 7 km long, between
Dunkirk and the Belgium border. South to southwesterly winds
(38-40%) are largely dominant on this coast, but fetch and coastal
orientation conditions restrict the incidence of southwesterly
waves. This fetch-limited environment both influenced by tides
and waves is characterized by short wave periods (5 to 6 s) and

Journal of Coastal Research, Special Issue 56, 2009
Spatial variability in post-storm beach recovery
significant wave heights below 1.5 m in offshore modal
conditions. Onshore winds and waves from the northern sector are
less frequent but they can induce storm surges responsible for
erosion of the foredune (V
ASSEUR
and H
EQUETTE
, 2000). The
coastal zone is dominated by an eastward-directed flood residual
flow inducing a net sediment transport towards Belgium
(H
ÉQUETTE
et al., 2008).
Cross-shore profiles were surveyed from the crest of the
established foredune to the low tide level of the day, including the
foot and the steep face of the eroding dune (Fig. 2). The database
from which this study has been constructed comprises 15 beach
profiles obtained from 3 transects using a very high resolution
laser electronic station, whose errors are within ± 3 mm for
distance and ± 0,0015° for direction. All the topographic
measurements were systematically referenced to the French
National Geodetic Datum (IGN 69). Three cross-shore beach-dune
profiles were surveyed before and after two storm events in March
2008, then two months and three months later. The four months
survey is divided into four distinct periods: period 1 (05/03 to
14/03/2008), period 2 (14/03 to 08/04/2008), period 3 (08/04 to
05/06/2008) and period 4 (05/06 to 16/07/2008). The first two
periods include the two stormy events, whilst the last two periods
were characterized by fair weather conditions. (Fig. 2A-B).
Maximal intertidal bar height was determined from residual values
of profile elevation plotted from a second-order polynomial curve
fitted to each intertidal profile using least-squares analysis,
following the method of M
ASSELINK
and A
NTHONY
(2001). Wind
(mean hourly wind speed and direction) and wave data (significant
wave height H
s
and mean period) recorded at a Belgium buoy
located 36 km offshore (Fig. 1) were obtained from the Flanders
Marine Institute (VLIZ, http://www.vliz.be).
RESULTS
In March 2008, two stormy events have occurred with different
wind and tidal conditions. During the first event, which began on
9 March 2008 and lasted about 4 days, mean offshore wind
velocity was up to 25 m/s from southwest to west, with gusts up to
31 m/s. Offshore significant wave height (H
s
) reached a maximum
value of 4.2 m on 12 March with wave period in order of 5 to 6.5
s. Although this event was combined with spring tides (tidal range
of 5.42 to 5.86 m) and mean wind speed in excess of 10 m/s
during 96 hours, the dune toe was not reached by waves during
this event. However, an upper beach lowering (up to 0.40 m) was
observed on the three profiles. The second event occurred between
20 and 22 March with a smaller tidal range (4.89 to 5.37 m), mean
wind velocities higher than 10 m/s during 64 hours from the west
to east-northeast sector and gusts of 25 m/s. These direct onshore
winds induced maximum wave heights (H
s
) of 4.5 m, recorded on
22 March with periods of 7 to 7.5 s. Due to these direct onshore
Figure 1. Location of the study area and cross-shore transects surveyed in this study. The aerial photograph was taken in 2005 and is
not representative of the beach morphology at the time of the study.

Journal of Coastal Research, Special Issue 56, 2009
Maspataud et al.
winds and lowering of the upper beach, dune front erosion
occurred during period 2. The morphological response to this
second storm event was nevertheless spatially variable, as
evidenced by the three beach transects (Fig. 2A). The western
profile (P10) showed significant dune scarping and retreat (>1 m)
of the dune front, with a net lowering of the upper beach and dune
toe. The central profile (P7) presented a dune-toe retreat of 1 m,
whereas the eastern profile (P3) exhibited a lowering of 0.70 m at
the dune toe and the formation of a micro-cliff. Dune scarping
was, however, limited on the central and eastern profiles. In terms
of morphological variations, the intertidal bar-trough systems did
not reveal strong adjustments. P7 appeared rather stable, instead of
P3 and P10 which revealed slight onshore bar crest migration.
Fair weather conditions prevailed during periods 3 and 4, with
maximum wind speed that usually did not exceed 10-15 m/s and
blew predominantly from southwest to northwest. The western
profile (P10), which showed the strongest erosion during storm
conditions, presented readjustments of the dune-scarp: collapse of
the cliff edge and accretion (0.20 m) at the dune toe that
disappeared after 5 June. The same readjustments occurred on the
eastern profile (P3) with a maximum accumulation of 0.42 m at
the dune-toe. On the central profile (P7) readjustments were less
pronounced and very little accumulation was observed at the dune
toe. Fair weather processes did not favour any recovery of the
upper beach on the three profiles. Bar-trough morphology
developed however during these periods with an onshore
migration of the intertidal bars on the western and eastern profiles
(Fig. 2B), whilst profile P7 remained smooth and stable.
Similarly, the analysis of the height and position of the highest
intertidal bar on each profile revealed a decrease of the maximum
Figure 2. Beach-dune profile changes of the three cross-shore transects (see Fig. 1 for location), with enlargement of the upper beach
and dune front, before and after (A) two stormy events in March 2008 and (B) during fair weather conditions.

Journal of Coastal Research, Special Issue 56, 2009
Spatial variability in post-storm beach recovery
bar height after the stormy events on all profiles (Fig. 3A). The
most significant decrease in bar height occurred on profile 3,
whilst the lowest bar crest elevations were observed on profile 10.
During the following fair weather periods (periods 3 and 4),
maximum bar heights tended to increase, without reaching initial
pre-storm elevations. However, the western profile (P10)
experienced a net decrease in maximum bar height by the end of
the survey period. The distance of the higher bar relative to the
dune-toe was variable for the western and eastern profiles,
contrasting with the central profile on which the highest bar
remained remarkably stable (Fig. 3B).
Volume changes, calculated over the first fifty meters of each
profile (Fig. 3C), show clearly the impact of the two storm events,
with maximum erosion recorded on P10. After a significant loss
during periods 1 and 2, the eastern profile (P3) underwent
noticeable post-storm recovery in fair weather conditions. The
most stable profile (P7) experienced minor recovery during
summer (period 4). The western profile showed little recovery and
a slight erosion by the end of the survey period.
DISCUSSION AND CONCLUSIONS
The results of this study confirm previous observations of the
morphodynamics of beaches and aeolian dunes on macrotidal
coasts, and notably their response to storm and fair-weather
processes. Our beach and dune surveys underline the stability of
intertidal bar morphology during storm events as shown by
A
NTHONY
et al. (2004) and R
EICHMÜTH
and A
NTHONY
(2008) on
beaches of the region. Our results also show that bar morphology
is more pronounced and tend to migrate onshore towards the upper
beach during fair-weather conditions, which is consistent with the
observations of R
EICHMÜTH
and A
NTHONY
(2008) who noted such
bar development during summer and similar seasonal variability in
intertidal bar behavior. The three profiles did not return to their
pre-storm position during fair weather conditions, particularly the
upper beach and the dune toe that did not recover to pre-storm
level. This can probably be explained by the short time period of
our survey as most studies conducted on post-storm beach
recovery showed that several years are usually necessary for
beaches to recover to their former morphology (T
HOM
and H
ALL
,
1991; M
ORTON
et al., 1994).
Another finding of this study is the large variability in beach and
dune front evolution over a short distance along the coast, with
one stable profile (P7) and two profiles characterized by greater
morphological changes (P3 and P10). The most variable upper
beaches and dune-front appear to be associated with the most
mobile bar-through systems. Conversely, the profile that
experienced less erosion on the upper beach and dune front
(Profile 7) is also the most stable in terms of intertidal bar
morphology and position.
Volumetric changes calculated from repetitive profile surveys
are weak, but may suggest that by the end of the survey period two
profiles (P3 and P7) tended to slightly recover at the dune toe,
whereas the third profile to the west continued to lose sediment
during summer (Fig. 3C). This evolution is not due to waves as
they did not reached the upper beach during summer fair weather
conditions. The summer period is usually more favorable to sand
accumulation on the upper beach and dune-toe due to more
aeolian sand transport, which is favored by a lower frequency of
high water levels and less rainfall, resulting in lower surface
moisture (R
UZ
and M
EUR
-F
ÉREC
, 2004). As shown by A
NTHONY
et al. (2006), however, on these beaches sand exchanges between
the upper beach and dune are limited by the narrow wave-tide
affected upper beach and the barred intertidal beach morphology
immediately adjoining the dune front. With a lower bar level (Fig.
3A), wave-breaking can occur higher on the upper beach and then
reduce significantly the potential aeolian fetch and possible
aeolian sand transfer to the dune. This can partly explain the
negative sand budget of the dune front for the western profile
(P10). As stressed by S
EDRATI
and A
NTHONY
(2008), the return of
eroded sand to the dune system to equilibrate the local sand
budget depends on the capacity of the beach to restore this
sediment through aeolian and/or fair-weather wave transport. An
additional factor that may explain the observed variability in
aeolian accumulation, leading to upper beach recovery during
summer, is the predominance of alongshore winds transporting
sediment eastward (R
UZ
and A
NTHONY
, 2008). The dominant
southwesterly winds could be responsible for sand transfer from
the western part of the beach (P10), which can represent a source
area, to the eastern part (P3) where aeolian accumulation was
recorded.
This study evinced incomplete volumetric and morphologic
upper beach-dune recovery four months after storm events. Failure
to attain pre-storm upper beach and dune-toe elevations is not only
due to the short time interval of this study, but also on the spatial
and temporal variability of the intertidal bar-trough systems.
Further surveys should therefore be carried out on annual or pluri-
annual time scales for assessing long-term recovery on such
macrotidal barred beaches.
ACKNOWLEDGEMENTS
This study was partly funded by the French “Agence Nationale
pour la Recherche” (ANR) through the project VULSACO
(VULnerability of SAndy COast systems to climatic and anthropic
changes). Aurélie Maspataud benefits from a PhD grant from the
French Ministry of Education. Thanks are due to Denis Marin for
improving the illustrations and Vincent Sipka for technical
assistance.
Figure 3. Temporal distribution of (A) maximum intertidal bar
height, (B) distance between the higher bar crest and dune toe, and
(C) volumetric changes (calculated over the first 50 meters of each
profile) for the three transects over the 4 month survey period.

Journal of Coastal Research, Special Issue 56, 2009
Maspataud et al.
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