Impacts of storm chronology within a storm cluster on beach/dune erosion are investigated by applying the state-of-the-art numerical model XBeach to the Sefton coast, northwest England. Six temporal storm clusters of different storm chronologies were formulated using three storms observed during the 2013/2014 winter. The storm power values of these three events nearly halve from the first to second event and from the second to third event. Cross-shore profile evolution was simulated in response to the tide, surge and wave forcing during these storms. The model was first calibrated against the available post-storm survey profiles. Cumulative impacts of beach/dune erosion during each storm cluster were simulated by using the post-storm profile of an event as the pre-storm profile for each subsequent event. For the largest event the water levels caused noticeable retreat of the dune toe due to the high water elevation. For the other events the greatest evolution occurs over the bar formations (erosion) and within the corresponding troughs (deposition) of the upper-beach profile. The sequence of events impacting the size of this ridge–runnel feature is important as it consequently changes the resilience of the system to the most extreme event that causes dune retreat. The highest erosion during each single storm event was always observed when that storm initialised the storm cluster. The most severe storm always resulted in the most erosion during each cluster, no matter when it occurred within the chronology, although the erosion volume due to this storm was reduced when it was not the primary event. The greatest cumulative cluster erosion occurred with increasing storm severity; however, the variability in cumulative cluster impact over a beach/dune cross section due to storm chronology is minimal. Initial storm impact can act to enhance or reduce the system resilience to subsequent impact, but overall the cumulative impact is controlled by the magnitude and number of the storms. This model application provides inter-survey information about morphological response to repeated storm impact. This will inform local managers of the potential beach response and dune vulnerability to variable storm configurations.
Natural coastal systems not only provide protection to coastal communities from flooding but also host both environmentally and economically important areas (Hanley et al., 2014). Foredunes are of importance to ecological habitats as well as of aesthetical value. Such sedimentary systems are at risk from naturally occurring coastal erosion and manmade intervention. For example, in the 1960s–1970s tourist urbanisation and road construction led to major alteration and destruction of extensive sand dune systems across Spain. The accelerated dune erosion was in response to interruptions of the littoral drift by harbour developments and sand mining for construction and agriculture, in addition to human trampling, refuse dumping, recreational pressure and cropping (Gómez-Pina et al., 2002). Across Europe, 25 % of sand dunes were lost during the 20th century and up to 85 % of the remainder may be threatened as a consequence of sea level rise and climate change (Hanley et al., 2014). In response to accelerated erosion artificial beach nourishment schemes have been widely implemented across Europe (Hanson et al., 2002).
Coastal storms are recognised as one of the most important driving agents responsible for the observed morphological changes within beach/dune systems (Tătu et al., 2014). Such systems can be viewed as adaptive through their beach/dune response to changes in energy from the forcing conditions (Hanley et al., 2014). It is therefore important to understand how the cross-shore beach/dune profile responds under temporal clusters in storm impacts to interpret the consequent changes in resilience and in turn the vulnerability of the dune system to repeat high energy shocks. To this end a case study of Formby Point (in the northwest of England) is used to assess sequences in storm impact on one of the largest dune systems in the UK. At this location, approximately 13 m of dune retreat was observed over the 2013/2014 winter period by the National Trust, the responsible authority for the management of this site (NT, 2014). Such information is therefore of importance to enable researched-informed shoreline management planning (Esteves et al., 2009).
The aim of this research is to investigate the cumulative change in beach/dune volume in response to the variation in the storm sequences to reduce the uncertainty in storm cluster impact. The impact of storm clusters has been investigated on a range of beaches by Ferreira (2005), Callaghan et al. (2008), Vousdoukas et al. (2012a) and Coco et al. (2013). Splinter et al. (2014) concluded that the cumulative cluster impact is insensitive to the sequence of events. This case study confirms these findings at this location but also assesses the change in dune impact from a single extreme event in response to a cluster of events evolving the ridge–runnel system on the lower beach face. This case study allows assessment of not only how a ridge–runnel system reduces dune erosion but also how this feature responses to a sequence of events of variable wave power. Analysis of a cross-sectional transect enables detailed analysis of how sediment is redistributed across the beach/dune profile in response to storms of varying strength. It is suggested that sediment lost from the dune system enhances bar growth on the beach face, forcing waves to break further offshore and preventing further degradation of the dune system (Hanley et al., 2014). Understanding the likely response of the beach/dune profile to a sequence of storms is crucial for the development of appropriate and sustainable strategies to manage coastal flood and erosion risks.
Formby Point is situated on the Sefton coast in Liverpool Bay and is one of the largest coastal dune systems in the UK (Fig. 1). Covering an area of 2100 ha, it extends 16 km alongshore and 4 km inland with dune heights reaching approximately 30 m (Esteves et al., 2012). It supports a diverse range of habitats, including protected species such as the red squirrel and natterjack toad within the dune system (Edmondson, 2010). While vegetation (e.g. marram grass) is present the dune frontage at the profile of interest is relatively free from the influence of plant root stabilisation. Such biotic factors can play an important role on the dune stability increasing slope steepness (Armaroli et al., 2013). In this region the nearshore is characterised by a series of symmetrical sand ridges which are separated from the dune complex by a planer slope and are between 0.5 and 1.0 m high with a wavelength between 150 and 500 m. These features are formed due to the large tidal range and wave dominance in shoreline evolution. Typically these features build up during calm periods and flatten during storms (Plater and Grenville, 2008).
The largest waves within Liverpool Bay reach 5 m and the coastal surges exceed 2 m (Brown et al., 2010). The mean spring tidal range is approximately 8.2 m at Liverpool (located at the southern extent of the Sefton coast; Esteves et al., 2012) and, when coinciding with veering winds from SW to W, gives rise to the most extreme combined wave and water level conditions in Liverpool Bay (Brown et al., 2010). The wind climate within this region and the convex coastline geometry cause waves to focus on Formby Point, located at the coastal apex, while the net onshore tidal transport of sediment diverges into a net north and south littoral drift at this point (Pye and Neal, 1994). Formby Point therefore experiences a negative sediment supply, making it susceptible to storm-driven erosion (Pye and Blott, 2008). Dune retreat of up to 20 m has been observed along the Sefton coast and at Formby Point. It is suggested that significant winter erosion is caused when water levels exceed 4.87 m OD (9.8 m CD) (Esteves et al., 2012).
Within this region, extensive coastal observations (Howarth et al., 2006) and shoreline monitoring by Sefton Metropolitan Borough Council (SMBC) has historically been carried out. At present, shoreline monitoring by SMBC of the coastal waves, circulation, beach profiles and shoreline position continues alongside an offshore wave rider buoy (WAV in Fig. 1), which forms part of the UK WaveNet system (maintained by Centre for Environment, Aquaculture and Fisheries Science – Cefas), that has been operational since 2005 in Liverpool Bay. A long-term tide gauge has also been maintained as part of the UK tide gauge network at Gladstone Dock, Liverpool (TG in Fig. 1). Using records of waves and water levels the recent storm cluster of December 2013–February 2014 has been found to consist of some of the most extreme conditions this coastline has experienced (Wadey et al., 2015). We therefore use this cluster of events to investigate how the chronology of wave events, with different wave power, causes variability in the system resilience to extreme events and the cumulative erosive impact on Formby Point.
Liverpool Bay with the locations of the studied Formby Point transect P14, on the Sefton coast, and points of used observations; WAV (offshore wave characteristics), TG (Liverpool Gladstone Dock, nearshore tide) and WN (Hilbre wind station).
The winter season 2013/2014 saw three events when the water elevation was able to
allow wave impact and soaking of the dune toe, while there were seven extreme
wave events impacting the shoreface. Beach surveys nearly 1 year later (8
October 2014) have shown that the dunes at the studied cross section have not
fully recovered over an annual cycle. The frontage is still setback by
approximately 5 m from the pre-storm state (surveyed 10 September 2013). The
ridge–runnel system has recovered, but sits slightly higher up the beach
face, although this position could be related to the tidal conditions around
the time of the surveys. This study uses three storm events (D1, D2 and J2)
from the storm cluster that occurred from December 2013 to January 2014 to
assess the impact of variable storm sequences on the ridge–runnel feature
within the beach profile that influences the resilience of the beach/dune
system at Formby Point. Such information will then be used to inform the
wider community of the possible erosive threats of storm sequences to natural
dune defence systems. The selected storms represent the first two that
occurred in December and the second event that occurred in January. The
extreme storm (D1, Fig. 2) is chosen due to the combination of large waves
combined with a total water level that allows impact on the dune system, i.e.
it exceeds the mean high water spring tide level (4.47 m OD at Liverpool
tide gauge) which the dune toe is typically located just above. This storm is
the most extreme during the winter 2013/2014 period and causes approximately
4 m of dune retreat for the considered profile. The other two (D2 and J2)
events are chosen to represent storms of different offshore wave severity
but with a clear linear relation between the event severity. These events do
not reach the dune toe, which typically has a mean winter position of 5.07 m
OD (Esteves et al., 2012), but they do inundate the ridge–runnel feature.
Selecting large wave events that can be related in terms of power is
important to assess the morphological response of the ridge–runnel system.
Unlike the dune response, the flattening of this feature is dependent on storm
activity rather than the total water level. This allows the wave impact of
different events on the ridge–runnel system to be assessed to identify whether the
consequent morphological dune evolution in D1 is controlled by the timing of
relative events. The relation between the wave power of all three events
allows assessment of whether the ridge–runnel response is proportional to wave
power of the number of repeated impacts. The first storm (D1 on 5
December) is the most powerful (266 m
Time variation of the wave height and water level within these events are
shown in Fig. 2 together with the storm threshold wave height used to
calculate the offshore storm wave power. In the first event (D1), which
persisted about 1 day, the peak storm wave height (4.6 m) coincides with
high water (6.2 m ODN) during spring tide and strong westerly wind (note:
wind characteristics are not shown here but are presented by Wadey et al.,
2015). The second storm (D2) spanned about 19 h and occurred during the
intermediate period between spring and neap tide. There were two peaks when
this storm exceeded the wave threshold, with the wave heights reaching 2.8 m
during the second peak. In this storm, the wind speed was higher at high
water than at low water. The high water elevations reached 4.2 and
3.9 m ODN. The third storm (J2) lasted 8 hours and the peak storm wave
height was 2.9 m. A large part of the J2 storm coincided with the high-water
spring tide (3.5 m ODN). Wind speed during this storm varied from 11 to
16 m s
The three selected storm events D1, D2 (in December 2013) and J2 (in January 2014) and their wave height and water level variations together with the storm threshold wave height.
Using the three storm events, six storm clusters of different wave chronologies were simulated (Table 1) to investigate their impacts on the cumulative beach/dune response of Formby Point.
Defined storm clusters using different storm wave chronologies of the three storm events (D1, D2 and J2).
The modelling system selected for this study is XBeach (Roelvink et al.,
2009), which is one of the latest developed
We focus on a 1-D profile at the apex of the Sefton coast, Formby Point
(transect
The pre-storm 1-D profile based on the observed data from survey location P14 (see Fig. 1). Calibration was performed over the transect length available from the post-storm survey.
Wave, wind and tidal forcings during each event are separately applied to
simulate the storms in the XBeach model. The extension of the model profile
offshore to a 20 m depth enables us to set up the model such that it is
forced with the observed waves at the offshore boundary (WAV in Fig. 1).
Water levels at the offshore boundary are those recorded by a nearby tide
gauge data at Gladstone Dock in Liverpool (TG in Fig. 1). This allows the
tide, surge and any interaction to be imposed. Any local surge generation
across the 1-D domain is assumed to be minimal and the tidal conditions are
likely to be similar to those experienced at Formby Point. The location of
the tide gauge in sheltered deep water within the Mersey estuary also means
wave set-up in the observed water level is likely to be minimal, allowing
XBeach to simulate this at the more open location. Wind speed and direction
during each storm were extracted using the observed data at WN (see Fig. 1).
The combination of these wave, wind and tidal characteristics provides the
full model forcing for the offshore boundary of the investigated transect
(
Initial model simulations were undertaken to calibrate the model settings comparing the measured post-storm profile on 9 December 2013 and that of the model prediction during the D1 storm event. The calibrated model was then separately used to obtain the cumulative morphological change during the storm clusters defined in Table 1. The transect assessed (see Fig. 4, cross-shore distance 75–400 m) corresponds to the post-storm survey data that assessed the beach elevation to the newly eroded dune frontage. In the second series of simulations, the post-storm model predicted profile of the previous storm was adopted as the initial bed topography in the subsequent simulation to enable the cumulative response of beach/dune evolution within a storm cluster to be modelled.
The morphodynamic prediction of XBeach is sensitive to a number of model
parameters (Pender and Karunarathna, 2013; McCall et al., 2010; Lindemer et
al., 2010). The sensitivity to parameter settings is known to increase with
stepper beach slopes (Vousdoukas et al., 2012b). Since this system is
dissipative with a gentle slope, many of the default settings are appropriate.
Only two parameters, found to cause the highest contribution to the
modelled morphological changes of beach/dune systems, are used in this
calibration: (1) the factor for time-averaged flows due to wave skewness
(
A series of simulations were undertaken by changing the values of these two
parameters systematically around the default settings. The optimised values
for
Comparison of measured and modelled profile evolution across transect P14 (13.65–14.00 km in Fig. 3) using the optimised calibration factors (facSk and facAs).
These calibrated coefficients were used in the subsequent model runs to investigate the cumulative response of the beach/dune system to the variable wave chronology within the storm clusters.
Here we compared the shape of profile evolution over the upper-beach and
lower-dune system from 75 to 400 m cross-shore distance in Fig. 4 (i.e.
from 0 m ODN, MSL, to
Profile evolution within the selected profile segment (from 75 to 400 m in Fig. 4) during each storm event within the six formulated storm clusters.
These results show the importance of the wave chronology enabling weaker storms to modify the beach profile when they are in close succession to other storms, which influences the system's resilience to dune erosion. This is due to the flattening of the ridge–runnel system reducing the wave dissipation and also the redistribution of sediment from this feature to form new features further up the profile. The larger the proceeding event, the less impact weaker storms that follow it have on the ridge–runnel system; however, when if the weaker storms come first they modify the systems resilience of the upper beach and dunes to later extreme events.
Bed level changes during each storm event in the upper-beach/dune area are compared within each storm cluster (Fig. 6). The highest bed level changes within all storm events correspond to the region of the ridge–runnel system and the dune toe in the case of D1. The ridge crests at 230 and 290 m experienced erosion while accretion occurred in the troughs located at 190 and 260 m cross-shore distance. The dune frontage at 400 m experiences erosion under D1.
The variable bed level change found for each storm event within the clusters
indicates that event evolution depends on the wave chronology. Over the
ridge–runnel system the magnitude of the bed level change corresponds to the
events position in the cluster. When it occurs first the evolution is
greatest and when it occurs last the evolution is smallest. These results
suggest that after two storms in close succession, no matter what the storm
power, this ridge–runnel system reaches a nearly stable (flattened) storm
beach profile and noticeable evolution in response to further storms occurs
only at the beach–dune interface if water levels allow. It is seen that when
a storm initiates the cluster two small bars towards the landward side of the
initial ridge–runnel system are initiated (at
At the dune, toe water elevation controls the storm impact as the waves either
can or cannot reach the dunes. The variable response of D1, the only storm
that can reach the dune system, is in relation to reduction of the
dissipation by the ridge–runnel system and also the increase in bed level
landward of the initial ridge–runnel system as new bars try to form further
landward under the elevated water levels. For D1, the erosion of the dune
frontage is quite consistent, demonstrating greater sensitivity to the water
elevation than the dissipative nature of the ridge–runnel system on the wave
conditions. The erosion is slightly increased when the ridge–runnel system is
flattened and decreased when a bar starts to form under D2 at higher
elevations on the beach (at
Bed level change from 75 to 400 m cross-shore distance during each
storm event within each storm cluster. A positive change indicates accretion
and negative is erosion. The erosion in
The time average of the absolute bed level change due to the three storm events run in sequence to form a cluster was separately analysed across the upper-beach region (Fig. 7). This represents the average effect of each storm sequence on the overall bed level change at select cross-shore locations representative of the ridge runnel features. The maximum event-average change of the bed level due to the clusters over the ridge–runnel system is about (0.12 m) half of that possible within a single storm event (0.25 m in Fig. 6). At the dune frontage the maximum time-averaged erosion is (0.26 m) is just less than a third of that possible within a single storm event (0.77 m in Fig. 6). This shows that while the ridge–runnel system evolution is influenced by approximately two storms the dune toe evolution is dominated by the single extreme event (D1).
Peak values in the averaged evolution (Fig. 7) correspond to the crests and troughs of the ridge–runnel formations of the initial profile (see Fig. 5), which experienced relatively large bed level change due to feature flattening compared with other locations across the profile. The first peak represents (0.06 m) erosion occurring on the bar located at 140 m cross-shore distance. The influence of all storm clusters is fairly similar at this location. The second peak at 190 m corresponds to the trough at 190 m cross-shore distance and its averaged bed change (0.12 m) is greater than that of the first peak, indicating strong deposition of slumped sediment from the bars at higher levels. The largest change at this location is found in cluster 4 while the lowest is given by the cluster 6. In both clusters, the most severe storm (D1) occurred at the end. The third peak at 230 m cross-shore distance shows the greatest erosional impact across the experienced at the bar (at 230 m) due to sediment at the crest being redistributed into the troughs either side. In this location, the largest average bed change is found under cluster 4 as well; whereas the smallest change resulted under the cluster 1 (i.e. D1 occurred initially). This is because D1 has the highest power, so once it has impacted this feature the latter storms that have less duration at this point in the profile due to lower water elevations and less power have less impact on the wider and lower feature. Deposition occurred in the trough located at 260 m cross-shore distance and is shown by the fourth peak. Cluster 2 produced the largest averaged bed change indicating the greatest deposition in this trough, while the lowest at this location was found in cluster 6. In these two clusters, the D1 event occurred at the beginning and the end of the sequence. The last peak at 290 m indicates erosion on the bar located at the landward end. All storm clusters resulted in similar averaged bed change at the fifth peak, implying a similar impact of storm clusters on the bed at this bar's location. This suggests the infill was dominated by one event (D1) with the most impact at the higher elevations, potentially accessing sediment from further up the beach system.
The averaged bed change from 300 m landwards to the dune area is dominated
by the erosion of the dune frontage which causes slumping at lower levels of
the dune face seen as accretion (to
The time-averaged absolute bed level change from the 75 to 400 m
cross-shore distance profile segment within each storm cluster. Dark grey
indicates erosion areas while light grey indicates accretion. The grey area
just landward of 325 m can accrete during the first event during the formation
of the embryo bars. The fifth cluster just exceeds the
These results indicated that the average effect of the storm chronology within a cluster on the bed level change slightly varies with location along the profile. Also, no clear criterion is found such that the timing of the most severe storm, either at the start, middle or end, within a cluster influences the peaks in average erosion or deposition over the ridge runnel system. However, the time-averaged impact at the dune toe is greater when D1 follows J2. This is due to the modified impact of D1 rather than a combined impact of events.
Volume change per unit cross-shore length was estimated during each storm
event by multiplying the change in bed elevation and the cross-shore distance
of grid cells along the selected profile segment from 75 to 400 m
cross-shore length (black bar in Fig. 8). The volume change in response to
the cluster of events was then found as the summation of the three storms
(white bar in Fig. 8). A negative value indicates erosion and thus all storm
events resulted in erosion over the upper-beach face. The event with the
highest storm power (266 m
When looking at each storm event in turn, the greatest erosion volume
associated with an event occurs when that storm is the initial event within a
storm cluster. However, D1 has the same impact when it follows J2, if J2
leads the sequence, as it does in isolation. This is due to J2 having the
least influence on the ridge–runnel system and no impact on the dunes. The
variation in volume change associated with each event varies very little when
the event is positioned differently in the storm sequence; however, it is
slightly reduced when the event occurs later within the wave chronology. The
amount of volume change is also found to depend on the storm wave severity of
the preceding event. Increasing the proceeding event severity leads to a
reduced erosion amount in the secondary event and decreasing the severity
increases the erosion within the secondary event (e.g. compared D1 in the
clusters 3 and 5) and thus shows the impact of storm wave sequence on the
event-driven bed evolution. This result is due to the features of the bed
profile being flattened by a variable amount, which then determines the
continued evolution until the profile is flat. The cumulative volume changes,
due to the three storm events within the clusters, indicate some variations
due to wave chronology, though they are not significantly large. The largest
cumulative volume change (
Comparison of the volume change from MSL to
Impacts of storm chronology in a storm cluster on beach/dune erosion were
investigated using a numerical model applied to Formby Point at the apex of
the Sefton coast in the Liverpool Bay, UK. Three storm events that impact the
ridge–runnel system with storm power values 266, 110 and
52 m
The most severe storm was used to calibrate the model.
Comparison of the predicted post-storm profile with that of the measured
profile resulted in an RMSE of 0.11 m and a correlation coefficient of 0.63
indicating a high storm model performance. Negligible variability in the cumulative impact of the storm
clusters occurred in response to different storm wave chronologies. However,
it was found that the event-scale ridge–runnel and dune face profile changed
depending on the storm severity and the magnitude of the change was modified
by previous events. Impacts of the storm clusters on bed change for this transect
are mostly in relation to the flattening of the ridge–runnel system and
slumping of the dune frontage. The largest event-driven bed level change occurred under the
forcing of the most powerful storm event when it initialised the cluster.
While the lowest bed level change occurred for the weakest event when it
ended the cluster. The ridge–runnel system that exists on the upper-beach face
at Formby Point lasts for about two storms in close succession, after which the
upper beach becomes more susceptible to erosion. If the initial storm is weak
the upper-beach face undergoes less evolution under later larger events, but
the dune frontage is typically more susceptible to impact during later
extreme events. Any morphological impact that occurs due to storms soon after
is minimal as the beach forms have already been flattened, further erosion of
the beach and cross-shore sediment exchange does not seem to occur. Continued
response may result when longshore transport is considered. The highest erosion during each storm event was observed
when that storm occurred as the initial event of a storm cluster. Within each
cluster the most severe storm always resulted in the highest erosion and the
weakest storm produced the lowest erosion no matter of its position within
all clusters. In a storm cluster, the highest erosion on the beach/dune
system was found when the storms increased in severity. The cumulative change
in the ridge–runnel system is similar as it flattens so the change is likely
to be related to a slight increase in erosion of the upper beach and the dune
system during the most extreme event. Although the first storms acted to flatten the ridge–runnel
system this had little influence on the volume change of the full profile in
the last event, although it did influence the local change experienced close
to the dune toe for the weaker storms when they occurred later. Interestingly for this case study, a reduction in maximum
water elevation during each storm event is consistent with a reduction in
offshore storm wave power. This suggests the fetch-limited conditions of the
Irish Sea and the orientation of this coast causes storms to generate
similarity in the severity of the water and wave elevations that occur
together. The storm events that were chosen to represent changing
severity of impact on the lower beach features demonstrate how dune impact
is more sensitive to events with high water levels than storm-driven changes
in the beach profile. The ridge–runnel system therefore provides little
increase in resilience for the dune system even when it is fully formed.
These results provide preliminary insights on the impacts of storm chronology within a storm cluster on the beach/dune erosion of Formby Point (Sefton coast). These findings will have important implications for the interpretation of the continued monitoring of the beach/dune erosion along the Sefton coast and will be useful to implement sustainable dune management strategies. Further model studies are now required to consider different profiles along the Sefton coast, storms with high water elevations and area simulation to get a comprehensive understand on the effects of the storm chronology. For other locations these results suggest that although wave chronology is important, influencing the event-scale morphological change, the cumulative impact is independent of the temporal sequencing.
The work presented in this paper was carried out under the project “FloodMEMORY (Multi-Event Modelling Of Risk and recoverY)” funded by the Engineering and Physical Sciences Research Council (EPSRC) under the grant number EP/K013513/1. The NOC COBS, BODC, NTSLF, the NOC marine data products team and CEFAS (WaveNet) are acknowledged for providing tidal and wave data. The Sefton Metropolitan Borough Council is acknowledged for providing access to relevant coastal monitoring used in this study. PD and HK also acknowledge the support of the Ensemble Estimation of Flood Risk in a Changing Climate project funded by the British Council through their Global Innovation Initiative.Edited by: P. Ciavola