The remnants of Hurricane Ophelia struck the British Isles in mid-October 2017 under the status of an extra-tropical storm. The Hurricane developed originally southwest of the Azores from a persistent low-pressure system, allowing it to travel to north-eastern Europe (NOAA, 2017). Storm Ophelia reached Ireland on the 16th October and was re-classified as a “post-tropical” cyclone (Guisado-Pintado and Jackson, 2018) but still retained gusts of up to 130 km/h were recorded. Storm Ophelia made landfall at 10am (1100 UTC) October 16th.

Storm Ophelia’s highest gust was 156 km/h with a SSE direction, which was recorded in Roche’s Point (Co. Cork) at 10:59 am, further north at Malin Head (Co. Donegal), a maximum wind gust of 106 km/h was reached at 19:30 h on the 16th of October with an overall direction of SSW. Storm Ophelia, despite initial expectations, lost its energy as it travelled north toward the study site. Around Malin Head (Figure 3A) significant wave heights ranged from 1.78 m on the 16th October at 9:00 am to 1.28 m when the storm had passed the study site (18th October) as shown in Figure 3C, with associated 4–5 s wave periods. The maximum wave height, 2.22 m, was reached at 6:00 pm (17th October, Figure 3B). Wave direction varied from 170 (S) to 220 (SW) during the storm event, which is parallel to main beach orientation of the study site.

Storm Hector occurred in the second half of June 2018 and was formed by an unusually strong jet stream forced by a cold air mass from Polar Regions meeting a warm air mass from the tropics. The storm reached the western Ireland coast on Wednesday the 14th June around midnight, producing 2 days of gales on the 13th and 14th, followed by frontal systems crossing toward Scotland on the 20^th June.

Storm Hector’s highest gust was 61 knots (113 km/h), the highest June gust since 2005. The month’s highest 10-minute mean wind speed was 48 knots (89 km/h), and its highest June 10-min wind speed since 2005.¹ In Northern Ireland, gusts of 64 knots (119 km/h) and 60 knots (111 km/h) were recorded.²

The strongest winds from Storm Hector over the northern coast of Ireland were coincident with high spring tides, leading to coastal flooding and wave overtopping in many coastal areas of Co. Donegal and Co. Galway. Storm Hector arrived at the study site around 3:00 am on the 14th of June (H_s = 2.5 m) and lasted until the 15^th June 6:00 am were significant wave heights dropped below 2.5 m (Figures 3E,F). The storm had reached a maximum significant wave height of 4.0 m south of Malin Head (14^th June 9:00 am) (Figure 3D). Mean wave period of 6 s were recorded closest to the study site. While offshore, the Marine Institute M4 buoy (54.99°N 9.9°W), recorded a wave height of 6.53 m (T = 8.9 s) at 9:00 am on the 14th of June. The storm showed a mean wave direction of 220–270° (W and WSW), perpendicular to the main coast orientation.

Topographic changes, driven by the storm events, both in the intertidal area and subaerial beach, were quantified using a FARO Focus 3D × 330 TLS. The TLS survey produced a dense point cloud, extending from the dune toe toward the intertidal environment. Surveys were undertaken during low tide conditions, revealing a large part of the intertidal area to around −1.5 m (see Figures 2A,B) and ranging landward over front sections of the dunes (up to +20 m).

Topographic surveys were performed along the northern section of Five Finger Strand (Figure 1B), selecting a 420 m × 250 m area of interest (≈105,000 m²). For Storm Ophelia pre- and post-storm surveys were 6 weeks apart (Guisado-Pintado and Jackson, 2018), whereas for Storm Hector a pre-storm survey was carried out 13 weeks before the event and the post-storm took place 8 days after the event (Table 1). A total of 29 scan station locations were established across the site for each event, ensuring good coverage. We used 6 target spheres as reference Ground Control Points (G) (Figure 1B) georeferenced into the Irish National Grid coordinate system using a RTK dGPS (Trimble 5800) with an accuracy of 0.03 m in x, y and z.

TLS-generated Point Clouds were post-processed to create Digital Surface Models (DSMs) from pre- and post- conditions of the storm events. The Geomorphic change detection (GCD) add-in in ArcGIS (Wheaton et al., 2010) allowed surface volume variations (DEM of Difference, DoD) and topographical comparisons (vertical changes) of each pair of surveys. The total volumetric change (dQ_total in m³) relates to the difference in volume of the post- and pre-storm surface utilising the DEM, and areal change (dA_total m²) is the amount of area undergoing erosion/deposition after each event and over subsequent TLS surveys during the recovery process. To guarantee the quantification of morphological changes at different beach sections as well as the comparison of driven impacts of both events (storms Ophelia and Hector), four zones were delimited following Guisado-Pintado and Jackson (2018). These relate to (1) lower areas from mean sea level (MSL) to lowest level surveyed by TLS, (2) an intertidal channel, (3) between MSL and the shoreline (dune toe) and; (4) the upper area from the shoreline to the top of the dune edge (Figure 2C). Finally, five cross-shore profiles (Figures 1, 2) extracted from DTM surveys where analysed to assess alongshore and cross-shore morphological changes between the two storm events.

Wind is one of the main environmental variables that dictates the characteristics of locally generated waves at the coast, with speed and direction of wind critical in driving potential beach impacts from a storm. Wind patterns and resulting wave climates during storms Ophelia and Hector were significantly different, with storm events that occurred between the pre- and post TLS surveys also being dissimilar in power, orientation and duration.

During Storm Ophelia, winds reached a maximum of 38 knots (70 km/h) at Malin Head station which were sustained for around 3 h (Figure 4A). However, as shown in Figure 4B, the wind direction was mostly shore-parallel and varied from a predominantly southern direction (about 180°) to SSW (220°) as the storm passed through the site (17th October 2017 at around 18:00 h). At Malin Head during the passage of the Storm Ophelia, sea level pressure reached a minimum of 979 hPa (Figure 4C). Storm Hector’s pattern was somewhat different. A maximum wind speed of 37 knots was recorded during the storm but lasted for only 1 h (see Figure 4A), although sustained winds over 18 knots were recorded for the remainder of the storm. Predominantly onshore winds with an average direction of 220–270° (WSW) were recorded over the site during the passage of Storm Hector. Minimum pressure at sea level never fell below 985 hPa (Figure 4C). In both cases, mean wind speeds during the storms were around 27 knots at Malin Head station (Figure 4A) and changes in wind orientation conditions produced a 5–10 knots decrease in wind speeds (Figures 4A,B).

The hourly evolution of Storm Ophelia along the western seaboard of Ireland (Figures 3A–C) reveals that the storm’s intensity gradually dissipated as it travelled north, whereas during Storm Hector energy conditions reached their maximum level at northern Irish latitudes as the storm moved toward the western seaboard (Figures 3D–F. In the case of Storm Ophelia, and according to Guisado-Pintado and Jackson (2018), three storm wave events (onshore directed with orientation varying from 318° to 324°) were recorded in between topographic and TLS surveys and before the occurrence of Storm Ophelia (see Table 2 and Figure 5A). The peak of the first storm (S1) occurred on the 2nd Oct at 03:00 am during high tide (1.21 m), producing a total water level (TWL) of 2.13 m. A second highest synchronisation between wave height and water levels took place on the 4th October between 16:30 and 18:00 h, where TWL peaked to 2.2 m (see Figure 5C). The second storm (S2: 10th Oct 18:00 h) began 5 days later and over its duration was coincident with several cycles of high tide (1.47 m) resulting in a maximum TWL of 2.5 m and thus above average dune toe height (2 m height as shown in Figure 2D). S2 peaked during the night of 10th October at 9:00 pm and couldn’t be recorded by the TLC but hours later (11th October between 09:00 and 10:00 am) a second peak resulted in TWL of 2.07 m as evidenced in Figure 5D, affecting the dune toe line in the most southern sections. Six hours after the end of S2, a third storm occurred (S3: 13th Oct 15:00 h). S3 presented two peaks, one on the 14^th at midnight with a maximum TWL of 1.86 m and a second the 14th at midday where TWLs were around 1.8 m, in both cases below mean dune toe height (Figure 5E). Twenty-four hours after the end of S3, at 3:00 am, Storm Ophelia struck south western Ireland. At the Five Finger site, on the 16th October at around 6:00 pm, the storm was coincident with a low tide, producing TWLs up to 1.4 m and hence below the average height of the dune toe at the time as shown in Figure 5F. Given the fast tracking speed of Storm Ophelia, sustained high wave energy conditions were limited and the average significant wave height was <2.5 m at Five Fingers Strand. Interestingly, using the standard storm analysis, which includes a certain threshold criteria (duration and 10% of exceedance of significant wave height), Storm Ophelia may not actually be characterised as a storm ‘event’. On the other hand, two of the three storm events (S1 and S2) that impacted Five Finger before Storm Ophelia, had the potential to produce dune trimming and shoreline retreat with the calculated TWLs (Figure 5B) and observed time-lapse images (Figures 5C,D). Neither Storm Ophelia nor the preceding storm (S3), however, had synchronised conditions (coincident with high tide and sustained onshore winds) with which to impact the dunes.

Storm Hector hit the western coast of Ireland during the 14th of June 2018. As the storm travelled north it increased in strength and the strongest winds were recorded across Northern Ireland before spreading eastwards across Scotland, northern England and north Wales. The storm lasted for 21 h and a maximum wave height of 4.06 m was recorded for the site. At the peak of the storm (14th June at 06:00 am), maximum water levels of 2.92 m were reached, given its coincidence with spring tide (2.21 m) and provoking the storm surge surpassing the average dune toe height for the site (Figure 2D). Wave run-up, as recorded by the time-lapse camera (see Figure 6D), reached the dune toe across the beach. However, in the spring of 2018, two other storms preceded Storm Hector at the site (Figure 6A and Table 2). The first storm (S1) began on the 17th April with its peak coincident with high tide (1.37 m) although TWLs achieved (2.3 m) were slightly below average dune toe height for north sectors but could have affected the dune toe in southern areas (see Figure 6B). One month later, S2 began on the 9^th of May and lasted for 57 h. Despite its duration, the storm was coincident with neap tides and although morphological changes in the intertidal and supratidal occurred, wave run-up did not effectively reach the dune base. The peak of S2 occurred at 3:00 am on the 10^th of May with a TWL of 2.02 m. A second peak took place 12 h later, resulting in maximum water levels below dune toe (TWL = 1.59 m) as can be seen in the time-lapse image (Figure 6C).

A key aspect of our study is the use of storm proxies to examine individual storm events in terms of intensity, duration and potential morphological impact. Resulting proxies were calculated for the seven events: three preceding events before Storm Ophelia, Ophelia itself, two storms preceding Storm Hector and for Storm Hector. Table 3 shows calculated proxies and some significant differences between storm events. Previous to Storm Ophelia, S1 displayed the greater SSI and SIP values, inducing significant dune trimming (SDT = 0.11) due to both its duration (more 100 h) and its coincidence with several cycles of high tides. Similarly, S2, which occurred 10 days before Storm Ophelia, had a Storm Severity Index of 721 with a strong potential morphological impact (SIP = 566.12) and impacted the dunes as confirmed with the SDT value and TLC images (Figure 5D). On the other hand, S3 and Storm Ophelia resulted in lower values of SSI and SIP. In both cases, storm duration was too short to cause morphological impact in the intertidal and dune field (SDT < 0.05), although Storm Ophelia with shore-parallel winds could have had an important role in sediment transport across the base of the frontal dune line and supratidal areas.

Storm proxies for the events preceding Storm Hector as well as Storm Hector itself, show that despite S2 (May 2018) presenting similar and greater values of SSI and SIP respectively than Storm Hector, the effective morphological impact on the dunes and subaerial beach was minor (SDT = 0.01). The S2 had an overall duration of 60 h but it coincided with neap tides resulting in lower TWLs. On the contrary, Storm Hector despite it only lasting a third of S2 duration, was synchronised with Spring tides resulting in a SDT of 0.9 and thus giving dune toe erosion (Table 3 and Figure 6C).

The analysis of the morphological response from both events was carried out using two pairs of DTMs from TLS point clouds at 0.5 m resolution. As described by Guisado-Pintado et al. (2019), this method helps compare topographic and volumetric changes between surveys across a beach/dune continuum. Total volumetric change (dQ_total in m³) relates to a difference in volume between the post- and pre-storm periods using the DEM, whereas areal change (dA_total m²) is the area with erosion/deposition.

The morphological response after Storm Ophelia shows that volumetric differences are significantly focussed in the alongshore, with a distinct North-South picture revealed (Figure 7A). The blue colour scale shows areas where deposition (gains) is prevalent, with a deposition volume of +9,464 m³ covering 0.39 km² (around 38% of the surface) and located around the intertidal channel with a net gains in volume of +7,309 m³ (Figure 9B) for the whole area (Guisado-Pintado and Jackson, 2018). This deposition is particular evident across profiles P1 and P2 and P4 where the differences in elevation are around +0.5 m. Red areas represent erosion/sediment loss and is represented by 62% of the total surface area. A total volume of 15,481 m³, representing 0.66 km², is ubiquitous over the lower (−7,140 m³) and upper beach area (−6,040 m³; Figure 9B) and represents −0.8 m in elevation change around profiles P3 and P5, as shown in Figures 8C,E. Finally, the southern dune face (at P4 and P5), where the minimum dune toe heights are found, accounts for a loss of −1,860 m³ due to dune trimming during the previous storms (around 0.5 m change in elevation) as shown in Figures 7A, 8D,E.

For Storm Hector, the spatial topographic changes detected in the DoD analysis suggest that a northern lobe of sand was stripped and relocated to the middle-north of the study area (Figure 7B), whereas southern profiles underwent erosion (Figures 8D,E). A general lowering of the southern area (≈−0.20 m of elevation change) and across all zones is found (with a net loss of −7,400 m³, Figure 9C) and 97% of area experiencing erosion. Further north (P1-P2), the blue colour scale indicate areas of deposition (gains) which dominates 86.5% of the northern part of the study site, with a total volume of deposition of +14,300 m^3. Across this area, changes in elevation reach up to +0.45 m, particularly across P1 as shown in Figure 8A. The area comprising the dunes shows a pattern of the higher portions of dune with erosion (red) suggesting that sediment had slumped to the base of dune (blue) with an exception at P3 (Figure 8C). This result is in line with previous results that showed that a maximum TWL of 3 m was above dune toe height in the majority of the coastal stretch (except around P1 and P2), invoking dune trimming during the storm. Finally, it is clear that sediment gains occurred in the lower beach (75% deposition with an average volume of 10,000 m³, Figure 9C) and in the dune area (91% deposition at P1 and P5 as in Figures 8A,E) whereas losses are evident in the upper beach around the southern part of the study site (P3–P5), where it accounts for −3,900 m³ (60% of the zone experienced lowering), and in the intertidal area (Figure 9B), except at P3 which small changes are detected.

Overall morphological changes induced by the seven storms analysed represent a positive net volume difference of 772 m³ in the lower beach area and 6,596 m³ in the intertidal as shown in Figure 9A. Negative net volume differences are found in the upper beach area, where after the balance of the erosion and deposition induced by the storms, represents a net volume of –6,859 m³. Finally, the dune facing area shows the smallest net volume difference accounting for the comparison of the two DoD with a value of +244 m³.

The duration of a storm (effectively the speed of the storm track) largely commands how long the high magnitude event will retain those energy levels at a particular site and thereby enhances the coincidence with high-tide. Storm Ophelia for example, was a rapidly tracking event, albeit of very high energy levels, which passed over the site relatively quickly, reducing its impact compared to Storm Hector which pushed its energy into a more temporally concentrated period to create more sustained conditions. Storms that are ‘held’ in place longer will naturally coincide with the regular high tide level cycles at sites and therefore impact, particularly dune toe erosion from wave run-up processes, will be magnified (e.g., Cooper et al., 2004). This ‘held’ erosive phase unfolded during Storm Hector, where we saw much more dune toe erosion than with Storm Ophelia (see Figure 7). The Storm Dune Trimming index (SDT) value for Storm Hector for example, was over four times greater than during Storm Ophelia, indicating that the storm was coincident with high tide levels (Figure 6D). Similarly, Storm Hector had a Storm Severity Index (SSI) over three times that of Storm Ophelia (Table 3) which suggests that overall, this induced a more severe morphological impact on the beach/dune which is in line with Figure 7B, where substantial changes in topography are more evident. In this sense, the proposed proxy seems to represent quite well the potential dune trimming from a storm event. Furthermore, storm events that occurred before Storm Ophelia (i.e., S1 and S2) showed high values of SSI, SIP and STD, suggesting that some of the intertidal changes observed in the post-storm survey could be associated with the passages of these two high-energy events as suggested by Guisado-Pintado and Jackson (2018).

The Storm Impact Potential (SIP) of Storm Hector was 1.67 times that of Storm Ophelia’s suggesting a much more effective sediment movement and displacement of intertidal and supratidal beach sand under Storm Hector conditions. This again is likely due to Storm Hector being effectively held over the site longer and forming a more effective direction (onshore winds with W to SW orientation) than Storm Ophelia, which gave time for waves to modify topography.

Comparison of pre- and post-Storms Ophelia and Hector surface topographies and their cross-shore profiles, the two major storms (Figures 7, 8), show localised displacement patterns that illustrate interesting responses of the beach surface. This finding indicates where waves have preferentially stripped and deposited sand as result of forcing in pre-storm and during storm conditions. For Storm Ophelia, the upper and lower sections of the beach show strong erosional patterns, with a central band of deposition running along North to South as previously discussed by Guisado-Pintado and Jackson (2018). For Storm Hector, sediment has been removed primarily from the northern and southern sections (upper and lower beach as seen in P2, P4 and P5) and then relocated further to the middle of the beach (around P3), depositing and building up the beach surface local to the erosion (Figures 7B, 8C). Further erosional lowering of the intertidal beach is then evident, again in the southern sections of the site (Figures 8D,E). Storm Hector’s ‘held’ position may have allowed more focussing of wave energy with a more onshore component of the storm compared to Storm Ophelia given that storm duration was coincident with onshore winds that were highly synchronised with high tide levels at the site. This resulted in a significant change on the front dunes, undergoing sediment loss due to dune toe trimming and resultant collapse that can be seen as depositional areas (91% of deposition) in the DoD (Figures 7B, 9C). In the case of Storm Ophelia, cross-shore sediment transport processes seem to be responsible for sediment lost in the upper beach and in the intertidal channel whereas morphological changes observed along the dune face are indicative of alongshore processes and exchanges. This finding shows that alongshore dune retreat is heterogeneous and driven by particular dune morphology present, antecedent beach volumes and likely nearshore bathymetric configurations.

Further, the observed vertical dune face differences in P4 (Figure 8D) between Storm Ophelia and Storm Hector seems to be the result of large-scale slumping and sediment cascading down the front dune face. Sediment is then re-worked and trimmed vertically to create the new profile found around the time of the Storm Hector event. In Profile 5 (Figure 8E), the lateral retreat of the dune face is due to wave erosion of the dune toe and sediment being moved offshore onto the back beach profile (vertical accretion pattern compared to Storm Ophelia cross-shore profiles). However, the changes found in the same profile, for instance the offset of P5, is likely the result of all intervening (smaller) storm and lower energy events occurring in the time period between the field campaigns (September 2017 to June 2018) and not just the product of the two storms, particularly Hector.

Differences in the alongshore response to high-energy events have been widely discussed. Several authors have argued that this range of responses can play out at different scales, from meters up to 1 km, depending on a combination of physical and dynamic variables (Harley et al., 2009, Gervais et al., 2012; Haerens et al., 2012; Houser, 2013; Loureiro et al., 2014; Splinter et al., 2018). A myriad of physical static factors such as coastline orientation and exposure to predominant wave direction, existence of emerged landforms (capes and rocky headlands), submerged morphological features (nearshore bars), underlying geological control (Jackson et al., 2005; Jackson and Cooper, 2009) as well as antecedent morphology conditions (e.g., beach slope, intertidal channels), and dynamic environmental variables such as wave height and tidal levels, wind speed, direction and duration and the synchronicity of all of them can lessen or enhance actual beach response. In terms of quantification of areal and volumetric changes (Figure 9), a noticeable variability alongshore (Figure 7) and in the cross-shore (Figure 8) exists. In Storms Ophelia and Hector, post-storm topography, an alongshore variability pattern is evident whereas the spatial configuration of sediment exchanges and actual distribution are dissimilar. This result suggests the important role of environmental variables (particularly wave energy and run up) not only in the generation of morphological changes but in the reorganising of sediment into different available spaces (Coco et al., 2014) along and across the beach/dune system. This is particularly the case for pre-Storm Hector intertidal beach configuration where summer conditions may have accentuated morphology by the presence of nearshore bars (qualitative TLC imagery reinforces the presence of bars).

Pre-storm events (S1-S2 for Storm Ophelia) may also have played a very important role in sculpting the intertidal topography so they cannot be ignored in the analysis. As a result, their cumulative effect may have added significantly to the actual Storm Ophelia event. Two of the three pre-Ophelia events have significantly higher SSI and SIPs and resulting SDTs than Storm Ophelia, sometimes by a huge margin. In addition, two of these events coincided with high tide and thus TWLs reached were above mean dune toe heights (Figures 5C,D and Table 2), particularly along the southern part of the site. Likewise, the two preceding storms of Storm Hector were (combined) at or exceeding the values of its SSI and SIP, albeit resulting in half of the SDT of the Storm Hector event itself, potentially due to neither of them accounting for TWL above the mean dune toe height or neither of them lasted beyond a high tide cycle whereas Storm Hector did (Figure 6D and Table 2). This highlights the importance of any pre-events that help adjust the antecedent surface beach conditions and possibly dictating future beach response, depending on pre-established surface heterogeneity.

It must also be recognised that any post-storm recovery of the intertidal and dune toe area may restore the beach topography into somewhat different configurations than its previous form and therefore subsequent hydrodynamics may undergo a different set of behavioural patterns than that of previous events.