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Article

Beach Nourishment Protection against Storms for Contrasting Backshore Typologies

by
Filipa S. B. F. Oliveira
*,
André B. Fortunato
and
Paula Freire
Laboratório Nacional de Engenharia Civil, Av. do Brasil 101, 1700-066 Lisboa, Portugal
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2024, 12(9), 1465; https://doi.org/10.3390/jmse12091465
Submission received: 23 July 2024 / Revised: 20 August 2024 / Accepted: 21 August 2024 / Published: 23 August 2024
(This article belongs to the Section Coastal Engineering)
Browse Figures
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Abstract

:
The protection against a storm event provided by nourishment to Costa da Caparica beaches near Lisbon, Portugal, is investigated numerically with a two-dimensional-horizontal morphodynamic model able to generate and propagate the longer infragravity waves. The beach has a groyne field and a multi-typology backshore. The nourishment of 106 m3 of sand was placed at the beach face and backshore. Pre- and post-nourishment topo-bathymetric surveys of the beach, which suffers from chronic erosion, were performed under a monitoring program. The morphodynamics of the pre- and post-nourished beach when exposed to a simulated historically damaging storm event and the post-storm morphologies were compared to evaluate the efficacy of the nourishment. Results indicate that the lower surface level of the beach face and backshore of the pre-nourished beach induces a larger erosion volume. The nourishment prevented the extreme retreat of the shoreline that occurred during the storm in the pre-nourished beach and reduced the storm-induced erosion volume by 20%, thus protecting the beach effectively against the storm. The beach backshore typology (seawall vs. dune) exerts differential influences on the sandy bottom. As a result, multi-typology backshores induce alongshore variability in cross-shore dynamics. The backshore seawalls exposed to direct wave action cause higher erosion volumes and a larger cross-shore extension of the active zone. The most vulnerable alongshore sectors of the beach were identified and related to the mechanisms responsible for the erosion phenomenon. These findings strengthen the importance of sand nourishment for the protection and sustainability of beaches, particularly those with a seawall at the backshore, where storm events cause higher erosion.

1. Introduction

Global warming-induced climate change and anthropogenic pressure are expected to increase coastal hazards such as erosion and flooding [1,2,3,4]. The former weakens beaches through sea level rise, growing storminess, and associated flooding events, and the latter weakens beaches through the limitation of their natural width variation due to territorial occupation with urban development and supporting infrastructures. Both factors contribute to the narrowing and reduction in the robustness of sandy beaches, which are critical for coastal erosion mitigation [5].
Beach nourishment has been increasing in popularity as a coastal management strategy against acute and chronic coastal erosion resulting from storms and longshore transport imbalances [6]. The nourishment can be placed in three areas within the cross-shore active profile: (i) the subaerial beach (i.e., beach face and backshore); (ii) the surf zone; and (iii) the shoreface (i.e., seaward of the surf zone). In practice, the choice of a nourishment location may depend on strategic considerations, such as economic viability based on cost–benefit analyses, desired outcome, the origin of the nourishment material, or the aim of the nourishment (e.g., repair of an eroded beach face or a long-term feed supply). For an offshore borrow area, nourishing the upper shoreface or the surf zone is generally cheaper than the subaerial beach.
Beaches experiencing chronic erosion and valued for recreation and bathing often undergo subaerial nourishment to mitigate erosion, promoting the long-term viability of these uses. However, knowledge gaps persist regarding the resilience of these nourishments during storms, their effectiveness in storm protection, and the influence of the backshore on overall beach response. Numerical coastal models serve as an indispensable tool to elucidate the underlying gaps, enabling, when appropriately calibrated, a vast array of tests under conditions far exceeding real-world scenarios.
The process-based numerical model XBeach [7] has been previously applied to predict the potential effects of various nourishment strategies to mitigate beach-dune erosion on the Dutch coast [8]. The authors investigated the impact of the nourishment location in the active zone of the cross-shore profile, assuming an alongshore uniform topo-bathymetry. They concluded that the nourishment reduces the dune erosion volume more effectively if added to the volume of the front dune instead of added (i) to the total active profile, or (ii) to the surf zone and beach face, or (iii) to reinforce the inner and outer bars.
This study assesses the morphodynamics of the Costa da Caparica beaches near Lisbon, Portugal, caused by an extreme maritime storm, for a pre- and post-nourished beach face and backshore through a locally validated morphodynamic numerical model, XBeach version 1.23.5526. The ultimate goal is to contribute to a better understanding of the impact of nourishment in the protection of beaches against storms, the response of beaches to storms, and the influence of the beach backshore typology (seawall vs. dune) in this response. Fulfilling this objective will provide a better understanding of the sediment dynamics of the Costa da Caparica beaches and scientifically-based guidance on future short- and medium-term nourishment-based protection strategies. This study provides insight into the impact of the nourishment along a coastal stretch with a multi-typology backshore: it includes a natural beach stretch with an active dune at the backshore, and a highly intervened beach stretch, where the sediment fluxes are influenced by a seawall at the backshore and a groyne field. The combination of the study site structures adds complexity to the hydrodynamics-structures-sediments interactions and consequently to the alongshore topo-bathymetric variability, making the acquired knowledge relevant to be generalized to other coastal regions with similar problems. The present work is an inspiring step for further and deeper investigation.
This paper is organized as follows. First, the study site is characterized, and its hydro-sedimentological dynamics are analyzed through the available literature (Section 2). The pre-storm topo-bathymetric conditions, the hydrodynamic storm conditions, and the numerical modeling system are described in Section 3. Section 4 describes the results of the spatial distribution of the nourishment that distinguishes the topo-bathymetry of the numerical tests, the wave climate and sea level in front of the beach, the morphodynamic model validation, and the storm impact in the pre- and post-nourishment conditions. Section 5 discusses the results. The conclusions are summarized in Section 6.

2. Study Site

The coastal zone under analysis is the Costa da Caparica beach, near Lisbon, Portugal. It is located south of the Tagus Estuary inlet, the second-largest European estuary on the western Portuguese coast. It is under the influence of the Atlantic Ocean, the Tagus Estuary, and the coast (Figure 1).
The Costa da Caparica shoreline is about 3.4 km long. The northwest sector, designated as S. João beach, has a 1.4 km long shoreline with a main orientation of 140° N and is limited alongshore by the groynes EV1 and EC7, at northwest and southeast, respectively (Figure 1). Throughout most of the backshore, a naturally active dune system prevails. However, a 460 m long rock armor seawall limits the backshore at the southeast extreme.
The central and southeast sectors, designated as urban beaches, have a 2 km long shoreline with a dominant orientation of 150° N. Both sectors have rock armor seawalls at the backshore. These urban beaches are limited alongshore by the groynes EC7 and EC1, at northwest and southeast, respectively, and are crossed by the five groynes EC6, EC5, EC4, EC3, and EC2 (Figure 1). The length of the beaches between groynes varies between 230 and 375 m. The length of the groynes varies between 100 and 250 m for groynes EC5 and EC6, respectively.
Significant morphological variations in the coastal zone of Costa da Caparica started to be observed in nautical charts in 1929 when the sand spit anchored at the northern extreme of the stretch started to retreat [9,10]. This erosive trend continued during the second half of the 20th century. The dune along Costa da Caparica beach retreated by about 100 m between 1957 and 1963, and its height decreased by 6 m [11]. Most of the coastal defense structures of the urban maritime frontage at the central and southeast sectors of Costa da Caparica were built in the years 1959–1971 in order to stabilize the shoreline and, tentatively, increase the beach width [12].
According to [13], between 1958 and 2010, the northern sector of Costa da Caparica exhibited a maximum retreat of −4.57 ± 0.2 m/year. These data agree with the findings of [14], who reported mean retreat rates of −3.09 ± 1.12 m/year at S. João da Caparica and 1.69 ± 1.94 m/year at the urban beaches from the assessment of shoreline evolution data between 1958 and 2013.
An erosion overtopping outbreak that demanded emergency interventions (such as the ripping and pushing of the sand from the lower beach face to the dune and the rehabilitation of the defense structures) took place in the winters of 2002/2003, 2003/2004, and 2006/2007.
In accordance with a comprehensive coastal management master plan [15], the Costa da Caparica beach underwent an extensive beach nourishment program. A cumulative sand volume of 4.5 × 106 m3 was placed in five phases (2007, 2008, 2009, 2014, and 2019) to nourish the upper shoreface and subaerial beach (encompassing both the beach face and backshore) (Table 1). The sand borrow site was the inlet channel banks, located 7 to 9 km from the project site. The dredging operations were executed by the Lisbon Port Authority for the maintenance of the navigation channel [16]. The project aimed to mitigate chronic erosion, protect the coastal structures, protect the maritime frontage of the town against coastal overtopping and flooding due to extreme hydro-meteorological events, and enhance the beach for recreational purposes.
The wave regime is dominated by long-period swells generated in the North Atlantic Ocean. During the maritime winter (from October to March), offshore mean wave conditions correspond to significant wave heights of 2.5 m, peak periods of 12.1 s, and mean wave direction of 305° N [17,18]. Due to protection provided by the Roca Cape at the north, the estuary is partially sheltered from waves incoming from the prevailing northwest direction but is fully exposed to waves coming from the southeast and west, which can reach significant wave heights of 10 m during extreme storms [19]. Tides are semi-diurnal, with ranges between 0.55 m and 3.86 m [20].
A two-year (2011–2013) beach monitoring program revealed that the Costa da Caparica beach is composed of medium sand, with a median grain diameter (D50) of 0.3 mm [21].
The Costa da Caparica coastal stretch experiences a northward net longshore sediment transport with an annual average magnitude on the order of 50 × 103 m3/year. This directional preference arises from an imbalance in the larger bulk transport volumes moving in both directions [22]. At the groyne-confined urban beaches, the bulk transport is lower than that at non-confined beaches due to the groynes blocking. At S. João da Caparica, inter-annual variability in the net longshore sediment transport rates and direction was identified despite the long-term average value being directed northward. The sediment dynamics of the Costa da Caparica beach are closely linked with the Tagus Estuary inlet dynamics, with complex sediment exchanges between the beach and the inlet southern sand banks. Through the establishment of a conceptual model for the dynamics of the morphological seasonal behavior of the inlet, tested with a two-dimensional-horizontal (2DH) process-based morphodynamic model combined with data analysis, it was concluded on the high probability that the Costa da Caparica beach can be nourished by sediments from the southern part of the ebb delta (from sediment deposits formed during the summer) under specific wave conditions (waves from the west with significant wave heights of about 4 m) [12].
Maritime storms have caused severe damage to the Costa da Caparica beach. In the northern sector, the S. João beach, they caused the retreat of the shoreline, the lowering of the beach face, and the frontal dune overwashing and inundation at the southern extreme (where a rock armor seawall was built later). In the central and southeast sectors, the urban beaches caused seawall overtopping, the inundation of the maritime frontage, and structural damages.
Despite the lack of rigorous quantitative knowledge on the effect of maritime storms on the morphology of the total active beach and on the volumes of inundation due to overtopping, the impact of the storm dated 19 January 2013 in the morphology of the subaerial S. João beach, between the mean sea level (MSL) and the high spring tide water level, was measured through a high-resolution topographic survey performed between 2 December 2012 and 21 January 2013. During this nearly two-month period, observations revealed a sand erosion volume of approximately 50 × 103 m3. Additionally, the mean retreat of the topo-bathymetric line at mean sea level (MSL) was 20 m, with a maximum retreat of 41 m recorded at the southeastern extremity of the beach. The observed retreat exceeded 2 to 3 times the variations observed in 10 quarterly consecutive topographic surveys performed during the monitoring period between March 2011 and June 2013 [23]. The morphological effects observed in the subaerial S. João beach revealed large variability alongshore.

3. Materials and Methods

3.1. Pre- and Post-Nourishment Topo-Bathymetric Conditions

A sand volume of 106 m3 was placed at the beach face and backshore between groyne EV1 and about 500 m south of groyne EC1 (along a 3.8 km extension) in August 2008. The topo-bathymetries of Costa da Caparica were surveyed before and after the nourishment, in July–August 2008 and November 2008, under the COSMO monitoring program [24]. A digital terrain model (DTM) was generated for each topo-bathymetric survey (Figure 2a,b). The differences in beach levels were obtained through the subtraction of the DTM of the pre-nourished beach from the DTM of the post-nourished beach. The result is presented and analyzed in Section 4.1.

3.2. Hydrodynamic Storm Conditions

The maritime storm that occurred in the first fortnight of January 2014, named Hercules, was forced at the offshore boundary and propagated over the pre- and post-nourished topo-bathymetries. The event was not a typical winter storm but rather an extreme storm that inflicted severe damage along the Portuguese coast. The hydrodynamic conditions were based on (i) the results of an implementation of the regional model WaveWatch III in the North Atlantic [25] for the wave climate offshore Costa da Caparica and (ii) a criterion of storm duration previously applied at the Portuguese west coast [26]. Following this criterion, the start of the event was considered as the date of the first of a record sequence with significant wave height, Hs, higher than 4.5 m during a minimum period of 24-h, and the end of the event was defined as the date of the last record with Hs higher than 4.5 m, providing that during the two consecutive days, values higher than 4.5 m are not registered. The selection of a two-day window for isolating and analyzing individual storm events is based on the assumption of their independence. This criterion aligns with established practices for the highly dynamic Portuguese west coast, characterized by significant inter-annual and seasonal variations in wave patterns. Furthermore, this approach is consistent with commonly employed metrics used to define wave height thresholds for storm event identification [18].

3.3. Numerical Modelling

Two geospatial scales, regional and intermediate, were used to model the hydrodynamics and calculate the ocean boundary conditions at the offshore boundary of the beach morphodynamic model (Figure 3). The generation and propagation of the wave climate at a large scale were simulated with the WaveWatch III model [27]. The model SCHISM [28] was used to propagate the tide at large scale [29]. The hydrodynamics at an intermediate (estuarine and coastal) scale, considering the tide, the atmospheric forcing (wind and atmospheric pressure), the river flow, and the wave climate, were simulated using the SCHISM-WWM model [17]. Waves and currents were fully coupled in the intermediate scale model, as tidal currents were shown to have a significant effect on wave propagation in this region [30,31].
The XBeach morphodynamic model version 1.23.5526 was applied in the 2DH surfbeat mode to assess the storm morphological effects over the pre- and post-nourished beach. The XBeach model (open-source) has been described extensively (e.g., [9,32,33]). The numerical modeling domain was sized to encompass the entire area covered by topo-bathymetric monitoring. This choice ensures sufficient resolution of the relevant physical processes. Specifically, it guarantees that short wind waves reaching the offshore boundary remain unbroken while still enabling the realistic simulation of infragravity wave generation, a phenomenon believed to be a significant driver of beach and dune erosion. Thus, two morphological domains were built over the same uniform grid, with cell size 5 × 5 m2. The domain extended 3410 m alongshore and 990 m cross-shore.
The surfbeat solver of the model was forced with the sea level and two-dimensional (frequency and direction) spectral wave series calculated by the estuarine model for the Hercules storm event at the absorbing-generating offshore boundary. Neumann conditions were prescribed at the lateral boundaries. The single_dir parameter was set to 1 in order to solve the short-wave direction at regular intervals using the stationary solver and then propagating the wave energy along the mean wave direction. This approach preserves the groupiness of the waves, thus leading to more forcing of the infragravity waves. The single directional bin for the instationary mode, dtheta, was computed considering the lower and upper directional limits, thetamin = 30° and thetamax = 150°, using the nautical convention. For the stationary solver, the bin size dtheta_s = 5° was considered. It was considered wave dissipation by breaking, as defined in [34]. To overcome the undesired effects of steepening of wave groups resulting from the application of the second-order upwind scheme used to solve wave propagation, the warmbeam scheme was applied. The latter implies a small additional diffusion term, which depends on the time step and group velocity and improves the surfbeat solver stability [35]. The morphological acceleration factor was set to 5 to speed up the computations.

4. Results

4.1. Spatial Distribution of the Nourishment

Sand volume changes were quantified between the reference bathymetric surveys of July–August 2008 and November 2008 using digital terrain models (DTMs). Calculations were performed within the overlapping area (approximately 3 km2) of these DTMs. Specifically, the volume of sand between horizontal planes at three elevations (−9 m CD, −1 m CD, and +2 m CD, relative to the nautical chart datum, CD) was computed. The comparison of these volumes showed that between surveys, the nourishment was kept above elevation 1 m CD, with most of the volume (640 × 103 m3) above +2 m CD (Table 2). The analysis of the spatial distribution of the sand nourishment revealed a slightly lower volume deposited at the northwest extreme of the beach compared to other sectors (Figure 4). Additionally, some of the nourishment sand filled the scour previously observed around the heads of the longest groynes in the pre-nourishment survey.

4.2. Wave Climate and Sea Level

The Hercules storm event lasted 3.88 days, from 2014-01-04 04:00 to 2014-01-08 01:10 (yyyy-mm-dd hh:mm). Two Hs peaks occurred offshore Costa da Caparica (Figure 5). The first, with a magnitude 5.9 m, occurred in 2014-01-04 16:50, 3.7 days after 1 January 2014, and the second, with a magnitude 7.1 m, occurred in 2014-01-06 18:50, 5.8 days after 1 January 2014. Figure 5 also presents the Hs values of two offshore buoys, located to the north and south (Leixões and Sines, respectively) of Costa da Caparica, and the values of the WaveWatch III regional model for the same locations.
The offshore wave and sea level boundary conditions of the morphodynamic model, series of sea level, significant wave height (Hs), peak period (Tp), and mean direction (Dir) were determined through the application of the hydrodynamic intermediate-scale model, SCHISM-WWM (Figure 6). The results show that the incident wave energy was uniform along the seaward boundary of the numerical domain. During the first half of the storm event, the incident wave direction was nearly shore-normal in the middle of the offshore boundary. This point appears to act as a divergence zone since towards the northwest of the offshore boundary, the incident waves rotated to the northeast (increase in Dir), and towards the southeast of the offshore boundary, the waves rotated to the northwest (decrease in Dir). The wave direction exhibits the influence of the tide, particularly during the first half of the storm event. This phenomenon might be related to the interaction between the wave-induced circulation and the ebb tidal jet. In the second half of the event, when the waves reached higher significant heights, the incident wave direction rotated towards northwest along the offshore boundary (decrease in Dir).

4.3. Morphodynamic Model Validation

The validation of the regional and intermediate (hydrodynamic) models has been described elsewhere [12,19,25,29]. Waves, validated for a time period during which Hs reached 5 m, were reproduced at a buoy in front of the Tagus Estuary with root mean square errors of 24 cm (Hs) and 1.3 s (Tp). Below, we focus on the validation of the local (morphodynamic) model.
The XBeach model was validated for the two-month period September–October 2014, after the 106 m3 sand nourishment intervention performed between 26 June and 25 August 2014. The validation period was less energetic (maximum Hs of about 3 m at the offshore boundary) than the Hercules storm (maximum Hs of about 5.5 m at the offshore boundary). The Hercules storm was chosen because it was a very energetic event. The calibration period was chosen because it was the shortest period in which two topo-bathymetric surveys were available for the entire domain and simultaneously because it was the start of the storm season. Topo-bathymetric or solely subaerial beach surveys were not conducted during the validation period of the model, but it is very unlikely that a recovery period occurred during these two months precisely for the start of the storm season.
The morphological evolution of the study zone observed between August 2014 and October 2014 (Figure 7) reveals that the northern and central sectors suffered larger morphological changes than the southern sector. The accretion of sand observed in the submerged part of the active beach between profiles PCC3 and PCC5 (marked in Figure 7) may have resulted from the transport of sand from the southern part of the ebb delta (accumulated during the summer season) towards the northeast to the Costa da Caparica beach, according to the conceptual model for the dynamics of the morphological seasonal behavior of the inlet suggested by [12].
Across the domain, the upper beach was eroded, particularly in the central sector (in the fourth, fifth, and sixth urban beaches counted from the southeast). These erosional trends likely resulted from a combination of factors. First, the commencement of the maritime winter season implies a transition to a higher wave energy environment. The beach profile is consequently adjusting towards a new equilibrium geometry suited to these increased wave forces. Furthermore, the recent beach nourishment operation may have impacted sediment consolidation, thereby altering the overall sediment distribution and bulk transport patterns throughout the beach-active zone. The most significant topo-bathymetric lowering was observed in the southeast extreme of the fifth urban beach counted from the southeast.
To validate the morphodynamic model’s performance, simulations were conducted in stationary mode, employing a phase-averaged solver for wave propagation. As the study area is typically exposed to moderate swell conditions during this period, it is reasonable to assume that infragravity waves exerted a minimal influence on the observed morphological changes on the beach.
The model was validated in one dimensional (1DH) profile mode because the large size of the study area, the need for high grid resolution (5 × 5 m2), and the large (two-month) simulation period make the computational cost of the 2DH model version prohibitive. To overcome this limitation without compromising the accuracy of the simulations, six cross-shore profiles (PCC3, PCC5, PCC7, PCC9, PCC11, and PCC14) with a uniform horizontal grid resolution of 5 m (Figure 7) were used. These six profiles were strategically distributed across the entire study area to achieve comprehensive coverage. Their placement aimed to minimize the numerical uncertainties of the 1DH model by reducing the influence of existing structures, specifically mitigating the potential for disruption of alongshore sediment transport processes and reduced exposure to oblique wave angles. The August 2014 profile simulations were driven by synoptic hydrodynamic data (sea level and wave conditions). This forcing scheme is consistent with the methodology employed for the storm event simulations. Although the 2DH model version is expected to yield more accurate results in areas dominated by 2DH processes (either because of the presence of cross-shore structures or alongshore morphological gradients), the results of the model validation proved the viability of the approach applied.
The morphodynamic model performance was evaluated through (i) a visual comparison between the numerical and observed profiles (Figure 8), (ii) two indicators of impact, and (iii) an indicator of error (Table 3). The indicators of impact used were the volume of erosion per meter of alongshore beach length and the retreat at the levels −1 m, 0 m, and +2 m CD. The Brier Skill Score (BSS) [36] was employed as the error metric for model evaluation.
Overall, the model either reproduced approximately or overestimated the retreat of the beach berm above the beach face. Although the maximum significant wave height (Hs) during the validation period was only 3 m, the two-month study period encompassed the transitional months between the maritime summer and winter. The retreat indicator suggests a good qualitative agreement between the numerical results and the observations, with the exception of profiles PCC9 and PCC14 at the +2 m CD level. Regarding the erosion volume indicator, the model only underestimated this metric for the northern profile, PCC3. It overestimated this indicator for the profiles PCC7 and PCC14 and calculated approximated values for the three other profiles. According to a classification for the performance of morphodynamic numerical models proposed in [36] (0.8 ≤ excellent < 1.0; 0.6 ≤ good < 0.8; 0.3 ≤ reasonable < 0.6; 0 ≤ weak < 0.3; bad < 0), and for an observation error (δ) of 0.15 m, the model’s performance varies between reasonable for the profiles PCC3 and PCC14, and good for the other four profiles.

4.4. Storm Impact in the Pre- and Post-Nourished Conditions

The pre-nourished beach experiences stronger erosion from the beach face than the post-nourished beach (Figure 9b,c). The calculated storm erosion volume in the total stretch is 33 × 103 m3 for the pre-nourished beach and 26 × 103 m3 for the post-nourished beach (Figure 10); that is, the nourishment reduces the storm-induced erosion volume by 21%. The erosion volume per meter alongshore within each cell (defined as a beach confined between groynes) varies between 8.1 and 13.2 m3/m for the pre-nourished case and 7.3–10.1 m3/m for the post-nourished case (Table 4).
At the S. João beach, the one at the northwest sector of the stretch, limited backshore mainly by an active dune, the erosion volume is 14 × 103 m3 and 11 × 103 m3 for the pre-nourished and post-nourished beach, respectively (Figure 10). The calculated erosion volumes are consistent with observations reported by [23]. These authors observed an erosion volume of approximately 50 × 103 m3 during a nearly two-month period (between 2 December 2012 and 21 January 2013) of the 2013 maritime winter, which included a one-day storm event on 19 January 2013. In both pre- and post-nourishment cases, the S. João beach numerical response to the Hercules storm reveals a large alongshore variability of the morphological effects (Figure 9), as previously observed by [23].
Regarding the urban beaches, limited backshore by a seawall, the erosion volumes for each cell are also higher for the pre-nourished beaches (Figure 10). The volumes vary between 1.8–3.3 × 103 m3 for the post-nourished beaches (minimum and maximum in cells EC2–EC3 and EC3–EC4, respectively) and 1.9–4.0 × 103 m3 for the pre-nourished beaches (minimum and maximum in cells EC2–EC3 and EC5–EC6, respectively).
The evolution of eleven inspection profiles, located approximately in the middle of the urban beaches and equidistantly distributed in S. João beach (Figure 9a), are shown in Figure 11. In the urban beaches, the pre- and post-nourished initial beach profiles are similar from offshore up to approximately the bed level −2 m CD. Since the same sediment grain size was considered for both cases, the cause of the higher erosion of the pre-nourished beach is the lower bed level of the beach face (Figure 11). The lower bed level near the seawall is also preceded by a slope that is smoother than the slope of the beach face in the post-nourished beach.
However, there are exceptional locations of lower erosion in the pre-nourished beaches caused by specific features of the pre-storm morphology. The profiles X = 50 and X = 150, located in the first and third beaches counted from the southeast, reveal a slightly lower erosion in the pre-nourishment case (Figure 12). That behavior is due to the initial morphology, which is also characterized by the presence of a nearly 30 m wide platform at level −2 m CD (Figure 13). Such morphological feature contrasts with the almost uniform slope observed between the bed level −2 m CD and the top of the profile in the pre-nourishment initial profile X = 350 (Figure 13), for which the erosion volume is the highest of all (Figure 12). An additional feature that contributes to the larger erosion of the pre-nourished profile X = 350 is the advance of the seawall relative to the position of the seawall in profiles X = 50, X = 100, and X = 150 (Figure 13). The reduction in the dissipation extension and consequent exposure of the seawall to direct wave action cause the generation of large undertow currents that push the sand further offshore. In fact, the more advanced position of the seawall, which is a characteristic of the fourth, fifth, and sixth beaches counted from the southeast (as can be seen in the aerial photograph of Figure 9a), causes larger sand erosion in the respective profiles X = 250, X = 300 and X = 350 in the pre-nourished beach. This trend is not observed on post-nourishment beaches because the nourishment avoids the direct wave action over the seawall.
The morphological changes in the eleven cross-shore inspection profiles (Figure 11) and the post-storm position of topo-bathymetric lines 0 m CD and +2 m CD (approximately the mean sea level) (Figure 14) reveal that the nourishment prevented the extreme retreat of the shoreline that occurred during the storm in the pre-nourished beach. The post-storm topo-bathymetric lines 0 m CD and +2 m CD exhibit more pronounced discrepancies between the pre- and post-nourishment cases, particularly for the urban beaches and the southeastern sector of S. João beach. This area is characterized by the presence of a backshore seawall. These observations support the hypothesis that the seawall plays a role in displacing sand seaward. While the presence of the seawall likely played a role in the observed morphologic differences, the potential influence of the nourishment distribution should be considered. Spatial variations in nourishment volume can be seen in Figure 4, with smaller quantities concentrated on the upper beach face at the northwest extreme of the S. João beach (Figure 11j,k). These spatial variations in nourishment may also have contributed to the observed discrepancies between the pre- and post-nourished scenarios.
The seaward limit of the active zone in the urban beaches is located between the heads of the longest groynes, EC6 and EC4, and the heads of the shortest groynes, EC5, EC3, EC2, and EC1 (Figure 9). The cross-shore extension of the active zone is larger in the urban beaches than in the S. João beach. These observations support the influence of the backshore typology (seawall vs. dune) on the behavior of the sandy bottom within the cell [37].
While the seawall effectively prevents shoreline retreat at the top of the beaches, it appears to contribute to a larger overall lowering of the profile within the cell. This is evident despite the presence of the extensive (460 m) seawall at the southeastern extreme of S. João beach. Notably, the S. João beach (1.4 km long, with active dunes along most of its backshore) exhibits the third-lowest pre-nourishment erosion and the second-lowest post-nourishment erosion among all beaches (Table 4). These findings highlight the potential trade-offs inherent to seawall implementation as a coastal protection strategy.

5. Discussion

Pre-nourishment beach morphology was primarily attributable to sand starvation [15], resulting in a lower beach and backshore profile and leading to increased erosion from direct wave action on the seawall [37,38]. However, this study’s results emphasize the additional influence of pre-storm morphological features, such as bed slope and the presence of temporary platforms, on wave transformation and dissipation processes, ultimately impacting beach erosion.
The beach nourishment intervention, with a volume of 106 m³ placed along a 3.8 km stretch of the beach face and backshore, demonstrated efficacy through several key mechanisms:
(i)
Shoreline retreat: the nourishment effectively reduced the retreat of the beach face observed in the absence of intervention, thereby protecting the shoreline.
(ii)
Enhanced beach resistance: a post-storm evaluation of the nourished beach revealed a significant volume of sand remaining on the beach face and backshore, suggesting that the nourishment project has been effective in resisting erosion.
(iii)
Seawall protection: by avoiding or limiting the direct wave action on the seawall, the nourishment mitigates potential structural damage and erosion.
(iv)
Safeguarding infrastructure: the nourished beach reduces the risk of overtopping and inundation events, which have historically affected the area [39], and provides a protective buffer for the beach and urban support infrastructure located landward of the seawall.
The uncertainties of this analysis are mostly due to the morphodynamic model application: the processes not accounted for by the model and the methodological application. The model has adequate predictive skills regarding the main coastal processes that occur in the site’s sandy beaches during storms because it includes long (infragravity) waves, runup and rundown of long waves (swash), and the slumping of sandy material from the dune face (avalanching) [7,32]. However, despite the application of the 2DH model version, diffraction (by the groynes) is not considered. Only sediment blocking and the shadowing zones protected from direct wave action by the groynes were considered. In the post-nourishment case, the beach evolution is likely to be affected by sediment consolidation, a phenomenon that the model also neglects.
The weak points of the model’s methodological application are the following:
(i)
The validation was restricted to the morphological evolution because of nonexistent measured synoptic hydrodynamic data. Despite the inherent hydrodynamic action of morphological evolution, the two components of morphodynamics should be validated separately to control the largest possible number of site-dependent parameters. For a study site 200 km north of Costa da Caparica, ref. [40] concluded that the numerical scheme used to propagate short waves and the breaking criterion were important calibration parameters to improve the surfbeat model performance. In our study, unlike [40], who used the original second-order upwind scheme, we used the warmbeam scheme to improve the surfbeat solver stability as suggested by [35], and we used the same formulation for the wave dissipation by breaking the [34] formulation, but without adjusting the breaking criterion parameter as performed by the authors.
(ii)
The current numerical domain is restricted by the limitations of the existing topo-bathymetric monitoring area. Expanding the monitoring area laterally and further offshore would be beneficial. This expanded data collection would enable a more accurate representation of the physical processes, particularly the influence of long waves [7,32].
(iii)
The selection of the 1DH stationary solver [7] for model validation was driven by a combination of factors: the study area’s size, the need for high grid resolution, the two-month simulation period, and the prevailing low-energy wave conditions. While strategically chosen application profiles minimized errors caused by longitudinal morphodynamic variability, the 2DH model version [7,32] is likely to yield more accurate results overall.
Still, with respect to modeling, the comparative nature of the analysis, focusing on the relative differences between the two cases, pre- and post-nourishment beaches, reduces the impact of uncertainties in the absolute values of the storm-induced morphological changes.
These results point out the importance of the sand nourishments for the protection and sustainability of Costa da Caparica beaches, particularly the urban beaches, where erosion is higher under storm events and the capacity to accumulate sand at the backshore due to aeolian processes is weaker due to the presence of the seawalls at the backshore.

6. Conclusions

This study assesses the impact of nourishment in the protection of a beach against an extreme storm and the influence of the beach backshore typology (seawall vs. dune) in this response. The beach suffers from chronic erosion. It has complex hydro-sedimentological dynamics, not only because of the sediment exchanges with the nearby inlet southern sand bank but also because it has a groyne field and a multi-typology backshore.
This study demonstrates the effectiveness of a 106 m³ beach nourishment in protecting the shoreline from the extreme retreat caused by the storm in unnourished scenarios. The nourishment resulted in a 21% reduction in storm-induced erosion volume.
The findings highlight the significant influence of alongshore variability in cross-shore dynamics during storms. The backshore typology also plays a critical role: seawalls exposed to direct wave action, often found in chronically eroded beaches, lead to higher erosion volumes and a larger active zone cross-shore extension. Additionally, pre-storm seabed features, such as bed slope and the presence of temporary platforms, were shown to influence beach erosion patterns. These combined observations enabled the identification of the study site’s most vulnerable beaches—those experiencing greater storm-induced erosion. The analysis revealed that despite the presence of a seawall limiting shoreline retreat, urban beaches with seawalls were more susceptible to erosion compared to the dune-backed beach. This suggests potential trade-offs associated with seawall implementation as a coastal protection strategy. All these findings offer valuable guidance for the development of future nourishment-based shore protection strategies to mitigate storm-induced acute erosion.
Building upon the findings of how pre-storm morphological characteristics influence storm-induced erosion volume, further investigations are warranted to elucidate the impact of nourished profile geometry, specifically the beach face and backshore slopes, on the resistance of nourished shorelines.
The results also revealed that a significant volume of sand nourishment remained on the beach face and backshore after this single storm event. Further numerical testing on the longevity of the project, that is, on the beach resistance to further storms typical of a winter impact, will provide valuable guidance for beach management.

Author Contributions

Conceptualization, F.S.B.F.O.; methodology, F.S.B.F.O. and A.B.F.; software, F.S.B.F.O. and A.B.F.; validation, F.S.B.F.O.; formal analysis, F.S.B.F.O. and A.B.F.; investigation, F.S.B.F.O.; resources, F.S.B.F.O. and A.B.F.; data curation, F.S.B.F.O. and A.B.F.; writing—original draft preparation, F.S.B.F.O.; writing—review and editing, F.S.B.F.O., A.B.F. and P.F.; visualization, F.S.B.F.O.; supervision, F.S.B.F.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by the Portuguese Environment Agency (APA: Agência Portuguesa do Ambiente).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank Fernando Brito of Laboratório Nacional de Engenharia Civil for the technical support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of Costa da Caparica (west coast of Portugal, Europe), with place names, topo-bathymetry of the Tagus Estuary (color gradient scale), and coastal depth contours 8, 16, and 30 m (adapted from https://geomar.hidrografico.pt/, on 8 April 2024).
Figure 1. Location of Costa da Caparica (west coast of Portugal, Europe), with place names, topo-bathymetry of the Tagus Estuary (color gradient scale), and coastal depth contours 8, 16, and 30 m (adapted from https://geomar.hidrografico.pt/, on 8 April 2024).
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Figure 2. Digital terrain models of the topo-bathymetric surveys of July–August 2008 (a) and November 2008 (b).
Figure 2. Digital terrain models of the topo-bathymetric surveys of July–August 2008 (a) and November 2008 (b).
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Figure 3. Scheme of the numerical models applied.
Figure 3. Scheme of the numerical models applied.
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Figure 4. Spatial distribution of the 106 m3 of sand nourishment performed in August 2008 (∆Z > 0: accretion; ∆Z < 0: erosion).
Figure 4. Spatial distribution of the 106 m3 of sand nourishment performed in August 2008 (∆Z > 0: accretion; ∆Z < 0: erosion).
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Figure 5. Significant wave height, Hs, in the first fortnight of January 2014: records of Leixões and Sines (at north and south respectively) buoys and WaveWatch III model results in the location of the buoys and offshore Costa da Caparica.
Figure 5. Significant wave height, Hs, in the first fortnight of January 2014: records of Leixões and Sines (at north and south respectively) buoys and WaveWatch III model results in the location of the buoys and offshore Costa da Caparica.
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Figure 6. Hydrodynamics at the entrance (northwest, middle, and southeast positions of the offshore boundary of the numerical domain) of the morphodynamic model (results of the intermediate scale model) during the storm Hercules: (a) sea level; (b) significant wave height (Hs); (c) peak period (Tp); and (d) mean wave direction (Dir).
Figure 6. Hydrodynamics at the entrance (northwest, middle, and southeast positions of the offshore boundary of the numerical domain) of the morphodynamic model (results of the intermediate scale model) during the storm Hercules: (a) sea level; (b) significant wave height (Hs); (c) peak period (Tp); and (d) mean wave direction (Dir).
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Figure 7. Comparison of the topo-bathymetric surveys of August 2014 and October 2014 and the location of the shore-normal profiles PCC3, PCC5, PCC7, PCC9, PCC11, and PCC14.
Figure 7. Comparison of the topo-bathymetric surveys of August 2014 and October 2014 and the location of the shore-normal profiles PCC3, PCC5, PCC7, PCC9, PCC11, and PCC14.
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Figure 8. Numerical and observed evolution of the shore-normal profiles (a) PCC3, (b) PCC5, (c) PCC7, (d) PCC9, (e) PCC11, and (f) PCC14, between August 2014 and October 2014.
Figure 8. Numerical and observed evolution of the shore-normal profiles (a) PCC3, (b) PCC5, (c) PCC7, (d) PCC9, (e) PCC11, and (f) PCC14, between August 2014 and October 2014.
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Figure 9. Site view with the location of the inspection profiles (a). Results of the morphological changes in Costa da Caparica during the Hercules storm: evolution over the pre-nourished topo-bathymetry (b), and evolution over the post-nourished topo-bathymetry (c).
Figure 9. Site view with the location of the inspection profiles (a). Results of the morphological changes in Costa da Caparica during the Hercules storm: evolution over the pre-nourished topo-bathymetry (b), and evolution over the post-nourished topo-bathymetry (c).
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Figure 10. Erosion and accretion volumes in the cells between groynes for the pre- and post-nourishment cases.
Figure 10. Erosion and accretion volumes in the cells between groynes for the pre- and post-nourishment cases.
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Figure 11. Results of the morphological changes in the eleven cross-shore inspection profiles, (a) X = 50, (b) X = 100, (c) X = 150, (d) X = 250, (e) X = 300, (f) X = 350, (g) X = 450, (h) X = 500, (i) X = 550, (j) X = 600, and (k) X = 650, for the pre- and post-nourishment cases.
Figure 11. Results of the morphological changes in the eleven cross-shore inspection profiles, (a) X = 50, (b) X = 100, (c) X = 150, (d) X = 250, (e) X = 300, (f) X = 350, (g) X = 450, (h) X = 500, (i) X = 550, (j) X = 600, and (k) X = 650, for the pre- and post-nourishment cases.
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Figure 12. Erosion in the eleven inspection profiles for the pre- and post-nourishment cases.
Figure 12. Erosion in the eleven inspection profiles for the pre- and post-nourishment cases.
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Figure 13. Initial profiles X = 50, X = 100, and X = 350 for the pre- and post-nourishment cases.
Figure 13. Initial profiles X = 50, X = 100, and X = 350 for the pre- and post-nourishment cases.
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Figure 14. Results of the morphological changes: post-storm position of the 0 and +2m CD bathymetric lines for the pre- and post-nourishment cases.
Figure 14. Results of the morphological changes: post-storm position of the 0 and +2m CD bathymetric lines for the pre- and post-nourishment cases.
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Table 1. Nourishment interventions performed in Costa da Caparica between 2007 and 2019.
Table 1. Nourishment interventions performed in Costa da Caparica between 2007 and 2019.
Intervention
Date		Volume Deposited in the Beach Face and Berm [m3]Location of the Deposit
August 20070.5 × 106between EV1 and EC4 (extension 2.4 km)
August 2008106between EV1 and about 500 m south of EC1 (extension 3.8 km)
August 2009106between EV1 and about 500 m south of EC1 (extension 3.8 km)
26 June–25 August 2014106between EV1 and about 500 m south of EC1 (extension 3.8 km)
August 2019106between EV1 and about 500 m south of EC1 (extension 3.8 km)
Table 2. Sediment volume between the horizontal planes at the elevations −9 m CD, −1 m CD, and +2 m CD in the MDT of the topo-bathymetric survey of July–August 2008 (reference survey).
Table 2. Sediment volume between the horizontal planes at the elevations −9 m CD, −1 m CD, and +2 m CD in the MDT of the topo-bathymetric survey of July–August 2008 (reference survey).
Volume × (103) [m3]
Topo-Bathymetric SurveyAbove −9 m CDAbove −1 m CDBelow −1 m CDAbove +2 m CDBelow +2 m CD
July–August 200816.031.7414.290.3015.73
November 200816.752.7314.020.9415.81
Table 3. Impact and error performance indicators of the morphodynamic model.
Table 3. Impact and error performance indicators of the morphodynamic model.
ProfilePerformance Indicators
Impact (Numerical/Observed)Error
Retreat [m] at the LevelsErosion Volume
[m3/m]		BSS
(δ = 0.15/0.3)
−1 m CD0 m CD+2 m CD
PCC320/40−55/−850/077/1410.5/0.6
PCC5−30/−15−25/−3525/30100/1130.7/0.8
PCC7−70/−75−45/−6515/10157/770.7/0.8
PCC9−95/−60−70/−40−10/25199/2150.7/0.8
PCC11−65/−35−45/−3020/15143/1530.6/0.6
PCC14−45/−30−30/−35−10/25135/860.4/0.6
Table 4. Storm-induced erosion per alongshore unit length in pre- and post-nourished beaches.
Table 4. Storm-induced erosion per alongshore unit length in pre- and post-nourished beaches.
Erosion [m3/m]
CellBeach Length [m]Pre-NourishmentPost-Nourishment
EC1-EC229510.69.5
EC2-EC32308.17.8
EC3-EC43759.58.9
EC4-EC528010.27.3
EC5-EC630013.28.8
EC6-EC730012.010.1
EC7-EV114009.97.7
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Oliveira, F.S.B.F.; Fortunato, A.B.; Freire, P. Beach Nourishment Protection against Storms for Contrasting Backshore Typologies. J. Mar. Sci. Eng. 2024, 12, 1465. https://doi.org/10.3390/jmse12091465

AMA Style

Oliveira FSBF, Fortunato AB, Freire P. Beach Nourishment Protection against Storms for Contrasting Backshore Typologies. Journal of Marine Science and Engineering. 2024; 12(9):1465. https://doi.org/10.3390/jmse12091465

Chicago/Turabian Style

Oliveira, Filipa S. B. F., André B. Fortunato, and Paula Freire. 2024. "Beach Nourishment Protection against Storms for Contrasting Backshore Typologies" Journal of Marine Science and Engineering 12, no. 9: 1465. https://doi.org/10.3390/jmse12091465

APA Style

Oliveira, F. S. B. F., Fortunato, A. B., & Freire, P. (2024). Beach Nourishment Protection against Storms for Contrasting Backshore Typologies. Journal of Marine Science and Engineering, 12(9), 1465. https://doi.org/10.3390/jmse12091465

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