*Article* **Morphological Development and Behaviour of a Shoreface Nourishment in the Portuguese Western Coast**

**Celso Aleixo Pinto 1,\* , Rui Taborda <sup>2</sup> , César Andrade <sup>2</sup> , Paulo Baptista <sup>3</sup> , Paulo Alves Silva <sup>4</sup> , Diogo Mendes <sup>5</sup> and Joaquim Pais-Barbosa <sup>4</sup>**


**Abstract:** Current coastal protection strategy in Portugal defines beach and shoreface nourishment as a valid measure to mitigate coastal erosion in some erosional hot-spots, being considered as an adaptation measure under the present climate change scenario, including the impacts of sea level rise. However, scant objective data on shoreface nourishments are available to evaluate performance of this type of intervention in mitigating beach erosion and managing coast risk. We present the first monitoring results of a <sup>≈</sup>2.4 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> shoreface nourishment on the Aveiro coast (Costa Nova—Ílhavo), the largest until now in Portugal, focusing on its morphological development, impacts on adjacent beaches due to alongshore spreading and cross-shore redistribution, and contribution to the sediment budget of the nourished sediment cell. The analyses are based on high-resolution coastal monitoring data, provided by the Portuguese COaStal MOnitoring Program (COSMO). A Multiple Monitoring Cell (MMC) approach was used to evaluate local and feeder efficiency of the nourishment, sediment budget exchanges within both the placement and wider survey domains (≈1 km<sup>2</sup> and 12 km<sup>2</sup> , respectively). Results show rapid (ca. 6 months) morphological change over the placement area, with a decrease of about 40% of the initial volume. Fast onshore sediment redistribution explains part of this change, placed sand having merged with the pre-existing bar system increased the volume of the shallower nearshore. Longshore transport is reflected by increasing the robustness of the bar downdrift of the placement area and also explains the negative sediment budget (0.75 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ) of the survey domain, which corresponds to losses through its southern boundary. Sediment spreading also induced accretion of the subaerial section of Costa Nova beaches in front of the placement area, reversing their long-term erosive trend. In contrast, this trend persisted at downdrift beaches. This suggests that the time lag of the subaerial beach response to this intervention increases with the distance to the placement area, and reversal of the erosive trend will only be noticeable in the following years. This study provides new insights on the time scales of beach response to high-magnitude shoreface interventions in high-energy wave-dominated sandy coasts, which will support decision making regarding similar operations designed to manage erosional hot-spots elsewhere.

**Keywords:** wave dominated coast; coastal erosion; shoreface nourishment; cross-shore and longshore processes; coastal protection strategy

### **1. Introduction**

Coastal areas are inherently dynamic, driven by meteorological, oceanographic, geological, and anthropogenic factors [1]. Sandy beaches occupy more than one-third of the

**Citation:** Pinto, C.A.; Taborda, R.; Andrade, C.; Baptista, P.; Silva, P.A.; Mendes, D.; Pais-Barbosa, J. Morphological Development and Behaviour of a Shoreface Nourishment in the Portuguese Western Coast. *J. Mar. Sci. Eng.* **2022**, *10*, 146. https://doi.org/10.3390/ jmse10020146

Academic Editor: Zhen-Gang Ji

Received: 16 December 2021 Accepted: 19 January 2022 Published: 22 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

world's coastline, with one-quarter under erosion [2], and this trend may be aggravated due to climate change effects, namely, sea level rise [3–5] and changes in frequency and duration of storminess [6,7]. Historical and ongoing development of the coastal area has induced coastal squeeze, leading to the loss of scenic values, recreational beach areas and habitats, and increasing hazards to people, built environments, and infrastructures, especially in densely populated and developed coastlines [8]. Management of these areas is particularly challenging, and can only be adequately addressed if grounded upon comprehensive knowledge of coastal changes and driving processes based on the existence of coastal monitoring data.

Beach nourishment (also referred to as sand replenishment or beach fill) comprises the addition of good quality sand to increase the width or volume of a specific beach or coastal stretch [9]. It is a coastal management technique used in risk reduction and adaptation to climate change worldwide [10]. Beach nourishment has been undertaken in emergency contexts, as a local and short-term remedy solution (e.g., in the aftermath of storm-induced erosion [11]) and as a regional and long-term management strategy to counteract erosive trends and reduce coastal vulnerability [10–14].

The nourishment type depends, among other factors, on coastal management objectives and on sand availability. Sediment is either placed above mean sea level (over the beach berm or foredune), or in the inner (upper) shoreface. While the former is typically called beach nourishment, the latter is often designated as shoreface nourishment. Borrow sediments are typically obtained from maintenance dredging of inlets or navigation channels associated with nearby harbours/fishing ports/recreational marinas. In addition, the outer (lower) shoreface may also provide excellent borrow areas [14–18].

In mainland Portugal, beach nourishment started in 1950, with up to 170 interventions concluded until 2020 (data updated from [14]). Altogether, they comprised placement of approximately 42 M m<sup>3</sup> , over the inner shoreface (thus landward of the depth of closure—DoC) and over the beach/berm and foredune. Only 8% of these interventions may be considered of high magnitude (i.e., volume > 1 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> [14]). To date, nourishments carried out have essentially resorted to sediment dredged from inlets and channels associated with port activities (commercial/fishing/recreation) (about 90% of all operations and of total volume involved). Dredged material was used to minimize erosion caused by port infrastructures, such as sand starvation downdrift of jetties, and enhanced accumulation updrift of jetties and in inlets and channels.

Extensive monitoring of sand redistribution following beach nourishments is crucial to assess project performance and impacts, in addition to improving understanding of the associated coastal dynamics [10,13]. In opposition to subaerial beach nourishment, behaviour of shoreface nourishment is still not well-understood [13]. This contrasts with the increasing importance of the latter worldwide, mostly due to its cost-effectiveness [13]. For example, 43% of nourishment operations carried out in mainland Portugal were of the shoreface type, and comprised 50% of the total placement volume [14].

So far, most data associated with shoreface nourishments have been acquired in a limited suite of environmental settings (e.g., The Netherlands [10,12,13] and USA [19]) with poor representation of high-energy wave dominated coasts, where tides and storm surge are secondary drivers.

The objective of this work is to provide new insights on the morphological evolution, behaviour, and efficiency of a high-magnitude volume shoreface nourishment, undertaken in a high-energy wave-dominated coast, downdrift of a stabilized tidal inlet (Aveiro, Portugal).

The study relies upon high resolution monitoring following the largest shoreface nourishment undertaken in Portugal, encompassing the downdrift domain potentially affected by short-term sediment redistribution. This work extends and complements previous study on the same field area [20], but it differs by introducing a sediment budget approach that allowed the understanding of post-nourishment sediment dispersion. In addition, we investigate the time scale required for the beach to acquire a condition of equilibrium replicating the equilibrium beach profile of Bruun's, following the imposition of a significant sand volume. equilibrium replicating the equilibrium beach profile of Bruun's, following the imposition of a significant sand volume.

affected by short-term sediment redistribution. This work extends and complements previous study on the same field area [20], but it differs by introducing a sediment budget approach that allowed the understanding of post-nourishment sediment dispersion. In addition, we investigate the time scale required for the beach to acquire a condition of

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 3 of 18

#### **2. Study Area 2. Study Area**

#### *2.1. Coastal Setting 2.1. Coastal Setting*

The study area comprises a low-lying beach–dune system that extends 20 km south of Aveiro inlet until Praia de Mira (MI) (north), including Barra (BA), Costa Nova (CS), Vagueira (VG1; VG2), Labrego (LB), Duna Alta (DA), Areão (AE), and Poço da Cruz (CZ) beaches (Figure 1). This coastal stretch has an NNE–SSW orientation, being included in coastal cell 1 (sub-cell 1b), according to an established classification [21]. Coastal defence works in the area include two jetties at Aveiro harbour entrance, nine groynes (E1 to E9), and three rock armour revetments (two in Costa Nova and one at Vagueira beach). The study area comprises a low-lying beach–dune system that extends 20 km south of Aveiro inlet until Praia de Mira (MI) (north), including Barra (BA), Costa Nova (CS), Vagueira (VG1; VG2), Labrego (LB), Duna Alta (DA), Areão (AE), and Poço da Cruz (CZ) beaches (Figure 1). This coastal stretch has an NNE–SSW orientation, being included in coastal cell 1 (sub-cell 1b), according to an established classification [21]. Coastal defence works in the area include two jetties at Aveiro harbour entrance, nine groynes (E1 to E9), and three rock armour revetments (two in Costa Nova and one at Vagueira beach).

**Figure 1.** Location of study area with detailed position of the dredging/borrow area and shoreface nourishment area (SNA). Beach profiles (BP), Beach shoreface profiles (BSP), and topo-bathymetric surveys (TBS) in the survey domain (SD) and southward until Praia de Mira. **Figure 1.** Location of study area with detailed position of the dredging/borrow area and shoreface nourishment area (SNA). Beach profiles (BP), Beach shoreface profiles (BSP), and topo-bathymetric surveys (TBS) in the survey domain (SD) and southward until Praia de Mira.

According to [22], the averaged offshore significant wave height (Hs) is = 2.36 m, with a monthly averaged value of 1.77 m during summer that increases to 3.04 m during winter. Mean wave peak-period (Tp) is 10.7 s, ranging between 8.7 s and 12.3 s during summer and winter, respectively. Mean wave direction is predominantly from NW (71%), with a mean value of 310.5° and monthly averaged value ranging from 298° to 324°. The number of storm events (Hs > 4.5 m) per year is, on average, 15 per year, with a mean value of Hs Max = 5.6 m and maximum values higher than 10 m. Tides are semi-diurnal, ranging from 1.2 m to 3.6 m during neap and spring tides respectively. According to [22], the averaged offshore significant wave height (Hs) is = 2.36 m, with a monthly averaged value of 1.77 m during summer that increases to 3.04 m during winter. Mean wave peak-period (Tp) is 10.7 s, ranging between 8.7 s and 12.3 s during summer and winter, respectively. Mean wave direction is predominantly from NW (71%), with a mean value of 310.5◦ and monthly averaged value ranging from 298◦ to 324◦ . The number of storm events (Hs > 4.5 m) per year is, on average, 15 per year, with a mean value of Hs Max = 5.6 m and maximum values higher than 10 m. Tides are semi-diurnal, ranging from 1.2 m to 3.6 m during neap and spring tides respectively.

Longshore sediment transport is about 1 × 106 m3/year directed towards south [22– 24], resulting from a southward and northward fluxes of circa 1.5 × 106 m3 and 0.5 × 106 m3, respectively. However, mean annual values show an irregular and noncyclical pattern, with yearly averages ranging from 0.11 × 106 m3/year to 2.24 × 106 m3/year between Longshore sediment transport is about 1 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year directed towards south [22–24], resulting from a southward and northward fluxes of circa 1.5 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> and 0.5 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> , respectively. However, mean annual values show an irregular and noncyclical pattern, with yearly averages ranging from 0.11 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year to 2.24 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year between 1953 and 2010 [24]. The authors in [22,25] also identified this annual inter-variability in longshore sediment transport rates, reporting values of 0.6 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year to 3.2 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year (1952 to 2010) and 0.16 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year to 1.52 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year (2000 to 2019), respectively.

Beach sediments consist of well and very well sorted medium sand (median grain size between 0.4 mm and 0.6 mm) [26], whereas shoreface sand is finer (medium to fine sand, median grain size between 0.35 mm and 0.21 mm). Sands of the beach and shoreface are essentially quartzic with minor contributions of carbonate bioclasts (shell fragments of bivalves).

### *2.2. Shoreline Evolution*

Shoreline evolution south of the Aveiro inlet, between Barra and north of Mira beach varies according to the considered time scale (i.e., long, medium or short-term). The authors in [27] mention retreat rates of −15 m/year between 1947 and 1954 in Barra-Costa Nova, and −5.2 m/year and −3.0 m/year in Vagueira from 1954 to 1990, while [28] reported a maximum retreat of 400 m of the waterline during 1948–2005 in Costa Nova-Vagueira. The authors in [29] obtained retreat rates between 1958 and 2018 of −1.0 m/year and −4.4 m/year in Barra-Costa Nova and Costa Nova-Vagueira, respectively. To the south until Mira beach, [30] obtained retreat rates ranging from −1.9 m/year to −4.5 m/year for the period 1958–2010. The above-mentioned results, obtained by different authors and methods, clearly show the existence of long-term erosion in this coastal sector. Causes are mostly related to: (1) the negative sediment budget that is verified towards south of the Aveiro inlet due to the reduction in sediment supply from the north (mainly Douro River) [21,31]; and (2) the interruption in longshore sediment transport caused by the north jetty of the Aveiro harbour, which blocked most of the available sand coming from the north [23].

More recently there is a decrease in erosion rates. Between 2010 and 2018, [29] obtained retreat rates of −0.40 m/year and −0.02 m/year in Barra-Costa Nova and Costa Nova-Vagueira sectors, respectively. Further south and until Mira, [29] obtained erosion rates ranging from −0.01 m/year to −0.57 m/year for the period 2013–2018. The most recent coastline (i.e., dune foot) comparison, from 2018 to 2020, carried out within the scope of this work using ortophotos and Digital Elevation Models (DEM) provided by the Portuguese COaStal MOnitoring Program—COSMO (https://cosmo.apambiente.pt, accessed date on 15 November 2021), shows a positive rate of 0.4 m/year between Barra-Vagueira, and 1.4 m/year from this point to the north of Praia de Mira, with 70% of the coastline being characterized as stable or accreting, according to the classification proposed by [2] (Figure 2). *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 5 of 18

The causes for the reduction of coastal erosion rates are most certainly related to previous nourishments performed in this coastal cell [14,32], which promoted localized

Since 1965, a total of 26 beach fills have been carried out between Barra beach and Mira beach, 20% on the dry beach/dune and 80% in the shoreface, comprising the deposition of 15.3 M m3 (36% of the total amount deposited in Portugal from 1950 to 2020), of which almost 50% was deposited in the last 10 years (data updated from [14]). This demonstrates the growing awareness regarding the beneficial use of sediments dredged by Port of Aveiro to counteract coastal erosion, and is a result of an established policy of integrated sediment management involving different stakeholders (i.e., Port and

Shoreface nourishment intervention comprised the dumping of ≈ 2.375 M m3 of sediment in May to early September 2020. Sand was dumped in front of Costa Nova beach (Figure 1 and Figure 2a), between 4 and 8 m below Chart Datum (CD—chart datum lies 2.0 m below mean sea level—MSL). The sand mound acquired a broadly trapezoidal shape with a flat summit at −4 m CD, and extended over 0.95 km2 (length = 1.9 km; width = 0.5 km). Sediments were dredged from a nearby borrow area (≈6.5 km) and consist of

Borrow materials are essentially made of quartz and consist of moderately well sorted, coarse and medium sand (mean grain size of 0.92 mm) [33], the mean size spanning over a wider size-spectrum than the beach. Coarser sand (about 1/3 of the samples analysed) is somewhat less well sorted than medium sand. All samples analysed (*n* = 36) for textural characterization of borrow materials yielded less than 1% mud (particles < 63 μm) contents. However, about 20% of the samples revealed non-negligible (higher than 5%) amounts of particles finer than 0.250 mm, in one case reaching up to 14% of the whole

**Figure 2.** Coastline evolution from 2018 to 2020. **Figure 2.** Coastline evolution from 2018 to 2020.

*2.3. Previous Nourishments* 

Environmental Authorities).

*3.1. Shoreface Nourishment* 

**3. Methods** 

dredge spoil stored within the Aveiro harbour (Figure 3).

The causes for the reduction of coastal erosion rates are most certainly related to previous nourishments performed in this coastal cell [14,32], which promoted localized replacement of the existing negative sediment budget.

#### *2.3. Previous Nourishments*

Since 1965, a total of 26 beach fills have been carried out between Barra beach and Mira beach, 20% on the dry beach/dune and 80% in the shoreface, comprising the deposition of 15.3 M m<sup>3</sup> (36% of the total amount deposited in Portugal from 1950 to 2020), of which almost 50% was deposited in the last 10 years (data updated from [14]). This demonstrates the growing awareness regarding the beneficial use of sediments dredged by Port of Aveiro to counteract coastal erosion, and is a result of an established policy of integrated sediment management involving different stakeholders (i.e., Port and Environmental Authorities). *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 6 of 18 sample. Assuming that the sampling scheme correctly represents the total volume of borrow materials, and that fine and very fine sand particles are especially prone to be

readily removed from the dump site to the offshore, we estimate that up to 8% of the sand

#### **3. Methods** volume dumped in the nourishment area could have been lost during (or shortly after)

#### *3.1. Shoreface Nourishment* the nourishment operations, slightly reducing the sediment volume in the dump area.

Shoreface nourishment intervention comprised the dumping of <sup>≈</sup> 2.375 M m<sup>3</sup> of sediment in May to early September 2020. Sand was dumped in front of Costa Nova beach (Figures 1 and 2a), between 4 and 8 m below Chart Datum (CD—chart datum lies 2.0 m below mean sea level—MSL). The sand mound acquired a broadly trapezoidal shape with a flat summit at <sup>−</sup>4 m CD, and extended over 0.95 km<sup>2</sup> (length = 1.9 km; width = 0.5 km). Sediments were dredged from a nearby borrow area (≈6.5 km) and consist of dredge spoil stored within the Aveiro harbour (Figure 3). An additional volume of ≈ 320,000 m3 (≈ 295,000 m3 after the above-mentioned textural correction) of sand was deposited between September 2020 and March 2021, adjacent to the seaward slope of the main dump, outside the pre-defined SNA of the main intervention, but within the SD. Finally, more 212,000 m3 were deposited between March 2021 and Aug 2021, at the location of the main dump, but that were excluded from sediment budget calculations within the SD, given that last Topo-Bathymetric Surveys (TBS) dates from March 2021.

**Figure 3.** (**a**) Dumping area and (**b**) dredging area. **Figure 3.** (**a**) Dumping area and (**b**) dredging area.

*3.2. Data Collection*  Three hourly deep water wave parameters (Hs—significant wave height of combined wind waves and swell; Tp—peak wave period; Ө—mean wave direction) covering the period from June 2018 to September 2021 were obtained for a point located broadly 60 km WNW of Aveiro harbour, at 9.5° W; 40.5° N from the Climate Data Store (https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-singlelevels?tab=form; accessed on 3 January 2022). Topographic and hydrographic data used for this study were provided by the COSMO Program [34] containing three types of datasets: Beach Profiles (BP), Beach-Shoreface Profiles (BSP), and Topo-Bathymetric Surveys (TBS), performed during the monitoring period, from July 2018 to September 2021 between Praia da Barra and Praia de Mira beaches (Figure 1). An additional Multi-Beam Hydrographic Survey (MBHS) was Borrow materials are essentially made of quartz and consist of moderately well sorted, coarse and medium sand (mean grain size of 0.92 mm) [33], the mean size spanning over a wider size-spectrum than the beach. Coarser sand (about 1/3 of the samples analysed) is somewhat less well sorted than medium sand. All samples analysed (*n* = 36) for textural characterization of borrow materials yielded less than 1% mud (particles < 63 µm) contents. However, about 20% of the samples revealed non-negligible (higher than 5%) amounts of particles finer than 0.250 mm, in one case reaching up to 14% of the whole sample. Assuming that the sampling scheme correctly represents the total volume of borrow materials, and that fine and very fine sand particles are especially prone to be readily removed from the dump site to the offshore, we estimate that up to 8% of the sand volume dumped in the nourishment area could have been lost during (or shortly after) the nourishment operations, slightly reducing the sediment volume in the dump area.

made by the Aveiro Port Administration in early September 2020, immediately after the main dump, covering only the deposition area (survey boundaries indicated in Figure 1). BP consist of cross-shore transects at several coastal locations (Figure 1) that were repeatedly surveyed using a GPS/RTK between a fixed reference onshore and extending at least to the +1 m (CD) contour line. Horizontal resolution was better than 1 m and vertical accuracy better than 0.05 m. BSP incorporate and extend BP into the shoreface down −20 m (CD) contour line, An additional volume of <sup>≈</sup>320,000 m<sup>3</sup> (≈295,000 m<sup>3</sup> after the above-mentioned textural correction) of sand was deposited between September 2020 and March 2021, adjacent to the seaward slope of the main dump, outside the pre-defined SNA of the main intervention, but within the SD. Finally, more 212,000 m<sup>3</sup> were deposited between March 2021 and Aug 2021, at the location of the main dump, but that were excluded from sediment budget calculations within the SD, given that last Topo-Bathymetric Surveys (TBS) dates from March 2021.

using a jet sky equipped with a GPS/RTK coupled with a single beam echo sounder, with

altimetry were better than 0.05 m. Hydrography extends into the beach submarine domain down to ca. −10 m (CD) and data were obtained using a single-beam (transect spacing of 30 m in the depth range of +1 to −3 m CD) and multi-beam echo sounders, the latter with

TBS results from the combination of topographic and hydrographic data. Topography was acquired using aerial photogrammetry techniques over stereoscopic imagery captured by a high-resolution camera mounted on a fixed wing UAV, equipped

planimetric and vertical accuracy similar to BP.

vertical accuracy of 0.05 m, the same as the MBHS.

*3.3. Data Processing and Analysis* 

#### *3.2. Data Collection*

Three hourly deep water wave parameters (Hs—significant wave height of combined wind waves and swell; Tp—peak wave period; —mean wave direction) covering the period from June 2018 to September 2021 were obtained for a point located broadly 60 km WNW of Aveiro harbour, at 9.5◦ W; 40.5◦ N from the Climate Data Store (https://cds. climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=form; accessed on 3 January 2022).

Topographic and hydrographic data used for this study were provided by the COSMO Program [34] containing three types of datasets: Beach Profiles (BP), Beach-Shoreface Profiles (BSP), and Topo-Bathymetric Surveys (TBS), performed during the monitoring period, from July 2018 to September 2021 between Praia da Barra and Praia de Mira beaches (Figure 1). An additional Multi-Beam Hydrographic Survey (MBHS) was made by the Aveiro Port Administration in early September 2020, immediately after the main dump, covering only the deposition area (survey boundaries indicated in Figure 1).

BP consist of cross-shore transects at several coastal locations (Figure 1) that were repeatedly surveyed using a GPS/RTK between a fixed reference onshore and extending at least to the +1 m (CD) contour line. Horizontal resolution was better than 1 m and vertical accuracy better than 0.05 m.

BSP incorporate and extend BP into the shoreface down −20 m (CD) contour line, using a jet sky equipped with a GPS/RTK coupled with a single beam echo sounder, with planimetric and vertical accuracy similar to BP.

TBS results from the combination of topographic and hydrographic data. Topography was acquired using aerial photogrammetry techniques over stereoscopic imagery captured by a high-resolution camera mounted on a fixed wing UAV, equipped with GPS/RTK, supported by several ground control points. RMS error in planimetry and altimetry were better than 0.05 m. Hydrography extends into the beach submarine domain down to ca. −10 m (CD) and data were obtained using a single-beam (transect spacing of 30 m in the depth range of +1 to −3 m CD) and multi-beam echo sounders, the latter with vertical accuracy of 0.05 m, the same as the MBHS.

#### *3.3. Data Processing and Analysis*

Wave data were used to characterize deep water wave regime over the monitoring period and to estimate potential longshore drift in the survey domain. Wave parameters at breaking were computed using Airy wave theory and Snell's Law. Longshore drift estimates were obtained by the energy flux method, using the CERC formula [35]. In agreement with the findings of [36], which reported an overestimation of 7.85 × regarding the CERC formula when spectral effects are not considered in computations of longshore drift, the formula was parametrized with a smaller empirical factor (*k* 0= 0.39/7.85). Bathymetric data were processed with QIMERA (multi-beam) and HYPACK software (single-beam) by AT-LANTICLAND (a consortium member of the COSMO Program). The obtained XYZ point cloud was interpolated to produce DEM with 0.03 m pixel resolution. For the topographic surveys it was used the AGISOFT software, with a processed point cloud of 100 points/m<sup>2</sup> , generating a DEM with a 0.01 m resolution (data processed by GEOGLOBAL, a consortium member of the COSMO Program).

In order to analyse the morphological development and behaviour of the shoreface nourishment, including spreading and diffusion processes over the survey domain (SD), a volumetric, cut-fill, and bed level changes analysis was performed over the TBS using ArcGIS and Globalmapper softwares. To analyse sediment budget exchanges in terms of cross-shore/longshore processes and evaluate feeder efficiency of the nourishment, the SD was divided in different areas, here designated as Multi-Monitoring Cells. Shoreface and subaerial beach response through time in relation to the nourishment, was based on the comparison of selected contour lines extracted from the TBS.

Shoreface and subaerial beach variability (i.e., volume and width) and its response to the nourishment was analysed through the available BP and BSP. For the BP, volume was

calculated above + 1 m (CD) up to a fixed height, where no variations occur; in turn, beach width variation was estimated measuring the horizontal displacement of the +2 m (CD). Shoreface morphological variability was addressed using BSP and evaluated through the assessment of horizontal displacement of selected contour lines down to the closure depth. To highlight the potential effect of the shoreface nourishment (SN), width and volume values obtained from BP and BSP were normalized by subtraction from the pre-nourishment time-averaged values, July 2018 to July 2020 and August 2018 to July 2020, respectively.

The depth of closure, seaward of which no significant morphological changes occur, according to the original definition proposed by [37] was estimated from statistics of bedlevel variability, as proposed by [36]. The value of 0.15 m was set as the threshold on the standard deviation (s) of the bed level. This is slightly higher than the vertical accuracy reported for the Jet Ski Single-Beam acquisition platform (0.10 m) and allows for some additional uncertainty due to wave conditions and water depth as suggested by [38].

A Multiple Monitoring Cell (MMC) approach was used to evaluate local and feeder efficiency of the SN and to assess post-nourishment pathways of sediment dispersion within survey domain (SD). This was applied to the SD by splitting this area into twenty rectangular cells, organized in four cross-shore rows and five longshore columns.

To quantify shoreline evolution, namely, comparison of contour lines of TBS), the Digital Shoreline Analyses System [39] was used, an extension in ArcGIS (ESRI) software that allowed for net shoreline movement and retreat rates calculations.

#### **4. Results**

#### *4.1. Wave Forcing and Longshore Drift in the Monitoring Period*

Figure 4 shows the time distribution of deep-water wave parameters over the monitoring period, highlighting the seasonality of wave forcing. Mean wave conditions (Hs = 2.23 m; Tp = 10.9 s) are in close agreement with long-term estimates indicated in Section 2.1. High-energy events essentially cluster in winter/autumn months with high directional variability in drift currents. In contrast, during summer/spring, wave height and period are lower and drift currents are more consistently directed southward. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 8 of 18

**Figure 4.** Time-series of deep-water parameters (Hs, Tp, Ө) during the monitoring period (July 2018 to September 2021). The highlighted blue region corresponds to the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021). The dotted line indicates shore normal direction. **Figure 4.** Time-series of deep-water parameters (Hs, Tp, ) during the monitoring period (July 2018 to September 2021). The highlighted blue region corresponds to the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021). The dotted line indicates shore normal direction.

**Figure 5.** Monthly longshore drift during the monitoring period (positive values indicate southward drift) The highlighted blue region corresponds to the period covered by the pre- and post-

In the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021) (highlighted blue region in Figure 5) the only reversal on longshore drift direction is observed in February 2021 in correspondence with persistency of westerly waves.

During the monitoring period, monthly longshore drift was almost exclusively directed towards south (Figure 5), with a grand total of 3.15 × 106 m3. This corresponds to a yearly average of 0.97 × 106 m3/year, which is remarkably similar to longer term estimates

nourishment TBS (June 2020 to March 2021).

During the monitoring period, monthly longshore drift was almost exclusively directed towards south (Figure 5), with a grand total of 3.15 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> . This corresponds to a yearly average of 0.97 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/year, which is remarkably similar to longer term estimates of mean annual longshore drift reported by [22–24]. **Figure 4.** Time-series of deep-water parameters (Hs, Tp, Ө) during the monitoring period (July 2018 to September 2021). The highlighted blue region corresponds to the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021). The dotted line indicates shore normal direction.

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 8 of 18

In the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021) (highlighted blue region in Figure 5) the only reversal on longshore drift direction is observed in February 2021 in correspondence with persistency of westerly waves. Notwithstanding this singularity, the net drift was directed southward with a magnitude of 0.87 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> , strongly influenced by high-energy winter waves (Figures 4 and 5). During the monitoring period, monthly longshore drift was almost exclusively directed towards south (Figure 5), with a grand total of 3.15 × 106 m3. This corresponds to a yearly average of 0.97 × 106 m3/year, which is remarkably similar to longer term estimates of mean annual longshore drift reported by [22–24].

**Figure 5.** Monthly longshore drift during the monitoring period (positive values indicate southward drift) The highlighted blue region corresponds to the period covered by the pre- and postnourishment TBS (June 2020 to March 2021). **Figure 5.** Monthly longshore drift during the monitoring period (positive values indicate southward drift) The highlighted blue region corresponds to the period covered by the pre- and post-nourishment TBS (June 2020 to March 2021).

#### In the period covered by the pre- and post-nourishment TBS (June 2020 to March *4.2. Beach Profiles (BP)*

2021) (highlighted blue region in Figure 5) the only reversal on longshore drift direction is observed in February 2021 in correspondence with persistency of westerly waves. Results (Figure 6) show the variability of subaerial beach volume and width related to short term (seasonal) wave forcing. Significant changes following the nourishment were only observed in (CS) (the profile closest to, and aligned with the SNA), with a consistent increase in width and volume of the subaerial beach over time. In this case, magnitude of changes clearly exceeds the range of seasonal variability over the monitoring period. All the remaining profiles, both updrift and downdrift, do not appear to have been substantially influenced by the nourishment.

### *4.3. Beach Shoreface Profiles (BSP)*

Figure 7 illustrates changes in cross-shore profiles extending from the foredune until −10 m (CD) over the monitoring period at the location where the shoreface nourishment was dumped, complementing data on subaerial beach changes mentioned above.

Depth of closure (DoC) used in this study was estimated from profile convergence (s < 0.15 m) and is—9.3 m (CD) (Figure 7). This depth is lower than previous wavebased empirical estimates of DoC for the same area, which can vary from −8.5 m CD to −17 m CD [28,40,41]. Discrepancies are mostly related to the different approaches (empirical versus morphological) and time scale of the analyses [42].

offshore).

*4.2. Beach Profiles (BP)* 

to have been substantially influenced by the nourishment.

**Figure 6.** Normalized beach volume and width variation along the monitoring period (**a**) Normalized volume in the SD; (**b**): Normalized width in the SD; (**c**): Normalized volume downdrift of SD; (**d**) Normalized width downdrift of SD. **Figure 6.** Normalized beach volume and width variation along the monitoring period (**a**) Normalized volume in the SD; (**b**): Normalized width in the SD; (**c**): Normalized volume downdrift of SD; (**d**) Normalized width downdrift of SD. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 10 of 18

Notwithstanding this singularity, the net drift was directed southward with a magnitude of 0.87 × 106 m3, strongly influenced by high-energy winter waves (Figure 4 and Figure 5).

Results (Figure 6) show the variability of subaerial beach volume and width related to short term (seasonal) wave forcing. Significant changes following the nourishment were only observed in (CS) (the profile closest to, and aligned with the SNA), with a consistent increase in width and volume of the subaerial beach over time. In this case, magnitude of changes clearly exceeds the range of seasonal variability over the monitoring period. All the remaining profiles, both updrift and downdrift, do not appear

**Figure 7.** Beach Shoreface Profiles (BSP) at Costa Nova between July 2018 and September 2021. The red dotted line of September 2020 represents the final construction profile of the shoreface nourishment (elevations referred to CD—Chart Datum). **Figure 7.** Beach Shoreface Profiles (BSP) at Costa Nova between July 2018 and September 2021. The red dotted line of September 2020 represents the final construction profile of the shoreface nourishment (elevations referred to CD—Chart Datum).

Depth of closure (DoC) used in this study was estimated from profile convergence (s < 0.15 m) and is—9.3 m (CD) (Figure 7). This depth is lower than previous wave-based empirical estimates of DoC for the same area, which can vary from −8.5 m CD to −17 m CD [28,40,41]. Discrepancies are mostly related to the different approaches (empirical versus morphological) and time scale of the analyses [42]. A complex longshore bar system dominates inner shoreface morphology and morphological changes. Inner bars (single or multiple), extend up to 500 m offshore, and develop between—4 m (CD) and 0 m (CD), the elevation offset between bar crest and trough reaching up to 2 m. After the SN, a plateau is evident in September 2020, from which a prominent bar is evident in November 2020 (800 m offshore). The inland migration of this bar is highlighted in Mar (500 m offshore) and September 2021 (350 m offshore).

which a prominent bar is evident in November 2020 (800 m offshore). The inland migration of this bar is highlighted in Mar (500 m offshore) and September 2021 (350 m

CD and −6 m CD contour lines, respectively (Figure 8a).

(**a**)

A complex longshore bar system dominates inner shoreface morphology and

The shoreface nourishment (illustrated by BSP September 2020 in Figure 7) created a 500 m wide, flat summited sand mound, with a broadly trapezoidal shape between −4 m CD and −8 m CD. This resulted in seaward displacement of 100 m and 300 m of the −8 m offshore).

(**b**).

20 m at − 6 m CD.

(Figure 8b).

The shoreface nourishment (illustrated by BSP September 2020 in Figure 7) created a 500 m wide, flat summited sand mound, with a broadly trapezoidal shape between −4 m CD and −8 m CD. This resulted in seaward displacement of 100 m and 300 m of the −8 m CD and −6 m CD contour lines, respectively (Figure 8a). 500 m wide, flat summited sand mound, with a broadly trapezoidal shape between −4 m CD and −8 m CD. This resulted in seaward displacement of 100 m and 300 m of the −8 m CD and −6 m CD contour lines, respectively (Figure 8a).

**Figure 7.** Beach Shoreface Profiles (BSP) at Costa Nova between July 2018 and September 2021. The red dotted line of September 2020 represents the final construction profile of the shoreface

Depth of closure (DoC) used in this study was estimated from profile convergence (s < 0.15 m) and is—9.3 m (CD) (Figure 7). This depth is lower than previous wave-based empirical estimates of DoC for the same area, which can vary from −8.5 m CD to −17 m CD [28,40,41]. Discrepancies are mostly related to the different approaches (empirical

A complex longshore bar system dominates inner shoreface morphology and morphological changes. Inner bars (single or multiple), extend up to 500 m offshore, and develop between—4 m (CD) and 0 m (CD), the elevation offset between bar crest and trough reaching up to 2 m. After the SN, a plateau is evident in September 2020, from which a prominent bar is evident in November 2020 (800 m offshore). The inland migration of this bar is highlighted in Mar (500 m offshore) and September 2021 (350 m

The shoreface nourishment (illustrated by BSP September 2020 in Figure 7) created a

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 10 of 18

nourishment (elevations referred to CD—Chart Datum).

versus morphological) and time scale of the analyses [42].

**Figure 8.** Temporal variation in the position of contour lines in the shoreface (**a**) and subaerial beach **Figure 8.** Temporal variation in the position of contour lines in the shoreface (**a**) and subaerial beach (**b**).

About two months later (November 2020) (Figure 7) the cross-section profiles show a landward translation and significant reshaping of the fill, with the development of two asymmetrical bars: a larger one atop the artificial sand mound and a smaller one merging with the pre-existing bar system. The seaward slope of the mound became milder, mimicking the pre-nourishment shape, and extended further seaward of the postconstruction profile until the DoC. This slope adjustment is manifested by additional 80 About two months later (November 2020) (Figure 7) the cross-section profiles show a landward translation and significant reshaping of the fill, with the development of two asymmetrical bars: a larger one atop the artificial sand mound and a smaller one merging with the pre-existing bar system. The seaward slope of the mound became milder, mimicking the pre-nourishment shape, and extended further seaward of the postconstruction profile until the DoC. This slope adjustment is manifested by additional 80 m seaward displacement of the −8 m CD, and limited opposite (landward) translation of 20 m at −6 m CD.

m seaward displacement of the −8 m CD, and limited opposite (landward) translation of

m CD (Figure 8a). Simultaneously, it was observed significant subaerial beach accretion by ca. 40 m seaward advance of the beach face (cf. contour lines + 2 m CD and + 4 m CD)

Seven months after the SN, almost all sand in the artificial mound moved landward,

Profile changes between March 2021 and September 2021 (Figure 7) are less pronounced landward o f−1 m CD, where limited landward displacement of the nearshore bar and beach face retreat occurred. Seaward of −1 m CD, lowering of the profile was

Profile changes in BSP may be taken as representative of sediment and morphological cross-shore dynamics in the area influenced by the SN. They suggest that SN at Costa Nova evolved rapidly, and essentially consisted of reshaping and landward translation of placed sand mound over the shoreface, the nourishment effects eventually affecting the

Analyses of TBS and MBHS allowed for the evaluation of volume changes and sediment budget within the SNA and SD. Moreover, it provides additional insights on

Comparison of TBS June 2020 (pre-nourishment survey) and MBHS September 2020 (post-construction final survey) in the SNA indicate a volume increase of 2.087 × 106 m3. This is 12% less than the volume of material dredged from the borrow area, as reported by the Port Authority (2.375 × 106 m3). This difference may be explained by: (i) permanent loss (8%) to the offshore of the finer size fractions of sediment (<0.125 mm); and (ii)

subaerial beach in about seven months.

observed, whereas sand accumulated between −5 m CD and −8 m CD.

*4.4. Topo-Bathymetric Surveys (TBS) and Multi-Beam Hydrographic Survey (MBHS)* 

sediment dispersion driven by both longshore and cross-shore process.

Seven months after the SN, almost all sand in the artificial mound moved landward, feeding and building a wider and shallower nearshore bar system. This behaviour is mirrored by a landward displacement of depth contour lines−8 m CD, −6 m CD, and −4 m CD (Figure 8a). Simultaneously, it was observed significant subaerial beach accretion by ca. 40 m seaward advance of the beach face (cf. contour lines + 2 m CD and + 4 m CD) (Figure 8b).

Profile changes between March 2021 and September 2021 (Figure 7) are less pronounced landward of −1 m CD, where limited landward displacement of the nearshore bar and beach face retreat occurred. Seaward of −1 m CD, lowering of the profile was observed, whereas sand accumulated between −5 m CD and −8 m CD.

Profile changes in BSP may be taken as representative of sediment and morphological cross-shore dynamics in the area influenced by the SN. They suggest that SN at Costa Nova evolved rapidly, and essentially consisted of reshaping and landward translation of placed sand mound over the shoreface, the nourishment effects eventually affecting the subaerial beach in about seven months.

### *4.4. Topo-Bathymetric Surveys (TBS) and Multi-Beam Hydrographic Survey (MBHS)*

Analyses of TBS and MBHS allowed for the evaluation of volume changes and sediment budget within the SNA and SD. Moreover, it provides additional insights on sediment dispersion driven by both longshore and cross-shore process.

Comparison of TBS June 2020 (pre-nourishment survey) and MBHS September 2020 (post-construction final survey) in the SNA indicate a volume increase of 2.087 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> . This is 12% less than the volume of material dredged from the borrow area, as reported by the Port Authority (2.375 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ). This difference may be explained by: (i) permanent loss (8%) to the offshore of the finer size fractions of sediment (<0.125 mm); and (ii) sediment transfer (4%) from SNA to the SD, caused by waves and currents over four months, during which numerous dumps added to produce the post nourishment measured volume. In agreement, a volume of 2.185 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> is considered a fair estimate of the amount of compatible sand placed at the SNA. Sediment budget calculations for the entire SD includes an additional amount of 295,000 m<sup>3</sup> . *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 12 of 18 sediment transfer (4%) from SNA to the SD, caused by waves and currents over four months, during which numerous dumps added to produce the post nourishment measured volume. In agreement, a volume of 2.185 × 106 m3 is considered a fair estimate of the amount of compatible sand placed at the SNA. Sediment budget calculations for the

Morphological changes from pre- to post-nourishment (June 2020 to March 2021) reveal a heterogeneous spatial pattern, with alternating accumulation and erosion patches (Figure 9), showing larger longshore continuity. Bed level changes range from −6 m to +5 m, with the largest changes occurring in the bar system and subaerial beach. The SNA, together with updrift and downdrift adjacent regions, show higher accumulation and increased spatial continuity, in contrast with the southern half of the SD where patches of accumulation and erosion are smaller and more fragmented. entire SD includes an additional amount of 295,000 m3. Morphological changes from pre- to post-nourishment (June 2020 to March 2021) reveal a heterogeneous spatial pattern, with alternating accumulation and erosion patches (Figure 9), showing larger longshore continuity. Bed level changes range from −6 m to + 5 m, with the largest changes occurring in the bar system and subaerial beach. The SNA, together with updrift and downdrift adjacent regions, show higher accumulation and increased spatial continuity, in contrast with the southern half of the SD where patches of

**Figure 9.** Morphological changes along the SD with respect to the pre-nourishment situation (June 2020) and after-nourishment (March 2021). **Figure 9.** Morphological changes along the SD with respect to the pre-nourishment situation (June 2020) and after-nourishment (March 2021).

The Multiple Monitoring Cell (MMC) approach was applied to SD by splitting this area in twenty rectangular cells, organized in four cross-shore rows and five longshore columns, as illustrated in Figure 10. Cross shore rows broadly correspond to the following The Multiple Monitoring Cell (MMC) approach was applied to SD by splitting this area in twenty rectangular cells, organized in four cross-shore rows and five longshore columns, as illustrated in Figure 10. Cross shore rows broadly correspond to the following

domains: (i) subaerial beach; (ii) bar system; (iii) depth range of SNA; (iv) depth range

**Figure 10.** Multiple Monitoring Cell (MMC) approach with sediment budget along the SD between

Cells 5, 6, 7, and 8 present a positive sediment budget in the entire beach domain, comprising the dry-beach and nearshore up to −10 m CD. South of SNA, namely, in cells 9, 10, 13, 14, 17, and 18, concentrated landward of −4 m CD (in the inner surfzone), they

Results shown in Figure 10 indicate that seven months after the nourishment, the

June 2020 and March 2021.

all have a negative sediment budget.

between seaward boundary of SNA and offshore limit of SD.

accumulation and erosion are smaller and more fragmented.


domains: (i) subaerial beach; (ii) bar system; (iii) depth range of SNA; (iv) depth range between seaward boundary of SNA and offshore limit of SD. Results shown in Figure 10 indicate that seven months after the nourishment, the SNA (cell 7) retained 1.3 × 106 m3 (60% of the dumped volume).

**Figure 9.** Morphological changes along the SD with respect to the pre-nourishment situation (June

The Multiple Monitoring Cell (MMC) approach was applied to SD by splitting this area in twenty rectangular cells, organized in four cross-shore rows and five longshore columns, as illustrated in Figure 10. Cross shore rows broadly correspond to the following domains: (i) subaerial beach; (ii) bar system; (iii) depth range of SNA; (iv) depth range

*J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 12 of 18

entire SD includes an additional amount of 295,000 m3.

accumulation and erosion are smaller and more fragmented.

between seaward boundary of SNA and offshore limit of SD.

2020) and after-nourishment (March 2021).

sediment transfer (4%) from SNA to the SD, caused by waves and currents over four months, during which numerous dumps added to produce the post nourishment measured volume. In agreement, a volume of 2.185 × 106 m3 is considered a fair estimate of the amount of compatible sand placed at the SNA. Sediment budget calculations for the

Morphological changes from pre- to post-nourishment (June 2020 to March 2021) reveal a heterogeneous spatial pattern, with alternating accumulation and erosion patches (Figure 9), showing larger longshore continuity. Bed level changes range from −6 m to + 5 m, with the largest changes occurring in the bar system and subaerial beach. The SNA, together with updrift and downdrift adjacent regions, show higher accumulation and increased spatial continuity, in contrast with the southern half of the SD where patches of

**Figure 10.** Multiple Monitoring Cell (MMC) approach with sediment budget along the SD between June 2020 and March 2021. **Figure 10.** Multiple Monitoring Cell (MMC) approach with sediment budget along the SD between June 2020 and March 2021.

Cells 5, 6, 7, and 8 present a positive sediment budget in the entire beach domain, comprising the dry-beach and nearshore up to −10 m CD. South of SNA, namely, in cells Results shown in Figure 10 indicate that seven months after the nourishment, the SNA (cell 7) retained 1.3 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> (60% of the dumped volume).

9, 10, 13, 14, 17, and 18, concentrated landward of −4 m CD (in the inner surfzone), they all have a negative sediment budget. Cells 5, 6, 7, and 8 present a positive sediment budget in the entire beach domain, comprising the dry-beach and nearshore up to −10 m CD. South of SNA, namely, in cells 9, 10, 13, 14, 17, and 18, concentrated landward of −4 m CD (in the inner surfzone), they all have a negative sediment budget. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 13 of 18

> Within the survey domain (SD), the June 2020 to March 2021 sediment budget is positive, and of 1.73 <sup>×</sup> <sup>10</sup> <sup>6</sup> <sup>m</sup><sup>3</sup> . Considering that an additional volume of 295,000 m<sup>3</sup> (September 2020 to March 2021) was added to the initial measured volume of 2.185 <sup>×</sup> <sup>10</sup> <sup>6</sup> <sup>m</sup><sup>3</sup> (deposited between June 2020 and September 2020), a loss of 750,000 m<sup>3</sup> (30 %) was observed in this period. Within the survey domain (SD), the June 2020 to March 2021 sediment budget is positive, and of 1.73 x 10 6 m3. Considering that an additional volume of 295,000 m3 (September 2020 to March 2021) was added to the initial measured volume of 2.185 × 10 6 m3 (deposited between June 2020 and September 2020), a loss of 750,000 m3 (30 %) was observed in this period.

> Regarding subaerial beach response to the nourishment in the adjacent beaches (i.e., Costa Nova) in front of the SNA and further south until Vagueira Beach, Figure 11 shows the horizontal displacement of the + 1 m (CD) contour line, and its variation in terms of advance or retreat between June 2020 and March 2021. Regarding subaerial beach response to the nourishment in the adjacent beaches (i.e., Costa Nova) in front of the SNA and further south until Vagueira Beach, Figure 11 shows the horizontal displacement of the + 1 m (CD) contour line, and its variation in terms of advance or retreat between June 2020 and March 2021.

**Figure 11.** Horizontal displacement of the + 1 m (CD) contour line and its variation in terms of advance or retreat. **Figure 11.** Horizontal displacement of the + 1 m (CD) contour line and its variation in terms of advance or retreat.

Results presented in Figure 11 confirm the above mentioned regarding subaerial beach variation, with a global positive net shoreline movement up to + 90 m within 0.5 km to the north and in front of the SNA. This trend changes further south, with a retreat of the +1 m contour line position, in average between −30 m to−60 m. The entire SD this contour line has an average displacement of + 9 m ± 44.5 m, which illustrates well the spatial variability of subaerial beach in response to the SN, because the standard deviation is much larger than the mean value. Results presented in Figure 11 confirm the above mentioned regarding subaerial beach variation, with a global positive net shoreline movement up to + 90 m within 0.5 km to the north and in front of the SNA. This trend changes further south, with a retreat of the +1 m contour line position, in average between −30 m to−60 m. The entire SD this contour line has an average displacement of + 9 m ± 44.5 m, which illustrates well the spatial variability of subaerial beach in response to the SN, because the standard deviation is much larger than the mean value.

Herein, we describe the impacts of a high-magnitude shoreface nourishment in the morphological evolution and behaviour of a wave-dominated, high-energy beach–dune

Coastal monitoring data show that cross-shore changes occur rapidly, with shortterm landward migration of the shoreface nourishment. Right after the intervention, the cross-shore shape of the sand mound skewed in onshore direction, by decreasing its seaward slope and increasing crest elevation, together with the formation of two bars that eventually merged with the pre-existing ones. Seven months after the intervention, the seaward slope of the SN is milder, more similar to the pre-construction native profile. This is interpreted as resulting from the dominance of onshore transport accompanied by progressive depletion of the sediment source at the seaward toe of the nourishment. This in agreement with observed post-nourishment profile responses in the Netherlands [13] that required two to four years to replicate the native configuration, whereas the timescale inferred in this study is much smaller (up to seven months). Concomitantly, cross-shore processes induced rapid subaerial increase in beach volume and width in front of the placement area (Costa Nova beach), while the dune profile remained unchanged (Figure

[20]. This has improved the understanding of the processes governing post-nourishment sediment dispersion within the survey domain. In addition, we provide a more detailed description of cross-shore changes along the nourished profile and evaluate the time scale required for the beach to acquire a new equilibrium profile following the placement of a

**5. Discussion** 

significant sand volume.

*5.1. Cross-Shore Processes* 

6 and Figure 7).

#### **5. Discussion**

Herein, we describe the impacts of a high-magnitude shoreface nourishment in the morphological evolution and behaviour of a wave-dominated, high-energy beach–dune system. We add a sediment budget approach to previous work targeting the same area [20]. This has improved the understanding of the processes governing post-nourishment sediment dispersion within the survey domain. In addition, we provide a more detailed description of cross-shore changes along the nourished profile and evaluate the time scale required for the beach to acquire a new equilibrium profile following the placement of a significant sand volume.

#### *5.1. Cross-Shore Processes*

Coastal monitoring data show that cross-shore changes occur rapidly, with short-term landward migration of the shoreface nourishment. Right after the intervention, the crossshore shape of the sand mound skewed in onshore direction, by decreasing its seaward slope and increasing crest elevation, together with the formation of two bars that eventually merged with the pre-existing ones. Seven months after the intervention, the seaward slope of the SN is milder, more similar to the pre-construction native profile. This is interpreted as resulting from the dominance of onshore transport accompanied by progressive depletion of the sediment source at the seaward toe of the nourishment. This in agreement with observed post-nourishment profile responses in the Netherlands [13] that required two to four years to replicate the native configuration, whereas the timescale inferred in this study is much smaller (up to seven months). Concomitantly, cross-shore processes induced rapid subaerial increase in beach volume and width in front of the placement area (Costa Nova beach), while the dune profile remained unchanged (Figures 6 and 7).

The observed rate of morphological responses is interpreted as mirroring the energetic wave conditions offshore Costa Nova, which are higher than the ones verified in the Netherlands [15] and California [19], where similar changes occur at larger timescales.

The magnitude of beachface seaward translation, represented by changes in beach width (Figure 6) measured between July 2020 and April 2021 is 76 m; the same estimate using October 2020 and January 2021 surveys is 60 m. These figures are in close agreement with the maximum seaward displacement (60 m) of the +3.05 m CD contour line observed between June 2020 and January 2021 in a nearby profile [20]. Moreover, our data suggest that beachface progradation slowed down or ceased after March 2021 (cf. Figure 7 for changes between March 2021 and September 2021). The magnitude of these changes is in close agreement with those yielded by solving Bruun's Rule (ca. 67 m), under the assumption that dumped sediment has been redistributed solely by cross-shore processes, aiming at restoring the equilibrium profile (replicating Bruun's equilibrium beach profile) up to the DoC. This similarity adds arguments to interpret earlier stages of post-nourishment morphodynamics as dominated by cross-shore processes. Following a convergence to the condition of equilibrium, additional increase in subaerial beach width is not expected.

### *5.2. Longshore Processes*

The patterns of morphological changes illustrated in Figures 7 and 8 indicate that a significant volume of sediment (<sup>≈</sup> 600,000 m<sup>3</sup> ) accumulated immediately southward of the SNA (cell 11 in Figure 10), suggesting that longshore currents over the shoreface nourishment fed the seaward slope of the pre-existing bar system. Feeding tends to fade out further southward (see Figures 9 and 10), due to the combined effects of diffusion and low rate of advection characterizing longshore sediment transport.

The bar system and subaerial beach downdrift of the SNA show a spatially consistent pattern of erosion, with remarkably high values immediately south and landward of SNA (cells 9 and 10 in Figure 10). This behaviour is interpreted by blockage or decrease in longshore drift landward of the SNA, induced by the "reef effect" of the dumped sand mound, as suggested by [43]. Lower wave heights shoreward of the SNA due to wave dissipation reduces longshore sediment supply to the domain located immediately

downdrift. Thus, a longshore drift gradient is created shortly after the intervention, leading to downdrift erosion. Smoothening of the morphological disturbance imposed by the nourishment will lead to the fading out of the "reef effect" and related erosive signal. These results suggest that subaerial domain of beaches facing and located downdrift the SNA are out of phase: while the former experience rapid accretion, the latter experience temporary enhanced erosion; this trend can only be reversed at larger time scales in tune with increasing importance of the longshore processes.

#### *5.3. Sediment Budget and Nourishment Lifetime*

Sediment budget calculations from June 2020 to March 2021 within the SD were based on the following assumptions:


Data collected in this study indicate that between June 2020 and March 2021 the sediment budget of the SD was positive and of about1.73 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> . Considering that the shoreface nourishment was the main sediment source to the SD, corresponding to 2.48 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> , a loss of 0.75 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> can be inferred. As the southern SD limit is the only open boundary this figure should correspond to a rough estimate of net longshore drift at that location. This conclusion is further supported by the independent assessments of potential net longshore drift performed in the scope of this work and by [22]. The former points to a value of 0.87 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> (cf. Section 4.1 and Figure 5) and the latter provides an estimate of 0.85 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> , which was obtained by adding up June to March monthly averages of longshore drift magnitude over a twenty-year period. The agreement between the computed losses and the potential longshore drift provides confidence in the results of the sediment budget.

Linear extrapolation into the future of the mean annual net longshore drift indicates that, despite the large magnitude of this intervention, the permanence of placed sand within the SD is of 2–3 years.

High magnitude of littoral drift suggests that large scale and frequent (e.g., every other year) renourishment operations may be required if updrift sand placement is to be maintained as single adaptation strategy regarding beach erosion along the sediment cell, which extends for 50 km south of the Aveiro inlet. However, large uncertainties remain regarding the optimal design of shoreface nourishments, including not only magnitude and frequency, but also plan and cross-section shape of the sand mound. Altogether, these parameters govern morphodynamic feedback of the construction over wave forcing and potential impacts on the adjacent coast.

#### **6. Conclusions**

This work addresses the impacts of a high-magnitude shoreface nourishment in the morphological evolution and behaviour of a wave-dominated, high-energy beach–dune system. Results show that cross-shore processes dominate the early stages of morphological evolution with significant onshore sand transport. The beach aligned with the sand mound rapidly (in about seven months) acquires a condition of equilibrium, replicating Bruun's equilibrium beach profile that corresponds to an increase in subaerial beach width.

Longshore transport of sediment sourced in the placement explains the intense growth of the downdrift adjacent bar system, which diffuses and fades out southward. At the subaerial beach southward of the SNA, longshore drift was temporarily influenced by the "reef effect" of the sand mound, enhancing the previous erosive trend. This trend is expected to reverse in time, as longshore processes gain relevance in net sediment transfers over the SD.

Observed rates of change at Costa Nova are significantly higher than the ones reported for similar interventions undertaken in lower wave energy coasts of the Netherlands and California. This highlights the importance of wave energy in regulating the timescale of coastal readjustment to large magnitude shoreface nourishments. In consequence, great caution should be exercised when extrapolating behaviour-based models to predict morphological evolution under conditions other than those for which they were developed.

Current coastal protection strategies define beach and shoreface nourishment as a standard procedure to mitigate coastal erosion in critical areas. This is considered as an adaptation measure under present climate change scenario, including sea level rise. New insights provided by this work are expected to support decision-making regarding similar high-magnitude interventions foreseen in other areas along the high-wave energy Portuguese western coast and elsewhere. This gains relevance considering that shoreface nourishments are much more cost-effective in replenishment of large sand volumes than traditional beach nourishments (≈half-price per m<sup>3</sup> ).

Additionally, results of this study highlight the importance of nourishment design to optimize mitigation of beach erosion in high-magnitude drift-dominated coasts. Further investigation of morphodynamic feedback of shoreface nourishments over wave forcing and assessment of potential impacts on the adjacent coast is required.

Accurate and repeatable coastal monitoring data have proven to be essential to perform this evidence-based analysis, highlighting the need to maintain systematic coastal monitoring programmes through time, like the current Portuguese COaStal MOnitoring Program (COSMO).

**Author Contributions:** Conceptualization, C.A.P.; formal analysis, C.A.P., R.T. and C.A.; funding acquisition, C.A.P. and P.B.; investigation, C.A.P.; methodology, C.A.P., R.T. and C.A.; validation, C.A.P., R.T. and C.A.; writing—original draft, C.A.P.; writing—review & editing, C.A.P., R.T., C.A., P.B., P.A.S., D.M. and J.P.-B. All authors have read and agreed to the published version of the manuscript.

**Funding:** Some of the collected data were co-funded by POSEUR (Ref. POSEUR-02-1809-FC-000051— Remoção dos Inertes da Zali do Porto de Aveiro para Reforço do Cordão Litoral a Sul da Costa Novas and are gratefully acknowledged. The Portuguese Coastal Monitoring Program (COSMO), developed and implemented by the Portuguese Environment Agency (APA), is co-funded by the Operational Program for Sustainability and Efficiency in the Use of Resources (POSEUR) (ref. POSEUR-02-1809- FC-000004) and are gratefully acknowledged. We acknowledge financial support of FCT through project UIDB/50019/2020—IDL. This work is a contribution to project SANDTRACK (POCI-01- 0145-FEDER-031779) funded by FEDER, through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI), and by national funds (OE), through FCT/MCTES. We acknowledge financial support to CESAM by FCT/MCTES (UIDP/50017/2020+UIDB/50017/2020+ LA/P/0094/2020), through national funds.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** C.A.P. acknowledge the company consortium (GEOGLOBAL and ATLATI-CLAND) for the acquisition and processing of topographic and hydrographic data under the COSMO programme service contract. Aveiro Port Authority (APA, S.A.), namely, Fátima Alves and Carla Garrido, is gratefully acknowledged for providing post-construction multi-beam survey and sediment characterization. C.A.P. also acknowledges André Inácio from APA for supporting data analyses and

calculations for Figures 6, 8 and 11. Images from Figure 3 were extracted from a video produced by JAN de NUL and INERSEL for Aveiro Port Authority/Portuguese Environment Agency during the shoreface nourishment intervention and are gratefully acknowledged. POSEUR is gratefully acknowledged for co-funding of the COSMO Program (ref. POSEUR-02-1809-FC-000004).

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


## *Article* **Beach Response to a Shoreface Nourishment (Aveiro, Portugal)**

**Diogo Mendes 1,\* ,† , Joaquim Pais-Barbosa <sup>2</sup> , Paulo Baptista <sup>1</sup> , Paulo A. Silva <sup>2</sup> , Cristina Bernardes <sup>1</sup> and Celso Pinto <sup>3</sup>**


**Abstract:** In Aveiro (NW coast of Portugal), a coastal monitoring programme was carried out in sequence of a shoreface nourishment intervention (over than 2 M m<sup>3</sup> ) performed in 2020. In this programme, almost one year of biweekly subaerial topographies and quarterly bathymetric surveys have been collected along a 10 km coastal stretch between June 2020 and June 2021. In this study, topographic and bathymetric surveys were analysed to assess the expectation that if the shoreface nourishment is located in sufficiently shallow water depths, its landward movement will feed adjacent beaches and, consequently, increase the subaerial beach volume. Results show that the subaerial beach volume is well correlated with the 1.05 m (above MSL) isoline displacement through time. While the seaward limit of the shoreface nourishment moved landwards about 200 m, the shoreline proxy (isoline of 1.05 m) displayed a maximum seaward displacement of 60 m. The displacement of the shoreline proxy was highly variable in space, along the 10 km coastal stretch, and also in time, during storm events. During such events, both landward and seawards displacement of the shoreline proxy took place, depending on the spatial position. Moreover, while beaches close to the initial shoreface nourishment intervention displayed faster accretion patterns than those located farther away, the well-defined onshore movement of the shoreface nourishment did not result in a considerable beach volume increase. The achieved results were also compared against case studies of shoreface nourishments with similar volumes performed worldwide.

**Keywords:** beach nourishment; field observations; storm; beach accretion

Accepted: 6 October 2021 Published: 13 October 2021

Received: 1 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **1. Introduction**

Beach nourishment constitutes a nature-based engineering solution commonly employed by coastal managers on sandy beaches [1]. It comprises the placement of large quantities of good quality sand on the beach to advance it seaward [2]. This advance is of key importance for tourism and recreation because it provides enough space on the dry beach for this type of activities. In general, urbanized beaches, such as those located or backed up by buildings and infrastructures, are the most prone to be improved with sand nourishments because a municipality tax revenue can be used for such interventions (e.g., [2]). The sand can be either placed on the subaerial beach or in the subtidal beach, as an underwater mound. While the former is usually referred in the literature as a beach nourishment, the latter can be referred as profile nourishment [2], berm nourishment [3], nearshore berm [4] or shoreface nourishment [5]. In this study, shoreface nourishment was used to designate the placement of sand in the subtidal zone of a beach profile.

Following Dean [2], a shoreface nourishment has two advantages compared to beach nourishment. First, the dredging-dumping operation is less expensive (e.g., [6]). Second, it is associated with less restrictive policies regarding sediment quality characteristics.

J.; Baptista, P.; Silva, P.A.; Bernardes, C.; Pinto, C. Beach Response to a Shoreface Nourishment (Aveiro, Portugal). *J. Mar. Sci. Eng.* **2021**, *9*, 1112. https://doi.org/10.3390/ jmse9101112

**Citation:** Mendes, D.; Pais-Barbosa,

Academic Editor: Rodger Tomlinson

92

Unlike beach nourishments, shoreface nourishments can use sediments that are dredged from nearby navigation channels, subtidal bars or offshore deposits. Though, there are some examples that sediments dredged from navigation channels are used for beach nourishment [7]. As an example, a large beach nourishment intervention (1.5 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ) was performed in Dunkirk, France, using sediments dredged from a nearby navigation channel but with a median grain size (*D*50) coarser than those of the native beach [8].

Beach nourishment has been employed throughout the world. In the US, beach nourishment has been used since 1923 with an exponentially growth in sand volume placement by the end of the last century [9]. In Europe, beach nourishment interventions started after 1950. In the last decades, it has been a gradual change from the use of hard to soft coastal protection/defense techniques both for short-term and long-term coastal planning [10]. In Australia, beach nourishment interventions are generally smaller in scale but more frequent and mainly begin in spring to promote beach accretion [11]. In China, beach nourishment was first introduced in 1990 and the number of beach nourishment interventions has also show an exponential increase between 1990 and 2010 [12]. Despite the overall use of beach nourishment on sandy beaches, this solution can also be used in complex reef environments associated with irregular bathymetries [13,14].

In Portugal, the first beach nourishment intervention was performed in 1950 [7]. A recent review and compilation of beach nourishment practice in Portugal has shown that the main objectives are shoreline stability and erosion mitigation [7]. Nourishment interventions have been mainly performed with sediments dredged from maintenance channels' dredging. Moreover, there has been an increasing tendency to use soft engineering techniques in opposition to hard engineering solutions [7]. The largest shoreface nourishment intervention ever made in Portugal, before that reported in this study, took place in Aveiro in 1996 where a sand volume of 1.7 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> was deposited nearshore. Sediments were dredged from the navigation channel of the Aveiro lagoon and were placed southwards at Costa Nova beach. Shoreline stability was the objective of this intervention. Previous works conducted in Aveiro associated with shoreface nourishments observed cross-shore volume variations of up to 1500 m3/m (e.g., [15]), which suggests that this coastal stretch is morphologically very dynamic.

Following Brutsché et al. [4], pioneer shoreface nourishment interventions, undertaken between 1930 and 1940 in the US, raised some doubts about their overall effectiveness. Besides the observed sediment accretion near the shoreface nourishment, sediment erosion took place near the shoreline. These initial results postponed the use of shoreface nourishments until late 1960. Later on, the effectiveness of shoreface nourishments started to become documented. As an example, observations indicated that a shoreface nourishment located in Durban (South Africa) provided shelter to beaches [16]. Those beaches that were located on the lee side of the shoreface nourishment experienced up to 25% less erosion than those that were not. Nowadays, shoreface nourishments constitute a viable solution from a technical perspective [2]. However, stakeholders and the general public can still be reluctant to this type of solutions because they cannot be easily seen from the dry beach. Even though, the large number of shoreface nourishment interventions after 1940, that were documented in [4], provides some confidence that they are becoming better accepted.

Beach nourishment design, construction and subsequent monitoring is well documented in the literature (e.g., [2]). On the contrary, shoreface nourishment interventions are less well understood. In particular, shoreface nourishments can behave in two ways [2]. The shoreface nourishment can be placed in sufficiently deep water depths so that it remains there in time, usually referred as a stable berm. The stable berm main objective is to reduce storm damage relative to the level of damage that would have resulted without the nourishment. The shoreface nourishment can also be placed in sufficiently shallow water depths so that it moves landwards, usually referred as a feeder berm. The main goal of a feeder berm is to feed adjacent beaches with sand. The hypothesis is if a shoreface nourishment is placed in sufficiently shallow depths, its onshore movement will continuously promote beach accretion, through an increase in the subaerial beach width or volume. In

this study, field observations associated with a coastal monitoring program were analysed to assess the spatial and time evolution of a large (2 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ) shoreface nourishment intervention that was deployed between −10 m and −6 m (Mean Sea Level, hereafter MSL) water depths in Costa Nova (Aveiro, Portugal). In particular, this dataset was used to test the hypothesis just mentioned.

This paper is structured as follows. A review of previous works on shoreface nourishment interventions with a considerable volume (>0.8 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ) is performed in Section 2. The characterization of the study site and of the shoreface nourishment intervention, together with a description of the field data collection, processing and analysis is presented in Section 3. The results of the shoreface nourishment subtidal evolution and of the subaerial beach shoreline proxy evolution are presented in Section 4. In Section 5, a discussion is performed in light of the hypothesis that shoreface nourishment interventions that moved landwards can increase the subaerial beach width or volume. The discussion in Section 5 also compares the results analysed in this work with other shoreface nourishments performed elsewhere. Conclusions are summarized in Section 6.

#### **2. Previous Works on Shoreface Nourishments**

Five shoreface nourishment interventions were reviewed in this Section. They were chosen from a recent review on shoreface nourishments [4] and from a review of beach nourishment experience in Europe [10]. The five chosen shoreface nourishments were selected based on two criterion. First, shoreface nourishments had an overall volume > 0.8 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> , to be comparable with that reported in the present study (2 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> ). Second, the chosen shoreface nourishments had ready-available and well-documented general characteristics, such as shoreface nourishment length (*L*) and placement water depths (*h*) (Table 1). It was noted that a shoreface nourishment of 8.2 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> was performed in Anglet, France [10] but, unfortunately, its general characteristics were not available. The shoreface nourishment of 2.0 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> performed in Rio de Janeiro, Brazil, [17] was also excluded because the project not only included a shoreface nourishment but also a beach nourishment. Consequently, the overall beach response was not solely due to the shoreface nourishment. Moreover, the interesting case study in Denmark [18], where a comparison between a beach and a shoreface nourishment was conducted, was also excluded because the shoreface nourishment volume was 0.25 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> .


**Table 1.** General characteristics of shoreface nourishments in previous works.

In 1992, a shoreface nourishment of about 1.0 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> was built at Newport Beach, California, US [19]. The shoreface nourishment was placed in water depths between −4 to −9 m (MSL). Offshore wave conditions along the California coastline are characterized by an averaged offshore significant wave height (*Hm*0) that ranges from 1.75 m to 3.5 m in summer and in winter, respectively. Large swell waves generated in the Pacific Ocean are common at this site with an averaged peak wave period (*Tp*) of 12.3 s [20]. During storms, *Hm*<sup>0</sup> can reach or exceed 5 m offshore but it is effectively reduced due to Channel Islands sheltering effect close to Newport Beach [20]. The wave-induced sediment transport is towards the southeast direction [19] and the tidal range is about 2 m during spring tides. Overall, this nourishment moved onshore likely due to wave action. While the outer limit of the shoreface nourishment moved about 180 m onshore, the MSL contour moved offshore (i.e., beach width increase) about 30 m in 2.5 yr (Figure 2 in [19]). Moreover, the

analysis of profile surveys displayed no indication of an alongshore shoreface nourishment movement [19].

All the other shoreface nourishments in Table 1 were performed in The Netherlands. The wave climate along the Dutch coast is associated with an average significant wave height of 1.0 m during summer, which increases to 1.7 m during winter [21]. Winter storms are typically associated with an *Hm*<sup>0</sup> of 4 to 5 m and a *T<sup>p</sup>* of 10 s. The net sand transport rate ranges between 0.25 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> up to 0.6 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> [22]. The Terschelling shoreface nourishment displayed a complex behaviour with both offshore and onshore movement of the intermediate bar after nourishment (Figure 7d in [6]). The other shoreface nourishments performed in the Dutch coast (Terheijde, Egmond and Wassenaar) displayed an overall onshore movement of about 100 m in 4.5 yr for Wassenaar, about 100 m in 2 yr for Egmond and about 150 m in 3 yr for Terheijde. The MSL contour varied less than about 20 m for all four shoreface nourishments.

The values associated with onshore and offshore shoreface nourishments' displacement are summarized in Table 2. The shoreface nourishment onshore displacement increases for a more energetic wave climate. The MSL contour seaward displacement is much smaller (<20%) than the outer part of the shoreface nourishment landward movement, independently of the offshore wave conditions.

**Table 2.** Shoreface nourishment onshore displacement (*Son*), MSL contour offshore displacement (*so f f* ), and respective migration rates.


#### **3. Case Study and Field Observations**

### *3.1. Brief Description of the Coastal Settings*

The study area is located south of the Aveiro harbour entrance (Barra), and extends up to 10 km towards south until Vagueira beach (Figure 1). The coastline orientation is approximately 15◦ N. Coastal defense works along the study area (Figure 1) comprise two breakwaters near the Aveiro harbour entrance (white), six groynes (blue) and two revetments (orange). The beach is delimited by groynes at Costa Nova (between Barra and G5) and it is backed-up by dune systems southwards of G5. At Vagueira, the beach is both confined by one revetment and one groyne. The reader is referred to [23] for a more in-depth characterization of the study area.

Based on an analysis of field measurements collected by deep-water wave buoys offshore mainland Portugal [24], the offshore *Hm*<sup>0</sup> has a monthly-averaged value of 1.7 ± 0.7 m during maritime summer and increases to 3.1 ± 1.3 m during maritime winter. The monthly-averaged values of mean wave period oscillate between 6.0 ± 1.0 s and 8.0 ± 1.7 s along the year, with large swell waves easily reaching 20 s of *T<sup>p</sup>* during winter. The most frequent mean wave direction is from NW, with some sea states displaying a mean wave direction from W. On average, five to fifteen coastal storms (*Hm*<sup>0</sup> threshold of 4.5 m) hit the study site per year [25]. Tidal range varies from approximately 1.2 m to 3.6 m during neap and spring tides, respectively. The most frequent northwestern wave conditions can promote a net potential sediment transport rate of about 1 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup>3/yr directed towards south [26].

According to numerical modelling results [27], the coastal stretch south of Aveiro after 25 years of reduced sediment supply, critical situations of imminent sand-spit disruption are expected, as well as an ultimate linkage between the sea and the lagoon. The scenarios of sea level rise (SLR) are less important than the scenarios of wave-climate change after 25 years. A slight increase in the relative frequency of higher waves would have greater effects than a pessimistic scenario of the SLR rate.

Morphodynamically, the beach profile can be characterized as intermediate [28]. A well-defined subtidal bar is often present on the shoreface which induces partial depthinduced breaking of incoming waves. On the foreshore, the beach slope is mild during low tide and steep during high tide. Intertidal beach sediments are typically composed by fine to medium sand with a median grain size between 0.4 mm to 0.6 mm [29].

#### *3.2. Shoreface Nourishment Intervention*

The shoreface nourishment intervention comprised the dumping of sediments, dredged from the Zona de Atividades Logísticas e Industriais (ZALI) deposition area located inside the Aveiro harbour, between groynes 3 and 5 (G3 and G5 in Figure 1). These groynes are located at Costa Nova beach. Consequently, the beach response to the shoreface nourishment at Costa Nova is of particular interest because it is located closer to the intervention. The intervention was materialized by an underwater mound with a length of 1900 m and a width of 500 m. The shoreface nourishment was placed in water depths between −6 m to −11 m (MSL). At the end of the intervention, the mean water depth at the flat top of the underwater mound was −6 m (MSL). Sediments that were dumped at that location were compatible with native sediments but slightly coarser because the former were dredged in part from the Aveiro harbour entrance channel [30]. According to the Aveiro harbour authority, dredging works have started in May and last until August 2020. The analysis of the available multibeam surveys performed on June 2020 and on August 2020 allowed to detect a volume increase of 1.9 <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> at that location. This calculation was performed based on the August 2020 survey coverage area with reference to the depth of closure which is −10 m (MSL) (see Section 4.1).

#### *3.3. Methodology*

#### 3.3.1. Data Collection

Bathymetric surveys were performed with the research vessel Nereide from Centro de Estudos do Ambiente e do Mar (CESAM). This vessel can be equipped with a single beam or a multi-beam echo-sounder. The multi-beam survey was performed in an area pre-defined by the Portuguese Environment Agency (hereafter APA). The single beam survey was performed along the study area through cross-shore transects with a 500 m spacing (Figure 1). The vertical accuracy of the single-beam surveys is approximately 0.10 m. Part of the surf zone was not surveyed due to wave conditions at the study site. The bathymetric surveys were performed on July 2020, on September 2020 and on January 2021 (see Figure 4, below).

Topographic surveys were performed using a quad-bike equiped with the INSHORE monitoring system [31,32]. This allows a horizontal and vertical accuracy better than 0.05 m (see Baptista et al. [31]). Topographic surveys were performed approximately biweekly from September 2020 to June 2021 along the 10 km coastal stretch (from Barra to Vagueira in Figure 1). These surveys were performed during low-water on spring tides and each took between 3 h to 4 h to be completed. Each topographic survey was performed through cross-shore beach transects with a spacing between 30 m to 80 m and through longshore transects that accounted for the water line, berm crest and dune base, with a maximum spacing of about 30 m. These topographic surveys were used to extract the 1.05 m isoline that was analysed in Section 4 (Figures 5–7, see below).

At approximately 55 km northwest of the study site, offshore wave conditions were obtained from the Copernicus Marine Service (CMEMS, https://marine.copernicus.eu/, accessed on 28 June 2021). The wave parameters were *Hm*0, *T<sup>p</sup>* and the mean wave direction (*MDir*). The numerically generated wave data was carefully compared against buoy and satellite observations (see CMEMS report and assessment [33]).

**Figure 1.** Study area. Coastal defense works include two breakwaters at the Aveiro entrance channel, five groynes at the northern part of the study area and another at Vagueira beach and two revetments one close to G4 and the other at Vagueira beach. The area associated with the shoreface nourishment intervention was between G3 and G5. Beach profiles and topographic beach profiles were obtained within the COSMO programme. Coordinates are referred to the ETRS89 system.

#### 3.3.2. Data Processing and Analysis

For the bathymetric surveys, the acquired data was processed with Caris software (version 9.5). The obtained XYZ coordinates were interpolated in the QGIS software to produce a digital elevation model (DEM) with a horizontal resolution of 5 m using a Triangular Irregular Network (TIN) interpolation method. For the topographic surveys, the acquired data was processed with a set of dedicated software, namely the Trimble Business Center (version 5.32, www.trimble.com, accessed on 10 October 2021) to GPS data process and the Matlab (version R2020a) to estimate the inclination angles (i.e., the attitude) of the GPS antennas in the quad-bike for each sample period. The obtained XYZ coordinates were interpolated in the ArcGIS software to produce a DEM with a horizontal resolution of 1 m using a Kriging interpolation method.

The analysis conducted in this study used a set of cross-shore transects with a spacing of 50 m that were extracted from the generated DEM between Barra and Vagueira. These transects were perpendicular to the main coastline orientation (15◦ N). The extraction of cross-shore transects with a 50 m spacing allowed to analysed in more detail the spatial variations of the beach response in time along the coastal stretch.

#### 3.3.3. Additional Data Sources

To complement the analysis conducted in this study, additional data sources were used. These sources were all the available topo-bathymetric surveys, beach profile surveys and topographic beach surveys obtained between July 2018 and November 2020 under the Portuguese COaStal MOnitoring Programme (COSMO), developed and implement by APA (https://cosmo.apambiente.pt/, accessed on 28 June 2021) [34]. Moreover, a bathymetric survey provided by the Aveiro harbour authority was also used in this study. The latter was performed right after the shoreface nourishment intervention on August 2020 and it covered the area in Figure 1 (dashed white line).

#### **4. Results**

#### *4.1. Depth of Closure and Beach Volume Based on a Shoreline Elevation Proxy*

The beach profile surveys, obtained before the shoreface nourishment through the COSMO programme, were used to estimate the elevation associated with the depth of closure in the study area. The beach profile surveys were conducted with a wave runner in the submerged part, thereby surveying the entire surf zone, and by foot on the intertidal and aerial part. In Figure 1, the guide lines of the submerged part of the beach profile survey are displayed in yellow. The available profiles were located near Costa Nova beach (between G3 and G4) and at Vagueira beach.

Figure 2 shows the beach profile surveys for Vagueira (left) and Costa Nova (right). The bottom panels display the standard deviation of the beach profiles. At Vagueira, the standard deviation rapidly increases from its offshore value of less than 0.20 m to values larger than about 0.50 m (Figure 2e). In more details, the standard deviation has an inflexion point for distance equal to 650 m. This inflexion point is associated with an elevation of −10 m (MSL). At Costa Nova, it is clear that the November 2020 profile has an influence of the shoreface nourishment (Figure 2b,d) and it was not used in the standard deviation calculation. A similar analysis performed on Costa Nova beach profile also suggests that the elevation −10 m (MSL) is associated with very small morphological changes (less than 0.20 m) (Figure 2d). Therefore, the analysis of the COSMO profiles allowed to estimate a depth of closure of −10 m (MSL) for this study site. Note that the shoreface nourishment intervention led to morphological modifications up to −11 m (MSL) at Costa Nova (November 2020 in Figure 2a,b) but this was due to the intervention and not due to the natural beach behaviour (see Figure 4 of [35]).

**Figure 2.** Beach profiles at Vagueira (panel (**a**)) and at Costa Nova (panel (**b**)) beaches (yellow lines in Figure 1) between 2018 and 2020. Zoom-in of the submerged beach profiles (panels (**c**,**d**)). Standard deviation of beach profile elevations for each cross-shore position (panels (**e**,**f**)).

Also available through the COSMO programme were four subaerial topographic profiles that can be found along the coastal stretch depicted in Figure 3 (see also red lines in Figure 1). The profiles were collected between August 2018 and October 2020 every 3 months. Therefore, the seasonal subaerial beach variability throughout is well captured. These field measurements were used to understand how well the displacement of an elevation in time can represent the beach volume temporal evolution. Beach profiles display an elevation (about 5 m, MSL) above which the morphological variations are smaller than 0.3 m (top panels in Figure 3). An exception is the beach profile Vagueira 2 because it is located in front of a revetment. The beach volume (per unit width) for Barra, Costa Nova and Vagueira 1 was determined as the area above MSL until the point in which the profiles converge (5 m, MSL), see Figure 3. For Vagueira 2, the beach volume (per unit width) was calculated as the volume above MSL that is delimited by the revetment toe (distance = 25 m, Figure 3). Next, it was assessed how well the displacement of several isolines (ranging from 0 m to 5 m) through time can describe the temporal beach volume variations. The middle panels of Figure 3 show the correlation coefficient between those two quantities. In general, the onshore-offshore displacements of isolines between 1 and 3 m are well correlated with the beach volume variations. For the Vagueira 1 beach, this is not the case and there is a well-defined maximum of the correlation coefficient around 1 m. Motivated by this maximum and since other beach profiles have also correlation coefficients higher than about 0.9 for 1 m, the following shoreline proxy was used: the 1.05 m (MSL) contour, for two reasons. First, this contour is capable of describing well the temporal variations of beach volume for the four beach profiles located across the coastal stretch (bottom panels of Figure 3). Second, the 1.05 m (MSL) contour is the averaged value of the high-tide at Aveiro tidal gauge, which corresponds to the shoreline [36]. Therefore, the displacement of the 1.05 m (MSL) contour will be used to characterize the subaerial beach response to the shoreface nourishment in time and in space.

**Figure 3.** Topographic beach profiles at Barra, Costa Nova, Vagueira 1 and Vagueira 2 beaches (red lines in Figure 1, from North to South) (**top** panels). Correlation coefficient between beach volume (per unit width) and the displacement of elevation contours (**middle** panels). Scatter diagrams between beach volume and the displacement of the 1.05 m (MSL) contour (**bottom** panels).

#### *4.2. Subtidal Beach Response to the Shoreface Nourishment*

Figure 4 shows the temporal evolution of the −10 m and of the −8 m (MSL) contours along the coastal stretch between June 2020 (previous to the nourishment) and January 2021. The initial sediment deposition zone, between G3 and G5, is obtained from the August 2020 multibeam survey provided by the Aveiro harbour administration.

Since the closure depth was disturbed by the shoreface nourishment intervention, its onshore or offshore displacement is associated with the shoreface nourishment movement through time. As an example, if the isobathymetric contour of −10 m (MSL) does not change in time, it means that the shoreface nourishment is relatively stable. On the opposite, if the −10 m (MSL) contour moves landward, it means that the shoreface nourishment moved towards the coast. To complement the analysis, the −8 m (MSL) contour was also used. The latter is expected to have a larger displacement in space than the −10 m (MSL). Other less deep contours were also envisaged but they were more influenced by the bar movement that occurred southward and northward of the initial shoreface nourishment area. Consequently, the shallower isobathymetric contours will not be considered hereafter.

Regarding the −10 m (MSL) contour, its offshore displacement ranged between 250 m at the southward end to 120 m at the northward end of the initial deposition zone (between June and September 2020). Between September 2020 and January 2021, while the onshore displacement at the northward end was smaller than 50 m, the onshore displacement reached 120 m at the southern end. Differences between the −10 m (MSL) contour between G1 and G2 are likely associated with the ebb-tidal delta shoal dynamics of the Aveiro inlet and to the different survey technologies (i.e., multibeam vs single-beam), and will not be discussed hereafter. The horizontal differences between the −10 m (MSL) contours are smaller than 50 m at 1600 m southwards (Y = 102,000 m in Figure 4a). Therefore, the

influence of the shoreface nourishment on the displacement of the −10 m (MSL) contour south of the initial deposition zone had an extent of about 1600 m.

**Figure 4.** Time evolution of the bathymetric contour associated with −10 m (MSL) (panel (**a**)) and with −8 m (MSL) (panel (**b**)). Coastal defense works over the study area (black) and the area associated with the shoreface nourishment intervention was between G3 and G5 (dashed white). Coordinates are referred to the ETRS89 system.

The patterns displayed by the −8 m (MSL) contour are more pronounced than those associated with the −10 m (MSL) contour. The offshore displacement between June and August 2020 ranged between 280 m to 350 m at the initial deposition zone. After one month (September 2020), the −8 m (MSL) contour shifted landwards between 50 m at the central area (in front of G4) and 120 m at the end points (in front of G3 and G5). The largest landward migration at the end points is in part explained by the very sharp contours that are rapidly eroded. On January 2021, the overall shape of the shoreface nourishment resembles a gaussian function (Figure 4b, January 2021), denoting the spreading of the nourishment in the longshore direction (Figure 4b). The onshore displacement of the −8 m (MSL) contour was 30 m between September 2020 and January 2021 at the central part (in front of G4). This onshore displacement increased farther away, reaching about 100 m in front of G3 and of G5. The influence of the shoreface nourishment was felt until about 600 m towards north (200 m north of G3). At south, the onshore or offshore displacements of the −8 m (MSL) contour can be associated with the subtidal bar movement, as seen in Figure 2a, thereby preventing to isolate the effect of the nourishment. The latter aspect is visible in front of Vagueira beach where the September 2020 −8 m (MSL) contour is not only located 60 m landward than the January 2021 contour but also about 40 m landward than the June 2020 contour.

#### *4.3. Subaerial Beach Response to the Shoreface Nourishment*

The analysis of the beach response to the shoreface nourishment was based on the 1.05 m (MSL) contour (see Figure 3 and Section 4.1). Figure 5 shows the spatial and temporal displacement of the 1.05 m (MSL) contour along the coastal stretch relative to its position on the beginning of June 2020, together with the the time series of wave parameters. While warm colors indicate an offshore displacement (i.e., beach volume increase), cold colors are associated with an onshore displacement (i.e., beach volume decrease). The horizontal black lines represent the five groynes (G1 to G5 in Figure 1) located at Costa Nova beach. The absence of colored circles means no data.

In general, three main features can be observed. First, the dominance of a landward displacement (blue circles) for distance between 3000 m and 4000 m. This displacement, that can reach locally −60 m, is located at the downdrift side of the shoreface nourishment. In more details, the shoreface nourishment was performed between G5 and G3. The retreat of the 1.05 m (MSL) contour relative to the June 2020 can be associated with modification of local wave parameters which drive divergency of the longshore sediment transport at the end parts of the shoreface nourishment. This erosion pattern was pointed out by [5] where the downdrift part can experience shoreline retreat in analogy to a detached breakwater. Second, the large seawards displacement (red circles) for distance equal to 2800 m between September and November 2020, and also on February and on April 2021. It is speculated that this seawards displacement is associated with the natural beach behaviour which increases in volume until late summer. Unfortunately, a comparison with older surveys is not possible because they are not available with the high-temporal and spatial resolution as those presented in this study. Third, the mild offshore displacement of the 1.05 m (MSL) contour located between G4 and G2. This offshore displacement through time is likely induced by the shoreface nourishment. The beaches delimited between G1 and G4 do experience the expected response due to the longshore drift until November 2020, which is from North to South at this study area during summer (see Figure 7 in [26]). Between September and October 2020, those beaches are associated with a seawards displacement northward of each groyne and with an onshore displacement southward (i.e., downdrift side) of each groyne. From November 2020 onward, this pattern changes and the onshore displacement is northward of each groyne, while the offshore displacement occurs at the downdrift side. This inversion in the expected beach erosion and accumulation associated with groynes is clearly observed for beaches between G2 and G5. Moreover, the mean wave direction become more confined to the WNW and W sectors (see January to February 2021 in Figure 5d). The maximum landward displacement of the shoreline proxy was −60 m at distance between 3000 m and 4000 m between January and April 2021. The maximum seawards displacement was +60 m at distance equal to 2800 m between September and October 2020.

**Figure 5.** Time series of offshore significant wave height (**a**), peak period (**b**) and mean wave direction (**c**) obtained from CMEMS (https://marine.copernicus.eu/, accessed on 28 June 2021), and 15-day moving average (red). Time-series of the relative 1.05 m (MSL) shoreline proxy advance seawards (warm colors) or retreat landwards (cold colors) (**d**) along the coastal stretch from Vagueira (0 m) to Barra (9500 m). The relative advance or retreat is in comparison with the shoreline proxy position surveyed on June 2020 (reference situation before shoreface nourishment). Horizontal black lines refer to groynes and to breakwater.

#### *4.4. Subaerial Beach Response to Storms and Subsequent Recovery*

The beach response to storms was analysed based on the relative displacement of the shoreline proxy (1.05 m, MSL) in relation to the previous topographic survey, as shown in Figure 6. As an example, the shoreline proxy displacement on 17 December 2020 is obtained as the difference between 17 and 3 December 2020 surveys.

**Figure 6.** Time series of offshore significant wave height (**a**), peak period (**b**) and mean wave direction (**c**) obtained from CMEMS (https://marine.copernicus.eu/, accessed on 28 June 2021), and 15-day moving average (red). Time-series of the relative 1.05 m (MSL) shoreline proxy advance seawards (warm colors) or retreat landwards (cold colors) (**d**) along the coastal stretch from Vagueira (0 m) to Barra (9500 m). The relative advance or retreat is in comparison with the shoreline proxy position surveyed on the previous survey. Horizontal black lines refer to groynes and to breakwater.

Looking at Figure 6d, the largest differences between consecutive surveys took place on December 2020. More specifically, the shoreline proxy moved seawards on 3 December and moved landwards on 17 December. Regarding the landward displacement, this occurred after storm Dora, which hit mainland Portugal on 4 December 2020. However, the beach response was very variable across the coastal stretch. The largest changes are more visible on natural beaches (south of G5) than on beaches close to coastal structures (between Barra and G5), which are under the influence of the shoreface nourishment. The offshore *Hm*<sup>0</sup> time-series clearly displays a very large increase on that day (up to 8 m) with an incident mean wave direction of about 320◦ N. The shoreline proxy retreat reach up to −50 m in some areas. The seaward displacement downdrift of G3 and G2 after storm Dora is likely associated with the presence of the shoreface nourishment. At that locations, the shoreline proxy advanced approximately 10 m. Regarding the seaward displacement that occurred on 3 December (south of G5), this beach accretion was likely associated with a reduction of the offshore *Hm*0, together with large *Tp*. The 15-day moving average window suggests that *Hm*<sup>0</sup> decreased from 4 m to about 2 m. This reduction accompanied by large

wave periods have most likely promoted the beach accretion (i.e., seaward displacement of the shoreline proxy). Again, the shoreline proxy advance is not uniform along the coastal stretch.

Beach accretion after storm Dora is displayed in Figure 7d. In this Figure, each shoreline proxy relative position is compared with the shoreline proxy position on 3 December (before storm Dora). Two major patterns emerged in Figure 7. First, the shoreline proxy 1.05 m (MSL) rapidly moved landwards at the downdrift side of G2 and of G3 from January 2021 onward. Second, the shoreline proxy continued to moved landwards on beaches backed up by dunes (those located southwards of G5). At those locations, there are no clear patterns of total recovery because the circles are still associated with negative values. The shoreline proxy close to Vagueira beach started to display some values close to 0 m on March 2021. This indicates that the 1.05 m (MSL) contour has recovered its 3 December relative position at that location, which can be in part ascribed to the Vagueira beach natural behaviour.

**Figure 7.** Time series of offshore significant wave height (**a**), peak period (**b**) and mean wave direction (**c**) obtained from CMEMS (https://marine.copernicus.eu/, accessed on 28 June 2021), and 15-day moving average (red). Time-series of the relative 1.05 m (MSL) shoreline proxy advance seawards (warm colors) or retreat landwards (cold colors) (**d**) along the coastal stretch from Vagueira (0 m) to Barra (9500 m). The relative advance or retreat is in comparison with the shoreline proxy position surveyed on 3 December 2020 (reference situation before storm Dora). Horizontal black lines refer to groynes and to breakwater.

#### **5. Discussion**

#### *5.1. General Characteristics of the Aveiro Shoreface Nourishment in Comparison with Previous Works*

The review of previous works presented in Section 2 (see Table 1) suggests that shoreface nourishments performed at locations with both larger significant wave heights and wave periods have a *V*/*L* value (ratio between nourishment volume and shoreface nourishment length) about a factor of 2 greater than those performed at locations characterized by milder and local generated waves. The shoreface nourishment movement landwards is performed with a migration rate about 50% larger than those in milder environments. In both mild or energetic wave conditions, the MSL contour displacement ranges between 13% to 20% of the shoreface nourishment onshore migration. Moreover, for the shoreface nourishment in California, Figure 2 in [19] suggests that the increase in beach volume above MSL is much smaller than the reduction of shoreface nourishment volume. For the Dutch shoreface nourishments, ref. [37] mentioned that the nourishment only acted partially as a feeder berm.

The Aveiro shoreface nourishment had an overall sediment volume (*V*) of about <sup>2</sup> <sup>×</sup> <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> and a length (*L*) = 1900 m, which gives a *<sup>V</sup>*/*<sup>L</sup>* = 1050 m3/m. Its <sup>−</sup>8 m (MSL) contour onshore displacement, ranged from 80 m to 230 m between August 2020 and January 2021. Although the observations available in this study do not coverage one year, the initial onshore displacement rate (rate *Son*) is between 200 m/yr to 500 m/yr. The local wave conditions at Aveiro are similar to those offshore because there are neither a sheltering effect promoted by islands nor dissipation by bottom friction over the continental shelf (about 80 km at Aveiro). In general, the Aveiro shoreface nourishment shares similar characteristics with that performed in California, US [19]. Wave conditions at California are associated with a larger *Hm*<sup>0</sup> than those at Aveiro (3.5 m in winter compared to 2.8 m) but the local wave conditions at California are likely milder due to the Channel Islands sheltering effect. Therefore, the large onshore displacement of Aveiro shoreface nourishment (200–500 m/yr), when compared to California (72 m/yr), can be attributed to the more energetic local wave conditions.

Regarding the Dutch nourishments [6], they were performed with a V/L ratio about a factor of 2–3 smaller than both Aveiro and California nourishments (Table 1). Their onshore migration rates varied for each nourishment, ranging from about 20 m/yr, for Wassenaar, to 50 m/yr, for both Egmond and Terheijde. These rates are smaller when compared with 72 m/yr for California and with 200–500 m/yr for Aveiro. In general, the wave climate along The Netherlands is milder and characterised by local generated waves. Even if the wave climate is different from that in Aveiro and California, often more energetic and characterized by swell waves, the differences between wave climates alone cannot explain the differences in the onshore migration rates. This is because for the same wave climate and with similar V/L ratios, the onshore migration rate of Wasenaar nourishment (22 m/yr) was about half of the rate of Egmond nourishment (50 m/yr). We suggest that other factors, such as tidal-induced velocities, beach or surfzone slopes, sediment grain sizes, coastal geomorphology and man-made structures (e.g., harbours) can contribute to explain these differences on the overall shoreface nourishment evolution.

The maximum seaward displacement of the 1.05 m (MSL) contour was about 60 m when compared to its reference position on June 2020. Although this displacement varied along the study area (see Figure 5), this gives an approximate offshore migration rate of 60 m/yr. Despite the fact that this offshore migration rate is larger than that of other shoreface nourishments, it is still about 30% of the onshore movement of the Aveiro shoreface nourishment (200 m/yr). Therefore, although the onshore and offshore displacements associated with the Aveiro shoreface nourishment are more pronounced than the shoreface nourishments performed elsewhere, the shoreline proxy advance seawards only accounted for 30% of the smallest shoreface nourishment outer limit advance landward. This suggests that the Aveiro shoreface nourishment behaved only partially as a feeder berm. In other words, the Aveiro shoreface nourishment did not contribute exclusively to a

subaerial beach volume increase. The remaining part of the sediment volume associated with the shoreface nourishment have likely dispersed alongshore driven by the littoral drift. The latter suggestion is supported by the gaussian shape of the shoreface nourishment −8 (MSL) contour in January 2021 (Figure 4b).

#### *5.2. Response of the Shoreline Proxy to the Shoreface Nourishment*

The results presented above clearly highlighted distinct behaviours of the shoreline proxy relative to its cross-shore position before the shoreface nourishment intervention (June 2020, in Figure 5) and after the major storm Dora (December 2020, in Figure 6). The results suggest that the most clear shoreline proxy displacement onshore occurs at the downdrift side of the shoreface nourishment intervention (cold colours in Figure 5d). This behaviour has been pointed out by Van Duin et al. [5]. In our case study, the extent of this effect was about 2000 m southwards of the initial deposition zone. Since the length of the Aveiro shoreface nourishment is 1900 m, it is suggested that the shoreline retreat associated with this shoreface nourishment is about the same length as the shoreface nourishment itself.

The longshore spreading and evolution of the shoreface nourishment is about their initial length (1900 m). The results show a southward displacement of the −10 m and of the −8 m isolines up to 1600 m (Figure 4). Towards north, the shoreface nourishment evolution is influenced by the ebb delta shoal of Aveiro inlet but it extends up to 600 m northwards. Based on the available bathymetric surveys, it can be expected that the shoreface longshore spreading is about its initial length during the first year of morphological evolution.

Beaches located closer to the shoreface nourishment were benefited. This effect was present in three comparisons: to its relative position on June 2020; after storm Dora; and also during subsequent beach recovery. As an example, during storm Dora beaches located near the shoreface nourishment (between G3 and G5) even experienced a shoreline proxy advance seawards. This effect is likely offered by the sheltering effect of the shoreface nourishment which is capable of reducing wave amplitude during storms. Moreover, beaches closer to the shoreface nourishment also experienced recovery much quicker than those farther away. Looking at Figure 7d, the shoreline proxy associated with beaches located between G4 and G2 displayed positive values after storm Dora (>20 m, yellow and red markers) from January until June 2021. On the opposite, the shoreline proxy associated with beaches located southwards of G5 displayed negative values (<−20 m, cyan and blue markers). There are some locations where this pattern was not so clear, such as beaches located updrift of G3 (where shoreline proxy retreat reached −40 m between February and March 2021) and also beaches located downdrift of G5 (where the shoreline proxy advanced up to 40 m on mid April 2021). Though, the general pattern is that the relative position of the shoreline proxy accreted more between G4 and G2 than at beaches located southwards of G5, where a shoreline proxy retreat was observed. Therefore, beaches located closer to the shoreface nourishment (between G4 and G2) achieved their position before storm Dora (warm colours in Figure 7d) quicker than beaches located southward of G5 (cold colours in Figure 7d).

On a seasonal scale (from September 2020 to April 2021), beaches located closer to the shoreface nourishment experienced a milder but steady advance seawards (see warm colours between G4 and G2 in Figure 5d). An interesting result is that beaches delimited by groynes have experienced a different morphodynamic pattern than that expected from the longshore drift. The expected pattern occurred until October 2020 on beaches located between G2 and G4, with erosion downdrift (cold colours in Figure 5d) and accretion updrift (warm colours in Figure 5d). From November 2020 on, but more clear in January and February 2021, this pattern changed and beach accretion occurred on the downdrift side of a groyne (warm colours in Figure 5d), while beach erosion took place on the updrift side of a groyne (cold colours in Figure 5d). This inversion is possibly linked to the nearshore circulation patterns induced by the shoreface nourishment [5], and also with the directional wave conditions. Future numerical modelling efforts may be used to understand the reason for this observed inversion.

The results presented in this study suggest that the shoreface nourishment was beneficial to the subaerial beach in different ways. Although the results presented above also allow to better understand the beach response to a shoreface nourishment at Aveiro, more coastal monitoring is definitely desirable to draw more firm conclusions. The use of simple techniques to assess shoreface nourishment expected evolution can be of interest for preliminary designs [38]. Additionally, the use of video cameras can be included to complement topographic and bathymetric measurements [39]. Moreover, the application of numerical models (either more simple, such as one-line models, or more complex, such as coastal area models) can be envisaged in future (e.g., [40]). These type of applications would not only allow to verify numerical models but also to test different shoreface nourishment options and configurations by varying its width, length, volume and distance to shoreline, and also to perform cost-benefit analysis of coastal protection strategies [41].

#### **6. Conclusions**

In this study, the beach response to a shoreface nourishment was analysed based on topographic and bathymetric surveys performed over one year along 10 km in Aveiro (Portugal). The analysis was based on the evolution of the −10 m and −8 m (MSL) contours and of the 1.05 m (MSL) contour. The former is associated with the displacement of the shoreface nourishment outer limit and the latter is associated with the temporal variation of the beach volume above MSL, for this study area.

While the onshore displacement of the −10 m (MSL) and of the −8 m (MSL) contour ranged between 80 m and 230 m, the maximum offshore displacement of the 1.05 m (MSL) contour was 60 m. Therefore, our results suggest that the shoreface nourishment intervention only acted partially as a feeder berm. This is in agreement with shoreface nourishment of similar volume magnitudes performed elsewhere.

The beach response (the onshore or offshore displacement of the 1.05 m contour) was highly variable in time and along the 10 km study area. Beaches located closer to the shoreface nourishment (delimited by groynes) exhibit a more stable behaviour, are not so vulnerable to wave conditions than those located south of G5 and display a large seaward displacement (i.e., beach volume increase). During storm Dora (December 2020), beaches located closer to the shoreface nourishment even display an offshore advance (i.e., beach volume increase). Moreover, the beach response is also quick on that beaches. A drawback associated with the shoreface nourishment was the beach retreat at the downdrift side of the nourishment. This effect occurred until a distance that is about the shoreface nourishment length.

**Author Contributions:** Conceptualization, D.M.; data collection, D.M.; methodology, D.M.; software, D.M.; formal analysis, D.M., J.P.-B., P.B., P.A.S., C.B. and C.P.; resources, P.B.; writing—original draft preparation, D.M.; writing—review and editing, J.P.-B., P.B., P.A.S., C.B. and C.P.; project administration, P.B. and C.P.; funding acquisition, P.B. and C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** Some of the collected data were co-funded by POSEUR (Ref. POSEUR-02-1809-FC-000051— Remoção dos Inertes da Zali do Porto de Aveiro para Reforço do Cordão Litoral a Sul da Costa Nova) and are gratefully acknowledged. Data were also obtained under the COSMO Programme-Coastal Monitoring Programme of Continental Portugal, of the Portuguese Environment Agency, co-funded by the Operational Program for Sustainability and Efficiency in the Use of Resources (POSEUR), https://cosmo.apambiente.pt, accessed on 28 June 2021.

**Acknowledgments:** The authors acknowledge Rita Cavalinhos (CESAM field technician) and Paulo Rosa (CESAM skipper) for their fantastic work during the bathymetric surveys. Rita Cavalinhos is also acknowledged for her work during the multibeam surveys pre-processing phase. Fábio Santos is also acknowledged for his help during the bathymetric surveys. The Aveiro harbour authority is warmly acknowledged for providing the multibeam survey. Comments and suggestions provided by two anonymous reviewers are acknowledged. Thanks are due to FCT/MCTES for the financial

support to CESAM (UIDP/50017/2020+UIDB/50017/2020), through national funds. SANDTRACK project (PTDC/CTA-GEO/31779/2017) funded by FEDER, through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI), and by national funds (OE), through FCT/MCTES.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

