1. Introduction
Coastal degradation has become a pressing problem due to changes in natural coastal conditions, mainly linked to population growth and an expanding global economy [
1,
2]. This degradation has been aggravated by climate change and rising sea levels, which have further deepened and accelerated this phenomenon [
3,
4]. As a result, coastal areas have been increasingly exposed to hazards such as erosion and flooding, which are expected to worsen in the coming years [
5,
6,
7,
8,
9].
The solutions that have been proposed to address these problems have been totally dependent on traditional structures, such as seawalls, breakwaters, and other structures [
10,
11,
12,
13,
14]. While these structures have solved specific problems of coastal protection in the short (several years) and medium term (one decade), they have been incapable of addressing issues such as the sea level rise in the long term (several decades) [
15,
16]. The cost of maintaining these structures and, in some cases, of retrofitting them, is so high that it makes them unsustainable in the long term [
17,
18,
19,
20].
Given the problems caused by the rigidification of the coastline, more sustainable alternatives have been evaluated in recent years to ensure the long-term effectiveness of coastal protection [
21,
22,
23]. A wide variety of marine ecosystems have been evaluated for their effectiveness against climate-change-related impacts, such as mean sea level rise [
24,
25], wave height variability [
26,
27,
28], or coastal erosion [
29,
30]. This has generated complementary alternatives to the hard engineering interventions that have dominated the urbanized coastal landscape for years [
31,
32,
33]. In addition, marine ecosystems have been able to deliver effective ecological [
34,
35] and economic value over time [
34,
36,
37]. Seagrass meadows have been an example of how these ecosystems can provide protection from climate-change-related events [
38,
39], by reducing wave currents, in order to support a variety of species that enrich the marine ecosystem [
40,
41].
The role of seagrass in coastal protection has been evaluated in both field [
42,
43,
44] and laboratory research [
26,
45,
46]. In recent years, experimental research with surrogate materials that mimic seagrass meadows has increased [
47,
48,
49], improving knowledge of the physical processes that govern the hydrodynamic changes produced by these ecosystems. However, studies have continued to focus on meadow zone processes, without monitoring the continuity of changes observed in onshore areas closer to the coastline [
26,
48,
49,
50,
51].
Experiments with natural plants have proven difficult to replicate due to the complex interplay of factors in their natural habitat, such as variations in light, salinity, nutrients, or water quality, which can trigger stress responses and alter plant properties such as buoyancy or stiffness [
52]. In addition,
Posidonia oceanica meadows are protected from extraction from their natural habitat. Therefore, the use of surrogate materials is required to conduct large-scale experiments. To create an accurate mimic of
Posidonia oceanica, a material was carefully selected to replicate the leaf behavior under wave action, as described by [
47]. This involved considering materials with densities and yield strengths similar to those of the actual leaves, while ensuring that the dimensions closely matched those of the species.
The work presented here aims to evaluate the process of velocity changes produced by a surrogate
Posidonia oceanica meadow in the nearest area behind the meadow. In addition, it assesses how these changes propagate along a beach profile up to the breaking location. Moreover, a sediment transport formula is used to corroborate the effect of velocity changes over the decrease in onshore sediment transport measured under the presence of the
Posidonia oceanica meadow [
47].
4. Discussion
The presence of a surrogate seagrass meadow in a series of mobile bed flume experiments leads to an increase in the current velocity (
Uc) measured in the area immediately in front of and behind the meadow. In front of the meadow,
Uc increases slightly by 0.012 ms
−1 (78.7%) on average over the experimental time (6 h of waves). Behind the meadow zone,
Uc increases significantly by 0.049 ms
−1 (365.4%) on average. Although the observed increase in values in front of the meadow are within the range of the sensor variability (
Table 2), the increase in
Uc behind the meadow is above this variability and can be attributed to a direct effect of the seagrass meadow studied. These results are consistent with previous studies that assessed velocities around the seagrass canopy in areas upstream [
63] and downstream of the meadow [
45,
49,
63,
64]. This
Uc could be explained by the velocity overshoot produced by a progressive wave above the boundary layer and by the progressive wave streaming expected within the boundary layer. Although the measurement point is not within the boundary layer, the presence of the oscillating bending long leaves and their associated turbulence is expected to increase the thickness of the wave boundary layer and therefore increase the shoreward velocity streaming produced by both effects. However, this experiment did not provide the necessary data to confirm such behavior.
The sensor variabilities shown in
Table 2 are slightly higher than the variabilities measured by the ADV in the comparatively quiescent background (−0.006 ± 0.006 ms
−1), calculated in the first 5 s after the start of each test. These values were associated with white noise or Doppler noise and are intrinsic to the ADV [
65]. However, the measured changes in peak velocities (
Table 5) are greater than the sensor variabilities (
Table 2). The variations of both maximum and minimum velocities are attributed to the presence of the meadow and the development of the breaker bar. The latter can be seen more clearly from t7 (3.5 h of waves), where the peak variabilities in front of the meadow increase their value (
Table 4) and the positive peaks increase their velocities even more with respect to previous times (
Table 5). The variability in velocity intensity and peaks can be observed in all tests and could be associated with the direct effect of the meadow.
In the area in front of the meadow, there is no appreciable change in Sk and As compared to the case without the meadow throughout all tests. The mean value of As for 12 tests was −0.004 for BR60 and −0.003 for R60, and the mean value of Sk was −0.087 for both layouts. Despite this, up to t6, there was an increase in positive peak velocities (mean 5.6%) and a slight reduction in negative peak velocities (mean 2.0%), without this affecting the magnitude of mean velocities (mean positive peak plus mean negative peak), resulting in changes of less than 0.010 ms−1.
Above the meadow, it is observed that
Sk values show little variation between layouts at the beginning, while
As was slightly reduced when the meadow was present. At the end of the meadow, the
Sk value increased for R60, while
As decreased. Studies such as that of [
49] have also observed a decrease in
As inside a mimic seagrass meadow patch and have found in their research that the larger the patch size, the greater the
As decrease. In addition,
Sk values increase when the surrogate meadow is present, but only under the most energetic waves, with an even greater increase when the patch covered a smaller area [
49].
In the area behind the meadow, a shift of the whole
u-series towards the positive axis can be observed, which is associated with the increase in
Uc. Additionally, there is an increase of 0.1 in
Sk compared to the BR60 layout, which is associated with a vertical deformation of the incoming waves. This effect is confirmed by the increase of positive peaks and the reduction of negative peaks under the waves (
Table 5), which adjusted to a new waveform when passing through the meadow. This implies that onshore velocities are higher, while offshore velocities are lower in magnitude than in the case without the surrogate meadow.
As values remain unchanged and behave similarly to the
As data obtained in front of the meadow.
The discrepancies in
Sk behind the meadow have been analyzed for the entire velocity series, comparing different layouts within the same test number and sensor position.
Figure 7a shows the velocity series measurements at the same location and time (time series t2) for both layouts. The upper and lower parts of
Figure 7a and
Figure 6b correspond to positions
X = 41.4 m and
X = 54.2 m, respectively. At
X = 41.4 m (
Figure 7a), a positive shift of the entire
u-series can be observed when comparing layouts BR60 and R60, indicating an increase in the magnitude of positive peak values and a decrease in the magnitude of negative peak values when the meadow is present. Additionally, this increase in
u is more pronounced in 4 points associated with wave relaxation periods. The upper part of
Figure 7b provides an example of this further increase in
u between times 123 and 137 s when the meadow is present, causing a significant rise in the velocities of low-magnitude flows. However, these changes in wave relaxation are not observed at more onshore positions, as illustrated in the lower part of
Figure 7b, which presents the velocity measurements at
X = 54.2 m.
As the position of the bar crest approaches the ADV control point, the value of Sk becomes more positive. At X = 45.7 m, layout R60 exhibits a slightly lower Sk compared to BR60, with both layouts showing lower magnitudes than when measured at positions in front of the meadow. The changes in peak velocities at this position experience a similar variation, with reduced maximum and minimum peaks in R60 with respect to BR60. At this location, the wave velocities take on a similar shape as in positions at X = 27.6 m but with a decrease in velocity magnitude in R60 relative to BR60.
At X = 50.2 m, the differences in Sk values between both layouts are even smaller than at X = 45.7 m, but their magnitude increases with respect to this position. The minimum and maximum peaks are similarly reduced. At t6, the peak velocities show a variation of less than 1.0% when comparing the two layouts, but at t8 the reduction of positive peaks is 8.6% and negative peaks decrease by 7.9% when the meadow is present. This velocity reduction is consistent with the results from the previous position.
Finally, at
X = 54.2 m, a general decrease in the maximum and minimum
u magnitudes (lower part of
Figure 7b) associated with a reduction in the magnitude of both maximum and minimum peaks can be observed when seagrass is present. Additionally, registered
Sk values turned positive and are higher at BR60 than at o R60, where both negative and positive peaks are reduced, except at t7 where they increase. This progressive increase in
Sk values reflects the change in wave shape as it passes through the shoaling to the breaking bar zone. However,
As values do not show significant variations in the same area, except at position
X = 54.2 m where it becomes slightly positive. It is worth noting that at t9 for the BR60 layout,
As presents a greater increase compared to other times and positions (
Figure 5). In this test, the position of the bar crest (
X = 56.4 m) is very close to the control point, so the wave starts to accelerate shorewards to generate a breaking wave. In the R60 layout, this process does not occur at this position but takes place two meters further onshore. The greater increase in
As of BR60 is attributable to a greater increase in sediment transport found where the bar is developing (See
Figure 6). This is consistent with previous studies [
66,
67,
68] that have shown that higher
As is correlated with a higher rate of sediment transport in the direction of wave propagation.
In summary, the presence of the meadow causes variations in velocities due to the enhanced resistance it offers to wave propagation. This resistance leads to lower
As values as the waves pass through the meadow, resulting in higher magnitudes of offshore velocities. The
Sk values increase as the waves pass through the meadow and become positive upon leaving the meadow (
Figure 4). This destabilizes the velocity series due to increased currents generated behind the meadow. As a result, there appear losses of potential energy (reduction of wave height) and kinetic energy (reduction of velocities), the latter of greater magnitude [
47]. In subsequent zones, the decreased velocity has stabilized, and variations of
Sk and
As are dominated by the displacement of the offshore bar crest (breaking wave area).
The transport capacity calculated by the SANTOSS formula has been compared with the volume of sediment transported at position
X = 54.2 m in tests t2, t7, and t9 for each layout (
Table 7). The sediment volume has been quantified from the bed evolution of each test, measured with a mechanical profiler, as detailed in [
47] Astudillo et al., 2022. The sediment transport obtained by both methods was in the onshore direction. The values obtained by the SANTOSS formula for the BR60 layout were close to those measured in the experiments. However, for the R60 layout, the calculated values exhibited larger discrepancies compared to the measured values. At t2, the measured sediment transport is greater than that estimated by the SANTOSS formula and is attributed to the sensor being located over one of the megaripple-like bedforms generated in the sandy profile. While at t7, it was observed that the presence of seagrass induces a larger amount of transported sediment, which is consistent with the peak velocities found in this test (
Table 5), where the R60 layout presents higher values than the BR60 layout. The increased velocity peaks in t7 cannot be conclusively explained within the scope of this work since they may be attributed to measurement errors or to the larger morphodynamic stability for dissipative profiles with bedforms.
Note that the sediment transport at t2 is higher at R60 compared to BR60 layout, again this is attributed to megaripple-like bedforms. These megaripple-like bedforms appeared first in R60 (t2), and then in BR60 (t5), and remained in the same position and shape until they were eventually absorbed by the main bar moving offshore (
Figure 8). Although the SANTOSS equation considers values from an empirical ripple generator, it should be emphasised that sediment transport in rippled bottoms is notoriously difficult to predict [
62]. The SANTOSS formula is unable to reproduce the position where the megaripple-like bedforms are generated, indicating that the sediment transport capacity at this point and test may not have been correctly estimated. However, the formula still provides valuable insight, suggesting that sediment transport in this area, after the formation of the megaripple-like bedforms, is very low.
The profile evolution, including megaripple-like bedforms, could be related to
Sk changes in the waves, as observed [
69] in the case of submerged berms subjected to irregular waves. Therefore, the formation of megaripple-like bedforms at t2 for the R60 layout could be associated with the
Sk changes induced by the presence of the meadow. Similarly, the development of the bar in the BR60 layout between t4 and t5, which in turn generates higher
Sk values, could potentially explain the subsequent appearance of these bedforms. However, it should be noted that the available data set is limited, and further investigation is needed to confirm this behavior.
5. Conclusions
The present study contributes to analyze the interactions between incoming waves and seagrass meadows, demonstrating the effect of the meadow on the velocities generated by irregular erosive waves. The analysis compares the velocity fields over a seagrass meadow, considering the hydrodynamic behavior up to the main sandbar. The results presented lead to the following conclusions:
The passage of the waves over the meadow causes a large decrease/increase in the magnitude of negative/positive velocity peaks. This interaction effect results in two important changes for the velocities: (i) a change in the way velocities propagate, generating higher velocities onshore and even lower velocities offshore; (ii) in the first few meters of the sandy slope, where the velocities returned to their original shape in front of the meadow but with a decrease in their overall magnitude. The observed interaction thus makes the incoming erosive waves less erosive with less sediment transport, resulting in a bar closer to the shoreline.
As the waves passed through the surrogate seagrass meadow, there is a gradual increase in Sk, with a local maximum value in the area behind the meadow. However, as the wave evolves along the sandy slope, the Sk value decreases, reaching a minimum value even lower than before the presence of the meadow. Finally, as the control position approaches the breaker bar, the Sk value gradually increases. In conditions without a meadow, the Sk values remain stable until the measurement station X = 50.2 m, where the Sk value increases because of its proximity to the position of the main bar.
The influence of the meadow on the As variation is found to be small, with only a slight reduction observed in the area above the meadow. However, a significant increase in As is observed when the control (measurement) point gets closer to the position of the bar. This increase is primarily attributed to the proximity of the waves’ breaking position and is not directly influenced by the surrogate meadow.
The decrease in velocities induced by the meadow diminishes the offshore sediment transport in the sand part of the profile. While the SANTOSS formula was able to provide a suitable estimate of sediment transport, it could not approximate the sediment volumes of the bedforms generated in the profile. Additionally, the changes in Sk caused by the presence of the meadow result in the earlier appearance of megaripple-like bedforms in the profile area in front of the main bar.