1. Introduction
Seawalls are coastal protection structures, employed for preserving the mainland from sea waves action. They are effective in stabilizing the coast from wave-induced erosion, but they are also used for contrasting overtopping and consequent flooding of the inland areas with possible damage to infrastructures (i.e., roads, and buildings). These aspects are crucial within the frame of coastal risk assessment (e.g., [
1,
2,
3,
4,
5]).
Seawalls are generally arranged parallel to the shoreline and often consist of either a vertical or curved concrete wall. One of the main problems connected with the seawalls is the increase of wave reflection in front of the structure, which could be estimated around 90–100% higher if compared to the undisturbed configuration, in the case of perpendicularly approaching waves [
6]. Standing waves could be then generated [
7], and the stability of the seawalls could be undermined by local scour at the toe (e.g., [
8,
9]).
To reduce wave reflection, sloping structures can be employed with artificial or rock armor units on the offshore side of the concrete wall [
10]. These type of structures, however, could require a large quantity of granular material, especially where water depths are high, and their cost could be considerable. This issue could be fixed by using well-graded mixture resulting from other civil works to create the core of the sloping structure. Then, a primary armor layer, similar to that typical of rubble mound breakwaters (RMBs), is required to assure the global stability of the structure. Indeed, permeable coastal structures, including RMBs, are employed for various purposes such as protection from coastal erosion, or to provide safe mooring in artificial ports by reducing sea waves energy. Given the importance of these structures, the study of their interaction with incident waves (e.g., reflection, dissipation, and transmission of waves energy) is of extreme importance [
11] and has received attention from past research studies (e.g., [
12,
13,
14]).
From a general point of view, the physical properties of structures and incident waves affect the behavior of RBMs (i.e., [
15,
16]). Indeed, their global performance strongly depends on berm height, so as to determine a primary classification between the so-called emerged and submerged RBMs. The differences between these two categories are noteworthy, both in terms of waves interaction [
17,
18] and environmental aspects, such as the impact on landscapes, natural surroundings (i.e., impacts on the water quality), and coastal ecosystems [
19].
Furthermore, the cross-sectional shape of RMBs has a significant influence on the wave–structure interaction. Analytical and theoretical studies have not proved always satisfactory to investigate the phenomenon, and several experimental studies have been conducted on various configurations of both impermeable (e.g., [
20]) and permeable (e.g., [
21]) breakwaters.
The permeability is a further important element for the main performance variables (i.e., waves reflection, transmission, and dissipation), the extent of which depends on the flow around and inside the structure [
18,
22,
23], and for their stability [
24,
25]. These aspects reveal that wave reflection, due to the armoring of seawalls with RMBs, is of crucial importance. For impermeable structures with the same shape, the roughness of the slope plays a crucial role, especially in terms of wave reflection and transmission [
17].
The design of RBMs has also to face some engineering problems concerning their global stability, intended as the ability to avoid heavy damages, failures, or collapses in both static and dynamic conditions [
24,
26]. This issue mainly affects the primary armor of a breakwater; it strongly depends on the characteristics of this layer, such as its slope [
27,
28], its shape [
15], the armor unit type (see, e.g., [
29,
30,
31]) for some of the more popular armor concrete units), and the number of the layers (see, e.g., [
32,
33]) for single and double layer, respectively), as well as on the seabed configuration (water depth and seabed slope of the foreshore).
The level of protection from incident waves provided by RMBs also depends on the characteristics of the waves. The reflection coefficient can, for example, be related to the spectral features [
34] or to the angle of incidence of waves [
35], whereas the stability and the overtopping phenomena (for emerged RMBs) are more tied to the waves’ steepness [
24,
26] or, mostly for seabed interaction, to sea currents [
36]. In these cases, the water level (i.e., related to the storm surge phenomenon) can also play a crucial role (e.g., [
37,
38,
39]). Finally, it should be noted how different the interactions of solitary [
40,
41] and cnoidal [
42] waves are from short-period waves.
As for the type of units used in the primary armor, it is important to highlight that the performances of artificial armor units are an open topic in scientific research. Due to the difficulty and the high costs of finding natural rocks of the suitable size to be used for breakwater primary armor, artificial concrete units use represents one of the most appropriate alternatives to armor rubble mound structures. In the past decades, many different shapes of units have been patented that are able to ensure high performances both in terms of dissipation of wave energy and global stability. For a comprehensive overview and a classification of the most common types of armor units with a detailed analysis of their performances, the readers are referred to the study of Bakker et al. [
43]. For all the new patented units, the need arises to verify if the available practical design formulas based on experimental tests (e.g., [
24,
26,
44,
45]) retain their validity. In particular, for the wave reflection by rubble mound structures, on which this paper is focused, several formulas have been tested for specific configurations of RMBs, in terms of layers number, the front slope of the primary armor, and typology of employed blocks [
46,
47,
48,
49,
50]. An extensive set of data about wave reflection on many types of structures have been provided [
51] from two research projects: the EU-projects DELOS [
52] and CLASH [
53]. In the last years, various composite-type breakwaters have also been tested using physical and numerical models to investigate their performance in coastal protection (e.g., [
54,
55]).
This study aims to extend previous works to seawalls protected by a rubble mound structure. Based on a 2D physical model experimental investigation, as described in
Section 2, the reflection coefficients of a concrete seawall protected by a rubble mound structure armored by a novel concrete unit arranged in a single layer, named MAYA, and characterized by a steep slope and a core constituted by a well-graded sediment mixture, is investigated for different incident wave conditions. The obtained results are illustrated in
Section 3, where a comparison between the reflection coefficients calculated using the widespread prediction formulas for RMBs and the observed reflection data is also discussed. Moreover, a novel formula, derived from an approach based on dimensional analysis, is proposed in
Section 4. Concluding remarks close the paper.
3. Results and Discussion
The numerous empirical formulas for the prediction of the reflection coefficient
of RMBs differ depending on the geometric characteristics of the structure as well as the type of units used for primary armor [
46,
47,
48,
49,
50].
The most widely used prediction formulas can be classified into two main categories: formulas based on surf similarity parameter [
51,
64], and relative depth [
50].
Nevertheless, Muttray et al. [
50] observed that the correlation between
and the surf similarity parameter is weak in the case of the steep slope of the armor. Since experimental data analysis reveals a strong dependence of
on both
and
, as reported in
Figure 5, in the present work, both approaches were followed.
Firstly, the linear wave reflection approach adopted by Muttray et al. [
50] was investigated considering that the dependence of
on the ratio
is noticeable (
Figure 5d).
Muttray et al. carried out experimental trials on RMBs characterized by an ACCROPO-DES™ primary armor slope equal to 2:3, with non-breaking waves (due to the foreshore), and in the absence of overtopping phenomena (
Table 3). All these conditions are comparable to the present experimental setup, except for the armor slope.
Several studies have dealt with the testing of the Muttray et al. formula by changing experimental conditions. In particular, Calabrese et al. [
65] carried out tests with ECOPODES™ armored RMB with two different front slopes: (i) a slope of 2:3 (as the Muttray et al. experiments [
50]); (ii) a slope of 3:4 (equal to that in the present paper).
Calabrese et al. also extended the experimental range of the surf similarity parameter
, which, in the experiments of Muttray et al. [
50], was set greater than 6 (
Table 3), and rewrote Muttray’s et al. formula as follows:
where
is the RMB armor slope, and
A and
B are two coefficients that depend on
m.
Buccino et al. [
49] further refined the estimation of
and
coefficients providing new values (
Table 4). These coefficients were estimated fitting the experimental data of
obtaining the best values of the coefficient of determination (
) and standard deviation (
) between measured and predicted
.
Figure 7 shows the comparison of the present experimental results with the curves by Muttray et al. (named as “
M 2:3”) and Buccino et al. with a slope of 2:3 (named as “
B 2:3”), and Buccino et al. with a slope of 3:4 (named as “
B 3:4”).
Figure 7 reports that the
B 3:4 curve is above the
B 2:3 curve, since the greater the slope is, the greater the reflected wave energy is, whereas small deviations of the curves can be found between
M 2:3 and
B 2:3. Buccino et al. already observed that those differences could be caused by the different scales used in the two experimental campaigns (the scale used by Muttray et al. [
50] was larger compared to [
49,
65]). Furthermore, the experimental ranges of the surf similarity parameter were also different: greater than 6 in [
50], 2.5 ÷ 9.5 in [
49,
65], and finally, the water depth at the toe of the structure in the experiments carried out by [
49,
65] was deeper than Muttray et al. [
50]. Hence, it is evident that the coefficients estimated by [
49,
50,
65] have to be considered as characteristic of the particular type of RMB armor unit and front slope.
The experimental investigation carried out in this work, characterized by wider experimental ranges of both
and
, represent an extension of previous research studies (as reported in
Table 2 and
Table 3). On the other hand, it is possible to argue the role of the limited (bulk) permeability of the structure.
Figure 7 shows that the experimental data of the present study lie above
curve, preserving the shape. The figure also shows that the reflection coefficient is not influenced by
for values (of the relative water depth) greater than 0.25.
Therefore, restricting the application of Equation (
3) to the experimental data of the present study with values of
in the range
, the coefficients
and
are updated based on the new experimental findings (
Table 5). The statistical values, reported in
Table 5, confirm a satisfactory fit:
assumes a high value (0.988), both Mean Absolute Error (
MAE) and
are low (0.012 and 0.014, respectively).
Figure 8 shows the experimental data of the present work with the theoretical curve of Equation (
3), using the refitted coefficients
and
, i.e., based on the experimental data of the study illustrated herein. Furthermore, in this case, the comparison shows a satisfactory agreement.
The tested structure seems to provide a higher reflection coefficient than RMB tested by Muttray et al. [
50] and Buccino et al. [
49]. This result is likely due to the presence of the seawall behind the rubble mound structure, the (bulk) permeability, and the novel concrete unit.
More recently, Diaz-Carrasco et al. [
66] proposed a new formula to estimate
in a wider range of variation of the relative water depth [
66]. The reflection coefficients are estimated exploiting a sigmoid function depending on the relative water depth and front slope
(Equation (
4))
where
and
are the higher and lower asymptotes of the function, respectively, and
a and
are coefficients representative of the shape of the sigmoid function.
As already observed in other formulas depending on the relative water depth, the sigmoid function must be fitted for each tested front slope, since the lower asymptotic value
clearly depends on the front slope angle, whereas
,
a, and
could be considered as constants [
66]. Diaz-Carrasco et al. suggested
= 0.8,
, and
for all slopes, whereas
ranges from
(for a 1:3 slope) to
(for a 1:1.5 slope) [
66].
The
data observed in the experimental tests carried out in this work were then used to fit Equation (
4).
Figure 9 shows that the experimental data are well represented by a sigmoid function, and, as expected, the lower asymptotic value
(=0.30) assumes a higher value than those estimated for milder front slopes. The coefficients
a and
were also refitted (
Table 6), and the estimated values are lower than those estimated by Diaz-Carrasco et al. [
66].
The influence of the surf similarity parameter upon the reflection coefficient
was also investigated.
Figure 10 shows the measured reflection coefficients represented as a function of the surf similarity parameter
. The representative curves of the formulas by Zanuttigh and Van der Meer [
63] and Seelig and Ahrens [
64] refitted by Zanuttigh and Van der Meer [
63] are also plotted. It can be observed that both formulas fit well the experimental points.
4. A New Approach to Estimate Reflection Coefficient
The experimental data, as reported in
Figure 5, clearly indicate that
strongly depends on both
and
. Hence, a more comprehensive approach is proposed herein to involve more than a parameter in
prediction.
Indeed, within the frame of a general dimensional analysis approach, the reflection coefficient should be expressed as a function of a long series of parameters from which a series of dimensionless groups could be identified (e.g., [
58]).
From the example of the application of dimensional analysis, owing to the Buckingham
theorem (e.g., [
58]), the organization of lab runs, and the analysis of experimental data are regulated by dimensionless groups. As shown by Barenblatt [
67], it is possible defining similar tests in the laboratory of the same physical phenomenon in which, although the numerical values of the dimensional quantities governing the phenomenon itself are different, the values of the corresponding dimensionless parameters are identical.
On the contrary, self-similar solutions are those for which a certain dimensionless parameter can be neglected, as it assumes very small or very large values. In other words, in some phenomena, it may happen that the dependence on an index number vanishes when the latter takes on very large or rather very small values. Barenblatt [
67] pointed out that self-similar solutions are called incomplete when the function
f tends to 0 or
∞ for
or
. In this case, it is necessary to maintain the dependence on
, and therefore, the following equation applies:
In Equation (
6), the power law with respect to the dimensionless parameter
must be also preferred for its simplicity. This is the case of the so-called incomplete self-similarity in the parameter
.
For the case at hand, the reflection coefficient can be expressed as a function of the relative water depth and surf similarity parameter as follows:
where the investigation of other parameters (i.e., bulk permeability, porosity of the armor, configuration of the foreshore) is out of the scope of this work, as the aim of the research effort is to address the behavior of a particular kind of structure (i.e., seawalls protected by a rubble mound structure armored with MAYA units).
The formal definition of the functional dependence of
from the identified dimensionless groups can be a hard and complex task. Indeed, it should depend on physical reasoning (as the formulation proposed by Diaz-Carrasco et al. [
66]) and must reveal the main feature of the phenomenon to be described.
Based on an in-depth sensitivity analysis, the reflection coefficient is proposed to be estimated by a power law as follows:
where the parameters were estimated by means of a non-linear least square method:
C =
(with a
confidence interval
),
a =
(with a
confidence interval
), and
b =
(with a
confidence interval
). The inspection of the parameters’ value reveals that the higher the relative water depth and the lower the surf similarity parameter are, the lower the reflection coefficient is. Furthermore, it can be observed that the role of the surf similarity parameter is larger if compared to the role of relative water depth.
Equation (
8) was tested against other widely used formulas for the estimation of the reflection coefficient, which take into account the dependence of
from the surf similarity parameter
[
63,
64] and the relative water depth [
66].
Figure 11 shows a satisfactory agreement between the experimental points and those calculated using Equation (
8), as also confirmed by the accuracy assessment estimated using the statistical parameters reported in
Table 7.
Since the coefficient of determination
is insensitive to additive and proportional differences between the predicted and observed data, an additional statistical parameter, the Nash–Sutcliffe coefficient of efficiency (NSE), was exploited to assess the performance of the proposed formula (e.g., [
68]). The NSE is defined as the ratio of the mean square error to the variance in the observed data subtracted from unity and ranges from minus infinity to 1.0. Given that the higher the values of NSE are, the better the agreement is, four model performance classes have been proposed by Ritter & Munoz-Carpena [
68]: unsatisfactory (NSE ≤ 0.65), acceptable (NSE = 0.65–0.8), Good (NSE = 0.8–0.9) and Very good (
NSE≥ 0.9).
A comparative analysis of the statistical parameters, shown in
Table 7, reveals that, although the existing equations based on the Iribarren parameter could be also used for providing reflection coefficient values for the tested structure, their performances are lower than those of the Equation (
8). The formula proposed by Diaz-Carrasco et al. [
66] provides the best results, but the coefficient in the equation needs fitting for each front slope, as the lower asymptotic value of the reflection coefficient (
) depends on the front slope. Hence, the proposed approach can be used without further parameter estimation for seawalls protected by a rubble mound structure for a relative water depth in the range [0.042÷0.709] and surf similarity parameters (
) in the range [2.91÷11.91].