Experimental Study on the Hydrodynamic Characteristics of a Fixed Comb-Type Floating Breakwater

Abstract

A comb-type floating breakwater is a new wave dissipation structure with particular force and dissipation performance advantages due to the two wave-reflecting surfaces. In this article, physical model experiments are used to study the hydrodynamic characteristics of a fixed floating comb breakwater and two structural optimization-based measures under the combined action of regular waves, irregular waves, and wave currents. The effects of factors such as the relative width, relative wave height, water flow velocity, and irregular waves on the transmission coefficient of the breakwater are analyzed. In addition, the characteristics of the transmission wave waveform are analyzed based on the time and frequency domains. The results show that (1) the wave transmission coefficient of a comb-type floating breakwater is lower than that of a rectangular floating box for long-period waves, while the transmission coefficient is larger than that of a rectangular floating box for short-period waves. (2) Under combined current and waves, the superimposition of bidirectional currents can increase the transmission coefficient, and the transmission coefficient increases with increasing current speed. The superimposition of the anti-directional current can decrease the transmission coefficient. (3) Moreover, with the same wave parameters, the transmission coefficient for irregular waves is larger than that of regular waves. (4) Finally, extending the bottom plate and adding lower baffles can effectively enhance the wave dissipation effect of the comb-type floating breakwater while also stabilizing the transmitted wave waveform.

1. Introduction

In past port construction projects, gravity breakwaters were the most commonly used type of breakwater. However, traditional gravity breakwaters require extensive materials and are difficult to construct in deep-water environments; thus, floating breakwaters have gradually been developed and increasingly applied. Compared to a gravity breakwater, a floating breakwater has the advantages of reduced water pollution in port and sea areas, convenient construction and demolition, a short construction period, requiring fewer engineering materials, and less stringent geological requirements. However, floating breakwaters also have limitations: the wave attenuation effects of long-period waves are not ideal, their durability is poor, and their maintenance costs are high.

In the past few decades, researchers have proposed various structural types of floating breakwater, which can be divided into two categories based on their working principles—reflective and energy dissipation—as well as composite floating breakwater structures that combine the two working characteristics. A pontoon-type floating breakwater is the most basic type of reflective breakwater. It is usually made of reinforced concrete and fixed onto the seabed using an anchoring system. The reflection of the pontoon on the waves reduces the effective wave height in the shielding area. Scholars at home and abroad have conducted numerous studies of wave attenuation performances, hydrodynamic characteristics, anchor chain forces, and other aspects of pontoon-type floating breakwaters.

Loukogeorgaki and Angelides [1] used an iterative procedure to couple the three-dimensional hydrodynamic model of a floating body with the static and dynamic models of mooring lines, ultimately clearly demonstrating that there is an “optimal” configuration for moored floating breakwaters in terms of wave elevation coefficients and mooring line forces. Yuan [2] studied the hydrodynamic characteristics of a pontoon-type floating breakwater that was secured by an anchor chain under the action of regular incident waves. This study showed that the breakwater width, anchor chain arrangement mode, and wave steepness are essential factors that affect the transmission coefficient. Zhang and Deng [3] numerically solved the diffraction and radiation problems of a pontoon-type floating breakwater, indicating that the position of the center of gravity has little effect on the wave attenuation performance of the pontoon-type floating breakwater. Increasing the pre-tension of the anchor chain can reduce the longwave transmission coefficient, though it will also increase the shortwave transmission coefficient. He et al. [4] determined through experiments that increase the width and draft of a rectangular square pontoon and the initial tension of its anchor chain can reduce the transmission coefficient. Gao et al. [5,6] deployed a series of sinusoidal bars outside the harbor model, demonstrating that Bragg resonant reflection can significantly alleviate harbor resonance and investigated the influence of transient wave groups characterized by high amplitudes and short durations on harbor oscillations.

Based on previous experience with gravity breakwaters, floating breakwaters can theoretically use a lower volume to achieve a practical shielding function while receiving less wave force. Therefore, different structural types of floating breakwaters have been proposed based on floating pontoons. Yao et al. [7] suggested several improved forms based on rectangular floating breakwaters. Moreover, the hydraulic characteristics of the sealing and opening of the front wall of a rectangular floating levee, breast wall on the outside, wave baffle or corrugated bottom plate on the bottom, left and right symmetrical side pontoons inside the floating levee, and rectangular or wedge-shaped pontoon body were compared. Wang et al. [8] established a numerical model of a square pontoon–vertical plate-based floating breakwater to study the effects of the vertical plate number, pontoon draft, and anchor chain stiffness on the transmission coefficient, motion response, and anchor chain force of a floating breakwater. He et al. [9] carried out model tests on a pontoon–plate composite-based floating breakwater, studying the effects of the depth of the pontoon entering the water, the distance between the bottom plate and the pontoon, and the length of the bottom plate on the transmission coefficient, structural motion response, and mooring force of the anchor chain and proposed structural optimization-based measures. Loukogeorgaki et al. [10] conducted an experimental study on both the hydroelastic and structural responses of a moored pontoon-type modular floating breakwater under the action of a perpendicular and oblique regular wave, concluding that the hydroelastic and structural responses of the floating breakwater depend strongly on the wave period, while the wave height and obliquity affect this response in the low-frequency range. Zhang et al. [11] researched the hydrodynamic characteristics of the protruding bottom plate of an inverted Π-type breakwater using the viscous CFD method. They confirmed the impact of the horizontal plates’ viscous effect on the breakwater’s wave absorption performance. By comparing the size of the vortices on the wave-facing and leeward sides of the breakwater, they proposed an L-shaped breakwater structure that had a horizontal protruding plate installed on the seaward side of the breakwater. Deng [12] investigated the hydrodynamic performance of a T-shaped floating breakwater using the matched eigenfunction expansion method (MEEM), which was based on linear water–wave theory, and examined the effects of the vertical screen’s height and installation position on the reflection and transmission coefficients, dynamic response, etc.

During the development of improved pontoon-type floating breakwaters, comb-type floating breakwaters were proposed as a new type of structure. The comb-type floating breakwater is based on a design concept proposed for a gravity comb-type breakwater. A wing plate-based structure was used to replace the main body of the caisson at a certain distance to obtain a comb-type breakwater at the waveward side. The total volume of the comb-type breakwater structure is small, and it applies to more considerable working water depths than a traditional vertical caisson breakwater. However, a comb-type breakwater has two wavefronts: the caisson’s central plane and the wing plate’s plane. When a wave force acts on the breakwater, a phase difference, which can effectively reduce the maximum wave force, will occur.

There are many relevant studies and engineering applications related to gravity comb-type breakwaters. Some scholars have studied the interactions between waves and permeable comb-type breakwaters using gravity breakwaters. Fang et al. [13] established a three-dimensional numerical wave tank using the source function wave-making method, compared and verified numerical simulation results against test results, and, based on the numerical results, offered an empirical formula for calculating the transmission coefficient of a perforated comb-type breakwater. Using linear potential flow theory, Wang and Liu [14] established a theoretical solution for interactions between a normal incident wave and a permeable comb-type breakwater. They clarified the influence of the relative width of the caisson, wing height, and length on the reflection and transmission coefficient of the porous comb-type breakwater. However, research into comb-type floating breakwaters is still rare. Chen et al. [15] studied the influence of the relative width and groove depth on the transmission coefficient, horizontal wave force, and vertical wave force regarding a fixed comb-type floating breakwater through physical model tests.

The above review shows that little research has focused on the hydrodynamic characteristics of comb-type floating breakwaters, especially under the combined wave and current conditions. In an engineering environment, waves often appear in the form of irregular waves, being accompanied by wave and current interactions. In the studies of Chen et al. [15] and Liang et al. [16], the floating breakwater was fixed at the water surface to study its hydrodynamic characteristics. Therefore, in this study, hydrodynamic model tests are carried out on a fixed comb-type floating breakwater; the effects of the relative width, relative wave height, wave–current interaction, and irregular waves on the wave dissipation performance of the breakwater are studied; and two structural optimization measures-based, which extend to the bottom plate and add lower baffles, are proposed. Furthermore, research into the transmitted waveforms in both the time and frequency domains is conducted.

2. Experimental Description

2.1. Experimental Setup

This model test was carried out in a large two-dimensional wave water tank with the following dimensions: 46 m long × 1.5 m wide × 1.6 m deep. A hydraulic servo push–plate wave maker can generate waves within a 0.5~5.0 s period. Figure 1 compares the measured wave and the target wave surface curves in the wave flume under four wave test groups.

Figure 1 shows that the measured wave surface elevation is in good agreement with the target wave surface elevation, which ensured the accuracy of the test conditions. The maximum flow speed for a 1.0-m depth of water could reach 0.20 m/s. The flow velocity at 0.3 m below the still water level was measured via acoustic doppler velocimetry (ADV). A wave-damping zone was installed at the end of the water flume further away from the wave maker to absorb waves and reduce the effects of wave reflections.

According to the similarity criteria of model tests in the codes, the physical wave model adopted a normal model and met the gravity similarity (Froude similarity). Considering the conditions of the flume test and the wave and current design parameters, the scale of the model test was set to 1:20. The primary purpose of this project was to study the essential wave dissipation performance of a comb-type floating breakwater. To accurately analyze the effects of the structural parameters, the model breakwater was fixed onto a stainless-steel frame in the experiment to reduce the influence of the movement of the breakwater on the wave dissipation, which was also widely used in previous studies.

The comb-type floating breakwater model consisted of three parts—a rectangular caisson, a bottom plate, and a back plate—that were rigidly connected using bolts. The rectangular caisson, bottom plate, and rear plate were all made of stainless-steel plates. The dimensions of the rectangular caisson were as follows: 0.4 m long, 0.22 m wide, and 0.4 m high, along with a spacing of 0.26 m between the two rectangular caissons. Therefore, two chambers that were 0.4 m long, 0.26 m wide, and 0.4 m high were formed. The bottom plate was 1.4 m long, 0.6 m wide, and 0.1 m high. The back plate was 1.4 m long, 0.1 m wide, and 0.4 m high, and the width of the model is W = 0.6 m. The total length of the model was 1.40 m, and a 0.05-m gap was reserved between the model and the flume wall on each side. In addition to the original model, the bottom plate was extended by 10 cm on both the wave-facing and leeward sides, and a 10-cm-long lower baffle was added to study the effects of these two structural optimization methods on wave dissipation. The diagrams and photos of the original model, as well as the structural optimization parts, are shown in Figure 2 and Figure 3.

Due to the complex geometry of a floating comb breakwater and its interactions with waves, the structure was preliminarily fixed at the water surface to investigate its hydrodynamic characteristics in the present study. The method of fixing the floating breakwater on the water surface was adopted in a few previous studies, such as Chen et al. [15] and Liang et al. [16]. In this method, the influence of the geometry of the structure on the wave dissipation performance was only focused, and the influence of the mooring system and motion of the structure could be excluded. As shown in Figure 3, the breakwater model was rigidly mounted on the stainless-steel frame, and the six legs of the stainless-steel frame were fixed onto the bottom of the flume using bolts. The vertical distance from the bottom of the bottom plate to the bottom of the tank was 0.67 m. The wave and current could pass below the comb-type floating breakwater model.

2.2. Test Conditions

The test water depth was set at d = 1.0 m, and the draft of the breakwater model was set at 0.33 m. The incident wave was set with a regular wave and a JONSWAP spectra irregular wave, the wave height was 0.06 m, and three kinds of flow velocities were set: forward 0.1 m/s and 0.2 m/s and reverse 0.1 m/s. A wide range of wave conditions was selected to ensure that the hydrodynamic characteristics of the fixed floating comb-type breakwater could be comprehensively understood. The wave period of the model test ranged from 0.8 to 1.6 s, and the wave height ranged from 0.02 to 0.10 m. The test conditions and their prototype conditions with a model scale of 1:20 are summarized in

Table 1. Summary of the test conditions.

Condition	Model Parameters		Prototype Parameters
	T (s)	H (m)	T (s)	H (m)
Regular waves	0.8~1.6	0.02~0.10	3.6~7.1	0.4~2.0
Irregular waves	0.8~1.6	0.06	3.6~7.1	1.2
Waves and current	0.8~1.6	0.06	3.6~7.1	1.2

. The wave period under prototype conditions ranged from 3.6 to 7.1 s, and the wave height ranged from 0.4 to 2.0 m, which are within the typical range of wave parameters found in oceans. According to the study by Cui et al. [17], the ratio of the breakwater width to the wavelength B/L was an important parameter that affected the performance of breakwater. The range of B/L in this experiment was 0.161~0.601.

Three wave height recorders were arranged in front of and behind the main body of the square box of the breakwater model, as shown in Figure 4. The wave height recorder was fixed on top of the water tank using a stainless-steel frame, which was connected to an SDA2000 sensor data acquisition system to measure the change in the wave surface in real time. Both the wave height recorder and the data acquisition system are sourced from Chengdu Yufan Technology Co., Ltd. (Chengdu, China). The sampling frequency of the wave altimeter was 30 Hz. After each experimental group was completed, the balance was cleared. To reduce the test error, we repeated each group of tests three times, and the average of the three groups of results was taken for analysis. The wave data directly behind the floating comb-style breakwater model were identified using the up-crossing zero method. The wave heights in the wave train were arranged in descending order, and the top one-third of the waves were taken as effective waves, thereby obtaining the effective wave height and calculating the transmission coefficient. Furthermore, the transmitted wave data were Fourier transformed to obtain the frequency spectra, allowing analysis of the transmitted wave waveforms from both the time and frequency domain perspectives.

3. Analyses and Discussion of the Transmission Coefficient

3.1. Comparisons with Existing Results

In this section, the test results of the comb-type breakwaters are compared with those of pontoon-type breakwaters detailed in published papers, including those in fixed and floating forms. Figure 5 shows a comparison between the results of the transmission coefficient of different structural forms and the change in the wave steepness H/L under a fixed wave height, in which the wave height of the fixed comb-type floating breakwater and the fixed pontoon-type floating breakwater is H = 0.1 m [16]. The width of the fixed rectangular pontoon floating breakwater is 0.5 m, the water depth is 0.6 m, and the draft depth is 0.16 m. Since the wave heights of the floating pontoon breakwater in the literature are H = 0.075 m and 0.125 m [18], the transmission coefficient under the condition of H = 0.1 m is taken as the average value to enable comparison. The width of the floating pontoon breakwater is 0.5 m, the water depth is 0.6 m, and the draft depth is 0.15 m. The ratio of draft depth to water depth of the rectangular structure is relatively close to that of the comb structure in the present study. Thus, both results are comparable. It is observed that the fixed floating breakwater has better wave dissipation performance than the floating breakwater, as it has a smaller transmission coefficient.

Regarding the fixed pontoon-type floating breakwater, with increasing wave steepness, the wave transmission coefficient decreases between 0.30 and 0.75. The transmission coefficient gradually increases in tandem with the increasing wavelength, and the wave attenuation effect worsens. The transmission coefficient of the fixed comb-type floating breakwater in this paper is significantly lower than that of the fixed pontoon-type floating breakwater when the wave steepness is less than 0.05, indicating that the wave dissipation effect of the fixed comb-type floating breakwater is better than that of the pontoon-type breakwater when the wavelength is more extensive. When the wave steepness is more significant than 0.05, the transmission coefficient of the fixed comb-type floating breakwater first increases and then decreases, ultimately tending to be stable. Moreover, this transmission coefficient is greater than the transmission coefficient of the fixed pontoon-type floating breakwater. Regarding the pontoon-type floating breakwater, the overall transmission of the breakwater is more significant than that of the fixed breakwater due to the influence of the movement of the breakwater body, and the variation range is 0.45~0.90, which conforms to the fundamental physical law. The above test results show that the wave dissipation effect of the comb-type breakwater is generally relatively stable, and the transmission coefficient varies from 0.40 to 0.65, especially in the case of long waves. This transmission coefficient is better than that of pontoon-type breakwaters, which shows that this structure has certain advantages in terms of longwave defense.

3.2. Effect of the Relative Width

In this subsection, the influence of the relative width on the transmission coefficient behind the comb-type breakwater is studied. The test water depth is d = 1.0 m, a regular wave is adopted, the wave height is H = 0.02~0.10 m, the period is T = 0.8~1.6 s, and the relative width range in the test is W/L = 0.15~0.6. Figure 6 shows the change curves of the transmission coefficient with relative width at different wave heights.

The test results show that within the range of the relative width value studied, the transmission coefficient first decreases, increases, and then decreases again. The overall trend is a horizontal S-shaped change. The minimum value appears when the relative width is between 0.20 and 0.27. The breakwater has the best wave attenuation effect, and the transmission coefficient has maximum values at relative widths of approximately 0.18 and 0.38. The extreme value of the transmission coefficient in the range of 0.20~0.27 occurs due to the phase difference between the reflected waves generated by the incident wave at the front of the box and the back plate. When the phase difference between the reflected waves from the main body of the square box and the rear plate is close to π/2, the waves are superimposed and offset at a certain distance from the breakwater, weakening the reflection (enhanced transmission). The experimental results are consistent with those of Wang et al. [19]. The changing trend of the transmission coefficient under different incident wave heights is the same, albeit with an increase in incident wave height, and the transmission coefficient also shows a trend of increase.

3.3. Effect of the Wave Height

Figure 7 shows the change curves of the transmission coefficient of the fixed comb-type floating breakwater with the wave height. The incident wave is regular, the water depth is d = 1.0 m, the period is T = 0.8~1.6 s, and the wave height range is H = 0.02~0.10 m. The test results show that the transmission coefficient gradually increases with increasing incident wave height because the breakwater model can weaken the breaking of waves when the incident wave height is small. The wave energy increases in tandem with increasing incident wave height, and the overtopping phenomenon is more prominent. The proportion of the wave energy that passes over the breakwater model increases, meaning that the transmission coefficient also increases, and the wave attenuation capacity decreases.

However, the amplitude of the transmission coefficient with the incident wave height varies significantly in different periods. When T = 0.8 s, 1.0 s, or 1.6 s, the transmission coefficient increases slightly in tandem with the wave height and maintains a stable trend. When T = 1.0 s or 1.6 s, the transmission coefficient reaches the maximum value between 0.55 and 0.70. When T = 1.2 s and 1.4 s, the transmission coefficient clearly increases with the wave height. These two periods correspond to W/L = 0.20~0.27 in Figure 6, and the transmission coefficient is set at a minimum, with a variation range of 0.15~0.40.

3.4. Effect of the Wave–Current Interaction

In this section, the effect of superimposed sea current waves on the wave dissipation performance of breakwaters is studied. The incident wave is regular, the wave height is fixed at H = 0.06 m, the period range is T = 0.8~1.6 s, and the corresponding relative width is W/L = 0.15~0.60. The current generation system in the flume generates water at an average velocity of u = 0.1, 0.2, or −0.1 m/s, where u = −0.1 m/s represents the opposite direction of the current and wave propagation. The influence of the current velocity on the model’s transmission coefficient is studied. Figure 8 shows the change curve of the transmission coefficient of a fixed comb-type floating breakwater with relative width under the action of regular waves and wave currents.

It can be seen in the figure that when the relative width is approximately 0.6 and other parameters of the breakwater remain unchanged, the overall distribution of the transmission coefficient is 0.50~0.90, indicating that the current velocity under the combined action of the wave and current has a more significant impact on its transmission coefficient. When the relative width is small, that is, the wavelength is large, the change in the transmission coefficient is not evident based on an increase in forward flow velocity, and the influence of the current on the transmission is small. When the relative width is significant, that is, the wavelength is small, with an increase in forward flow velocity, the transmission coefficient increases significantly, and the influence of the current on the transmission is significant.

The downstream current in the same direction as the wave will enhance the transmission of the wave. The higher the velocity, the greater the transmission coefficient. The reverse current that moves in the opposite direction to the wave direction will weaken the wave transmission, because in the case of a significant wave height, the downstream current will also allow the energy of the wave trough to pass under the breakwater model. Moreover, the greater the current velocity, the stronger the promotion effect. At the same time, when the wave interacts with the uniform current, the wavelength of the wave increases in tandem with the increase in the velocity of the downstream current. With the increase in the velocity of the reverse current, the wavelength of the wave decreases [20], and the change in the wavelength will also affect the wave absorption performance of the breakwater to some extent.

3.5. Comparison between Regular and Irregular Waves

In this section, irregular waves based on the improved JONSWAP spectra presented by Goda [21] are generated, and the influence of irregular waves on the transmission coefficient of the model is studied. The spectra broadness is γ = 3.3, the symmetry parameter for frequencies below 1/T_p is σ_a = 0.07 and for frequencies above 1/T_p, the symmetry parameter is σ_b = 0.09. The water depth is d = 1.0 m, the wave height is H_s = 0.06 m, and the spectral peak period change is T_p = 0.8~1.6 s, which is consistent with the regular wave period. Figure 9 shows the change curve of the transmission coefficient of a fixed comb-type floating breakwater based on the relative width W/L under regular and irregular wave conditions.

The test results show that the transmission coefficient of the JONSWAP spectral irregular wave is approximately 19% higher than that of the regular wave. They also show that the wave dissipation effect of the breakwater on irregular waves is weaker than that of regular waves. In the case of irregular waves, waves with different periods and wave heights will not be equally attenuated. Some wave heights are small, resulting in a feeble weakening effect, while others are large, exceeding the top of the breakwater, thus increasing the wave height behind the breakwater. Therefore, if the test is carried out under the action of regular waves, as shown in the engineering design, the influence of irregular waves on the wave attenuation effect should be considered in the application.

3.6. Effect of Extending the Bottom Plate

In this section, based on the breakwater model, both the wave-facing and leeward sides of the bottom plate were extended by 10 cm, as shown in Figure 3b, to investigate the effect of the extended bottom plate on the model’s transmission coefficient.

The water depth was set to d = 1.0 m; the wave heights were set to H = 0.06 m and H = 0.1 m, with periods T ranging from 0.8 to 1.6 s; and the relative widths varied from W/L = 0.15 to 0.60. Figure 10 presents the variation curves of the transmission coefficients for the originally fixed comb-type floating breakwater model and the model with the extended bottom plate under both wave height conditions as a function of the relative width W/L.

The experimental results show that extending the bottom plate can effectively enhance the wave dissipation effect of the fixed comb-type floating breakwater. When the relative width is smaller, i.e., the wavelength is larger, the impact of extending the bottom plate on the transmission and reflection effects is relatively small. Conversely, when the relative width is more significant, i.e., the wavelength is smaller, the influence of extending the bottom plate on the transmission and reflection effects is greater.

Extending the bottom plate increases the contact area between the waves and the bottom plate, strengthening the interaction between the waves and the breakwater structure. The new formation of the vortex at the edge of the seaward plate occurs when the wave along the seaward side of the breakwater moves downward from its maximum crest position or moves upward from its minimum trough position, generating greater vorticity and dissipating more wave energy, with this phenomenon being particularly noticeable at the bottom plate on the wave-facing side in Zhang et al.’s study [11].

3.7. Effect of the Adding Lower Baffles

In this section, based on the breakwater model, one 10-centimeter-long lower baffle was installed on both the wave-facing and the leeward sides, as shown in Figure 3c, to investigate the effect of adding the lower baffle on the model’s transmission coefficient.

The water depth was d = 1.0 m, and the wave heights were set to H = 0.06 m and H = 0.1 m, with period T ranging from 0.8 to 1.6 s and the relative widths varying from W/L = 0.15 to 0.60. Figure 11 presents the variation curves of the transmission coefficients for the originally fixed comb-type floating breakwater model and the model with the added lower baffle under both wave height conditions as a function of the relative width W/L.

The experimental results show that adding a lower baffle has a significant impact on the transmission coefficient of the fixed comb-type floating breakwater because the lower baffle can increase both the submersion depth of the breakwater model and the reflection area while also blocking part of the wave transmission.

The wave dissipation effect of the lower baffle is weaker when the relative width is smaller, i.e., the wavelength is larger, and it has a more substantial impact when the relative width is larger, i.e., the wavelength is smaller. The experimental results are generally consistent with the findings of Deng et al. [12] regarding the variation in the transmission coefficient under different lower baffle lengths for T-shaped floating breakwaters.

4. Analyses and Discussion of the Transmitted Waveform

4.1. Analysis of the Transmission Wave Waveform of the Original Breakwater Model

In this section, the waveform of the transmitted waves of the original floating comb-type breakwater model will be analyzed from two perspectives: the time and frequency domains. Figure 12 and Figure 13 show the measured waveform and frequency spectra of transmitted waves under regular wave conditions, with periods of 0.8 s and 1.2 s and wave heights of 0.04 m and 0.1 m.

The measured waveform shown in Figure 12 and the frequency spectra in Figure 13 demonstrate transmitted wave characteristics. The frequency spectra in Figure 13 show that significant high-frequency harmonics are generated during the interaction between the breakwater and waves. The generation of these high-frequency harmonics is positively correlated with the period and wave height of the incident wave. As the period and wave height increase, the breakwater model can interact more effectively with the waves, resulting in more high-frequency harmonics. The comb-type breakwater has two wavefronts: the caisson’s central plane and the wing plate’s plane. When a wave acts on the breakwater, a phase difference, which can result in the superposition of transmitted waves at different locations, will occur. This complexity in wave morphology may contribute to the generation of high-frequency harmonics. In addition, the breaking and overtopping of waves, as well as the reflection from the sidewalls of the water tank, may contribute to the generation of high-frequency harmonics to a certain extent.

Furthermore, the measured waveform in Figure 12 reveals the influence of high-frequency harmonics on the transmitted wave waveform. Firstly, we note that the average period of transmitted waves is smaller than that of incident waves because some wave energy is transferred from the low-frequency band to the high-frequency band [22], which can reduce the impact of specific frequency-range wave energy on the coastline or marine engineering facilities, thereby increasing its protective effect. Secondly, in the case of longer periods, we noted the appearance of secondary peaks in the transmitted wave shape. Secondary peaks are small peaks between the peaks, which occur when the wave steepness is greater than a certain critical value, as shown in Figure 12, at T = 1.2 s, t = 0.8 s, and 1.9 s. These secondary peaks increase as the wave height increases, and due to the different propagation speeds of the secondary and main peaks, they make the entire waveform more complex. The appearance of the secondary wave peaks indicates that the comb breakwater model can further disperse wave energy, decomposing a single large-scale wave into multiple smaller waves. A portion of the energy of the wave is concentrated at the main wave peak, while other energy is dispersed at other wave peaks, thereby reducing the impact of the waves on the structure and the coastline.

In addition, the following phenomena were observed: (1) The peak width of the transmitted wave is greater than the valley width, which is more pronounced under conditions with larger periods. This result may be due to changes in the waveform of the waves during their interaction with the breakwater. (2) When the wave height is small, the noise at the peak is more pronounced. This result may be because under these conditions, the energy of waves is relatively low and concentrated at the broader peaks, making them more susceptible to the scattering effect of the breakwater, resulting in complex waveforms that manifest as noise. However, the impact of the breakwater on the trough is relatively small and insufficient to generate observable noise. (3) As the period increases, the peak intensity at the second harmonic frequency (which corresponds to twice the incident wave period) exceeds the peak intensity at the fundamental frequency. However, as the period continues to increase, the peak intensity at the fundamental frequency again exceeds that recorded at the second harmonic frequency, which may be due to the more significant impact of waves on the interaction with the breakwater under specific conditions, resulting in the original fundamental frequency being masked by frequency-doubling fluctuations.

4.2. Analysis of the Transmission Wave Waveform of the Improved Breakwater Model

In this section, the transmitted waveforms of the two structural optimization-based measures—extending the bottom plate and adding lower baffles—will be analyzed in both time and frequency domains. Figure 14 and Figure 15 show the measured waveform and frequency spectra of the transmitted wave under regular wave conditions, respectively. The period is 0.8 s, and the wave height is 0.06 m. Similarly, Figure 16 and Figure 17 represent the same parameters, but the period is 1.2 s, and the wave height is 0.06 m.

By comparing the original model, we can observe that under the optimization measures of extending the bottom plate and adding bottom plates, the intensity of high-frequency harmonics in the transmitted wave is significantly reduced, and the number of secondary peaks in the transmitted wave is reduced substantially. The intensity ratio of high-frequency harmonics (frequency components greater than the fundamental frequency) was calculated for the original breakwater and improved breakwater models. Under the conditions of T = 0.8 m and H = 0.06 m, the intensity ratio of high-frequency harmonics for the original model was 0.640, while for the models with extended bottom plates and added lower baffles, the ratios were 0.603 and 0.563, respectively. Under the conditions of T = 1.2 m and H = 0.06 m, the intensity ratio of high-frequency harmonics for the original model was 0.794, while for the models with extended bottom plates and added lower baffles, the ratios were 0.708 and 0.657, respectively.

These results indicate that extending the bottom plate and adding bottom plates can effectively improve the transmitted wave’s stability, making the breakwater’s wave attenuation effect more uniform and stable. These observations validate the effectiveness of our structural optimization-based measures and provide essential references for subsequent breakwater design.

5. Conclusions

In this paper, an experimental study was carried out regarding the wave dissipation performance of a new comb-type floating breakwater. The breakwater model was fixed in the experiment to reduce the impact of the breakwater movement. The change law of the wave transmission coefficient was studied by changing the parameters, such as the incident wave height, wave period, superimposed current, and irregular wave. Based on the analysis of the experimental results, the following conclusions were obtained:

(1)

Compared to a rectangular caisson breakwater, the wave dissipation performance of the comb-type floating breakwater in this paper is relatively stable, and the transmission coefficient changes in the range 0.4~0.6. When the relative width is W/L < 0.45, that is, the wavelength is large, the transmission coefficient of the comb-type floating breakwater is smaller than that of the rectangular caisson breakwater of the same size. When the relative width is W/L = 0.20~0.27, the wave dissipation performance of the comb-type floating breakwater is optimal.

(2)

The incident wave height greatly influences wave dissipation when the relative width is small, and with an increase in the wave height, the transmission coefficient increases. Under the combined action of waves and currents, the velocity greatly influences the wave dissipation performance of the fixed comb-type floating breakwater. The downstream current will enhance the wave transmission. Moreover, the faster the velocity, the more pronounced the effect, while the reverse current has the opposite effect. For all factors, the relative width and current velocity are more important to the transmission coefficient than the wave height.

(3)

Compared to regular waves, the fixed comb-type floating breakwater has a poor wave attenuation effect for irregular waves. In practical engineering applications, it is necessary to consider the influence of irregular waves on the wave attenuation effect.

(4)

Extending the bottom plate and adding a lower baffle can effectively enhance the wave dissipation function of the floating comb-type breakwater, and the effect is more pronounced when the relative width is more significant, i.e., the wavelength is smaller. This research can, thus, help to improve existing breakwater structures and their wave dissipation performance.

(5)

The floating comb-type breakwater model can reduce the wave period. Furthermore, it interacts effectively with waves when the period and wave height are relatively large, generating high-frequency harmonics that contribute to creating secondary wave peaks. Both structural optimization-based measures—extending the bottom plate and adding lower baffles—can effectively reduce the intensity of these high-frequency harmonics, thereby enhancing the stability of the transmitted waves.

The experimental results on the fixed floating comb-type breakwater design process followed in this study can provide fundamental knowledge of the hydrodynamic characteristics of this type of structure. As for its application in practical engineering, comprehensive investigations into a flexible floating comb-type breakwater are still needed in future work.