Buffer Blocks as Wave Energy Dissipators: Flow Depth Reduction

Abstract

High-energy events such as tsunamis pose significant threats to coastal infrastructure and buildings, necessitating effective mitigation strategies to minimize damage. Compared to massive construction measures, buffer blocks are increasingly recognized as a potential solution for reducing the impact of such events. Understanding their effectiveness and optimizing their placement configurations is crucial for enhancing coastal resilience. The present study aims to experimentally evaluate the influence of buffer blocks on the depth of tsunami inundation. By comparing different configurations of buffer blocks, this study seeks to identify the most effective one for reducing the inundating flow depth. The laboratory tests revealed that the buffer blocks significantly influence flow depth and its characteristics. For ratios of flow depth to block height (R) below 1.5, the buffer blocks exhibited a delay in the arrival of the flow and effectively reduced the flow depths. However, for R values greater than 1.5, the effectiveness reduced as the flow began to overtop the obstructions. The increase in the flow depth at the buffer blocks was the least for a single-row configuration, while the multiple-row configurations with wider spacing offered higher flow resistance and increased the flow depths in front of the blocks. These findings suggest that strategically configured buffer blocks can substantially enhance coastal protection against high-energy flow events during natural coastal hazards. This study provides critical insights into the design and optimization of buffer block configurations, contributing to improved coastal infrastructure resilience and disaster mitigation strategies.

1. Introduction

Coastal areas are of crucial importance for the settlements and industries. Neumann et al. [1] showed that in the year 2000, the average population density in low-elevation coastal zones, which are coastal areas less than 10 m above sea level [2], was more than five times higher compared to the global mean population density. Predictions indicate an increase in the population in low-elevation coastal zones from 680 million people in 2019 to over one billion by 2050 [3].

This rapid population growth involves a corresponding expansion of infrastructure, change in land use, and, eventually, the emergence of economic agglomerations [4,5]. As a result, the exposure of coastal inhabitants and economic assets to coastal hazards has increased, thereby leading to higher vulnerability and, thus, risk of extreme events along the coast, such as storm surges or tsunamis [1,6].

Tsunamis, in particular, can cause devastating damage due to their destructive nature, as are two striking examples over the past two decades, resulting in huge human and economic losses. The Indian Ocean tsunami, IOT of 2004, cost the lives of nearly 230,000 people in 14 countries around the Indian Ocean and caused total economic damage of about USD 9.9 billion [7], making it the deadliest tsunami hazard recorded in history. The Tohokuku tsunami, TOT of 2011, has been reported as yet another dreadful natural disaster, resulting in total economic damage of more than USD 200 billion [8] and approximately 20,000 deaths [9].

These catastrophic events have spurred research efforts in tsunami risk management, mitigation, and protection, as many coasts exposed to this risk, especially in structurally weak regions, do not have adequate protective measures against extreme coastal hazards. To mitigate tsunami damages, hard (e.g., sea walls) or soft (e.g., early warning systems) countermeasures can be implemented, with each of the measures having its specific benefits and shortcomings [10].

In light of the above, developing advanced countermeasures, mitigation strategies, early warning systems, and emergency response plans tailored to the needs of coastal communities is crucial for future coastal management. This paper aims to conduct a comprehensive quantification of the performance and effectiveness of buffer blocks (which are traditionally used for reducing wave run-up), describes the possibility of using buffer blocks as energy dissipators to mitigate the impact and resulting damage, and highlights the need for further research.

2. Tsunami Mitigation Techniques

2.1. Shortcomings of Traditional Approaches

Japan, which is the country with the highest susceptibility to tsunamis, has acquired considerable knowledge and experience in the field of tsunami risk mitigation [11]. In response to a significant tsunami event in 1993, the Council on Earthquake Disaster Prevention (CEDP) of Japan proposed ten countermeasures for mitigating the impact of future tsunamis [11,12]. These proposals were later supplemented by proposals from the National Oceanic and Atmospheric Administration of the USA (NOAA [13]) and UNESCO [14], as depicted in Figure 1.

Depending on the expected intensity of tsunami hazards, the establishment of these (partial) barriers can be achieved using four distinct strategies, as outlined by NOAA [13]. The traditional protection strategy includes blocking structures (e.g., massive sea walls) that act as a permeable or non-permeable barrier to withstand the incoming flow due to the extreme event along the coast. Another strategy employs steering structures (e.g., angled walls) to divert the tsunami force away from vulnerable buildings and coastal communities. A less conventional technique, the slowing strategy, aims to reduce the wave impact by implementing macro-roughness elements, which can be easily integrated into densely developed areas. Alternatively, in line with the philosophy of accommodating, buildings can be constructed on higher ground or elevated structures such as piers or podiums or far enough from the shore to avoid the inundation area [13].

The two most commonly applied measures are sea walls (blocking) and breakwaters (blocking and slowing) [10]. Although these measures have the potential to significantly reduce the damage caused by tsunami hazards to coastal communities and could easily be implemented in developed areas, there are crucial shortcomings that need to be considered.

Table 1. Overview of shortcomings of tsunami mitigation strategies following Oetjen et al. [10] and references therein.

Strategy	Targeted Tsunami Level	Financial Effort	Area of Application	Shortcomings
Blocking	Level 1 *	Very high	Developed area	Increased hazard by failure of structure, wave overtopping, wave reflection, wave amplifications, or redirection of wave energy into unintended directions Intervention into water ecology Disconnection of settlements from sea (fisheries, tourism, etc.) False sense of security
Steering	Level 1		Not developed area	Increased hazard for subsequent buildings Increased flood velocities due to neighboring steering structures
Slowing	Level 1	Reasonable	Developed area	Insufficient knowledge No design guidelines exist
Avoiding	Level 1	High	Not developed area/in development	Subsequent enforcement of coastal areas impossible
Retreating	Level 1, Level 2		Not developed area/recently affected areas	Enormous intervention for local communities

* Level 1 = tsunamis with a return period of 50–160 years with inundation depths below 10 m; Level 2 = tsunamis with a return period of hundreds to thousands of years with inundation depths above 10 m.

presents the primary findings from the comprehensive review of structural tsunami countermeasures by Oetjen et al. [10].

2.2. Macro-Roughness Elements

In light of the aforementioned shortcomings of current tsunami mitigation strategies, there is a clear need to revise existing design guidelines based on recent tsunami hazards and to develop advanced mitigation techniques. The complex interaction between tsunamis and countermeasures requires further research and understanding [10]. To contribute to filling this research gap, this study supports the proposal of Percy et al. [15] of a space-saving approach based on the idea of inducing energy dissipation and flow deceleration.

The concept of using buffer blocks as a tsunami mitigation measure is based on the installation of salient structural elements along the coastline in front of vulnerable structures and areas. These elements, which function as macro-roughness elements, are designed to induce wave energy dissipation by creating additional turbulences and reflections in the flow, ultimately reducing the incoming inundation distance and height and, subsequently, the potential damage (Figure 2 and Figure 3). The key idea is to create disturbances in the streamline and inhibit the flow using its own energy. Furthermore, the buffer blocks should be considered multi-functional coastal structures since tsunamis are rare events. These blocks can serve as relaxing spaces for beachgoers without obstructing the aesthetics of the coastal view. Hence, the number of rows and the height of the buffer blocks should be optimal to enhance the aesthetics of the beach, as seen on the coast along the Island of Nordeney (Figure 4).

Unlike massive and continuous coastal protection structures, buffer blocks do not have to withstand the full impact energy as the flow is allowed to propagate through and over the blocks. This permeable design requires less material and space, making buffer blocks comparably economical and avoiding critical shortcomings of other conventional measures.

The buffer block concept shares similarities with the performance of vegetation belts as a mitigation measure due to their attenuation effect, which has been observed during the IOT 2004 and TOT 2011 and has been investigated in several studies [16,17,18,19,20,21,22,23,24,25,26,27].

Buffer blocks serve as an artificial low-maintenance substitute for vegetation in areas where vegetation zones are neither feasible nor desirable. In areas where vegetation is beneficial as a mitigation measure, buffer blocks can act as a reliable reinforcement and a secondary defense line behind vegetation belts or vice versa.

The concept of buffer blocks as a tsunami protection measure and also to serve as a multi-functional coastal structure is a new approach, and its development is in an early stage of research. To date, buffer blocks have only been implemented as roughness elements on dikes and as space-saving reinforcement measures to existing dikes in the context of storm surges (as shown in Figure 4), allowing the dissipation of wave energy during overflow [28]. Along the North Sea Island of Norderney, straight elements measuring 5.5 m long and curved elements measuring 7.7 m long, both with a height of 1.3 m, successfully endured multiple storm surges over several years without suffering any damage [29].

The application of buffer blocks in the context of tsunamis has been discussed in general by several authors [15,29,30,31,32,33,34,35], but empirical relationships between wave impact and buffer block configuration have not yet been published.

Oumeraci [29] suggested that the concept of discontinuous damping elements could be modified by wider and more stable buffer blocks that prevent ecological and social interference with the coastal ecosystem and allow significant attenuation of the incoming tsunami flow.

In his experimental studies, Goseberg [4,30] investigated the wave run-up reduction in non-breaking long waves due to the interaction with macro-roughness elements, representing coastal urban development that is not fully submerged during the run-up. The experiments were carried out in a closed-circuit wave flume with a sloped beach inclined at 1:40. Based on his experiments, Goseberg [4,30] concluded that the run-up height is mainly influenced by the element configuration (arrangement and direction of wave impact) and the long-shore obstruction ratio (distance between the blocks), whereas the cross-shore obstruction ratio (number of rows) only have a minor influence.

Similar investigations were conducted by Giridhar and Reddy [36] regarding the effect of differently shaped buffer blocks (rectangular/semi-circular/trapezoidal) on wave-run up and wave reflection. For trapezoidal, rectangular, and semi-circular blocks, a trend in decreasing wave run-up was reported. Both Goseberg [4,30] and Giridhar and Reddy [36] did not consider the flow depth and force reduction behind the macro-roughness elements.

In contrast, the research carried out by Rahman et al. [31] focused on the attenuation of wave forces on coastal buildings fronted by continuous sea wall setups of different heights and a perforated sea wall setup. The results indicated a significant decrease in both flow heights and forces exerted on the building model with and without a sea wall. Notably, the case without sea wall protection showed greater increases in wave heights and resulting forces. A maximum of 41% force reduction was observed with the higher wall configuration, compared to 27% with the smaller wall, suggesting that an increase in wall height can provide more substantial attenuation of forces. Walls closer to the model were more effective in reducing forces, indicating that the proximity of the sea wall to the onshore structure is a highly relevant factor. The performance characteristics of both solid and perforated walls are similar, Rahman et al. [31] recommended the use of perforated sea walls in view of the fact that the overtopped water is enabled to flow back to the sea.

The experimental study by Thomas et al. [32] is one of the first three-dimensional works investigating the influence of buffer blocks on the wave run-up and induced forces on coastal structures. Thomas et al. [32] observed that smaller wake clearance angles generally reduced forces (up to 75%), whereas larger angles increased them (up to more than 100%) due to wave deflection and focusing. Angles between 20° and 35° tended to increase the forces on the structures. Angles less than 15° led to substantial decreases in wave run up and velocity. The clearance angle is defined as the one between a line perpendicular to the wave propagation direction and centered on the instrumented structure, and a line connecting the front center of the instrumented structure to the front inside corner of the nearest buffer block.

In line with the idea of flow disturbance, Ono and Hiraishi [37] concluded through physical modeling that increasing obstacle heights has a significant effect on the impact of force reduction and demonstrated that obstacles that were both higher and less permeable were more effective in mitigating wave forces.

Similar to the investigations by Goseberg [4,30], the experimental study by Moon et al. [34] examined the influence of nearby buildings (macro-roughness elements) on the impact force of tsunamis on buildings (structure of interest), providing insights into the hydrodynamic interactions in between the buffer blocks. Compared to a non-protected scenario, Moon et al. [34] pointed out that the presence of seaward blocks reduced the forces and pressures on the front face of the structure of interest up to 38% and 30% on the back face. Landward blocks generally increased the maximum back-face pressure, emphasizing the potential negative effects of improperly positioned buffer blocks.

Ishii et al. [35] also pointed out the significance of the block placement, examining their shielding effects on onshore structures during wave run-up. Both the three-dimensional experimental and numerical studies involved cuboid elements, representing coastal buildings and differing in their blockage ratio (single structure/structures parallel/perpendicular to shoreline) and spacing. They concluded that onshore obstruction elements have a significant influence on the flow characteristics, as the surface velocity was reduced by 40% in an area extending a minimum of three times the structure’s width. The study reported that blocks placed directly behind others offered significant protection against the impact, suggesting potential strategies for urban planning in tsunami-prone areas [35].

Most of the studies above and listed in

Table 2. Overview of existing studies on the interaction between roughness elements and tsunami-like flows.

Study	Type of Buffer Blocks	Characteristics Varied in Buffer Block Configurations	Wave Generation	Scale	Bed Type and Condition
Goseberg [4,30]	Cubic elements	Arrangement (staggered/aligned) Orientation (perpendicular/45° oblique) Obstruction ratio (cross-shore/long-shore)	Single sinusoidal waves (pump system)	Not specified	Sloped (1:40), Dry
Giridhar and Reddy [36]	Rectangular/semi-circular/trapezoidal elements	Distance between blocks Porosity	Monochromatic waves (wave paddle)	Not specified	Sloped (1:35), Dry
Rahman et al. [31]	Continuous and perforated elements	Height Position Permeability (26% perforated/solid)	Tsunami-like waves (dam-break setup)	1:35	Flat, Dry
Thomas et al. [32]	Cubic elements	Wake clearance angle	Long-period waves (wavemaker)	1:25	Flat, Dry
Ono and Hiraishi [37]	Continuous elements	Height Material (wood/artificial raisin)	Solitary waves (piston-type wavemaker)	1:40	Flat, Dry
Moon et al. [34]	Cuboid elements	Arrangement (staggered/aligned) Presence of seaward/landward roughness	Solitary waves (dam-break setup)	1:100	Flat, Dry
Ishii et al. [35]	Cuboid elements	Blockage ratio (single/parallel/perpendicular)	Tsunami-like waves (pump-driven dam- break setup)	1:80	Flat, Dry
Percy et al. [15]	Cuboid elements	Number of rows Distance between rows	Hydraulic bores (dam-break setup)	1:8	Flat, Dry
Present study	Cuboid elements	Number of rows Distance between rows	Hydraulic bores (pump system)	1:16	Flat, Dry

refer to building structures and their interactions with tsunami flows, which can broadly be considered as macro-roughness elements, indicating the potential of buffer blocks as tsunami mitigation measures. However, detailed knowledge regarding the effectiveness of these buffer blocks—which are eventually smaller and thus interact differently with flows, particularly in terms of their overtopping, reflection, and dissipation characteristics—is still limited.

Therefore, the study by Percy et al. [15] focused on roughness elements of smaller heights, providing additional time for emergency evacuation with decreased forces on coastal buildings. Using OpenFOAM, the study numerically examined the relationship between buffer block characteristics and their attenuation effect on the flow depth and velocity in a 1 m wide flume. The work involved buffer blocks placed in a staggered arrangement with differing numbers of rows and spacings between the rows and a dam-break scenario generating tsunami-like flows. Percy et al. [15] identified that a single row of buffer blocks reduced the momentum flux by 5 to 40% and the three-row configuration by 15 to 65%, depending on the flow characteristics. Moreover, it was stated that increasing the number of rows leads to a significant reduction in flow depth behind the blocks, compared to the single-row configuration (10 to 40%). As more rows relate to more obstructions in the flow, the flow depth in front of the buffer blocks increases due to the build-up of water. Wider inter-row spacings lead to less pressure on the rear side of the blocks, less turbulence, and slower flow reflection than closer spacings. Percy et al. [15] stated that the performance of the blocks depends on the incoming flow and the resulting flow characteristics. In case of flow depths smaller than the block height, the blocks were reported to be more effective in reducing the flow depth behind them.

By extending the examination of smaller-height roughness elements, this study initiates preliminary physical experiments aimed at understanding the flow characteristics in the presence of the buffer blocks. The present study focuses on five buffer block configurations that vary by the number of rows and the spacing between rows. Unlike the dam-break scenario used in Percy et al.’s [15] research, the present study utilizes a pump-based method for generating the bores, which can simulate tsunami inundation in the long term. Additionally, this study includes an experimental examination of the single-row configuration explored by Percy et al. [15] and a continuous block setup analogous to a sea wall. This approach enables a systematic comparison in terms of both flow generation techniques and scale effects. This research targets more extreme events, focusing on pure overtopping scenarios on the buffer blocks where the ratio of flow depth to block height (R) is greater than or equal to 1, compared to the conditions examined by Percy et al. [15], where 0.7 ≤ R ≤ 1.5.

3. Materials and Methods

To evaluate how and to what extent buffer blocks reduced the tsunami flow energy, a comprehensive study employing both physical and numerical modeling is being conducted as part of a collaborative research project (buffer blocks as wave energy dissipators: BB-WEnDis) between the Institute of Hydraulic Engineering and Water Resources Management (IWW) at RWTH Aachen University in Germany and the Department of Ocean Engineering (DOE) at the Indian Institute of Technology Madras in India. The reproduction of similar experimental and numerical investigations at both laboratories (IIT Madras and RWTH Aachen) involved in the present research provides the basis for this and enables a technical, methodical, and well-controlled comparison.

3.1. Experimental Facility

The physical experiments for the present study were conducted in the large tilting flume of the laboratory at the IWW, utilizing a pump-based bore generation method (Figure 5). The flume, measuring 33 m in length, 1 m in width, and 1 m in height, was used for numerous tsunami-related studies [38,39,40,41,42]. Similar to the previous studies, the tsunami-like flow was simulated as a broken bore. Two remote-controlled pumps, which are switched on and off with a time interval of 2 s, extract water from an underground water reservoir, which is then directed upwards through a metal perforated plate and a flow straightener consisting of a stack of pipes arranged in parallel (Figure 6). The perforated plate is intended to minimize water splashing out of the flume, while the flow straightener ensures the homogenization of the flow and thus creates two-dimensional flow characteristics. After the homogenization, the flow encounters the edge of the 20 cm high false bottom (i.e., a pedestal; see Figure 5 and Figure 6) and, finally, flows in the experimental flume. The broken bore then propagates across the 24.5 m long false bottom and impacts the buffer blocks 14 m from the start of the false bottom. Finally, the water flows back into the underground reservoir via the outlet. For the generation of bores varying in heights, lengths, and velocities, the pumping time (duration) and the percentage of valve openings were adjusted. This allowed the generation of well-controlled bores.

3.2. Model Setup

This study aims to investigate the interaction between extreme flow, such as that due to a tsunami, and buffer blocks and to establish correlations between different buffer block configurations and flow characteristics. The prototype buffer block used in this study measures 0.8 m in height, 1.2 m in width, and 0.4 m in length, dimensions slightly smaller than those along the East Frisian Island of Nordeney of the North Sea coast of Germany. The height of the buffer blocks (h) was chosen to ensure access to the sea while also providing a functional seating area on the beach under normal conditions and to integrate well with the beach landscape. The dimensions, with the width being 1.5 times the height and the length half the height, were derived following the USBR guidelines, as previously specified by Percy et al. [15].

The first phase of experiments, conducted on a scale of 1:16, involved buffer blocks of PVC of the dimensions (7.5 × 2.5 × 5 cm). For a systematical investigation of the dissipation capacity of buffer blocks, five different configurations (C1–C5) in a staggered arrangement were investigated. These configurations varied in number of rows (one, two, or three), and the distance between the rows was investigated (see Figure 7). The first and third rows comprise six complete blocks and two blocks partially intersected by the flume wall. Furthermore, a reference scenario without any buffer blocks was also tested for comparison. The five configurations of buffer blocks are depicted in Figure 7. It is important to note that the distance between the rows was defined from the front of one block to the front of the next and was set to be equal to or double the height of the buffer blocks (h), as illustrated in Figure 8. In order to limit the range of parameters investigated to a reasonable number, the model setups were restricted to a flat shore model. It should, therefore, be noted that real applications in practice may partially deviate from this “idealization” of a flat shore and that corresponding hydraulic effects are neglected in the present study. Nevertheless, after the beach slope, the ground would be mostly flat. We are more focused on the flat region where the tsunami flow already overflowed the beach slope.

The research plan at the IITM (scale 1:8) includes the execution of physical experiments using a dam-break setup within a flume spanning 2 m in width, subsequent to the preliminary numerical analyses. Since the tilting flume at IWW is only half the width, the scale was set to be half of those to be conducted at IITM. This way, the same long-shore obstruction rate was maintained, allowing a 1:1 comparison of the results. The long-shore obstruction rate was calculated as the sum of the widths of buffer blocks (b) divided by the width of the flume (w):

$$\frac{{\mathrm{b}}_{\mathrm{I}\mathrm{W}\mathrm{W}}}{{\mathrm{w}}_{\mathrm{I}\mathrm{W}\mathrm{W}}}=\frac{2\times 12.5 \mathrm{m}\mathrm{m}+6\times 75 \mathrm{m}\mathrm{m}}{1000 \mathrm{m}\mathrm{m}}=\frac{475 \mathrm{m}\mathrm{m}}{1000 \mathrm{m}\mathrm{m}}=0.475$$

$$\frac{{\mathrm{b}}_{\mathrm{I}\mathrm{I}\mathrm{T}\mathrm{M}}}{{\mathrm{w}}_{\mathrm{I}\mathrm{I}\mathrm{T}\mathrm{M}}}=\frac{2\times 25 \mathrm{m}\mathrm{m}+6\times 150 \mathrm{m}\mathrm{m}}{2000 \mathrm{m}\mathrm{m}}=\frac{950 \mathrm{m}\mathrm{m}}{2000 \mathrm{m}\mathrm{m}}=0.475$$

Previously, Percy et al. [15] investigated tsunami-attenuation characteristics of the buffer blocks by carrying out numerical simulations with the dam-break flow (Table 2). Taking advantage of the fact that the width of the flumes in both studies was consistent at 1 m, additional experiments in a 1:8 scale were conducted using buffer blocks (test setup C1*) with dimensions identical to those used by Percy et al. [15] in their numerical study (35 × 5 × 10 cm). This setup allowed the investigation of the flow characteristics and the comparison of the outcomes from the physical model tests with those obtained from the numerical model. The C1* blocks were arranged in a single row, symmetrically to the center of the flume cross-section and perpendicular to the flow direction (Figure 7). Similarly, to the prior experiments, the lateral spacing between the blocks was set equal to their width (15 cm), thereby maintaining a 1:1 ratio of the lateral gap to the block width. To investigate the impact of permeable structures in comparison to a massive sea wall, a 10 cm high continuous block (CB) extending across the full width of the flume was also tested in comparison to the buffer blocks.

3.3. Test Program

Prior to the experiments, free-flow tests with different input settings were conducted. These initial tests facilitated the identification of four distinct design flows. The preliminary tests revealed that with a pumping time shorter than 10 s, the water level did not rise above the false bottom. Hence, the pumping time period was always varied between 15 s and 20 s.

Table 3. Selected flow depth to buffer block height ratios (block configuration in Figure 7).

Free-Flow Depth at Buffer Block Position H [cm]	Free-Flow Depth to Block Height Ratio R [−]
	For Setups C1 to C5 (1:16 Scale)	For Setups C1* and CB (1:8 Scale)
5	1	0.5
7.5	1.5	0.75
10	2	1
15	3	1.5

provides an overview of the parameters considered for the experiments with a systematic variation in the flow parameters, as well as test setups. Each experiment was repeated three times, resulting in 96 experimental runs.

A ratio of the flow depth to the buffer block height (R) below 1 indicates that the buffer block has emerged, while a ratio greater than 1 signifies a submerged condition. The selection of the four design flows was based on their profiles and specific flow depths at the buffer block position under free-flow conditions, as shown in Figure 9 and Table 3. Given this study’s focus on more extreme events than those considered by Percy et al. [15], the design flows were chosen to ensure their height exceeds the block height (5 cm), leading to selected flow depth to block height ratios of 1, 1.5, 2, and 3, corresponding to flow depths of 5, 7.5, 10, and 15 cm respectively. For the additional experiments to investigate it at a 1:8 scale (Figure 7) (i.e., C1* and CB configuration), the exact same design flows were employed (Table 3), as they correspond to R values ranging from 0.5 to 1.5, which align with the ratios explored in the study by Percy et al. [15].

3.4. Instrumentation

As mentioned earlier, the experiments were carried out with and without buffer blocks to derive their influence on flow depth reduction. In this first set of experiments, the focus lay on investigating the flow properties in front and behind the buffer blocks. Therefore, only flow depth was measured. Six ultrasonic depth sensors (WG) (6 micro-sonic pico+35/l) were placed every 3.5 m from the start of the false bottom to measure the variation in flow depth across the entire flow path. WG 1 served as the reference point for the time (t = 0 s). Figure 10 shows the positions of the structures and instruments.

4. Results and Discussion

4.1. Experiments with Configurations C1 to C5

The flow depth profiles over time produced in the presence of the buffer blocks are depicted in Figure 11. For R = 1 and R = 1.5, the buffer blocks delayed the arrival of the flow at WG 6 by about 1 s compared to the free flow, corresponding to the results from the first set of experiments. In the case of R = 2 and R = 3, the delay in flow arrival is hardly noticeable, suggesting that the buffer blocks might be less effective for flow depths that are at least double the height of the block. Overall, the flow depth profiles with buffer blocks are comparable to those of the free flow, with no significant difference detectable from the different configurations. Only for the least flow tested (R = 1) does the single row configuration appear to have the least reduction in flow depth, while all other configurations show a slightly greater reduction.

Additionally, oscillations in the measured flow depth behind the buffer blocks could be detected, indicating an interaction between the flow and the buffer blocks. For R ≤ 1.5, these oscillations are slightly below the free flow curve, whereas for R ≥ 2, they oscillate slightly above it. A fast Fourier transform analysis was performed to understand whether the frequencies of the flows relate to the buffer block configurations. However, due to the irregular nature and low amplitude of the oscillations, it proved difficult to establish a well-defined relationship. Visual observations suggest that the oscillations might stem from the interference of streams deflected through the buffer blocks, as depicted in Figure 12. This phenomenon appears to be a local phenomenon, possibly due to the proximity to the buffer blocks.

A closer examination of the flow depth profiles in front of the buffer blocks reveals a consistent alignment of flow peaks across all test cases, proving reliable reproduction and comparability of the incoming flows. Additionally, the reflected flows depict similar profiles across different flow depth to block height ratios. Notably, the two-row configuration with a 2 h gap (C4) results in the fastest and highest reflected flow. This is followed, in decreasing order of flow reflection, by the three-row configuration with 2 h gap (C5), the three-row configuration with 1 h gap (C3), the two-row configuration with 1 h gap (C2), and finally, the single-row configuration (C1). To analyze and quantify the influence of buffer blocks on flow depths, the dimensionless parameter relative flow depth ($\frac{H_b}{H_{ff}}$) defined as the ratio of the maximum flow depth with buffer blocks (H_b) to the maximum flow depth in the free flow scenario (H_ff) is introduced (Figure 13). It is observed that the presence of buffer blocks consistently increased the flow depth in front of the blocks, ranging from 1.3 to 2.25 times higher than in the free flow case. The lowest increase in flow depth was noted for the single-row configuration (C1), whereas the highest increase was observed for the multiple-row configuration with wider spacing (C4 and C5). This could be because the single-row configuration offers less obstruction to the flow, resulting in minimal disruption and lower flow depth increase. In contrast, the multiple-row configurations with wider spacing create more significant obstructions, causing greater flow resistance and a subsequent increase in flow depth. The wider spacing between the rows of buffer blocks allows for more turbulent flow interactions and energy dissipation, leading to greater accumulation of water and, subsequently, higher flow depths. Additionally, for most of the buffer block configurations, the relative flow depths appear to increase until R < 1.5 and decrease for R > 1.5. This trend suggests that as the incoming flow depth rises, the influence of buffer blocks diminishes as the flow begins to overtop the blocks. The data for R = 3, which is less than for R = 1, also supports this observation.

Behind the buffer blocks, the relative flow depths are found to be in the range of 0.78 to 1.34. The values at WG 5 are generally found to be higher than those at WG 6, indicating that the relative flow depths decrease with increasing distance from the blocks. This is due to the rooster tail formed behind such structures, as explained by Harish et al. [39]. However, in most cases, the relative flow depths are greater than 1, signifying higher flow depths compared to the scenario without blocks. Only for the R ≤ 1.5 cases do the flow depths decrease. This observation suggests that the influence of the buffer blocks is strongly dependent on the ratio of flow depth to block height. Further, it could also be realized from Figure 13 that the C5 configuration performs better in reducing the flow depth behind the buffer blocks compared to the other configuration.

To quantify and compare the flow depth reduction in relation to the incoming flow and the position, the reduction in flow depth is similarly plotted against the flow depth to block height ratio R for WG 4, 5, and 6 (Figure 14). The reduction in flow depth due to the presence of the buffer blocks was calculated as $\frac{dH}{H_{ff}}=\frac{H_{ff}-H_b}{H_{ff}}$ [−]. Herein, ‘H’ indicates the flow depth, ‘ff’ depicts the free flow case, and ‘b’ is the case in the presence of the blocks. In all the test scenarios, the results display negative values for the flow depth reduction in front of the buffer block position. Confirming with the observations above, these negative values signify an increase in water level, which can be attributed to the obstruction caused by the buffer blocks, leading to the accumulation of water in front of them. The increase in water levels ranges from 125% for the multiple-row configurations with wider spacing (C4, C5) to 30% for the single-row configuration (C1). Similar to the profile of the relative flow depth, the data for the flow depth reductions show different behavior for R > 1.5. This aligns with the earlier finding that for R values above 1, the flow predominantly overtops rather than passes through the blocks. Consequently, this results in the bore rolling over the blocks and, thus, inducing a relatively lower accumulation of water in front of the structure. The experiments with configurations C1 to C5 indicate that the single-row configuration (C1) results in the least increase in flow depth. As the number of rows and the gaps between them increase, so does the magnitude of the flow depth.

Leeside of the blocks, the flow depth reduction becomes greater with an increase in the distance to the blocks. Overall, the buffer blocks appear to have a negligible impact on the flow depth, with a slight increase in flow depth compared to the free flow observed downstream. The values at WG 6 range from −20% to +22%. This arbitrariness of the data may be attributed to the previously discussed local phenomenon of oscillations and turbulences influencing the WG 6 measurements. Therefore, conducting measurements far away from the buffer blocks might offer a clearer understanding of the flow regime unaffected by this localized phenomenon.

In order to enhance the understanding of how the rows and their spacing influence flow depth reduction, the reduction values are plotted against the number of rows in Figure 15. The flow depth reduction is calculated in comparison to the free flow depth at position 6. A pattern emerges wherein increasing bore heights correlate with a diminished reduction in flow depth, while an increased number of rows tends to enhance the reduction slightly. Regarding the row spacing, observations from the experiments indicate that for configurations with two rows, a gap of 2 h yields a greater reduction than a gap of 1 h at low R values.

To assess whether the size of the prototype buffer block (1.2 × 0.4 × 0.8 m) should potentially be doubled, the performances of the C1* and C1 configurations were compared, as shown in Figure 16. In a single-row configuration and with the same incoming flow parameters, there is no significant difference in flow depth reduction whether small (C1) or larger buffer blocks (C1*) are implemented. However, the reflection of flow caused by larger blocks is higher and faster (−5 s), as can be observed on the right-hand side of Figure 16. The larger buffer block height (C1) serves as the reference point for calculating the ratio R in this comparison.

4.2. Additional Experiments with Configuration C1* and CB

The flow characteristics over time provide an initial insight into the influence of permeable structures such as buffer blocks compared to continuous structures such as a sea wall. Figure 17 compares the flow depths over time behind the structure for the different incoming flow characteristics due to buffer blocks or continuous blocks, as opposed to the scenario without any structure (free flow). Both the continuous block and the buffer blocks delay the arrival of the flow at WG 6 by approx. 1 s (Figure 17; left-hand side), in comparison to the free flow. Across all ratios of R, the flow depths resulting from the continuous blocks were consistently lower than those generated by the buffer blocks and for the free flow case. This indicates that in the case of a continuous block, most of the flow is reflected, and a lesser part of the energy is transmitted due to the impermeable nature of the structure. Oscillations in the time history can be observed in the downstream flow profile due to the presence of the buffer blocks. These oscillations tend to be greater for the submerged condition (R ≥ 1) than for the emerged scenarios (R = 0.5 and R = 0.75). For ratios above 1, the flow mainly propagates over the blocks, which could lead to more turbulence than the flow propagation along the block.

Focusing on the reflection component in front of the buffer blocks, it is to be noted that the flow reflection at a continuous block has a larger bore height than the one at a buffer block. Based on the distribution of the flow peaks, it can also be seen that the reflection of flow induced by a continuous block propagates approx. 3 s faster than the flow induced by the buffer blocks. This is in line with the results mentioned above. Since more water is reflected in the case of a continuous block, the reflecting bore height is greater than in the case of buffer blocks.

The numerical study by Percy et al. [15] presents flow profiles at the buffer block position, depicted in Figure 18, that bear a notable resemblance to those observed in the IWW experiments. In the experimental data, there is a pronounced initial sharp peak, which is then followed by a subtler, more gently rounded peak. On the other hand, the flow profiles generated by the numerical model exhibit a smoother, more prolonged ascent in water level. The sharp peak in the flow depth might be a splash captured by the ultrasonic sensor, which is not being captured in the numerical model due to the numerical gauge extraction method; however, such a splash was noticed in the numerical model results [15].

Examining the relative flow depth in front of the buffer block (Figure 19), a trend similar to that observed in C1 to C5 configurations is evident. For the continuous block, a substantial increase in water level is observed, with a rise of 250% for R = 0.5 and 150% for R = 0.75 (Figure 20). For higher ratios of flow depth to block height, such as R = 1.5, the increase is comparatively lower at 70%. This is due to significant flow overtopping. In the case of the buffer block configuration (C1*), the increase in water levels noted is less, with a rise of 140% for R = 0.5 and 115% for R = 0.75. The study by Percy et al. [15] reported even smaller increases (80%) than those measured in these experiments. This discrepancy could be attributed to the differences in extreme flow generation methods. In the present study, the flows were produced by pumps, whereas Percy et al. [15] used a finite-length dam break setup. The finite-length dam break setup could lead to differences in flow characteristics, such as energy and velocity, which might result in varying impacts on flow depth increases. For the continuous block configuration, the reduction in flow depth behind the block is also found to be higher with an increasing distance to the blocks for all investigated ratios R. The most significant reduction of 24.5% was observed at 7 m behind the block (WG 6) for R = 0.75, while the reduction due to buffer blocks (C1*) is minimal with 0.06%.

5. Conclusions and Future Research

The rapid population growth in coastal areas has resulted in increased infrastructure and economic concentrations, making coastal inhabitants and assets more vulnerable to hazards such as tsunamis. The study shows that buffer blocks present a promising solution for mitigating tsunami impacts by dissipating flow energy and overcoming the limitations of conventional measures. Different configurations of buffer blocks were tested at a 1:16 scale by varying the number of rows and spacing between them. The following vital conclusions arrived:

• Buffer blocks effectively delay flow arrival and reduce flow depths for flow depth to block height ratios (R) below 1.5. However, their effectiveness reduces for higher R as the flow overtops the blocks. The flow overtopping, in most cases, resulted in increased flow depth immediately behind the buffer blocks and, hence, behaves negatively.
• Increasing the number of rows enhanced the flow depth reduction behind the buffer blocks and increased the flow depth in the vicinity of the buffer blocks on its upstream side due to the increase in the obstruction.
• Multiple-row configurations with wider spacing achieved the best flow depth reduction behind the buffer blocks, hence necessitating sufficient space between the buffer blocks’ rows. C5 configuration yielded the best results out of the tested configuration when R ≤ 1.5.
• Comparisons with continuous block structures revealed that buffer blocks perform better in flow depth reduction by inducing more turbulence and oscillations. Thus, buffer blocks can be realized as an alternative to the conventional sea walls of low heights.

Despite the present study identifying that increasing the spacing between the rows and increasing the number of buffer block rows can yield a better reduction in flow depth, the tested configuration was limited considering the bi-functional nature (i.e., both tsunami-resistant and beach aesthetics) of these buffer blocks, as argued in Section 2.2.

While flow depth measurements provide valuable insights, they offer a limited analysis of energy dissipation. Nevertheless, the assessment of flow depth information is more critical as it forms the integral component of flow energy and momentum evaluation, which are necessary for tsunami hazard and risk mapping. Further, such information is necessary to establish the minimum vertical evacuation height required for evacuating the people. The research highlights the complexity of accurately predicting the performance of such protection measures under varying conditions. To address these limitations, future studies should include detailed measurements of flow velocity and incorporate force sensors to quantify the forces exerted on buffer blocks and the structures behind them. These data are crucial for designing buffer blocks that can withstand the impact forces of high-energy events. Future experimental or numerical experiments should also investigate the beach slope, which may have a significant impact on the buffer block functionality.

Future research should focus on refining the understanding of buffer blocks’ performance under different flow conditions and integrating the measurement of velocity and forces into improved design guidelines and planning strategies. This will help ensure that coastal communities worldwide are better protected against the increasing threat of tsunamis and other extreme flow events. By advancing the knowledge and application of buffer blocks, we can enhance coastal resilience and safeguard vulnerable populations and infrastructure.