Wave Reflection of Concrete Armour Unit: A 2D Physical Model Study

A breakwater is a coastal structure that breaks the wave energy coming towards the beach. When a wave hits an object, it will be reflected entirely or partially. Initially, the primary layer units on the coastal protection structure were only composed of large piles of natural stone. It is becoming increasingly difficult to find a natural stone of the right size, and to prevent its scarcity; natural stones are commonly replaced with concrete blocks. This study conducted the reflection coefficient analysis on the BPPT Lock concrete armor unit. Physical model testing was carried out on the wave channel of the Coastal and Port Infrastructure Laboratory, Department of Ocean Engineering ITS. From this study, it was found that the reflection coefficient on BPPT Lock ranges from 0.512 to 0.828 at a wave steepness of 0.002088 to 0.0117 with a structural slope angle of 1: 2. The reflection coefficient value is influenced by wave height, wave period, and the slope angle of the structure.


Introduction
Abrasion is a common problem often experienced by the average country with a coastal area.This abrasion occurs naturally, but what exacerbates the impact is human activity.The retreat of the coastline to damage residential areas and urban infrastructure is the impact of a moderately severe abrasion.To overcome the effect of this abrasion, researchers and experts have designed several types of coastal protection structures [1].
The beach is a land area that is directly adjacent to the ocean.When the beach has been provided with coastal protection building facilities, the incoming waves will be absorbed and reflected in their energy so that the beach can withstand abrasion.On the other hand, the coastline will be damaged if the impact of sea waves cannot be minimized.Destructive waves can be identified based on the height of the wave.When a big wave comes and breaks near a beach that does not have protective buildings, the beach will be easily damaged [2].
Good coastal protection structures design can dampen and reduce the energy of incoming sea waves.This wave energy reduction can be made by building the breakwater parallel to the coast.The incoming waves will break long before the shore so that the wave energy the beach feels is only on a small scale.Therefore, it is necessary to identify and understand the causes of damage to coastal areas, both short-1250 (2023) 012002 IOP Publishing doi:10.1088/1755-1315/1250/1/012002 2 term and long-term, so that the protective structure that will be designed will be appropriate to overcome the problems that occur [3].
The primary layer of a structure is an essential part of the plan's design.A miscalculation in the weight design of the armor unit may result in structural failure.The typical layout of coastal structures was made of large piles of natural stone and weighed several tons [4].It is becoming increasingly difficult to find a natural stone of the right size, and to prevent its scarcity; natural stones are commonly replaced with concrete blocks with specific shapes, such as tetrapods, macropods, cubes, and xbloc [5].The Coastal Dynamics Assessment Center (BPDP) proposed a new design innovation for a protective layer unit called BPPT-Lock.The BPPT Lock armor unit is said to have a higher stability value than tetrapod, xbloc, and dolos armor [6].

Figure 1. BPPT Lock Armour Unit [6]
Based on this information, a study was conducted on wave reflection on the breakwater armor layer unit using a pile of concrete blocks BPPT Lock.Breakwater and BPPT Lock were physically modeled in the flume tank of the Department of Ocean Engineering, ITS, Surabaya.

Research Methods
Testing the physical wave reflection model on the BPPT-Lock protective layer unit was carried out on a 2-dimensional wave channel of the Coastal and Port Infrastructure Laboratory, Department of Marine Engineering, ITS.The wave flume has 20 m long, 2.5 m high, and 2 m wide.In this test, the generated waves are irregular with a JONSWAP spectrum.This test will determine the value of the reflection coefficient that occurs.In addition, wave height and wave period data were obtained from recording changes in water level elevation by the wave probe.
Table 1 below is a test scenario with variations in the height and period of the generated waves.The test starts by generating the smallest wave to the largest wave.The test was repeated three times to obtain sufficient data for processing and analysis, each consisting of a series of tests.

Figure 2. Side view of wave probes and breakwater model placements
Measurement of the reflection coefficient used the method proposed by Goda & Suzuki [8].The technique used is by utilizing two wave probes as wave data takers.These data show a deviation to get the reflection and incident wave heights.According to [8], a reflection wave measurement can be obtained by placing at least 0.2 of the wavelength wave probes against the structure.The wave probes were placed at a distance of 1 m from the structure, while the distance between the wave probes was 0.5 m.

Results and Discussion
The reflection coefficient is obtained using the algorithm's equation [9], which was solved using Matlab software.The column height and wave period contain the recording results of the wave height sensor, and the column  is the Irribaren number obtained using the algorithm equation [9].

The Effect of Wave Steepness on The Reflection Coefficient
As parameters representing the wave characteristics, the wave steepness influences the reflection coefficient.

Figure 3. Comparison Wave Steepness to Reflection Coefficient
Figure 3 shows that the value of the reflection coefficient will be higher with increasing wave steepness.This research produces a reflection coefficient value (Cr) range of 0.071 -0.229.

The Effect of Irribaren Number on The Reflection Coefficient
The Iribarren number is used to describe its effect on the reflection coefficient.The Iribarren number is a non-dimensional parameter used to describe the impact of waves on coastal protection structures.The comparison graph shows the influence of wave steepness on the reflection coefficient.From figure 5 shows that the BPPT Lock has an Iribaren Number ranging from 1.765 -3.190 with a wave steepness of 0.004 -0.013.The value of the reflection coefficient will increase as the Iribaren number increases.BPPT Lock has the smallest reflection coefficient compared to other types of protective stones.This is beneficial in reducing waves and creating a calm and safe harbour for ships.

Conclusion
The reflection coefficient value of the BPPT Lock protection layer varies from 0.071 to 0.24 at wave steepness ranging from 0.004 to 0.013 with a slope angle of 1: 2, according to the findings of testing the physical model.The structure's rise in inclination, wave height, and wave period all have an impact on this reflection coefficient.Given that BPPT Lock has the lowest reflection coefficient when compared to Tetrapod, Shed, Diode, and Stabit, it is advised that coastal constructions in Indonesia utilize it as a armour unit.
Figure 2 below  shows the position of the model placement in the wave channel.The model tested has a scale size of 1: 43 so that the laboratory facilities can generate waves of up to 5.8 m on the prototype.A water level elevation sensor (wave probe) measures the wave height.If the device is immersed in water, the electrode measures the conductivity of the water volume.The conductivity changes proportionally according to variations in changes in water level elevation.The number of wave probes used in this study are four pieces, which refers to the physical experiments conducted by Neelamani & Rajendran[7].

Table 1 .
Test Wave Model Scenario Test Scenario study was conducted to determine the wave character of the interlocking unit of the BPPT Lock armor unit.