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Effects of Roughness and Vertical Wall Factors on Wave Overtopping in Rubble Mound Breakwaters in Busan Yacht Harbor

Coastlines are protected by breakwater structures against the erosion of sand or other materials along beaches due to wave action. This research examined the use of physical modeling to determine the effects of the tetrapod size and vertical walls of a rubble mound on the volume of wave overtopping under irregular wave conditions in coastal areas in Busan Yacht Harbor. In this analysis model, the structures were studied using irregular waves and the JONSWAP wave energy spectrum. To understand the effects of the tetrapod size and heights of the vertical wall, the study considered vertical walls of 0, 1.78, 6.83, and 9.33 cm with armor double layered material tetrapods of 8, 12, 16, and 20 tons. An extensive number of experiments covering a relatively large range of variables enabled a comprehensive discussion. First, in the presence of a short vertical wall, the water level played a key role in the overtopping discharge. In such circumstances, the values of the wave overtopping discharge decreased with increasing freeboard size. In the presence of a tall freeboard and middle, the value of the wave overtopping discharge was equally influenced by the vertical wall factor. Moreover, the tetrapod size decreased by an increase in the vertical wall factor, and relationship between them resulted in a short wall height. From an engineering point of view, considering a small water level may allow the choice of a shorter vertical wall, which would ultimately provide a more economical design.


Introduction
Coastal walls are structures that are parallel to and near the shoreline to protect beaches and building and facilities for achieve coastal security. The coastal wall shape and type of aggregate, are divided into different types, which include rubble mound breakwaters. The rubble mound type can be separated to two categories: static stability (traditional) and dynamic stability. One of the most important parameters in a breakwater is wave overtopping. Breaking waves play a significant role in virtually all coastal engineering problems. A large number of factors influence the design of the crest freeboard. These factors are related to the breakwater geometry, structural material and hydrology statistics. The geometrical parameters in coastal engineering design play important roles; the parameters include the free board, crest and width shape, slope of the tetrapod layer, and water depth.
Over the past few decades, a variety of breakwaters have been developed. Since these varieties were implemented, the results have indicated that the structures are far from optimal. The freeboard parameter is also the main parameter for the design of a breakwater. The rubble mound breakwater has been considered in South Korea's, Busan yacht Harbor. The East, Middle and West rubble mound breakwaters in this area of the beach are the most suitable design for this investigation into the effects irregular waves (Busan Port Authority, 2011). In Busan Yacht Harbor, in the case of the East, Middle and West breakwater structures, the design analysis has been complicated by physical processes that depend on the hydro dynamical situations (waves, wave height, wave period, etc.) and the type of breakwater. To the best of the authors' knowledge, there is still lack of information on the effects of vertical wall and tetrapod weights within the rubble mound structures in response to complex 2 dimensional incoming waves to the harbor. Among the existing literature, (Park et al., 2014;Zanuttigh and Van der Meer, 2006) examined the effects of vertical wall in a rubble mound structure and in the presence of various types of tetrapod weights but they did not   (Owen, 1980;Goda, 1985), and more recent analysis of Rubble Mound Breakwaters includes (Aminti and Franco, 1988;Van der Meer and De Waal, 1992;Van der Meer et al., 2005;Bradbury and Allsop, 1988;Muttray et al., 2006;Holthuijsen, 2007). Concerning rubble mound breakwaters in comparatively deep water, Jensen and Juhl(1987) examined consequences from an irregular wave model tests and some different projects were approved at DHI, and his work was later reanalyzed by Jensen and Juhl(1987). While this work is beneficial, it is limited by the condition of geometry and the specific conditions of the tested rubble mound breakwaters. Pullen et al.(2007) recommended the installation of vertical walls on the rubble mound breakwater structure to reduce the overtopping discharge. They suggested that the wall height to be in the same elevation of the armor crest level because wave action forces on the vertical wall will go up severely by action waves and not hidden behind the armor crest if directly were attacked, Pullen et al.(2007). In this study, physical model tests

Experimental Setup
The 2D physical experiments were completed in the wave flume of the Coastal & Ocean and Harbor Engineering Laboratory. Length, width and height of the test flume was 30m, 0.60m and 1 m, respectively. All tests included three kinds of water levels (Table 1) in the water levels (low level of water, mean sea level and high water level), based on a JONSWAP(Joint North Sea Wave Project) spectrum. The wave generator paddle in a 0° angle wave attack provided mechanically irregular waves on the flume, which is controlled by a personal program. The program on the computer can be used to generate regular, irregular and a variety of other wave conditions. After finding the irregular waves precisely, the computer program was used to make waves with a generator (wave period, wave height) according to a model in the scale of the prototype. Figure 3 shows a photograph of the four tetrapods used in the present study (8, 12, 16 and 20 ton). Table 2 compares the weights of the tetrapods between the actual and physical model. The present experiment made used 4 wave gauges (Mansard and Funke, 1980), the first wave gauge was in front of the generator (5 channel), the second wave gauge was 2 meters in front of the crest rubble mound (7 channel

Definition of Overtopping
The present data was analyzed with the Van der Meer (2002) formula to complete the certainty of the results for wave overtopping in this paper, that characterize the wave overtopping discharge for irregular waves. A connection of the experimental consequences with Van der Meer's formula, Eq.
(1) displayed that the present wave overtopping discharge estimator administers good reaction. For each tetrapod in the existence of no vertical wall the roughness factor (  ) achieved, (Park et al., 2014).
The complete formula (Van der Meer, 2002;Pullen et al., 2007) is (1) if Rc = Ac, the maximum overtopping can be expressed as Equation 2.
Furthermore, if gamma   = 1 Eq. (2): (2) where q is the wave overtopping discharge (m 3 /m/s), H1/3 is the significant wave height at the toe of the rubble mound (m), ɡ is the gravitational acceleration (m/s 2 ),   is the roughness factors (  ),   is the angle of wave action factors (  =1),   is the effect of toe factors,   is the vertical wall factors (  ), and   is the breaker parameter that this research confirmed under the breaking wave action. Estimating that the roughness factor did not change for each tetrapod weight, this inquiry checked out the influence of different vertical walls ( Figure 6) and relationship between them on the overtopping discharge and Eq. (3). (3)

JONSWAP Energy Spectrum
An empirical relationship that defines the distribution of energy with frequency within the wave action was the JONSWAP spectrum (Hasselmann et al., 1973). (Goda, 1988), JONSWAP spectrum modified Eq. (4) to be substantiated in terms of the peak wave period (Tp) and significant wave height (H1/3): (4)

Where
Here, H1/3 is the significant wave height, H1/3 is the significant wave period (Table 3), Tp is the peak period, ƒ represents frequency and ƒp is the peak wave frequency,  the so-called peak enhancement parameter, which disciplines the sharpness of the spectral peak, б is a JONWAP spectral parameter and related to on whether the wave frequency is equal, higher or lower than peak wave frequency,  J is the form parameter in JONWAP spectrum.    Table 3 presented that there was no vertical wall   (Park et al., 2014) (case 4) above the crest freeboard and Rc equals Ac. Four Ac and Rc were prescribed in this analysis for case 1, 2, 3 and 4 (Table 4)  in no vertical wall while the roughness factor in this case had a high repercussion and plays very large aspect in felling to wave overtopping and it means that is more assertive than the effect of the roughness factor (f) when the vertical wall is equal or short than 1.78 cm. Consequently, the tetrapod size played a meaningful role in this case due to the short vertical

Conclusions
The consequences of the physical model for the effects of link between roughness and vertical wall factors on wave overtopping tests, which were performed as part of the design in Busan Yacht Harbor, Busan, South Korea, were used to test the expression on different vertical wall height along with variation of the tetrapods weights. The results of this study suggest that a decrease in the freeboard and size of tetrapod established a new hydraulic condition for the overtopping mechanism and consequently, sudden growth was observed in the amount of overtopping discharge. First, the tetrapod size played an important role due to the short vertical wall; this means that the effect of the influence of the roughness factor (  ) is more dominant than the effect of the vertical wall factor (  ) in presence of short vertical walls.
Next, the effect of the water levels from LLW to HHW on the wave overtopping discharge increased with decreasing tetrapod size for a presented dimensionless discharge and a steady vertical wall factor. In other words, relationship between (v) and (  ) demonstrated that electing a shorter vertical wall by using a larger tetrapod size would allow boomed wave overtopping but will lead to a more charge impressive design. From an engineering point of view, considering a small water level may allow the choice of a shorter vertical wall that would ultimately provide a more economical design.