Experimental investigating on the reflected waves from the caisson-type vertical porous seawalls Mehdi Esmaeili 1 , Maryam Rahbani 2 *, Ali Karami Khaniki 3 1 Marine Sciences Colleges, Chabahar Maritime University, Chabahar 99711, Iran 2 Faculty of Marine Science and Technology, University of Hormozgan, Bandar Abbas 79131, Iran 3 Soil Conservation and Watershed Management Research Institute, Tehran 1398, Iran Received 19 October 2017; accepted 9 February 2018 © Chinese Society for Oceanography and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The hydrodynamic efficiencies of caisson-type vertical porous seawalls used for protecting coastal areas were calculated in this study. Physical models were developed to compare the wave reflection from vertical plane, semi-porous, and porous seawalls caused by both regular and random waves. Tests were carried out for a wide range of wave heights, wave periods, and different water depths (d=0.165, 0.270 and 0.375 m). The performance regarding the reflected waves from porous and semi-porous seawalls showed improvement when compared with those from the plane seawall. The reflection coefficients of the porous and semi-porous seawalls were calculated as 0.6 and 0.75, respectively, while the coefficient for the fully reflecting plane vertical wall was significantly higher (0.9). It was also observed that the reflection coefficient decreases with increase in wave steepness and relative water depth. In addition, the reduction in the reflection coefficient of porous and semi-porous seawalls, as compared to that of a plane seawall, was observed for both regular and random waves. New equations were also proposed to calculate the reflection coefficient of different types of seawalls with the aid of laboratory experiments. By verifying the developed equations using some other experimental data, it was validated that the equations could be used for practical situations. The results of the present study can be applied to optimize the design of vertical seawalls and for coastal protecting schemes. Key words: reflection coefficient, porous seawall, regular wave, random wave, flume Citation: Esmaeili Mehdi, Rahbani Maryam, Khaniki Ali Karami. 2019. Experimental investigating on the reflected waves from the caisson- type vertical porous seawalls. Acta Oceanologica Sinica, 38(6): 117–123, doi: 10.1007/s13131-019-1386-6 1  Introduction Seawalls are structures that can be used to protect shorelines. They are built to reduce the destructive effects of wave action, protect the shore from excessive erosion, and guard coastal struc- tures against damage. Any seawall placed in a wave field reflects part of the incident wave energy away from the structure, which in turn creates steep and unstable waves. This can then lead to increasing wave disturbance and wave loads in adjacent areas and harbors. As a result, for harbor management and shore pro- tection, it is extremely important to identify the cause and im- pact of wave reflections and discover preventive solutions. Porous seawalls could be used as a solution to the problem associated with increasing wave disturbance. Porous seawalls are common shore protection structures around the world. They can dissipate the incident wave energy and reduce the reflected wave height. Vertical porous seawalls are considered as effective re- placement for traditional impermeable seawalls, especially for the reduction of wave energy in harbors and fishing ports. The amount of wave energy dissipation depends on the porosity of the seawall (n). In addition, the shore side of the vertical porous seawall can be used for vessel berthing activities. Many theoretical and experimental studies have been con- ducted on different hydrodynamic aspects of waves, such as re- flection, transmission, and dissipation, and their effects on vari- ous seawall structures. For instance, Zhu (1999) examined the wave interactions of a porous seawall to estimate the reflection of waves by employing theoretical analyses, numerical modeling, and experimental investigations. Suh et al. (2001), using an ana- lytical model, deduced that the reflection of irregular waves radi- ated normal to a caisson-type seawall. They conducted some laboratory tests to investigate the performance of the proposed model. In this regard, varying flume widths were employed. They showed that the frequency-averaged reflection coefficients de- rived from the measurement were in line with those obtained from calculations. Zhu and Chwang (2001) analytically studied the interaction between waves by developing a linear wave, using a slotted sea- wall with a specific expansion function method. Their investiga- tions indicated that the characteristics of the reflected waves mainly depend on the porosity of the seawall gap and height of the incident wave. Neelamani and Sandhya (2003) conducted an experimental study to compare the reflection characteristics of vertical and sloped planes and dentate and serrated seawall models. In their research, they considered regular and random waves across a myriad of heights and periods. They measured the reflection of different waves to assess the dissipation effect of the seawalls. They claimed that the serrated seawall showed better efficiency, compared with the plane and dentate seawalls, for reduction of the reflection of waves. Based on their measurements and mul- tiple regression analyses, they offered some equations to calcu-   *Corresponding author, E-mail: maryamrahbani@yahoo.com   Acta Oceanol. Sin., 2019, Vol. 38, No. 6, P. 117–123 https://doi.org/10.1007/s13131-019-1386-6 http://www.hyxb.org.cn E-mail: hyxbe@263.net