Abstract
Impulse waves are actually a type of tsunami waves. The most important factor in creating these waves is the transfer of momentum from the sliding mass to the water. Impulse waves are the cause of dam failure and downstream flood. Therefore, knowing the factors affecting this phenomenon has always been the concern of researchers and hydraulic engineers. This study was conducted using the Flow-3D numerical model and a laboratory data (Breguli, 2017) was used to validate the model. In this research, for the first time the effect of surface tension on wave propagation in the three-dimensional Flow-3D modeling of waves was considered. The results show that considering the surface tension force is necessary for high accuracy results and it reduces the modeling error by an average of 5% while other simulation conditions such as the initial and boundary conditions and the mesh size are the same. Also, the depth of still water level in reservoir and the porosity of the sliding mass have an effect on the properties of the impulse wave. According to results, with the increase of about two times the depth of the water, the speed and length of the created wave increases by nearly 30 and 48%, respectively. In addition, with a 10% decrease in the porosity of the sliding mass, the wave velocity decreased by 14% and the wavelength decreased by 10%.

Introduction
The most destructive consequence of large solid mass sliding into water is the generation of very high waves. This phenomenon has consistently resulted in significant damages worldwide. Among the most important of them, we can point to the sliding event in the reservoir of the Vajont dam in Italy and Lituya Bay mega tsunami in Canada and a fatal landslide in New Guinea, which caused many casualties and damages. Various researchers have investigated this phenomenon with various analytical, numerical and experimental methods and have explained the effective factors from different geotechnical, geological and hydraulic aspects. The review of previous researches shows that despite the existence of experimental and numerical models, the factors of the depth of still water in the reservoir and the effect of surface tension force and porosity of the sliding mass have not been studied. These factors can have important effects on the forming and restraining forces of the phenomenon characteristics.
In this research, an attempt has been made to study these parameters in a three-dimensional study using Flow-3D software. In particular, information about the initial water depth, sliding mass porosity and numerical modeling considering the surface traction force in the wave has not been published. The innovation of this research is the investigation of the impact of these parameters in the Flow-3D model and also the validation of the mentioned model using laboratory data (Breguli et al., 2017).

Methodology
The three-dimensional modeling of impulse wave caused by the mass sliding has been done using the Flow-3D. v11.02 model, which is one of the most powerful tools for the use of computational fluid dynamics. This model solves three-dimensional equations of Navier-Stokes using volume fraction method and volume of fluid for incompressible fluid. In addition, consistent turbulence equations in the study of this phenomenon is renormalization group (RNG). In order to validate the numerical model, the laboratory data (Breguli et al., 2017) which measured the impulse wave by mass sliding in still water in 3D has been used (Fig.1). This model sensitivity has been measured with five different mesh sizes (Tab.2). In the laboratory study of this phenomenon, since the dimensions and geometries are small, there will be scale effects such as the effect of surface tension, which can be ignored in the real scale of this phenomenon, and it is a false assumption if this effect is not considered in the modeling. As shown in Figure 3, modeling by activating surface tension physics in the Flow-3D model has better accuracy in all mesh sizes. In this research, 19 models using five still water levels in two different physics and two different types of masses have been implemented (Tab.4). To study the effect of mass porosity, the physics of surface tension is active and three still water level have been used.
Results and Discussion
The characteristics of the first wave formed after the mass impact the water surface and its propagation mechanism have been studied by investigation the wave parameters.
As shown in Fig.5, by activating surface tension physics in the model, the wave velocity has decreased compared to its inactive state. By considering the effect of surface tension and boundary layer turbulence in the air-water interface, changes in fluid viscosity are considered in the surface tension equations, which causes the loss of wave energy.
The depth of water affects the characteristics of wave. Therefore, according to the Fig.6 and Tab.7, characteristics of the wave extracted from the numerical model are given. As the water depth increases, the wave velocity increases and waves with shorter amplitude and longer wavelength are generated.
As shown in Fig.8 and 9 results show that by increasing the porosity of the sliding mass, a smaller wave with less energy is generated. The cause of this problem is the pore in the mass, which absorbs the energy of the collision and reduces the transfer of momentum to the water surface. In fact, the porous mass has a geometry similar to a damper.

Coclusion
Increasing the water depth increases the velocity and wavelength and decreases the sharpness and height of the wave. Applying the surface tension force is necessary for small-scale modeling and surface tension in the generation and propagation of waves cannot be ignored. As the porosity of the sliding mass increases, the height and velocity of the wave decreases and the wavelength increases.

Keywords
Surface tension, Mass sliding, Impulsive wave propagation, Flow-3D model.