### 1. Introduction

*R*) that describes the stability in the fluid motion was used for the calculation (e.g. Blumberg, 1977; Pacanowski and Philanders, 1981). Currently, various turbulence models have been developed, and the eddy viscosity coefficient calculated from the turbulence model is generally used for the horizontal diffusion coefficient, and vertical diffusion coefficient is recalculated with application of Schmidt Number (Sc) or Prandtl Number (

_{i}*P*), which is described as the ratio between the eddy viscosity and diffusion, according to the type of turbulence model, grid system, etc (e.g. Farhanieh et al., 2001; Lesser et al., 2004). In addition, Sato et al.(2006), who used the 3-D numerical model, selectively applied eddy viscosity coefficient calculated with large eddy simulation (LES) and diffusion coefficient that is calculated by substituting

_{r}*R*and

_{i}*S*.

_{c}### 2. Experimental Setup and Procedure

### 2.1 Description of Hydraulic Model

### 2.2 Experimental Conditions and Measurement

#### 2.2.1 Experiment using Flow Visualization

*d*) are 0.2 mm and 2.5 mm, and the inclinations (

_{p}*B*) are 1:0 (cliff), 1:2, 1:3, 1:5 and 1:∞ (horizontal plane). The initial salinity is 10 psu, 20 psu, and 30 psu. In addition, the initial conditions are listed in Table 1 for the experiments related to the temperature, salinity and density of water. Here, the water density is calculated by empirical formula which was proposed by Gill(1982).

_{s}*S*,

_{0}*T*, and

_{0}*ρ*mean the salt, temperature and density of saltwater, and

_{0}*T*and

_{f}*ρ*mean the temperature and density of freshwater, respectively.

_{f}*g*` is a reduced gravity and calculated as

*Δg/ρf*(

*Δρ*is the density difference between the saltwater and freshwater; and

*g*is the gravitational acceleration).

#### 2.2.2 Experiment using PIV System

^{-5}cm and the particles are high porous type synthetic adsorbents with specific gravity of 1.01 g/cm and effective size of 2.5 mm.

^{®}) for 2-D fluid analysis is used for the particle video and the velocity component (

*u*and

*w*) is calculated in

*x-z*direction for certain unit area of 32 pixel × 32 pixel (1.25 cm × 1.25 cm) with the resolution of 0.39 mm/pixel.

### 3. Experimental Results

### 3.1 Advection-Diffusion Characteristics of Saltwater

#### 3.1.1 Characteristics due to Initial Salinity

*h*is the initial depth and

_{0}*x*is the length of a compartment.

_{0}*g*’, which affects the spreading rate. Thus, when

*g*’ increases, the acceleration influencing the saltwater increases so that the advection speed of saltwater increases until the pressure gradient is balanced. In other words, bigger the density difference between the two fluids is, higher the reduced gravity resulting in faster movement of saltwater is. Comparing Figs. 5(a)-5(b), Karmann vortex occurs because of Kelvin-Helmholts instability when the reduced gravity (density difference) becomes higher. According to this, more saltwater spread is expected, and it will be discussed through the comparison and analysis of flow/vorticity fields and vertical salt concentration, which are obtained from PIV system.

#### 3.1.2 Characteristics due to Components of Bottom

*S*20 psu; the reduced gravity is

_{0}*g*’ 16.52 cm/s

^{2}; the red indicates saltwater; and the black indicates freshwater.

#### 3.1.3 Characteristics due to Bottom Slope

### 3.2 Flow and Vorticity Field Characteristics on Density Current

^{®}),

*x-z*flow components (

*u*and

*w*) are calculated in a unit area of 32 pixel × 32 pixel (1.25 cm × 1.25 cm). The estimated flow velocities are applied to Eq. (1) of Raffel et al.(1998) and Raffel et al.(2007) in order to calculate the vorticity in the

*x-z*plane rotating around

*y*-axis. Here, the clockwise vorticity is expressed in a positive value and the counterclockwise one is expressed in a negative value.

#### 3.2.1 Characteristics due to Initial Salinity

*x/x*

_{0}2.5, Fig. 9(a) is

*t*7.92 sec and Fig. 9(b) is

*t*4.44 sec. Here, the red corresponds to the clockwise vorticity and the blue corresponds to the counterclockwise one.

*g*` = 24.74 cm/s

^{2}) with higher reduced gravity, the inflow speed of freshwater increases because of the high-speed of saltwater. In addition, Karmann vortex develops along the interface of saltwater and freshwater because of Kelvin-Helmoholtz instability, and there is strong vortex and shedding phenomenon in case of Fig. 9(b) CASE3 (

*g*` = 24.74 cm/s

^{2})with higher spreading rate of saltwater. This phenomenon implies that there is an active saltwater spread.

#### 3.2.2 Characteristics due to Bed Condition

*x/x*

_{0}2.5; Fig. 10(a) is

*t*5.75 sec and Fig. 10(b) is

*t*4.34 sec. Here, the red shows the clockwise vorticity and the blue shows the counterclockwise one. As mentioned above, the spreading rate of saltwater proceeding on the permeable ground by density difference decreases due to the shear stress applied in/out of the surface influenced by the bottom roughness and the saltwater seeping into the ground. In Fig. 10(a), the flow velocity on the bottom decreases and the head shape of saltwater changes, resulting in the distribution of flow field and vorticity field different from the impermeable bottom condition (Fig. 9). Furthermore, because of the decreased spreading rate of saltwater, less vortex shedding can be observed.

### 3.3 Vertical Distribution of Salinity

*S*) according to the initial salinity in a compartment for the impermeable bottom condition, where the salinity is measured at every 1 cm interval from the bottom when there is pressure balance. Here, the black circles (●) are results for the initial salinity of 10 psu (CASE1); the blue squares () are results for the initial salinity of 20 psu (CASE2); and the red diamonds () are results for the initial salinity of 30 psu (CASE3).

_{0}*S*of compartment increases and due to the spread of saltwater by Karmann vortex and vortex shedding at the interface. According to these experimental results, saltwater spread is governed by the vortex and vortex shedding at the interface, and it has great influence on the vertical distribution of salinity.

_{0}### 4. Concluding Remarks

*g*`) increases (density difference increases).