# Experimental Study on the Near-Red Flow Characteristics of Alluvial Channel with Seepage
## 3 Results and Discussion  
In order to detect the flow characteristics when downward seepage occurs and at moderate Reynolds number, turbulent flow parameters, including the time-averaged velocity, Reynolds shear stresses (RSS), turbulence intensity, and the skewness and kurtosis of velocity fluctuations, were measured at the centerline of the channel. The structure of turbulence was achieved based on the ADV data. Figure 5 presents the outlines of the time-averaged velocities with the non-dimensional flow depth, \(z/h\) (where \(z\) is the distance from the bed surface and \(h\) is the flow depth), for all the experiments. The solid and open squares relate to no seepage and downward seepage, respectively. It can be seen that the vertical velocities (\(w\)) were more affected by seepage than the streamwise velocities (\(u\)). The experimental data almost overlap in the case of \(u\), while they are far apart in the case of \(w\), denoting an increase in turbulence level. Previous investigations on the downward seepage, although in different contexts from the one under consideration, have also shown that the velocity profile moves downward and, hence, a higher velocity is achieved in the near-bed region [8; 16]. RSS values were achieved for both no-seepage and downward seepage runs. Specifically, the trends for the values of RSS with time at \(z=0.05\)\(h\) and the vertical profiles of RSS for the non-dimensional flow depth \(z/h\) are presented in Figure 6. Data for no seepage and downward seepage are represented by solid and open squares, respectively. In general, it was found that RSS increases towards the channel boundary, which specifies the transfer of greater momentum between the flow and the sediment bed. The RSS attains its peak magnitude in the vicinity of the inner layer and then it reduces in the direction of the channel bed because of the presence of viscous force or a roughness sub-layer. The vertical profiles of RSS are similar for no seepage and downward seepage, though with different magnitude. In particular, the magnitude of near-bed (\(z/h<0.15\)) RSS is amplified in the case of seepage, indicating greater momentum transfer between the flow and the sediment bed. As a result, the turbulence increases when downward seepage occurs. It can also be observed in the literature [1; 8] that downward seepage increases the RSS, which leads to the mobility of sediment particles, in contrast to the case of no seepage. Figure 5: Vertical profiles of (**a**) time-averaged streamwise velocity (\(u\)) and (**b**) vertical velocity (\(w\)) for all experiments in this study. The vertical distributions of \(\sigma(u)\) and \(\sigma(w)\) for no seepage and downward seepage are presented in Figure 7. Data for no seepage and downward seepage are signified by solid and open squares, respectively. The peak values of \(\sigma(u)\) and \(\sigma(w)\) were observed close to the channel boundary while the least values were found near the liquid free surface owing to the decreasing trend of RSS. At the near-bed (\(z/h<0.15\)) \(\sigma(u)\) and \(\sigma(w)\) increase when seepage occurs, indicating that seepage would amplify the velocity fluctuations in the vicinity of bed surface. This observation is in agreement with previous researches [20,28], although carried out in different environments. The outlines of turbulence intensities are substantial in the inner zone of the flow and the degree of near-bed (\(z/h<0.15\)) streamwise and vertical turbulence intensity was found increased approximately up to 20% and up to 12%, respectively, when downward seepage occurs. Figure 8 displays the individual profiles of the skewness for no seepage and downward seepage. In this case, a clear distinction between no seepage and downward seepage is not evident, especially in Figure 8b. However, the following comments could be made. \(S(u)\) and \(S(w)\) start with small positive and negative values in the vicinity of the bed surface for both flows subjected to no seepage and seepage.