## 4 Results and Discussion  
### Characteristics of Turbulence Intensity Distribution along the Flow Direction  
The changes of turbulence intensity in the proximity of the unit reef tend to be consistent under different working conditions, as displayed in Figures 6 and 7, which show the distribution of turbulence intensity under the worst scenario for the velocities tested and each geometrical setup; therefore, tests 0-5, 1-5, 2-5, and 3-5 displayed in Table 2 show the average values for the longitudinal and vertical turbulence intensity over the whole study range. Figure 6: Longitudinal turbulence intensity for cases 0-5, 1-5, 2-5, and 3-5. Figure 7: Vertical turbulence intensity for cases 0-5, 1-5, 2-5, and 3-5. Figures 6 and 7 show that, except for condition 0-5, where no artificial reef is applied, the turbulence intensity has an obvious boundary between the turbulent and non-turbulent regions, which gradually rises with the flow direction. When the flow was introduced into the flume inlet and passed through the artificial reef structure, it was apparent that the flow patterns in the upstream area of the flume and in those around the reef group A, the initial set of artificial reefs, were largely similar despite using different reef configurations. These were understandable as initial flow conditions and are usually typical of each artificial reef shape used. It can be noticed that comparing the same geometrical configuration, the longitudinal turbulence intensity is significantly larger than the vertical one, in terms of magnitude and areas characterized by high values. In fact, the characteristic length of the larger longitudinal turbulence intensity (>2.0) is about 5-7 times the height of the reef, while the larger vertical turbulence intensity (>0.16) is about 2-4 times the height of the reef. Moreover, the location of the area with larger longitudinal turbulence intensity is higher than that of the vertical turbulence intensity, as expected. Before entering the reef area, the longitudinal turbulence intensity gradually increases from the bottom at about 2.8 times the height of the reef in front of it, and along the flow direction, the turbulence height increases, which is close to the position where the current uplifts. The vertical turbulence intensity begins to change from the bottom at about 1 time the length in front of the reef. These results indicate that the blocking effect of artificial reefs is mainly reflected in the longitudinal velocity. Analyzing the longitudinal turbulence intensity under different working conditions, the magnitude of the numerical values and the area affected can be classified as follows: condition 2-5 > condition 1-5 > condition 3-5, but in terms of average turbulence intensity, this is the final order: condition 3-5 > condition 1-5 > condition 2-5. Analyzing the vertical turbulence intensity calculated for different geometrical and hydraulics of the worst scenario (higher velocities), the magnitude of the numerical values obtained and the length of the area affected follow this order: condition 1-5 > condition 2-5 > condition 3-5, but in terms of average vertical turbulence intensity, the order changes to condition 1-5 > condition 3-5 > condition 2-5. If values of longitudinal and vertical turbulence intensity are combined, the performance follows this order: condition 1-5 > condition 2-5 > condition 3-5; that is, \(L_{1}>L_{2}>L_{3}\), which indicates that when the spacing is small, the impact of reef on the flow is greater and the turbulence is stronger. Nevertheless, it can also be observed that due to the presence of the artificial reefs, the whole unit reef area is characterized by a relatively large turbulence intensity. Higher values of turbulence intensity initiate above the reef, following in parallel the flow streamlines and spreading outward with a strip shape, which is obviously greater than the surrounding turbulence intensity.