## 4 Results and Discussion
### Characteristics of Turbulence Intensity along the Entire Flow Depth  
Although the maps produced for the turbulence within the artificial reefs can clearly show the overall variation of longitudinal and vertical turbulence intensity, due to the difference between unit reef spacing setup for the experiments conducted, it is not convenient to compare and analyze the turbulence intensity at the same relative position because it may be not related to exactly the same location. In order to accurately compare the changes of turbulence intensity in the vertical line under different velocity conditions, 16 sections were identified as displayed in Figure 8. More in detail, Section A and Section B are 60 mm (1 times reef length) and 30 mm (1/2 times reef length) before the first group (Group A, as introduced in Figure 5) of artificial reefs; Section C is 30 mm after the first monocase of Group A; Section D, Section E, and Section F are 30, 60, and 120 mm after the last monocase of Group A; Section G, Section H, and Section I are 120, 60, and 30 mm in front of the second group of artificial reefs, Group B (as introduced in Figure 5); Section J is 30 mm after the first monocase in Group B; Section K is 30 mm after the second monocase in Group B; Section L is 30 mm after Group B; Section M is 30 mm before the final group of artificial reefs, Group C (as previously displayed in Figure 5); Section N is 30 mm after the first monocase in Group C; Section O and Section P are 30 mm and 60 mm after Group C.  
According to the location of these sections relative to the unit reefs, the 16 sections are classified into four categories as follows:  
* A and B are the sections not entering the reef area (Type I);
* G, H, I, and M are the front sections of the reef groups (Type II);
* C, J, K, and N are the sections between the two reef monocases (Type III);
* D, E, F, L, O, and P are the rear sections of the reef groups (Type IV). Figures 9 and 10 show the comparison of longitudinal and vertical turbulence across each section for the worst scenario tested (3) and all the velocities replicated (1-5). It is possible to notice that on the same section, the trends of turbulence intensity are highly coincident, despite the different velocities tested. A general pattern can also be observed: along the flow direction, outside the reef area, the longitudinal turbulence intensity gradually decreases with the increase of the water level, while this behavior cannot be noticed for the vertical turbulence intensity. After the flow enters the reef area, two inflection points appear in the turbulence intensity on the vertical line. In fact, after the flow enters the reef area, the turbulence intensity firstly increases to the maximum value with the increase of water level (1 st inflection point); then, it decreases sharply with the increase of water level (2 nd inflection point) and then decreases slowly. The 1 st inflection point appears at the highest point where the water flow is affected by the reef on the vertical line. In the test area, the height of the 2 nd inflection point increases gradually along the flow direction. Figure 8: Cross-section distribution of artificial reefs. Figure 9: Longitudinal turbulence intensity for each section under geometrical configuration 3 and five different velocities tested. The subplots refers to the values measured at each of the 16 sections identified (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P). intensity appears at 1.7 times the reef height, and the position of the 2 nd inflection point is not clearly identifiable. The 1 st inflection point of the vertical turbulence intensity appears at 1 times the height of the reef, and the 2 nd inflection point is higher, about 3.2 times the reef height. However, in the sections affected by back vortex, such as C, J, K, and N, the 1 st inflection point of longitudinal turbulence intensity appears at 1.25 times the reef height, and the 2 nd inflection point appears at about 2 times the reef height.