## 5 Laboratory Tests
### Testing Results
The measurement results that were recorded in different scenarios when the testing time was 180 min are shown in Figure 20. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Testing Point**} & **Erosion Depth before** & **Erosion Depth after** & **Reduction Rate (\%)** \\  & **Using “Fishnet” (cm)** & **Using “Fishnet” (cm)** & **Reduction Rate (\%)** \\ \hline
1 & 4.7 & 4.2 & 10.64 \\
2 & 5.0 & 3.9 & 22.00 \\
4 & 5.5 & 3.5 & 36.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Effectiveness investigation results of the “fishnet” technology. Figure 19: Scour pit: (**a**) before using the fishnet device; (**b**) after using the fishnet device. In principle, the smallest erosion depth indicates the optimal size of the fishnet. However, from Figure 20, it was interestingly found that the smallest erosion depth in different directions was observed at different sizes of the fishnet. In particular, the erosion depth at testing point 2 showed an extremely low value when the size of the fishnet was 2.5 cm due to either measurement error or other uncertainties in the laboratory test. Such an outlier disturbed the judgment of the optimal size of the fishnet. Despite this, it can still be seen that a generally satisfactory erosion reduction effect could be achieved when the size of the fishnet was 1.0 (based on point 1) or 1.5 cm (based on point 4). Secondly, the optimal diameter of the fishnet thread was investigated. In the experiments, the size of the fishnet was 1.5 cm, the length of the fishnet device was still 20 cm, the diameters of the fishnet thread were 0.6 mm, 0.8 mm, 1.2 mm, 1.5 mm, and 1.8 mm. The erosion depths that were measured in different scenarios when the testing time was 180 min are shown in Figure 21.  
From Figure 21, it can clearly be seen that the smallest erosion depth was obtained in all three directions when the diameter of the fishnet thread was 1.2 mm. Thus, it can be said that 1.2 mm should be the optimal diameter of the fishnet thread. Finally, the optimal length of the fishnet device was investigated in the laboratory. In the experiments, the size of the fishnet was selected to be 1.5 cm, and the diameter of the fishnet thread was 1.2 mm, while the lengths of the fishnet device were selected to be 10 cm, 20 cm, 30 cm, 40 cm, and 50 cm. The erosion depths that were measured in different scenarios when the testing time was 180 min are shown in Figure 22.  
Figure 21: Erosion depth when fishnet thread had different diameters. Figure 20: Erosion depth when fishnet had different sizes. From Figure 22, it can be seen that changes in the length of the fishnet device minimally affected the erosion depth on both sides of the monopile, as its efficiency was maintained at about 38.2%. However, the earlier numerical study disclosed that the erosion reduction efficiency of the proposed \"fishnet\" technology was only about 14%. On the other hand, according to Reference [21], an efficiency of about 46.3% could be observed from the spiral technology when the spiral angle was \(15^{\circ}\). Obviously, there is no consensus on the efficiency of this kind of technology. The difference may be related to many variables in both numerical calculations and laboratory tests. However, despite these differences, it can be concluded that the \"fishnet\" technology studied above can indeed reduce scouring, and its contribution to erosion reduction can be further improved by performing parameter optimization.