## 3 Results
### Turbidity  
Turbidity is an index reflecting the water quality and can be used to assess the sediment deposition areas to a certain extent [47]. When the sediment-laden flows pass through a cascade permeable dam, sediment particles are deposited at different locations, resulting in differences in turbidity in the water behind the dam. By measuring the turbidity, a preliminary analysis is carried out to evaluate the effectiveness of the cascade permeable dam in water purification. As shown in Figures 7 and 8, under different discharge conditions, the turbidity of different locations increases monotonically with increasing sediment concentration overall. By measuring the collected dates and building the corresponding scatter plot, the black linear model was fitted to a total turbidity removal rate in Figures 7 and 8 [48,49].  
Figure 7: Turbidity variation curve at different locations in the main flume and at the outlet for small particle size (the black line is the fitted curve for the total turbidity removal rate).  
In the experimental groups of the small particle size, the turbidity of the water before 1# dam is always the highest. Part of the sediment particles in the sediment-laden flows flocculates and infiltrates into the dam, resulting in a gradual decrease in turbidity values behind 1# dam. The turbidity removal rate of 1-4# dam is 2.7-21.8%, \(-\)4.1 to 16.3%, \(-\)5.7 to 11.9%, and \(-\)9.4 to 5.0%, respectively, indicating that each dam in the cascade permeable dam can purify the water and the turbidity removal rate is decreasing step by step. The turbidity removal rate of the whole cascade permeable dam ranges from 15.9% to 39.8%. In addition, the removal rate gradually decreases with time. At the sediment concentration of 2.0 g/L, the turbidity decreases significantly behind 1# and 2# dam under different discharge conditions, and decreases relatively little behind 3# and 4# dam. In this condition, the total turbidity removal rate of the cascade permeable dam tends to decrease and then increase with increasing discharge. The pore volume of the dam is large at the beginning of the experiments, and 1# dam shows more obvious fluctuations of turbidity value at 30 min, when the total turbidity removal rate varied between 18% and 37%. The 2#, 3#, and 4# dam shows a smaller change and more intersection in the turbidity value, which is mainly due to the impact-blocking impact cycle that occurs when the sediment inside the dams moves hydraulically, thus causing the turbidity to vary repeatedly.  
The sediment adhering to the surface of the dam material causes local flow disturbances, resulting in irregular changes in the turbidity value. In the large particle size, the turbidity fluctuates more frequently behind the dam. At the beginning of the experiments, the turbidity value increases slightly under some working conditions. The water level in front of the dams decreases along the flow direction, and the deposited sediment carries out the secondary deposition distribution. At the end of the experiments, the turbidity at the outlet of the flume was significantly reduced, and the turbidity removal rate of 1-4# dam is \(-\)0.41 to 6.66%, \(-\)7.94 to 11.29%, \(-\)5.43 to 8.09%, and \(-\)5.56 to 13.44%, respectively. However, the turbidity values and the total turbidity removal rate of the dam always varied  
Figure 8: Turbidity variation curve at different locations in the main flume and at the outlet for large particle size (the black line is the fitted curve for the total turbidity removal rate).  
slightly within a certain range at different discharges, indicating a stable filtration effect of the permeable dam.