The following is the first draft of an academic paper. Please help me polish the language.
<4 Discussion
4.13D morphological features of the pipe
The 3D geometry of the pipe is the foundation of building pipe modeling in numerical research, and it is of great significance for numerical research of BEP. This study found that the pipe has obvious 3D morphological characteristics. In order to establish a mathematical model of the pipe, the cross-sectional shape of the pipe was first needed to be generalized. It is found that the cross-sectional shape of the pipe could be approximated as a rectangular cross-section based on the experimental results. Afterwards, the average depth and average width of pipe cross-section were further analyzed. The results indicate that the average width and depth of the pipe showed a linear decreasing trend as the pipe length increased, which is consistent with previous results [22]. Since seepage concentrates towards the pipe, and the continuous scour in the pipe causes deepening and widening of the pipe cross section as the pipe gets longer. The absolute values of the linear correlation coefficients between pipe width and length in Test-1 and Test-2 are both greater than 0.8, indicating a highly linear correlation. Hence the relationship of the pipe depth and width with the pipe length could be fitted by linear functions. A complete 3D pipe numerical model can be obtained by the formulas (1) and (3) for the pipe in Test-2. The compactness of the sand layer has a certain impact on the geometric size of the pipe. As the compactness of the sand increased, the depth and width of the pipe decreased to a certain extent.

4.2 The influence of cover characteristics on pipe geometry
In dike engineering, the contact surface between the sand layer of the dike foundation and the cohesive blanket is not smooth. However, smooth acrylic plates were directly used to simulate the clay layer [6,13] in many experiments. This set-up has a certain degree of distortion in the characterization of the contact characteristics between the sand layer and clay layer. To investigate the effect of smooth cover plate on the 3D geometric shape of the pipe, comparative experiments were conducted. In the control group (Test-2), a silicone plate was placed between the sand layer and the acrylic plate to simulate cohesive blanket [20].
Compared with the control group, the tortuosity of the pipe decreased when the cover plate was smooth (Test-1). The variation patterns of pipe depth and width in Test-1 and Test-2 were similar, but there were certain differences in magnitude. The average depths of pipes in Test-1 and Test-2 were 3.5 and 4.0 mm, respectively, while the average depth of pipes in Test-1 and Test-2 were 28.3 and 31.1 mm, respectively. The depth and width of the pipe in Test-1 were lower than those in Test-2, and the specific mechanism has not been fully understood. The cross-section of the pipe was approximately rectangular under different roughness levels of the cover plate. In order to make the results more reasonable, the selection of top plate materials should be more cautious in future research, and the more realistic characteristics of the contact surface between sand layer and cohesive blanket should be considered.

5 Conclusion
This paper explored the 3D geometric characteristics of pipe that just reach upstream level under different conditions through 3D laser scanning and other methods, and the following main conclusions were obtained: 
The curvature of the pipe increased with the increase of the sand layer compactness and the roughness level of the cover plate. The cross-section of the pipe was approximately rectangular, with a scale in the horizontal direction much larger than that in the vertical direction.
The depth and width of the pipe gradually decreased as the pipe length increased, and the relationship of the pipe depth and width with the pipe length could be fitted by linear functions. Width-to-depth ratio of the pipe in different experiments was similar, with an average value of 6-8.
The roughness level of the contact surface between the sand layer and the cover plate had a significant impact on the geometry of the pipe. As the compactness of sand layer increased, the depth and width of the pipe decreased to a certain extent.>