## 2 Experimental Setup and Methods
### Experimental Design and Methods  
Prior to initiating the experiment, a 10 cm thick layer of experiment sand was placed in the experiment section of the flume. To preserve the bedforms, the tailgate was initially closed, the flow rate at the inlet was set to a low value, and reverse flow from downstream was utilized. Once the water depth in the flume reached approximately 5 cm, the speed of the flume pump was adjusted to achieve the desired discharge for the experiment. Subsequently, the tailgate was adjusted to equalize the flow levels upstream and downstream of the flume, and the formal experiment commenced following the establishment of uniform flow. Each experiment lasted between 14 and 20 h, with the exact duration depending primarily on the stability of the bedforms. The height of the bedforms was monitored and recorded every half hour. Once the bedforms stabilized, the experiment concluded. At the end of each experiment, the tailgate was closed, and the remaining water in the flume was gradually drained to prevent damage to the bedforms. After draining all the water, the bedforms were measured. The conventional measurement methods for dune bedforms provide only basic data, such as dune length and height, which limit the depth of spatial analysis that can be conducted on the dunes. In contrast, a 3D laser scanner, specifically the Faro S350, offers high accuracy, speed, and a large volume of data. Practical experience has demonstrated that when the scanner head is oriented upwards, it can be obscured by the surrounding bedforms, creating a blind zone that hinders measurements at the flume bottom. Consequently, in this study, the scanner head was oriented downward, as illustrated in Figure 3.  
In this study, six white target balls were uniformly distributed along the inner side of the experiment section in the flume. Four measurement positions were established to assess the bedforms across the entire experiment section. The topographic data from these positions were integrated using the target balls to create a complete point cloud representation of the bedforms of the experiment. Data integration was performed using Faro SCENE software (v. 2024.0), resulting in a point cloud containing over 100 million points. To enhance the data processing efficiency, while maintaining measurement accuracy, we employed a spatial thresholding method, wherein data points spaced less than 1 mm apart were excluded. The experimental setup included three slope gradients: 1%, 2%, and 4%. For each slope, seven trials were conducted, i.e., twenty-one experiments in total. The parameters for each experiment are detailed in Table 1. The minimum flow rate at which the bedload movement and partial bedforms formation started was determined experimentally. Initially, the bedforms in each experiment were flat. The flow rate plays a key role in the formation  
Figure 3: Illustration of scanning range of scanner and obstruction by bedforms. (**a**) Bedform measurement in flume, upside down scanner, and target spheres, and (**b**) illustration of scanning range of scanner and obstruction by dunes. of bedforms. Specifically, when the open channel flow moves at a certain rate, the bedforms are subjected to shear stress from the water flow, causing sediment particles to move in a regular pattern, eventually forming bedforms with distinct characteristics. Ripples are the initial stage of bedform development, and the size of the ripples is generally small. As the flow rate increases, the ripples continue to develop and gradually increase in size, forming dunes. There is no clear distinction between ripples and dunes, and both belong to the shape of bedforms. In this study, we focused on the shape characteristics of bedforms, and therefore made no distinction between the ripples and the dunes. The water depth \(\mathrm{h}\) was read using a steel ruler mounted on the outer wall of the glass flume. Twelve steel rulers were installed along the glass flume for the easy reading of water depth.