# River Surface Space-Time Image Velocimetry Based on Dual-Channel Residual Network
## 4 Experiments and Discussions
### Vertical Average Velocity Comparison Experiment
#### 4.4.2 Experimental Results  
The comparison period is from 8:25 to 9:25 on 21 August 2020. As shown in Figure 23, the water level of the river is maintained at 998 m, which belongs to high water conditions. The corresponding velocity coefficient is 0.89. During this period, the flow tracer and surface velocity are relatively stable. The experiment uses a 30 s video taken at 9:00 to measure nine testing lines from 55 m to 175 m, including normal (2-5), vortex (1, 9), and obstacle (6-9) scenarios. The corresponding STIs are shown in Figure 24.  
The obtained MOT and vertical average velocity results are listed in Tables 10 and 11, respectively. Table 10 reveals that the AE is minimized under the normal scenario, whereas higher AE values are observed in both obstacle and vortex scenarios. Analysis of each STI reveals that while testing line 1 experiences vortex interference, its effective texture features  
Figure 23: Section diagram. Figure 24: STIs of different testing lines. (1–9 correspond to the starting distances of 55 m, 65 m, 90 m, 105 m, 120 m, 135 m, 155 m, 165 m, 175 m, respectively.) remain distinct. Similarly, testing lines 5-6, despite partial occlusion by the current meter and black cable, maintain clear and consistent tracer-generated textures. Consequently, both the FFT and DCResNet methods demonstrate accurate MOT detection for testing lines 1-6. However, testing lines 7-9 present greater challenges due to concurrent cable obstruction and far-bank vortex effects, leading to texture blurring, inconsistent flow directionality, and multi-oriented interference patterns. Furthermore, testing line 9 suffers from reduced resolution owing to its far-field position. These factors cause substantial FFT-derived deviations (3.29-33.82\({}^{\circ}\), maximized at line 9), whereas DCResNet maintains better performance through robust texture feature extraction. By ensuring a reliable MOT estimation, DCResNet consistently limits the AE below 2\({}^{\circ}\), effectively preventing the gross errors observed in the FFT results. Figure 25 presents the vertical average velocity distribution, revealing distinct performance characteristics between the two methods. In the 55-135 m range, both FFT and DCResNet demonstrate good agreement with the current meter measurements, maintaining an RE below 6%. However, significant differences emerge in the 155-175 m section, where vortex and obstacle interference become more pronounced. In this region, the FFT results exhibit abnormal fluctuations, with an exceptionally high RE of 256.36%, demonstrating the method's failure under these challenging conditions. In contrast, DCResNet successfully mitigates gross errors, maintaining a consistent velocity distribution with current meter measurements across the entire 55-175 m range. The method's MRE of 4.08% further confirms its reliability for practical velocity measurement applications, which is better than the FFT method. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Number**} & **Starting** & **FFT** & **DCResNet** & **Label Value** & \multicolumn{2}{c}{**AE (\({}^{\circ}\))**} \\ \cline{3-6}  & **Distance (m)** & (\({}^{\circ}\)) & (\({}^{\circ}\)) & (\({}^{\circ}\)) & **FFT** & **DCResNet** \\ \hline
1 & 55 & 82.71 & 82.91 & 82.60 & 0.11 & 0.31 \\
2 & 65 & 81.66 & 81.72 & 81.54 & 0.12 & 0.18 \\
3 & 90 & 80.61 & 80.83 & 80.69 & 0.08 & 0.14 \\
4 & 105 & 79.32 & 79.53 & 79.40 & 0.08 & 0.13 \\
5 & 120 & 78.18 & 78.52 & 78.38 & 0.20 & 0.14 \\
6 & 135 & 77.55 & 77.32 & 77.41 & 0.14 & 0.09 \\
7 & 155 & 73.10 & 70.21 & 69.81 & 3.29 & 0.40 \\
8 & 165 & 70.52 & 58.45 & 56.80 & 13.72 & 1.65 \\
9 & 175 & 72.02 & 37.69 & 38.20 & 33.82 & 0.51 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: MOT detection results.