## 3 Results
### Effectiveness of the Pressure Method
#### 3.1.3 Gravel Transport Mass  
This section is to measure the bedload transport mass and to examine whether the measured bedload mass using the GPVS increases when the bedload on the GPVS increases. To this end, a gate is installed at the end of the flume and can be adjusted during the experiment to ensure that the bedload on the GPVS gradually increases with time. Because the sediment is taken from the upper reach of the Yangtze River and has a wide grain size distribution, ranging from 1 cm to 15 cm, it is difficult to find a suitable threshold velocity or critical shield. Therefore, a simple method is adopted by starting the test with a low flow rate (200 L/s) and increasing it to 400 L/s to observe bedload transport. The observation of these two tests shows that, under these two flow discharges, little or no bedload is transported and the mass of moving gravel measured by GPVS fluctuates around 0, as shown in Figure 7. When the flow discharge reaches 600 L/s, gravel gradually accumulates  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Method** & **1** & **2** & **3** & **4** & **5** \\ \hline GPVS & 24.78 kg & 30.92 kg & 28.63 kg & 21.94 kg & 26.32 kg \\ Bedload trap & 25.98 kg & 32.05 kg & 29.87 kg & 22.63 kg & 28.74 kg \\ \hline \hline \end{tabular}
\end{table}
Table 1: Results of different methods of measuring gravel transport mass (600 L/S).  
Figure 5: Variation of the estimated pressure with gravel weight placed at various status: (**a**) flat; (**b**) diagonal tiling (10\({}^{\circ}\)); (**c**) opposite side tiling (10\({}^{\circ}\)); and (**d**) overturning.  
on the GPVS plate, causing the measured value of the GPVS to rise. Figure 8 shows that, for a flow discharge of 600 L/s, the measured gravel mass by the GPVS increases with time and then reaches a quasi-constant value, indicating that the mass of gravel entering the plate of the GPVS and exiting the plate of the GPVS in a short period of time is virtually the same. Because there is a large amount of gravel movement at only 600 L/s flow discharge, the following analysis and discussion focus on 600 L/s.  
Based on the above analysis, the calculated pressure using the recorded acoustic signals of the GPVS can be used to estimate the gravel transport mass. Defining T\({}_{i}\) and T\({}_{i+1}\) as the adjacent recording times, and the corresponding pressures as \(X_{i}\) and \(X_{i+1}\), the gravel mass passing the GPVS is then calculated as:  
\[M_{i}=\left|X_{i+1}-X_{i}\right|/g, \tag{1}\]  
where \(g\) is the gravity acceleration; and \(i\) is \(i\)th observation. The total mass of transported gravel will be:  
\[M=\sum_{1}^{n}M_{i}, \tag{2}\]  
where \(n\) is the total number of recordings.  
Figure 6: Effect of water depth on pressure recording.  
Figure 7: Photograph of bedload particles in motion.  
The mass of five groups of pebbles measured by the GPVS and a sand pit under the 600 L/s water flow condition is calculated using the above approach. Table 1 shows that the mass of gravel collected by the downstream bedload trap is slightly greater than that measured by the GPVS. The difference in the mass of moving bedload sediments estimated by the two approaches ranges from 3% to 8%. The main reason is perhaps because the trap is placed one meter downstream of the GPVS, and as such, there is a one-meter gravel flume bed between them. Under the condition of no sediment input from the upstream inlet, this one-meter gravel bed makes an additional contribution towards the gravel collected by the bedload trap downstream of the GPVS. Eventually, this leads to a slightly larger bedload mass measured by the trap than by the GPVS.  
In summary, the calculation of gravel transport using the GPVS can reflect the gravel transport mass for a period to some extent. This indicates that the developed GPVS has potential application in the field.