## 3 Hydraulic Prototype Observation and Measuring Point Arrangement
### Observation Method and Measuring Point Arrangement  
Flow state observation  
The flow pattern in the open channel was observed using a digital camera and a video recorder, with a focus on adverse water flow phenomena, especially the occurrence of transverse backflow in the open channel.  
For convenience, seven sections along the guide wall on the left side of the open channel were chosen as observation points: 0 + 060.0 m, 0 + 090.0 m, 0 + 130.0 m, 0 + 170.0 m, 0 + 215.0 m, 0 + 271.5 m, and 0 + 323.5 m, as shown in Figure 3. Total-station and laser rangefinder instruments were employed to measure the elevation of the free water surface during flood discharge. The final measurement data was taken as the average value from the two observations. The total station measurement device used was a ZPS-12R, manufactured by Hi-Target, with an angle accuracy of 2\" and a distance accuracy of 3 mm + 2D \(\times\) 10\({}^{-6}\) (D is the measured distance). The laser rangefinder was a Leica DISTO D2, with a measuring range of 100 m and a precision of \(\pm\)1.5 mm. However, not all laser rangefinders can measure the free water surface position, and some may yield unfavorable results. In addition, the free water surface fluctuates violently during water discharge, and the measurement time is asynchronous in different observation methods, which can lead to a variance in the measurement results of the free water surface position. Therefore, when the state of the discharge flow becomes relatively stable, the position of the highest water level could be observed 3-5 times according to the fluctuation of water waves. The average value is then taken as the highest water surface elevation in that particular flood discharge condition.  
During the initial construction period, no facilities were buried at the base of sluice No.5#\(\sim\)7# for observation tests. In this HPO test, the pulsating pressure sensors were only embedded into the continuous sill and reinforcement base slab during the technical reconstruction to avoid damage to the original concrete of the sluice dam body. To understand the change in pulsating pressure within the continuous sill, nine measuring points were set up at three sections (i.e., K-1, K-2, and K-3) perpendicular to the axis of the continuous sill on three different elevation levels (i.e., 983.5 m, 986.0 m, and 988.5 m). These measuring points were numbered as FFK1.1-FFK1.3, FFK2.1-FFK2.3, and FFK4.1-FFK4.3, as shown in Figure 4. To observe the change in pulsating pressure in the base slab, three measuring points were set on the reinforcement base slabs of B-4, B-5, and B-6 along the axis of the open channel, at the same elevation level of 983.5 m. Their positions along the guide wall were 0 + 148.80 m, 0 + 158.60 m, and 0 + 168.40 m, respectively, numbered as FFK2.4-FFK2.6, as shown in Figure 4.  
In this study, the pressure sensors and data collection system, XL3402, developed by \(\times\)XIELI Technologies Inc. (Wenzhou, China) were utilized to collect the pulsating pressure. It is essential to electrify the measuring system for 30 min before data collection to ensure the stability and reliability of the data collection system. The original value of all measuring points should be recorded at the beginning of the test. Then, the sluice gate should be opened slowly according to the opening regulation of the HPO test. Pressure data should not be collected until a stable state is achieved for each flood discharge condition, i.e., at least 5 min after the sluice gate opens to maintain stability. The data collection is made at a frequency of 200 Hz. Three data groups are collected for each working condition to obtain their average value. Furthermore, at the end of all flood discharge conditions, when the sluice gates are closed, and the water level in the open channel slowly recovers to its original state before the test, the final value of all measuring points should be re-recorded to calibrate the zero drift of the data collection system.