**5. Experimental Results and Analysis**

Flood discharges from a reservoir and flood duration are key factors for investigating scouring processes in the sediment replenishment area. On the basis of data regarding flood discharges from the Shihmen Reservoir in the past 10 years, we observed that the flood discharges were higher than 500 and 1000 m<sup>3</sup>/s for 38 and 22 h, respectively, in all flood events. Apart from 2-, 10-, and 20-year return period flood discharges, we considered lower discharges of 500 and 1000 m<sup>3</sup>/s as the inflow discharges for practical purposes in the physical model tests. These discharges generated overbank flow to flood over the replenishment area on the floodplain.

Figure 10 illustrates the outflow sediment concentration measured against time in test cases involving various inflow discharges (from 500 to 6100 m<sup>3</sup>/s) at the downstream end (Section 85) of the physical model. In this figure, the hydrographs in the first 1.5-h period are enlarged to enhance the visualization of changes in the measured sediment concentration. In the test case that involved a *Q* value of 6100 m<sup>3</sup>/s, the peak sediment concentration (12,000 mg/L) was observed at 0.07 h because a higher discharge could generate a higher flow velocity to scour most of the replenished sediment. For comparison, in the test case that involved a *Q* value of 500 m<sup>3</sup>/s, the peak sediment concentration (24,000 mg/L) occurred at 0.7 h. Although a higher discharge could generate a higher flow velocity to scour the replenished sediment, the concentration of suspended sediments may not have increased further due to the larger water volume. The peak sediment concentration for the test case that involved the *Q* value of 6100 m<sup>3</sup>/s was lower than that for the test case that involved the *Q* value of 500 m<sup>3</sup>/s. However, the peak sediment discharge (product of flow discharge and sediment concentration) should be considerably increased due to higher flow discharge. Moreover, the hydrographs revealed that major variations obviously occurred in the first 1.5 h. After 1.5 h, the measured sediment concentrations of all the test cases were lower than 6000 mg/L. This concentration can be managed by the Bansin water treatment plant. Consequently, only the first 1.5 h would be required for complete scouring in the replenishment area.

The volume of scoured sediment could be obtained from the sediment concentration measured at the downstream boundary in the physical model. The scour ratio could be defined as the ratio of the cumulative volume of the scoured sediment to the total amount of replenished sediment. As plotted in Figure 11 for each given discharge, except for 6100 m<sup>3</sup>/s, the variations in the scour ratio exhibited a linear trend approximately after 3 h. This indicates that the replenished sediment was scoured at a constant rate with time. According to the observed linear trend, the regression equation could be derived (Figure 11), and the corresponding R-squared value revealed good agreement. By using the regression equation for each given discharge, we estimated the time required for complete scouring (i.e., 100% scour ratio) of replenished sediment (with water content of 40%); the estimation results are listed in Table 2. The results indicated that the scour ratio increased with the discharge; complete scouring was achieved within 8 h. Moreover, when the *Q* value was 6100 m<sup>3</sup>/s, the entire amount of the replenished sediment was scoured (i.e., scour ratio = 100%) within 3 h. These results demonstrate that the scour ratio depends on the flood discharge and scour duration.

**Figure 10.** Hydrographs of concentrations of sediment with a water content of 40% at various reservoir flood discharges.

**Figure 11.** Trend of the scour ratio against time for sediment with the water content of 40% at various reservoir flood discharges.

**Table 2.** Scour ratio, experimental duration, and estimated duration for achieving a scour ratio of 100% for each given discharge at different water contents.


Figure 12 illustrates hydrographs of the measured sediment concentration against time for the test cases that involved replenished sediment with a water content level of 80%. Because of the lower erodibility of the replenished sediment, the experimental duration was too short to execute the physical model for a *Q* value of 6100 m<sup>3</sup>/s. Therefore, the test cases were conducted under only discharge values ranging from 500 to 3500 m<sup>3</sup>/s. In the test case that involved a *Q* value of 3500 m<sup>3</sup>/s, the peak sediment concentration (14,500 mg/L) was observed at 0.07 h. For comparison, in the test case that involved a *Q* value of 500 m<sup>3</sup>/s, the peak sediment concentration (42,000 mg/L) was observed at 0.7 h. Similarly, the peak sediment concentration observed in the test case that involved the *Q* value of 3500 m<sup>3</sup>/s was lower than that observed in the test case that involved the *Q* value of 500 m<sup>3</sup>/s; however, the peak sediment discharge in the test case of 3500 m<sup>3</sup>/s was higher. Furthermore, as revealed in the hydrographs, major variations obviously occurred in the first 1.5 h. After 1.5 h, the measured sediment concentrations for all test cases were lower than 3000 mg/L.

**Figure 12.** Hydrographs of the concentrations of sediment with a water content of 80% at various reservoir flood discharges.

As plotted in Figure 13, the variations in the scour ratio after approximately 2 h presented a linear trend, indicating the steady rate of the scour ratio at each given discharge with time. Accordingly, the regression equation at each given discharge could be obtained, and the corresponding R-squared values demonstrated good agreemen<sup>t</sup> with the measured data. By applying the regression equation to each given discharge value, we estimated the time required for complete scouring of the replenished sediment (with water content of 80%); the estimation results are presented in Table 2. Except for the test case that involved the *Q* value of 3500 m<sup>3</sup>/s, the experiments were conducted within 8 h, and the scour ratio increased with the discharge. When *Q* was 3500 m<sup>3</sup>/s, 4.25 h was required to scour the entire amount of the replenished sediment (i.e., scour ratio = 100%). The experimental results reveal that a higher water content was associated with a higher scour ratio. The estimated time required to achieve a scour ratio of 100% was also longer when the water content was lower. On the basis of these experimental results, we can conclude that the scour ratio depends on the flood discharge, flood duration, and water content of replenished sediment.

**Figure 13.** Trends of scour ratio against time for sediment with a water content of 80% at various reservoir flood discharges.

#### **6. Influence of Scouring on Downstream Water Intake**

Scouring replenished sediment may influence the water quality at the downstream water intake section. Using the derived experimental data regarding outflow sediment concentrations, we investigated the influence of scouring on downstream water intake by conducting a 2D numerical simulation of suspended sediment transport in the downstream river. The Yuanshan weir is located downstream the Shihmen Reservoir, which is approximately 16.5 km away from the replenishment area. Water is typically withdrawn from the water intake section at the Yuanshan weir to the Bansin water treatment plant, which has a concentration limit of 6000 mg/L. According to the experimental results, the outflow sediment concentrations measured in all the test cases when the water content was 40% were rather high during the first 1.5 h. This may affect the suspended sediment concentration at the water intake and exceed the concentration limit stipulated by the Bansin water treatment plant. Considering the facilities of the treatment plant, the water turbidity can be treated if the sediment concentration decreases to 6000 mg/<sup>L</sup> or lower [34].

To understand the effect of the scouring of replenished sediment on the downstream water intake, the reduction ratio of sediment concentration should be investigated. By using field measurement data obtained from 2007 to 2016, we derived the relationship between the suspended sediment concentrations at the Yuanshan weir and afterbay weir (nearby replenishment area), and the results are plotted in Figure 14. For modelling suspended sediment transport along the downstream river reach, we applied the 2D numerical model to solve the shallow water equations coupled with the advection–diffusion equation. According to the peak sediment concentration observed for each test case, we simulated the transport of suspended sediment and dissipation of replenished sediment under steady-flow conditions within the river reach, as described in Section 3.2. Figure 14 shows a plot of the simulated sediment concentration in the water intake section of the Yuanshan weir against the concentration in the replenishment area; the figure also shows a plot of the regressed relationship between the field data measured during flood discharge at the Yuanshan weir and the afterbay weir (around the replenishment area). By comparing the regression equations, we observed the simulated results to be close to the measured results. Clearly, the sediment concentration reduction ratio ranged from 91% to 92%. Thus, the reduction ratio of the concentration of suspended sediment scoured from the replenishment area to the concentration of suspended sediment at the Yuanshan weir was

approximately 90%. According to the suspended sediment concentration limit (6000 mg/L) imposed at the Bansin water treatment plant, the outflow sediment concentration from the replenishment area was limited to 6667 mg/L.

**Figure 14.** Relationship between sediment concentration in replenishment area and that at the downstream water intake at Yuanshan weir.
