**7. Summary and Conclusions**

Many reservoir managemen<sup>t</sup> strategies and countermeasures have been implemented for reducing sedimentation effects. Dredging is a commonly used measure to remove mechanically deposited sediment. However, the disposal of dredged sediment can be costly because of engineering, permission, or placement site problems. Nevertheless, dredged sediment can be considered a beneficial resource that can be added to downstream rivers to compensate for dam-induced interruptions of sediment supply. The sediment replenishment method is an experimentally determined strategy in which sediment dredged or excavated from a reservoir can be used for riverbed replenishment to prevent downstream riverbed degradation, reduce coastline erosion, and execute estuary restoration for aquatic habitats. Nevertheless, high concentrations of fine replenished sediment may be scoured during reservoir flood discharges, which may deteriorate the water quality at the water intake in downstream river reaches.

This study selected the Shihmen Reservoir located in Northern Taiwan to investigate the feasibility of reservoir replenishment using fine sediment. Previous studies on sediment replenishment in field or laboratory experiments have mainly focused on coarse materials, with few studies providing examples for fine sediment. In this study, candidate replenishment areas were first analyzed using a validated 2D numerical model to simulate flow velocity fields and water depths. To simulate suspended sediment transport in the model, the 2D shallow water equations along with the advection–diffusion equation was solved numerically in the framework of the finite volume method. On the basis of the analysis of the critical scour velocity, we adopted a flow velocity of 1.5 m/s along with a sufficient water depth to evaluate the incipient motion of both very fine and coarse replenished sediment. Although the concept of critical scour velocity is used herein for illustrative purposes, it indeed provided the preliminary criterion to assess the most feasible replenishment area. By comparing the simulated velocity fields and water depths in the candidate replenishment areas through hydraulic assessment, we identified

the most feasible replenishment area, namely Zone 1, as the experimental domain (Figure 6) for the physical model.

For satisfying the similitude of model scaling, the physical model was developed to mimic the scouring process of replenished sediment in the downstream river reach of Shihmen dam. As mentioned, similarities in sediment transport mechanisms engender di fficulties in combining coarse and fine materials because of the di fference in sediment particle sizes. The dimensionless shear parameter in the Shields diagram was employed for scaling the incipient particle motion by using Equation (8) for coarse sediment in the filter structure. For determining the scouring similarity of the fine replenished sediment, the relationship between flow-critical velocity and sediment dry density was regressed through flume experiments by using Equation (10). Using di fferent water content levels (40% and 80% corresponding to dry densities of 1.2 and 0.84 <sup>t</sup>/m3, respectively) of the dredged fine sediment, we performed experiments in the physical model to investigate the incipient motion of replenished sediment with cohesiveness. On the basis of dynamic similarity during scouring, the sediment concentration scale ratio was equal to the dry density scale ratio presented in Equation (11). The time scale ratio of the scouring process was the same as that of the flow similarity derived from the Froude number. This indicates that the scouring time scale of flood flow is equal to that of sediment transport.

According to flood discharge data and frequency analysis data available for the Shihmen Reservoir, five inflow discharge values ranging from 500 to 6100 m<sup>3</sup>/s were used in the physical model tests as the upstream boundary conditions, which generated overbank flow during a flood in the replenishment area on the floodplain. The hydrographs obtained from the test cases that involved the water content of 40% (Figure 10) revealed that the variations in the sediment concentration apparently occurred in the first 1.5 h. After 1.5 h, the measured sediment concentrations for all test cases were lower than 6000 mg/L. This concentration value was determined to be suitable for the downstream water intake stipulated by the Bansin water treatment plant. For the test cases that involved the water content of 80% (Figure 12), the measured sediment concentrations after 1.5 h were lower than 3000 mg/<sup>L</sup> for all test cases. Thus, for all test cases, the entire scouring operation took less than 1.5 h in the replenishment area to a ffect the downstream water treatment plant.

In the scouring process, the scour ratio was defined as the cumulative amount of scoured volume to the total amount of replenished sediment. Through experiments, the scour ratio was analyzed using relevant factors, including flood discharge and duration and water content of the replenished sediment. The scour ratio presented a linear trend, which provided the basis for obtaining the regression equation for each given discharge value (Figures 11 and 13). Using the regression equation, we estimated the time required to scour the entire amount of the replenished sediment (i.e., scour ratio = 100%), as presented in Table 2. As the scour ratio increased, the inflow discharge increased, and the time required to scour the entire amount of the replenished sediment decreased.

According to the experimental results obtained for the study site, high concentrations of replenished sediment in the outflow could a ffect the suspended sediment concentrations at the water intake and exceed the concentration limit stipulated by the Bansin water treatment plant. The reduction ratio of sediment concentration was investigated by establishing the relationship between the suspended sediment concentration data obtained at the Yuanshan weir and those obtained at the afterbay weir near the replenishment area (Figure 14). Through the use of the 2D numerical model, the peak sediment concentration in each test case was provided as the input to obtain the simulated sediment concentration at the water intake of the Yuanshan weir. By comparing the regression equations, it was found that the sediment concentration reduction ratio ranged from 91% to 92%. The reduction ratio of the concentration of the suspended sediment scoured from the replenishment area to the concentration of the sediment at the Yuanshan weir was approximately 90%. Thus, the concentration of the outflow sediment produced at the replenishment area was limited to 6667 mg/L, thus matching the concentration limit of 6000 mg/<sup>L</sup> stipulated by the Bansin water treatment plant.

**Author Contributions:** F.-Z.L. developed the study methodology, conducted the experiments, and wrote the manuscript. J.-S.L. provided research directions and comments and revised the manuscript. W.-D.G. assisted in the numerical simulations and result analysis. T.S. was responsible for data collection regarding sediment replenishment and for providing practice suggestions.

**Funding:** This study was funded by the Ministry of Science and Technology, Taiwan, under gran<sup>t</sup> numbers MOST 107-2625-M-002-002, MOST 105-2221-E-002-063-MY3, and MOST 105-2625-M-002-025-MY2.

**Acknowledgments:** The authors acknowledge the Northern Region Water Resources Office, Water Resources Agency, and Ministry of Economic Affairs, Taiwan, for the valuable data provided for this study. The authors also appreciate the Hydrotech Research Institute of National Taiwan University for providing facilities and technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
