**2. Description of Study Site**

The Shihmen Reservoir was built in 1964 in northern Taiwan (Figure 1). Shihmen dam is a 133.1-m-high embankment dam and is equipped with spillways, flood diversion tunnels, a bottom outlet, a power plant inlet, a sediment-sluicing tunnel, and an irrigation inlet. The design capacity of the three spillways is 11,400 m<sup>3</sup>/s, that of the flood diversion tunnels is 2400 m<sup>3</sup>/s, that of the bottom outlet is 34 m<sup>3</sup>/s, that of the power plant inlet is 56 m<sup>3</sup>/s, that of the sediment-sluicing tunnel is 300 m<sup>3</sup>/s, and that of the irrigation inlet is 18.4 m<sup>3</sup>/s. At a normal water level of EL. 245 m, the reservoir pool is approximately 16.5 km in length and forms a water surface area of 8 km2. The initial storage capacity is 30.9 × 10<sup>6</sup> m3. The reservoir is multifunctional and is used for water supply, irrigation, electric power generation, and flood mitigation; it is also a tourist attraction. In particular, the reservoir supplies water to 2.1 million people in Taoyuan City and New Taipei City for daily use.

**Figure 1.** Location map and geometry of Shihmen Reservoir.

According to the data of a survey conducted in 2018, the reservoir's storage capacity was estimated to be 65.72% of its initial capacity. The average annual deposited sediment volume from 1963 to 2018 was approximately 1.93 × 10<sup>6</sup> m<sup>3</sup>/year. Except for hydraulic desilting measures, mechanical excavation constitutes the strategies adopted for maintaining the Shihmen Reservoir. The mechanical excavation includes the dredging and dry excavation. The dredging is mainly performed under the water surface, and the main purpose of dry excavation is to remove exposed sediment. The dry excavation site in this study is located at the Lofu gauge station, and the averaged exaction volume between 2009 and 2018 was approximately 0.47 × 10<sup>6</sup> m<sup>3</sup>/year. The dredging site is near the dam, and the dredged sediment volume between 2009 and 2018 was nearly 0.36 × 10<sup>6</sup> m<sup>3</sup>/year. Fine sediment deposited in front of Shihmen Dam is typically removed through a pipeline by a hydraulic dredging system that applies siphon dredging; before executing the dredging process, the system calculates the head di fference between the reservoir water surface and the pipeline outlet. However, finding a disposal location for the dredged sediment is a problem. Therefore, one of the implemented reservoir-desilting strategies entails conducting sediment replenishment by placing the reservoir deposits on the downstream floodplain, which may be used in the Shihmen Reservoir.

Referring to the particle size distribution presented in Figure 2, the dredged sediment near the dam site is almost entirely composed of silt and clay; thus, it is classified as a cohesive material. The sediment excavated at the Lofu gauge station is coarse sand. In this study, sediment obtained from both dredging and dry excavation was used for sediment replenishment investigation on the downstream floodplain. According to the regulation of the water pollution control action [19], the replenished material should be placed at the floorplan and confined by a filter structure that serves as a permeable barrier to avoid the replenished fine sediment from polluting the water in nearby main channels during low-flow periods.

**Figure 2.** Sediment grain size distribution sampled in the field and used in the physical model.

The available flood discharges from the reservoir and the flood duration are key factors for implementing sediment replenishment. Based on historical records from 1911 to 2017, three typhoons hit Taiwan each year on average. According to a frequency analysis, the 2-, 10-, and 20-year return period floods have peak discharges of 1700, 3500, and 6100 m<sup>3</sup>/s, respectively [20].

## **3. Hydraulic Assessment of Sediment Replenishment**

If the flow intensity exceeds a specific value and sediment particles begin to move, the flow condition that corresponds to a critical value is called incipience. The main parameters of the incipience condition are usually related to the flow velocity and water depth. Therefore, in this study, the flow velocity fields and water depths were simulated using the 2D numerical model developed by Guo et al. (2011) [18]. The advection–diffusion equation is commonly used to describe the suspended sediment transport phenomenon. For determining the process of suspended sediment transport using the developed numerical model, the 2D shallow water equations coupled with the advection–diffusion equation served as the governing equations. Flow velocity fields and water depths in the candidate replenishment areas were compared, as described in this section. Subsequently, the feasible area for the physical model test was identified and selected.
