**1. Introduction**

For reservoirs worldwide, storage preservation through sustainable sediment managemen<sup>t</sup> operations is a critical issue due to the severe problem of reservoir sedimentation. The available total storage capacity of global reservoirs has been decreasing in recent years because of the lack of e ffective sediment managemen<sup>t</sup> practices [1]. Sediment trapping in a reservoir or dam interferes with the course of sediment transport through a river system; such interference exerts a considerable influence on the sustainability of future water supplies from the reservoir, engenders channel erosion, and negatively a ffects the downstream ecosystem. Morphological e ffects on downstream river channels include riverbed incision, riverbank instability, damage to embankments and levees, and channel width variation [2–5].

Studies have investigated many managemen<sup>t</sup> strategies and e fficient countermeasures for reducing the influence of sedimentation [3,6–8]. Dredging is a commonly used measure to remove mechanically deposited sediment from reservoirs. The disposal of dredged sediment is costly due to the involved engineering, permission, and placement site requirements. However, dredged sediment can be

considered as a resource that provides effective environmental benefits and may be added to the downstream river of a dam to compensate for the lack of sediment supply. Accordingly, an approach called sediment replenishment (sediment augmentation) can be adopted for the effective replenishment due to dam interruption. [3,9,10]. Since the 1980s, sediment has been added into rivers to replenish the downstream river reaches of dams. The sediment replenishment method has been implemented to prevent downstream riverbed degradation and to improve the suitable living habitat of aquatic animals. Field investigations and laboratory experiments have been conducted for rivers in America, Europe, Japan, and Taiwan to improve knowledge about the transport processes during sediment replenishment [11–13].

In field practice, the sediment replenishment method can be planned and practiced in specific hydrological and geographic areas. Okano et al. (2004) [14] investigated reservoir sedimentation managemen<sup>t</sup> by depositing coarse sediment (mainly sand and gravel sizes) with a volume of 0.3–25 × 10<sup>3</sup> m<sup>3</sup> for replenishing downstream areas. They summarized sediment replenishment projects conducted at eight dams in Japan from 1999 to 2003. Moreover, researchers have investigated the influences of sediment replenishment conducted using appropriate grain sizes on the downstream river of Yahagi dam and the upper basin of Kizu river [15,16]. Researchers also analyzed the interaction between the relative flow field and morphological evolution during field experiments of sediment replenishment [13]. The conceptual idea of the replenishment method is to place sediment in the downstream floodplain before the arrival of floods. If the replenishment areas experience floods, the replenished sediment can be scoured and transported further downstream. As mentioned, field tests in Japan have focused on the deposition of coarse materials in reservoirs to subsequently replenish downstream rivers of the dams in order to create a suitable environment for aquatic organisms. The applied replenishment method requires a sufficient amount of discharge in order to scour replenished sediment; replenished sediment is thus always placed on floodplains before the discharge of water from a reservoir during the wet season. Sediment deposited in a reservoir can be periodically dredged or excavated and then temporarily deposited on the floodplain for transportation to the downstream river of a dam. Moreover, the replenished volume is based on the sediment transport capacity of the channel and environmental conditions. According to reports in Japan regarding the replenished volume, up to 10% of the annual deposition volume in a reservoir could be successfully executed [6,7]. Ock et al. (2013) [17] reviewed methods in the context of sediment replenishment and compared implementation activities undertaken in the Nunome river of Japan and Trinity river of California, USA. According to sediment placement or injection types, sediment replenishment methods were implemented with mechanical rehabilitation for re-creating gravel or sand bar features through fluvial processes. This comparative study provided useful information to adopt proper methods corresponding to river specific high-flow and sediment regimes. In 2009, a field test for coarse sediment replenishment was conducted at the downstream floodplain of Shihkang dam across the Dachia river in Central Taiwan [12]. The total replenished volume of the coarse sediment excavated (median grain size: 80 mm) from the Shigang Reservoir was approximately 50 × 10<sup>3</sup> m3. The field experiment revealed that the replenished sediment was entirely scoured further downstream in a typhoon flood event with a peak discharge of 5400 m<sup>3</sup>/s in 2009.

On the basis of the aforementioned studies, field and laboratory experiments for sediment replenishment have mainly focused on coarse materials, with few fine sediment examples being provided. In fact, fine replenished sediment dredged from reservoirs is valuable for coastline or estuary restoration and for the respiration of aquatic organisms. However, if a high concentration of fine replenished sediment is scoured by a reservoir flood discharge, the water quality of water treatment plants connected to the relevant reservoir may deteriorate.

In the present study, the Shihmen Reservoir was used as the study site to investigate the feasibility of fine-sediment replenishment. First, possible areas for replenishment were analyzed through a numerical simulation conducted using a two-dimensional (2D) numerical model [18]. On the basis of the framework of the finite volume method, the 2D numerical model was used to solve the shallow water equations coupled with the advection–di ffusion equation in order to simulate the suspended sediment transport phenomenon in the river. A physical model was built to mimic the scouring process of replenished sediment in the downstream river reach of Shihmen dam. Experiments involving di fferent water content levels of dredged fine sediment were conducted in the physical model to investigate the incipient motion of the replenished sediment with cohesiveness. Through the experiments, the scour ratio was analyzed using relevant factors such as flood discharge, flood duration, and water content of the replenished sediment. The e ffect of the scouring of fine sediment from the replenishment area on the downstream water intake was also investigated.
