**Scouring of Replenished Sediment through Reservoir Flood Discharge A** ff**ects Suspended Sediment Concentrations at Downstream River Water Intake**

#### **Fong-Zuo Lee 1, Jihn-Sung Lai 1,\*, Wen-Dar Guo 2 and Tetsuya Sumi 3**


Received: 5 August 2019; Accepted: 20 September 2019; Published: 25 September 2019

**Abstract:** Dredging is a commonly used sedimentation managemen<sup>t</sup> strategy to remove mechanically deposited sediment from reservoirs. However, dredged sediment disposal is costly. Dredged sediment can be considered a beneficial resource and used for riverbed replenishment to prevent downstream riverbed degradation and improve aquatic habitats. This study investigated the feasibility of using dredged deposits with cohesive sediment for replenishment at the Shihmen Reservoir. Using the criterion of critical scour velocity, we conducted hydraulic assessments and identified the feasible replenishment area as the experimental domain. A physical model was developed to mimic the scouring process in the replenishment area. By applying dynamic similarity for scouring fine replenished sediment, we derived the regression relationship between flow-critical velocity and sediment-dry density, and used it for model ratio scaling of the grain size, dry density, and concentration in the physical model. Scoured sediment concentrations were measured to study the scour ratio at various flood discharges. Experimental results indicated that the scour ratio was related to factors such as flood discharge, flood duration, and water content of the replenished sediment. The reduction ratio of the concentration of sediment scoured from the replenishment area to the concentration of sediment at the downstream water intake was approximately 90% in the present study.

**Keywords:** dredging; reservoir sedimentation; replenishment; cohesive sediment; scour
