Efficiency of the Drawdown Flushing and Partition Desilting of a Reservoir in Taiwan

Abstract

Sedimentation limits the benefits of storage reservoirs, especially in areas with higher sediment yields, such as Agongdian Reservoir in southern Taiwan. Although drawdown flushing is a known strategy that releases large amounts of fine sediment into a downstream channel, there is limited information on the long-term monitoring and multiple metrics being used to evaluate flushing efficiency. The objectives of this study were three-fold: (1) to continue collecting valuable long-term observed data, since Agongdian Reservoir is one of the few reservoirs currently conducting sediment flushing operations; (2) to evaluate and identify the hydrological parameters that are highly related to the flushing efficiency; (3) to execute numerical simulations of different reservoir flushing scenarios at multiple water levels to discuss potential strategies to improve the flushing efficiency. The findings of this study revealed that long-term monitoring data was valuable for identifying factors highly related to the flushing efficiency, which included the initial water level; average water level; average velocity. Based on simulations, compartmentalizing the reservoir is a proposed strategy that has demonstrated high levels of improvement in terms of the flushing efficiency, depending on particular scenarios involving partition desilting, empty flushing, or a combination of both. Recommendations to increase the flushing efficiency include lowering the initial water level, creating a narrower gorge-like geometry by partitioning, and further considering to modify the operation rules.

Application of artificial countermeasures to enhance desilting efficiency in a reservoir under normal and extreme events

“…2D models hold significant value for reservoir and water managers due to their capacity to account for intricate topographies and flow characteristics. The Sedimentation and River Hydraulics (SRH-2D) model, a numerical tool adept at portraying twodimensional hydraulic and sediment transport phenomena, has proven its utility in this domain (Huang et al 2019;Wang et al 2020). The model's capabilities extend to the simulation of various flow conditions, encompassing steady, quasisteady, and unsteady flows.…”

“…Examples include solid obstacles and permeable obstacles at the reservoir bottom (De Cesare et al 2006;Oehy & Schleiss 2007;Baghalian & Ghodsian 2020), aiming to enhance the desiltation effect of the reservoir. However, the flushing channel, when left empty, demonstrates relatively better desilting efficiency (Chen & Tsai 2017;Wang et al 2020;WRA 2020a). The application of the main channel characteristic concept to the flushing channel is also employed during highwater-level conditions (Chen 2022).…”

“…The application of the main channel characteristic concept to the flushing channel is also employed during highwater-level conditions (Chen 2022). Nevertheless, the location and configuration of the flushing channel change with the hydrological and hydraulic characteristics of inflow and outflow (Lai & Shen 1996;Wang et al 2020). To mitigate uncertainty and facilitate maintenance, this study introduces an innovative concept.…”

“…The 2D model research related to the establishment of a desilting channel at the bottom of the Zengwen Reservoir including bottom dredged channels was adapted to simulate the sediment concentration and desilting efficiency (Chen 2020). Wang et al (2020) proposed a strategy through 2D model simulations that involves compartmentalizing the reservoir. This strategy has shown significant enhancements in terms of flushing efficiency.…”

“…The degree of improvement varies based on specific scenarios, encompassing partition desilting, empty flushing, or a hybrid approach that combines both methods. In addition, the studies of Chen and Tsai (2017) and Wang et al (2020) indicate that creating a narrower gorge-like geometry is helpful to increase desilting efficiency. A 2D numerical model can also be used to simulate the weir area to explore the effect of different types of hydraulic structures on flood control and deposition reduction (Lee et al 2021).…”