Using two-dimensional modeling to evaluate strategies of sediment reduction and evacuation for Nanshi river under Guishan dam operations

摘要

This paper presents a unique case study of Nanshi River in Taiwan which may be able to serve as the basis for decision making and river maintenance operations or as a reference for other cases. This study area is composed of regions from upstream alluvial channel to delta-front and the Guishan Dam reservoir, with a surplus of material supply to transport capacity in the whole region. Dredging is a standard method by which to deal with the accumulation of sediment within a watercourse; however, our understanding of hydrodynamics and sedimentation is limited by the complexity of the sedimentation process. In addition, most of the similar geomorphological studies did not pay attention to the upstream effects of reservoirs on alluvial channels. Similar examples are more often observed in estuaries and coastal zones. Our objective was to evaluate the morphological impact of river dredging implemented in conjunction with downstream gate operations by employing a 2D hydraulic sediment model, SRH-2D. With the assistance of the model simulations, we revealed that the deposition in the study area is under the interaction among three main mechanisms: headward deposition of the dam, channel width contraction in midstream, and progressive deposition of sediment from the confluence. Accordingly, this study suggested that dredging focuses on the creation of deep trenches in the midstream and upstream sections. It is also important to coordinate gate operations at Guishan Dam with peak flows in order to facilitate the transport of sediment to the reservoir for subsequent discharge via empty flushing. Regarding the effect of Guishan Dam removal, it would have a more pronounced effect on head cutting which leading to erosion in the reservoir and the downstream transport of substantial quantities of sediment, but its effect would be limited to a certain distance. However, dam removal is not considered a viable option considering the need for local water storage and the benefits of the dam in terms of tourism.