Water quality simulation of an empty flushing reservoir using a two-dimensional variable grid model

• Main Authors: Fei-YuChiang, 蔣斐羽
• Other Authors: Chih-Hua Chang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/j56k25

碩士 === 國立成功大學 === 環境工程學系 === 102 === Agongdian Reservoir is the only reservoir in Taiwan that regularly uses an empty flushing strategy for desiltation. The empty flushing method involves the use of reservoir inflow to wash away sediment from the reservoir bottom by transferring the sediment further downstream. Main sources of Agongdian Reservoir water inflow include: cross-watershed diversion, rainfall runoff, direct channel precipitation, and water coming from the ZuoShui and WangLai Rivers within the reservoir’s catchment. The objective of this study is to build up a suitable water quality model for an empty flushing reservoir. The methodology for this study was therefore split into three stages. First, the modeling strategies in the Agongdian Reservoir were divided into two periods alternating between the storage period and the empty flushing period. Second, a variable grid was established for each period of water quality modeling. Third, the assimilative capacity of the reservoir to receive pollutants at different periods were then compared and analyzed. The results of this study show: (1) Using acceptable water levels (i.e. RMSE〈 0.539) and water temperatures (i.e. AME 〈 1.678) is a good indicator that simulated water quality results in this study are robust. (2) Based on the water quality model, rainfall is the major source of inflow pollution during periods of empty flushing. In addition, during the water storage period, cross-watershed diversion is another important factor to consider. (3) The empty flushing period has a relatively lower carrying capacity compared to the water storage period, where sediment has time to settle on the bottom. The lower carrying capacity is due to multiple factors including: sediment remaining suspended in the water column, re-suspension of sediment settled on the reservoir bottom, which thereby causes release of phosphate and nitrogen into the water body. Based on these findings in the Agongdian Reservoir, this study suggests that because of lower carrying capacity, improvement of water quality during empty flushing period is an unnecessary additional cost. So, unlike other reservoirs in Taiwan, eutrophication analysis for the Agongdian Reservoir should be considered only during the water storage period.