水庫空庫排砂操作時間點選定之研究

Abstract

水庫的淤砂問題在環境以及經濟發展上是個相當嚴重的問題，泥砂的淤積顯著使水庫的庫容以及水庫的效益減少，現行有許多方法可供水庫清淤，如上游產砂的控制、利用洪水事件排砂、水力排砂、機械清淤等方法，其中尤以水力排砂中的空庫排砂的排砂效率最高，空庫排砂利用水庫的入流量經過水庫底部的排砂孔道挾帶大量泥砂排出水庫，將水庫的水理狀態變為一般河川的水理狀態，因此其排砂量相當高，根據過去研究發現其排砂效率可達2400~5000%。 當進行空庫排砂，必須負擔未來的缺水風險；因為水庫的蓄水全部用來當作排砂用水，沒有水儲存下來供未來使用，當預期的洪水不符合預期，那麼對於水資源的運用面臨相當大的困難。因此空庫排砂的時間點選定就是個相當重要的課題，在未來的用水風險最小的狀況下進行空庫排砂。 本研究利用石門水庫當作研究案例建立石門水庫供水系統之數理模型，將一年分作36旬進行操作，藉由GAMS（General Algebraic Modeling System）最佳化軟體分配各用水標的各旬的配水量，接著在各旬進行空庫排砂模擬排砂過後對於未來用水端的影響，模擬時間長度以30年為一周期，比較各旬進行空庫排砂下的缺水指標，發現在5月第2旬進行空庫排砂對於供水端影響最小且排砂量並非最少，利用SAMS2007（Stochastic Analysis Modeling and Simulation）生成合成流量，模擬流量不確定性對於空庫排砂最佳操作時間的影響，發現最佳操作時間並不改變，而次佳的操作時間發生在8月第2旬，此時間點可當作第二選擇。並與機械清淤費用作比較，證明空庫排砂屬於較經濟的排砂手段。 本研究的操作屬於非常大膽的操作，但隨著水庫淤積情況加劇，空庫排砂有實行的空間，屆時時間點的選定就相當重要，本研究可當作參考依據。

Reservoir sedimentation is a serious problem with large environmental and economic implications. The sedimentation significantly decreases the reservoir capacity and reduces benefits. To maintain the reservoir capacity, several kind sediment management practices can be considered, including, reduction of sediment yield, sediment bypass routing, sediment flushing, and sediment removal. Among these practices, sediment flushing, especially empty flushing, is the most effective one which remove the reservoir sediment by increasing flow velocity and then transported through drainage outlet. Empty flushing involves the opening of bottom outlets to completely empty the reservoir and allow stream flow to scour sediment deposits. The sediment releasing efficiency could reach 2,400-5,500% for empty flushing. When performing the empty flush in a reservoir, a concern is the increasing risk of water shortage. Because the reservoir is empty, no extra water stored for future use. When the expected inflow is not coming, it will cause problem in water supply. As a result, when to perform empty flush is important. This decision involves the tradeoff between sediment drainage quantity/efficiency and water shortage risk. This paper investigates the issue of optimal time for reservoir empty flushing. A water resource management model is developed to simulate the increasing risk due to empty flush. A combined objective function is proposed by considering this problem. This conceptual framework is applied to Shihmen Reservoir as a case study. According to the knowledge learned from the analysis, this study will identify the decision making for optimal time for reservoir empty flushing.