應用異重流二維層平均數值模式分析曾文水庫出水工之出流泥砂濃度及排砂效率

Abstract

水庫防淤經營管理策略主要可分為集水區保育、水庫通砂減淤、回復水庫庫容，以及調適策略，其中水庫通砂減淤及回復水庫庫容則主要以水力排砂及機械清淤為主要工程技術。當水資源缺乏時，於水力排砂中，則以異重流排砂較較為常用；因此本研究透過水中浚渫方式，於曾文水庫防淤隧道入口端往庫區上游浚渫形成導流槽，且考量不同寬深的導流槽，擬集中及增加到達壩前異重流之渾水水體，進而達到增加防淤隧道排砂效率之目的。此外，本研究採用二維層平均數值模式(Two-dimensional layer-averaged model, 2DLAM)，作為模擬異重流在曾文水庫運移之工具，並透過五場歷史洪水事件檢定驗證入庫渾水到達壩前之泥砂濃度，同時引用比較不同出流泥砂濃度迴歸經驗公式，率定曾文水庫出水工之出流泥砂濃度係數，作為改進2DLAM模式出流泥砂濃度計算方法之參考。本研究根據2DLAM模式演算結果發現導流槽往上游延伸長度越長或越寬，則防淤隧道排砂效率越大；但水位變化及導流槽深度變化，則對於排砂效率影響不大；當入流量越大，由於防淤隧道設計出流量有限之因素，因此大部分流量與泥砂量將經由溢洪道排出，此時反而會造成防淤隧道排砂效率降低。

Reservoir desilting operation and management strategies can be classified into four parts: water and soil conservation, reservoir sedimentation reduction, reservoir capacity restoration, and adjustment strategies. Among them, the outflow discharges from the reservoir and the restoration of the reservoir capacity are primarily based on hydraulic desiltation and mechanical dredging. The application of turbidity current venting in hydraulic desilation is commonly-used in the field cases, especially the reservoir water is valuable. In this study, the venting efficiency of the desilting tunnel has been investigated through various design guiding channels in the Zengwen Reservoir. In the guiding channel dredged in different sizes in front of the desilting tunnel, the turbidity current may travel along the guiding channel and reaches the dam. This study uses the numerical model, 2DLAM, as a tool to simulate the migration of turbidity current in the Zengwen Reservoir. Based on the data from five historical typhoon events, outflow sediment concentrations are simulated and compared with the measured data obtained from the physical model as well as the field. In addition, several empirical formulas have been quoted to estimate the outflow sediment concentration at various outlets. In the Zengwen Reservoir, the simulated sediment concentration at the intake of each outlet has been used as a reference for estimation of the outflow sediment concentration. According to the simulated results, the length and width of the guiding channel can affect the venting efficiency of the desilting tunnel. However, the changes in the reservoir water level and the depth of the guiding channel have little effects on venting efficiency. When inflow discharges increase and larger than that of the outflow design discharge, part of the outflow discharge will pass through the spillway and the reduction of the venting efficiency at desilting tunnel of the tunnel can be observed.