多方式特性法應用於非均勻質河床沖淤之研究

Abstract

本研究以李氏(1992)、陳氏(1994)及臧氏(1995)所發展之非均勻質多方式 特性法模式為基礎，加以改良並進行淤沖交替模擬之研究。在數值方法上 ，由特性波傳遞之方向，推求特徵值及特徵向量，以改進以往底床沖淤變 化後，水理條件驟變所造成質量不守恆之問題；並擴充模式適用於天然河 川之任意斷面形狀之情況；以及修正上下游邊界條件之處理方法，使模式 更趨於穩定。另納入不同之非均勻質沉滓個別粒徑輸砂量計算方法，以反 應遮蔽因子(hiding factor)與顯露因子(exposure factor)之效應。為檢 視模式之準確度與適用性，採用顏等(1987及1990)非均勻質試驗資料，進 行平衡、超載、減載及清水沖刷試驗之模擬，探討淤積及沖刷情況下，底 床高程及粒徑組成變化之情形，並加以比較分析，所得結果頗令人滿意。 This study, based on the coupled nonuniform-sediment multimode characteristics model developed by Li (1992), Chen (1994), and Tsang (1995), conducts simulations under the condition of alternating aggradation and degradation of the channel beds. The model used previously has a shortcoming of failing to meet the law of mass conservation due to the sudden change of hydraulic conditions. To avoid this shortcoming, the numerical method adopted in this study compute the eigen-values and eigen-vectors along the propagation of the characteristic waves. This model is also extended to consider the cases with irregular cross- sections in natural rivers. To enhance the stability of the model, a modification was made to handle the upstream and the downstream boundary conditions. Moreover, the computation of transporting nonuniform sediment of different sizes was done to reflect the effects of the hiding factor and the exposure factor.Experimental data based on the laboratory studies of Yen et al. (1987 & 1990) were used to test the accuracy and the applicability of the model. According to the experiments, the model was used to investigate the bed elevation change and the bed material gradation under a sequence of equilibrium, overloading, underloading and clean water degradation stages. An assessment then was made through the analysis and comparison between the experimental data and the simulation results, which show a satisfactory prediction of this model.