二維水深平均模式應用於河川變遷過程之研究

Abstract

非正交曲線座標顯式有限解析法動床模式之特點為:(1)引介貼壁座標系統，以描述天然河道不規則變化的邊界；(2)引介有限解析法，此一方法有別於習見之有限差分法或有限元素法，該法原應用在流力問題上且成果甚佳；(3)輸砂通量考量非均勻沉滓之懸浮載及河床載運移，以及兩者間交換之機制。 為使此水平二維動床模式功能更趨完備，以符合台灣河川治理規劃之需，本研究延續模式既有架構，發展具模擬河道岸壁側向變遷之功能，以便較正確掌握河道或渠道在水流作用下之垂向沖淤與側向變遷之行為。目前已建制完畢之側向變遷機制計有非凝聚性沈滓之岸壁沖刷機制及凝聚性沈滓之圓弧及平面崩坍機制。 經由測試案例模擬發現上游入流量對於非凝聚性沈滓之岸壁沖刷影響較大；岸壁土壤臨界剪應力則是對於凝聚性沈滓之圓弧及平面崩坍機制影響較大。現地模擬案例常遭遇乾濕互見之渠槽底床，目前能針對河道幾何變化未較複雜者進行模擬，未來可朝此問題再做深入探討，以增加模式之實用性。

The special features of the 2-D explicit finite analytic depth-averaged mobile bed numerical model derived on non-orthogonal curvilinear coordinate system include: (1) the development of a generalized body-fitted coordinate system to describe the irregular boundary of natural river; (2) the adoption of the finite-analytic method, which is different from the common finite-difference or finite-element method, and is originally proposed for solving the fluid-mechanics problems with great success; (3) non-uniform sediment transport in form of suspended and bed load, with the consideration of exchange mechanic between them. In order to expand the model applicability, the study established lateral bank migration mechanics in the previous framework. The established mechanics would clarify the later mobile migration behavior. The model had developed bank erosion mechanism of non-cohesive sediment and rotational and planar failure mechanisms of cohesive sediment. Through the simulation of testing cases, it had shown that the upstream water discharge played an important role in the bank erosion mechanism of non-cohesive sediment, while the soil critical shear stress had most effect in the rotational and planar failure mechanisms of cohesive sediment. In the simulation of practical cases, it had suffered enormous challenge of alternative dry and wet bed problem. At the present time, the model could simulate the cases of less complex channel morphology. It is suggested to pay more attentions to this problem to enhance the model practicability.