以數值模擬探討大甲溪卵礫石辮狀河川之河相演變機制

臺灣溪流於中下游常呈現沙洲滿佈及流路分歧的辮狀型態，因為辮狀河床之深槽擺盪及沙洲遷移頻繁，使其河床演變不易掌握。本文以Nays2DH二維水理輸砂模式，探討大甲溪石岡壩下游辮狀河段在2012年至2013年洪峰期間的河相演變，並分析其河床沖淤特性、河道縱橫向穩定性以及河相受河工結構物之影響。由分析結果得知，數值模式可呈現與實際河川相近的地貌形態和演變過程，如：河心洲生成、沙洲裂解及深槽改道等機制；這些機制也連帶影響河道的斷面高程、河寬及主深槽位置；此外，位處河川凹岸的縱向構造物若採用糙度較低的材料，在洪峰時容易吸引水流對凹岸的沖蝕，而橫向構造物則因攔阻砂石改變河川原有的土砂收支，促使上下游的河床形態局部不連續。

關鍵字

辮狀河川；數值模式； Nays2DH ；大甲溪；河工構造物

並列摘要

Because of steep slopes, rapid flow, and large amounts of sediment, rivers in Taiwan are mainly braided rivers, which comprise several bars and exhibit bifurcated flow. Channel shifting behavior and bar migration in natural conditions create difficulties for river engineering work. In this study, the Nays2DH model is used to simulate the braiding processes and the morphological changes downstream of the Shih-Kang Dam in Da-jia River from 2012 to 2013. Furthermore, the characteristics of riverbed erosion, channel stability in its longitudinal and transverse directions, and the impact of river structures on its morphology were analyzed. The numerical simulation results revealed common braiding processes and statistical topographic characteristics, as exhibited by natural rivers. The scour pools mostly occurred at the apex of the river bend and downstream from bars (confluence) in braided rivers, which means that bar migration is the main factor altering the scour position. Under the assumption of sediment transportation in equilibrium at the upstream inflow, the width of the channel determines whether the river undergoes longitudinal erosion or transverse changes. The results of this research serve as a reference for future research on river morphology evolution, river engineering, and river spatial planning.