水平二維動床模式於複式河槽洪水位壅高之研究

Abstract

台灣下游段河川多為主深槽與相鄰高灘地所構成之複式河槽，在平時枯水時期僅主深槽有基流量之水流動，當汛期洪水來臨時，水位將從主深槽上升漫過高灘地，此主深槽與高灘地間底床粗糙度之變化勢必對洪水位壅高造成影響，而台灣河川中下游兩岸人口密集，此洪水位壅高之影響評估對於防洪安全實屬刻不容緩之研究課題。 本研究以許（2002）發展之二維顯式有限解析法模式為基礎，程式處理技巧方面參考蔡氏等（2000）提出一套乾濕點修正方式，在演算流程上有所不同，並將乾濕點修正判斷方式精簡為八種類型，克服模擬現況變量流案例水位升降所遭遇之乾濕點問題。以實驗室及曾文溪中下游河段為模擬對象，探討高灘地糙度改變對洪水位壅高之影響。 由模擬結果得知，水位能夠符合實際流況在主深槽與高灘地間升降。藉由高灘地不同糙度之改變，可看出改變糙度之區域沿程水位有明顯抬升。而水位上升程度隨著高灘地糙度增加而增加，當流量加大時，上升值亦隨之增加；從上升百分比來看，實際案例中水位最大可上升到平均水深之7%，此上升程度對於防洪安全具有一定之風險性。

The cross-section of downstream natural rivers is usually composed of a deep main channel and adjacent shallow floodplains. During the low flow period, water only flows in the main channel, while during the flood season, the floodplains would be inundated. The differences of roughness at the deep main channel and the floodplain cause the variations of flood stages. In Taiwan, the middle and lower reaches of the rivers are densely populated. The study of flood stage variations of compound channels and the changes of roughness at the main channel and floodplains are important and urgent topic in the study of flood prevention. In this study, a mobile-bed numerical model using the explicitly finite analytic method developed by Hsu in 2002 is adopted to simulate the flood stage variations in the compound channels. A wetting and drying treatment method is built into the model to overcome the dry-bed problem. It is based on Tsai’s (2000) theory which is different in algorithm and simplified to eight types. The model is applied to study the effect of the changes of roughness at the main channel and floodplains on flood stage variations of Tseng-Wen Creek, Taiwan. From the simulated results, the water surface can rise and fall between the main channel and the floodplain. By changing the roughness of the floodplain, the water level rises with increasing roughness and discharge, respectively. In terms of percentage of rise of water stage in the study reach of Tseng-Wen Creek, the maximum increase of water stage is 7% of the average depth; which results in the high risk in flood prevention.