淡水河在過去因過量之採砂造成河床下陷嚴重,河川之地形有了大幅的改變,導致二重疏洪道之分洪量由原先之9,200cms降至6,500cms,差異甚大,可知淡水河河床之沖淤,對於遭逢洪水時之疏洪道分洪量影響甚大。為避免因河床持續下降對橋樑與堤防構成威脅,政府全面禁止淡水河系開採砂石,在無人為之干擾下,淡水河河床經由自然沖淤有逐漸回淤之現象。 因此本研究利用SMS軟體進行二維流場之數值模擬,以淡水河最新之河床斷面作為基礎,模擬目前淡水河河床在持續淤積之情況下,兩種不同之淤積範圍,從上游淤積至台北橋與淤至關渡橋,以及在淤積範圍內四種不同之淤積高度,分別為0.5公尺、1公尺、1.5公尺及2公尺,在遭逢兩百年之洪水時,對二重疏洪道之分洪量、洪水位及流速各有何影響,以瞭解疏洪道在未來是否仍有其防洪效果,以及對整個淡水河流域內防洪工程所帶來之問題。模擬結果顯示,當淤積範圍至關渡橋且淤高至2公尺時,所得到之分洪量最大也最接近規劃初期之水準,但同時上游之洪水位卻有可能面臨溢堤之危險;因此淡水河河床之淤積確實有助於分洪量提升,但過度之淤積卻反而可能為淡水河流域帶來危害。
The Tanhsui river have three main tributaries, includes Tahan river, Hsintien river and Keelung river, the river flows through Kuantu and flows into the Taiwan straits from Tanhsui, the Tanhsui river is the first largest rivers in the north of Taiwan, the length of river is 159 km and drainage area is 2,726 km². Three main tributaries gather flow rate in the basin of Taipei within the range of 10 km, only one outfall in Kuantu can release floodwater. In this study uses the latest(A.D. 2005) data of watershed of Tanhsui river, the data including topography, hydrology and flow, and using SMS to simulate, then uses the same data of the topography section, and different silt range and silt height, uses the same models to simulate too. We can get the result and find difference between them. We are known by the result, the same topographical condition but different river section profundity, the discharge capacity to Er-Chung floodway will cause influence too, the result hopes to be consulted by scholar and hydraulic expert, when they assess the dredge questions of Tanhsui river and efficiency question of floodway.