台灣地處板塊交接處，由於受到歐亞板塊及菲律賓海板塊之擠壓，山區地勢陡峭，河流湍急。此外，台灣位於亞熱帶地區，雨量豐沛，又因地處颱風主要路徑之中，其長延時、高強度之降雨易對台灣山區造成災害，包括崩塌、地滑、土石流及天然壩災害。其中，天然壩災害其壩體後方積蓄大量水體，一旦壩體潰決，大量水流將對下游聚落造成莫大威脅，又因天然壩自形成至潰決之時間可能極短，導致下游保全對象對災害之應變時間不足，因此若能對天然壩之形成有一快速且有效的偵測方法，將對天然壩災害之防治和減輕有莫大貢獻。
於天然壩災害發生時，由於上游水流受壩體阻塞，無法正常供應至下游，導致下游水位產生一異常下降之現象，前人於研究中使用此水位異常下降之現象及簡單波方程式來推估天然壩形成之位置，但由於忽略底床磨擦及坡度之影響，其應用於自然河川中將產生一誤差；本研究透過加入底床磨擦以及坡度因子之推導公式與室內物理模型進行比較，於水深比(y⁄y_0 )>0.5時，其水位變化之誤差率較低，約小於15%，而使用此區間水位進行天然壩位推估時置，期誤差約小於20%，相較於簡單波方程式，本研究所得之結果較為良好。

Taiwan lies on the edge of the Eurasian Plate and the Philippine Sea Plate. It is easy for earthquakes to occur due to plate collisions. In addition, because Taiwan is located in a subtropical zone, heavy rainfall and typhoon events occur frequently. These conditions make it easy for Taiwan to experience landslide dam disasters. Usually, a landslide dam forms in mountainous areas that are difficult for people to reach. In addition, the location of a landslide dam is difficult to ascertain. Therefore, we usually miss opportunities to have a reaction to such disasters and to develop an evacuation plan.
Due to the landslide dam occurrence, a negative surge will propagate to downstream and exhibit an unusual water depth decrease. This study uses an equation derived from the de Saint Venant equation by using the method of characteristic to estimate downstream water depth changes. The applicability of the equation is verified through a physical model experiment. There are a total of 36 cases in the experiment. The results show that the accuracy of the equation used in this study is better than the simple wave equation which has been used in previous studies.

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