上游野溪河道在山崩、地滑、土石流或高含砂水流的衝擊下，常造成大量土石淤積，若無適當土石清疏，當再次遭遇颱風暴雨時，很容易造成河岸及下游人民身家財產之損失，而到底需清除
多少土方量，方不致造成土石災害，則是一個值得推估的課題；本研究以宜蘭縣寒溪集水區為例，推估50 年頻率降雨條件下，河道土砂之淹沒深度、範圍及體積；首先收集歷年降雨量，推估降雨量及降雨雨型，隨後以HEC-HMS 水文分析模組進行集水區水文演算，得到洪水流量歷線，最後以FLO-2D 模式，進行二維水理及土石堆積量計算，提供土石清疏量體之依據。

Landslide, debris flow and flood with high sediment concentration usually induce a great quantity of sediment deposits in the upstream river. If the sediment deposits cannot clean up in time, the people lived beside the river may fall into the danger of debris flow while storm happens. There are a interesting topic is: How much volume of sediment deposits should be clear away, so as to prevent the sediment disaster? This study select the Hanxi watershed, Yilan County as the example, to estimate the depth, range and volume of sediment deposit in stream under a rainfall depth with 50 year regression period. Firstly, the historical rainfall depth is collect to estimate the rainfall volume and the rainfall time distribution. Secondly, the hydrological model HEC-HMS is adopted to process the hydrological routing and to obtain the flood hydrograph. Finally, the FLO-2D model is used to finish the two-dimensional hydraulic routing and sediment deposit volume estimation, which will be a basis of sediment cleanness.