The Influence of Accumulated Precipitation on Debris Flow Hazard

Debris flow warning system in Taiwan uses effective accumulated precipitation as the warning criteria. Little research has studied the affected area associated with different levels of accumulated precipitation. The Taipei DF024 potential debris flow torrent is used as an example to study the relation between an affected area and accumulated precipitation. Three different amount of accumulated precipitation 300, 500 (official warning criteria) and 700 mm in 24 hours are chosen for comparison. Average rainfall intensity per hour is calculated from accumulated rainfall through the Monobe formula. Total water flow rate is estimated using the Rational formula and a discharge hydrograph. Maximum debris flow volume is determined by the equilibrium concentration formula. All sources of debris are distributed on landslide areas and river bed from field investigation. DEBRIS-2D is used to simulate debris flows. The results show that the hazard area is proportional to precipitation, and the thickness of maximum debris flow accumulation is between 3 and 3.5 m for all three cases. The relation between accumulated precipitation and hazard area can provide officials with additional information related to resident evacuation.土石流預警在台灣是以有效累積雨量為基準，但在不同累積雨量下的土石流災害範圍，卻少有學者討論。針對這個問題，本文以北市DF024 為例，用其警戒雨量500 毫米做為基準，再選300 和700 毫米的累積雨量做影響範圍的比較。透過物部公式 (Monobe formula) 將累積雨量轉換為降雨強度，並搭配合理化公式和流量歷線去估算集水區產生的總水量，再以土石流平衡濃度公式推估土石流體積量。並經過現場調查，將料源分佈於崩坍地與河床崩積層，最後採用DEBRIS-2D 去模擬土石流影響範圍。在北市DF024 的案例中發現，土石流影響範圍會隨著累積雨量的增加而擴大，但最大土石堆積深度皆在3~3.5 公尺之間。透過此方法得到的土石流影響範圍不只可以連結與累積雨量的關係，並且可提供相關單位一個修正警戒雨量或疏散 範圍決策的參考