在防災應用上，土石流潛勢溪流之溢流點及影響範圍的劃設與人民生命財產最為相關，而
溢流點位置更直接決定了影響範圍劃設的區域。本研究之溢流點劃設方式採用現地勘察為主，
並將地形圖、航照資料納入進行判釋。本研究之嘉縣DF042 土石流潛勢溪流位於嘉義縣阿里山
鄉來吉村，在2009 年8 月9 日莫拉克颱風時，大雨使得集水區內發生大規模土石崩塌，而崩塌
材料進入河道中更引發土石流災害，並造成下游與阿里山溪匯流口左岸之保全住戶遭沖毀。在
2010 年8 月莫拉克颱風後之調查，將溢流點修正位於溪流匯入阿里山溪口上游約200 公尺處之
出谷口轉彎處。本研究發現，單一溪流中可發現多處符合溢流點地形條件之位置，但劃設區域
若設置於最下游之地點，易低估其影響範圍，故建議後續溢流點及影響範圍應採以較保守之方
式進行劃設。

In debris flow hazard mitigation, the safety of life and property were highly related to the location of overflow point and inundation area. Furthermore, the location of overflow point determined the region of inundation area. In this study, we used topographic map and aerial photo, with field investigation to re-exam the location of overflow point. The Chiayi DF042 debris flow potential torrent is located in Laiji Village, Alishan Township, Chiayi County. During Typhoon Morakot of Aug. 2009, a
debris flow event occurred. Heavy rainfall induced massive landslides in the watershed, and the material went into the torrent together with runoff created a debris flow which struck the house in downstream. After the investigation in Agu. 2010, the overflow point was relocated to the notch of valley and river bending point. In this study we found out that there might be many locations that satisfied the terms of overflow point. For hazard mitigation purpose, the most conservative position and range of overflow point and inundation area should be considered.