第四紀礫石層為臺灣地區最容易發生土石流的地層之一，此種地層廣泛分佈在西部山麓和平地接壤的丘陵或臺地等聚落或人口密集之處，一旦發生土石流經常導致重大災情，因此值得深入研究。本研究選定南投縣九九峰礫石層區的乾溪、田尾坑溪兩個集水區進行研究。將研究區的土石流依其規模大小及發生區位分成邊坡型和山谷型兩種型態，然後利用野外調查，輔以室內航空照片判識和地形圖的比對，針對邊坡型土石流與其他邊坡崩塌型態在地形特徵上的差異進行比較，另就所有山谷依土石流災害發生的有無加以區分，分析可能影響這土石流發生的地形因素。 野外觀測發現邊坡型土石流大多發生在具有內聚凸坡坡形的邊坡上，其崩塌處下方通常會出現4至6公尺寬的隘口。資料分析結果顯示，邊坡型土石流的崩塌區的主軸長度、崩塌深度、崩塌面積和集水面積均較岩屑滑崩和岩屑滑落為大，且發生邊坡型土石流的邊坡集水區面積均達1,000平方公尺，且為崩塌面積的2到6倍。而發生土石流的山谷集水區面積與崩塌面積佔集水區面積的比率皆較未發生土石流的山谷為大，並以崩塌面積佔集水區面積的比值的差異最為顯著，其次為集水區面積，最後才是坡度。研究結果顯示以15,000平方公尺的集水區面積和崩塌面積佔集水區面積20% 為閾值時，可以有效評估山谷型土石流的潛在危險性。

Conglomerate formations are broadly distributed in the populated western foothills of Taiwan. Since conglomerates are subject to debris flows, many disasters have taken place in areas covered by this type of geologic formations. This study aims to deduce the geomorphic thresholds for debris flows from data derived from field investigation and aerial photograph analysis of the Dry Creek and the Tien-wei-keng Creek watershed in the Ninety-Nine Peaks area in Nantou. The debris flows took place in the study area were classified into two categories according to their size and location. One is the slope-type debris flows, the other is the valley-type debris flows. The geomorphic characteristics of the former were compared with those of the other two most commonly seen slope movements, i.e. debris slides and debris avalanches, while valleys with debris flow deposition in their valley mouths are compared with those without. This study shows that the slope-type debris flows always took place on converged convex slopes. They usually had a bowl-shaped appearance, with a constriction of 4 to 6 meters wide below the sliding area. The main axis length, the depth of the slide and the size of both the sliding area and its watershed of slope-typed debris flows are all significantly greater than those of the other two types of slope movements. It is discovered that when the contributing area of a landslide exceeds 1,000 square meters and is larger than 2 to 6 times of the sliding area, debris flows are very likely to take place. The watershed size and the total area of landslides contained within the watershed where valley-type debris flows took place are significantly larger than those without such phenomena. Valley-type debris flows tend to occur in watersheds that are larger than 15,000 square meters with 20% of its area occupied by landslides.