On May 5, 1998, after 30 h of continuous rainfall (100–180 mm), large areas of the tephra-rich colluvial cover 12–18 km east of Vesuvius volcano (Italy) failed, generating a series of debris flows that killed more than 150 people in the Sarno area. Some 37 basins were affected almost simultaneously by this phenomenon. The Sarno disaster shows that air-fall deposits can create debris-flow hazards that may extend for several decades or even centuries after eruptions. In the Sarno area, the affected basins have the following features: (1) a funnel shape, debouching at the base of the hillslope; (2) an integrated drainage network with a central subrectilinear channel; (3) slopes exceeding 25° on average; and (4) a thin (∼0.5–2.0 m) volcanic-rich colluvial cover related to the past ash fallout of Vesuvius. The stratigraphy of downstream alluvial-fan deposits records the repeated occurrence of large debris flows. On the basis of the 1998 case history, we propose an approach to evaluate hazard potential related to similar debris flows for the neighboring Clanio Valley. This approach is mainly based on morphometric data derived from a digital elevation model and is of potential worldwide application.