Sediment hazard zoning, such as delineating areas prone to debris flow or flooding, is a crucial task in disaster prevention and land management in mountainous watersheds. In this study, distributed Melton ratio (MR) was used as a dimensionless geomorphic parameter together with a geographic information system and digital elevation
model to evaluate the types of sediment disasters. When the MR is greater than 0.3 at the local point, debris flow occurs if abundant loose sediment is located upstream. When the MR is smaller than 0.3, flooding occurs at this location. The MR values for the confluence of upstream trunk rivers and tributaries and for upstream spilling points, such as bridges or valleys, should be examined to evaluate the corresponding risk of sediment-related disasters in these locations. Typically,the threshold drainage areas for channel initiation are within the ranges of 0.1-2.5 and 0.5-20.0 ha for gravelly valleys and sandstone–shale valleys, respectively. The distribution of MR values along a trunk river can be classified into six categories, with the majority of watersheds belonging to categories III and IV. Case studies of Pingguan Creek in New Taipei City and Talowan River in Nantou County revealed that distributed MR models can identify the types of sediment-related disasters that occur along the main channel in mountainous watersheds.
Key Words: Potential debris-flow torrents, Melton ratio (MR), Geomorphological analysis, Sediment-related disasters