Study on a Geomorphic Indicator for Evaluating Sediment Transport Capacity in Mountainous Rivers

Flooding of water, sediment, and driftwood and its associated landform changes often occurs downstream of debris-flow deposition zones in small, mountainous river basins. This type of flooding occurs due to a sudden decrease in the sediment transport capacity of river channels, resulting in considerable damage. Despite the significant need for appropriate countermeasures to mitigate damage, it has been a challenge to plan and implement these across all river basins at risk of these hazards. However, if a simple but reliable tool is available to identify hazardous areas in advance, this would be extremely useful for prioritizing such areas and designing and implementing the required measures. To this end, this paper examines the geomorphic characteristics of recent flood events in Japan and proposes a method for identifying hazardous areas due to inundation by flood flows with sediment. We first explain the occurrence of sediment inundation by focusing on longitudinal changes in sediment transport capacity using the geomorphic indicator defined as the product of the drainage area A and bed slope i. In addition, we investigate the longitudinal profiles of the sediment transport capacity along the study channels using a one-dimensional governing equation for flood flows and, based on the results, discuss that finding that the computed longitudinal profiles of the sediment transport capacity are consistent with the longitudinal distribution of the Ai. Our findings indicate that the Ai is an effective indicator for identifying areas prone to inundation by flood flows with sediment.