Early identification of river blocking induced by tributary debris flow based on dimensionless volume index

Abstract

River-blocking events induced by tributary debris flows occur frequently in mountainous areas. If a dam formed by a river-blocking debris flow were damaged, a massive outburst flood could induce downstream hazards. Therefore, early identification of areas potentially at risk of river blocking is essential for hazard mitigation, particularly in extensive mountainous areas. This study proposed a dimensionless volume index (DVI) to evaluate river-blocking formation by considering the relationship between the deposition volume of a tributary debris flow in a river and the minimum river-blocking volume. The values of both parameters can be established based on the theory of debris flow run-out distance, deposition width, and deposition thickness with consideration of the effects of drag force, hydrostatic force, and buoyancy. The efficacy of the proposed method was demonstrated through two case studies of mainstream blocking/nonblocking by tributary debris flow events in Taiwan during Typhoon Morakot. The results indicated that early identification of river blocking could be achieved using the DVI based on a pre-established database of potential debris flow hazard areas, including local site information, rainfall intensities at potential debris flow hazard areas under varying recurrence periods, and mainstream discharge at varying recurrence periods.