Numerical investigation of sediment-charged flash flood due to a cascade of natural dam failures

Landslide natural dams are commonly formed in a river valley of mountainous areas due to heavy rainfall or earthquake, which can be a complete or partial blockage. Different from conventional man-made dams, natural dams typically comprise unconsolidated and poorly sorted material, and are vulnerable to failure and breaching in short period due to overtopping or seepage. For those small sediment blockage in a river valley, their failures frequently occur during high intense rainfalls, which will induce a large flash flood with high-concentrated sediment downstream in a short period, and the magnitude is likely to be amplified along the flow direction due to the inclusion of a large amount of sediment. This can result in significant and sudden debris flow or high sediment-charged flash flood in the downstream for human life and property. Cascade failures of a series of natural dams in a gully have been considered to be a primary reason for the enlargement of high sediment-laden flash flood. In general, cascading natural dams can be formed along the sloping channel due to the randomness and unpredictability of landslides, which complexes the hydraulics of landslide dam failures.

This study evaluates the formation and development of sediment-charged flash floods due to cascading failure of natural dams through detailed hydro-morphodynamic modelling. The model used is based on shallow water theory and it has been successful in predicting the flow and morphological process during sudden dam-break, as well as full and partial dyke-breach.  The study first calibrates the model with experiemntal data of a cascade of partical blockage dam failures. Then the calibrated model is applied to two types of natural dam failure cases: (1) straight steep slope channel with a series of small partial blockage dams; (2) bend channel with steep slope including a series of partical blockage dams. For both cases, various scenarios are modelled, including: (1) failure of a single dam in a sloping channel, (2) failure of two dams in a sloping channel, (3) failure of multiple landslide dams (four) in a sloping channel. Based on the detailed model results, the study systematically explores the tempo-spatial evolution of sediment-charged flash floods (discharge, flow velocity, and flow concentration) and geomorphic properties along the steep sloping channel.  The effects of in-channel erosion and flow-driven sediment from dams on the evolution of flood dynamic process are analysed.  The results improve the understanding of the formation and development mechanism of flash floods due to cascading landslide dam failures.  The findings are beneficial for downstream flood risk assessment and developing control strategies for landslide-induced floods.