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Owing to extensive construction of dams, the impact of backwater on flow may lead to navigation or flood control issues in curved channels. To date, the impact of backwater on the water surface profile in curved channels remains unknown and requires investigation. In this study, experiments were conducted in a glass-walled recirculating flume with a length of 19.4 m, a width of 0.6 m, and a depth of 0.8 m, and the impact of backwater on the water surface profile in a 90° channel bend was investigated. The experimental results showed that the backwater degree had a significant impact on the transverse and longitudinal flow depth distributions in the bend. The transverse slope of the flow (Jr) increased linearly with an increase in the Froude number of the approach flow upstream of the bend. Jr increased with the longitudinal location parameter ξ when −0.2

Characterized by large scale, high frequency, and strong destructiveness, debris flow has become the most noticeable geohazards throughout the world, especially in the mountainous areas of southwestern China. On August 20, 2019, large-scale heavy rainfall pummeled Wenchuan County, Sichuan Province, Southwestern China, which resulted in a cluster of debris flows (the “8·20” clustered debris flows event), and caused considerable economic losses (approximately 3.4 billion RMB were lost) and heavy casualties (48,862 people were displaced, 16 people died and 22 people went missing). Based on field investigation, image data interpretation, mechanism analysis, and other methods, this study reveals the formation mechanism, dynamic evolutionary process, and impacts of human activities on the “8·20” clustered debris flows event. Results from a comprehensive analysis indicate that the occurrence of short-term, high-intensity rainfall and the excessive supply of solid material were the main factors that triggered this catastrophic event. With the debris flow flowing into the main river, this event presented an extremely apparent disaster chain effect. It is also found that improper site selection and inadequate design of human activities played a crucial role in the movement process of the debris flow that directly aggravated the losses. Finally, to improve debris flow prediction and prevention, some early warning and mitigation measures are discussed.

Mountain flood hazards are now among the most studied hazards, given that they cause loss of life and damage to property and infrastructure throughout the world. This paper takes the 2015 Xuyong flash flood/debris flow event as a case study to examine the impacts of human activities on the occurrence and amplification of mountain flood hazards. Field investigations, the interpretation of observational data and mechanistic analyses are carried out as part of a comprehensive analysis of the processes by which mountain flood hazards develop and become amplified, the contribution of excessive sediment supplies to mountain flood hazards and the effects of human activities on mountain flood hazards. Results of this comprehensive analysis indicate that some human activities contribute to the occurrence of the initiation of mountain flood hazards. These human activities fall into two main categories: (1) deforestation and improper construction damage the natural environment and may increase the occurrence of mountain flood hazards; (2) improper choices of residences, local congestion of engineering structures and some other factors may directly amplify the losses caused by mountain flood hazards. Finally, some hazard prevention and mitigation measures for mountain flood hazards are discussed.