The formation and geometry characteristics of boulder bars due to outburst floods triggered by overtopped landslide dam failure

Abstract

Abstract. Boulder bars are a common form of riverbed morphology that could be affected by landslide dams. However, few studies have focused on the formation and geometry characteristics of boulder bars due to outburst floods triggered by landslide dam failure. In such a way, eight group landslide dam failure experiments with a movable bed length of 4 to 7 times the dam length with 25 boulder bars were carried out. In addition, 38 boulder bars formed in the field triggered by four landslide dam failures were investigated. The aim of this paper is to study the formation and geometry characteristics of boulder bars along the riverbeds. The results show that boulder bars are formed after peak discharge of outburst flow. The number of boulder bars is 0.4 to 1.0 times the ratio of riverbed length to dam bottom length. Besides, boulder bars have the characteristic of lengthening upstream during the failure process. A boulder bar's upstream edge has a more extensive development than a boulder bar's downstream edge. The length of a boulder bar along the channel changes faster than the boulder bar's width and height. After the dam failure, the boulder bar's length is about 8 to 14 times its width. The relationship between the ratio of boulder bar length to width and the boulder bar's dimensionless length could be described with a hyperbolic equation. The dimensionless area of the boulder bar increases linearly with the dimensionless area of the river section, and the linear ratio is about 0.5. With the field data, this demonstrates that the formation and geometry characteristics of boulder bars in tests are consistent with the field boulder bars. Therefore, the results in this paper are credible and can be applied to the riverbed's geomorphological characteristics analysis triggered by overtopped landslide dam failure. The plentiful experimental and field data could contribute to the community boulder bar research.

Effects of soil properties and geomorphic parameters on the breach mechanisms of landslide dams and prediction of peak discharge

“…This bar first occurred near the dam toe and then gradually extended in the stream direction. These phenomena are consistent with the observations of laboratory tests and the Yigong and Tangjiashan dams in the field [16,36]. It shows that the results are credible and can provide a reference for the evolution of river morphology.…”

“…The armoring layer in Fig. 8 is termed a 'boulder bar' by Jiang et al [16]. The boulder bar was formed after peak discharge because the breach discharge gradually reduced.…”

“…The failure mode is regulated by debris strength and its permeability [9,26]. The influence of debris type on the failure mode for landslide dams has been investigated through a series of laboratory tests [16,17]. The infilling stage from dam formation to breaching largely depends on the geomorphic parameters of a dam and backwater alters debris strength by the seepage.…”

Sedimentary records of giant landslide-dam breach events in western Sichuan, China

“…Therefore, new sedimentary evidence is needed to support these issues (Lord and Kehew, 1987;Cutler et al, 2002;Borgohain et al, 2020;Wang et al, 2023a). An abundance of ODs was discovered to be preserved in the Diexi Reach of the Upper Minjiang River and Tangjiashan Reach of the Tongkou River Basin (Ma et al, 2018;Jiang and Wei, 2020;Jiang et al, 2021;Ma et al, 2022). These ODs were formed by LLOFs induced by a giant ancient landslide-dammed lake and the Tangjiashan dammed lake, respectively, and they have good comparability in terms of deposition distribution, sedimentary characteristics and profile structures.…”