The formation processes and development characteristics of 1 sandbars due to outburst flood triggered by landslide dam 2 overtopping failure 3

12 Sandbars are an essential form of riverbed morphology which could be affected 13 by landslide dams. However, few studies have focused on the formation processes and 14 development characteristics of sandbars triggered by outburst flood. In such a way, 15 eight group dam failure experiments with 4 to 7 times of dam length movable bed is 16 carried out to study the temporal and spatial distributions of 25 sandbars along the 17 riverbeds, the sandbars geometric characteristics, and the influence of outburst flow 18 hydraulic characteristics on developments of sandbars. The results show that sandbars 19 are formed after peak discharge of outburst flow. The number of sandbars is 0.4 to 1.0 20 times the ratio of river bed length to dam length. Besides, sandbars have the 21 characteristic of lengthening towards upstream during the failure process. Sandbars' 22 https://doi.org/10.5194/esurf-2020-92 Preprint. Discussion started: 16 December 2020 c © Author(s) 2020. CC BY 4.0 License.

upstream edges have a more extensive development than sandbars downstream edges. 23 The length of a sandbar along the channel changes faster than the sandbar's width and 24 height. The sandbars' length and width are about 10 to 80 and 1 to 7 times of average 25 height, respectively, and the average heights of sandbars are about 1 to 3.5 times the 26 maximum particle size. Sandbars' lengths make a more significant impact on sandbars' 27 volumes than widths and heights. It found that the Froude number has a significant ). Sandbars are shaped siltation bodies with exposed water surfaces formed by 54 rivers, lakes, and seashores (Chien et al., 1987). Moreover, sandbars are a feature of the 55 transition zone between aquatic and terrestrial, which have essential impacts on 56 transportation and species habitation using river corridors (Lin, 1990; Tracy-Smith et    And they discussed that flow unsteady property seemed to change the growth 112 mechanism of sandbars. Besides, for this type of sandbars, the upstream sediment is 113 mainly supplied by the dam material, which is different from other types of sandbars.   Table 1.  The dam materials used in this study were mixtures of sand and graves, with a 156 median particle size D50 of 3.8 mm. Due to the flume space limitation, the maximum 157 sediment particle size was set to 20 mm. The riverbed was movable, which consisted 158 of the same material as the dam model. The thickness of the riverbed was set to 0.06 m.

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The gradation curve of material particles' sizes is shown in Fig. 1.

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In the experiment, the flow velocity was measured based on the reference object.              Jiang and Wei (2020) discussed the relationships between the lengths and the 380 maximum widths and heights of sandbars when the dam was failed entirely, but the 381 relationships with sandbars lengths, average widths, and heights had not been involved.

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It found that the average heights after the dam failure were about 1 to 3.5 times the 383 maximum grain size. The ratios of lengths to average heights were basically between 384 10 to 80, and the rate of average widths to average heights were basically between 1 to 385 7 (Fig. 7).
where ce is bedload sediment carrying capacity, qb is the unit-width bedload transport where θ is the Shields number, which can be obtained according to Eq. (4), and θc is the    The sums of (c-ce) at the sandbars' upstream and downstream edges are used as 520 the criterion for judging the sandbars' length variation. The relationships between the 521 sums of (c-ce) and zero determine the increase or decrease of sandbars' lengths. Suppose 522 the sums of (c-ce) are greater than zero. In that case, it means that the outburst flow 523 cannot transport all the sediments. The excess sediments are deposited in the sandbars 524 areas, corresponding to the increase in sandbars' lengths and volumes; otherwise, 525 sandbars' lengths and volumes are reduced. Figure. 12 shows the relationships between 526 the sums of (c-ce) at the sandbars' edges and 0. By combining Figs. 12 and 6, it can be 527 seen that when the sums of (c-ce) are greater than 0, sandbars' lengths and volumes are 528 increased. It reveals that the relationship between the sums of (c-ce) and 0 can be used 529 to judge the trend of sandbars' lengths and volumes.