# Experimental Investigation into the Number of Phases in Debris Flows
## 2 Sampling Sites  
The sandy-silt debris-flow sample [44] used in this study is collected from the well-preserved debris flow deposit (27\({}^{\circ}\)38\({}^{\prime}\)21.31\({}^{\prime\prime}\) N, 98\({}^{\circ}\)44\({}^{\prime}\)4.74\({}^{\prime\prime}\), Figure 1 in [PERSON] et al. (2024) [45]) about 500 m upstream of the Dongyuege (DYG) Creek (a tributary to the Nu River) fan. The DYG event, which occurred on 18 August 2010, was typical of Nu River debris flows. The Nu River catchment area is the most prone to sudden and catastrophic debris flows, in China.  
Geologically, the Nu River catchment is hosted in the N-S extensional Nu River fault zone, which is \(\sim\)500 km long and 20-30 km wide in Yunnan, China, and is dominated by medium- to high-grade metamorphic rocks [46]. The strong downcutting of the Nu River and its hundreds of tributaries, driven by the uplift of the Tibetan Plateau in the Cenozoic, and abundant loose sediment fed highly fragmented bedrock, preconditioned a multitude of debris-flow-producing basins.  
The DYG debris flow has the following characteristics: high magnitude [35, 47], high elevation of the initiation area [48] and large relief of the flow track, long reach, high mobility, and high competence [45, 49, 50, 51, 52, 53].  
(i) Approximately 6 \(\times\) 10\({}^{5}\) m\({}^{3}\) of sediment was delivered to the alluvial fan, killing 96 people. (ii) The event initiated at elevations of 3000-3500 m a.s.l., near the snow line, whereas the fan ranges in elevation from 1400 m to 1450 m a.s.l. (iii) Its total travel and runout distances [12, 39, 54] reached 11 km and 9 km, respectively. (iv) It traveled up to 3.3 km in a low-gradient channel (2\({}^{\circ}\)-5\({}^{\circ}\)) upstream of the mouth of the DYG Creek.  
(v) Three large boulders, 7-9 m in size, were transferred from the stream channel \(\sim\)8 km upstream of the fan apex onto the fan in addition to common boulders of many tens of centimeters in the deposits. (vi) The \(<\)0.005 mm fraction of the deposit contained no typical clay minerals [21; 55], and the plasticity index of the \(<\)0.075 mm fraction was only 6.9, which was governed by basin bedrock composed predominantly of granite, schist, genesis, quartzite, and marble.  
The DYG event is representative of common alpine debris flows [39; 56; 57], with a predominant sandy silt texture.  
The Jiangjia Gully (JJG) debris flow (26\({}^{\circ}\)14\({}^{\prime}\)50.65\({}^{\prime\prime}\) N, 103\({}^{\circ}\) 8\({}^{\prime}\)4.57\({}^{\prime\prime}\) E, Figure 1 in [PERSON] et al. (2024) [45]) sampled in this study is known for extreme debris-flow activity and has received the most intensive study [4; 6; 28; 29; 37; 58], especially in China [59; 60; 61; 62; 63; 64; 65; 66; 67; 68]. As expected, the magnitude of the JJG flow was relatively low, with the sediment mobilized by a single surge commonly having volumes of thousands of m\({}^{3}\) to tens of thousands of m\({}^{3}\). It is interesting to note that the largest clasts in the JJG debris-flow deposits rarely have a maximum dimension of greater than 0.5 m (generally \(<\)0.3 m). This phenomenon can be ascribed to bedrock geological controls, that is, lithological and structural predisposition. The JJG basin is underlain by highly fragmented Proterozoic slate and is also part of the active Xiao River fault zone, which is of regional importance and is several kilometers wide and \(\sim\)350 km long. The Xiao River watershed has an area of approximately 2950 km\({}^{2}\), is preconfigured by the fault zone, and has been repeatedly subjected to debris flows.  
Profoundly different from the DYG flow, the \(<\)0.005 mm fractions of the JJG debris-flow deposit are dominated by typical clay mineral (illite), and the plasticity indices of the \(<\)0.5 mm and \(<\)0.075 mm fractions can reach 12.0 and 12.4, respectively [52; 53]. Therefore, the JJG deposits can be thought of as the product of muddy debris flows [26; 38; 39; 40].