Significance of coarse clasts in viscous debris flows

Highlights

• A combination of rheometrical and flow tests, pore pressure measurements, 3D laser scanning and FESEM observations is used.

• The role of coarse clasts (0.005–10 mm) in debris flows is evaluated in a comprehensive manner.

• Coarse clasts greatly affect debris slurrying potential and resultant slurries' bulk behavior.

• Shape and surface texture of coarse clasts influence liquefaction potential of debris.

• Coarse clasts can be classified as platy/flaky and equidimensional.

Abstract

Debris flows are flowing mixture of water and sediments (from clay to boulder-sized particles). The sediments control both occurrence and characteristics of debris flows, however, the relevant literature has concentrated mainly on fine debris. Considering their large fractions in debris flows, the significance of coarse clasts (≥0.005 mm) is investigated by comparing the crude debris (CD, the fractions of <10 mm and <5 mm of the Jiangjia Gully debris-flow deposit) with the reconstituted debris (RD, prepared by replacing the coarse clasts of the CD with the corresponding sizes of the Dongyuege debris-flow clasts following the gradation of the CD). Rheometrical tests, propagation tests, and pore-pressure measurements are made on the CD and RD samples in a systematic manner.

Aggregation of data collected in 39 tested debris-water mixtures leads us to conclude that the coarse clasts can greatly affect the slurrying susceptibility of debris masses and bulk behavior of slurries. The debris-flow indices (I_d) from the CD to RD slurries with maximum grain sizes of 10 and 5 mm decrease by 29% and 17%, respectively. The CD slurries possess higher yield strength and viscosity than their RD counterparts with the same solid volumetric concentrations (C_vs). The areas inundated by 10 CD slurries with maximum grain sizes of 10 and 5 mm are approximately 17%–39% of those of their RD counterparts. The excess pore pressures in the RD slurries dissipate more quickly than those in the CD slurries. The shape and surface texture of coarse clasts regulate slurrying of debris masses, as well as influence their liquefaction potential, which in turn has an impact on the mobility and persistence of subsequent slurries.

The platy/flaky debris-producing mountain catchments generally experience lower-magnitude/higher-frequency, lower C_v debris flows of relatively less destructive power, and the debris flows dominated by equidimensional clasts are often more powerful and hazardous, despite their potentially lower frequency.

Introduction

Debris flows are sudden and strongly destructive mass movements, that occur when poorly-sorted debris, mixed with water, move down valleys in response to gravitation (Iverson, 1997). Both debris and water exert a strong influence on the rheological properties and propagation of debris flows, which distinguishes debris flows from related phenomena such as rock avalanche-debris flows and sediment-laden water floods (Iverson, 1997). The debris typically consists of grains ranging in size from clay to boulder, which commonly composes most of the mass of debris-flow bodies (Johnson and Rodine, 1984). Sufficiently available loose debris is a major contributing factor to the processes that trigger debris flows (Jakob et al., 2005) and, perhaps more significantly, the availability of debris material resulting from rock collapses is the main triggering factor of high magnitude debris flows (Gregoretti et al., 2018). Debris flows have diverse flow behavior and dynamics (Sohn, 2000; Sanvitale and Bowman, 2017), which can be governed by many factors, such as the triggering liquid discharge (Lanzoni et al., 2017) and the debris characteristics in particular (Iverson, 1997; Morton et al., 2008; Pudasaini and Miller, 2012). Hence, in-depth studies on multi-scale debris are needed for a better understanding of debris-flow processes.

Debris is generally divided by grain size into coarse and fine clasts (Rodine and Johnson, 1976). However, there is no generally accepted size guideline in the literature for the distinction between them. The threshold grain size used to differentiate fine clasts and coarse clasts primarily include 0.005 mm (Zhou et al., 2018; Tang et al., 2018), 0.04 mm (Coussot and Meunier, 1996; D'Agostino et al., 2010; Kaitna et al., 2007), 0.05 mm (Remaître et al., 2005, Remaître et al., 2011), 0.063 mm (Pérez, 2001; Phillips, 1988), 0.074 mm (Godt and Coe, 2007; NEH-3, 2012; Hürlimann et al., 2015) and 2 mm (Bernard et al., 2019).

The fine clast contents have always been considered as a key factor affecting the initiation and movement of debris flows in previous studies. Large-scale flume experiments performed by Iverson (1997) showed that increased fine particle (silt plus clay) contents were effective to reduce the frictional resistance of debris flows and enhance debris lobe spreading. de Haas et al. (2015) found in a small-scale flume experiment that the runout distance of laboratory debris flows varied with fine-clast (clay sized grains) contents, which was also observed in the laboratory by Hürlimann et al. (2015). Pierson (1981) underscored that the presence of fine particles (silt and clay) could help sustain high pore pressures, which in turn played a governing role in debris-flow behaviors (Wang and Sassa, 2003; Iverson et al., 2010; Kaitna et al., 2016; Zhou et al., 2018). Moreover, according to O'Brien and Julien (1988), changing the clay-sized particle contents would greatly modify the order of magnitude of rheological parameters, thereby affecting the mobility of debris flows (Bisantino et al., 2010; Kang and Kim, 2017). This finding is in good agreement with Major and Pierson (1992) as well as Iverson (1997), who indicated that fine debris (clay plus silt) could strongly control the rheological behaviors and mobility of flow materials.

To explore the role of fine clast contents in experimental debris flows is essential for research on debris flows, but it is well known that most debris flows are dominated by coarse clasts (Pierson, 1980; Major and Pierson, 1992; Iverson, 1997) and that the fine clasts (especially the particles <0.005 mm) commonly constitute less than 10% of the mass (Pierson, 1995; Major, 1997; Pérez, 2001; Remaître et al., 2011). Coarse clasts may have a strong influence on the behaviors of debris flows. Unfortunately, the effect of coarse clasts on debris-flow behaviors is often neglected, and systematic research on how coarse clasts influence both debris-flow behavior and mobility remains unavailable.

This paper first assesses in a quantitative way the effects of coarse debris (particle diameter exceeding about 0.005 mm) on debris-flow performances. This is carried out by using two types of tested materials with the same composition of fine fractions and grain size distribution but with different composition of coarse fractions, which are prepared by the most representative debris-flow materials in China. The influences of coarse clasts on the debris flow behaviors are uniquely evaluated with the aids of laboratorial data and geometric measurements/micro-observations of coarse debris. The results not only provide compelling experimental evidence that coarse clasts exert a significant influence on debris flow behaviors, but also are helpful to improve our understanding of the entire debris-flow processes and beneficial for hazard evaluation as well.