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Subaerial debris flows are important geomorphological processes that occur in steepland valleys and mountainous regions worldwide (Stock and Dietrich, 2003). They are extremely powerful events and pose a serious natural hazard. They can claim hundreds of lives every year and cause millions of dollars worth of property damage (Takahashi, 1981; Costa, 1984;

Davies, 1986; Matthews et al., 1999; Breien et al., 2008; VanDine and Bovis, 2002;

Mangeney et al., 2010; Jakob et al., 2013; Gartner et al., 2015). Japan, Canada, North America, Indonesia, Tasmania, Costa Rica, India, New Zealand and Norway are some of the countries subject to the frequent devastating affects from debris flows, and therefore understanding initiation conditions, event-frequency and run-out is important to evaluate potential human impact (Costa, 1984).

Subaerial debris flows tend to occur in areas of steep drainage and are typically caused by short periods of intense rain falling (flash floods, for example linked to the El Niño-La Niña climatic oscillations) onto already saturated soils during severe storms, leading to soil instability (Davies, 1986; Breien et al., 2008). They can also form from collapsed glacial lakes and natural dams (VanDine and Bovis, 2002; Breien et al., 2008). Figure 2.10 shows a typical debris-flow-prone subaerial steepland valley environment producing a network of debris flows as they flow and accumulate downslope.

Figure 2.10. Subaerial debris flow valley network in the Oregon Coast Range, USA. Debris flows were initiated from a severe storm in 1996, which initiated at the valley heads and scoured sediment to expose the Tyee sandstone bedrock (white areas). Road at top right indicates scale. From Stock and Dietrich (2003).

2.4.1.1 Field observations of erosive subaerial debris flows

Takahashi (1981) first documented large erosive scarps, resulting in the evacuation of sediments that formed debris flows. However, damage from these flows was not observed in the underlying road paving downstream of the flow, therefore Takhashi (1981) suggested that fully freighted/loaded debris flows appeared to have little erosive effect away from the source area. Zicheng (1987) also described how maximum erosion occurred near debris flow formation in the upper reaches of the valley, but also noted when enough water was present in the flow large quantities of unconsolidated loose sediment present in the channel valleys was entrained into the flow body.

Other studies have highlighted a gap in our understanding of subaerial debris flow processes. They illustrate how these dynamic events can cause huge amounts of erosion not

only up-dip but also down-dip in subaerial settings (Davies, 1986, 1990). Davies (1986) described the behaviour of debris flows observed in the field reporting an unsteady, ‘pulsing’

nature between periods of relatively low flow rate or zero-flow, an observation true of many other eye-witness accounts. Flow properties between the head (pulse) and tail (between pulses) of each debris flow surge were markedly different, where the head of the debris flow was observed to have the highest density, viscosity and velocity and was seen to scour the channel bed (Table 2.3).

Characteristic Pulse (head of flow) Between pulse (tail of flow) Flow density High > 2.1 T/m³ Low < 1.6 T/m³

Sizes present above bed Bimodal; fines and gravel Unimodel; fines dominant Location of coarse load Throughout flow depth Bedload only

Appearance Laminar Turbulent

Viscosity High Lower

Velocity High; ~ 3 – 10 m/s Low; ~ 1 – 2 m/s

Effect Strongly erosive Depositional

Table 2.3. Characteristics of the ‘head’ and ‘tail’ of subaerial debris flows. From Davies (1986).

Debris flows are now known as one of the primary processes that sculpt and erode subaerial steepland valleys as they catastrophically flow down-slope (Mangeney et al., 2010).

Debris flow magnitude is calculated as the total volume of material moved to the depositional area during an event (Hungr et al., 2005). Hungr et al. (1984) described a measure of erosion defined as the yield rate; the volume eroded per metre of channel length. Entrainment of sediment can potentially accelerate or decelerate a flow depending on the nature of underlying erodible material, topography, and dynamics of the flow, which is typically limited to a weak erodible layer (Mangeney et al., 2010). In subaerial environments, the weak erodible layer is typically defined by the presence of loose material in a channel that has been remobilised by previous events. Stock and Dietrich (2006) studied episodic debris flows in

the subaerial steepland valley of Joe’s Canyon, Wasatch Range, Utah, USA. The valley bedrock comprises Palaeozoic-age quartzite, known as the Oquirrh Formation. This study shows abundant abrasion marks and decimetre-sized blocks missing from jointed bedrock along the length of the valley channel (Figure 2.11). At the site of deposition, boulder-fronts were found at the terminal margins and leveés of the debris-flow deposits, which were interpreted to originate from plucked sandstones from the valley channel. Other examples of

‘block-plucking’ have also been observed in the Mesozoic granite and diorite valley floor, San Gabriel Mountains, Southern California and in the Santa Cruz Mountains, Western California. These present-day examples that show active bedrock erosive processes occurring during debris flow passage (Stock and Dietrich, 2003).

Until recently, relatively few physical parameters of debris flows were documented.

Although many field observations and video recordings of debris flows existed (e.g., Davies, 1986, 1990), these data sets did not quantify the amount of sediment erosion and entrainment through debris-flow processes. However, with increased technology, such as the use of automated sensor networks, erosion bolts and force plates drilled in the bedrock of steepland valleys, recent studies have quantified the distribution of basal forces and the complex erosional-depositional processes of debris flows. These studies have helped to understand the physical processes that occur in subaerial environments (Stock and Dietrich, 2003, 2006;

Huggel et al., 2004; Hungr et al., 2005; Jakob et al., 2005; Breien et al., 2008; Santi et al., 2008; Mangeney et al., 2010; Berger et al., 2011; Schürch et al., 2011; McCoy et al., 2013).

Breien et al. (2008) provide a detailed quantitative case study from glacial lake outburst floods, Fjærland, Western Norway. Digital terrain models were generated pre- and post debris-flow events to quantify the differences in elevation change along the passage of the channel path. Figure 2.12-A shows the debris flow track divided into 6 sections, termed the ‘upper-flat’, ‘cliff’, ‘upper-steep (6a)’, ‘mid (6b)’, ‘lower-1 (6c)’ and ‘lower-2’ sections, which are located progressively down-dip from the source area. Cross-sectional profiles are shown from sections 6a, 6b and 6c (Figures 2.12-B, C and D, respectively) to show changes in the width and depth of the channel after erosive debris-flow events.

Figure 2.11. Evidence for bedrock lowering from Stock and Dietrich (2006). (A) Impact lowering resulting in removal of decimetre-sized blocks of quartzite. (B) Plucking and abrasion (C) Tensile failure of quartzite (D) Removal of sandstone grains (~ 0.5 – 1 mm in diametre). (E) 1-3 mm deep groove indicating sustained sliding contact of particle for at least 190 mm along the bed. (F) Post-weathering ‘tent’ feature after debris-flow passage.

Figure 2.12. (A) Profile of debris flow track. Gradient varying from 4° at the top of the valley to 60° at the steep rock-cliff, to ~ 12° at the base of the valley. Dashed lines indicate areas of erosion and deposition. (B, C, D) Cross-sectional channel profiles progressing downslope (from B to D, respectively) of the debris flow gully pre- and post-debris flow. Modified from Breien et al. (2008).

The initial 60° slope at the ‘cliff’ section allowed the flow to gain momentum and velocity. Incipient erosion developed at the base of the steep cliff face (location 6a shown in

Figure 2.12-A). As the debris flow continued to propagate downslope, it eroded the channel bed. The amount of vertical erosion is not evenly distributed throughout the channelised flow length. The lowest yield rates are located in the mid-section of the flow path (80 m³/m) and the highest yield rates (212 m³/m) are recorded in the lower-section of the flow path, occurring just before deposition (Figure 2.13).

Figure 2.13. Graph showing increase in vertical erosion and yield rate at the break-of-slope in a subaerial environment. Data re-plotted from Breien et al. (2008).

The slope gradient (green line, Figure 2.13) decreases at the mid-point position along the slope profile. At this break-of-slope, both the average yield rate (m³/m) and the amount of vertical erosion (m) increase from 96 to 213 m³/m and 1.3 to 5.12 m, respectively. From initiation to the point of deposition, the debris flow volume increased from 25,000 – 240,000 m³ recording a significant increase of sediment entrained into the flow (215,000 m³). This study recorded that incision increased at the change of gradient, suggesting that a change in angle of slope may affect the way that debris flows erode the bedrock. The highest yield rate occurred near deposition, at the end of the flow. Breien et al. (2008) suggest that high vertical and longitudinal shear forces are translated to the underlying bed to produce a positive feedback effect; therefore, debris flows with a higher sediment volume are able to cause more erosion due to higher basal-drag and frictional forces.

Berger et al. (2011) provided a similar detailed quantitative case study from field data on erosive debris flows in the Illgraben catchment, western Switzerland. Using scour sensors, the magnitude of channel-bed erosion was measured downslope from debris-flow initiation.

Erosion was measured up to 0.5 m towards the end of the Illgraben debris-flow fan that deposited at the base-of-slope. Figure 2.14 shows channel-width cross-sectional profiles from topographic surveys before and after a debris-flow event. These results are similar to those presented in Breien et al. (2008). Measurements of pressure fluctuations against the scour sensors in the Breien et al. (2008) and Berger et al. (2011) case studies suggest that inter-particle collisions impact, fracture and loosen the bedrock enabling efficient erosion where shear rates were higher. These studies also found that the rate of erosion depended on sediment volume, meaning erosion increased as the unit discharge increased.