How memory effects, check dams, and channel geometry control erosion and deposition by debris flows

Abstract

Debris flows can grow greatly in size and hazardous potential by eroding bed and bank material, but effective hazard assessment and mitigation is currently hampered by limited understanding of erosion and deposition dynamics. We have collected high-resolution pre- and post-flow topography for 6 debris flows over a 3 km long unconsolidated reach of the Illgraben channel in the Swiss Alps with drone-based photogrammetry. We show that the spatio-temporal patterns of erosion and deposition in debris-flow torrents are highly variable and dynamic. Check dams strongly control the spatial patterns of erosion and deposition. We identify a memory effect where erosion is strong at locations of strong deposition during previous flows and vice versa. Large sediment inputs from subcatchments initially result in new channel erosion through the subcatchment deposits and simultaneous upstream deposition, likely as a result of backwater effects. It is generally believed that erosion increases with debris-flow magnitude, but we show that there is a limit to debris-flow bulking set by channel geometry. These findings provide key guidelines for flow volume forecasting, emphasizing the importance of memory effects and the need to resolve both erosion and deposition in predictive models.

Insights From the Particle Impact Model Into the High‐Frequency Seismic Signature of Debris Flows

“…However, changes in ξ are likely only significant when erosion exposes bedrock (Kean et al., 2015), which is not the case on Illgraben's debris fan in the Rhone valley. Moreover, at Illgraben, changes in ξ concentrate near check dams (de Haas et al., 2020), such that spatial variations in ξ tend to cancel each other out leaving ξ unchanged between events. We observe an empirical relationship between ξ and distance r, which evolves over time depending on the flow front velocity (Figure 2d).…”

“…This also marks the confluence of the Illgraben channel and a steep lateral slope to the east of it, called Sagenschleif (Figure 1d). In recent years, Sagenschleif has produced mass wasting events, including minor debris flows, providing sediments to the Illgraben channel (de Haas et al., 2020). Entrainment of these sediments can increase the debris‐flow volume and impact forces during runout events in the main canyon.…”

“…In our data set, such shallow events are small volume events (Table ) explaining decreasing AIFS along the debris‐flow runout (Figures 3b and 3d). For the small June 10, 2019 events, drone‐derived imagery did confirm unusually strong deposition between 1,200 and 700 m upstream of CD29, locally increasing bed elevation by more than 2 m (de Haas et al., 2020). Furthermore, accounts of debris flows that stop before CD29 (such as the June 20, 2019 event) suggest that small events decelerate as they approach the force plate.…”

Climate Change Impacts on Sediment Yield and Debris‐Flow Activity in an Alpine Catchment

“…The debris-flow force plate is situated just before the confluence of the Illbach with the Rhône river under the bridge of the main road. This location is relatively far from the debris-flow initiation area (∼5 km) and erosion and deposition along the channel on the fan has been observed (Berger et al, 2011a;de Haas et al, 2020;Schürch et al, 2011). We assume this erosion-deposition effect to be negligible compared to total debris-flow volumes and over longer time scales (years) because the channel is stabilized by many check dams.…”

Mountain surface processes and regulation

“…Tichavský et al 20 found that dry spells and extreme precipitation were the main triggers of landslides in the Outer Western Carpathians of Central Europe, and Stoffel et al 21 found that small rockfalls near Monte Fitz Roy in Argentina, could be triggered by earthquakes of moderate intensity, at large distances from their epicenter. De Haas et al 22 identified a memory effect whereby erosion was stronger at locations where there had previously been strong deposition from debris flows. Erosion did not increase with debris-flow magnitude due to a limit of debris-flow bulking set by channel geometry.…”