Spatio-temporal patterns of debris-flow erosion and deposition in the Illgraben torrent

Debris flows can grow greatly in size and hazardous potential by eroding bed 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 with drone-based photogrammetry in the Illgraben channel in the Swiss Alps. We present erosion and deposition patterns as a result of six debris flows and intensive subcatchment activity over a 3.3 km long unconsolidated reach with check dams, and interpret these erosion and deposition patterns with in-situ flow measurements. We show that the spatio-temporal patterns of erosion and deposition in natural debris-flow torrents are highly variable and dynamic. 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 subcatchments deposits and at the same time upstream deposition 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. Large flows that overtop their channel deposit large amount of sediment in levees and on overbanks, leading to net deposition despite strong thalweg erosion, and thus a decrease in flow volume. These findings provide key guidelines for flow volume forecasting, emphasizing the importance of memory effects and the need to resolve both erosion and deposition for accurate flow volume estimation.