In-channel landslide deposits and future debris flows

Debris flows/floods are natural hazards occurring in steep mountain catchments. Debris material mainly derives from processes of channel/channel head, bed erosion, bank destabilization or shallow landslides. More rarely landslide deposits within the channel could be sources of debris. Some studies pointed out the potential increment in debris flow magnitude because the flow may increase its volume and peak discharge after the impacts against an in-channel deposit. The objective of this investigation is to estimate the potential consequences of a debris flow impacting a landslide deposit located in the channel bed.

The project has been developed analysing the rio Rudan catchment (Belluno province, North-eastern Italy), characterized by a frequent occurrence of debris flows in the last decades. In the rio Rudan a wide shallow landslide, highly connected to the transport channel reach,   occurred on the 15^th December 2020 and deposited the majority of the volume within the channel. The landslide was capable to generate only a low magnitude debris flow (of the order of 10’000 m³). Most of the released material (40’000 m³) remained in the channel close to the slope failure zone. In order to analyse the effects of following different types of debris-flows encountering the deposit, different scenarios have been simulated considering the landslide deposit as an entrainable layer. We created five triangular shaped input debris flow hydrographs characterized by different peak discharge (20, 40, 60, 80 and 100 m³ s^-1) and a flow hydrograph representing a debris flood (peak of 20 m³s^-1). Simulations have been performed using the r.avaflow model (version 2.4) for which we employed the two-phase routing model together with the empirical erosion model.

Results of the simulations showed that the magnitude of possible future debris flow events was reduced due to the presence of the landslide deposit. In particular, the peak discharges of the simulated output debris flow hydrograph was reduced of 60-70% compared to the input hydrograph. Even if the coefficient of erosion was set to high values, the quantity of entrained material was low and, surprisingly, most of the solid component of the simulated debris flows deposited in the upper part of the landslide deposit due to the decrease in slope. Most of the erosion process occurred in the lower part of the deposit for the increase in slope. Conversely, in the numerical simulation of the longer-duration debris flood event (or even characterized by multiple peak discharge), the landslide deposit has proved to furnish a constant input of debris material, magnifying the total volume of the event but not the peak discharge. Looking at the results of the simulated case study, we can conclude that the big landslide deposit within the Rudan channel could have a mitigation effect in reducing the peak discharge of future debris flow events considering those debris flows with an important (return periods of 20-30 years) but not extreme magnitude. This highlights the importance of a dedicated modelling in companion cases to avoid excessive costs for interventions and to correctly assess residual risks in case of non-interventions.