Limitations of RAMMS:Debrisflow as a slushflow simulation tool

Abstract

Slushflows are defined as water-saturated snow (slush), that moves rapidly downslope as a gravitational mass flow. The high-water content of the flow results in a high density, posing a significant danger to construction, infrastructure and people in its path. To reduce this danger, it is important to predict where slushflows may initiate and travel in the terrain. Numerical modelling of the runout process is a valuable tool for both hazard mapping as part of the spatial planning process, and to identify areas that may require early warning systems or mitigation. RAMMS:Debrisflow is a gravitational mass flow runout simulation tool that has been parameterized for debris flows. To use RAMMS:Debrisflow to simulate slushflows the assumptions made in simulation must be checked and the parameterization must be adapted. The parameterization must be adapted from the friction and density of sediments to slush material. To establish a parameter set for slushflows a catalog of slushflow events are needed for model calibration and validation. Several parameter sets have been proposed already for slushflow simulations with RAMMS:Debrisflow, however these were only built on a few events, so further validation is needed. To this end, three events from winter/spring 2023 in Northen Norway were digitized for simulation of the runout. While the parameters represent slushflow runout to some extent, a shortcoming is the simulated runouts are sensitive to the input data. The slushflows released in gentle slopes, however, the model cannot replicate these starting conditions. The solution was to create release areas for events in the part of the slushflow where it rolled over into steeper terrain. To analyze the sensitivity of the flow path to different release conditions we simulated the known slushflow events with varying slope gradients, surface areas and fracture depth. We conclude that input parameters (release areas and release volume) are too sensitive to truly test the capabilities of RAMMS:Debrisflow to simulate slushflows. There must be more work done to standardize the method to delineate the release areas.