Investigating the interaction between debris flows and groundwater and implications for debris flow erosion

Along their flow paths debris flows can grow as they entrain sediment from the bed and banks of their channels. This entrainment has come into focus in the simulation of debris flows, in order to achieve more accurate predictions of debris flow behaviour. One of the factors that influences the entrainment process is the water saturation of the channel bed. Könz (2023) has built a model that integrates this saturation into the RAMMS debris flow model. That model relies mostly on antecedent precipitation to calculate the water saturation. While precipitation is an important factor in soil moisture content there are other elements that result in the water saturation of the banks and bed of the channel. This thesis aims to improve the understanding of the importance of infiltration of water from the creek and from the debris flow itself on the water saturation of sediments in a debris flow channel. It also investigates the links between the saturation and erosion patterns. To achieve this a case study was conducted on the situation at the Illgraben. Field investigations and simulations have been conducted. From the data that was gathered a conceptual model of the water saturation around the channel has been constructed. The ERT measurements have been instrumental in understanding the hydrogeological situation. They suggested that there is a saturated zone below the channel, that does not extend to the surface. There is also no groundwater table visible down to 20 m. The depth to which this zone extends matches the inverted groundwater table that is seen in the simulation. The resulting conceptual model suggests that there is a perched groundwater table beneath the channel. This groundwater table is partially connected to the stream. The groundwater table is disconnected from the deep groundwater table. It is limited in its depth by either an impermeable layer or an inverted groundwater table. Comparing this model to data on erosional patterns, observed at the Illgraben, it provides a possible explanation for the proportionally higher rate of erosion seen with larger debris flows. It would be possible that large debris flows erode the unsaturated layer above the groundwater and reach the saturated sediments. These more erodible sediments would then lead to an overall deeper erosion. Overall, the results of this thesis suggest, that examining the groundwater beneath debris flow channels could help in furthering the understanding of debris flow erosional patterns and thus improve debris flow modelling.