Debris flows are one of the most critical geomorphic modifiers in mountain regions, with potentially disastrous consequences when they occur in developed areas. They involve the gravitational movement of a solid-fluid mixture, with sediment distributed throughout the entire flow depth. When the debris flow channel has a gradient higher than 10%, energy dissipation structures are often installed to reduce the momentum of the debris flow and to stabilize the channel geometry. The Firschnitzbach in East Tyrol, Austria, is a debris flow-prone channel with a series of energy dissipation structures (bed sills) spaced throughout the channel passing through the village of Virgen, Austria. On 4 August 2012, intense rainfall occurred in the Firschnitzbach catchment which triggered two debris flow events, characterized by a series of surges that overtopped the channel and damaged adjacent structures. A series of hydraulic simulations were performed using the Flow 3D software to better understand the processes occurring in the channel, the surge-generating mechanisms and the effectiveness of the countermeasures. The simulations were defined by steady-state conditions, flood hydrographs, and rheological parameters for both, clear-water conditions and debris flows. The simulations provided reasonable results for the overall hydraulic situation and showed that the removal of the countermeasures increased the freeboard, which reduced the overtopping potential, but did not reveal the surge-triggering mechanisms. However, video footage of the event showed that the debris surges with intense sediment concentration periodically stalled, changing the channel geometry through a negative depositional gradient between the bed sills, or bankfull deposition of debris flow material in the channel. The debris flow instabilities that occurred in the Firschnitzbach were caused by the impact and the rapid displacement of the incoming surge into the deposited material of the previous surge, which was caused by the reduction in channel gradient due to the installed countermeasures. As a result, bed sills were removed near critical infrastructure to improve debris flow conveyance and reduce the potential for the unique countermeasure-debris flow interactions that occur in the stepped portion of the channel.