This report provides the results of a project called Impact of hillslope debris flows (HDF) funded by the Swiss Federal Office for the Environment (FOEN). The project investigated impacts of HDF on buildings in Switzerland to better estimate damages and optimize future protective measures. HDF are spontaneous mass movements that can cause significant damage, as evidenced by the 2005 storm, which resulted in damages amounting to 150 million CHF. The specific objectives were:• To develop the necessary foundations and components for a practical recommendation to estimate the velocityof HDF.• To precisely evaluate damages to buildings caused by HDF depending on construction types and materials.• To review the current pressure calculation formula for HDF in the SIA Norm 261/1 and compare it with otherexisting formulas.• To provide the foundation for improving existing intensity criteria for HDF, which are currently more focusedon static processes. Key findings include:• The DebrisInterMixing simulation model demonstrated promising results in reproducing the runout distancesfor 45 historical HDF events with release volumes ranging from 22 m3 to 2592 m3. In addition, simulationswere carried out with modi ed release volumes and water contents. Finally, a total of 94 HDF events weresimulated and statistically analysed.• The simulated runout distance varied between 15 and 337 metres. The median of the simulated runoutdistance for the reproduced historical events is 99 m and therefore very close to the median of the observeddistances (96 m).• The maximum front velocity of the HDF reached approximately 15 m/s - the majority were between 4 and12 m/s.• The front velocity of HDF was strongly correlated with the slope gradient, the water content, and theproportion of ne particles.• Over 50% of buildings in Switzerland are constructed of concrete, about 40% of masonry, and 10% ofwood. Historical buildings (pre-1919) are predominantly masonry, making them more susceptible to dynamicimpacts.• Load limit curves for masonry and wood showed signi cant weaknesses under high-pressure loads (>30 kPa),whereas reinforced concrete walls demonstrated good resistance.• The current pressure formula used in the SIA Norm 261/1 was deemed less suitable for HDF. The formulaproposed by Scheidegger (1975) appears to be the most e ective.• The introduction of pressure-based intensity criteria for the hazard assessment of HDF is recommended.