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Debris flows may run into urban areas where the flows are affected by the presence of buildings and urban hard surfaces. This study aims to simulate debris flows in the urban environment by considering different natural and urban superficial geomaterial conditions and building blockage effects. The analysis model adopts shallow water equations to describe debris flows, including the bed erosion and debris deposition processes. A target analysis area is characterized using a three-dimensional digital elevation model (DEM) and discretized into cells with different bed material parameters. A computational scheme is then developed to take into account erosion on different surface beds and building blockage effects. The analysis model and the parameters are evaluated by two historical debris flow cases on Hong Kong Island. Using the verified model and parameters, detailed simulations of debris flows in an urban catchment are conducted. The buildings increase the maximum depth and the maximum velocity as the debris tends to run up and deposit in front of the buildings and the flow path tends to be narrowed due to building blockage. The entrainment of the underlying surface materials increases the debris-flow intensity significantly, leading to larger affected areas and travel distances. [ABSTRACT FROM AUTHOR]