A physically-based approach for debrisflow hazard mapping is developed to assess the potential hazard with an 50 years return period on an urban area at downstream of Shengou ravine which goes through the Dongchuan city of Yunnan province in southwestern china and has a long historical debris-flow activity. The governing equations are twodimensional Saint-Venant Equations. Particle model and marker-and-cell computational technique are used to calculate distributions of debris-flow velocity and flow depth, and predict the affected area by a debris-flow event with 50 years frequency on the digital elevation grid of the basin. The momentum of debris flows which is related to its impact force is calculated by the product of the velocity and flow depth. The inundation area is divided into four hazard classes from low to very high. Three hazard maps zoned by debris-flow velocity, flow depth and momentum show big differences, especially in the middle of inundation region affected by overbank flow. The velocity and flow-depth mapping indexes overestimate the dangerous area. Compared with the other two variables, the momentum is better as the hazard zoning index in the urban area. According to the hazard map, three maneuvers for reducing the hazard of the debris-flow event are proposed.