<p>Due to extreme rainfall in 2010 in the Lusatian Neisse River catchment area, a flood event with a return period of over 100 years occurred, leading to the failure of the Niedów dam. The earth-type dam was washed away, resulting in the rapid release of nearly 8.5 million m<sup>3</sup> of water and the flooding of the downstream area with substantial material losses. The paper analyses the conditions and causes of the dam’s failure, with special attention given to the mechanism and dynamics of the compound breaching process, in which the dam’s upstreeam slope reinforcement played a remarkable role. The paper also describes a numerical approach for simulating a combined flood event along the Lusatian Neisse River with the use of a two-dimensional hydrodynamic model (MIKE21). The flood event occurred downstream from the dam. Considering the specific local conditions and available data set, an iterative solution of the unsteady state problem is proposed. This approach enables realistic flood propagation estimates to be delivered, the dam breach outflow to be reconstructed, and several important answers concerning the consequences of the dam’s failure to be provided.</p>