The design of an efficient Early Warning System for floods in a specific region requires the knowledge of the hydrodynamic behavior of floods that affects flood arrival times and thus the available warning times. In the present work, we attempt to answer the following research question “Is a combined 1D/2D model sufficient to simulate adequately flood propagation in an urban area?” To answer this question, we apply the combined 1D/2D HEC-RAS model to simulate the propagation of the disastrous flash flood that occurred in Mandra in Attica, Greece, in November 2017. Calculations showed that maximum water depths in the two streams of Mandra Soures and Agia Aikaterini ranged from 1.2 to 6.0 m, while along Koropouli Street that was one of the main streets, which suffered severe flooding, they ranged from 1.00 to 2.65 m and were generally in good agreement with measurements. The calculated inundation area showed a practically excellent agreement with the observed area, while calculated flood arrival times were close to the actual values.