Van Coc Lake located in the Dan Phuong and Phuc Tho, Hanoi, Vietnam has an area of 30.83 km2. It is usually dry and includes arable land and residential areas. However, to protect Hanoi City from catastrophic floods, in emergency situations, it receives the floodwater from the Red River as a regulating reservoir and drains the floodwater to the downstream Day River through the Day weir. We performed numerical simulations to assess the movement of floodwater in this lake and evaluated the impact of floodwater on its residential areas. A two-dimensional depth-integrated hydrodynamic model was constructed to simulate inundation in this lake. The model was validated and the effects of Manning’s coefficient of roughness were examined. In case of emergency, it has to receive an inflow of 2,500 m3/s into the area. Inflow rates of 1,200 and 600 m3/s were also examined to compare the results. The simulation results indicate that this lake was entirely inundated in 13, 28, and 56 h by inflow rates of 2,500, 1,200, and 600 m3/s, respectively. Between 66% and 99% of residential areas were inundated. The areas with highest velocity were primarily focused behind the gate with a velocity of 1.5–1.8 m/s.