Its pretty similar to how some tsunamis manages to wreak havoc in low lying towns ... a small initial wave inundates the area with a few centimetres of floodwat

Its pretty similar to how some tsunamis manages to wreak havoc in low lying towns ... a small initial wave inundates the area with a few centimetres of floodwater, paving the way for the following large surge, which encounters little friction due to the existing water.