Rodney says "There is a tsunami heading towards the city." Tsunamis hundreds of feet high do not occur in open water. In open water they travel fast but are only small. They only grow only when they hit shallower water.
The wave is similar in size and destructive power to a tsunami near the shore, without actually being one. This was a hurricane wave charged by extreme wind over a much larger area than Earth can provide.
If you look closely at the trees "blowing in the storm" out the window of the gate room you can see the stage hand shaking them to create this effect.