The stadium has been flooded like that 3 times. There was a game in the 90's where the water in the lower bowl backed up 3 rows. It was over the seats in the 1st row. After that they made changes so the water would drain faster to the field. It wasn't so bad yesterday considering the monsoon rain.
The field itself actually withstood the heavy rain very well especially for a grass field. If it had been Pittsburgh, they would've been playing in mud.