if something opens the floodgates, or if the floodgates open, it suddenly becomes possible for a lot of things to happen which were prevented from happening before:
If this case is successful it could open the floodgates for thousands of similar claims.
There are fears that the floodgates will open and large numbers of parents will take their children out of school.
if the floodgates open, or if something opens the floodgates, someone begins to cry and show their emotions after keeping them hidden:
His display of kindness to her opened the floodgates again, and she began to sob loudly.
a gate that is used to control the flow of water from a large lake or river