真的那道水閘
這是字面的用法，也是整個字的源頭畫面；先看得到這道門，後面的比喻才有東西可以借。
noun · C1
A gate that can be opened or closed to control how much water flows out of a large lake, river, canal or reservoir.
水閘門
核心畫面
You can picture a floodgate as one heavy gate in a dam wall, with a whole lake pressing against the other side of it. While the gate is down, nothing moves and the lake stays where it is. Lift it, and everything that was waiting arrives at once, fast, and not in any order you chose. Every other use of this word is that same gate, with something other than water behind it.
floodgate 可以想成水壩牆上那一道厚重的門，門的另一邊壓著一整座湖。門關著的時候，什麼都不動，湖就待在原地。…更多收起
floodgate 可以想成水壩牆上那一道厚重的門，門的另一邊壓著一整座湖。門關著的時候，什麼都不動，湖就待在原地。門一拉起來，等在後面的東西一次全到，又快又不照你安排的順序。這個字其他的用法，都可以想成同一道門，只是後面壓著的不再是水，而是案件、要求或忍了很久的情緒。From the word’s history
Wiktionary records floodgate as Middle English flodegate, that is flood plus gate. The compound is transparent: it is literally the gate for a flood, which is exactly the picture the figurative uses borrow.
Wiktionary 記的是中古英文 flodegate，也就是 flood 加 gate。這個合成詞字面很直接——擋洪水的那道門，而後來所有的比喻用法借的就是這個畫面。
Wiktionary entry for 'floodgate' (kaikki:3f6f69a190036ae4:1386668231:10724): 'From Middle English flodegate, flodgate, flodeyate, floodȝate, flodȝete, equivalent to flood + gate.'
Three branches
一開就擋不住的一大波
把門後面的水換成案件、申請、抱怨；門一開，被擋住的一次全湧進來。這是理解上的比喻連結，不是說這個用法比字面的閘門更早出現。
noun · C1
If something opens the floodgates, or if the floodgates open, it suddenly becomes possible for a lot of things to happen all at once which were held back before — for example claims, complaints, applications or requests.
開了先例，一發不可收拾
忍住的情緒潰堤
這一支把門後面的水換成一個人憋著的情緒，開門的往往只是一句好話。
noun · C1
If the floodgates open, or if something opens the floodgates, a person who has been keeping their feelings hidden suddenly starts to cry and let those feelings out.
情緒潰堤