To me "barrier will open" sounds more correct. I am not 100% sure if "barrier will be opened" is acceptable. What do you think?
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They are both entirely grammatical, but one may be more suitable, depending on the context.
"When the water-level reaches a critical threshold, the barrier will open" suggests that the barrier is opening without anyone actively doing it — or that it doesn't really matter how it opens: the water pressure may be enough to open it. The important point is that a closed barrier opens.
"When the water-level reaches a critical threshold, the barrier will be opened" suggests that someone is actually and consciously opening the barrier. The important point is that someone has to do something to open the barrier.
The barrier will be opened (passive voice) indicates that the barrier is the object of the verb open and that there is definitely an agent doing that. The barrier will open (active voice) doesn't necessarily indicate that anyone or anything is opening the barrier, simply that it will open.
Open is an ergative verb, that is, one that allows the subject in one clause to be the object in another clause. So, we can say ‘The barrier will open’ as well as ‘The port authorities will open the barrier.’ When such a verb is transitive, as in the second sentence, it can form a passive clause: ‘The barrier will be opened by the port authorities.’