As already stated the road side drains were once routinely cleaned/flushed out with one of those sucker pipe things on a big truck.
Apparently at one time there were a few of them designated to the Wrexham area, but now there are only a couple and virtually the only time that they are deployed is when a member of the public rings the Council or a councillor and tell them that a road side drain is blocked.
I know this from personal experience and to be fair once an issue has been highlighted the drains are unblocked.