What is the purpose of the weirs in a tower?

A weir is designed to deal with floods or potential floods. Weirs are barriers over which water flows in an open channel. Flow rates in small streams can be conveniently monitored using weirs. A weir in a tower sounds like something from a fantasy classic, but I have discovered that Anglesey, a small set of islands off the coast of England has a smoke tower and weir.

Ynys Gorad Goch consists of two small islands linked by a stone causeway, the smaller having a smoke tower at one end. Two stone weirs form funnel-shaped areas into which fish are swept when the tide is flooding. The force of the tide traps the fish. Upward-sloping grilles across the narrow openings finish the job.

Weirs in a tower allow water to overflow to the tray below. It’s basically making a waterfall effect. However, I cannot be certain that this is the case, as I have no idea what B.M R.E.C.C.N. means.

The only weirs I could find only were William D. Weirs and the kind of weirs that are installed in dams to change the height of a river or the lake behind a dam. I’m pretty sure these are not the kind of weirs this question wants to hear about.

That said, I cannot definitely say. Perhaps the dam one is right. I can almost certainly say that this question does not pertain to William D. Weirs, though.