Tidal locks Lock (water navigation)

sea lock @ bude, cornwall

a tidal lock lock connects tidal non-tidal water. includes lock between tidal river , non-tidal reaches, or between tidal river , canal, or sea lock. however, term refers lock method of operation affected state of tide. examples:

a canal joining river levels lower canal. needed ordinary lock, gates pointing canal. lock used long tide high enough float boats through lower gates. if near low tide lock becomes unusable, gates can barred (and become reverse flood gate , holding water in canal). arrangement applies sea locks (e.g. bude canal).
a canal joining river below it, can rise above (at high tides, or after heavy rain). 1 pair of gates can made bidirectional, i.e. inward-pointing gates supplemented pair pointing out river. when river higher canal, normal gates drift open, additional pair of gates can closed protect canal, , prevent navigation river. in effect, have added flood gate.
as above, safe navigate when river higher canal. lock bidirectional (two pairs of oppositely pointing gates @ each end) allow boats pass @ normal river levels. @ extreme low or high tides unsuitable navigation, appropriate sets of gates barred prevent passage.