#tide  the periodic rise and fall of the sea level under the gravitational pull of the moon
  supertype:  #periodic_event__recurrent_event__recurrentevent  an event that recurs at intervals
  subtype: {#high_tide #low_tide} {#ebbtide #rising_tide}  #slack_water
  part:  #tidal_flow__tidalflow__tidal_current__tidalcurrent  the water current caused by the tides
  part:  #riptide__rip_current__ripcurrent  a strong surface current flowing outwards from a shore
  part:  #undertide__undercurrent  a current below the surface of a fluid

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