aboideau air lock direct tide dock gate ebb ebb and flow ebb tide flood flood tide flood-hatch floodgate flow flux flux and reflux full tide gate head gate high tide high water lock lock gate low tide low water lunar tide neap neap tide opposite tide penstock refluence reflux rip riptide sluice sluice gate solar tide spring tide thalassometer tidal amplitude tidal current tidal current chart tidal flow tidal range tide tide chart tide gauge tide race tide rip tidewater tideway water gate weir