Definition of ebb and flood (tide)

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Definition of ebb and flood tides:
The following two definitions of ebb and flood can be found in the literature[1]:
  1. Ebb is the tidal phase during which the tidal current is flowing seaward (ebb current) and flood is the tidal phase during which the tidal current is flowing inland (flood current);
  2. Ebb is the tidal phase during which the water level is falling and flood the tidal phase during which the water level is rising.
The two definitions do not coincide. The first definition is more usual for tidal inlet systems: estuaries, tidal lagoons and tidal rivers; the second definition is more usual for the open coast.
This is the common definition for ebb and flood tides, other definitions can be discussed in the article


In coastal oceanography, the terms ebb and flood refer to tidal currents. These terms are particularly relevant for tidal inlet systems: estuaries, tidal rivers and lagoons. The ebb current corresponds to seaward flow and the flood current to landward flow. The ebb current does not fully coincide with the falling tide i.e. the period of decreasing water level. The ebb current reverses some time after low water; the water level at sea starts rising already before the inlet system is emptied. Likewise, the flood current reverses some time after high water.

On the open sea the terms ebb and flood are not well defined. The tidal current vector rotates during the tidal cycle; the tidal current is never zero. On the contrary, high water (HW) and low water (LW) are well defined. At the beach, the period from LW to HW (rising tide, corresponding to a landward advancing waterline) is called flood. The period from HW to LW (falling tide, corresponding to retreat of the waterline) is called ebb. However, close to the shoreline, the tidal current is almost unidirectional along the coast. In the period around HW, the tidal current flows with the coast to the right on the N Hemisphere and with the coast to the left on the S Hemisphere; this is called the flood direction. The opposite holds for the tidal current in ebb direction in the period around LW.


Related articles

Ocean and shelf tides
Coriolis and tidal motion in shelf seas


Reference

  1. Darwin, G.H., 1962. The Tides, Publisher:W.H.Freeman and Company