Definition of ebb and flood (tide)

Definition of ebb and flood tides: The following two definitions of ebb and flood can be found in the literature: 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); Ebb is the tidal phase during which the water level is falling and flood is the tidal phase during which the water level is rising. The two definitions do not coincide. The first definition is commonly used for tidal inlet systems: estuaries, tidal lagoons and tidal rivers; the second definition is commonly used 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. Ebb and flood currents do not generally coincide exactly with falling and rising water level. In a tidal inlet system, ebb flow can continue for some time after low water at sea because water stored in the basin is still draining seaward. Similarly, flood flow can continue after high water at sea. The transitions between ebb and flood occur at slack water: high-water slack for the transition from flood to ebb flow and low-water slack for the transition from ebb to flood flow.

In the open sea, the tidal current generally rotates during the tidal cycle rather than reversing through slack water; the terms ebb and flood current are therefore not generally well defined. By contrast, high water (HW) and low water (LW) are well defined. At the beach, the period from LW to HW is called flood; the tide is rising and the waterline generally advances landward. The period from HW to LW is called ebb; the tide is falling and the waterline generally retreats seaward. However, close to the shores of tidal seas, the tidal current is almost everywhere unidirectional along the coast. In the period around HW, the tidal current flows with the coast to the right in the Northern Hemisphere and with the coast to the left in the Southern Hemisphere; this is called the flood direction. The opposite holds during ebb around LW.

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