The cyclical movement of ocean water is governed by the gravitational forces of the Moon and the Sun. These astronomical influences cause a regular rise and fall in the vertical sea level, which is what most people associate with tides. Simultaneously, this vertical movement drives a horizontal flow of water, known as the tidal current or tidal stream, which is particularly noticeable along coastlines, within bays, and in river estuaries. Understanding this dual nature of tidal movement is important for coastal navigation, marine biology, and various human activities near the shore.
Defining the Flood Tide
The flood tide is the period during the tidal cycle when the water level is actively rising relative to the land. This phase begins at low water, the point of minimum vertical height. The sea level steadily increases until it reaches high water, its maximum vertical elevation. The entire period between low water and high water is referred to as the flood tide. During this time, the shoreline advances as the ocean encroaches onto the land. In a common semi-diurnal tidal system, the time it takes for the water to rise is approximately six hours and 12.5 minutes.
Tidal Current and Ebb Comparison
While the flood tide describes the rising water level, the term also refers to the flood current, the horizontal movement of water associated with this rise. The flood current is characterized by water flowing toward the land or moving upstream into a bay, harbor, or river. This current is a physical mass of water moving horizontally, driven by pressure differences created by the rising tide.
The flood current is the direct opposite of the ebb current, the seaward or downstream flow of water. The ebb current happens during the ebb tide, the phase between high water and low water. The strength of both the flood and ebb currents varies, typically reaching maximum speed around the midpoint of their cycles.
The transition between the flood current and the ebb current is marked by slack water. Slack water is the brief period when the tidal current velocity is near zero, meaning the horizontal movement comes to a near standstill before reversing direction. This period of minimal current occurs twice in a tidal cycle. This moment of current reversal is a crucial marker for navigators, as it is often safer to move through channels and inlets when the water is relatively still.
Key Terminology and Markers
The flood tide is bounded by two specific vertical markers: low water and high water. Low water marks the end of the ebb tide and the beginning of the flood tide. High water is the maximum height the sea level reaches, indicating the conclusion of the flood tide phase.
The total vertical distance the water travels between low water and the following high water is called the tidal range. This range can vary widely based on location, from less than one meter in open ocean areas to over 16 meters in funnel-shaped bays. The tidal range is highest during spring tides, when the gravitational forces of the Moon and Sun align, and smallest during neap tides, when these forces are at right angles.
It is important to distinguish between high water (the maximum vertical height) and high water slack (the minimal horizontal current). High water refers to the vertical movement, while high water slack refers to the horizontal current velocity. These two events do not always occur at the exact same time, particularly in channels and estuaries, where the momentum of the moving water can cause the current to continue flowing for a short time after the water level has peaked.