Online Tide Charts
Slack Tide Info
What is the relationship between "Tides" and "Tidal Currents"?
The vertical rise and fall of the tides, created by the gravitational force of the Moon and Sun acting on the oceans water, also creates a horizontal motion of the water in the bays, harbors and estuaries. These are tidal currents. In general, as the tides rise there will be a current flowing from the oceans into the bays, harbors and estuaries; this is termed a "flood current". As the tides fall there will be a current flowing towards the oceans; this is termed an "ebb current". There are also periods when there is little or no horizontal motion of the water; this is called "slack water".
Many professional and recreational users of tide and tidal current information have a "rule of thumb" to assume a relationship between the times of high/low tides and the times of the currents. That the times of slack water will be at the same time as the high and low tides, and that the flood and ebb current will occur between the high and low tides. Unfortunately, this assumed "rule of thumb" does not hold for most locations.
The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three "base case" conditions. The first is a "standing wave" type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid way between the high and low tides. The second is a "progressive wave" current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide. The third case is a "hydraulic current". In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same.
Hydraulic currents occur at a limited number of locations. Some examples would be:
• the Chesapeake and Delaware Canal, which connects the Chesapeake Bay and Delaware Bay
• the East River in New York, which connects Long Island Sound to New York Harbor
• certain sections of the Intra Coastal Water Way (ICWW)
• between barrier islands which create different tidal conditions on opposite sides of the island
Progressive currents are most common at the oceanic entrance to many bays and harbor. Standing wave conditions are most common at the head (most inland point) of larger bays and harbors. Most areas of the coast will fall somewhere in between a progressive and standing wave current. [Example (jpg)] The exact relationship between the times of high and low tides and the maximum current or slack water is unique to each location and cannot be determined from a generic "rule of thumb".
Because the tidal currents are created by the same forces which cause the tides, the currents can be predicted in much the same way as the tides. Observational data on the currents at a location can be analyzed using the same methods employed to analyze tides, and the results of that analysis can be used to generate predictions of tidal currents. However, because the relationship between tides and tidal currents is unique to each location, tide predictions and tidal current predictions are generated separately.
• Tide predictions provide the times and heights of the tides.
• Tidal current predictions provide the times and speed of maximum current and times of slack water.
It is up to the user to insure that they are using the correct type of predictions for their activities.
What is the relationship between "Tides" and "Tidal Currents"?
The vertical rise and fall of the tides, created by the gravitational force of the Moon and Sun acting on the oceans water, also creates a horizontal motion of the water in the bays, harbors and estuaries. These are tidal currents. In general, as the tides rise there will be a current flowing from the oceans into the bays, harbors and estuaries; this is termed a "flood current". As the tides fall there will be a current flowing towards the oceans; this is termed an "ebb current". There are also periods when there is little or no horizontal motion of the water; this is called "slack water".
Many professional and recreational users of tide and tidal current information have a "rule of thumb" to assume a relationship between the times of high/low tides and the times of the currents. That the times of slack water will be at the same time as the high and low tides, and that the flood and ebb current will occur between the high and low tides. Unfortunately, this assumed "rule of thumb" does not hold for most locations.
The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three "base case" conditions. The first is a "standing wave" type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid way between the high and low tides. The second is a "progressive wave" current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide. The third case is a "hydraulic current". In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same.
Hydraulic currents occur at a limited number of locations. Some examples would be:
• the Chesapeake and Delaware Canal, which connects the Chesapeake Bay and Delaware Bay
• the East River in New York, which connects Long Island Sound to New York Harbor
• certain sections of the Intra Coastal Water Way (ICWW)
• between barrier islands which create different tidal conditions on opposite sides of the island
Progressive currents are most common at the oceanic entrance to many bays and harbor. Standing wave conditions are most common at the head (most inland point) of larger bays and harbors. Most areas of the coast will fall somewhere in between a progressive and standing wave current. [Example (jpg)] The exact relationship between the times of high and low tides and the maximum current or slack water is unique to each location and cannot be determined from a generic "rule of thumb".
Because the tidal currents are created by the same forces which cause the tides, the currents can be predicted in much the same way as the tides. Observational data on the currents at a location can be analyzed using the same methods employed to analyze tides, and the results of that analysis can be used to generate predictions of tidal currents. However, because the relationship between tides and tidal currents is unique to each location, tide predictions and tidal current predictions are generated separately.
• Tide predictions provide the times and heights of the tides.
• Tidal current predictions provide the times and speed of maximum current and times of slack water.
It is up to the user to insure that they are using the correct type of predictions for their activities.
