Therefore, you can expect to see two high slacks and two low slacks in a hour period in most places. Peak currents are therefore at the midpoint between high and low tide.
During larger than normal tides, you will also see larger currents. Currents are usually greater in the mid-channel or deepest part of the channel and weaker over shallower water.
How can you tell which way the current is going? One way is to look at a buoy and see where the wake forms behind it. We also happened to time our trip so we were returning to Vancouver against the current, however we used the shallower water near the sides of the channel where the current is weaker to get under the bridge.
Video: Sail against current. Another thing to consider is the interaction between wind and current. When the wind and current are travelling together, the seas are calmer than when they are opposing each other. The turn of the tide can cause a relatively flat but windy channel to build into rolling waves and whitecaps.
When fast-flowing tidal currents are forced to pass through narrow channels, they can create dangerous tidal rapids with standing waves, large back eddies and intimidating whirl pools. At first sight, a passage at peak flood with raging rapids might look impassable, when in fact, you just showed up at the wrong time.
These passages can be navigable in a sailboat, but only at slack current. If you miscalculate the slack and arrive too late, the current may have already become too strong and you may have to wait for hours before the next window of opportunity to pass.
Here, the narrow opening between the Strait of Georgia and Sechelt Inlet sees flows of up to High slack zero current and high tide high sea-level elevation occur at the same time. They often appear to happen at different times in the tide and current tables because tide and current stations are not located in the same place. Thus, the difference in time merely represents the time lag between one station and the other as the water moves between them. High slack refers to when the tide has come all the way up and pauses for a moment, causing current to drop to almost nothing, before reversing direction and going out as the tide drops.
As the tide rises, water moves toward the shore. This is called a flood current. As the tide recedes, the waters move away from the shore. This is called an ebb current.
The movement of water toward and away from the shore is illustrated by the movement of the green seaweed. Log in. Page Discussion. Read View source View history. Jump to: navigation , search.
Definition of ebb and flood tides:. The following two definitions of ebb and flood can be found in the literature [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 ; 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. After clicking on a state, select a tidal monitoring station and click on it. Predicted readings for an area will appear in blue, and actual observations will appear in red. To view a good example of a diurnal tidal cycle, select a tidal station in Louisiana, Mississippi or Alabama. To see semi-diurnal tides, select a coastal monitoring station from a northeastern state such as Delaware or Maine. Note that the tidal range increases as you move further north.
To observe mixed semidiurnal tides, select any one of the tidal stations on the west coast. After selecting a particular harbor, you can view the types and geographic placement of different sensors in that area by clicking on the sensor locations highlighted on the area maps. To view specific data for a particular sensor in graphic format click on the image of data graph you wish to view.
To view the data in ASCII, or text format, scroll down the page of the graphic presentation and click on the link to the data set you want to see.
You can select a particular monitoring station by choosing a state and clicking on it. A map will appear showing the geographic locations of all the monitoring stations in the state. Select one of these stations to view a graph of water levels in real time at that site for a hour period. To view the data in ASCII, or text format, scroll down the page and click on the link to the data listing. Click on the year for which you want tidal prediction data.
This opens a new page with a list of states and other areas for which there are tide stations on the left side of the page. Clicking on a state name displays a list of regions within that state.
Clicking on a region name will present a list of the tidal current stations in the area. These stations are listed geographically. This will make finding the location you are interested in, or a nearby station, simpler. Unlike tide stations, which are normally located along the shoreline, most tidal current stations are located offshore in channels, rivers, and bays.
These stations are often named for the channel, river, or bay in which they are located or for a nearby navigational reference point. A map or some personal knowledge of the area may help you identify stations. Similar to the tidal current prediction Web site above, this site provides you with water level predictions for more than water level stations. The list of stations may be selected from a listing on the main part of the page, or from a sub-listing of stations broken down by state which can be accessed on the left hand side of the page.
You can also use the interactive map feature accessible from the page to search for a tidal station geographically. Skip to content As the tides rise and fall, they create flood and ebb currents.
The relationship between the masses of the Earth, moon and sun and their distances to each other play critical roles in affecting tides. As these two celestial bodies increase and decrease their angles to the Earth, so do the tidal bulges. Frequency of Tides — The Lunar Day Unlike a hour solar day, a lunar day lasts 24 hours and 50 minutes. This occurs because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. This map shows the geographic distribution of different tidal cycles.
Coastal areas experiencing diurnal tides are yellow, areas experiencing semidiurnal tides are red and regions with mixed semidiurnal tides are outlined in blue.
The shape of bays and estuaries, geographic location and weather patterns all can affect local tidal intensity. The Importance of Monitoring the Tides and Their Currents The ability to predict tides and currents is essential for people who rely on the sea for their livelihood.
Marine commerce is one area in which tide and current predictions are critical. How are Tides Measured? Special tide houses were constructed to shelter permanent water level recorders, protecting them from harsh environmental conditions. A monitoring station attached directly to a pier. Today, tide monitoring stations are very accurate, require little maintenance, and are part of a larger nationwide network. References Ross, D. Introduction to Oceanography. Tides Roadmap to Resources These online resources are meant to guide students and educators to topics presented in the online tides and water levels tutorial.
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