More Than You Want To Know About Currents & Tides

#1
I've taken/borrowed/purloined paragraphs from a couple of NOAA sites, rather than link. If the words are black, they are not mine (the sites do not attribute authors, alas). Not sure how important current is to oceanic or southern California anglers, but up here in the Bay, I would have to say it's crucial data, just about as important as tides. Saltwater species are tidal dependent and many of us Bay fisherfolk are keen on water level and water speed.


Tides involve water moving up and down; currents involve the movement of water back and forth. Currents are driven by several factors. Tides are one of these. Wind, the shape of the land, and even water temperature are other facts that drive currents.

A horizontal movement of water often accompanies the rising and falling of the tide. This is called the tidal current. The incoming tide along the coast and into the bays and estuaries is called a flood current; the outgoing tide is called an ebb current. The strongest flood and ebb currents usually occur before or near the time of the high and low tides. The weakest currents occur between the flood and ebb currents and are called slack tides. In the open ocean tidal currents are relatively weak. Near estuary entrances, narrow straits and inlets, the speed of tidal currents can reach up to several kilometers per hour

Q: What is the relationship between "Tides" and "Tidal Currents"?
Comparison of tidal conditions vs. tidal current phases


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. (See graphic for a vivid example of SF Bay differences.)

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.

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. 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.



richtideA.jpg

Monday, April 12 predictions for Richmond, California. Note that the current is often not too close to the tide data. Think of a cooking pan. Prop it up at a slight angle and slowly pour water onto the raised side. Watch it collect and rise at the other end. That's a basic model. If you have nothing else to do, put in tiny islands and obstructions like straits and channels (this will have to be a huge pan! :)) and see how the speed and rate of rise are affected.
 
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Stickman

Well-known member
#4
Good information. I can attest to the importance of tidal currents as I fish the waters of the "West Delta" ( Rio Vista, Isleton, Franks Tract, and west of there to Sherman Lake) some 30 times each year. I NEVER go out on the water without consulting the Tides4fishing web site. There are actually locations that fish well during and "outgoing" current that that are a bust when the current is coming in - and vice-versa. Knowing how tidal currents affect your game plan for fishing is just another piece on the chess board. Without taking the time to learn how these conditions factor in to your quest to find and catch fish, your rate of success drops into the "beginners luck" percentages. They say that 90% of the fish are caught by 10% of the fishermen. Those 10% are the ones that never stop studying the craft, and put in their time. And even at that, we can (and do) get skunked from time to time. If it wasn't challenging, we would soon look for something else to do.