Slack tide how long

Eventually the current would stop slack but the tide would continue to rise from the ocean. After a while the river would run backward upstream. Well here is an example where the slack and high tide does not coincide. Conversely, a tidal current ebbs when the flow is seaward and the water levels fall. In order to understand the relationship between maximum current, high and low tides and slack water, consider the situation in the open ocean. When the tide flows pulled by the gravitational force of the moon and the sun , the maximum rate of flow occurs during the two hours of mid-flow; as it approaches high water the rate of flow slackens until there is no appreciable movement.

The same pattern occurs on the ebb. It can be seen that slack water coincides with high and low tides and that maximum flow occurs half way between them. Now consider the situation at the Golden Gate see graph at right. Note that maximum current again occurs roughly half way between two periods of slack water, but that slack water no longer coincides with high and low tides.

In this example, slack water after high tide is approximately 1 hour and 40 minutes later; slack water after low tide is approximately 2 hours and 10 minutes later. The principle reason for this is inertia: it takes time to fill and drain the Bay through the narrow opening of the Golden Gate. In other words, that "bulge" of water takes 2 hours to travel the 25 miles between the Golden Gate Bridge and the Carquinez Strait. The flatter the curve the less current, the steeper the curve the greater the current.

I have forgotten all I ever knew about calculus, but I know there's a formula in there somewhere The animated image shown at right illustrates the relationship between maximum current, high and low tides, and slack water. In order for the image to repeat indefinitely, it uses a twelve hour cycle rather than the actual twelve hours and forty minutes between successive high tides. The size of the arrows is intended to illustrate the varying current flowing past the north tower of the Golden Gate Bridge.

The arrows also indicate that slack water is the result of the equal and opposite flow of water, rather than the absence of flow, per se. The cycle begins with high tide at 1 AM illustrating that although the water level begins to recede, it is still moving into the Bay until the rate of flood diminishes to the point where it is balanced by water beginning to flow out of the Bay, and which produces slack water at 3 AM.

Thereafter, the ebb increases to a maximum of 4. The ebb then diminishes until it is balanced by the growing flood at AM. The flood increases and the cycle repeats endlessly. Current —The flowing of water in one direction. Measured in knots. Currents may be affected by tides, wind, barometric pressure, temperature and geography.

Knot —One nautical mile per hour. The misused phrase "knots per hour" is not only incorrect because it's redundant, but it makes you sound like a land lubber. Lubber n. Big clumsy fellow, lout; clumsy seaman.

Basically, tides are very long-period waves that move through the oceans in response to the forces exerted by the moon and sun. Tides originate in the oceans and progress toward the coastlines where they appear as the regular rise and fall of the sea surface. When the highest part, or crest of the wave reaches a particular location, high tide occurs; low tide corresponds to the lowest part of the wave, or its trough.

The difference in height between the high tide and the low tide is called the tidal range. 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.