PART IIIA. Background on tides and currents

Because estuaries are partially isolated from the open ocean by land, they are, for the most part, protected from the onslaught of strong wave action. In San Francisco Bay, swell waves from the ocean enter the Bay through the Golden Gate (south of the Central Bay label in Figure 1), but they dissipate quickly and do not propagate throughout the rest of the Bay. Nevertheless, local winds often generate short-period sea waves that can produce quite rough, choppy conditions at the Bay's surface.

The action of the daily tides are generally a more important process in San Francisco Bay than the action of waves. Twice each day the level of the ocean rises and falls with the changing tidal cycle. Along open-ocean beaches, we experience the changing tide as a simple rising and falling of the water level. In the Bay, however, the rising and falling water must all enter through the narrow opening at the Golden Gate, where strong tidal currents are generated. Each day, an enormous volume of saltwater moves in and out of the estuary with each tidal cycle. This quantity is known as the tidal prism and is equal to nearly one-fourth of the Bay's total volume. Throughout much of the year, San Francisco Bay is a partially-mixed estuary. If needed, return to the diagram in Part I that shows the different types of estuaries.. Look at the salinity distribution of the water and imagine this package of water moving 3 to 9 km (2 to 6 miles) seaward and landward twice each day. This tidal action causes strong currents that move water landward during rising (flooding) tides and seaward during falling (ebbing) tides. Like at the beach, the water level of the Bay also rises and falls.

Anyone who goes onto the Bay must know something about tidal currents, which vary greatly from place to place and from time to time. To understand how tidal currents vary from place to place, we must look at the Bay's shape. Figure 1 (below) is a map of San Francisco Bay that also shows variations in depth (bathymetry). The amount of water that moves back and forth with the tides is greatest in the deeper areas, where tidal currents tend to be strongest. Currents also tend to be stronger as one gets closer to the Golden Gate, where water from the ocean enters.

 

 

 

Figure 1. Map showing shape of San Francisco Bay and water depths along its length (see scale at bottom). Image is from the San Francisco Physical Oceanographic Real-Time System (SFPORTS) web site, operated by the U.S. Geological Survey and the National Oceanography and Atmospheric Administration (NOAA).

1. Describe the bathymetry of the Bay, that is, the depth variations. What is the range of depths and where are the areas of greatest depth and the areas of shallowest depth?

2. Look at the Golden Gate, which has the greatest depths in the Bay. How deep is the water at the Golden Gate in meters? in feet? (1 meter = 3.3 feet) Why is this location the deepest part of the Bay? (Hint: it has something to do with the tides.)

To understand how tidal currents vary from time to time, we must look at the characteristics of the tidal wave. Characteristics to look for are the times of the maximum high tidal level and the minimum low tidal level and the range between the height of the high tide and low tide.

Along the California coast, we have mixed semi-diurnal tides, meaning that there are two unequal high tides and two unequal low tides in each roughly 24-hour period (Figure 2 below). The tides are caused by the gravitational pull of the moon and the sun, with the tidal range changing in a regular pattern as the moon circles the earth every 28 days. The tides with the greatest range, called spring tides (they happen during all seasons of the year), occur during the full and new phases of the moon (Figure 2A). Neap tides, with the least tidal range, occur during the first and third quarter phases of the moon (Figure 2B).

Note that tides are measured with respect to the Mean Lower Low Water (MLLW), which is the average of the lower of each day's two low tides each day. Thus minus tides (less than 0, the MLLW daum) are times of extremely low water levels (Figure 2A).

Figure 2B. Spring tides at Ocean Beach during the full phase of the moon.

Figure 2A. Neap tides at Ocean Beach during the first quarter phase of the moon one week earlier. Graphic output for A and B was created using the WWW Tide and Current Predictor, developed by Dean Pentcheff and David Flater at the University of South Carolina.

3. Examine Figures 2A and 2B. What is the maximum tidal range (that is, the difference in elevation between the highest and lowest tidal levels of the day) in Figure 2A (note that with minus tides you must add the elevation below 0 to the elevation above 0)?; in Figure 2B? What generalizations can you make about the difference in tidal range during neap tides and spring tides?

If tides had true 24-hour periods, each low tide would be 12 hours after the last, and every high tide would be 12 hours after the last.

4. Look at the times of the two high tides and the two low tides on Figure 2A. What is the time difference between successive low tides? between successive high tides? What generalization can you make about the relationship between the length of the tidal period and the 24-period of a day?


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