These exercises were created by Dr. Karen Grove (© 1998 Grove) for use in the Introduction to Oceanography course offered by the Department of Geosciences at San Francisco State University. Other educators are welcome to use these exercises in their own classes. Please send comments to kgrove@sfsu.edu. Course home page: http://geosci.sfsu.edu/courses/geol102/home.html Commercial use is prohibited.
Voyage 6: Tides and currents in San Francisco Bay
Instructions: First print copies of the Lag-time questions and Real-time questions. Next read the text and examine the images on the computer and write answers to the questions on your paper copies. When you are satisfied with your answers, type them into the computer and submit You may also wish to print the text and images as a study aid.
Don't forget to check out the course web site where you can obtain preparatory information before each class and summaries that are posted after the class meets. Click here to bring up a new browser window with the course site. You can also use this site to display the text and images part of the voyage on a second window so that you don't have to print anything.
Estuaries such as San Francisco Bay are partially isolated from the open ocean by land and are, for the most part, protected from the onslaught of strong wave action. The action of daily tides is therefore generally a more important process in the Bay than the action of waves. Twice each day the level of the ocean rises and falls with the changing tidal cycle. In the Bay, the rising and falling water must all enter through the narrow opening at the Golden Gate, where strong tidal currents are generated. Tidal currents move water landward during rising (flood) tides and seaward during falling (ebb) tides.
Along the California coast, we have mixed semi-diurnal tides, meaning that there are two unequal high tides and two unequal low tides during each day (Figure 1 below). The tides are caused by the gravitational pull of the moon and the sun, and the tidal range changes in a regular pattern as the moon orbits the earth every 28 days. Tides with the greatest range between high and low levels are called spring tides, although they occur during all seasons of the year. Spring tides occur twice each month, during the full and new phases of the moon (Figure 1A). Tides with the smallest range between high and low levels occur during the first and third quarter phases of the moon (Figure 1B) and are called neap tides.
Figure 1A. Spring tides at Ocean Beach during a full phase of the moon (8-9 August 1998). Note that tides are measured with respect to the Mean Lower Low Water (MLLW), which is the long-term average of the lower of each day's two low tides. Thus minus tides (less than 0, the MLLW datum, illustrated by a white horizontal line on the graph) are times of extremely low water levels. The yellow horizontal line is the mean of all tidal measurements. The vertical axis is in feet, while the horizontal axis is in hours.
Figure 1B. Neap tides at Ocean Beach during the first quarter phase of the moon one week earlier than Figure 1A (1-2 August 1998).
Large volumes of water move into and out of San Francisco Bay as the tidal level of the Pacific Ocean just outside the Golden Gate changes each day. When the tide is changing from low to high levels, a flooding current moves water inland from (and through) the Golden Gate. When the tide is changing from high to low levels, an ebbing current moves water from inside the Bay toward (and through) the Golden Gate.
To help people predict the currents more precisely, the U.S. Geological Survey (USGS) and the National Oceanography and Atmospheric Administration (NOAA) have set up a San Francisco Physical Oceanographic Real-Time System (SFPORTS). The web site contains continually updated data that are measured by instruments deployed within the Bay. SFPORTS is designed to provide crucial real-time information to mariners, oil spill response teams, managers of coastal resources, and others about San Francisco Bay's water levels, currents, salinity, and winds.
Figure 2. Physical oceanographic measurements are made at 5-10 locations within the Bay. These measurements are input into a computer model that is used to generate maps of current direction and speed throughout the Bay, not just at the measurement locations. The map above illustrates bathymetric (depth) variations throughout the Bay that are used in the computer model. Map colors correspond to depth in meters (see scale at bottom of image). The black area is land above water. The map has axes that show horizontal distance in hectometers (divide the number by 10 to get kilometers).
The diagrams below are examples obtained from the SFPORTS site during rising (flood) and falling (ebb) parts of the tidal cycle. The tide charts show how measured and modeled tidal levels compare. The maps depict the tidal current direction and speed generated by the model for each part of the tidal cycle.
The Flooding Tide
Figure 3A. This plot was collected at 9:30 AM on 5 August 1998 during a rising (flood) tide (note the vertical light blue line). The full moon was on 7 August, so the tidal range (elevation difference between the highest and lowest tides) was increasing toward spring-type (large) ranges.
Figure 3B. This model of the Central Bay was generated from measurements collected during the part of the cycle delineated (by the light blue line) in Figure 3A. The arrows are vectors; that is, their orientation shows the direction of current flow, and their size shows the speed of the current. The arrow in the upper right corner has a speed of 4 knots (about 2 meters/sec or 4.5 feet/sec). Smaller arrows have proportionally smaller speeds.
The Ebbing Tide
Figure 4A. This plot was collected 6 hours after Figure 3A, at about 15:30 (3:30 PM) on 5 August during a falling (ebb) tide. Although the tidal range was large during this time of month (spring tides), the larger fall was in the middle of the night (about midnight to 4:00 AM), when the currents would have been stronger.
Figure 4B. This model of Central Bay currents was generated from measurements collected during the part of the cycle illustrated in Figure 4A.
What's Happening Right Now?
Click here to go to the SFPORTS web site and see what the tides are doing in San Francisco Bay now. Click on the "Get Data" button on the left side of the page to obtain a plot of the tidal cycle (as in Figures 3A and 4A). Do not change any of the plot settings. NOTE: This site may take a minute or two to load. If you cannot access the site, look at the tidal charts in today's newspaper or go to the first web site listed in the credits, where you can get tidal information for any location.
Click here to see a map with measurement locations and the most recent wind and tide data from the Bay. For each location, the left part of the plot shows wind direction and speed (in knots), and the right part of the diagram shows the water level, with arrows showing whether the level is rising (upward-pointing arrow), falling (downward-pointing arrow), or slack (horizontal arrow) during the tidal cycle. A chart at the bottom of the diagram shows the data from each station in words. If you cannot access this site, go to the tidal charts in the course reader to see the predicted tides for the day.
OPTIONAL: Click here to see a map with measurement locations and the most recent current data from the Bay. For each location, the left side of each plot shows the direction and speed of the tidal currents (in knots), and the right side of each plot shows the water level, with arrows pointing upward for flooding currents and downward for ebbing currents. A horizontal arrow shows a slack current.