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# Tides and Water Levels

## The Importance of Monitoring the Tides and Their Currents

Predicting tides has always been important to people who look to the  sea for their livelihood. Commercial and recreational fishermen use  their knowledge of the tides and tidal currents to help them improve  their catches. Depending on the species and water depth in a particular  area, fish may concentrate during ebb or flood tidal currents. In some  areas, strong tidal currents concentrate bait and smaller fish,  attracting larger fish. In addition, knowledge of the tides has also  been of interest to recreational beachgoers and surfers.

*On June 14, 2002, these four industrial cranes, valued at approximately $1.25 million each, arrived in San Francisco Bay from Shanghai, China. Designed to rapidly hoist 40-foot-long containers from super-sized cargo ships, they had to be transported beneath the Oakland Bridge to reach their final destination, the Port of Oakland. The tidal range of San Francisco Bay when these cranes were transported was 4.1 feet and the bridge had a motion of approximately 6 inches. With light chop on the bay and winds blowing at around 10 mph, there was little room for error. With detailed knowledge of the tidal cycle and skillful piloting of the vessel, the cranes cleared the bottom of the bridge by about 6 feet.*

Navigating ships through shallow water ports, intracoastal waterways  and estuaries requires knowledge of the time and height of the tides as  well as the speed and direction of the tidal currents. This was  particularly critical to sailing ships because they had to take  advantage of the tides and currents to maneuver correctly. Knowledge of  tides and currents is still critical because today’s vessels are much  larger than the old sailing ships. The depths and widths of the  channels in which they sail, and the increased marine traffic leaves  very little room for error. Real-time water level, water current, and  weather measurement systems now are being used in many major ports to  provide mariners and port operators with the latest conditions.

Coastal zone engineering projects, including the construction of  bridges, docks, etc., require engineers to monitor fluctuating tide  levels. Projects involving the construction, demolition or movement of  large structures must be scheduled far in advance if an area  experiences wide fluctuations in water levels during its tidal cycle.  Habitat restoration projects also require accurate knowledge of tide  and current conditions.

Scientists are concerned with tides, water levels and tidal currents as  well. Ecologists may focus on the tidal mixing of near-shore waters,  where pollutants are removed and nutrients are recirculated. Tidal  currents also move floating animals and plants to and from breeding  areas in estuaries to deeper waters. Oceanographers or atmospheric  scientists may study tidal fluctuations to better understand the  circulation of the ocean and its relationship to world climatic changes.

*Marine commerce is an area in which tide and current predictions are critical. In June 2002, these four marine cranes valued at $5 million cleared the Oakland Bridge in San Francisco Bay by approximately 6 feet. If you look carefully in the center of the image, you can see a shadowed figure between the crane and the bridge. This is one of the mariners standing on top of the crane and touching the bottom of the bridge as the barge passes beneath it.*

### Tides and Water Levels

              - Welcome
              - What are Tides?
              - What Causes Tides?
              - Gravity, Inertia, and Bulges
              - Changing Angles and Tides
              - The Frequency of Tides
              - Tidal Variations
              - Types and Causes of Tidal Cycles
              - What Else Affects Tides?
              - Monitoring the Tides
              - Old System
              - New System
              - References
              - Roadmap to Resources 
              - Subject Review

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