A NOAA Tool for Safe Navigation --
Accurate and timely water level and current information result in significant cost savings and revenue increases. Consider that by weight, 99% of U.S. transport moves by ship and is valued at $500 billion. And in our global economy, U.S. exports are estimated to grow to over 30% of Gross Domestic Product by the year 2000. Containerships might increase revenue from $6,000 to $24,000 per inch from increased draft gained by using better water level information. Relying on better forecasts of water levels and currents, shippers can also minimize delays in entering and exiting harbors. Idle ships cost from $72,000 to $700,000 per day.
NOS has a variety of methods for providing water level and current information and predictions. A new approach, with a potential accuracy higher than what is now possible, will be ready soon. This method uses numerical oceanographic model-based nowcast and forecast systems for bays and harbors and makes use of the latest real-time observational data inputs and the most accurate atmospheric forecasts available.
For decades, U.S. mariners have depended on NOAA's Tide and Tidal Current Tables for the best estimate of expected water levels and currents. These Tables provide accurate predictions of the astronomical tide and tidal currents (i.e. the change in water level and the currents due to the gravitational effects of the moon and sun and the rotation of the Earth). An explanation of the astronomical tides can be found at http://www.opsd.nos.noaa.gov/restles1.html.
The Tide and Current Tables, however, cannot include the important effects of wind, coastal surges and river flow which vary daily. For example, when a storm and accompanying high winds move across a bay, changes in water level can be dramatic (see Figure 1 below).
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| Figure 1. The observed (orange) and astronomically predicted (blue) water level at Baltimore during a storm in October. The rapid draining of the Harbor beginning October 3 was caused by strong northwesterly winds. |
Real-time measurement systems such as Physical Oceanographic Real-Time Systems (PORTS) are a big improvement over tide and tidal current predictions but are useful only for that immediate time. Mariners and others in the marine community still need to know what the water level and currents will be over the next day since it can take several hours to transit a bay or load a ship. Forecast currents will also make oil spill clean-up operations more efficient by improved prediction of spill movement and will improve safety by supplying better predicted drift information for search and rescue operations.
NOS is developing a new approach to scientific forecasting. The agency is in the process of implementing regional, high-resolution hydrodynamic numerical models to simulate, by computer, the flow of water throughout specific bays caused by tidal forcing, winds, buoyancy and changes in bottom depth. An example of water level forecasts in Chesapeake Bay and surface currents in New York Harbor are shown below. Forecast currents can be used by mariners to maneuver in confined channels.
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| Figure 2. Observed and forecast water levels for Baltimore, Maryland (July 29-30, 1998). |
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| Figure 3. Predicted surface currents in New York Harbor in the vicinity of the Bayonne Bridge. |
Regional forecast models are now under development for Chesapeake Bay, New York Harbor and Galveston Bay. Forecast boundary conditions will be provided by one of the coastal models such as the U.S. East Coast Ocean Model or the Gulf of Mexico Ocean Model, each driven by National Weather Service's atmospheric forecast model. Relatively recent developments in real-time telecommunications (including the Internet), improved computing power needed to run the models and the availability of real-time observations will contribute to a successful system.
Regional Models
o Chesapeake Bay
o Port of New York and New Jersey
o Galveston Bay
o Narragansett Bay
Coastal Models
o East Coast Ocean Forecast System
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Last Updated July 2003
