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Tides and Currents

Modern ship drafts come close to the channel bottom

Ships have greatly increased in size over the years, thus drastically reducing the margin for error in navigation. This image shows how close ship drafts come to the channel bottom as they approach port. Accurate water-level information is crucial for safe navigation in the modern world.

NOAA's National Ocean Service (NOS) has been monitoring sea-level variations for many years. For some U.S. locations, sea-level records exist for more than 100 years. The nation uses water-level data for a variety of practical purposes, including hydrography, nautical charting, maritime navigation, coastal engineering, and tsunami and storm surge warnings. Mariners use the information to advantageously time their approach to and exit from ports. Long-term applications include marine boundary determinations, tidal predictions, monitoring sea-level trends, oceanographic research, and climate research. Bridge, breakwater, and deep-water channel construction also are affected by tidal and current changes.

Water currents are more difficult to measure. In the past, observations of currents were made for only a few days at a time at any particular location. More recently, however, continuous current observations are being made at several locations along the nation's coasts. However, these observation stations are subject to corrosion, marine fouling, and other damage, and are expensive to maintain.

current meter

Current meter installation near Sitka, Alaska. The current meter is an acoustic sensor that measures the speed of particles going by, thus determining the current in a given location.

Within NOS, the Center for Operational Oceanographic Products and Services (CO-OPS) is primarily responsible for predicting and measuring water levels and currents and disseminating this information. CO-OPS collects, analyzes, and distributes such data to maintain safe maritime navigation and waterborne commerce.

PORTS

The Center manages the national network of Physical Oceanographic Real-Time Systems (PORTS®), which reside in major U.S. harbors. PORTS® provides real-time information to ship masters and pilots to avoid groundings and collisions. Such information includes water levels, currents, air gap (clearance between the water surface and bridges), and other oceanographic and meteorological data from bays and harbors via telephone voice response and the Internet.

physical oceanographic real-time system

PORTS® provides real-time data and other navigation products to promote safe navigation for ships. This drawing shows the inter-relationship among the different components of the system.

Water Levels

CO-OPS also manages the nation's National Water Level Observation Program (NWLON). NWLON provides basic water-level information to determine U.S. coastal marine boundaries and to create nautical charts.

It also supports climate monitoring activities, tsunami and storm surge warning systems, coastal processes, and tectonic research. The network consists of 200 continuously operating water-level measurement stations along the U.S. coasts and the Great Lakes regions, many of which have been operating for more than 50 years.

airgap sensor

CO-OPS newest navigational tool, the "air gap" sensor, is installed at the Desmond Bridge in California. These sensors measure the clearance between the water surface and the lowest part of the bridge, thus assisting large ships in determining bridge clearance.

CO-OPS also provides real-time tidal and storm surge information and mean sea level variations and trends for U.S. coastal areas, as well as water level information for the Great Lakes region. In addition, the Center provides predictive information about tides and currents for more than 3,000 locations.

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