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Participants in the annual Great Chesapeake Bay Swim will get help from the National Oceanic and Atmospheric Administration again this year. The Commerce Department agency will use high-tech hardware to set a start time which avoids dangerous currents that have plagued swimmers in past years. The event has previously drawn hundreds of participants who endeavor to swim 4.4 miles from Sandy Point State Park to Kent Island on the Eastern Shore.

In 1992, swimmers encountered strong currents and scores had to be plucked from the bay. Only 48 out of 331 entrants finished. In 1991, 720  swimmers out of 884 had to be taken from the water by rescue boats. Because the predicted low tide does not necessarily coincide with the time that currents are weakest, and because high winds can substantially affect the water surface, the intensity of currents and even timing of the tides, NOAA experts will gather information on weather and current speed in the days immediately before and during the swim.

NOAA's coastal and estuarine oceanography branch will use a device called an acoustic Doppler current profiler. The instrument, mounted on a small catamaran and towed behind a research vessel, is used to measure speed and direction of current from the water surface to the bottom. This information can show where the strongest currents will occur during the race. In early June, NOAA tide experts, with help from the Maryland Geological Survey, will use the profiler to measure currents from one side of the bay to the other at the race course.

The day before the race NOAA will make similar measurements to verify its earlier reading. NOAA'S National Weather Service will forecast wind, waves, and local weather on race day using a Doppler radar system. The measurements will be made in cooperation with Bay Bridge Aviation, WBAL-TV and Automated Weather Service Company.

The NOAA Chesapeake Bay office in Annapolis, Maryland, which organized the agency's assistance in the swim, will have a booth set up at the finish line to distribute information about the bay's tides, weather, and currents, and efforts to restore the bay. Up-to-date wind and weather information can be obtained from these National Weather Service forecast offices: Marine forecast: Weather information from Baltimore Harbor to the Patuxent River, (703) 260-0505. D.C. Metropolitan area: Local and extended forecast, including the Chesapeake Bay, (703) 260-0305.

NOAA Predicting the Tidal Currents for a Safer Swim

The National Oceanic and Atmospheric Administration (NOAA) is helping the hundreds of participants in the Great Chesapeake Bay Swim. When the starting gun goes off, hundreds of swimmers can feel safer, knowing that NOAA has provided a comprehensive forecast of weather, water temperature, tides and currents to assist in setting the optimum starting time for the swim.

The event has previously drawn swimmers from across the country who endeavor to cross the 4.4 mile stretch. In 1993, NOAA became involved in the Great Chesapeake Bay Swim by providing accurate tidal current velocity data along with up-to-date weather forecasts to aid the race organizers in establishing a safe time. With the aid of this new information, 504 of 521 swimmers finished the race. In 1994, results were similar with all but five swimmers completing the strenuous swim.

NOAA's efforts will be coordinated by its local Chesapeake Bay office in Annapolis, Maryland. In the past, NOAA's Coastal and Estuarine Oceanography Branch, in addition to providing tidal current predictions, has used their towed Acoustic Doppler Current Profiler (ADCP) to measure surface currents in real time across the entire Bay. The ADCP computes current velocities throughout the water column by measuring the Doppler shift of a sound transmission. Sound scatters in water due to plankton, sediment, and other particles reflecting the sound back to the ADCP in the form a "backscattered" echo. The ADCP can measure currents in the velocity range of 0 to 5 knots and has a long -term theoretical accuracy of 0.01 knots. This year velocity data will be collected using the University of Maryland - CEES current profiler.

The data gathered from this device, and from NOAA tide gauges during the first few years of NOAA's involvement will allow oceanographers to make comparisons between predicted currents and observed, real time conditions. Predicted current velocities, supplemented with weather information, will allow the swimmers to take to the water with a good, strong ebb tide (1.0 knots maximum), continue through the slack and end early into a relatively weak flood (0.6 knots).

If there happens to be strong wind or heavy rain before and during the race, the currents could be stronger. NOAA's  National Weather Service will be on hand with a direct connection to the forecast office in Sterling, Virginia, to keep tabs on any changes in race time conditions. With all of this information, the start time will be adjusted (within logistical constraints) by the race organizers to minimize the effects of the currents and to make the swimmer's passage across the Bay as safe and rewarding an experience as possible.

Chesapeake Bay Tidal Changes

The primary force causing tides in the Chesapeake Bay is the progression of the tide through the southern entrance from the Atlantic Ocean. A secondary source in the upper Bay is through the Chesapeake and Delaware Canal from ocean tides which have progressed through Delaware Bay. The modification of the characteristics of the ocean tide that take place in the Bay are dependent on the width, depth, and configuration of the estuarine basins and tributaries.

The Chesapeake Bay is fairly unique because it is long enough to contain one complete wave length of the dominant semidiurnal tide: i.e., when one high tide is reaching the head of the Bay near Havre de Grace, the next high tide is just entering the Bay near the Chesapeake Bay Bridge Tunnel. The mean range of tide (the elevation difference between high water and low water) in the Bay varies from 2.8 feet at the Atlantic Ocean entrance, slowly decreasing to 1.0 foot near Annapolis, and then increasing to nearly 2.0 feet near Town Point, Maryland.

An interesting feature of the tide in mid-Bay is that the range of the tide is generally higher on the Eastern Shore than the Western Shore. For example, the range of the tide at Smith Point (mother of the Potomac River) is around 1.0 foot while the range across the Bay in Tangier Sound is around 1.2 feet. The ranges of tide in the tributaries on the western and eastern sides of the Bay show significant increases proceeding up the rivers. For instance, in the Potomac River, the range of tide near the entrance is just about 1.0 foot, while the range of tide at Washington, D.C. is just over 2.6 feet.

The average speeds of maximum flood and maximum ebb currents show similar distribution with the highest average speeds of less than 0.5 knots in the mid-Bay from Windmill Point up to Bloody Point Light. The average speeds are then between 0.5 and 1.0 knots up to the head of the Bay.

There are two kinds of tidal cycles in the Bay. In the southern Bay and in Tangier Sound, the tides and tidal currents are semidiurnal, while in the central and northern Bay, they are classified as mixed. Semidiurnal tides generally have two high and two low waters each day. Diurnal tides generally have one high and one low per day.

Mixed tides are a combination of semidiurnal and diurnal tide types. A characteristic of mixed tides is that on days when two high waters and two low waters occur, the two high tides and the two low tides will be of unequal height. The tides and tidal currents in the Bay described above can be significantly modified by the effects of local and large  scale meteorological patterns. These effects include strength and duration of wind speed and direction, barometric pressure changes, and river runoff.

The tides in the Bay are highly susceptible to modification because the Bay is generally shallow and the tides and the tidal current are very weak. The effect on the tides is highly dependent on the direction of the winds relative to the orientation of the basin, and the scale of the meteorological event. For instance, a local squall line might have dramatic local effects on the tide for a short duration, while a large scale East coast storm affects the entire Bay for several days, especially with added effects of river runoff from the tributaries. However, the characteristics of the Bay are such that even relatively frequent meteorological patterns and associated changes can significantly affect the tides.

Tides, Wind, and Currents

The word "tides" is a generic term used to define the alternating rise and fall in sea level with respect to the land, produced by the gravitational attraction of the moon and the sun. The term tide correctly refers only to a relatively short period, astronomically-induced change in the height of the sea surface: the expression tidal currents relates to accompanying periodic horizontal movements of the ocean water, both near the coast and offshore (but distinct from the continuous, stream flow type of ocean current.)

These astronomically driven tidal waves which move up and down the Chesapeake Bay are easily predicted. Tide tables contain these predictions and are essential tools for marine navigation, fishing, and other water-dependent activities. Additional non-astronomical factors, such as the configuration of the coast line, local depth of the water, bathymetry, and other hydrographic and meteorological influences play an important role in altering the range interval between high and low water, and the times and arrival of the tides.

The meteorological influence, in particular, may skew tidal predictions since weather events are not predictable over long periods of time and cannot be accounted for in tidal predictions. It is not uncommon for actual tides to differ from published tide tables. NOAA is making real time water level data available by phone through Tides ABC, a computer program that provides users with access to real-time information. High tides are produced in the ocean waters by the "heaping" action resulting from the horizontal flow of water toward two regions of the earth representing the positions of maximum attraction of the combined lunar and solar gravitational forces. Low tides are created by a compensating maximum withdrawal of water from regions around the earth midway between these two tidal humps.

The alternation of high and low tides is caused by the daily (or diurnal) rotation of the solid body of the earth with  respect to these two tidal humps and two tidal depressions. The tidal force produced by the moon's gravitational attraction is accompanied by a tidal force of considerably smaller amplitude produced by the sun. The position of this  tidal force shifts with the relative orbital position of the earth in respect to the sun. Because of the great differences between the average distance of the moon (238,555 miles) and sun (92,900,000 miles) from the earth, the tide raising force of the moon is approximately 2.5 times that of the sun.

The gravitational attractions (and resultant tidal forces) produced by the moon and sun reinforce each other at times of new and full moon to increase the range of tides, and counteract each other at first and third quarters to reduce tidal range. When the moon is at new phase and full phase (both positions called syzygy), the gravitational attractions of the moon and the sun reinforce each other. Since the resultant or combined tidal force is also increased, the observed high tides and higher and low tides are lower than average. This means that the tidal range is greater at all locations which display a consecutive high and low water. Such greater-than-average tides resulting at the syzygy positions of the moon are known as spring tides, a term which merely implies a "welling-up" of the water and bears no relationship to the season of the year.

At the first and third quarter phases (quadrature) of the moon, the gravitational attractions of the moon and sun upon the waters of the earth are exerted at right angles to each other. Each force tends in part to counteract the other. High tides are lower and low tides are higher than average. Such tides of diminished range are called “neap tides,”  from a Greek word meaning "scanty." The inflowing, or high tides, are also called “flood tides,” while the outgoing, or low tides are called “ebb tides.”

The movement of the sun and moon are known so we can predict the tides and rely on those predictions in the oceans, but other factors are necessary to consider when predicting the tides of estuaries. The shape of the estuary's  shoreline and bottom can have a dramatic effect on tides. The opening of an estuary is narrow and restricts movement in and out. Weather can also have a dramatic effect on tidal predictions, particularly in a broad, shallow estuary such as the Chesapeake Bay. "Storm surges," caused by low barometric pressures, often accompanied by a continuous strong flow of winds either onshore or offshore, may superimpose their effects upon tides to cause either heightened or diminished tides, respectively.

High pressure atmospheric systems may also depress the tides. The National Ocean Service, a component of the National Oceanic and Atmospheric Administration, maintains a continuous control network of approximately 140 tide gauges, 12 in the Chesapeake Bay, for use in recording and predicting tides.