In order to operate safely and profitably in New York Harbor, Mariners need to predict three things about the currents. They need to know the time, direction, and velocity. The tides in New York Harbor are semidiurnal reversing (Bowditch p 139), which means twice daily floods and ebbs. There are a number of traditional publications available to independent operators and larger companies often prepare and provide proprietary tables for their fleets. The most popular publication is "Eldridge Tide and Pilot Book”, and is published on a yearly basis. The reference contains a visual representation of surface currents associated with the times of High and Low water at the Battery. It also contains tables of current changes at important geographic locations. These tables document the predicted start of a current flow and the predicted maximum velocity. One such location is the Verrazano Narrows. Recently, the availability of tablet applications has added to the references that can be used. All references are based on predictive models and tables originally produced by the National Ocean Service, a division of the National Oceanographic and Atmospheric Administration. Monitoring devices cannot be installed where they would interfere with safe navigation. For this reason there are no monitoring capabilities in the middle of the Verrazano Narrows. The predictions that can be made for a given location in the harbor are fairly coarse. The current diagrams contained in Eldridge are in intervals of one hour.
The observations were taken on Saturday December 2, 2012. The ebb was scheduled to begin at the Verrazano Narrows at 11:52 AM
There are seven locations for the sampling. All are on shore except one which is the northern-most can in the Buttermilk Channel. They are:
Because buoys are anchored by chains and the chain has to be long enough for the highest water level that may be encountered, buoys are constantly changing position within a range defined by the chain length. The factors that contribute to the position a buoy takes in a current are: 1) the ratio of the length of anchor chain to the water depth at any given time. There must be enough length to the chain that there is some slack at mean high water spring, the average level of high water that occurs at the time of spring tides (Bowditch p 138). This is the level achieved when all factors combine to produce the highest annual tide 2) the velocity of the current at the time 3) the weight of the chain and 4) the size of the buoy.
There are a number of ways to measure the velocity of a current. The oldest are logs, which mariners have used over the centuries to measure the speed of ships through the water. They are cylindrical plugs with curved fins. As the log rotated through the water, the number of complete rotations was recorded. Calculating how many rotations occurred in a given period of time gave the speed. Modern observations can be made using multiple instruments. One is an Acoustic Doppler Current Profiler sensor (Blumberg, et al 1999). These work when immersed in the water and are good for getting velocities throughout the water column at a single spot. When installed together with magnetic sensing both current velocity and direction can be recorded. We used our GPS device to document the surface velocity at the observation location in Buttermilk Channel. The observer at Pier 15 also documented the current velocity using the GPS built into his IPhone. Documenting positions using GPS devices Recording sequential positions using a GPS device does not generate identical values. For each documented position a series of positions have been recorded in order to get an accurate final position.
An observation guide was produced for each of the volunteers. Each was asked to document the conditions of the tide at three times, 30 minutes before the start of the ebb, at the time of the start of the ebb, and 30 minutes after the start of the ebb. They were also asked to document the time they observed the ebb starting.
There were three observation techniques employed depending on the nature of the observation point. At the Verrazano Narrows there are eddies along the shore at the times of current change with the actual change taking place in the center of the channel (Manhattan Waters Surface Currents). This makes it impossible to accurately judge the time of the change from a point on the shore. For this reason the observer used clues to mark the time change. There was a container ship anchored at the south end of Stapleton Anchorage which is just off the coast of Staten Island, on the western edge of the main channel. She was positioned approximately 1.5 km north of the Verrazano Bridge. The vertical angle of the ships anchor-chain reveals the conditions of the current in which the ship lays at any given time. Figures 3 through 6 show how the angle of the anchor chain and the ship’s orientation reveal the ambient current behavior.
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A container ship is large and heavy. When it is riding on an anchor in a strong current the anchor chain will be pulled tight. the ship acts like a weather vane and points in the direction the current is coming from. |
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When the tide goes slack, the weight of the chain causes it to drop to the bottom while pulling the ship closer to the anchor. This results in the chain hanging straight down. |
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When the current first starts to flow in the opposite direction the ship will not experience a swinging impluse until it has slid forward enough to create some tension on the slack chain. This why the chain will actually lead aft initialy. once some tension is on the chain the hull will begin to swing around. |
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Once the ship has finished swinging around and the velocity has risen the chain is stretched out and the ship points in the new direction that the current is coming from. |
At pier 15 and Louis Valentino Pier, direct visual observations were made using visual aides such as floating objects. At Can #7 in the Buttermilk Channel, was also used. However the wake caused by the water passing around the buoy indicated movement even in the absence of floating objects.
The position of the #7 can was taken at three times. Since the observation was planned for a switch from flood to ebb, the order of observations was: 1) during the flood, 2) at slack water, and 3) during the ebb. Ideally the flood and ebb times would have been at the height of the respective cycles but this was not possible due to time constraints. For this reason the flood and ebb positions were documented at one hour before and after the predicted time of slack water. To record a position the side of the boat was positioned approximately 1 meter away from the buoy as any contact made with a buoy is to be reported to the Coast Guard.
The GPS device was used to measure the velocity of the current during the ebb. This was done after marking the buoy’s ebb position so it reveals the velocity that resulted in the ebb position. The tender was put into neutral so that it drifted in the ebb. The wind was negligible and so did not impact the measurement. The track was loaded into the application that accompanies the GPS device. The start and end times and the distance traveled came from the application. It also displayed the velocity but included no fractional component. The velocity was calculated using distance and time to get a more accurate value. No drift track was taken during the time of flood.
Eight points were recorded for each stationary position proposed as an observation location. Each point was recorded more than one minute later than the previous. The positions of the #7 can recorded while aboard the tender were limited to five due to time constraints. The reported position was calculated by averaging the latitudes and longitudes.
The indicated time for the start of the ebb (Eldridge 2012) was 11:52 AM. Slack water began at the Narrows and at Pier 15 at the same time, 12:01 PM. Slack water began at Can #7 in Buttermilk channel at 12:05. The start of the ebb was observed at pier 15 at 12:12. Slack water was called at Louis Valentino pier at 12:16. The start of the ebb was called for the Narrows at 12:19 using the orientation of the ship’s anchor chain on the ship itself. It is reasonable to assume that the ebb actually began earlier because the great bulk of the ship and its chain mean that the forces of the current need to build beyond a certain amplitude before the affects would become visible. The beginning of the ebb at Can #7 in the Buttermilk channel came at 12:25. An obvious start of the ebb was never observed at Louis Valentino pier in Red Hook. The start of the ebb is observed at Pier 15 exactly 20 minutes after the scheduled start of ebb at the Verrazano Narrows. For the following diagrams, Green indicates a flood, Yellow indicates an observed slack, and Red indicates an observed ebb.
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| Initial Slack 12:01 | Initial Ebb 12:19 | All Ebb 12:25 |
The distance between the buoy’s flood position and its slack position was 8.254 meters. The distance between the buoy’s slack water position and its ebb position was 17.574 meters. There was 59 minutes between the flood and slack observations and 1:13 between the slack and ebb observations. The slack/ebb distance was more than twice the flood/slack distance.
The current velocity was measured by using the GPS device. After shutting down the engine and waiting a few minutes to settle into making no way through the water, the track as recorded started at 13:01, ended at 15:14, and traveled 81 m. The velocity (81m/133s) is 0.609 m/s or 2.192 km/h.
The hypothesis that the tide changes at pier 16 twenty minutes after the change at Verrazano Narrows has been confirmed. Despite the inability to confirm the exact start of ebb at the Narrows, a mariner wishing to know what the timing is at pier 15 does not need to know if the tide tables are accurate for the Narrows, only if adding twenty minutes to the schedule for the Narrows gives an accurate schedule for Pier 15. The hypothesis that the current change propagates along the Brooklyn shore is proven false. For a brief time the current was ebbing at either end of the Buttermilk channel while the channel itself is still slack. It is reasonable to conclude that no communication is required between the East River and the Narrows through the channel for the ebb to flow in the East River. The “Manhattan Waters Surface Currents” model agrees with observation. It shows a disorganized flow in the upper bay between Governors Island and New Jersey with the primary source of ebb flow into the Narrows coming from Newark Bay through the Arthur Kills. Eventually the entire upper bay became organized in southerly ebb.
The elapsed time between the recording of the flood/slack A distance and the ebb/slack B distance was roughly equal. However the B is more than twice the size of A. This result suggests stronger ebb than flood. The “Manhattan Waters Surface Currents” model for a nearby location in the Upper Harbor (Buoy 32) shows a similar result for the observation day. The ebbs have a maximum velocity of over 2 knots while the maximum velocity for flood is just over 1 knot. The 32 buoy pattern can be explained by the fact that it is directly downstream of the Hudson River. This relationship cannot explain the results in Buttermilk Channel because in ebb the flow through the deep water channel is east-to-west blocking any Hudson River ebb from entering Buttermilk Channel. The most likely explanation is a resonance between New York Harbor and Long Island Sound resulting in a general flow over time of water from the sound into the area of the New York Bight.
Recording the velocity of a drifting boat is not a good way to document current velocities. It does, however, demonstrate different capabilities of GPS devices and technology. This method is useful for filling in gaps that exist with other deployed recording devices. This method can be translated into floating, self-contained devices that record their position over time using GPS receivers.
In following map shows the eight positions recorded for documenting the postion of pier 15 (Blue markers). The averaged reported position can be seen to be central to the cluster of recorded points (Red Marker). The green marker marks the actual position of the GPS device at recording time. It is unclear if the discrepancy is due to the recording device/system or inaccuracies between the image and the actual latitude/longitude within the mapping software.
The New York Harbor School was very helpful to this project by allowing the use of their vessel “Indy 7” for the observations made in the “Buttermilk Channel”, along with someone to operate her.
I also want to thank the team of volunteers who help record the observations. I also want to thank the team that helped evaluate the observation sites
National Imagery and Mapping Agency. (2002). The American Practical Navigator – Bowditch
Blumberg, A., Khan, L., St. John, J. Three-Dimensional Hydrodynamic Model of New York Harbor Region
The Center for Maritime Systems – Stevens Institute of Technology Manhattan Waters Surface Currents
White, R. E. (Ed.). (2012). Eldridge Tide and Pilot Book
Another resource on Tides and Currents is the NOAA site Physical Oceanographic Real-Time System (PORTS)