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					                                                Surface Current Mapping in the Lower Chesapeake Bay
INTRODUCTION
                                                                                                                                                                                                 DATA PRODUCTS & RESEARCH OPPORTUNITIES
High frequency RADAR antennas are used to observe the
surface circulation patterns in the lower Chesapeake Bay and                                                                                                                                     The basic products are hourly surface velocity maps (Fig 4).
provide near real-time hourly surface current velocity maps to                                                                                                                                   Other data products include 50 hour averaged (sub-tidal)
the public, maritime and scientific communities. This velocity                                                                                                                                   currents (Fig. 5), time series (Fig. 6), tidal ellipses (Fig. 7), and
data may be applied to search and rescue operations, maritime                                                                                                                                    particle trajectories. Special data products tailored to specific
navigation, pollution tracking, beach erosion management,                                                                                                                                        users may be developed on a trial basis. Combining the velocity
fishing and research of coastal ocean processes.                                                                                                                                                 data with concurrent observations such as wind and water level
                                                                                                                                                                                                 measurements from the National Oceanic and Atmospheric
The most widely used current meters to date have gathered                                                                                                                                        Administration and river discharge from the U.S. Geological
velocity data at a single point or along a vertical/horizontal profile                                                                                                                           Survey will allow the study of how circulation patterns vary under
in the water column. The radio surface mapping of a wide                                                                                                                                         different forcing conditions.
coverage area offers a unique perspective allowing for                        Figure 3. Radial current velocities produced by CBBT (left) and Ocean View (right) antennas. The
                                                                              vectors represent the speed of the current going towards or away from the antenna. They are spaced at
development of new data products to a wider range of users.                   5 degree intervals in direction and divided into 1.5 km range bins. The combination of data at these two
This project aims to facilitate the use of this data by identifying           stations produces the total surface current vector map shown in Figure 4 below.                                    Figure 7. Tidal ellipses in the
potential users and incorporating their input into the                                                                                                                                           Lower Chesapeake Bay calcu-
                                                                                                                                                                                                 lated over a 14 day period. The
development of data products.                                                                                                                                                                    major axes of the ellipses are
                                                                                                                                                                                                 oriented along the channel. The
The Center for Coastal Physical Oceanography at Old Dominion                                                                                                                                     area of tidal ellipses is affected
                                                                                                                                                                                                 by the river confluence, coastal
University, through a project funded by CIT and NOAA, operates                                                                                                                                   curvature and changes in the
two CODAR standard range (25 MHz) HF RADAR antennas in                                                                                                                                           bathymetry. Smaller tidal ellipses
the lower Bay: one located at Ocean View (Fig. 1) and another                                                                                                                                    occur near the mouth of the
                                                                                                                                                                                                 Elizabeth River. Also note that
on the 4th island of the Chesapeake Bay Bridge Tunnel (Fig. 2)                                                                                                                                   along the coast of Ocean View
(Green markers on Figure 8.)                                                                                                                                                                     (southern Bay coastline), the tidal
                                                                                                                                                                                                 energy is concentrated almost
                                                                                                                                                                                                 exclusively along the major axes.



                                                                                 Figure 4. Example of hourly average surface current vectors during flood tide (left) and ebb tide
                                                                                 (right) with a dominant SSW wind.


                                                                                                                                          a)       100 cm/s




                                                                                                     a)
                                                                                                                                                                                                                                              Chesapeake Bay Bridge




 Figure 1. CODAR Antenna located      Figure 2. CODAR Antenna located on
 on the Community Beach at Ocean      the 4th Island of the Chesapeake Bay                                b)

 View.                                Bridge Tunnel.                                                                                      b)      100 cm/s


HF RADAR MAPPING
                                                                                                                                                                                                                   Ocean View


The antennas transmit radio signals and listen for strong returns
reflected off of sea waves. The speed derived from the Doppler
shift of the return signals represents the combined speed of the               Figure 5. Sub-tidal (residual) surface current in
wave and the surface current underlying the wave. Using the                    the Lower Chesapeake Bay (50 hours                       Figure 6. Time series current velocity for 50 hours at
                                                                                                                                        the two points marked on Figure 5 (black circles).       Figure 8. Mean flow over 25 hours (sub-tidal flow) shows surface outflow
principles of Bragg scattering and the deep-water dispersion                   average). The flow shows a counter-clockwise
                                                                               recirculation in the north area and a clockwise          The flow shows the semidiurnal component of the          in the study area that is typical of estuarine circulation. This is an
equation, the theoretical phase speed of the wave is calculated                recirculation in the south area. At the                  tide and indicates a maximum speed of 71 cm/s            example of the real time visualization through the National HFRADAR
and subtracted out, leaving the speed of the surface current.                  Chesapeake Bay mouth, the currents show                  during ebb. The tidal current at point (a) shows a       Network Gateway. The green markers indicate antenna locations.
                                                                               typical estuarine outflow. During the averaging          nearly 1 hour delay with respect to point (b).
                                           In an HF RADAR network, radial      period, the dominant winds were 10 knots from
                                           maps (Fig. 3) from two or more      the SSW.
                                           remote sites are combined at a
                                           central station to make a total     ACKNOWLEDGMENTS                                                                ACCESS TO DATA                                                      CONTACT
                                           current vector map (Fig. 4).
                                                                               This project is supported by the Center for Innovative                         Data are freely available to anyone on the project                  Old Dominion University, Norfolk, VA
                                           Communication between the           Technology (CIT) and the National Oceanic and                                  website (www.lions.odu.edu/org/cbc) or on the                       Center for Coastal Physical Oceanography
                                           remote site and the central site    Atmospheric Administration (NOAA). Thanks to CODAR                             National HFRADAR Network Gateway                                    •Teresa Garner (garner@ccpo.odu.edu)
                                           is through a computer modem         for technical support, and to the City of Norfolk and
                                           or Ethernet network (CODAR                                                                                         (http://cordc.ucsd.edu/projects/mapping/).                          •Jose Blanco (jlblanco@ccpo.odu.edu)
                                                                               Chesapeake Bay Bridge Tunnel for use of their facilities.
                                           manual).                                                                                                                                                                               •Larry Atkinson (latkinso@odu.edu)

				
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posted:10/3/2011
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