Surface Current Mapping in the Lower Chesapeake Bay
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
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
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 (email@example.com)
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 (firstname.lastname@example.org)
Chesapeake Bay Bridge Tunnel for use of their facilities.
manual). •Larry Atkinson (email@example.com)