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PROGRAM TITLE: NEARSHORE and OPEN-LAKE PHYSICAL PROCESSES - FY96/97 Update
Wind, waves, and thermal structure are primary determinants of water movements, mixing, and circulation in large lakes and (along with tides) in coastal ocean areas. The movement and mixing of water in natural systems affects water quality, biological community structure and productivity, and both sediment and contaminant transport, especially in nearshore areas. This program supports research on physical processes in large lakes and the coastal ocean, their relationship to the biology, chemistry, and geochemistry of the ecosystem, and development of models to identify, forecast, and assist in managing and/or mitigating water quality and natural resource problems. At present, this program incorporates one major ERL Research Task: GLERL 02 - Nearshore Hydrodynamics (Task Leader: James Saylor, Jim Saylor).
Most studies of the Great Lakes from the 1950's through the 1980's focused
on either whole-lake or open-lake (i.e., off-shore) areas. However, some
programs, such as the 1976-1982 Pollution Through Land Use Activities
(PLUARG) Program and the 1980's Connecting Channels Study, were
oriented towards the nearshore area. These programs found that the nearshore
area is a critical link between land and open lake ecosystems. In the
aquatic environment, few areas are more profoundly affected by human activities
than the coastal areas, especially near large population centers and in
bays, harbors, and the Great Lakes, where circulation and flushing are
restricted. A better understanding of this environment is needed for reasons
that range from improved weather forecasting to prediction of the movement
of contaminated sediments. Effective remediation and management of diverse
coastal areas depends on an accurate assessment of present conditions
and an understanding of the hydrodynamic processes controlling sediment
remobilization and transport.
Relatively little is known about the hydrodynamics of nearshore areas.
In the Great Lakes, because lake-scale physical processes, such as surface
and internal seiches, topographic waves, and wind-driven circulation drive
so many of the coastal hydrodynamic phenomena, it is often necessary for
local studies with a nearshore focus to extend into the open lake to determine
causative physical forcing.
In the early 1990's, GLERL started reorienting its research and information
gathering activities towards a greater overall focus on the nearshore
area. The long term objectives of this program are to: 1) synthesize the
results of research studies on wind and wave dynamics, thermal structure,
currents, biological processes, and water chemistry of the nearshore region,
and apply them to practical problems of coastal environmental management
and planning; 2) study the relationship between the physical processes
and forces driving lake circulation and the frequency and characteristics
of sediment resuspension in varying water depths and sediment types; 3)
conduct research to measure, define, and describe the whole-basin circulation
of large lakes; and 4) guide application of basic and applied scientific
research to critical coastal environmental problems requiring unique expertise
available in academic and NOAA laboratories
FY96/97 Accomplishments and
Plans Cover Page
Research Overview page
Project Index
GLERL 02 - Nearshore Hydrodynamics
- Milwaukee Nearshore Study
- Cross-shore Transport Processes (new project)
- Lake Circulation and Bottom Boundary Layer Studies
- Time-frequency Study of Nearshore Wind and Wave
Processes (new project)
- Sediment Resuspension and Transport in the Great Lakes
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ERL Research Task: GLERL 02 - Nearshore Hydrodynamics
It has become apparent that although most environmental problems occur
or originate in nearshore areas, these areas have been much less studied
because they are so complex, and thus, their dynamics are much less understood.
Implemented in 1993, this Research Task is aimed at providing quantitative
understanding of the influence of hydrodynamic processes on the transport,
transformation, and fate of the contaminated materials and sediments in
the coastal nearshore region of the Great Lakes. When this Task was established,
Congressional language attached to appropriations required that it be
conducted in nearshore waters of the State of Wisconsin. Therefore, two
workshops were held with scientists and resource managers from the Wisconsin
Department of Natural Resources, Wisconsin Office of Coastal Zone Management,
U.S. Geological Survey and Wisconsin academia, to design and focus the
initial studies. During those workshops, the nearshore area of Milwaukee,
Wisconsin was chosen and several projects were defined. It was also at
this time that an outbreak of drinking water contamination by the protozoan
Cryptosporidium in the spring of 1993 caused illness in over 400,000
Milwaukee residents and over a hundred deaths. The Cryptosporidium
problem thus became a major focus of the work conducted under this Task
from 1993 through 1995. A synopsis of this work, which has been completed
except for the final documentation, is provided below under the heading
Milwaukee Nearshore Study.
In 1992, Congress added funding and a requirement for studies on Lake
Champlain. The research activity developed under that mandate was administratively
housed in an existing GLERL research project, Lake Circulation and
Bottom Boundary Layer Studies, which was later moved to the Nearshore
Hydrodynamics Task in 1993.
In early 1997 this Task was reorganized to reflect the completion of
the Milwaukee Nearshore Study and the addition of several
new projects.
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and Plans Cover Page]
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page]
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Milwaukee Nearshore Study - summary
Principal Investigators: P. C. Liu (paul.liu@noaa.gov) , A. Bratkovich (deceased), G. Miller (Gerald Miller)
Collaborating Scientists: A. Brooks, C. Sandgren, K. Lee, E. Christensen (University of Wisconsin at Milwaukee)
The Milwaukee Nearshore Study has been completed and final documentation
is being prepared. This research study and modeling effort was initiated
between GLERL and the University of Wisconsin - Milwaukee shortly after
the implementation of GLERL's Nearshore Hydrodynamics Task, which coincided
with Milwaukee's drinking water Cryptosporidium contamination crisis
in the spring of 1993. The study was conducted in conjunction with the
City of Milwaukee during 1994-1995 in order to develop safeguards to prevent
future occurrences of events like the Cryptosporidium contamination.
The primary tasks were to: 1) investigate water quality at the existing
water intake; 2) evaluate alternatives for improving quality of the source
water, and 3) identify and evaluate possible new water intake locations.
By combining numerical model simulations with field data, it was found
that the spring 1993 contamination was associated with highly turbid,
contaminated river water that discharges into the harbor and periodically
flows from the harbor as a plume that covers the site of the present water
intake. A computer-based model of the flow field showed that the water
intake was located in an area with a high probability of being impacted
by the harbor plume, but that moving or extending the intake less than
a mile would place it at a location with a much lower probability of receiving
highly contaminated water. As a result, in order to prevent similar contamination
events from impacting on the water intake in the future, it was recommend
that the present Texas Avenue Water Intake be relocated by adding a 4,000
ft extension pipeline, and that the municipal water filtration system
be upgraded. The city of Milwaukee adopted these recommendations in 1996.
A new water intake that will bring significantly better quality raw water
is under construction for service to start in 1997.
FY97 Plans
- Complete final documentation of this study as a NOAA Technical Memorandum.
Cross-shore Transport Processes (new project,
FY97)
Principal Investigators: Gerald Miller and Nathan
Hawley
During the winter when large areas of the Great Lakes become isothermal,
they are characterized by energetic currents penetrating to depth, with
extensive mixing. However, winter ice cover, which is often confined to
the coastal regions, tempers these currents by inhibiting momentum transfer
from the wind field to the water. Alternately, a lack of ice cover permits
strong currents to transfer energy all the way to the lake bottom, especially
in nearshore areas. During the summer when the Great Lakes are stratified,
there are ubiquitous internal oscillations (long internal gravity waves)
that occur at various time scales ranging from minutes to days. These
oscillations, including internal seiches, upwelling and downwelling, inertial
oscillations, progressive and standing Poincare' waves, and Kelvin waves,
are responses to wind forcing. The presence of land and shallow depths
influence their characteristics in the nearshore area. During strong stratification,
currents generated by coastal upwelling and downwelling events may be
important in the vertical and horizontal redistribution of nutrients and
biota and in the transport of materials across the coastal margin. If
wind forcing is sufficiently strong, the thermocline intersects the surface
and, with continued wind forcing, drives the thermocline front farther
offshore. However, the geostrophic readjustment that takes place when
the wind stress relaxes is not well understood. During large downwelling
episodes, the leading waves form internal surge fronts which may play
a significant role in both nearshore and offshore transport. Thus, the
annual transition between vertically well mixed (isothermal) conditions
and vertically stratified conditions in the Great Lakes can dictate the
nature, timing and duration of cross-shore exchange events. These processes
may play a significant role in structuring the Great Lakes ecosystem.
These processes may also play an important role in the offshore transport
of fine-grained material. Previous studies in Lake Michigan have shown
that sediment is rarely resuspended in the offshore areas during the stratified
period, yet vertical profiles show that the total sediment load in both
the benthic nepheloid layer and in the total water column varies considerably
over relatively short time intervals.
The objectives of this projects are to 1) identify the physical processes
responsible for the cross-shore exchange of materials in the Great Lakes;
2) increase understanding of the dynamics of upwelling and downwelling
and its role in cross-shore transport; and 3) investigate the physical
mechanisms responsible for both the formation and the spatial and temporal
variability of the bottom nepheloid layer (BNL) in the Great Lakes.
FY97 Plans
- Analyze current and temperature data obtained at the southern Lake Michigan sequential sediment trap moorings, November 1994-May 1996, including the 1996 spring plume event.
- Continue work on current/sediment flux relationships at the 100m station off Muskegon, 1994-95.
- Two Acoustic Doppler Current Profilers that were procured in FY96 will be field tested in Lake Michigan. If possible, this will be done in conjunction with the testing of GLERL's redesigned Vector-Averaging Current Meters.
- Construct platform for an underwater camera system and acoustic current meter and test.
- Design, construct and test in-situ profiling unit.
- Attempt to recover moorings that were previously lost in Green Bay and Whitefish Bay.
Lake Circulation and Bottom Boundary Layer Studies
Principal Investigator: James Saylor (Jim Saylor)
Collaborating Scientists: Gerald Miller (GLERL); Thomas Manly (Marine
Research Corporation); Patricia Manly (Middlebury College)
Historically, this project has encompassed a long-term study of the bottom
boundary layer in Lake Michigan. It is based on measuring bottom currents
and sediment resuspension to study the physics of the bottom boundary
layer, and to relate the frequency of sediment resuspension in varying
water depths and sediment types to the causative forces driving lake circulation.
In 1992 the continuing work in Lake Michigan was deferred and resources
and staff were shifted to respond to a Congressional requirement to conduct
work in Lake Champlain, the sixth largest body of fresh water in the United
States. After several meetings with the local research community, it was
decided that understanding the physical processes that exert primary control
on circulation within the lake and subsequent incorporation of this information
into a whole-lake modeling effort, would be of paramount importance to
developing management strategies to address many of the environmental
issues concerning Lake Champlain. Therefore, GLERL established a new research
activity in collaboration with scientists at Middlebury College (Vermont),
focused on determining the circulation and sediment resuspension dynamics
of Lake Champlain. The objectives of our Lake Champlain research activity
are: 1) understanding of the currents and circulation within the main
lake basin of Lake Champlain and its complex interactions and water mass
exchanges with the peripheral sub-basins; 2) development and verification
of conceptual and numerical models of water mass thermal structure and
current flows; and 3) quantification of resuspension potential in areas
of contaminated bottom sediments and the forces causing the sediment erosion.
FY96 Progress and Accomplishments
Completed data analyses confirmed the propagation of gravity waves and
internal bores northward along the bottom of Lake Champlain. These wave
forms are associated with high speed currents flowing in the deep bottom
layers of lake, resuspending bottom sediments and redistributing them
throughout the basin. In some areas of the lake the sediments are contaminated
with toxic materials, and this sediment resuspension process reintroduces
the materials into the water mass.
An additional field experiment to measure water mass exchange processes
between the main lake and one of its sub-basins was implemented.
FY97 Plans
- Deploy five ADCP/T-chain moorings equipped with sediment traps at or south of Thompson's Point in Lake Champlain.
- Deploy up to three thermistor-chain or temperature-sensor mini-moorings in the shallow area directly north of Bulwaga Bay in Lake Champlain.
- Complete and document analyses of the data collected in previous years in the main basin of Lake Champlain.
Time-frequency Study of Nearshore Wind and Wave
Processes (new project, FY97)
Principal Investigator: Paul Liu (paul.liu@noaa.gov)
Collaborating Scientists: David Schwab (GLERL)
Modern studies of wind and wave processes have evolved from the conjecture
that the random stochastic nature of waves can be considered as the composite
sum of a complete spectrum of simple harmonic waves with different frequencies
and energies. All the available models for wind-wave prediction were developed
within the framework of a frequency wave spectrum. The validity of the
frequency wave spectrum is hinged on the concept that wave processes are
fundamentally stationary. This concept, however, clearly contradicts the
well-known phenomenon that waves occur more prevalently in intermittent
groups. This contradiction between the wave spectrum and wave grouping
concepts, often ignored by most of the available wave models, undoubtedly
contributes to the inaccuracies of the wind-wave model results.
In order to move towards reconciling the conceptual difficulties in the
conventional wind-wave analysis and modeling, we propose to apply time-frequency
analysis to the wind waves instead of simple frequency analysis. Specifically,
we will 1) make long-term wind and waves time series measurements to collect
and monitor currently lacking wind and wave data in the nearshore area;
2) advance the time-frequency analysis approach to the available and newly
measured wind and wave time series data to gain insights of processes
in the time-frequency domain; and 3) develop exploratory models to formulate
realistic approaches toward accurate, rational, and judicious wave prediction.
FY97 Plans
- Contract with the National Data Buoy Center for the development, preparation, and deployment of a nearshore instrumented buoy in Lake Michigan.
- Collect and process available wind and wave time series data from the Great Lakes and initiate time-frequency analysis of the data.
Sediment Resuspension and Transport in the Great
Lakes
Principal Investigator: Nathan Hawley (nathan.hawley@noaa.gov)
Collaborating Scientists: Chang-Hee Lee (CILER); Bruce Brownawell,
Roger Flood (S.U.N.Y. at Stony Brook)
The processes responsible for the transport, deposition, and resuspension
of solids are of considerable importance in both the Great Lakes and in
the coastal ocean in general, since this material can significantly affect
both biological productivity and the cycling of pollutants. Although longterm
patterns of sediment deposition and movement in the Great Lakes have been
deduced from the distribution of radionuclide tracers, the temporal resolution
of these studies is of order 30 years. The response of the lake bed to
individual events is not well-understood and the processes responsible
for the maintenance of the Benthic Nepheloid Layer (BNL), which is found
in many ocean basins and all of the Great Lakes are also not well known.
Further, the physical properties of the particles in the BNL are not well
known. Whatever its origin, the BNL appears to be important in the crossshelf
transport of chemical substances.
Extending our understanding of these processes, and properly representing
them in models, requires detailed and long-term measurements of currents,
particulate concentration and sediment properties both within the bottom
boundary layer (BBL) and the water column as a whole. Acquiring such measurements
and using these to guide model development are the long-term goals of
this project.
This project consists of one or more subprojects; at present there are
two active subprojects:
- Seasonal Resuspension Of Contaminated Sediments In Southwestern Lake
Ontario
- Sediment Resuspension And Transport In Lake Michigan - this subproject is part of the EPA-sponsored Lake Michigan Mass Balance Study (LMMB).
FY96 Progress and Accomplishments
Lake Ontario
This project was completed. Documentation of our analyses of the data collected during the fall 1992 and spring 1993 from Lake Ontario was completed (see Products). By combining the chemical analysis of material collected in a set of sediment traps with time series measurements of the bottom currents, several episodes of bottom resuspension due to current action were identified, and it was shown that the material caught in the sediment traps must have come from at least three different sources. Although no observations of suspended sediment concentration were available, this is the first report of bottom resuspension in deep water (below wave base) in the Great Lakes that is supported by current velocity observations. All previous reports of resuspension in deep water have been based on either trap data alone, or on model calculations.
Products
HAWLEY, N., and C.R. Murthy, 1995. The response of the benthic nepheloid layer to a downwelling event. J. Great Lakes Res., 21, 641-651.Lake Michigan
HAWLEY, N., X. Wang, B. Brownawell, and R. Flood, 1996. Resuspension of bottom sediments in Lake Ontario during the unstratified period, 1992-1993. J. Great Lakes Res., 27, 707-721.
The field program was completed with the successful retrieval of 12 moorings
in Lake Michigan in October, 1995.
A number of data sets for use in the analysis of the mooring observations were obtained:
- hourly observations of water temperature and suspended sediment concentration from the Muskegon water intake plant for 1994 and 1995;
- water discharge and sediment concentration data collected by the US Geological Survey from the Muskegon and Grand Rivers for 1994 and 1995;
- all CTD profiles made by the EPA during their 7 cruises in Lake Michigan during 1994 and 1995, and
- hourly weather observations from the met station located at GLERL's Muskegon facility.
An in situ flume was successfully deployed from the Lake Guardian
16 times last fall. Of the 16 deployments, 10 resulted in bottom resuspension
data.
Data from the 3 time series deployments were examined, corrected where
necessary, and supplied to the EPA. The data (water temperature, current
velocity, and water transparency) are of very high quality and the records
are virtually 100% complete during the summer and fall deployments. The
only large gaps in the coverage are during the winter (when ice formed
on some of the transparency sensors) and during the spring (when the moorings
were being serviced). These observations are the most extensive set of
time series observations of suspended sediment concentrations ever collected
in the Great Lakes, with measurements made at three stations at a total
of 12 elevations for over 8 months.
The results from the wave rider observations made in 1995 were compared
to the wave parameters computed by the GLERL wave model. Although the
wave heights agree quite well, the wave model over-estimated the wave
periods. Since the depth to which surface waves affect the bottom is a
function of the wave length, using the wave model results in a sediment
resuspension model will cause the model to predict too many instances
of resuspension. Thus, the wave model may have to be significantly modified
in the future.
Analysis of the winter tripod data was begun. The observations show unequivocal
evidence of sediment resuspension at depths below wave base (about 30
m) in the Great Lakes. The observations also show that material resuspended
inshore due to storm action is advected offshore during the period following
the storm. These results were presented at the 1996 Ocean Sciences meeting.
The data collected during the stratified season is being analyzed to
determine the mechanisms responsible for offshore transport during that
period. The result to date suggest that sediment is resuspended inshore
and then transported offshore during downwelling events.
FY97 Plans
- Continue analysis of the time series data collected in 1994-1995 in Lake Michigan to determine what physical conditions are required for bottom resuspension at different water depths, and to determine the response of the benthic nepheloid layer to different hydrodynamic forcings.
- Complete in situ flume deployments in Lake Michigan and analyze the data to determine the bottom shear stress required to erode different bottom sediments.
- Begin experimental measurements of the bulk shear strength of different bottom sediments in Lake Michigan.
FY96/97 Accomplishments and Plans Cover Page
FY96/97 Research Overview page
Last updated: August 19, 2002 mbl