Abstract
My project attempted to determine the resuspension
threshold (i.e. the particle size at which resuspension will occur) assuming
wind driven resuspension to be the major natural factor controlling the
grain size distribution.Fetches
were calculated at ten transects of Lower Green Bay perpendicular to the
most probable prevailing wind direction (from 3 years of wind direction
and velocity data) and then used in the SMB equations to calculate wave
heights.Bottom velocities were estimated
assuming a universal velocity defect law and the direction of bottom velocities
were assumed to be parallel to the bottom contours.The
surface wind stress was also assumed to be parallel to the bottom contours.I
computed resuspension thresholds for various wind speed events to get an
idea about which events would pose the most serious resuspension problems.Although
my results were a bit askew due to technical programming difficulty that
I encountered, I learned a great deal about compiling and analyzing data,
and have a better understanding for how difficult it really is to predict
something as “simple” as dirt being lifted from the bottom of a bay.
Motivations
and Objectives of the Project
“Great Lakes Areas
of Concern (AOCs) are severely degraded geographic areas within the Great
Lakes Basin.They are defined by
the U.S.-Canada Great Lakes Water Quality Agreement (Annex 2 of the 1987
Protocol) as ‘geographic areas that fail to meet the general or specific
objectives of the agreement where such failure has caused or is likely
to cause impairment of beneficial use of the area’s ability to support
aquatic life.’The U.S. and Canadian
governments have identified 43 such areas; 26 in U.S. waters, 17 in Canadian
water (five are shared between U.S. and Canada on connecting river systems)”
<http://www.great-lakes.net/places/aoc/aoc.html>.
The Lower Green
Bay is one such area.The Great Lakes
Water Quality Agreement calls for Remedial Action Plans (RAPs) to restore
and protect 14 “beneficial uses” in the Areas of Concern.An
impaired beneficial use means a change in the chemical, physical or biological
integrity of the Great Lakes system sufficient to cause any of the following:
restrictions on
fish and wildlife consumption
tainting of fish
and wildlife flavor
degradation of fish
wildlife populations
fish tumors or other
deformities
bird or animal deformities
or reproduction problems
degradation of benthos
restrictions on
dredging activities
eutrophication or
undesirable algae
restrictions on drinking
water consumption, or taste and odor problems
beach closings
degradation of aesthetics
added costs to agriculture
or industry
degradation of phytoplankton
and zooplankton populations
loss of fish and wildlife
habitat
10
of the 14 impairments listed above have been identified in the Lower Green
Bay.My objective in this study is
to determine the particle size that poses a potential threat to Lower Green
Bay if it is contaminated, since if it is contaminated and moving, it will
be that much harder to remove.
The
Coastal Environment of Targeted Area
The
coastal environment of the targeted area is quite diverse.Near
the city of Green Bay, much of the shoreline has been developed with sea
walls (for the large cargo ships to dock near), residential shore protection
(revetments typically), and piers.The
land along either shore as one travels northeast however becomes less inhabited
and more “natural.”There is much
less human activity along the east and west shores as there is near the
mouth of the Fox River.The land
along the coasts is mostly sandy soil with some larger rocks mixed in.Since
the Green Bay is a part of the great lakes system, the water is obviously
fresh and salt-free.Lastly, the
meteorology of the area can be generalized as being temperate.Green
Bay experiences cold winters where the surface of the bay freezes over,
and it experiences warm-mild summers where the temperatures rarely go above
90°F.The
bathymetry of the lower Green Bay can be viewed at:
The
bottom contours are rather gentle and it does not get very deep until one
is a good distance from the city of Green Bay.
Approaches
to the issues
My
approach to the question of resuspension threshold was multifaceted.First,
I needed to acquire wind data for the Lower Green Bay.Since
Green Bay does not have a buoy that collects meteorological data, I had
to resort to the airport’s weather station data.I
downloaded and assembled wind speed and direction data for 1997-1999.I
then constructed separate probability plots for both wind direction and
wind speed to determine the most probably speed and direction (see Wind
Speed Probability Plot). Using this data, I used the SMB method to
determine the wave height at 10 transects.I
measured the fetch at each transect and plugged in the appropriate fetch
for each calculation.Next, I followed
a series of equations presented in Brassard et al. in order to determine
the maximum bottom velocity and horizontal movement of a general particle.Then,
I downloaded grain size distribution data from a NOAA web page and plotted
grain size distribution to determine the median diameter for the sediment
actually present in Lower Green Bay (see Grain
Size Distribution Plot). It was
at this point that things began to go awry. At this point what
SHOULD happen is for each transect, I would determine the different particle
diameters that would be resuspended at given depths, using the developed
equations and my grain size information. This however did not work
out as well as I had hoped...
Results
My
results were quite odd. Not only did the particle diameters not make
much sense, but they did not seem to follow any pattern of error either--which
made it rather difficult to locate programming errors (See example
excel page for a sample calculations page from my program). In
the end, I had to accept the program with it's errors and begin to write
my presentation and this web page. I had expected to find that finer
particles would be more easily lifted in lighter wind conditions and in
deeper waters, while it would take heavier wind conditions and shallower
waters to lift larger particles.
Discussion
Despite
not returning the results that I had expected, going through the process
of collecting, organizing, and analyzing large amounts of data has proven
to be a tremendous learning experience for me. I have learned that
not only does an analysis such as this take much more time than I had anticipated,
but that the real tedious part of the process is not the actual calculations,
but formatting the data into a useable form. Since I am only a novice
programmer and also a novice researcher, this project took me a considerable
amount of time to finish. In fact, just putting together this web
page represents a great deal of time and effort. In all, I would
say that despite not getting "acceptable" results, this project was a huge
success for me and that next time I undertake a project of this magnitude,
I will not only be more aware of what is required to complete such a project,
but I will know the right questions to ask to get the help I need along
the way. Chin, THANKS FOR A GREAT SEMESTER!!
References
Brassard,
Pierre, et al.1997. Resuspension
and Redistribution of Sediments in Hamilton Harbor.J.
Great Lakes Res. 23(1):74-75.
Sorensen,
Robert M.1997.Basic
Coastal Engineering.Chapman &
Hall, New York.
