U.S. Department of the Interior
U.S. Geological Survey
FS-156-96
Issue |
Sediment Transport Processes |
Long-Term Consequences |
USGS Research |
Project Goals |
Information
Recent algal blooms and seagrass mortality have raised concerns
about the water quality of Florida Bay, particularly its nutrient
content (nitrogen and phosphorus), hypersalinity, and turbidity.
Water quality is closely tied to sediment transport processes
because resuspension of sediments increases turbidity, releases
stored nutrients, and facilitates sediment export to the reef
tract. Over decades to centuries, bathymetric changes due to
erosion or sediment deposition affect water circulation and
hypersalinity. The effect on circulation depends on the
interplay between sediment accumulation and sea-level rise. The
goal of this U. S. Geological Survey project is to document and
quantify short- and long- term processes associated with sediment
transport so that the influence of sediments on water quality can
be better defined and later integrated with numerical modeling
efforts conducted by cooperating agencies.
Although much is known about the origin of the carbonate
sediments within Florida Bay, the development of the bay's unique
bank and basin morphology remains poorly understood, in part
because sediment transport processes are not well understood.
In the most general sense, previous research suggests that
long-term processes are not uniform in Florida Bay. The bay may
be subdivided into three regions: in the eastern bay mud banks
are eroding; in the central bay, mud banks are migrating; in the
western bay, mud banks are accreting. While this
characterization is generally correct, erosion and accretion
occur in all regions of the bay, and additional detail is needed
for more accurate quantification.
Predicted sea-level rise (red) is greater than past
sea-level rise as measured at Key West (blue).
Rising sea level has lead to considerable speculation about the
future of Florida Bay. Some researchers, without considering
sedimentation, have suggested that sea-level rise will inundate
the southern Everglades, enlarging Florida Bay. Others, not
considering sea-level rise, predict that the bay will fill with
sediments. More likely, some combination of these processes will
occur. The sediment transport mechanisms that keep mud banks
near sea level may continue to do so, while the processes that
remove mud from the basins will cause them to deepen. The result
may be continued restriction of the bay with increasingly deeper
basins between the banks, and larger volumes of water retained in
each basin. Water management agencies are already incorporating
predicted sea-level rise for the next century into planning for
Everglades restoration. This project will allow the agencies to
incorporate sedimentation into the planning as well.
Because of the spatial complexity of sediment composition and
consistency, as well as the distribution of seagrass, a
three-fold approach has been adopted for the study
of transport processes. Initially, wave propagation
through the bay will be modeled under varying wind and water
levels. Model results will be used to quantify transport
processes produced by winds observed within the region.
The second portion of the study will focus on sediment
properties of the bay and how they relate to entrainment and
resuspension. An extensive survey of sediment properties,
including mud, water and organic content; particle size
distribution and sorting; viscosity, and bulk density will be
conducted. In addition, with an instrument successfully used in
other environments, we hope to directly measure the response of
the sediment bed to turbulence. A map of bottom type and
resuspension potential throughout Florida Bay will be constructed
based on the results of this portion of the study. The
bottom-type map will also be used to integrate bottom friction
into the wave model.
Sediment transport is the result of water flow over the bay floor
and the response of underlying sediments. This response, termed
entrainment, is a function of the shear stress produced by flow,
and properties of the sediments and sedimentary surface. Within
Florida Bay, near-bottom flow is produced by tides, waves, and
changes in the adjacent ocean. Residual tidal flows appear weak
except in a few areas near channels or around islands. In
general, it is believed that wave- driven flow causes
resuspension and, when combined with residual circulation,
results in sediment transport.
It is not clear which of the complex and variable properties of
the sediment surface within Florida Bay control the potential for
suspension or how sediments actually behave under fluid flow.
Broad variations in mud, organic, shell, pellet, and water
content lead to widely varying compositions and consistencies.
Further complications are produced by the presence of varying
densities and types of seagrass, algae, bioturbating organisms,
oxidation and dissolution processes, algal and bacterial mats, as
well as periodic dewatering during mud bank emergence.
Consequently, classic entrainment criteria based on grain size,
settling velocity, sorting, or shape are inappropriate for
Florida Bay sediments. This study intends to define and
quantify the relationship between flow and the entrainment of
Florida Bay sediments.
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Sediment elevation is measured at survey stations like that shown in the photograph. |
Because of the complexity of the sediment surface in Florida Bay,
existing empirical formulas for sediment transport are not
appropriate. Box core samples of Florida Bay sediments will be
collected and placed in a laboratory flume. Experiments will be
performed in the flume to investigate the shear stress required
to initiate transport in the bottom types found during the
bay-wide survey. Findings will then be compared to modeled
wave-induced shear and shear produced by unidirectional currents
(hydrodynamic modeling and current measurements).
The objective of this research is to provide a better
understanding of how and when sediments within Florida Bay are
resuspended and deposited, to define the spatial distribution of
the potential for resuspension, to delineate patterns of
potential bathymetric change, and to predict the impacts of
storms or sea-grass die off on bathymetry and circulation within
the bay. By combining these results with the findings of other
research being conducted in Florida Bay, we hope to quantify
sediment export from the bay, better define the nutrient input
during resuspension events, and assist in modeling circulation
and water quality. Results will enable long-term sediment
deposition and erosion in various regions of the bay to be
integrated with data on the anticipated sea-level rise to predict
future water depths and volumes. Results from this project,
together with established sediment production rates, will provide
the basis for a sediment budget for Florida Bay.
For more information contact:
Related information:
SOFIA Project: Sediment Properties and Transport Processes in
Florida Bay
U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
The third research task addresses long-term sedimentation
and erosion rates in the bay through geochemical studies of
cores, multiyear surveys from monitoring stations, and analysis
of historic maps and aerial photographs. Fifteen stations
have been established in three transects across mud banks. These
stations provide the basis for determining long- term accretion
or erosion rates by using techniques similar to those used for
measuring sedimentation rates in wetlands. Ongoing studies using
geochemical dating methods and naturally occurring temporal
markers have identified areas of long-term accretion in the bay.
These methods use naturally occurring and man made radionuclides
to determine sedimentation rates during the last century. This
project will continue to identify bay sediments that are
appropriate for geochemical analysis of sedimentation
rates.
Planned Products
Anticipated Schedule
Robert B. Halley
U. S. Geological Survey
600 4th St. S.
St. Petersburg, FL 33701
(727) 803-8747
SOFIA Project: Sedimentation, Sea-Level Rise, and Circulation
in Florida Bay
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Last updated: 11 October, 2002 @ 09:31 PM
(KP)