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U.S. Department of the Interior
U.S. Geological Survey
FS-156-96

Sedimentation, Sea-Level Rise, and Circulation in Florida Bay

Issue | Sediment Transport Processes | Long-Term Consequences | USGS Research | Project Goals | Information

Issue

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.


Sediment Transport Processes

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.

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.


Long-Term Consequences

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.

graph illustrating that predicted sea-level rise is greater than past sea-level rise as measured at Key West

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.


USGS Research

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.
photo of scientist measuring sediment elevation at a survey station
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 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.


Project Goals

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.

Planned Products


Anticipated Schedule
  • 1995: Establish bank transects
  • 1995-97: Study cores; analyze geochemistry of core samples
  • 1995-99: Prepare salinity maps
  • 1996-97: Model wave propagation, survey bank transects, survey sediment and bottom
  • types, analyze and compile survey results, incorporate varying bottom friction into wave
  • modeling, measure sediment elevations
  • 1997-98: Study sediment entrainment properties, compile report on sediment survey and wave modeling, continue transect measurements
  • 1998-99: Compile and synthesize



For more information contact:

Robert B. Halley
U. S. Geological Survey
600 4th St. S.
St. Petersburg, FL 33701
(727) 803-8747

Related information:

SOFIA Project: Sedimentation, Sea-Level Rise, and Circulation in Florida Bay

SOFIA Project: Sediment Properties and Transport Processes in Florida Bay



U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
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Last updated: 11 October, 2002 @ 09:31 PM (KP)