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Sediment Properties and Transport Processes in Florida Bay

Project Proposal for 1998

Program: Fragile Environments
Project Title: Sediment Properties and Transport Processes in Florida Bay
Location of Study Area: Florida Bay
Project Start Date: FY 1997
Project End Date: FY 1999
Project Number: 7242-37656
Project Chief: Ellen Prager
Region/Division/Team/Section: Eastern/ Geologic/Marine and Coastal/St. Petersburg
E-mail: no longer available
Phone: no longer available
Fax: no longer available
Mailing Address: no longer available
Program Element(s)/Task(s) 1. Florida and Biscayne Bay, Task 1.4 Physical properties of sediment and processes of transport
Panel:
Collaborators, Clients:
Army Core of Engineers
Our results will be used jointly to determine varying bottom friction coefficients throughout the Bay for incorporation in wave and circulation models. Additionally, the data from this study will provide the means to add sediment resuspension and transport to the water quality and hydrodynamic models.
South Florida Water Management, Florida Marine Research Institute and other academic researchers
Data supplied by this project will be used in conjunction with nutrient analyses to determine the contribution of nutrients from the sediments during resuspension events.
Department of Environmental Protection
Data provided by this study will aid in understanding the population dynamics of benthic mollusks found in Florida Bay.
National Park Service
The Park Service is interested in the consequences of seagrass mortality and propeller scars, our findings will provide a means to quantify sediment resuspension from varying bottom types within the Bay. Data will also suggest which areas of Florida Bay are most vulnerable to erosion and where and when turbidity events are most likely to occur.
Other Environmental Organizations and Businesses
Widespread interest in Florida Bay restoration exists among national environmental organizations and local business leaders. Results of our project will provide a scientific basis on which to form educated opinions and make sound policy decisions regarding planned structural changes and the long-term effects of such changes.


BACKGROUND NARRATIVES

Project Summary: Within Florida Bay, increased turbidity, hypersalinity events, seagrass mortality, and reoccurring algal blooms have resulted in a national effort in ecosystem restoration. Sediment resuspension is inherently tied to the issues prompting restoration efforts; resuspension of sediments is the main cause of increased turbidity, decreased light due to suspended sediments may adversely effect seagrass growth, and nutrients stored within the sediments and released during resuspension events may induce algal blooms. This project proposes to examine the controls on sediment resuspension in Florida Bay, quantify its occurrence on an annual basis, and provide the overall understanding and information which is needed to incorporate sediment resuspension and transport into circulation and water quality models.

Project Justification: Beginning in the year 2000, in one of this nations largest environmental restoration attempts, an extensive infrastructure will be built by the ACOE and State of Florida to restore water quality to Florida Bay. Prior to that time, these agencies would like to understand the processes which have caused the decline in water quality in Florida Bay and using models, predict the success of alternative designs for restoration efforts. Recent evidence suggests the significance of sediment resuspension in the issues of increased turbidity, seagrass die-offs, and reoccurring algal blooms. Consensus among consulting experts is that sediment resuspension must be incorporated into a developing water quality model and if possible, sediment transport should be added to the circulation model being constructed. The data, maps, and understanding provided by this project is essential if resuspension of sediments in Florida Bay is to be incorporated into either model, and on a more fundamental level, to delineate the relationship between resuspension events and ecologic changes in the Bay.

Project Objectives: The objectives of this research are to provide a better understanding of how and when sediments within Florida Bay are resuspended, to define the spatial distribution of sedimentary properties and bottom morphology which control resuspension, quantify resuspension on an annual basis and provide the empirically-derived data needed to incorporate sediment resuspension or transport into numerical models. Through our collaborative efforts with other agencies and researchers, we will also provide the data necessary to incorporate varying bottom friction into a circulation model. Furthermore, these data will contribute information required to delineate patterns of bathymetric change, predict the impacts of storms or sea grass mortality on turbidity, assess the contribution of nutrients during resuspension events and estimate sediment export as part of an overall sediment budget for Florida Bay.

Overall Strategy, Study Design, and Planned Major Products: A three-fold approach has been adopted to study sediment resuspension processes in Florida Bay and to provide the information necessary for incorporation into numerical models. The first portion of the study focuses on the forcing of flow within the Bay which results in sediment resuspension. The second investigates the spatial variability of the sediments and morphology of the sea floor in the Bay. And the third task is to empirically define how these properties relate to the potential for resuspension and transport. Products to be produced from this study include a summary report of findings, several articles in peer reviewed journals, presentation at scientific and regional meetings, a map of sediment properties and bottom types in Florida Bay, and a map and summary of resuspension potential. Data produced will also enable transport and water quality modeling to be performed, and help to quantify a sediment budget for the region.

WORK PLAN

Overall: Initially, the focus of this study has been to simulate the patterns of flow responsible for widespread sediment resuspension in Florida Bay. Satellite imagery of turbidity suggests that resuspension events are closely linked to increases in wind strength, therefore we believe that resuspension of sedimentary materials in the Bay is caused primarily by wind-driven waves. To quantify the velocity of near-bottom flow produced under a variety of wind strengths and directions, a shallow-water wave model (HISWA, Holthuijsen et al, 1989) is being applied to Florida Bay. The model incorporates detailed bathymetry, measured wind conditions, and data from a survey of sediments and bottom cover to determine varying bottom friction. To aid in assessing the extent of each bottom type, aerial photos, satellite imagery, and local experts have been consulted. A map of bottom types is in production (Figure 1) and the appropriate bottom friction coefficients are being assigned to each bottom type for incorporation into the wave model. In laboratory flume experiments, seagrass has been shown to significantly dampen incoming wave energy. Consequently, mapping the distribution of seagrass throughout the Bay is crucial to correctly simulating wave propagation. The measurement of actual wave damping in the Bay by seagrass is greatly needed to correctly estimate the associated bottom friction in the model. Results will be compared to satellite imagery of turbidity events and wave data from pressure sensors deployed within the Bay. Using an equation specifically designed to take into account the accelerations produced by wave-induced motion (Grant and Madsen, 1979; Signell et al, 1990), bed shear stress will be calculated from near-bottom orbital velocities produced by the model and quantified on an annual basis. It will also be possible to add the effects of unidirectional flow to the calculations using the output of the ACOE hydrodynamic model. We also plan to simulate increased water levels or changes in bottom friction due to seagrass mortality in order to examine the impact on resuspension processes and turbidity.

To examine the spatial variation in sedimentary properties related to entrainment, an extensive survey of sediment types within Florida Bay has been undertaken. Field and laboratory measurement of grain size distribution, mud, water, organic, and carbonate content has been performed. The location of sampling sites was chosen in an attempt to cover as much of the Bay as was logistically possible and to ensure that areas identified as critical are included. Critical areas were identified in satellite imagery for their significance during turbidity events or areas in which collaborating investigators are also taking samples (nutrient analyses). Statistical analyses of variation and similarity between sample sites will be performed in an effort to identify a finite number of sediment types within the Bay.

Once a finite number of sediment types are defined, experiments using a portable resuspension device (PRD) will be conducted to measure the susceptibility of the sediments to resuspension. This instrument has been used successfully in freshwater environments, and can provide the empirical data needed to compare the potential for resuspension in various sediment types in the Bay and to incorporate sediment resuspension into a water quality or hydrodynamic model (Zeigler and Nisbet, 1994, Gailani et at, 1991; Tsai and Lick, 1986).

Based on results from the research outlined above, we expect to quantify the resuspension of sediments in Florida Bay on an annual basis, delineate the relationship between sedimentary properties and entrainment, and supply the necessary information for sediment transport/resuspension modeling. Data from this project will also be used to better explain widespread patterns of turbidity observed using remote sensing techniques (task 1.1). By combining our results with data on sedimentation patterns (task 1.3) and rates (task 5.4), as well as that from the study of historical bathymetric change (task 1.2), we expect to further our understanding and ability to predict the future bathymetry of Florida Bay and begin estimates for a sediment budget. All data will be stored in both digital and hardcopy format and access to information will be made available through the USGS South Florida website with links from other collaborating agency websites.

Timeline:
1997
1. Survey bottom types and sediment sampling (Prager and Halley)
2. Produce map of bottom types (Prager, Halley and tech. help)
3. Begin wave modeling, pressure sensor deployment, incorporation of varying bottom friction into model (Prager and intern - Orton)
4. Sediment analyses (Prager, Halley, tech, and interns)
5. Begin PRD experiments (Prager and Orton)
1998
1. Deploy transect of pressure sensors for damping experiment and model verification (Prager, Orton, Reich, and Hickey)
2. Complete wave modeling (Prager and Orton)
3. Complete PRD experiments (Prager, Halley, Reich, and Hickey)
4. Compare PRD results with wave modeling and sediment properties (Prager and Halley)
5. Map resuspension potential (Prager, Halley, and tech.)
1999
1. Synthesis of results (Prager and Halley)
2. Produce several publications (Prager and Halley)

Planned Deliverables/Products: This project will produce two maps for the scientific community, client agencies, and the general public: 1) a map of bottom types in Florida Bay, and 2) a map of resuspension potential in Florida Bay. We also expect to produce several scientific publications and will continue to present this work at local, regional and national meetings. Data and information produced during the study, such as the digital version of the bottom friction grid, will be exchanged with client agencies including the ACOE, FMRL SFWM, NOAA and other academic researchers.

Planned Outreach Activities: The results of this project will be disseminated through presentations at meetings and publications. In addition, several field trips will utilize the information we have produced and we are currently discussing with CNN Science and Technology a possible piece on our research in Florida Bay. Continued meetings and discussions are planned with other researchers examining related topics to facilitate the exchange of data, information and ideas. We are also updating our USGS South Florida web site with results from research as it becomes available.

Prior Accomplishments in Proposed Area of Work:

New Directions, Expansion of Continuing Project (if applicable):

ACCOMPLISHMENTS, OUTCOMES, PRODUCTS, OUTREACH

Accomplishments and Outcomes, Including Outreach: As a result of our study thus far, the importance of sediment resuspension and its complexity, have become recognized by the panel of experts reviewing the overall science program in Florida Bay. Consequently, great interest in our project has been generated and in the reviews for plans to develop a water quality model, our work is cited for inclusion. After sending out drafts of the bottom type map for review, we have received multiple requests for the final draft by numerous agencies, researchers and outreach coordinators.

Deliverables, Products Completed:
Prager, E.J., Halley, R.B., and M. Hansen, 1996, Sediment Transport Processes and Sea-Floor Mapping in Florida Bay, 1996 Florida Bay Science Conference, Program and Abstracts: 68 - 69.

Halley, R.B., and E.J. Prager, 1996, Sedimentation, Sea-level Rise and Circulation in Florida Bay, USGS Fact Sheet 156-96.

A draft map of bottom types in Florida Bay has been completed and reviewed by 6 outside scientists working in Florida Bay. We are in the process of completing a final draft of the map and expect publication by the end of the year.

NEEDS

Required Expertise:
1997 - Expertise in carbonate sedimentology, survey and sampling techniques, mapping/GIS capabilities, wave modeling, pressure sensor deployment and analysis, laboratory analysis of sediments, and development and use of PRD.
1998 - Wave modeling and pressure sensor deployment and analysis of data, core sampling and use of PRD for measuring resuspension potential, laboratory analysis of suspended sediments concentration, mapping/GIS capability.
1999 - Ability to synthesis all of the results into a final report, articles and presentations.

Names of Key Project Staff:
1997 - Prager, Halley, Orton
1998 - Prager, Halley, Orton, Hickey, Reich, Byrd
1999 - Prager, Halley

Major Equipment/Facility Needs:
1997: Portable Resuspension Device, use of laboratory facilities in the Florida Keys and St. Petersburg, Boat use, I workstation equivalent for modeling and I laptop computer for field work.
1998: 3 pressure gauges are needed to measure wave heights in shallow water, two of which having directional current measurement capabilities to examine spectral direction and flow magnitude, I laptop computer for data analysis from the pressure gauges.
Justification: During the 1997 FY three pressure gauges were borrowed from the ACOE to measure wave height and damping in Florida Bay. The equipment borrowed did not resolve the wave phenomenon as needed and we had no opportunity to attempt any redeployment because they were borrowed with very limited time for use. Consequently, we would like to purchase our own pressure gauges which would 1) be available for multiple deployments, 2) resolve the waves to an appropriate accuracy, and 3) result in additional information regarding propagation direction and current flow. This data is essential for our modeling efforts both for verifying the model output and to validate the bottom friction coefficients (wave damping by seagrass) we have used in the model. This data has also been requested by other researchers working in the Florida Bay science program. Boat use and laboratory facilities will also be needed during FY 1998
1999: No major equipment purchases are expected.


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