properties and transport processes in florida bay >
Project Proposal for 1998
Program: Fragile Environments
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.
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.
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.
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:
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.
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