1. Numerical Modeling Assessment of Dredging Spoils Dispersion and Deposition, Fort Lauderdale, FL
Julio A. Zyserman, DHI, Solana Beach, California
Christopher G. Creed, Olsen Associates, Inc., Jacksonville, Florida

2. Outline of Presentation
Project background
Problem definition
Selected modeling approach
Description of model setup
Presentation and discussion of results
Conclusions and closing remarks
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October 24, 2012

3. Project Background
Miami
Port Everglades Inlet is a complete barrier to littoral drift
Net southward sediment drift 56,000 yd3/yr. (42,800 m3/yr.)
Sediment accumulation along North beach and Inlet channel, significant erosion along South beach
Planned sediment bypass program:
Sediment trap on north side of channel
Remove relict spoil shoal
Improvements to north jetty
Periodic bypassing every 2-3 years
Sponsored by Broward County and FDEP
Construction planned for
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October 24, 2012

4. Problem Definition Need to dredge sand trap Need to remove spoil shoal
Sediments to be excavated includes significant % of fines
Direct (dredging) and indirect impacts (sedimentation and turbidity)
Potential impacts from sediment spreading and deposition:
Degradation of water quality
Temporary and permanent hardbottom habitat loss:
Scleractinian (stony) corals and Octocorals (soft) habitat (one of few coral reef areas in the continental US!)
Temporary and permanent seagrass and seagrass habitat loss
Modeling study required
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October 24, 2012

5. Selected Modeling Approach
Hydrodynamic model MIKE 21 HD to compute water levels and velocities
Lagrangian discrete particle model MIKE 21 PA to compute fate of spilled sediments
Lagrangian approach in PA allows individual tracking of particles of different sizes, from different sources, spilled at different positions along the water column, etc.
Standard output from PA: 2D time series of suspended sediment concentration (SSC) and deposited mass for every sediment fraction
Additional post-processing of PA results to obtain maximum/average values
Maximum thickness of deposited sediment layer, time required to reach maximum thickness and associated average rate of sediment deposition also calculated by post-processing PA results
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October 24, 2012

6. HD Model Setup Nested Cartesian grids, 30m and 10m resolution
Time-varying tidal water levels at North & South boundaries
Calibration against measured water levels and ADCP discharges
Water level differential to mimic Florida Current
Production runs cover neap-spring tidal cycle + 1 day model spin-off
Seven scenarios: pure tide + 6 intensities of Florida Current (3 north- + 3 southward)
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October 24, 2012

7. PA Model Setup
69,000 yd3 (52,750 m3) of sediment to be removed from sand trap, 118,000 yd3 (90,200 m3) from spoil shoal
9,000 yd3/day (6,900 m3) dredged at both sites, distributed over 540 bucket lifts, 2 minutes/cycle duration  dredge 18 hours/day
Sediments finer than 150 microns will be spilled
Sediment lost at grab/lift and by wash during lift along water column
Three sediment classes in PA: 40, 100 and 150 microns
Spill rates range from 0.10 kg/s to 0.55 kg/s
Losses at lift: 0.5 m above seabed, losses by wash at free surface
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October 24, 2012

8. Model Results (1/2)
Average
MIKE 21 PA output consists of 24 parameters (3 sediment sizes, two locations, two types of losses, SSC/deposited sediment)
Results for three sediment sizes and two types of losses integrated
Maximum and mean values calculated
Additional post-processing performed to calculate:
Thickness of deposited sediment layer (mm)
Rate of sediment deposition (mm/day)
Time to maximum thickness of deposition layer (days)
Max.
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October 24, 2012

9. Model Results (2/2) Influence of velocity of Florida Current
Average
Max.
Influence of velocity of Florida Current
Average
Max.
Influence of position of dredger
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October 24, 2012

10. Summary and Conclusions
Sand bypassing at Port Everglades Inlet is considered to be a feasible and cost-effective
inlet and beach management strategy for Broward County
The project will serve as a reliable long-term sand source for the beaches south of the inlet and will reduce sand shoaling in the Federal navigation entrance channel
Viewed as an environmental enhancement to the County’s beach management program, the project will decrease demand on offshore sand resources located adjacent to reefs
Model results will be used as justification to expand mixing zones that will be required
to comply with State water quality standards during construction
Model results will be used to specify the spatial extent of sedimentation and turbidity
monitoring as well as coral and seagrass community characterization and monitoring
Given the sensitivity of marine habitat and communities around the project site,
the dredge plume fate modeling is an essential tool for the planning of project
resource protection efforts
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October 24, 2012

11. Thank you very much for your attention! Any questions?
Julio A. Zyserman, DHI, Solana Beach, California
Christopher G. Creed, Olsen Associates, Inc., Jacksonville, Florida
© DHI
October 24, 2012