International Dredging Review

International Dredging Review
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In March, the U.S. Army Corps of Engineers Jacksonville District awarded a $4.4 million contract to Environmental Tracing, LLC for a Sediment Morphodynamic Assessment Project to examine sediment transport in southeast Florida over the next two years. The project will include in-water measurements of sediment transport, waves and currents, numerical modeling and sediment tracing, in order to analyze the hydrodynamic, sediment transport and morphology changes across the region.

During storms or high wind and wave events, sediment is constantly moving in the system, north and south along the beach, in the nearshore east to west, and in and out of inlets and harbors.

“Understanding and being able to predict how the system is moving the sediment naturally,” said Corps Jacksonville District Project Manager Laurel Reichold, “will allow us to better predict shoaling concerns inside our inlets and harbors and in predicting the erosion of our beaches.”

She said the district’s flood risk management and navigation missions require the analysis of sediment movement around southeast Florida’s inlets and harbors. “In addition to knowing where and how sediment is moving naturally, the projects also require an understanding of sediment movement during or as a result of dredging, how dredged sediment that is secondarily dispersed into the water column behaves, and optimal channel features and beach placement design,” Reichold said.

Protecting the nearshore and offshore hardbottom habitat ecosystems requires the Corps to evaluate the potential impacts of displaced sediment to those resources.

Reichold said another challenge for the Corps’ mission in southeast Florida is a lack of sediment for flood risk management projects. “Given the scarcity of resource, extraction and relocation of existing sediment, resources must be evaluated to determine the secondary and long-term effects and ensure the sustainability of flood risk management projects,” she said.

Numeric modeling will fill the gaps between the existing and planned monitoring data. It can simulate scenarios under different hydrodynamic conditions within the study area. “Hydrodynamic, sediment transport, and morphology change monitoring and modeling that includes a large area, will provide the requisite information to evaluate multiple missions and authorized projects in the area,” Reichold said.

Environmental Tracing, LLC will monitor sediment transport using sediment tracers, to mimic silt and sand, to map the movement of material over time in different locations. It will use DELFT – 3D numerical modeling software, which specializes in hydrodynamic and sediment transport predictions.

In addition, field data for turbidity, salinity, wind, waves, current magnitude and directions, will be used to formulate a conceptual understanding of the system and existing conditions, Reichold said.

The project will concentrate on three primary study areas: Lake Worth Inlet, Port Everglades Harbor and Miami-Dade County. For each area, Reichold identified the problem with each location, the goal for the study, and how the Corps plans to achieve that goal.

Lake Worth Inlet: Channel Shoaling
Problem: Chronic shoaling plagues Lake Worth Inlet. Material typically encroaches into the channel from the north, filling the existing sediment traps and eventually, impacting the navigation channel.

Goal: To understand ambient sediment transport and suggest engineering solutions to minimize shoaling within the channel.

Objectives: To collect measurements to describe the ambient sediment transport processes in and around Lake Worth Inlet; to identify where shoaled material ends up in the channel; and to suggest specific improvements to prolong dredging requirements, such as the optimal locations for sediment traps or other maintenance measures to decrease shoaling in the channel.

Port Everglades Harbor: Dredge Plume Dynamics
Problem: Port Everglades requires routine maintenance dredging and is also undergoing final review for an authorized capital improvement project (deepening and widening portions of the channel). A non-federal sand bypass project is undergoing regulatory review. Dredging projects in Port Everglades could deposit sediment on nearby hardbottom resources.

Goal: To understand dredge plume dynamics within Port Everglades to better inform capital dredging operations.

Objectives: To collect measurements to describe the overall circulation and ambient sediment transport processes, including fine grain material, which may be released during dredging; to leverage any dredging events in the area to collect information on dredge plume dispersal and spillage; in order to better understand the potential of sediment movement and dredge plume dispersal, develop a calibrated and validated sediment transport model for Port Everglades for simulating proposed dredge events; to use numeric modeling to suggest mitigation strategies to protect local resources.

Miami-Dade County: Sediment Resource Atlas
Problem: Miami-Dade County suffers from beach erosion that degrades the barrier island’s main storm protection feature, which has been routinely renourished for the last 40 years (the Beach Erosion Control and Hurricane Protection Project). More recently, offshore sediment sources have been depleted, requiring sand from elsewhere for the beaches, which increases cost and does not provide a long-term solution. The Corps needs to know from where and why the sand is eroding, where it is going, and what sustainable management techniques can better manage sediment resources. Baker’s Haulover Inlet and Government Cut Inlet also require maintenance dredging to remove shoaled material and little is known about how these two inlets are hydrodynamically connected and how those dynamics may be influencing sediment transport throughout the county.

Goal: To produce a Miami-Dade County sediment resource atlas, which details sediment pathways and possible sediment resources, serves as a model of county-wide hydrodynamics and offers sustainable management techniques.

Objectives: To collect measurements to describe the overall county-wide circulation and wave climate and ambient sediment transport processes in and around the inlets and beaches; to develop a sediment transport and morphology model for the county; to identify and report any previously released sediment tracers from Miami Harbor; to evaluate best management techniques (i.e., nearshore berms, coastal structures, inlet management, backpassing) to reduce renourishment needs, frequency and cost via numeric modeling and other analysis.