Modeling dredging plumes in an estuary

Overview

The construction of a submarine outfall required trench excavation and backfilling within an estuarine environment. These activities inevitably generate suspended sediment plumes, which can move with the local currents and potentially affect water intakes, navigation areas, and nearby coastal zones. Understanding the magnitude and extent of these plumes under realistic environmental conditions was essential for planning the works and defining appropriate monitoring measures.

HCS carried out a numerical modeling study to simulate the formation, evolution, and dispersion of turbidity plumes during dredging operations. The analysis supported decision-making by predicting the spatial footprint of suspended sediments under different combinations of tides, winds, and sediment characteristics.

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The Challenge

Dredging temporarily increases suspended sediment concentrations above natural background levels. The estuary where the outfall was to be built is characterized by:

• Variable tidal currents
• Episodic wind-driven flows
• Spatially heterogeneous background turbidity
• Fine sediments that can remain suspended for long periods

Given the proximity of sensitive sites—including water intakes and recreational coastal areas—it was necessary to assess:

• How far the plume could extend under typical and adverse conditions
• How long elevated turbidity would persist
• Whether dredging activities could increase risks for nearby users
• What monitoring program should accompany construction

These questions required a site-specific, scenario-based modeling approach.

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Our Contribution

HCS developed a hydrodynamic and sediment transport model tailored to the study area, incorporating site bathymetry, sediment properties, and relevant environmental forcing.

1. Hydrodynamic Simulation

A numerical model of the estuary reproduced the tidal and wind-driven circulation patterns that control plume transport. The model resolved local flow variability around the trench area and along the adjacent coastline.

2. Sediment Resuspension and Transport

Suspended sediment dynamics were represented using a transport module that accounted for:

• Local release of fine sediments during dredging
• Settling and deposition processes
• Transport by ambient currents
• Interaction with the background turbidity field

3. Scenario Analysis

Multiple simulations were carried out to reflect operational and environmental variability, including:

• Different dredging intensities
• Variations in sediment release volumes
• Tidal cycles and representative wind conditions
• Sensitivity tests for sediment settling velocities

These scenarios provided a range of possible plume behaviors rather than a single deterministic estimate.

4. Monitoring Strategy

Based on model outcomes, HCS proposed a monitoring plan that included:

• Real-time turbidity sensors placed at strategic locations
• Complementary observations during critical construction stages
• Trigger levels for adaptive management

This ensured transparency during operations and allowed timely response to atypical turbidity events.

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Figure 1: Model mesh, general view

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Key Findings

The modeling revealed:

• The plume remains concentrated near the dredging area under typical tidal conditions, with gradual dilution downstream.
• Under certain wind and tide combinations, suspended sediments can be transported farther alongshore, although remaining within a manageable extent.
• Turbidity elevations are temporary and decrease substantially after dredging stops, as settling processes dominate.
• Model results provide a clear framework for identifying the periods and locations of greatest sensitivity.

These findings helped determine whether construction could proceed safely relative to nearby users and infrastructure.

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Outcomes and Client Value

This study provided:

• A scientifically grounded assessment of potential turbidity impacts
• Scenario-based predictions to support planning and scheduling of dredging
• A practical monitoring strategy aligned with modeled plume behavior
• A defensible methodology for communicating environmental performance to regulators and stakeholders

The modeling system delivered by HCS offered clear, quantitatively informed guidance for managing suspended sediment during construction in a complex estuarine setting.