Dredging for dilution: A simulation based case study in a Tidal River.

Tidal rivers present complex hydrodynamic conditions that influence pollutant dispersion. Prior research has shown that tidal cycles and sub-grid turbulence affect particle movement. However, the combined impact of dredging and outfall relocation on dilution remains unclear. Existing models often focus on Eulerian frameworks alone, missing the full picture of pollutant transport. This gap motivated the integration of Lagrangian and Eulerian methods to evaluate dredging scenarios. No prior work had resolved the combined effects of dredging and outfall positioning on dilution performance. Tidal advection and sub-grid turbulence are known to influence pollutant flushing but remain poorly quantified in dredged systems. The need for a dual-method approach to assess dredging impacts is well recognized but rarely implemented. This paper addresses the lack of comprehensive simulation-based evaluations in tidal river management.

The study aims to evaluate how dredging and outfall relocation affect pollutant dilution in a tidal river. The specific problem involves a wastewater treatment facility discharging into Oyster River, New Hampshire. The motivation is to determine if dredging alone or in combination with outfall relocation improves dilution efficiency. The study uses a 2-D hydrodynamic model with a Lagrangian particle module to simulate pollutant transport. The goal is to compare three scenarios: current conditions and two dredged configurations. The model incorporates tidal advection and sub-grid turbulence to simulate realistic particle movement. The dual Eulerian and Lagrangian approach allows for a more complete analysis of hydrodynamic and dilution effects. This method enables a detailed comparison of dredging scenarios in a tidal river system.

The study employs a 2-D hydrodynamic finite element model with a Lagrangian particle module. The model simulates the M2 tidal component and includes mud flat flooding and drying effects. A Lagrangian particle tracking method is used with tidal advection and a horizontal random walk to represent sub-grid turbulence. Three scenarios are simulated: a base case and two dredged channel/outfall configurations. Continuous pollutant releases are modeled from the wastewater treatment facility outfall. Eulerian hydrodynamics and Lagrangian dilution improvement ratios are calculated for each case. The model tracks 16,000 particles to ensure statistical significance. The combination of Eulerian and Lagrangian methods allows for a more robust assessment of dilution and flushing.

The simulated hydrodynamics align with observed conditions in the tidal river. Eulerian and Lagrangian residuals indicate an outward path of pollutant flushing on time scales longer than the M2 tidal cycle. Simulated dilution maps show that dredging alone does not significantly improve dilution. Outfall relocation into the dredged main channel is necessary for improved dilution performance. The combination of dredging and outfall relocation yields the best dilution results. The model highlights the importance of sub-grid turbulence in pollutant dispersion. The use of 16,000 particles enhances the statistical reliability of the simulation. The dual method approach provides a more accurate representation of pollutant transport in tidal systems.

The study concludes that dredging alone is insufficient to improve pollutant dilution in tidal rivers. Outfall relocation into dredged channels is required to achieve better dilution performance. The combination of Eulerian and Lagrangian methods provides a more accurate assessment of hydrodynamic and dilution effects. The model's statistical significance is enhanced by using a large number of particles. This approach can be applied to similar tidal river systems worldwide with minimal effort. The methodology improves upon prior studies by integrating Eulerian analysis with Lagrangian tracking. The results suggest that dredging and outfall relocation should be considered together in management decisions. The study emphasizes the value of dual-method simulations in environmental fluid dynamics.