Hydrodynamic Impacts of Expanding Aquaculture

Project Description

The projected increase in the demand for aquacultured seafood presents an economic opportunity to expand aquaculture production in Maine. Many studies show that aquaculture farms reduce tidal current magnitude at the surface and bottom, which impacts nutrient and waste transport. Without considering the impact of a farm on flow, and hence nutrient supply and waste removal, carrying capacity may be overestimated. This project aims to study the hydrodynamic impact of aquaculture farms by utilizing field observations, numerical simulations, and analytical models.

Cross-sections of current velocity, turbulence, density, turbidity, and fluorescence are collected before and after a farm. These observations are used to understand how the farm impacts systems dynamics. A computational fluid dynamics (CFD) model, with large eddy simulation (LES), is used to mimic the observed farms and evaluate hydrodynamic impact. A bulk drag coefficient varying with farm size and aquaculture type, derives the observations and simulations. The numerical model and field measurements develop farm layout guidelines for optimal flow. Additionally, an analytical model framework is combined with a laboratory experiments to understand the interaction between waves and floating oyster farms.

Results and Accomplishments

In work performed in 2017-2018, it has been determined that a floating oyster farm modifies the vertical velocity profile and causes lateral flow. The farm induced friction near the surface has been quantified based on field-collected data and can be implemented as a bulk drag in numerical models.

It is observed that a floating oyster farm enhances turbulence dissipation and mixing, which is beneficial to material transport. During flood phases of the tide, flow travels under the farm. This results in flow acceleration near the bottom that can mobilize sediment and scour secondary channels. During the ebb phase of the tide, due to the shallowness of the water levels, flow is diverted laterally around the farm. This indicates an asymmetry in sediment transport below the farm between flood and ebb.

Summary of Data Being Collected

Data	Type	Quantity	Location
Velocity and turbulence	Model	12 hour surveys in upper, lower and mid-reach lateral transects, 8 field surveys total	Damariscotta River
Current velocity	Field	12 hour surveys in upper, lower and mid-reach lateral transects, 2300 profiles total	Damariscotta River and Maine coastal area
Turbulence kinetic energy dissipation rate	Field	12 hour surveys in upper, lower and mid-reach lateral transects, 2300 profiles total	Damariscotta River and Maine coastal area
Reynolds Stress	Field	12 hour surveys in upper, lower and mid-reach lateral transects, 2300 profiles total	Damariscotta River and Maine coastal area
Suspended sediment concentration	Field	12 hour surveys in upper, lower and mid-reach lateral transects, 2300 profiles total	Damariscotta River and Maine coastal area
Fluorescence	Field	12 hour surveys in upper, lower and mid-reach lateral transects, 2300 profiles total	Damariscotta River and Maine coastal area