Date of Award

Spring 5-30-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Advisor

Kimberly Huguenard

Second Committee Member

Lauren Ross

Third Committee Member

Shaleen Jain

Additional Committee Members

Huijie Xue

David W. Fredriksson

Abstract

Climate change may potentially change aquatic systems and bring certain risks for aquaculture development. Understanding interactions between aquaculture and the environment helps to ensure aquaculture expansion is sustainable in the future. It is critical to determine how farms influence tidal flow patterns, turbulence, mixing and material transport in estuaries. This research aims to determine the flow response of an oyster farm, predict how expanding farms and farm placement will alter estuarine dynamics, and understand how the design of a farm influences material transport.

The hydrodynamic response of a floating oyster aquaculture farm in a low inflow estuary (the Damariscotta River estuary) is investigated using hourly field observations covering both neap and spring tidal conditions and an idealized numerical model. Given the importance of lateral processes in estuaries, particularly those with channel complexities such as channel bends, we hypothesize that the farm-imposed drag force will affect the nearby dynamics in the channel and that the farm effects will not be localized to the farm area. A bulk drag coefficient for the whole farm, as well as for a single oyster cage was derived and implemented into an idealized regional scale model to qualitatively repeat patterns observed from field. The qualitative consistency between field observation and idealized model results provides valuable insight into the hydrodynamic response of a floating oyster farm.

The field observations also depicted a reversal in subtidal flow patterns compared with those typically expected in an elongated estuary. To better understand the mechanisms driving subtidal flow reversal, a semi-analytical model for a low inflow estuary with farm drag force was developed. The model captured surface flow reduction and flow bypassing, consistent with field observations. Without the farm, subtidal flows were laterally sheared with inflow on the right-hand side and outflow on the left. The reduction of tidal flow from drag force in farm area resulted in tidally averaged along channel advection at the seaward and landward farm boundaries that drove subtidal flows into the farm. Inside the farm, the reduced tidal currents near the surface combined with upwelling and downwelling at the channel-shoal interface to produce tidally rectified flow, which altered the subtidal flow structure compared to the case with no farm. The transport of Lagrangian particles demonstrated how various farm expansion scenarios hindered seaward long-term transport in the estuary portions upstream of the farm.

The semi-analytical hydrodynamic model combined with a material transport model was further applied to investigate the sensitivity of farm layout to food uptake, which showed that a bluff layout (wide and narrow) was optimal since more nutrients can be transported into the farm through wider landward or seaward boundary. Expanding individual farm size decays filtration per unit area in the farm, where the filtration over a tidal cycle per unit area yields a logarithmic decay with length expansion and hyperbolic decay with width expansion. Therefore, the feedback between the hydrodynamics and the farm can deteriorate the food supply. Additionally, in shallow estuarine locations, bottom generated turbulence can overcome weak stratification to transport bottom sediment upward, resulting high near surface turbidity that might negatively impact oyster growth. Based on the linkage between near surface turbidity to tidal mixing, and stratification a critical depth for farm siting was proposed to minimize surface water turbidity.

Outcomes from this work highlight the importance of understanding interactions between aquaculture and environments. The hydrodynamic and hydrographic conditions that control species growth factors are highly variable and site specific, therefore acquiring detailed environmental data and thoughtfully evaluating interactions between aquaculture and the environment are beneficial approaches for farm planning. Both a field data collection strategies and modeling tools from this work can be used to promote environmental and economical sustainability in aquaculture expansion in the future.

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