Impact of Environmental Change on Material Transport

Project Description

Estuarine circulation is the mechanism governing long-term material transport in estuaries, such as plankton, suspended sediment, and pollutants, and is important to understand for aquaculture sustainability in estuaries under environmental change. Many studies have shown the lateral variability of this transport can vary significantly across the estuary and during different forcing conditions. The underlying physics of this transport needs to be determined observationally before it can be accurately modeled.

In 2016 (dry season), project personnel conducted four tidal cycle surveys across a lateral transect at different locations (upper and lower reaches) in the Damariscotta River, during spring and neap tidal forcing conditions. This field campaign was repeated in wet and dry seasons in the Bagaduce River in October 2017 and April 2018. Researchers collected cross-sections of current velocity, turbulence, density, suspended sediment concentration, and plankton biomass. These data are being used to quantify residual transport patterns and to predict how environmental change (increasing precipitation and sea level rise) will change system dynamics. These data are being provided to Theme 1 for validation of the FVCOM model and to ensure the accuracy of residual transport processes.

Results and Accomplishments

The main findings during 2017-2018 for the Damariscotta River were:

1. Subtidal flow exhibit varying patterns: vertically sheared in the lower reach and a mix of laterally and vertically sheared in upper reaches.
2. The complexity of the river geometry, including channel curvature and bathymetry, impact the subtidal momentum balances, which shape subtidal flow patterns.
3. Turbulence and mixing is more prominent up river due to these complexities, though the events are more intermittent than closer to the mouth, where geometries are more complex.
4.  A curved underwater channel in the northern reach of the Damariscotta Estuary generates a centripetal force that creates a secondary flow. This flow is expected to be a single cell circulation. This centripetal force produces an eddy that dominates the subtidal flow pattern. The eddy also causes faster water to flow over slower water, and vice versa, resulting in vertical shears in velocity.
5. The eddy north of Glidden ledges creates a phenomenon called an overtide. This results in velocities that have a quarter diurnal periodicity, which vary in location between spring and neap. During spring, overtides are generated near bottom in the channel due to bottom friction and advection. During neap tide, overtides are generated near the surface, purely from advection from the eddy.
6. Overtides are an important factor in sediment suspension but only if they persist to the bottom of the water column (i.e. during spring). However, the near surface overtides may be important for flow through aquaculture farms that are located near the surface during neap tides.

The main findings during 2017-2018 for the Bagaduce River were:

1. The subtidal flow structure near the mouth of the Bagaduce River features an interesting laterally sheared structure, with inflow on the right hand side (looking into the estuary) and outflow looking seaward.
2. The greatest density stratification during the dry season was observed during the flood phase of the tide due to salinity intrusion and greater turbulence was observed during ebb.

Summary of Data Being Collected

Data	Type	Quantity	Location
Current Velocities	Field	TBD	Damariscotta River and Bagaduce River
Density	Field	TBD	Damariscotta River and Bagaduce River
Turbulent kinetic energy dissipation rate	Field	TBD	Damariscotta River and Bagaduce River
Suspended sediment concentration	Field	TBD	Damariscotta River and Bagaduce River
Fluorescence	Field	TBD	Damariscotta River and Bagaduce River