The Generation of Overtides in Flow Around a Headland in a Low Inflow Estuary

The Damariscotta River in midcoast Maine is a weakly stratified estuary characterized by several constrictions and channel bends that affect tidal asymmetry and material transport. Microstructure and current velocity measurements were collected at a cross-channel transect in the northern reach of the estuary during spring and neap tidal conditions to study the effects of a channel bend immediately upstream of a constricted sill on intratidal dynamics. During the flood phase, a counterclockwise gyre is formed upstream of the headland, which enhances landward-directed flow in the channel and is countered by seaward-directed flow over the shallow shoal. Semidiurnal and quarter-diurnal patterns of lateral advection and stress divergence emerge in the surface layer because of these secondary flows. Lateral advection effects dominate the dynamics in neap tide, and although they are stronger in spring tide, they are partially obscured by bottom friction forces that are proportional to the along-channel velocity squared. A novel harmonic decomposition technique is introduced to determine the relative importance of advection and stress divergence on quarter-diurnal velocity generation, and their implications to neap/spring variability in estuarine water quality are discussed. Plain Language Summary The Damariscotta River in midcoast Maine is a weakly stratified estuary characterized by several constrictions and channel bends that affect tidal asymmetry and material transport. This article is an in-depth study on how a headland near the Glidden Ledges constriction affects the flow dynamics of the northern estuary. Microstructure and current velocity measurements were collected at a cross-channel transect in the northern reach of the estuary during spring and neap tidal conditions. During the flood phase, a counterclockwise gyre is formed upstream of the headland, which enhances landward-directed flow in the channel and is countered by seaward-directed flow over the shallow shoal. These flow dynamics affect tidal asymmetry through both lateral and vertical velocity gradients. They must also be considered alongside frictional forces between the estuary current and sediment along the bottom. This research helps to explain what makes the upper estuary so effective at retaining warm water and nutrients vital to the oyster aquaculture industry.