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

Abstract

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.

Intratidal Variability of Water Quality in the Damariscotta River, Maine

“…The along channel and across channel velocity are defined as u and v, respectively, on a y-z (across channel-depth) axis in several time slices. Turbulence and dissipation data were processed as per Lieberthal et al [6].…”

“…The D4 overtide attenuated in the region between Clark Cove and Briar Cove and amplified as it was forced through the Glidden Ledges constriction near Briar Cove, reaching a maximum amplitude of 2.9% of D2 at New Castle. Since the D4 overtides are caused by a variety of forces, including bottom friction, depth variational friction, and, to a lesser extent, advection and nonlinear continuity [41], the influence of constrictions on this constituent are varied and often unintuitive [6]. On the other hand, the D6 overtide amplitude, as it was solely caused by bottom friction, was inversely proportional to the estuary depth.…”

“…Further upstream, at the Glidden Ledges constriction, the flow dynamics were heavily influenced by the 600 m constriction, in which flow was forced through the constriction and into the eastern channel during flood phase [6]. In this region, elevated was observed throughout the water column but only in the flood phase.…”

“…Previous research into the Damariscotta estuary explored the unique features of the headland located north of a constricted sill at Glidden Ledges, and its effects on tidal dynamics [6]. A gyre forms upstream of the headland during the flood phase, which enhances inland flow over the eastern channel and is countered by seaward flow over the western shoals.…”

Observations of Near-Surface Mixing Behind a Headland

“…What has not been previously reported is if lateral processes generated by along-channel variations in bathymetry can cause near-surface mixing. Near-surface mixing is clearly important for the vertical exchange of momentum and is increasingly being reported in various systems (e.g., [20][21][22]).…”