Role of straining and advection in the intratidal evolution of stratification, vertical mixing, and longitudinal dispersion of a shallow, macrotidal, salt wedge estuary

A month of flow observations in the Snohomish River Estuary reveals the complex intratidal and fortnightly stratification, mixing, and dispersion dynamics in this macrotidal, shallow, salt wedge estuary system. Both salt wedge propagation and concomitant straining of the density field dominate temporal and spatial variations in stratification leading to intratidal variability of shear and mixing that differs in important ways from observations in partially mixed estuaries. Bottom-generated turbulent kinetic energy production is enhanced during spring tides and acts in concert with straining to counteract advection and minimize vertical stratification during the spring flood tides. This bottom-generated mixing contributes to a buoyancy flux near the top of a well-mixed layer during strong flood tides. During strong ebb tides, interfacial shear production and buoyancy flux occur along the sharp straining-enhanced interface just before the system becomes well mixed. Longitudinal dispersion is less sensitive to the spring/neap cycle yet exhibits strong intratidal variability. Reduced longitudinal dispersion is observed during the large floods relative to the rest of the tidal cycle, behavior we attribute to a lack of vertical shear. Overall, ebb tide advection and straining enhance stratification and longitudinal dispersion and allow for interfacial mixing. Intratidal variability, which varies on the spring/neap scale, is a dominant feature of this estuary, suggesting the importance of intratidal processes and tidally varying mixing coefficients in similar strongly stratified, strongly forced estuaries.