Mixing layer dynamics in separated flow over an estuarine sill with variable stratification

We investigate the generation of a mixing layer in the separated flow behind an estuarine sill (height H ∼ 4 m) in the Snohomish River, Washington as part of a larger investigation of coherent structures using remote and in situ sensing. During increasing ebb flows the depth d and stratification decrease and a region of sheared flow characterized by elevated production of turbulent kinetic energy develops. Profiles of velocity and acoustic backscatter exhibit coherent fluctuations of order 0.1 Hz and are used to define the boundaries of the mixing layer. Variations in the mixing layer width and its embedded coherent structures are caused by changes to both the normalized sill height H / d and to a bulk Richardson number Ri h defined using the depth of flow over the sill. Entrainment E T and the mixing layer expansion angle increase as stratification and the bulk Richardson number decrease; this relationship is parameterized as E T = 0.07 Ri h −0.5 and is valid for approximately 0.1 < Ri h < 2.8. Available comparisons with literature for inertially dominated conditions ( Ri h < 0.1) are consistent with our data and validate our approach, though lateral gradients may introduce an upwards bias of approximately 20%. As the ratio H / d increases over the ebb, the free surface boundary pushes the mixing layer trajectory downward, reduces its expansion angle, and produces asymmetry in the acoustic backscatter (coherent structures). Three‐dimensional divergence, as imaged by infrared video and transecting data, becomes more prominent for H / d > 0.8 due to blocking of flow by the sill.