The role of the shoal – salt mixing mechanisms in a partially mixed tidal estuary with channel-shoal geometry

The estuarine exchange flow is inherently linked to salt mixing which transports water across isohaline surfaces. For the partially mixed, northwest German Elbe estuary, such mixing occurs predominantly at the channel-shoal transition. Here, we aim to identify the mechanisms leading to such near-shoal mixing. To this end, a highly resolved numerical simulation of the Elbe estuary was accounted for in a process-oriented model performance evaluation and then applied for a selected spring-neap cycle, assessing elevated mixing in space and time. Throughout the spring-neap cycle, the timing of mixing was limited by the availability of stratification rather than tidal energy, leading to an early mixing peak at neap. For the corresponding tidal cycle of most intense mixing, we find two main mechanisms of near-shoal mixing despite a high, topography-related spatial variability: First, lateral straining during flood leads to the formation of a salty bottom layer above flat shoals. This layer can be reinstated during ebb via longitudinal straining. Strong shear leads to intense mixing at the halocline during flood and ebb. Second, the intersection of a sufficiently strong halocline with the turbulent channel sidewalls results in intense boundary layer mixing. Under strong stratification, the intensity of near-shoal mixing relative to channel mixing increases as persistent stratification stabilizes the channel throughout the entire tidal cycle while sidewall mixing intensifies.