An exploratory study of the variability of currents and density in the Marsdiep

Observational data of tidal currents and vertical stratification in a periodically-stratified estuarine basin are presented in this study. The data were collected between 2010 and 2012 during multiple surveys in the Marsdiep basin, which is part of the Western Dutch Wadden Sea. The aim of this study is to better understand the spatial variability of the tidal currents and the occurrence of vertical stratification in the Marsdiep basin. A harmonic analysis is applied to the depth-averaged velocity to obtain estimates of the tidal ellipse parameters of the M2 tidal constituent along the surveys. The spatial variation in magnitude of the tidal currents is complex but seems mainly related to water depth. This relationship varies in time on a daily to fortnightly timescale, which results in a variation of the lateral velocity shears over time. Furthermore, the tidally-averaged lateral density gradient is of the same order of magnitude as the longitudinal density gradient, which implies that cross-stream processes are important in the Marsdiep basin. At some locations, vertical stratification is only observed during the flood phase which seems mainly driven by lateral processes. This pattern is inconsistent with the classical alongstream tidal straining mechanism, where vertical stratification develops during ebb and diminishes during flood. It is hypothesized that vertical stratification during flood increases from neap to spring tidal conditions due to increased differential advection. The strength and timing of vertical stratification is shown to be highly spatially variable in the Marsdiep basin, which might have implications for the residual circulation patterns.