Variability of secondary currents in a weakly stratified tidal inlet with low curvature

Long-term ferry acoustic Doppler current profiler (ADCP) and surface density data collected in the period 2000-2002, and ADCP data and vertical profiles of density collected during several 13-h surveys in the period 2003-2006 provide insight in the variability of instantaneous secondary (transverse) currents in the 4 km wide and 27 m deep Marsdiep tidal inlet, the Netherlands. The application of the data to the transverse momentum balance indicates that centrifugal and Coriolis accelerations and baroclinic pressure gradients govern the observed secondary circulations. In well-mixed conditions, local curvature (|Rs|≈10km) dominates over Coriolis forcing in driving secondary currents. In stratified conditions transverse density gradients become more important than centrifugal and Coriolis accelerations. The data suggests that the well-mixed Marsdiep inlet falls in the regime where stratification still enhances secondary currents. In the inlet the vertical stratification of maximally O(1kgm-3) is in part set up due to transverse density gradients. The stratification dampens turbulent mixing, which allows for surface secondary currents up to 0.4ms-1 (∼30% of the streamwise currents).