Vertical and temporal fluid mud dynamics during spring-neap tidal cycles

Abstract. The hyper-turbid Ems estuary has undergone extensive channel deepening since the 1980s resulting in distinct tidal asymmetry and substantial sediment accumulation which led to the occurrence of a fluid mud layer. This might cover up to 60 % of the water column, creating density-driven stratification that significantly affects hydrodynamics, ecology, and navigability. During a six-week monitoring period we investigated fluid mud dynamics with the focus on occurrence, thickness, density, and driving processes to obtain insights about formation and break-up of the fluid mud layer. We identified two superimposed recurring cycles of fluid mud occurrence. On a shorter timescale, we observed distinct differences over the semidiurnal tide while on a longer timescale fluid mud occurrence shows a distinct spring-neap tide relation. We conducted dedicated measurement campaigns around spring and neap tide to further investigate spring-neap variability of fluid mud occurrence. One main finding of those campaigns is that during neap tide, the majority of the water column is covered by a dense fluid mud layer, thus reducing the hydrodynamic cross-section. This leads to increased current velocity and entrainment at the lutocline, and reduced stability of the fluid mud layer. In contrast during spring tide, the hydrodynamic cross-section remains wider, leading to less friction at the lutocline and thus enhances stability of the fluid mud layer and thus longer persistence of fluid mud.