Regime Shift to Hyperturbid Conditions in the Loire Estuary: Overview of Observations and Model Analysis of Physical Mechanisms

The Loire estuary (France) was extensively deepened during the 20th century. Coincidentally, suspended sediment concentrations increased drastically from ∼0.1 g/l to ∼1–5 g/l at the surface and the estuarine turbidity maximum (ETM) moved upstream. In this study we, for the first time, brought together a century of observations of estuary bed level, tidal amplitude, and sediment concentration to demonstrate these large changes. Next, we analyzed a minimal set of physical mechanisms that explain the dramatic increase in sediment concentration. To this end, we used the iFlow model representing dynamic equilibrium conditions in the Loire. Novel in the model is that it dynamically resolves salt stratification and corresponding damping of turbulence. For conditions representing the year 2000, high sediment concentrations were found with satisfactory correspondence to observations. Low sediment concentrations were found when using the year 1900 bed level but keeping all other model parameters the same. Varying the bed level gradually between these two extremes, the equilibrium solution suddenly increases for intermediate bed level, constituting an abrupt regime shift. Robustness of this result was established in an extensive sensitivity study featuring 13,200 model experiments. The regime shift is enabled by a feedback between increasing sediment concentration, reducing turbulence due to sediment and salt stratification, and increasing sediment importing capacity of the estuary. The essential sediment importing mechanisms in this feedback are related to the tidal asymmetry and gravitational circulation. This is the first time gravitational circulation and salt stratification are shown to be important factors in a transition to hyperturbidity.