Turning the tide: The effect of river discharge on estuary dynamics and equilibrium

Abstract

The impact of human intervention on long-term morphological evolution of estuaries is unknown. Numerical models fail to predict natural dynamics and are sensitive to constitutive parameters. On the other hand, physical models were hampered by scaling problems in the past century. Recently, is has been discovered that a periodically tilting flume generates dynamic tidal morphology. This enables experimental investigation to complement numerical modelling. I studied the effects of upstream river discharge on the small-scale channel-shoal dynamics and large-scale equilibrium planform and dimensions of estuaries. A behaviour-oriented model was used to predict the effect of river discharge on equilibrium dimensions of estuaries. The model results were further tested in scaled laboratory experiments in which I systematically varied the amount of river inflow. Typical values of river flow velocity divided by tidal flow velocity were 0.015-0.030. Tilting amplitude was 3-4 mm with a flume of 3.8 m length. River inflow increases the width of the upper estuary and estuary mouth, resulting in larger total surface area and larger tidal prism. Furthermore, river inflow increases small-scale dynamics of estuaries, particularly migration and shifting of channels and reversals in ebb and flood dominance of channels. This shows that river inflow is one of the key drivers for estuarine morphodynamics and must be included in any long term prediction. The results of this study have economic and ecological implications for management of estuaries, including dam construction, land reclamation and mining activities, as well as scientific implications for example for palaeoreconstruction of the Rhine-Meuse delta.