Long-term morphodynamics of a large estuary subject to decreasing sediment supply and sea level rise

Abstract Globally the sediment supply from rivers to estuaries is decreasing and the mean sea level is rising, while the effects of these changes on the long-term estuarine morphodynamics have not been fully explored. An idealized one-dimensional model was utilized to investigate the long-term morphodynamics of a large estuary subject to the changes in the sediment supply and sea level. Simplifications involved the use of a 560 km long funnel-shaped channel with fixed banks, constant input water and sediment fluxes, a single grain size and a semi-diurnal tide. A range of values of changes in the sediment supply (50–90% reduction) and sea level (1–5 mm/yr increase) were considered. Starting from an equilibrium state for an initial sediment supply, the estuary shifts to a new equilibrium for the considered changes. The shift is completed on a timescale of millennia, while 50% of the bed level change occurs within several hundreds of years. A larger decrease in the sediment supply results in a stronger bed erosion, while the corresponding adjustment time has minor changes in its range for the considered sediment reduction. In addition to the reduced sediment supply, under sea level rise the erosion is weakened and the adjustment time is shortened. The equilibrium under the considered rates of sea level rise is characterized by a bed level keeping pace with the sea level and a significant amount of sediment being trapped in the estuary. Additional numerical experiments that use the real geometry and more realistic forcing of the Yangtze Estuary show that overall erosion of the estuary is expected in the coming centuries.