Effects of mud supply on large-scale estuary morphology and development over centuries to millennia
Lisanne Braat1, Thijs van Kessel2, Jasper R. F. W. Leuven1, and Maarten G. Kleinhans11Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, the Netherlands 2Deltares, Boussinesqweg 1, 2629 HV Delft, the Netherlands
Received: 07 Mar 2017 – Accepted for review: 20 Mar 2017 – Discussion started: 24 Mar 2017
Abstract. Alluvial river estuaries consist largely of sand but are typically flanked by mud flats and salt marshes. The analogy with meandering rivers, that are kept narrower than braided rivers by cohesive floodplain formation, raises the question how large-scale estuarine morphology and late Holocene development of estuaries are affected by cohesive sediment. In this study we combine sand and mud transport processes and study their interaction effects on morphologically modelled estuaires on centennial to millennial time-scales. The numerical modelling package Delft3D was applied in 2DH starting from an idealised convergent estuary. The mixed sediment was modelled with an active layer and storage module with fluxes predicted by the Partheniades-Krone relations for mud, and Engelund-Hansen for sand. The model was subjected to a range of idealised boundary conditions of tidal range, river discharge, waves and mud input. The model results show that mud is predominantly stored in mudflats on the side of the estuary. Marine mud supply only influences the mouth of the estuary whereas fluvial mud is distributed along the whole estuary. Coastal waves stir up mud and remove the tendency to form muddy coastlines and the formation of mudflats in the downstream part of the estuary. Widening continues in estuaries with only sand while mud supply leads to a narrower constant width and reduced channel and bar dynamics. This self-confinement eventually leads to a dynamic equilibrium where lateral channel migration and mud flat expansion are on average balanced. However, for higher mud concentrations, higher discharge and low tidal amplitude the estuary narrows and fills to become a tidal delta.
Braat, L., van Kessel, T., Leuven, J. R. F. W., and Kleinhans, M. G.: Effects of mud supply on large-scale estuary morphology and development over centuries to millennia, Earth Surf. Dynam. Discuss., doi:10.5194/esurf-2017-14, in review, 2017.