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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union

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Earth Surf. Dynam., 5, 617-652, 2017
https://doi.org/10.5194/esurf-5-617-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
09 Oct 2017
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. Kleinhans1 1Utrecht University, Heidelberglaan 2, 3584 CS Utrecht, the Netherlands
2Deltares, Boussinesqweg 1, 2629 HV Delft, the Netherlands
Abstract. Alluvial river estuaries consist largely of sand but are typically flanked by mudflats and salt marshes. The analogy with meandering rivers that are kept narrower than braided rivers by cohesive floodplain formation raises the question of how large-scale estuarine morphology and the 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 estuaries on centennial to millennial timescales. The numerical modelling package Delft3D was applied in 2-DH 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 in which lateral channel migration and mudflat expansion are balanced on average. However, for higher mud concentrations, higher discharge and low tidal amplitude, the estuary narrows and fills to become a tidal delta.

Citation: 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., 5, 617-652, https://doi.org/10.5194/esurf-5-617-2017, 2017.
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Short summary
Mud raises concern in the short-term management of estuaries, but it is not known whether cohesive mud affects the long-term development of estuaries. We discovered that a small supply of mud from the river confines the estuary by forming stable mudflats on the sides in centuries, whereas estuaries with only sand continue to grow. Mudflats also reduce the shifting of channels and bars. This implies that changes in mud supply in estuaries may have led to changes in shape and dynamics in the past.
Mud raises concern in the short-term management of estuaries, but it is not known whether...
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