Holocene evolution of tidal systems in The Netherlands: Effects of rivers, coastal boundary conditions, eco-engineering species, inherited relief and human interference

Estuaries and tidal embayments are partly enclosed coastal bodies of water with a free connection to the open sea at their tidal inlet and with minimal (tidal embayments) or substantial fluvial input (estuaries). Their tidal inlets can only remain open over multiple centuries to millennia when (1) the formation of accommodation space exceeds infilling or (2) the inlet system is in dynamic equilibrium (i.e., sediment input equals output). Numerical modeling studies often suggest that estuaries and tidal embayments can develop toward a dynamic equilibrium under constant boundary conditions and consequently remain open over centuries to millennia, whereas in the Holocene sedimentary record many estuaries and tidal embayments are observed to have filled up and closed off. This raises the questions whether and how tidal inlets can remain open over long timescales (centuries to millennia), and what the effects are of river inflow and sediment supply. Here we compare the long-term evolution of contrasting tidal systems along the Dutch coastal plain to empirically identify the most important factors that control their long-term evolution. We study tidal systems along the Dutch coast because of (1) high data density, (2) abundant well-preserved and well-described estuaries and tidal embayments with contrasting boundary conditions and morphodynamic evolution and (3) their low-sloping setting with soft boundaries. This makes contrasting estuarine dimensions and development largely dependent on initial conditions, boundary conditions and internal biogeomorphological processes.

In the Middle Holocene, Dutch estuaries and tidal embayments were mainly formed by rapid relative sea-level rise. In the late Holocene, they were predominantly the result of natural and human-induced subsidence in coastal plain peatlands. Tidal inlets connected to rivers (estuaries) persisted and attained dynamic large-scale equilibrium while tidal embayments without or with a marginal fluvial inflow were unstable and closed off under abundant sediment supply. Estuaries probably attained a quasi-stable configuration wherein sediment input equaled export due to river-enhanced ebb flow, until fluvial influx was cut off by upstream avulsion causing transition to an embayment and system closure. Long-term net import of sediment from the sea into Dutch tidal embayments is favored by strong, flood-dominated, tidal asymmetry along the Dutch coast, the shallow sand-rich floor of the North Sea, erosion of inherited coastal promontories, and the abundance of mud in the coastal area supplied by the Rhine and Meuse rivers. While sandy tidal embayments without fluvial feeders and with fixed boundaries may obtain dynamic equilibrium and remain open over long timescales, we hypothesize that an abundance of mud and eco-engineering species often culminates in continuous embayment filling with fine sediment and the growth of intertidal and supratidal areas, eventually resulting in closure of the embayment.