An estuary out of equilibrium: The importance of dredging in determining the net sediment flux in the Rhine-Meuse Estuary

Sediment budgets summarize the complex sediment dynamics and hydrodynamics present in coastal and fluvial systems. By documenting the net gain or net loss of sediment in the region, a sediment budget determines whether an outward building delta or ingressive estuary system exists at the fluvial-tidal transition.A lack of sediment in a delta leads to a myriad of problems including extreme channel deepening, a loss of ecologically rich areas and increased flooding (Best 2019). It is important to understand the processes and controls which determine the sediment budget in this complex system of tides, rivers and human interference: which parts of the budget critically impact the sediment budget and which are of less importance? A detailed sediment budget which provides insight into the sediment fluxes can help to devise strategies to sustainably manage sediment. Frings ten Brinke (2018) clearly summarize the main reasons to continue to use sediment budgets for river and delta management and in particular how they are useful in the Rhine-Meuse Estuary (RME). Here, a sediment budget for the period 2000-2018 is calculated for the Rhine-Meuse Estuary (RME) in the west of the Netherlands. The sediment budget was created by two methods: 1) By considering the fluxes in and out of the system, dredging and dumping volumes and suspended and bedload concentrations 2)By using multi-beam bathymetric data. Both methods compute an overall negative budget which is double the average of the previous budgets for the RME due to changes in the mouth area since 2012. This is largely due to the doubled volume of dredging in ports and harbours in the RME since 2015. Intense dredging is the main control of the sediment budget: without dredging the RME would net gain sediment. Changing dredged volumes near the mouth are directly linked to the flux of sediment at the North Sea boundary, previously assumed to be stable over time, a further key factor in determining the sediment budget. This calls for a new estimation of the incoming sediment at the North Sea to verify if the net import of sediment has increased and to what extent it has increased.