Modelling estuarine variations in tidal marsh sedimentation: response to changing sea level and suspended sediment concentrations

Abstract

This study presents the validation of a zero-dimensional time-stepping physically based model (MARSED) to simulate the varying rates of long-term (10–100 years) tidal marsh accumulation within an estuary. First, field data on long-term tidal marsh accumulation were collected for 25 marsh sites scattered along the Scheldt estuary (NW Europe), based on old topographic data and radiometric and paleoenvironmental dating of sediment cores. The field data showed that estuarine marshes accumulate at highly varying rates depending on (1) the age of the marsh, (2) estuarine variations in mean high water-level (MHWL) rise, and (3) variations in suspended sediment concentrations (SSCs). As a general mechanism, young low marsh surfaces accumulate quickly and asymptotically up to an equilibrium level around MHWL. After this, high old marshes accumulate much slower at rates that are comparable to local MHWL rise. Furthermore, marsh accumulation rates are higher in the inner part than in the outer part of the estuary. This difference can be attributed to the faster MHWL rise and higher SSC values in the inner estuary. Second, the MARSED model was validated against the field data. The model was able to simulate the observed variations in marsh accumulation rates with good accuracy. Furthermore, the model allows to quantify the combined effect of sea-level change and SSC on variations in accumulation rates: they showed that tidal marshes can maintain their equilibrium elevation around MHWL, only if incoming SSC is high enough. Finally, the model allows to predict marsh accumulation rates in response to changing environmental conditions. Simulations for the next 100 years suggest that the tidal marshes in the Scheldt estuary will be able to keep up with the rising MHWL, unless MHWL rise would increase and SSC would decrease importantly.