Insights in marsh forest boundary transgression in response to storms and sea-level rise from modelling

Salt marshes provide critical ecosystem services at the land-sea interface and the lateral extent and vertical stability of salt marshes to rising sea levels have been shown broadly to be functions of interacting drivers and ecogeomorphic feedbacks. As a result, nonlinear behavior can emerge in these systems including how lateral salt marsh loss due to wave erosion may provide inorganic sediment to the marsh platform helping to maintain vertical stability. This lateral loss can be offset by relative sea-level rise (RSLR) providing an opportunity for marsh expansion through upland migration. In order to explore the interaction of ecogeomorphic feedbacks and stochastic drivers, a two-dimensional transect model was developed to examine changes in marsh lateral extent, vertical stability and the potential for marsh transgression inland forced by synthetic stochastic time series of water level deviations and winds. While marsh upland transgression is broadly controlled by slope and RSLR, actual rates are related to the timing and extent high water events which can lead to sudden large marsh expansion. Persistence of this marsh expansion is dependent on forest recovery rates from RSLR and further flooding events. The model demonstrates that in contrast to the relatively steady erosion of the seaward marsh edge, or a steady slope dependent landward migration for steeper slope environments; when encountering moderate and low slope environments, the landward marsh edge is controlled by the interaction of the timing and frequency of extreme events and forest recovery rates, resulting in so called punctuated transgressive events. However, as RSLR rates increase, the importance of this interplay diminishes, implying that for high RSLR rates, migration rates again respond to a slope-RSLR dominated process.