Long-term marsh growth and retreat in an online coupled hydrodynamic, morphodynamic and ecological model

Salt marshes are widely regarded as pristine ecosystems serving multiple ecological functions. Moreover, submerged salt marsh have the ability to attenuate wave energy, which makes them suitable as a natural way of flood protection. Given these positive characteristics coastal engineers, managers and policy makers are looking to preserve and restore these salt marshes in salt marsh creation and restoration projects. However, salt marshes are also known as highly dynamic ecosystems and significant changes in their total covered area have been observed in the past. Even for marshes located in relatively close proximity to each other (a few kilometres apart only) both growth and retreat have been observed over decadal timescales. Numerical models are often used to simulate the long-term development of salt marsh ecosystems. In most of these models the establishment of new vegetation is either neglected or included as an idealized stochastic function. In such models, each grid cell has an equal chance of vegetation establishment, even though their physical properties i.e. the hydro- and morphodynamic characteristics are different. Recent field and flume experiments suggest that the chances of seedling establishment (i.e. the establishment of new vegetation) may be a function of bed level dynamics. These theories state that if sediment is eroded underneath a seedling with a rate greater than the growth rate of their roots, seedlings will be uprooted and subsequently fail to establish. On the other hand, if sedimentation occurs over a rate which is greater than the growth rate of the seedlings, seedlings will be buried and thus fail to establish as well. The Windows of Opportunity concept accounts for, and describes, this relation between bed level dynamics and seedling establishment. This concept was implemented in a numerical model (D-Flow FM) in order to create a process based vegetation establishment model. The Windows of Opportunity establishment model was combined with a model which governs the growth and decay of established salt marsh vegetation over time. Together, they form a dynamic vegetation model in which the vegetation field is modelled in a process based way. The model results suggest that the establishment of pioneering vegetation on bare unvegetated mudflats is an important process for the formation of tidal channels. Once a few patches of vegetation are established, flowing water concentrates between these laterally expanding patches of established vegetation. The flowing water increases the bed shear stress leading to erosion which initiates the formation of tidal channels. Due to the erosion between these vegetation patches seedlings are unable to root within these first tidal channel outlines. The rate at which seedlings establish subsequently affects the spatial layout of the tidal channels as well as their width and depth. In case of high vegetation rates, narrow and deep tidal channels are formed which follow the direction of the tidal flow. With slower vegetation rates wider tidal channels are created which are eroded less quickly and show a clear meandering pattern. The presented model may serve as a reference case for further process based vegetation modelling which could include time-varying physical forcings such as waves or river discharges.