Modeling the Dynamics of Salt Marsh Formation

Salt marshes are important habitats and coastal defenses. Their valuable ecosystem services are spurring projects for marsh restoration and expansion around the world. However, it is difficult to forecast the final extension of the vegetated area in a restored marsh and what drivers control vegetation cover. Herein, we use a 3D fully coupled vegetation-hydrodynamic-morphological modeling system (COAWST), to simulate marsh formation in a typical configuration used in land reclamation projects. The final vegetation cover and the timescale to reach it are derived from simulations with various forcing conditions. We found that marsh formation can be divided in three distinctive phases: a preparation phase characterized by sediment accumulation in the absence of vegetation, an encroachment phase in which the vegetated area grows, and an adjustment phase in which the vegetated area remains relatively constant while marsh accretes vertically to compensate for sea level rise. High sediment concentration and settling velocity generally help marsh expansion and reduce the time needed to reach equilibrium, while high rates of sea level rise hinder marsh formation. Large tidal ranges increase the area covered by the marsh at equilibrium while 1~2 meters of tidal range induces rapid expansion. The Unvegetated-Vegetated Ratio (UVVR) was already related to sediment budgets in degrading marshes (Ganju et al 2017). Our simulations show that UVVR also relates to sediment budget in marsh formation under most conditions.