Beyond marsh drowning: The many faces of marsh loss (and gain)

The morphodynamics of tidal marshes is investigated using a novel model (MarshMorpho2D) that couples 2D tidal dynamics, wave dynamics, mass-conserving sediment transport, edge erosion, pond dynamics, and marsh upland migration. By only changing tidal range, sediment supply, and RSLR rate, the model reproduces a great variety of spatial patterns in marsh loss. For all tidal ranges, an increase in RSLR triggers channel widening, whereas a decrease in sediment supply triggers tidal flat deepening and increases wave edge erosion. Marsh loss by pond runaway expansion dominates for intermediate RSLR rates, especially for small tidal ranges and for small sediment supplies. Drowning only takes place in the microtidal setting for high RSLR rates and extremely low sediment supply. Drowning is restricted to the landward portion of the marsh, and lacks the variety of spatial scales associated with pond runaway. For a large tidal range, the amount of marsh loss by drowning and pond runaway is negligible, thus increasing the relative importance of edge erosion and channel widening. Upland migration deepens the channels, which release sediment and thus help sustaining the existing marsh. Nonetheless, marsh upland expansion compensates for existing marsh loss only for a large tidal range or for a high sediment supply, and thus microtidal marshes are likely to experience net loss. Marsh loss is highly heterogeneous, and is not well represented by the dichotomy between marsh survival and marsh loss based on vertical accretion thresholds. Marsh loss can take place by a variety of mechanisms, hindering the ability to generalize predictions about marsh loss.