Eco-morphodynamics of coastal wetlands

We survey the problem of the response of coastal wetlands to sea level rise. Two opposite views have traditionally been confronted. According to the former, on the geological time scale, coastal lagoons would be ‘ephemeral’ features. The latter view maintains that marshes would keep pace with relative sea level rise as, increasing the rate of the latter, the sedimentation rate would also increase. In any case, the timescale of morphodynamic evolution is of the order of centuries, which makes it not easily perceived. For example, in Venice, the diversion of the rivers debouching into the lagoon undertaken in the Renaissance has taken centuries to display its consequences (shift from depositional to erosional environment). This process accelerated in the last two centuries due to effects of the industrial revolution and of an enhanced sea level rise. Recent research has employed powerful computational techniques and advanced models of marsh vegetation. Zero-order modeling suggests that marsh equilibrium is possible, provided the rate of relative sea level rise does not exceed a threshold depending on the availability of minerogenic sediments, quantified through a loosely defined ambient sediment concentration. Analysis of the morphological interaction between adjacent morphological units suggests that the ‘equilibrium states’ identified by zero-order modeling correspond to marshes which either prograde or retreat, i.e., are not in equilibrium. Results suggest that available techniques, e.g., artificial replenishment of salt marshes or search for more productive halophytic species, will hardly allow Venice wetlands to keep up with a strong acceleration of sea level rise.