Exposed salt marsh morphodynamics: An example from the Danish Wadden Sea

Abstract

In the Danish Wadden Sea, exposed salt marshes grow in the form of sequences of salt marsh platforms which terminate at the tidal flat with an erosional cliff. On the tidal flat adjacent to the salt marsh, a marsh-parallel small depression is usually formed which acts as a tidal channel (landpriel) creating levees on its landward side, on which salt marsh from the mainland spreads seaward with an uneven topography. On the seaward side of the landpriel, patches of slightly higher elevations form as a result of wave action. As plants get a foothold on these patches salt marsh growth is initiated. Along with this process a salt marsh cliff develops against wave attack from the tidal flat during storms. The unit consisting of levee, landpriel and patches of higher elevation eventually silts up and buries the former cliff on top of the levee, so this now stands as a small (≈ 0.25 m) ridge separating the new platform from the older one. Thus, the salt marsh accretes through a sequence of salt marsh platforms, each with its own landpriel, levee and erosional cliff. The landpriel and levee can be recognised later as topographic lows and highs, respectively, in the sand beneath the fine-grained salt marsh sediments, while the erosional cliff can be recognised in the salt marsh topography. Based on these observations, a conceptual evolutionary/accretionary model for exposed salt marshes with associated erosional cliff, landpriel and levee is presented. The deposition across the salt marsh platform decreases in an exponential manner away from the salt marsh edge at a lower rate than that found on a lee side salt marsh at the Skallingen backbarrier north of the study area. This is interpreted as a result of wave action.