Salt marsh stability and patterns of sedimentation across a backbarrier platform

Abstract

Long term observations of clay thicknesses from 1949 to 2007 and measurements of the bulk dry density of salt marsh on the backbarrier of Skallingen (west Denmark) formed the basis of constructing a space distributed model of salt marsh deposition. The deposition potential (an empirical constant, β) was measured at 32 locations across the studied salt marsh platform. It was found to correlate with two independent variables: (1) distance to marsh edge and (2) distance to creeks of 2nd or higher order. They explained in combination 71% of the variation. The derived algorithms for estimating the salt marsh deposition made it possible to tune in a model in a 4 by 4 m grid covering the studied salt marsh area of app 3 km 2. This was used to describe the general pattern of deposition on the salt marsh platform. Running the model with a constant sea level revealed that balance between the inner and the outer salt marsh deposition cannot be achieved within a time scale of 1000s of years. Modeling salt marsh sedimentation under rising sea level scenarios revealed that only one specific sea level rise provides equilibrium for a given location. With a higher sea level rise, the marsh at the specific location will eventually drown, whereas it – with a sea level rise below this level – will grow towards the top of the rising tidal frame. Short term scenarios (100 years) showed that the Skallingen salt marsh will survive a sea level rise of 4 mm yr − 1 and become unstable in the inner part with a sea level rise of 6 mm yr − 1 .