Abstract:
The erosion potential over bedforms in a tidal flat of the East Frisian Wadden Sea was studied by conducting erosion and physical and
biological sediment property measurements on the crests and troughs of bedforms. Five stations along a cross-shore transect of 1.5 km length
from immediately below the salt marsh to the mid tide-level of the tidal flat were visited during two field campaigns in June and September 2002.
Measurements of sediment erodibility were made on both crests and troughs using an EROMES erosion device and quantified in terms of critical
erosion shear stress and erosion rate. Surface sediment scrape samples (upper 1 mm layer) were taken from crests and troughs to determine
various physical and biological properties of the sediment. The results show that crests are generally more stable (i.e. higher critical erosion
shear stresses and lower erosion rates) than troughs. In general, crests contained more chlorophyll a, colloidal carbohydrate, and EPS (extracellular
polymeric substance) than troughs. Median grain-size, water content and wet bulk density of the crests showed no statistically significant
difference from those of the troughs with the exception at the most landward station immediately below the salt marsh margin, where crests had
significantly lower water content and higher wet bulk density than troughs.
Two different processes were identified for the difference in erodibility between crests and troughs: (1) At stations with emersion times less
than 6 h, the higher benthic diatom biomass (measured as chlorophyll a concentration) on the crests increases the amount of EPS, which is likely
to stabilize the sediment surface of these features; (2) in a saltmarsh transition area (most landward station), physical processes such as surface
drying and compaction seem to enhance in a synergistic way the sediment stability on the crests.
