Conditional effects of tides and waves on short-term marsh sedimentation dynamics

Salt marshes are in danger of degradation due to human impact and climate change. A thorough understanding of mechanisms controlling sedimentation and erosion in salt marshes is essential for their conservation and restoration. To understand short-term dynamics of sediment availability and deposition around marsh edges, two contrasting marshes, Rattekaai and Sint Annaland, were studied in the Oosterschelde (southwest Netherlands). Suspended sediment concentration (SSC) was measured by siphon samplers along four transects perpendicular to the marsh edge in each marsh, during nine flood tides between March and December 2013. Each transect was comprised of four sampling sites (-10m and -1m on the mudflat and +1m and +10m on the marsh plateau, relative to the marsh edge). Sediment deposition was measured along the transects on the marsh, at +1m and +10m from the marsh edge, over seven c. 14-day intervals during the same 10-month period. Two types of sediment traps were used, one measuring gross sediment deposition (TTD - tube trap deposition) and one measuring net sediment deposition (FTD - filter trap deposition). Wave loggers were deployed 10m away from the marsh edge on the mudflat at each marsh. The results showed that both SSC and sediment deposition varied greatly through space, both between the two marshes and within each marsh along the marsh edge. The SSC and gross sediment deposition were much higher at Rattekaai than at Sint Annaland. SSC was significantly correlated with wind speed during sampling. Sediment deposition rates (TTD and FTD) and retention ratio (FTD/TTD) were significantly correlated with cumulative wave energy during the measurement period. A conceptual model of local sediment dynamics is proposed to explain the sediment dynamics around the marsh edge. This study highlights the importance of incorporating local sediment dynamics when evaluating marsh vulnerability and stability.