Composition and size distributions of local and advected sediment trapped over a tidal flat during moderate and storm conditions

Abstract

Geografisk Tidsskrift, Danish Journal of Geography 106(1):1–11, 2006

Vertical profiles of settling rates in an array of traps trapping suspended matter on a micro tidal flat indicated a large vertical resuspension flux up to about 1 m above the bed during moderate conditions. Size distributions of trapped material were used to fingerprint the origin of the trapped material. The lowermost trap always contained samples with bimodal distributions. A pronounced peak at 200 μm indicated local tidal flat resuspension of the sand which made up more than 95% of the trapped sand and a peak at 10–20 μm (similar to the nearby salt marsh sediments) indicated advection of fine-grained material which also dominated the upper traps. Following storm surge conditions the local sand was trapped at all depths above the bottom and made up approximately 1% of the material in the uppermost trap (2.45 m above the bed). Finer sand was also deposited in the uppermost trap. The size distribution of this sand indicated a supply in near uniform suspension from the exposed salt marsh fringes. It is noteworthy that following such conditions, sand-sized material made up 18% of suspended matter trapped 2.45 m above the bed.

Keywords:

• Grain size distributions
• mixing
• sediment traps
• tidal flat
• Wadden Sea