Complex Sediment Transport Pattern on a Spit-Platform in the Danish Wadden Sea

Abstract

Spit-platforms have not drawn much attention, despite their expected importance for understanding sediment transport mechanisms leading to siltation of tidal channels. In this article, the complex sediment transport pattern on a shallow spit-platform in the Danish Wadden Sea is described based on measurements from four current meters deployed on the spit-platform in a low- and a high-energy period. The hydrodynamic characteristics at the four stations are different in terms of impact from tidal and wave-induced currents. The sediment enters the spit-platform as sandbars oriented perpendicular to the shore migrating alongshore, because of wave activity, until the sediment reaches an ebb-dominated tidal channel. In and near this channel, the sediment is transported downdrift by the ebb-tidal currents until the sand enters a shallow dissecting ebb channel, where the sand is transported obliquely offshore and deposited on a small ebb-tidal delta. This zigzag pattern of sediment transport continues until all ebb channels (up to four) are bypassed. On the subtidal flats between the most downdrift ebb channel and the main tidal channel, the sediment is transported the remaining distance into the main tidal channel or onto the tidal flat in the lagoon landward of the spit-platform. During higher-energy conditions, the sediment transport is more affected by wave-induced currents, leading to high transport rates in the shallow parts of the spit-platform and less transport in the horizontally restricted channels.