Tidal Sedimentation in the Wadden Sea
Sedimentological studies have recently been carried out in the Spiekeroog back-barrier area (southern North Sea coast of Germany) to explore the interrelationship between the surficial sediment distribution patterns, energy levels, and transport processes. The sediment distribution patterns show that the sediment generally becomes finer landwards (north-south), irrespective of the tidal channel orientations (east-west). a closer examination, however, shows two distinct patterns. The first pattern is a general landward-fining of the sediments within the inlet from about 1.0 phi (0.50 mm) in the inlet throat to about 3.5 phi (0.088 mm) on the landward reaches of the inlet. This pattern is a result of the decrease in current velocity from the inlet throat landwards. The second pattern, which is the most conspicuous on the mean grain-size map, shows a distinct shore-normal (north-south), landward sediment fining across the tidal flats from about 2.0 phi (0.25 mm) on the islands to 2.5–3.0 phi (0.25–0.125 mm) on the tidal flats to as fine as 3.5 phi (0.088 mm) along the dike (mainland coast). This shore-normal sediment fining has been found to be a result of the shore-normal energy gradient (flow velocity) associated with overbank flow from one channel to the next.
The analysis of skewness evolution has been shown to be a powerful tool for the interpretation of transport pathways in a tidal environment. Skewness distribution patterns indicate that the inlet areas act as sources of sediment from which sediment is transported landwards during the flood tide and seawards during the ebb phase. In the whole area, however, the fine sediment fraction (population) whose energy niche is the landward margin of the backbarrier areas, and the coarse fraction characterizing the inlet throat areas, undergo a range of population mixing as well as progressive sorting of the individual populations. Progressive sorting appears to be dominant normal to the shore and across the tidal flats whereas mixing processes are more pronounced along the main channel of the tidal inlets.