Multi-fraction sediment sorting and entrainment at the Prins Hendrikzanddijk

Abstract

Since the 1990’s sand nourishments are increasingly used to improve coastal safety at the wave-dominated North Sea coast. Generally, the nourished sediment is kept similar to the naturally occurring sediment and in most cases consists of well-sorted sand. However, recently, sandy beaches are also used to strengthen dykes in regions that are not wave-dominated. The Prins Hendrikzanddijk (PHZD) is an example of such a sandy retrofit, located on the leeward side of the Dutch barrier island of Texel. Additionally, a layer of coarse sediment was added on top of the PHZD in order to limit erosion. This has resulted in a complex spatiotemporally varying sediment composition in a mixed wave-current environment. It is expected that this combination of factors results in highly variable and complex sediment entrainment and transport processes, which are not yet fully understood. This research focuses on the spatiotemporally variability in the sediment composition and how this affects sediment entrainment transport at the PHZD in an aim to improve our fundamental understanding of these processes.

During a 6-week (SEDMEX; mixed SEDiments in Mixed Energy eXperiment) field campaign in early autumn at the PHZD, instruments were deployed at 6 alongshore locations, of which two had additional instruments in the cross-shore. The collected data consist of measurements on waves, currents and sediment (composition and transport). A total of 139 sediment samples were collected in the intertidal area at different cross-shore locations and over varying time intervals. Wave and current shear stresses were combined with the sediment composition to do a multi-fraction mobility analysis which is then compared to measured turbidity from Optical Backscatter Sensors (OBS).

The sediment mixture was coarse (average 1032 µm) and poorly sorted (σG 2.3 – 2.7) across the entire cross-shore location L2. Finer grain sizes were observed during calmer periods (430 – 850 µm) and coarser (1350 – 2000 µm) during energetic conditions. Wave related shear stresses were on average 5 times larger than that of currents. The results from the multi-fraction mobility analysis showed that the Shields number was often close to the critical Shields for incipient motion which resulted in preferential transport of the finer sediments. Over time this led to coarsening of the bed. Energetic conditions were able completely mix up the bed increasing the availability of fine sediments and flattening the beach profile. A conceptual Figure and model illustrate the how the processes affect each other and highlights how the storm frequency is an important factor in the availability of the fine sediments.