Coastal nourishments are performed to manage coastal systems against threats like sea level rise, extreme weather conditions, and increasing population demands. Sand used in shoreface nourishments is neither uniform in size nor has the same size distribution as its surrounding environment (see Huisman et al. (2016) and Guillén & Hoekstra (1997)). Sand transport is an indispensable link between hydrodynamic conditions and morphological changes in coastal areas. However, sand transport models cannot accurately estimate the critical bed shear stress for various particle sizes, especially when they interact with finer or coarser grains.
When different sand grain sizes are combined, it is called a sand mixture. According to Van Rijn (2007c), sand particles behave differently when they are in a mixture. The coarser grains in a sand mixture are more easily mobilized compared to same-size grains when part of a uniform sand, whereas the finer grains in a mixture require a higher critical bed shear stress than in a uniform fine sand. Existing studies have focused on incipient motion processes for sand-gravel mixtures under steady flow conditions (currents). The results do not account for sand-sand mixtures, and do not explain to what extent the results can change if they are exposed to waves instead of currents. As a result, existing formulations cannot reliably predict the critical shear stress of the coarse and the fine fractions, potentially resulting in errors in the accuracy of the sand transport models.
As a result, as part of the SOURCE project, two types of experiments will be conducted in the laboratory flume of WaterProof in Lelystad. The first set of experiments will be performed under only waves, and the second set will be performed under currents only. Bimodal sand mixtures (consisting of a finer fraction of 0.1 mm and a coarser fraction of 0.4 mm) with different size compositions (size distributions) will be placed in a test bed in the middle of the flume (see Figure‑1). Wave and flow characteristics will be recorded using wave gauges and an ADV. Concentrations will be measured using OBS3+, and suction pipes and sand traps will be used to sample sand transport rates and to analyze the size distributions of the transported sand.
Figure‑1 Schematic representation of the flume at WaterProof.
Incipient motion of different mixtures will be compared to understand the size composition effect on the incipient motion of the two sand fractions. Results obtained under wave-only and current-only conditions for the same sand mixtures will be compared to understand to what extent the incipient motion of sand-sand mixtures is different for waves and currents. Finally, empirical formulations on sand mixture incipient motion will be improved through analyzing the experimental data.
The objective of this research is to observe, understand and quantify sand grain size effects on the initiation of motion of mixed sands under waves and currents. To achieve this goal, there are three steps:
- Assisting in conducting experiments in WaterProof laboratory in Lelystad (experiment period: October - November 2025).
- Processing and analyzing experimental data collected from the flume
- Comparing the experimental results with the results from empirical formulations
We are seeking a student who is excited about conducting experiments, performing data analysis, and drawing insights from the results. The student will gain practical and valuable laboratory experience in Lelystad under the direct supervision of a PhD candidate. Coding skills and background in hydraulics, wave dynamics, and sediment transport are required.
References
- Shields, A. (1936). Application of Similarity Principles and Turbulence Research to Bed-Load Movement (PhD dissertation). Technical University of Berlin, Berlin, Germany.
- Soulsby, R. (1997). Dynamics of marine sands: A manual for practical applications. Thomas Telford.
- Van Rijn, L.C. (2007) Unified view of sediment transport by currents and waves. III: Gravel beds. Journal of Hydraulic Engineering, 133(7), 761–775. https://doi.org/10.1061/(ASCE)0733-9429(2007)133:7(761)
- Guillén, J., & Hoekstra, P. (1997). Sediment Distribution in the Nearshore Zone: Grain Size Evolution in Response to Shoreface Nourishment (Island of Terschelling, The Netherlands). Estuarine, Coastal and Shelf Science, 45(5), 639-652. https://doi.org/10.1006/ecss.1996.0218
- Huisman, B. J. A., de Schipper, M. A., & Ruessink, B. G. (2016). Sediment sorting at the Sand Motor at storm and annual time scales. Marine Geology, 381, 209-226. https://doi.org/10.1016/j.margeo.2016.09.005
Supervision
Are you interested in this assignment? Contact the Master thesis coordinator.
