Shaping the beach: Cross-shore sand transport in the swash zone

This paper summarizes the main results from the three main activities of Shaping The Beach research project (2018-2023), and gives recommendations for further studies into swash zone sand transport. The project involved a unique and integral combination of detailed wave flume experiments with advanced measuring techniques, sophisticated intra-swash numerical modelling, and engineering, practical sand transport modeling. Shoreline retreat/advance is driven by high-/low-energy wave conditions, independent of the beach slope. Milder beach slope does decrease the shoreline retreat and support more accretion. Both effects are captured by relating beach volume change to the slope-corrected Dean number as proposed by Hattori and Kawamata (1980). Depth-resolving VoF RANS models like OpenFoam are key to unravel the vertical flow and suspended sand concentration structures within the swash zone. These models only work well if air-flow interactions and turbulence are carefully represented. Existing practical models have skill in predicting swash zone transport rates and bed level changes. However, they do not perform equally well for erosive and accretive conditions and require heavy data fitting, which makes them not fully predictive (yet). Knowledge on swash zone sand transport is essential to understand and cope with coastal erosion and sedimentation by natural processes and human interferences. This knowledge is used to assess climate change impacts on beach stability, to design cost-effective beach protection measures such as sand nourishments, and to design coastal structures.