F. Galiforni-Silva1*, S. Dan2, S. de Vries1, C. Sevindik1, C. Hallin3, B. Huisman4, M. Maarse4, F. van Rees4, A. Reniers1
1 Delft University of Technology, The Netherlands; 2 Flanders Hydraulics, Belgium; 3 Lund University, Sweden; 4 Deltares, The Netherlands
* Corresponding author: f.galifornisilva@tudelft.nl
Introduction
Hybrid nature-based solutions are increasingly considered in coastal management projects. It combines the stability of hard coastal structures with the adaptability of dunes and vegetation, offering promising pathways for sustainable coastal protection. Yet, the long-term feedback between hard and soft elements remains poorly understood, and modelling tools able to simulate beach-dune development and dike interaction are scarce. To fill this gap, the DuneFront project was designed to improve our understanding of hybrid nature-based solutions and optimize the design of future solutions. A key challenge is developing a schematization of the coastal profile that provides sufficient information to predict the long-term evolution of hybrid coastal dunes at scales of interest for stakeholders, while maintaining appropriate levels of accuracy and within relatively short simulation times.
The objective of this study is to adapt the cross-shore morphodynamic model CS-Model (Hallin et. al., 2019) to simulate the long-term evolution of hybrid dune–dike systems by including a schematization for hard structures and vegetation development.
Objective and Methods
The CS-Model is a semi-empirical cross-shore model designed to simulate decadal to centennial beach-dune evolution. Beach-dune evolution is simulated by changing pivot points along a schematized profile to reaccommodate the sediment exchange within the different profile compartments (i.e., beach, dune, bar). The model includes formulation for aeolian transport, hydrodynamic erosion, sea-level rise, and nourishments. Hard structures are implemented as non-erodible compartments over which dunes can develop. The evolving dune volume at each time step are satisfied by moving pivot points along the hybrid profile that meet geometric constraints using a root-finding function. Vegetation effects are introduced via an effectiveness parameter based on Nield and Baas (2008), which modulates aeolian transport through wind reduction, and a trapping efficiency coefficient representing the fraction of potential transport locally trapped by vegetation. We test the model in a series of synthetic cases, as well as test cases in Oosteroever, Belgium, where three years of monitoring data are available, and Katwijk, the Netherlands, where we compared the model with an XBeach simulation for an extreme storm.
Results
Synthetic tests that include dike interaction show that sediment is conserved across all configurations, with mismatches below 2% at maximum, but consistently below 0.5%. As the dike intrudes further into the back-dune, the model produces clear trends: the lee dunetoe moves landward, crest growth increases, and less volume is needed to fill the ramp. Vegetation has a strong influence on where sand is deposited, with overall patterns following the literature (Wasson and Nanninga, 1986; Walker et. al., 2017). For bare dunes, most deposition occurs on the crest and lee as acceleration over the ramp limits deposition. With vegetation, increasing cover and dune height strongly reduce transport to the lee. Results for the Oosteroever site show good agreement regarding dune growth, while vegetation variability is lower than observed. In Katwijk, the model showed qualitative coherence with expected patterns such as an increase in beach width due to erosion and overwash deposition. The adapted CS-Model shows potential as a first-order, long-term tool for exploring hybrid dune–dike interactions. It is a first step towards adapting existing models to work in these hybrid environments.
References
Hallin, C., Larson, M. & Hanson, H. (2019). Simulating beach and dune evolution at decadal to centennial scale under rising sea levels. PLoS One, 14, e0215651.
Nield, J. M. & Baas, A. C. W. (2008). The influence of different environmental and climatic conditions on vegetated aeolian dune landscape development and response. Global and Planetary Change, 64, 76-92.
Walker, I. J., Davidson-Arnott, R. G. D., Bauer, B. O., Hesp, P. A., Delgado-Fernandez, I., Ollerhead, J. & Smyth, T. A. G. (2017). Scale-dependent perspectives on the geomorphology and evolution of beach-dune systems. Earth-Science Reviews, 171, 220-253.
Wasson, R. J. & Nanninga, P. M. (1986). Estimating wind transport of sand on vegetated surfaces. Earth Surface Processes and Landforms, 11, 505-514
