Simulating sand nourishment strategies

Abstract

Sandy coasts worldwide face increasing pressure from sea level rise, ecological constraints, and intensified human use. Sand nourishment is widely applied as a nature-based alternative to hard coastal defence, yet its long-term morphological and functional impacts remain insufficiently understood—particularly over decadal timescales and under varying design strategies. This thesis develops, validates, and applies a modelling framework to simulate the multi-decadal evolution of nourished sandy coasts under different nourishment strategies and sea level rise scenarios, supporting adaptive and multifunctional coastal planning.
The research was structured around four interlinked subcomponents. First, the multi-decadal cross-shore profile evolution of repeatedly nourished sandy coasts was simulated, focusing on equilibration timescales, profile change, and shoreline migration. This included the development of a cross-shore behavioural model, Crocodile, that comprises diffusion-based formulations and incorporates site-specific parameters governing profile shape, depthdependent diffusion timescales and alongshore transport losses. Validation against three decades of bathymetric data from nourished Dutch coasts demonstrates that Crocodile accurately reproduces shoreline position, beach width, and profile volume, enabling differentiation between nourishment strategies.
Second, the model was used to assess how strategies differing in placement volume, frequency, and policy objectives perform under varying sea level rise rates. Results reveal nonlinear and depth-dependent responses. Larger nourishment volumes can induce profile steepening and reduce nourishment lifetimes by up to 30%. Strategic choices produce differences of up to 75% in total sand demand over 50 years. High-frequency “hold-the-line” approaches may require biannual interventions under accelerated sea level rise, whereas proactive volume-based strategies risk over-nourishment. These findings highlight the importance of flexible design in timing and scale to preserve future adaptation options.
The third part of the study explored the depth-dependent alongshore dispersion of mega nourishments. Crocodile was coupled with the one-line shoreline model ShorelineS to simulate the depthand time-dependent dispersion of Gaussian-shaped mega nourishments over 50 years. Results showed a two-phase evolution: an initial phase of roughly a decade during which sand redistributes both by cross-shore equilibration and alongshore dispersion, followed by longer-term phase dominated by alongshore dispersion. Sand placed at lower bed elevations remains largely immobile, indicating that functional outcomes such as beach width and dune development do not scale linearly with nourishment volume.
Finally, a conceptual framework was developed to link morphological model outputs to coastal functions including recreation, ecology, and flood protection. Application in case studies demonstrates how trade-offs and synergies can be quantified to support multifunctional strategy design.