Modelling of bed sediment composition changes at the lower shoreface of the Sand Motor

Large perturbations in the coastline, such as the 'Sand Motor' nourishment (∼21 million m3) at the Holland coast, can initiate considerable spatial and temporal changes in the median grain size (D50) of the sea bed on the lower shoreface. The relevance of hydrodynamic conditions for the development of the heterogeneity in D50 at large-scale nourishments was assessed with a numerical model (Delft3D), which required a validation against 2.5 years of D50 measurements. A good representation of the observed spatial pattern of D50 was obtained independent of a 2DH or 3D approach and initial condition for the D50 of the bed. Five sediment size fractions and a multi-layer administration of the bed composition were used. The extent and magnitude of the coarsening of the bed is related to the velocity of the horizontal tide, while a far less pronounced coarsening takes place during energetic conditions (i.e. Hm0≥ 3 m). Differential suspension behaviour between the size fractions, which are all mobilized at the bed, causes a preferential transport of fine sediment (in alongshore direction) away from the Sand Motor at the lower shoreface (i.e. seaward of MSL -6 m). Storm conditions may induce a partial removal of the coarse top-layer due to mobilization of all of the size fractions and mixing with the relatively fine substrate material. Simulations also show that transport of the fine sand fraction extents to much deeper water than for the medium and coarse sand fractions. Models with multiple sediment fractions are therefore required for the assessment of environmental impacts of large-scale coastal structures or land reclamation's and sediment transport on the lower shoreface.