Exploring the bio-geomorphological evolution of mega nourishments with a cellular automata model

Mega nourishments are innovative Nature-based Solutions to protect against coastal erosion and flooding. By upscaling the volume of sand nourishments, mega nourishments enhance coastal safety while creating new space for recreation and nature development. The application of such large nourishment volumes can significantly alter natural beach-dune morphology, with an unclear effect on the long-term evolution of the beach-dune system. The DuBeVeg model, a bio-geomorphological Cellular Automata model, was extended to incorporate the typical surface armouring and longshore coastline development of mega nourishments. The extended model was successfully validated with morphodynamic data from the Sand Motor mega nourishment and was used to explore the long-term dune development of idealized bell-shaped mega nourishments. Over a 50-year time span, the typically artificially high and wide beach of a mega nourishment resulted in the seaward expansion of the foredune zone whilst a more natural, lower beach elevation resulted in a greater volume of the foredune zone in the long-term. A faster diffusion of the initially bell-shaped mega nourishment gave rise to a lesser advance and a more alongshore-uniform width of the foredune zone. Furthermore, the armour layer that develops when the nourishment material is less well-sorted and coarser than native beach sand, led to the emergence of more scattered and isolated dunes compared to a nourishment with native beach sand. These model results show potential for investigations into design optimisation of the subaerial development of Nature-based Solutions for sandy coasts

Model versus nature: Hydrodynamics in mangrove pneumatophores

Water flows through submerged and emergent vegetation control the transport and deposition of sediment in coastal wetlands. Many past studies into the hydrodynamics of vegetation fields have used idealized vegetation mimics, mostly rigid dowels of uniform height. In this study, a canopy of real mangrove pneumatophores was reconstructed in a flume to quantify flow and turbulence within and above this canopy. At a constant flow forcing, an increase in pneumatophore density, from 71 m⁻² to 268 m⁻², was found to cause a reduction of the within-canopy flow velocities, whereas the over-canopy flows increased. Within-canopy velocities reduced to 46% and 27% of the free-stream velocities for the lowest and highest pneumatophore densities, respectively, resulting in stronger vertical shear and hence greater turbulence production around the top of the denser pneumatophore canopies. The maximum Reynolds stress was observed at 1.5 times the average pneumatophore height, in contrast to uniform-height canopies, in which the maximum occurs at approximately the height of the vegetation. The ratios of the within-canopy velocity to the free-stream velocity for the pneumatophores were found to be similar to previous observations with uniform-height vegetation mimics for the same vegetation densities. However, maxima of the scaled friction velocity were two times smaller over the real pneumatophore canopies than for idealized dowel canopies, due to the reduced velocity gradients over the variable-height pneumatophores compared to uniform-height dowels. These findings imply that results from previous studies with idealized and uniform vegetation mimics may have limited application when considering sediment transport and deposition in real vegetation, as the observed turbulence characteristics in nonuniform canopies deviate significantly from those in dowel canopies