Bastiaan Wijnand Borsje

The role of suspended load transport in the occurrence of tidal sand waves

Tidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics, sediment transport, and geomorphology. Field data from the southern North Sea reveal that sand waves are absent where suspended load transport is the dominant transport mode. In order to understand the mechanisms responsible for the absence of sand waves, we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process-based geomorphological model (Delft3D). Model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable (k-e turbulence model). First, it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment, while for a relatively small grain size the flat bed is stable and no sand waves evolve. Second, it is shown that suspended load transport causes the suppression of long sand waves, resulting in a finite range of wavelengths that experience growth. Finally, by varying flow velocity amplitude and grain size, critical conditions for sand wave formation are found, i.e., conditions for which sand waves are marginally generated.

Do salt marshes survive sea level rise? Modelling wave action, morphodynamics and vegetation dynamics

Abstract This paper aims to fundamentally assess the resilience of salt marsh-mudflat systems under sea level rise. We applied an open-source schematized 2D area model (Delft3D) that couples intertidal flow, wave-action, sediment transport, geomorphological development with a population dynamics approach including temporal and spatial growth of vegetation and bio-accumulation. Wave-action maintains a high sediment concentration on the mudflat while the tidal motion transports the sediments within the vegetated marsh areas during flood. The marsh-mudflat system attained dynamic equilibrium within 120 years. Sediment deposition and bio-accumulation within the marsh make the system initially resilient to sea level rise scenarios. However, after 50–60 years the marsh system starts to drown with vegetated-levees being the last surviving features. Biomass accumulation and sediment supply are critical determinants for the marsh drowning rate and survival. Our model methodology can be applied to assess the resilience of vegetated coast lines and combined engineering solutions for long-term sustainability.

Biogeomorphology of coastal seas. How benthic organisms, hydrodynamics and sediment dynamic shape tidal sand waves

There is growing recognition of the importance of feedbacks between organisms and physical forces in landscape formation; a field labeled biogeomorphology. Biogeomorphological processes typically involve so-called ecosystem engineering species, which are organisms that modify the abiotic environment via their activity or physical structures and thereby create, modify and maintain habitats. Biogeomorphological processes are known to shape a broad range of landscapes. However, in the underwater landscape these interactions have received little attention, despite the high abundance of ecosystem engineering species in the bed of coastal seas. This thesis aims at understanding the interaction between ecosystem engineering species, hydrodynamics and sediment dynamics in the formation of the underwater landscape. The most dynamic large-scale seabed patterns are tidal sand waves, with wavelengths of several hundreds of meters, heights of several meters and migration rates up to tens of meters per year. Within this dynamic landscape, ecosystem engineering species have a large impact on both the hydrodynamics and the sediment dynamics. In order to understand this interaction, we followed a model approach. Additionally, field observations and flume experiments are executed to obtain input parameters and validation data for the model studies.