Modelling the two-way coupling of tidal sand waves and benthic organisms:a linear stability approach

We use a linear stability approach to develop a process-based morphodynamic model including a two-way coupling between tidal sand wave dynamics and benthic organisms. With this model we are able to study both the effect of benthic organisms on the hydro- and sediment dynamics, and the effect of spatial and temporal environmental variations on the distribution of these organisms. Specifically, we include two coupling processes: the effect of the biomass of the organisms on the bottom slip parameter, and the effect of shear stress variations on the biological carrying capacity. We discuss the differences and similarities between the methodology used in this work and that from ‘traditional’ (morphodynamics only) stability modelling studies. Here, we end up with a 2×2 linear eigenvalue problem, which leads to two distinct eigenmodes for each topographic wave number. These eigenmodes control the growth and migration properties of both sand waves and benthic organisms (biomass). Apart from hydrodynamic forcing, the biomass also grows autonomously, which results in a changing fastest growing mode (FGM, i.e. the preferred wavelength) over time. As a result, in contrast to ‘traditional’ stability modelling studies, the FGM for a certain model outcome does not necessarily have to be dominant in the field. Therefore, we also analysed the temporal evolution of an initial bed hump (without perturbing biomass) and of an initial biomass hump (without perturbing topography). It turns out that these local disturbances may trigger the combined growth of sand waves and spatially varying biomass patterns. Moreover, the results reveal that the autonomous benthic growth significantly influences the growth rate of sand waves. Finally, we show that biomass maxima tend to concentrate in the region around the trough and lee side slope of sand waves, which corresponds to observations in the field

Experiment-supported modelling of salt marsh establishment

Recently, the use of salt marshes in front of hard structures is increasingly proposed as a more sustainable coastal protection measure. Yet, salt marsh restoration and creation is often hampered by the lack of a thorough understanding of initial vegetation establishment. Recent studies highlight the importance of bed level change for salt marsh development. In this study we continue the examination of the impact of bed level change on salt marsh development, focussing on the prediction of salt marsh establishment and the implications for coastal management. First, a test with Spartina anglica seedlings (Cordgrass) in a wave flume showed that long-term (seasonal) bed level change is more important for seedling survival than direct wave impact at the shoot. Therefore, we subsequently incorporated bed level change in the Windows of Opportunity (WoO) framework. Lastly, this revised WoO framework was applied to the design of the Marconi pioneer salt marsh (The Netherlands). Combining the WoO framework with a morphodynamic model (Delft3D) showed its potential for salt marsh design. The framework can be used to determine whether salt marsh establishment is possible, to find out which conditions are limiting establishment and to design engineering measures creating the conditions that facilitate salt marsh establishment

Modeling biogeomorphological interactions in underwater nourishments

To prevent sandy coastlines from further erosion, nourishments are executed in which sand is usually put underwater at the foreshore. Waves and currents transport the sand on the beach and in this way stabilize the coastal profile. Little is known about the interactions of these so-called shoreface nourishments with the benthic populations inhabiting the coastal strip. Benthos is affected by the nourishments, but benthic populations could in turn affect the morphological evolution of the nourished coast. Monitoring has shown that the benthic community will mainly recovery after ca. 1 year. However, the impact of benthos on the sediment dynamics and hydrodynamics is unknown. In this paper we focus on tube building worms, which have a large abundance in the foreshore, live in patches of several square meters in diameter and protrude several centimeters from the sediment in the water column. Tube building worms are included in a numerical modeling tool (Delft3D), by explicitly accounting for the influence of cylindrical structures on drag and turbulence by an extra source term of friction force in the momentum equation and an extra source term of Total Kinetic Energy (TKE) and turbulent energy dissipation in the k-ε equations respectively. The model is validated against field and flume experiments and it shows a significant influence on flow velocities near the bed, bed shear stress and bed-load transport rates. Moreover, model results reveal that tube building worms are able to stabilize nourishments and slow down the migration of the outer breaker bar. Present model explorations indicate that future research should focus on the measurement of the patchy distribution of bio-engineers in the foreshore and their impact on the sediment dynamics and hydrodynamics. Such knowledge will enable process based modeling of the spatial and temporal variation in biological activity on the morphological development of the coastal profile and also it will lead to validation of the proposed model with field measurements.</jats:p

Mechanistic modeling of marsh seedling establishment provides a positive outlook for coastal wetland restoration under global climate change

While many studies focus on the persistence of coastal wetlands under climate change, similar predictions are lacking for new wetland establishment, despite being critical to restoration. Recent experiments revealed that marsh seedling establishment is driven by a balance between physical disturbance of bed-level dynamics and seedling root stability. Using machine learning, we quantitatively translate such finding in a new biogeomorphic model to assess marsh establishment extent. This model was validated against multiyear observations of natural seedling-expansion events at typical sites in the Netherlands and China. Subsequently, synthetic modeling experiments underscored that seedling expansion was primarily determined by controllable local conditions (e.g., sediment supply, local wave height, and tidal flat bathymetry) rather than uncontrollable climate change factors (e.g., change in sea-level and global wave regime). Thus, science-based local management measures can facilitate coastal wetland restoration, despite global climate change, shedding hope for managing a variety of coastal ecosystems under similar stresses

Resilient Flood Defenses

Flood risk in deltaic regions is increasing due to a combination of more economic activities and an increase in flooding probability [...

Artificial Structures Steer Morphological Development of Salt Marshes: A Model Study

Salt marshes are increasingly recognized as resilient and sustainable supplements to traditional engineering structures for protecting coasts against flooding. Nevertheless, many salt marshes face severe erosion. There is a consensus that providing structures that create sheltered conditions from high energetic conditions can improve the potential for salt marsh growth. However, little proof is provided on the explicit influence of structures to promote salt marsh growth. This paper investigates how artificial structures can be used to steer the morphological development of salt marshes. A morphological model (Delft3D Flexible Mesh) was applied, which enabled the analysis of various artificial structures with realistic representation. A salt marsh in the Wadden Sea which has seen heavy erosion (lateral retreat rate of 0.9 m/year) served as case study. We simulate both daily and storm conditions. Hereby, vegetation is represented by an increased bed roughness. The model is able to simulate the governing processes of salt marsh development. Results show that, without artificial structures, erosion of the salt marsh and tidal flat continues. With structures implemented, results indicate that there is potential for salt marsh growth in the study area. Moreover, traditional structures, which were widely implemented in the past, proved to be most effective to stimulate marsh growth. More broadly, the paper indicates how morphological development of a salt marsh can be steered by various configurations of artificial structures