Long-term Dynamics and Stabilization of Intertidal flats: A system approach

Decreasing sediment availability, in combination with sea level rise and human fixation of the coastline, results in losses of the intertidal environment (lying in-between the mean low water and mean high water spring tide). This means a loss of biodiversity and an increased coastal vulnerability to extreme events and sea level rise. Thereforeit is of utmost importance to understand the dynamics of the intertidal wetlands; their response to sea level rise and to different types of human interferences. The better we understand the processes that underlie the evolution of the intertidal system, the more effectively we can manipulate the system, to stimulate its rise and maintain its elevation relative to mean sea level.The long-term morphodynamics is difficult to understand due to the interdependencies of the underlying processes; the morphology is shaped by the hydrodynamic forces,while it influences these forces at the same time. Due to the feedback loops, the components are strongly entangled and the whole system cannot be reduced to the sum of itsparts and solved by the traditional reductionist method.In this thesis, system theory and system analysis are applied to get towards an understanding of ‘the intertidal morphodynamical system’. This is the philosophy that states arise that are understandable and possible to determine exactly, despite the many interactionsbetween the variables and the apparent complexity of systems. To describe these states, I follow a top-down approach, where I learn from the observed system behavior. Hence, the observation of conserved properties leads to the important question:‘why are they conserved?’ The answer to this question can reveal much of the system’s dynamics.Water ResourcesCoastal Engineerin

Evolution of the Yangtze Tidal Flats: A One-Dimensional Approach (abstract)

The Yangtze river (China) is one of the biggest rivers in the world in terms of length, area, discharge and sediment load. The large sediment supply by the Yangtze river resulted in the development of large coastal wetlands in its estuary, which form rich and important ecosystems. Human activities are increasingly influencing the Yangtze estuary, for instance by the construction of plentiful dams in the river. The precise consequences of human interventions and the effects of climate change like global sea level rise, for the tidal flats are uncertain. In the last decades, the flats have been increasing in size, while measurements from recent years indicate a stop of this growth. Investigation of the development of the tidal flats is important for preserving nature, planning further land reclamation projects and coastal defence issues.Hydraulic EngineeringCivil Engineering and Geoscience

Progradation Speed of Tide-Dominated Tidal Flats Decreases Stronger Than LinearlyWith Decreasing Sediment Availability and LinearlyWith Sea Level Rise

We use the results of a one-dimensional morphodynamic model and the basis of the “Lagrangian equilibrium state” (Maan et al., 2015, https://doi.org/10.1002/2014JF003311) to derive a quantitative relationship between the progradation speed of tidal flats and the suspended sediment concentration in their adjacent waters and show that the speed increases more than linearly with the concentration. We also show that horizontally prograding flats rise vertically with sea level rise at the expense of their horizontal speed via a linear relationship. If accretion rates are insufficient to keep up with sea level rise, however, the intertidal flat submerges and retreats landward at the same time. We apply the obtained relationships to the Yangtze Estuary to estimate the critical sediment concentration level below which a shift from progradation to retreat can be expected.Coastal EngineeringEnvironmental Fluid Mechanic

Morphodynamic Feedback Loops Control Stable Fringing Flats

We apply a 2-D horizontal process-based model (Delft3D) to study the feedback mechanisms that control the long-term evolution of a fringing intertidal flat in the Western Scheldt Estuary. The hydrodynamic model is validated using a comparison with measurements on the intertidal flat and the sediment transport module is calibrated against long-term morphology data. First, the processes that lead to net sediment exchange between channel and flat are studied. Then, long-term simulations are performed and the dependency of sediment fluxes on the tidal flat bathymetry, and the corresponding morphodynamic feedback mechanisms are explained. In the long run, relatively stable states can be approached, which are shown to be typical for wave-dominated fringing mudflats. The system behavior can be explained by the typical feedback mechanisms between the intertidal bathymetry and the hydrodynamic forces on the flat. In the subtidal domain, the impact of small (5–10 cm) wind waves increases with a rising elevation due to decreasing water depths. In the intertidal domain, the wave impactincreases with increasing cross-sectional slope due to wave shoaling. These relationships result in negative (stabilizing) morphodynamic feedback loops. The tidal current velocities and tide-induced bed shear stresses, on the other hand, are largely determined by the typical horizontal geometry. A stabilizingfeedback loop fails, so that there is no trend toward an equilibrium state in the absence of wind waves.Coastal EngineeringEnvironmental Fluid Mechanic

Root-soil inter actions adaptation strategies of plants in response to soil water availability

We study the coupled action of water uptake and root development of maize in potting soil under greenhouse conditions. To this end, we apply subsurface irrigation strategies that are constant over weeks. We perform synchronous realtime measurements of the co-evolving soil moisture fields and root distributions. Will constant irrigation regimes eventually lead to constant root distributions and soil moisture profiles? In this contribution we report on the preliminary results of a study on the soil-root system behavior and underlying feedback loops. Understanding of the feedback loops between the soil moisture distribution and root development opens new pathways for boosting natural adaptation and climate resilience of plants. We compare two soil-root-systems that differ in irrigation depth; one with a constant irrigation depth and one with a step wise increasing irrigation depth. We also compare a bare soil system without roots.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Water ResourcesAtmospheric Remote Sensin

Do intertidal flats ever reach equilibrium?

Various studies have identified a strong relation between the hydrodynamic forces and the equilibrium profile for intertidal flats. A thorough understanding of the interplay between the hydrodynamic forces and the morphology, however, concerns more than the equilibrium state alone. We study the basic processes and feedback mechanisms underlying the long-term behavior of the intertidal system, restricting ourselves to unvegetated intertidal flats that are controlled by cross-shore tidal currents and wind waves and applying a 1-D cross-shore morphodynamic model. The results indicate that by an adjustment of the profile slope and shape, an initial imbalance between deposition and erosion is minimized within a few decades. What follows is a state of long-term seaward progradation or landward retreat of the intertidal flat, in which the cross-shore profile shape is largely maintained and the imbalance between deposition and erosion is not further reduced. These long-term trends can be explained by positive feedbacks from the morphology onto the hydrodynamic forces over the flat: initial accretion (erosion) decreases (increases) the shear stresses over the flat, which induces further accretion (erosion). This implies that a static equilibrium state cannot exist; the flat either builds out or retreats. The modeled behavior is in accordance with observations in the Yangtze Estuary. To treat these unbalanced systems with a one-dimensional numerical model, we propose a moving (Lagrangian) framework in which a stable cross-sectional shape and progradation speed can be derived for growing tidal flats, as a function of the wave climate and the sediment concentration in deeper water.Hydraulic EngineeringCivil Engineering and Geoscience

Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Plant roots are highly adaptable, but their adaptability is not included in crop and land surface models. They rely on a simplified representation of root growth, which is independent of soil moisture availability. Data of subsurface processes and interactions, needed for model setup and validation, are scarce. Here we investigated soil-moisture-driven root growth. To this end, we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length × width × height of 45 × 7.5 × 45 cm). The development of the vertical soil moisture and root growth profiles is tracked with a high spatial and temporal resolution. The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that the extension of the maximum rooting depth will stop if the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the water input flux (irrigation), can be convincingly improved by implementing root growth driven by soil moisture availability.Water ResourcesAtmospheric Remote Sensin