Biogeomorphology shaping coastal and estuarine systems
Authors:   , Boechat Albernaz Marcio 
Published: November 2023

Coastal and estuarine environments are home to a wide variety of habitats and ecosystems. They provide ecosystem services such as nursery grounds and habitats for (non-)commercial fishes and other aquatic species, and offer natural protection to coastlines, shelter for harbors, tourism and leisure. Understanding the morphodynamics of coastal areas is vital as hundreds of millions of people live near the coast around the globe. Due to human pressure and climate change, many deltas and coastal areas are under threat. Therefore, the ability to understand and predict the long-term evolution (i.e. decades to millennium) of these systems is crucial for supporting policies and management towards more sustainable and safe usage of coastal environments. Coastal, fluvial and estuarine landscapes develop through interaction of water, sediments and biota. These landscapes are an ever evolving product of hydrodynamic forces, such as river discharges, tides and waves, sediment transport and interactions with biota on the pre-existing morphology. As such, a landscape is a product of the initial conditions (e.g. the geological legacy), the boundary conditions (tides, waves, fluvial discharges, sediment supply and longer-term sea level fluctuations) and internal conditions (roughness, friction, inertia and turbulence steered by channels, shoals, bars, bedforms and vegetation) controlling the hydrodynamics and sediment transport which in turn modify the morphology and the abiotic conditions for biota. Biota affect the hydrodynamics in various ways and may also directly change morphology by organic material accretion. Thus the landscape is a result of dynamic biogemorphodynamic interactions between water, sediment, morphology and biota. Two major but contrasting methodologies of studying long-term geomorphology and morphodynamics are: historical-paleogeographical reconstructions and morphodynamic modelling. Both methods have their own advantages and disadvantages in reconstructing past conditions, forecasting future conditions and understanding the morphodynamic drivers. Yet, scientific communities tend to concentrate on either method with limited interaction in spite of studying the same systems and processes. The reconstruction and hindcast of past conditions as well as the forecast of future scenarios are challenges for scientists. For example, paleogeographical reconstructions are commonly hampered by the ability to isolate variables and testing alternative hypotheses from often limited past data. On the other hand, morphodynamic models often need to be simplified in their initial and boundary conditions and in the acting mechanisms, in comparison to nature, due to a combination of limited available data (in terms of detailed model inputs), limited physical process formulations and limitations of computational power. That means, predicting the evolution of beaches, tidal basins and estuaries is challenging due to the limitations on our knowledge about the individual and combined effects of changing forces, such as sea level rise and sediment supply, within the morphodynamic feedback; as well as due to shortcomings in our knowledge of physics and the uncertainties of predicting future hydrodynamic conditions, sediment fluxes and composition and the biota composition for decades to centuries ahead. Consequently, there is an urgent need to identify and address important knowledge gaps between paleogeographical reconstructions and morphodynamic models regarding the response of coastal-estuarine systems under varying sediment supply, fluvial discharges and increasing sea level, especially in combination with vegetation. The objective of this thesis is to systematically determine the long-term and large-scale development of coasts, estuaries and tidal basins under combinations of tides, waves, river discharges, sea level rise, sediment supply and vegetation. Hypotheses posed by paleogeographical reconstructions were tested with numerical models.