Microplastic (MP) contaminates terrestrial, aquatic, and atmospheric environments. Although the number of river sampling studies with regard to MP concentrations is increasing, comprehension of the predominant transport processes of MP in the watercourse is still very limited. In order to gain a better process understanding, around 500 physical experiments were conducted to shed more light on the effects of particle shape, size and density on the rise and settling velocities of MP. The determined velocities ranged between 0.39 cm/s for polyamide fibers (settling) and 31.4 cm/s for expanded polystyrene pellets (rise). Subsequently, the determined velocities were compared with formulas from sediment transport and, as there were large differences between theoretically and experimentally determined velocities, own formulas were developed to describe settling and rise velocities of MP particles with a large variety of shapes, sizes and densities. This study shows that MP differs significantly from sediment in its behavior and that a transfer of common sediment transport formulas should be treated with caution. Furthermore, the established formulas can now be used in numerical simulations to describe the settling and rising of MP more precisely.

Microplastic (MP) has been detected in marine, limnic, terrestrial, and atmospheric environments. However, rivers are often only seen as transport paths for MPs from inland sources to the oceans, although transport rates in rivers can hardly be determined yet. MP in rivers can either be transported, or it settles to the bottom of the river and either remains there or is remobilized again at higher flow velocities. This remobilization, also known as erosion, depends on the critical shear stress of a particle and is influenced by the particle properties and the sediment bed. In this study, the critical shear stresses of 14 MP particles with different shapes, densities, and particle sizes on different sediment beds were experimentally determined and subsequently compared with the basic principles of erosion from sediment transport. Critical shear stresses of the MP particles were between 0.002 and 0.233 N/m 2 , depending on particle and sediment properties. Furthermore, the hiding-exposure effect was transferred to MPs and an equation was developed to determine the critical shear stress of different MP particles on natural sediment beds.

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Over recent years, many coastal engineering projects have employed the use of soft solutions as these are generally less environmentally damaging than hard solutions. However, in some cases, local conditions hinder the use of soft solutions, meaning that hard solutions have to be adopted or, sometimes, a combination of hard and soft measures is seen as optimal. This research reviews the use of hard coastal structures on the foreshore (groynes, breakwaters and jetties) and onshore (seawalls and dikes). The purpose, functioning and local conditions for which these structures are most suitable are outlined. A description is provided on the negative effects that these structures may have on morphological, hydrodynamic and ecological conditions. To reduce or mitigate these negative impacts, or to create new ecosystem services, the following nature-based adaptations are proposed and discussed: (1) applying soft solutions complementary to hard solutions, (2) mitigating morphological and hydrodynamic changes and (3) ecologically enhancing hard coastal structures. The selection and also the success of these potential adaptations are highly dependent on local conditions, such as hydrodynamic forcing, spatial requirements and socioeconomic factors. The overview provided in this paper aims to offer an interdisciplinary understanding, by giving general guidance on which type of solution is suitable for given characteristics, taking into consideration all aspects that are key for environmentally sensitive coastal designs. Overall, this study aims to provide guidance at the interdisciplinary design stage of nature-based coastal defence structures.