Concerns regarding plastic pollution arise as large quantities of plastic enter the ocean and affect wildlife and possibly human health. Rivers form a dominant pathway for macro plastic particles into the ocean. Plastic pollution in rivers can result in blockage, where it affects water quality and ecology. This increases risks of flooding and provides health concerns. To prevent further dispersion of plastics into the ocean and reduce health and flood risks, reduction of plastic concentrations in rivers is desired. Buoyant macro plastics are a dominant polluter and understanding their cross-sectional distribution in rivers could promote areas for efficient extraction or monitoring. As field measurements are expensive and time consuming it is desired to accelerate research with modelling techniques. To improve modelling of buoyant macro plastics and translate cross-sectional distributions it is necessary to model relative motion of buoyant macro plastic particles to the flow and one another. Previous research has shown that the understanding of macro plastic forces and transport is limited. The range of macro plastic properties complicates modelling approaches to macro plastic transport. To simplify, a classification of macro plastics is introduced according to cornerstones in behaviour. A basic force model is constructed (derived from riverine wood transport studies) to provide an understanding of the effect of size, density, wind and discharge on transport of different macro plastic types. This new understanding is applied to a variety of flow features induced by a range of river elements (river bend, channel widening, river confluence and groyne) to develop a hypotheses on the cross-sectional distribution of the different macro plastic objects in these environments. These hypotheses are tested in a numerical transport model. A theoretical background study on particle response times and Stokes numbers is performed to identify a relation between the particle response time and relative particle dispersion (compared to the flow). This provides a possibility for translating the transport of the different macro plastic objects into the numerical model. To define numerical modelling input of different buoyant macro plastic objects, particle response times are calculated and translated to horizontal particle dispersion coefficients in D-WAQ PART (software package of choice). Each of the previously formulated river elements is modelled as well in the numerical model. The modelling results show that river bends cause an increase in particle concentrations along the river bend. Channel widening increases the concentration along the boundaries. A river confluence increases the concentration along the tributary side of the channel. A groyne increases the macro plastic concentration along the groyne-free side of the channel. These results show that particle response times and their relation to Stokes number and particle dispersion propose an interesting tool for the modelling of relative particle motion towards the flow and the different macro plastic objects. This study identifies the significance, diversity and complexity of macro plastic particles in riverine environments. It highlights the need for classification and simplification to facilitate modelling and still be able to obtain relevant results. Describing macro plastics according to cornerstones in behaviour provides an efficient tool to simplify modelling. Coupling of different macro plastic classes to a range of riverine environments and understanding the differences in their cross-sectional distribution provides practical use.

For the past years large quantities of plastic waste have been accumulating on the beaches in Southwest Bali. This has economical, environmental and health implications in this area. This event mainly occurs between December to March. The objective of this report is to map the plastic waste problem in Southwest Bali and look for efficient solutions to mitigate plastic washing ashore at its beaches. To reach this objective the main drivers for the plastic waste accumulation are investigated. Furthermore, research on the distribution and composition is provided. The local waste management system and its shortcomings are assessed. Personal waste treatment is analyzed using a survey. Background information on the problem was gathered with the use of a literature study and area analysis. A conceptual model is introduced to reach the research objective. This conceptual model is a schematization, in order to better understand the research area and the behavior of the plastic in the case area. In order to quantify the model further research is required. This consist of four research disciplines. Namely, ocean, boundaries and bottom; river measurements; beach measurements and waste management & social studies. Via literature studies and interviews, research on the ocean, boundaries and bottom is performed. Three rivers where measured over the course of 2 months. Research was done to determine river parameters. Plastics were caught by a manta trawl and visual waste counting was performed to understand the plastic flux in rivers. Research on the beaches was done on the composition and quantity of various plastic sources namely oceanic, terrestrial and river inputs. Via interviews with local stakeholders the waste management system the case area is mapped. With the use of a survey, personal awareness and motivation among plastic pollution is analyzed. The plastic accumulation is largely dependent on the climate and local waste treatment. As waste management facilities are lacking and local awareness and motivation is limited, large quantities of plastic enter local rivers. Together with increased rain conditions large quantities of plastic end up in the ocean. Wind then transports plastic waste towards the beaches. This debris is mainly 'pushed' towards the beaches in Southwest Bali due to the coastal shape. During the research single use plastics are found to be the most common plastic type. Four solutions are proposed to reduce the plastic accumulation in the beaches of Southwest Bali. Intercepting plastics in rivers prevents further dispersion into the ocean and thus limits plastics accumulating on the beaches. Improved waste management could tackle the source of plastics pollution and reduce the amount of plastics entering our nature. Increased education on plastic pollution can result in improved personal waste management, especially in remote areas. As a lot of stakeholders are involved in the problem an improved communication network is desired in order to achieve a combined goal and work more efficiently.