Plastic debris in water systems is a major challenge for our ecosystem, because it is extremely persistent in the environment. Apart from the importance of reducing the amount of plastic entering the ocean, clearing the rivers from debris is important for societal concerns, such as flood risks. Plastic waste accumulation at trash racks leads to a rise in upstream water level and may increase urban flood risk. Until now, most studies of riverine debris accumulation predominantly focused on organic accumulations at trash racks and bridge piers. In this study, flume experiments were used to study the behavior of plastic and mixed debris accumulations. One of the key findings from this study is that plastic debris causes a faster blockage than organic matter, as the plastic blockage contains fewer voids and therefore has a higher blockage density. In addition to the flume experiments, field measurements were performed in the Cikapundung River (Indonesia). This river is one of the tributaries of the Citarum River, which is considered one of the world’s most heavily polluted rivers. Combining the results of the flume experiments and field measurements demonstrated that a backwater rise of 1 m/h is plausible for a blocked trash rack in the Cikapundung River, illustrating the additional flood risk caused by plastic pollution. Our results emphasize the need for further quantifying riverine (plastic) debris and investigating its relation to changes in the water system behavior, including its influence on urban flood risk.

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Many rivers in the world carry loads of debris to the sea. Typically in urbanizing areas with limited waste collection systems, the amounts of plastic amongst the organic material can be substantial. This has an impact on e.g. water quality, clogging of the (urban) water infrastructure, and deterioration of habitats. Flood risk is increased when debris is accumulating at bridges, weirs and trash racks. Understanding the behaviour of plastics in rivers and their impact on river conveyance is important for assessing both water quality issues and flood risks. In this study samples of waste in the Citarum river basin near Bandung, Indonesia where taken using single and double trawls. The analysis showed that plastic content of those samples can amount to more than 40% of the total debris weight. Additional laboratory measurement were performed on a debris flow through a trash rack in a straight flume, to assess the effects of plastic content on debris behaviour and consequent increased energy loss in flows through a trash rack. Though limited by scale effects and simplifications in the experiment, it was concluded that the plastic content in the debris increased the rate of clogging as well as the flow resistance for a certain amount of debris. The rapidly increased resistance is considered a problem in areas with intermittent, heavy rainfall where plastics that accumulated on river banks are mobilised and flushed into the water system. The carpet formed by plastic-containing debris appears to be more coherent and to contribute more to flow resistance and a higher risk of flooding. Future research will include a wider variation of flow geometries and debris composition.@en