Plastic debris in water systems form a major problem for our ecosystem because it is extremely persistent in the environment and the global plastic production is still increasing without adequate measures to prevent pollution. 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 health related issues and flood risks. The interaction between riverine debris and the hydrodynamics of water systems plays an important role in the construction of trash racks that can block water flow in the river. These clogged trash racks lead to an increase in upstream water level and can cause regional flood risks. This study was set up to investigate the amount and accumulation of riverine debris in the Cikapundung River in Indonesia, one of the tributaries of the Citarum River.

The Citarum River is one of the world’s most heavily polluted rivers, predominantly caused by industries and households dumping their waste directly in the river. With a length of 270 kilometers, this is the longest river of West-Java and it is the main source of water for 27 million people. The river flows through three reservoirs and the area upstream of these reservoirs is called the Upper Citarum River Basin. The main contribution of pollution comes from this area, as the city of Bandung is located in this area. The Cikapundung River flows through the city of Bandung and is one of the tributaries of the Citarum River. This river was selected for the field measurements in this study as it allowed measurements to be performed in river parts with rural and urban areas neighboring the river.
Up until now, many studies of riverine debris accumulation are predominantly focused on organic debris accumulations at bridges and gates. This study also investigated plastic debris accumulations, as it forms a significant part of the debris in the Cikapundung River and other Asian rivers.

Field measurements in the Cikapundung River were performed with a single and double trawl, to determine the riverine debris composition and flux. Scaled laboratory tests were carried out, to (1) monitor the blockage growth process for different debris compositions, (2) investigate the influence of different parameters on the upstream water level and (3) determine the impact of a blocked trash rack on regional flood risks.

From the field measurements, an increase in downstream direction was found for the debris flux, but the debris composition varied in both time and space. The plastic debris mass varied between 11\% and 78\% of the total debris mass.
Based on the laboratory tests, the behavior of riverine debris is studied during normal flow conditions and in front of a trash rack. One of the key findings from this study is that plastic debris causes a faster blockage than organic debris, as the plastic blockage contains fewer voids and therefore has a higher blockage density. In the formation of a blockage in front of a trash rack, a mix of organic and plastic debris behaved more similar to plastic debris. The shape of the blockage was also found to be different for plastic and organic debris: a high amount of plastic in the debris lead to an angular blockage shape, whereas mainly organic debris produced a curved blockage shape.
The most important indicators for flood risks related to debris accumulations were found to be the debris load, loss coefficient, initial flow velocity and initial flow depth. By rescaling the results to the case study location in Bandung, it was found that a backwater rise within the hour of O(1 m) is plausible for a blocked trash rack.

This study forms the stepping stone to further quantifying riverine (plastic) debris and investigating its relation to changes in the water system behavior, including its influence on regional flood risks.