"Plastic pollution is a threat for all ecosystems due to its effects on people, animals, and environment. Rivers are estimated to transport around 0.5 millions tons of plastic per year. When plastic enters a river system, it is transported downstream towards the sea but it is also likely to accumulate at specific cross sections and locations, including hydraulic structures, eventually increasing the risk of floods. Gates, locks, weirs, and bridges are commonly present in rivers and canals and have several functions, including water level regulation, flood safety, and inland water shipping. These can also be found in water treatment plants, hydropower stations as well as debris/plastic collection systems. Riverine plastic accumulation is also known to cause geomorphic changes. In-depth knowledge on how plastic particles accumulate upstream of hydraulic structures is therefore crucial to understand the processes that affect plastic transport, its influence on the safety and functionality of hydraulic structures and their effects on the hydro- and morphodynamic conditions of the flow. In this research experiments were performed using simplified plastic particles to analyse the processes that lead to the instability of accumulated particles upstream of a simple gate."@en