2.1. Fluvial floods

Fluvial floods occur when the river discharge results in water levels that exceed the embankment of the river cross-section, leading to inundation of the floodplains. In case settlements are built on the natural floodplains, which is the case in many rivers around the world, urban areas can flood. The flow direction during fluvial floods can change direction during the event. When river discharge increases, the flow is generally directed from the river toward the floodplain, and reverses when the discharge decreases again. Fluvial flood can therefore have two effects. The inundation of floodplains and urban areas leads to mobilization of plastic, which may be transported toward or parallel to the river. However, if the flow from the river toward the floodplain already contains plastic, this may also be retained during inundation, for example, in vegetation or other obstructions of the flow and debris. Roebroek et al. (Reference Roebroek, Harrigan, Van Emmerik, Baugh, Eilander, Prudhomme and Pappenberger2021a) provided a first global assessment of the potential plastic mobilization (PPM) from inundated floodplains, by combining the best estimates of mismanaged plastic waste [kg/y/km²] (Lebreton & Andrady, Reference Lebreton and Andrady2019) with flood extent maps for events with return periods from 5 to 500 years. Globally, the PPM from floods was found to be ten times higher than during non-flood conditions. In several regions that are both flood prone and heavily polluted, factors as high as 40 were found. Hauk et al. (Reference Hauk, van Emmerik, van der Ploeg, de Winter, Boonstra, Löhr and Teuling2023) looked at the effect of the 2021 flood in the Meuse on 100 specific plastic item types, demonstrating the response varies considerably between categories. Smaller items were generally flushed and larger items were mainly deposited during the flood. This is in line with Hurley et al. (Reference Hurley, Woodward and Rothwell2018), who found that microplastic in river sediments decreased with 70% after a flood event. Fluvial floods may lead to both additional mobilization and retention of plastic, depending on the direction of the flow and the plastic characteristics.

2.2. Pluvial floods

Pluvial floods occur when the rainfall intensity exceeds the drainage or infiltration capacity, leading to ponding on the land surface or on the streets and subsequently surface runoff. Plastics can be mobilized through both the pressure forces of the water layer on their surface, and through flotation and suspension when the inundation depth is large enough. The eventual travel distance is determined by a combination of the driving forces and the resisting forces, the latter mainly influenced by surface roughness and physical obstructions. Plastic items that remain mobile can enter rivers through direct entry of surface runoff, and through urban drainage networks that discharge in rivers (Treilles et al., Reference Treilles, Gasperi, Saad, Tramoy, Breton, Rabier and Tassin2021). Especially in urban areas that are closely connected to waterways, pluvial floods may result in an increase in plastic pollution in rivers. Research on such travel dynamics in urban areas remains scarce. A recent study on Amsterdam found no significant correlations between rainfall and floating plastic transport in either of the six measured canals (Tasseron et al., Reference Tasseron, Begemann, Joosse, van der Ploeg, van Driel and van Emmerik2023). For the Huveaune River in Marseille, Tramoy et al. (Reference Tramoy, Blin, Poitou, Noûs, Tassin and Gasperi2022) found that 36% of the annual macroplastic transport occurred during heavy rainfall events. Treilles et al. (Reference Treilles, Gasperi, Saad, Tramoy, Breton, Rabier and Tassin2021) estimated that 20–36% of the total annual plastic transport in stormwater was discharged through untreated stormwater, which mainly occurs when the precipitation included runoff exceeds the urban drainage capacity. Pluvial floods are mainly associated with increased plastic transport, especially through urban drainage systems.

2.3. Flash floods

Flash floods can occur when high-intensity precipitation falls on a relatively small surface area. Water level may rapidly increase, leading to high flow velocities, and therefore high plastic mobilization (Moore et al., Reference Moore, Lattin and Zellers2011). During the 2021 European floods, floating plastic transport was found to have increased with a factor over 100 compared to non-flood conditions, which was partly explained by flash floods that occurred in several tributaries of the Meuse (van Emmerik et al., Reference van Emmerik, Frings, Schreyers, Hauk, de Lange and Mellink2023a). Flash floods may result in high forces that mobilize items and objects that are generally not found in or around rivers. Bayón et al. (Reference Bayón, Valero and Franca2023) showed that besides large plastic items, caravans, furniture, garbage bins, and even larger objects can also be mobilized. Anecdotal evidence from the 2021 European floods confirms that indeed much larger items were found on riverbanks and floating at the surface than under non-flood conditions (van Emmerik et al., Reference van Emmerik, Frings, Schreyers, Hauk, de Lange and Mellink2023a). When flash floods occur in areas close to river mouths, these may lead to flushing of accumulated plastics. Pierdomenico et al. (Reference Pierdomenico, Ridente, Casalbore, Di Bella, Milli and Chiocci2022) found increased burial of plastics in sediment around river mouths after flash floods in the mouth of the Mazzarrà River. In some cases, flash floods flush large amount of plastics to submarine canyons near river mouths (Pierdomenico et al., Reference Pierdomenico, Casalbore and Chiocci2019). Flash floods are events that mobilize additional, and larger plastic items and objects, and specifically can cause a flushing effect of accumulated plastic in river systems.

2.4. Coastal floods

Coastal floods can be caused by storm surges or tsunamis, and are characterized by progressing and retreating flow direction. Similar to fluvial floods, coastal floods may therefore lead both to mobilization and deposition of plastic. As many of the world's largest cities are located in delta areas, coastal floods are potentially high-impact events for plastic transport. However, only few work has been done to quantify the impact of coastal floods on plastic pollution. Several efforts have been made to study the impact of 2011 Tohoku tsunami that hit Japan. Lebreton and Borrero (Reference Lebreton and Borrero2013) estimated that an amount equal to thousands years of normal plastic leakage entered the ocean during this extreme event. Murray et al. (Reference Murray, Maximenko and Lippiatt2018) estimated that the tenfold increase of beach plastic density in North America and Hawaii could be attributed to this single event. Coastal floods mainly lead to mobilization of plastics, and large additional input into the ocean. Over time this may also result to increased beached plastic on shorelines elsewhere.

2.5. Impact of flood-driven plastic transport

Here the three main additional direct impacts of flood-driven plastic transport are identified. First, floods lead to direct additional impact of plastics on the environment. Larger plastic items, especially foils and other flexible items, can accumulate around infrastructure. Honingh et al. (Reference Honingh, Van Emmerik, Uijttewaal, Kardhana, Hoes and Van de Giesen2020) combined field measurements during high discharge and lab experiments to estimate that plastic pollution blockage in the Cikapundung River around Bandung, Indonesia, could lead to a water level increase of 1 m per hour. This increase was much higher than blockages caused by organic debris, and the studied event was not even under (extreme) flood conditions. Plastic blockage of bridges and other types of infrastructure was also reported by Tjia (Reference Tjia2020) in urban rivers in Kumasi, Ghana. Small rivers draining densely populated areas are assumed to be among the most polluted globally, and exactly those rivers are sensitive for blockage by plastic pollution. The Odaw River, draining Ghana's capital Accra, was found to transport similar amounts of plastic as the Saigon, Vietnam, and Chao Phraya, Thailand (Pinto et al., Reference Pinto, Bogerd, van der Ploeg, Duah, Uijlenhoet and van Emmerik2024; van Calcar & van Emmerik, Reference van Calcar and van Emmerik2019). However, the Odaw is around 5–10 times narrower, and full of bridges, weirs, and other locations where plastic can easily accumulate. In turn, the increased inundation depths upstream may lead to additional plastic being mobilized.

Second, floods have a large impact on the annual plastic budget of river systems. In the United Kingdom, it was estimated that the 2015 floods resulted in a 70% microplastic concentration in river sediments (Hurley et al., Reference Hurley, Woodward and Rothwell2018). It was unknown how far these particles traveled in response to the flood. For the 2021 Meuse flood, the total plastic transported during the six-day flood event equaled one-third of the annual plastic transport (van Emmerik et al., Reference van Emmerik, Frings, Schreyers, Hauk, de Lange and Mellink2023a). In this specific case study, the observations were done as part of an ongoing one-year monitoring campaign, allowing to rapidly and safely mobilize observers to relevant locations. However, during other large-scale and long-term monitoring efforts, no observations were done during extreme events. Not accounting for flood-driven plastic transport may therefore introduce large errors in the assessment of annual plastic budgets, and understanding of transport and retention dynamics.

Finally, flood-related plastic deposition may threaten vegetation species globally. Gonçalves et al. (Reference Gonçalves, Schmid, Andrade, Andrades, Pegado and Giarrizzo2020) found flooded forests to be large accumulation zones for riverine plastic pollution. As introduced in Section 2.1, fluvial floods may also lead to deposition of plastics in vegetation on the floodplains, known as the Christmas tree effect (Williams & Simmons, Reference Williams and Simmons1999). Although the effect of plastic pollution on vegetation is not globally quantified, complete coverage of root zones of, for example, mangrove trees leads to tree death (van Bijsterveldt et al., Reference van Bijsterveldt, van Wesenbeeck, Ramadhani, Raven, van Gool, Pribadi and Bouma2021).