Quantification of floating riverine macro-debris transport using an image processing approach

Abstract

A new algorithm has been developed to quantify floating macro-debris transport on river surfaces that consists of three fundamental techniques: (1) generating a difference image of the colour difference between the debris and surrounding water in the CIELuv colour space, (2) detecting the debris pixels from the difference image, and (3) calculating the debris area flux via the template matching method. Debris pixels were accurately detected from the images taken of the laboratory channel and river water surfaces and were consistent with those detected by visual observation. The area fluxes were statistically significantly correlated with the mass fluxes measured through debris collection. The mass fluxes calculated by multiplying the area fluxes with the debris mass per unit area (M/A) were significantly related to the flood rising stage flow rates and agreed with the mass fluxes measured through debris collection. In our algorithm, plastic mass fluxes can be estimated via calibration using the mass percentage of plastics to the total debris in target rivers. Quantifying riverine macro-plastic transport is essential to formulating countermeasures, mitigating adverse plastic pollution impacts and understanding global-scale riverine macro-plastic transport. Quantifying the transport of macro-debris floating on the world's rivers, which are major sources of ocean debris, is essential in formulating countermeasures to mitigate the adverse impacts of land-based loads. In particular, the adverse impacts on aquatic ecosystems of plastics containing toxic chemicals (e.g., persistent organic pollutants (POPs)) 1-4 are recognized as a serious concern in the global aquatic environment 5-8. Many of the plastics in the oceans originate from land 9 , and thereafter, macro-plastics (>25 mm in diameter) 10 are evenly broken down into smaller plastic fragments known as meso-(5−25 mm in diameter) and micro-plastics (<5 mm in diameter) 10 due to photo-and thermo-oxidative degradation 6. Micro-plastics are rarely removed from the aquatic environment when released and are thus gradually transported far away due to ocean currents. Hence, to formulate countermeasures against oceanic plastic pollution, the macro-plastics in rivers must be efficiently captured before being fragmented into smaller pieces and/or being released to the oceans. In particular, because most macro-plastics float on the surface, it is important to understand how floating macro-debris is transported via rivers and then released into the oceans. Recently, a few studies have attempted to estimate the plastic waste emissions from land 9,11. Jambeck et al. 9 , for instance, estimated that 4.8 to 12.7 million tonnes of mismanaged plastic waste could be entering the oceans from 192 coastal countries in 2010 by considering the waste management level in each country and the coastal population. In their estimate, 0.02 to 0.06 million MT of plastic waste can enter the Pacific Ocean from Japan. Moreover, Lebreton et al. 11 estimated that between 1.15 and 2.41 million tonnes ...

Macroplastic Storage and Remobilization in Rivers

“…To date, the transport of macroplastic debris has been quantified using detection of plastic particles from aerial photographs taken by a drone from low heights (5 and 15 m; [35]). Macroplastic debris transport was also measured by visual counting from bridges, video approaches, trackers, net sampling, or from infrastructure or riverbanks (e.g., [13,18,26,27,40,56]). In future works, more details may be determined using a combination of a visual counting method with hydrodynamic modelling [57].…”

“…Similar to the transport phase, the output phase may also be temporarily modified by hydropeaking or sediment flushing events occurring downstream from dams. The intensity of an output phase may be measured in the same way as for the transport phase (e.g., [27,40,56]). Finally, it can be also hypothesized that a proportion of surface-stored macroplastic debris may be transported by wind outside the storage/remobilization zone, hence being removed from the area potentially influenced by fluvial processes (Table 1).…”

“…From this perspective, precise information is still required on the spatial and temporal extent of the storage of macroplastic debris in river deposits (both surface and subsurface) and riparian vegetation, and the estimation of the potential for macroplastic remobilization caused by water inundation and erosion processes. Recent works showed that macroplastic debris can be detected through visual analysis of present and past sediments [1,21,34], and monitored and mapped with remote sensing methods (e.g., aerial photographs, UAV surveys) [35][36][37][38][39][40][41], thus creating a promising methodological perspective for future research. However, a conceptual framework and a theoretical background for research on macroplastic storage and remobilization in fluvial systems are not available.…”

High-Resolution Mapping of Japanese Microplastic and Macroplastic Emissions from the Land into the Sea

“…In particular, a method of measuring MacP during flooding, when various types of suspended matter flow downstream, has not been established. In order to solve such problems, Kataoka and Nihei [55] have proposed a MacP survey method that combines video image captures and image analyses of the river water surfaces, along with safety considerations. In the future, it will be essential to collect MacP concentration data using these methods to improve the accuracy of plastic emissions evaluations.…”

Crowd-Based Observations of Riverine Macroplastic Pollution

“…New technological advances can facilitate the upscaling of data collection across time (long-term time series) and space (more rivers, and more locations along single rivers). For example, Geraeds et al (2019) and Kataoka and Nihei (2020) demonstrated the potential of using Unmanned Aerial Vehicles (UAVs) and cameras to measure floating plastic and plastic on banks in river systems.…”