10:15 AM - 10:30 AM
[MIS21-06] Investigation of Plastic Litter Quantity and Behavior in the Oyabe River Using Video and Numerical Analysis
Keywords:annual number of plastic litters, time lapse camera, numerical simulation, particle tracking model
In recent years, plastic litter transported from rivers to the ocean has become a major global issue, making its mitigation an important international concern. In Japan, one of the coasts of Toyama Prefecture has been identified as a hotspot for drifting litter, with Rokudoji Beach near the mouth of the Oyabe River recording the highest amount of debris per 100 m³. Due to this situation, numerous volunteer efforts have been carried out to reduce plastic waste on the beach. However, their effectiveness remains limited without appropriate countermeasures within the river itself.
To address this issue, it is essential to quantify the amount and behavior of plastic litter within the river. However, the exact quantity and timing of plastic litter transport in the Oyabe River over the course of a year have not been well understood. Therefore, to clarify the number and characteristics of transported plastic litter in the Oyabe River, we conducted a field survey using time-lapse video recording, followed by particle tracking numerical analysis.
For the field survey, a fixed-point time-lapse camera was installed at Nijo Bridge, located 6.0 km from the river mouth. The camera operated from June to March, capturing 10-second recordings at 5-minute (or 10-minuite in winter season) intervals from 4:00 to 20:00. From the recorded footage, the number of plastic litter pieces passing through the river in each 10-second interval was counted and averaged over 30-minute periods. Using this data, we estimated the total number of plastic debris pieces transported during the video recording period. Additionally, a correlation equation between water level (flow rate) and the number of plastic debris pieces passing in 10 seconds was derived. This allowed us to estimate passage counts during periods when no video recordings were available.
As a result, the number of plastic litter pieces was estimated using two methods: camera observations and a correlation equation between flow rate and litter count. The correlation equation was used to estimate litter flow during nighttime and missing video periods. Combining these estimates, it was determined that approximately 640,000 pieces of plastic litter are discharged into the river mouth annually. Additionally, the monthly plastic litter count was found to be lower in winter than in summer. Furthermore, a comparison between morning and afternoon showed a significant difference (p < 0.05) based on the U test, with a higher litter count in the morning.
For the numerical analysis, the flow of the Oyabe River was simulated to evaluate plastic litter transport paths. The free software iRIC Nays2D+ was used for flow calculations. To model the transport of suspended particles, a particle tracking model was incorporated into Nays2D+, enabling the specification of arbitrary supply locations and quantities as part of the analysis. The results showed that during normal discharge conditions, particles remained suspended longer in the summer season (discharge ~30 m³/s) than in the winter season (discharge ~60 m³/s). During flood discharge, particles settled rapidly during the discharge recession stage. Additionally, hotspot areas where plastic debris accumulated were identified, including the outer bank of meanders, downstream regions of cross-sectional obstacles, and other specific locations.
To address this issue, it is essential to quantify the amount and behavior of plastic litter within the river. However, the exact quantity and timing of plastic litter transport in the Oyabe River over the course of a year have not been well understood. Therefore, to clarify the number and characteristics of transported plastic litter in the Oyabe River, we conducted a field survey using time-lapse video recording, followed by particle tracking numerical analysis.
For the field survey, a fixed-point time-lapse camera was installed at Nijo Bridge, located 6.0 km from the river mouth. The camera operated from June to March, capturing 10-second recordings at 5-minute (or 10-minuite in winter season) intervals from 4:00 to 20:00. From the recorded footage, the number of plastic litter pieces passing through the river in each 10-second interval was counted and averaged over 30-minute periods. Using this data, we estimated the total number of plastic debris pieces transported during the video recording period. Additionally, a correlation equation between water level (flow rate) and the number of plastic debris pieces passing in 10 seconds was derived. This allowed us to estimate passage counts during periods when no video recordings were available.
As a result, the number of plastic litter pieces was estimated using two methods: camera observations and a correlation equation between flow rate and litter count. The correlation equation was used to estimate litter flow during nighttime and missing video periods. Combining these estimates, it was determined that approximately 640,000 pieces of plastic litter are discharged into the river mouth annually. Additionally, the monthly plastic litter count was found to be lower in winter than in summer. Furthermore, a comparison between morning and afternoon showed a significant difference (p < 0.05) based on the U test, with a higher litter count in the morning.
For the numerical analysis, the flow of the Oyabe River was simulated to evaluate plastic litter transport paths. The free software iRIC Nays2D+ was used for flow calculations. To model the transport of suspended particles, a particle tracking model was incorporated into Nays2D+, enabling the specification of arbitrary supply locations and quantities as part of the analysis. The results showed that during normal discharge conditions, particles remained suspended longer in the summer season (discharge ~30 m³/s) than in the winter season (discharge ~60 m³/s). During flood discharge, particles settled rapidly during the discharge recession stage. Additionally, hotspot areas where plastic debris accumulated were identified, including the outer bank of meanders, downstream regions of cross-sectional obstacles, and other specific locations.