17:15 〜 18:45
[MIS09-P03] Visual surveys and numerical modeling on marine debris in the Gulf of Thailand
キーワード：Floating Marine Debris, Particle Tracking Model, Gulf of Thailand
The purpose of this study was understanding/predicting the distribution of floating debris in the Gulf of Thailand (GoT) between northeasterly (NE) monsoon and transition monsoon from southwesterly (SW) to NE monsoon, based on visual observations from vessel. We thereafter attempt to reproduce the observed results using numerical particle tracking model approach.
The study area was covering the upper and center GoT between 99-104E and 11-14N in longitude and latitude, respectively. The field surveys were conducted using research vessels belonging to the Southeast Asian Fisheries Development Center in NE monsoon (Dec 2019 and Jan 2022) and transition monsoon (Oct 2023). The observers recorded flotsam position, types shown below, and observed date when they found them from the vessel. The flotsam types were categorized to foam, food package, glass, metal, plastic bag, plastic bottle, plastic crake, fishing net, fishing floating buoy, and unknown object.
In the particle tracking model (PTM), we gave monthly winds at 10m above sea surface with 1°x 1°resolution from J-OFURO from year 1993-2018 (Tomita et al., 2019), monthly HYCOM current and Stokes drift with 0.08° x 0.08° resolutions from year 1993-2018 (Isobe et al., 2019) and plastic debris emissions from rivers surrounding the GoT (Lebreton et al., 2017). We use the forward in-time PTM by using the equation as
Xt+Δt=Xt+UΔt+R2KhΔt
Where X= x,y represents the particle location, Δt is the time increment, R is a random number generated at each time step, Kh is diffusivity (=0.01*Δx4/3 ; where Δx means the spatial resolution of ocean currents given to the model), and U is a linear combination of surface ocean currents, Strokes drift and windage. The windage was calculated by
W=ρaρwAaAwCdaCdw ·Ws,
When W is windage, ρ is density, A is particle’s projected area, Cd is drag coefficient, Ws is wind speed, and subscripts a, w denote the variables on the air and water sides, respectively. The PTM includes beaching/re-drifting processes of particles. We give modeled particle for big rivers (>90% discharge rates in the GoT) at the bay head to represent riverine ocean plastics.
The total number of floating debris observed in the field surveys in NE monsoon was 4,068 items, divided into foam 1,299 items (31.93%), other plastic 2,581 items (63.45%) and fishing gear 188 items (4.62%) while we found the floating debris 1,978 items divided into foam 318 items (16.08%), other plastic 1,642 items (83.01%) and fishing gear 18 items (0.91%) in the transition monsoon.
The PTM showed that the floating marine debris was carried to the northern and eastern part of GoT during Apr to Sep, and were carried to the western part during mid-Oct to Mar. Debris’ distribution was well explained by the current and wind patterns, which are SE wind and current generated during May to Sep and are replaced by the NE wind and current from Nov to Mar. In transition monsoon, large numbers of both floating debris and modeled particles were shown on the western side: the highest number were revealed near Bangpakong river mouth. Meanwhile, in the NE monsoon, the highest number of floating debris was observed near Bangpakong river mouth, but although the number was low in the GoT. The number of modeled particles on the beach showed very high in both periods. The westward deviation in transition monsoon was found both in modeling and observation. Therefore, the floating debris found in the visual surveys was regarded as riverine plastics. In NE monsoon, modeling was found low number of particles in GoT although observations revealed high number of floating debris. It is suggested that the debris was not derived from rivers in the same season, but was carried from the beaches by strong off-shoreward winds and currents in that period.
The study area was covering the upper and center GoT between 99-104E and 11-14N in longitude and latitude, respectively. The field surveys were conducted using research vessels belonging to the Southeast Asian Fisheries Development Center in NE monsoon (Dec 2019 and Jan 2022) and transition monsoon (Oct 2023). The observers recorded flotsam position, types shown below, and observed date when they found them from the vessel. The flotsam types were categorized to foam, food package, glass, metal, plastic bag, plastic bottle, plastic crake, fishing net, fishing floating buoy, and unknown object.
In the particle tracking model (PTM), we gave monthly winds at 10m above sea surface with 1°x 1°resolution from J-OFURO from year 1993-2018 (Tomita et al., 2019), monthly HYCOM current and Stokes drift with 0.08° x 0.08° resolutions from year 1993-2018 (Isobe et al., 2019) and plastic debris emissions from rivers surrounding the GoT (Lebreton et al., 2017). We use the forward in-time PTM by using the equation as
Xt+Δt=Xt+UΔt+R2KhΔt
Where X= x,y represents the particle location, Δt is the time increment, R is a random number generated at each time step, Kh is diffusivity (=0.01*Δx4/3 ; where Δx means the spatial resolution of ocean currents given to the model), and U is a linear combination of surface ocean currents, Strokes drift and windage. The windage was calculated by
W=ρaρwAaAwCdaCdw ·Ws,
When W is windage, ρ is density, A is particle’s projected area, Cd is drag coefficient, Ws is wind speed, and subscripts a, w denote the variables on the air and water sides, respectively. The PTM includes beaching/re-drifting processes of particles. We give modeled particle for big rivers (>90% discharge rates in the GoT) at the bay head to represent riverine ocean plastics.
The total number of floating debris observed in the field surveys in NE monsoon was 4,068 items, divided into foam 1,299 items (31.93%), other plastic 2,581 items (63.45%) and fishing gear 188 items (4.62%) while we found the floating debris 1,978 items divided into foam 318 items (16.08%), other plastic 1,642 items (83.01%) and fishing gear 18 items (0.91%) in the transition monsoon.
The PTM showed that the floating marine debris was carried to the northern and eastern part of GoT during Apr to Sep, and were carried to the western part during mid-Oct to Mar. Debris’ distribution was well explained by the current and wind patterns, which are SE wind and current generated during May to Sep and are replaced by the NE wind and current from Nov to Mar. In transition monsoon, large numbers of both floating debris and modeled particles were shown on the western side: the highest number were revealed near Bangpakong river mouth. Meanwhile, in the NE monsoon, the highest number of floating debris was observed near Bangpakong river mouth, but although the number was low in the GoT. The number of modeled particles on the beach showed very high in both periods. The westward deviation in transition monsoon was found both in modeling and observation. Therefore, the floating debris found in the visual surveys was regarded as riverine plastics. In NE monsoon, modeling was found low number of particles in GoT although observations revealed high number of floating debris. It is suggested that the debris was not derived from rivers in the same season, but was carried from the beaches by strong off-shoreward winds and currents in that period.