12/9/2023 0 Comments Tidal riverIf, instead the absolute volumes of the ebb and flood flow are taken, then the equation becomes: V_tidal should be the average of the ebb and flood volumes: (Vebb-Vflood)/2. This makes the calculation of the delivery ratio unclear. But in that case, the integral of the tidal volumes, V_ebb and V_flood, should also be positive and negative. In line 155, the authors mention that ebb flow is positive and flood flow is negative. So, until this unclarity is solved, we cannot judge if this conclusion is correct. But the calculation of the tidal volumes and efficiencies is, unfortunately, doubtful because of erroneous or unclear mathematics. Also the conclusion that the plastic travels faster than the fresh water component of the tidal flow is relevant. It is well written and reports on extensive field observations in the Saigon tidal river, which I consider very valuable. With this paper we emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean. Our results demonstrate the crucial role of tidal dynamics and bidirectional flows in net plastic transport. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (11–17 % vs 31–39 %), suggesting the retention time strongly depends on item characteristics. The plastic net transport rates alternate between positive (seaward) net transport and negative (landward) net transport, as a result of the diurnal inequality in the tidal cycles. Such factors include wind, varying plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The net transport of plastic is higher than the net discharge (27–32 % and 18 %, respectively), suggesting that plastic transport is governed by other factors than water flow. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's r = 0.87, R 2 = 0.75). We show that the net plastic transport is about 27–32 % of the total plastic transport. We applied our method to the highly polluted Saigon river, Vietnam, throughout six full tidal cycles in May 2022. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of plastic transport and discharge during full tidal cycles. Here, we provide a first observation-based estimate of net plastic transport on a daily time scale in tidal rivers. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bidirectional flows and varying water levels, can lead to even higher likelihood of long-term retention. Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. As a consequence, flow velocity direction and magnitude can change diurnally. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. However, the processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. Rivers are assumed to transport land-based plastic into the ocean, and the fluvial and marine transport processes have been relatively well studied to date. Plastic is an emerging pollutant, and the quantities in rivers and oceans are expected to increase.
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