Effective Application of a Mechanical Circulator for Reducing Hypoxia in an Estuarine Trench

Sasaki, Jun; Kanayama, Susumu; Nakase, Kota; Kino, Seiji

doi:10.1142/s0578563409002053

Cited by 6 publications

(5 citation statements)

References 26 publications

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“…To reduce occasional errors, two replicate samples were collected; (4) The pump was started. The downwelling flow impinged the sediment-forming scour hole and the shape of the hole was nearly unchanged after 4 h; (5) The pump was closed and water samples were collected again. For the convenience of measuring the shape of the scour holes, water was carefully drained from the flume by syringes; (6) The needle water-level gauge was used to approximately measure the shape of the scour hole.…”

Section: Methodsmentioning

confidence: 99%

“…Stigebrandt et al [2] proposed the idea of using an erect pipe to generate artificial downwelling. Sasaki et al [5] developed a mechanical circulator driven by electricity to transport the surface water to the bottom through a draft tube, and its effectiveness was verified by field tests. The field tests indicated that the quality of both the water and sediment was improved in the short term by the mechanical circulator, while long-term sediment processes were not addressed.…”

Section: Introductionmentioning

confidence: 99%

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Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

et al. 2020

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Artificial downwelling, which is an ecological engineering method, potentially alleviates bottom hypoxia by bringing oxygen-rich surface water down below the pycnocline. However, the downward flow is likely to disturb sediments (or induce sediment resuspension) when reaching the bottom and then have unwanted side effects on the local ecosystem. To evaluate this, our paper presents a theoretical model and experimental data for the sediment resuspension caused by artificial downwelling. The theoretical model considers the critical conditions for sediment resuspension and the scour volume with the downwelling flow disturbing sediment. Experiments with altered downwelling flow speeds, discharge positions relative to the bottom, and particle sizes of sediment were conducted in a water tank, and the results were consistent with our theoretical model. The results show that the critical Froude number (hereinafter Fr) for sediment resuspension is 0.5. The prevention of sediment resuspension requires the downwelling flow speed and the discharge position to be adjusted so that Fr < 0.5; otherwise a portion of sediment is released into the water and its volume can be predicted by the derived formulation based on the Shields theory. Furthermore, sediment resuspension has side effects, such as a water turbidity increase and phosphorus release, the magnitudes of which are discussed with respect to engineering parameters. Further study will focus on field experiments of artificial downwelling and its environmental impacts.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

et al. 2020

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“…The bottom anoxic waters often contain hydrogen sulfide generated by sulfate reduction (Kodama and Horiguchi, 2011;Furukawa, 2015;Amunugama and Sasaki, 2018). In addition, navigation channels and borrow pits (dredged for the foreshore reclamation) exist at the bay head, in which anoxic waters with a high concentration of hydrogen sulfide often appear (Sasaki et al, 2009a;Sasaki et al, 2009b). The upwelling of such anoxic waters causes discoloration of surface water due to the oxidation of hydrogen sulfide generating particulate sulfur, which scatters the sunshine and the surface watercolor turned to milky blue called blue tide in Tokyo Bay (Otsubo et al, 1991;Sasaki et al, 2009b;Higa et al, 2020;Wang et al, 2022).…”

Section: Study Areamentioning

confidence: 99%

“…The decomposition of organic matter increases dissolved inorganic carbon (DIC) and decreases pH, resulting in increasing pCO 2 in the lower layer. In addition, waters in navigation channels and borrow pits (mining sediments for the foreshore reclamation) in the inner part of the bay are extensively stagnant, causing the appearance of anoxic waters with high concentrations of sulfides (Sasaki et al, 2009a;Sasaki et al, 2009b). Although CO 2 is thought to be emitted to the atmosphere during coastal upwelling (Feely et al, 2008;Norman et al, 2013;Tokoro et al, 2021), the actual status of CO 2 emissions and DIC dynamics are not fully understood because coastal upwelling, as well as anoxia and hypoxia, is an episodic event in the bay (Sasaki et al, 2009b).…”

Section: Introductionmentioning

confidence: 99%

A practical pCO2 estimation and carbonate dynamics at an event of hypoxic water upwelling in Tokyo Bay

et al. 2023

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Urban bays have been considered to have a high CO2 absorption function due to the high nutrient load and resultant primary production. It is expected to enhance the function by promoting a blue carbon policy co-beneficial with strengthening ecosystem services such as fisheries. Estimates of CO2 absorption in urban bays have been based mostly on fragmentary information from shipboard observations, and an evaluation based on continuous observation of water quality is necessary considering the large spatiotemporal variability of such bay environment. In particular, Tokyo Bay has a specific feature of water pollution problem of hypoxia and anoxia leading to emitting high CO2. Bottom hypoxic and anoxic waters develop from early summer to autumn in the central part of the bay and enclosed areas such as navigation channels and borrow pits. It is known that pCO2 becomes very high in these waters, and their upwelling (called blue tide in the bay from the discoloration of the sea surface) is thought to cause high CO2 emissions; however, the actual situation is unknown. We developed a practical method for continuous estimation of pCO2 by appropriately combining continuous observation of water quality using sensors and measurements of carbonate parameters by water sampling. The results show that a highly reproducible and practical method for continuous estimation of pCO2 was possible by combining in situ salinity and pH meters and the total alkalinity and calc. pH measured by a total alkalinity titrator for water samples. This method was then applied to the duration of blue tide that occurred in the head of the bay in the summer and autumn of 2021. The pCO2 in the surface water was found to increase significantly and exceed 2000 µatm due to the upwelling of anoxic bottom water containing high pCO2. Mean CO2 emissions of approximately +2150 and +1540 µmol m-2 h-1 were observed at two stations during the upwelling period. The mean values rose to +2390 and +2190 µmol m-2h-1 with the blue tide and lowered to +810 and +1120 µmol m-2 h-1 without it, suggesting that high CO2 emissions may occur due to upwelling, especially with blue tides.

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“…Tokyo Bay surrounded by the metropolis of Japan with the largest population density and economic accumulation, represents one of the most polluted shallow coastal waters in the world. Red tides [27] occur in the bay and have increased in frequency from spring to autumn with different composition of species, resulting in large quantity of organic material to sediment and severe oxygen depletion in the bottom water [6,[28][29][30][31][32]. Moreover, water stratification occurs owing to the warm temperature and fresh water discharge from spring to autumn, thus forming oxygen-depleted bottom water.…”

Section: Introductionmentioning

confidence: 99%

Numerical Modeling of Long-Term Biogeochemical Processes and Its Application to Sedimentary Bed Formation in Tokyo Bay

Amunugama

Sasaki

2018

Water

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Even though models of water quality have become increasingly detailed over time, their applicability to analyze long-term effect on sedimentary bed formation is yet to be clarified. Hence, an integrated, layer-resolved, process-based, sediment-water coupled, long-term robust, three-dimensional (3D) ecosystem model, including realistic sedimentary and pelagic processes, was developed. The constituents of the integrated model included a multi layered ecosystem model, a quasi-three dimensional hydrodynamic model, a wave hindcasting model, and a sediment deposition and erosion model. Because numerical modeling difficulties arise in setting the initial conditions, especially for a sedimentary bed, this novel modeling approach suggests to initiate the model with the simplest initial conditions of no particulate organic carbon content (POCC) and uniform inorganic sediment distribution. The model was applied to Tokyo Bay and quasi-steady state POCC in the sediment was obtained through long-term computation with realistic sedimentary and pelagic processes. Wave induced bed shear stress (WBSS) is critical for sediment erosion and spatial movement of sediment pollution, while a stable morphology is determined by the balance between the current induced bed shear stress (CBSS) and WBSS in Tokyo Bay. This novel modeling approach, with the simplest sedimentary initial conditions and realistic sedimentary and pelagic processes, provides a great tool for long-term ecosystem modeling in future studies.

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Effective Application of a Mechanical Circulator for Reducing Hypoxia in an Estuarine Trench

Cited by 6 publications

References 26 publications

Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

Experimental Study on the Effects of a Vertical Jet Impinging on Soft Bottom Sediments

A practical pCO2 estimation and carbonate dynamics at an event of hypoxic water upwelling in Tokyo Bay

Numerical Modeling of Long-Term Biogeochemical Processes and Its Application to Sedimentary Bed Formation in Tokyo Bay

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