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

Abstract

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.

Review of Artificial Downwelling for Mitigating Hypoxia in Coastal Waters

“…Downwelling technologies may have potential side effects, including the destruction of the stratification of water and changes in the water cycle and the natural biochemical cycle process. Studies on the disturbance of submarine sediments by downwelling indicate that if the critical Froude number associated with the downwelling flow speed and the discharge position is greater than 0.5, it will cause the resuspension of the sediment, with an increase of water turbidity and phosphorus release in the water column [25]. Potential impacts of the warm surface water and particulate organic matter transporting down need to be studied; downflow efficiency may be reduced due to biofouling from long-term operations and competition for space and predation of aquatic organisms may be disturbed [78].…”

An experimental study of fluidization post impinging fluid in granular bed for breaking sedimentation

“…The fluid jet is shot to the granular form and immersed location to break the granular. The jet produces an upward push and changes the granular arrangement [11,12].…”

Simulations of the Concentration Fields of Rosette-Type Multiport Buoyant Discharges Using Combined CFD and Multigene Genetic Programming Techniques

“…In recent years, complementing the experimental studies [8][9][10][11][12], the simulation of the mixing processes of jets and plumes using fully physics-based computational fluid dynamics models has become popular because of the improvements in computing capabilities [13][14][15][16][17][18]. One shortcoming of the CFD approach is that it typically requires heavy computing resources and long simulation time, and it is thus helpful to propose a new technique that can make predictions faster with lower requirements for computing resources.…”