A tidal pump for artificial downwelling: Theory and experiment

Abstract

Coastal hypoxia is on the rise around the world, reducing secondary production and ecosystem services in estuarine and coastal regions. We propose a tidal pump consisting of a 90° bend and a vertical downwelling pipe that potentially mitigates hypoxia in bottom waters. Downwelling kinetic head presents an approximate linear dependence on incident kinetic head. Experiments were carried out on a physical model at 1:80 scale covering a wide range of current speeds and density difference heads. Experimental findings confirm the theoretical model. Downwelling flow rate mainly depends on tidal current speed, relative density difference, the immersed length of the pump below the pycnocline and the pump geometry, the latter two of which allow engineers actively regulate downwelling flow. In the Changjiang estuary where hypoxia frequently occurs below the pycnocline during late summers, surface oxygen-rich water could be pumped into a depth 15.36 m below the pycnocline at a flow rate of 0.4 m3/s for 1.2 m pipe diameter. Further study will focus on the deployment of the tidal pump to obtain the optimal dissolved oxygen (DO) concentration field in bottom waters.