Abstract
As part of restoration of the breakwater at the entrance of Ofunato Bay, which was damaged by the 2011 Tohoku Earthquake Tsunami, flow pipes with a diameter of 3.5m were placed through the bottom of the breakwater to enhance water exchange between the inner and outer bay. The aim of this research is to understand the effect of these pipes on the occurrence of low oxygen water based on field experiments conducted after the completion of the breakwater. These field experiments indicate that the main effect on water exchange due to the flow pipes was the discharge of low temperature, high density water masses inside the breakwater to the outside of the breakwater, with flow speeds in the pipes sufficient to discharge the entire volume below the top mound of the breakwater (-16 m depth) in about 10 days. The effect of this enhanced exchange was to allow easy intrusion of low temperature water masses that appeared in the outer bay into the lower layer of inner bay. Since the temperature inside the mounds had been increasing, the cold water intrusion prevented the inner part of bay developing a persistent low oxygen environment.
