Hydraulics of seawater purging in tunneled wastewater outfall

The mechanism of seawater purging in a tunneled outfall is studied using a 1:83 hydraulic scale model of Boston's proposed wastewater outfall. Purging requires high rates of effluent flow, which is particularly problematic for Boston because of the wide range of expected flow rates caused by combined sewers and seasonal ground‐water infiltration. This study explores some potential solutions to this problem. Model results show riser purging at flows of 5–10% less than predicted for risers (Munro criterion) and tunnel purging at flows about 20% less than predicted (full pipe flow criterion based on riser section invert slope), suggesting that the theoretical criteria are conservative. Construction of a tunnel constriction (Venturi section) just upstream from the risers substantially reduces seawater penetration in the tunnel by creating a condition of densimetric critical flow. Theoretical calculations of the minimum flow rate without tunnel penetration are consistent with observations when downstream mixin...