Hydraulics of Seawater Purging in Tunneled Wastewater Outfall
Publication: Journal of Hydraulic Engineering
Volume 120, Issue 2
Abstract
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 mixing is considered. Further tests show that, in combination with the Venturi section, a short‐term increase in effluent flow caused by intermittent dumping of the chlorine contact tanks could significantly reduce the riser purging requirement as well.
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Information & Authors
Information
Published In
Journal of Hydraulic Engineering
Volume 120 • Issue 2 • February 1994
Pages: 209 - 226
Copyright
Copyright © 1994 American Society of Civil Engineers.
History
Received: Sep 21, 1992
Published online: Feb 1, 1994
Published in print: Feb 1994
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