Modeling of transient pneumatic events in a combined sewer overflow storage tunnel system: A novel approach for designing mitigations

The City of Portland, Oregon recently completed construction of the Willamette Combined Sewer Overflow tunnel system to contain combined sewage during storm events and prevent unauthorized release of sewage to the Willamette River. The Willamette CSO Tunnel system includes a network of consolidation conduits designed to intercept combined flows upstream of CSO outfalls and convey the combined flows to drop shafts which discharge to deep CSO tunnels. As the CSO Tunnel system was brought on line, the challenges of consolidating and redirecting flows for the most extreme events became apparent as the effect of pressure surges were observed at various locations along the conduits and interceptor sewers. The events were manifested by the displacement of manhole covers at two locations and, at one location where the manhole cover was bolted down, by pavement deformation. A subsequent modeling investigation focused on simulation of these target transient events to better understand the nature of the events and to provide a risk-based understanding of them for mitigation design.

The target transient events in the consolidation conduits were modeled using LimnoTech’s SHAFT model framework, which is a 1-dimensional finite volume model formulated specifically for rapidly varied, mixed flow systems. Inflows to the system were produced by a collection system model, using rainfall data collected during the modeled events. The downstream boundary condition of the SHAFT models was defined by drop shaft water level data, collected during the events.

Preliminary modeling results did not show flooding to grade for the observed events, but did indicate pressurization of the air space underneath manhole covers. A routine was added to SHAFT to dynamically simulate pressurization of the air space in chambers above surcharged sewers, including venting of air via an orifice equation. Application of the enhanced model provided confidence that pressurization was the source of the observed events, and that additional venting would suffice to safely relieve pneumatic pressures. A subsequent design effort will focus on providing pneumatic relief to the systems.

This paper will describe the observed transient events, and then provide details of the analysis approach including a description of the modeling framework. Results of the modeling will be shown along with an explanation of how the results were interpreted to isolate the most likely cause of the observed events. The topic is expected to be of interest to owners, operators and designers of CSO control systems, both as a forensic tool for existing systems and as an improved approach for systems currently under design or contemplated for the future.