A Sewer Tunnel System Replacement & Contingency Plan
As a result of the STAG monies obtained through the Corps of Engineers, Hussey Gay Bell and Black & Veatch teamed to produce a Master Plan for the replacement of Charleston Water System’s failing sewer tunnel system. Hussey Gay Bell’s task also included the preparation of a Contingency Pumping Plan for the Ashley and Cooper River legs of the tunnel system in the event of a failure in the existing tunnels. This plan included pump and force main sizing and force main routing for multiple scenarios of potential tunnel failures.
Prior to the development of the master plan, the Charleston Water System (CWS) experienced several catastrophic events in their deep level sanitary sewer tunnel conveyance system. As a result of severe corrosion to the concrete carrier pipe, several partial tunnel collapses nearly resulted in the complete blockage of almost 10 mgd of flow in the Ashley River Tunnel leg of the system. Potentially hundreds of thousands of gallons of raw sewage were within several feet of overflowing into the City’s streets. This occurrence along with similar events ultimately spurred CWS to replace the Ashley and Cooper River tunnel system over the following six-year period. However, prior to this replacement project CWS retained Hussey Gay Bell to develop a citywide “Tunnel System Contingency Wastewater Pumping Plan” to mitigate a catastrophic event.
Due to the potential for a tunnel collapse, Hussey Gay Bell was tasked with developing a plan of action to by-pass wastewater flows between access points in the system at drop pipes and shaft locations. For both the Ashley and Cooper River branches of the tunnel fifteen emergency by-pass pumping scenarios were identified. To prepare for potential failure event(s), a detailed “Implementation Plan” was developed for each scenario to include: a work procedure, key personnel to contact, flow data estimates, required pumping capacity and head conditions, force main sizing and lengths quantified, necessary road and railroad crossings and a detail routing plan for placement of an above ground temporary force main.
The worst-case peak flow scenario included preparing for the by-pass pumping of almost 10 mgd through 3,000 LF of dual 12-inch diameter force mains through the streets of the Charleston peninsula. As a proactive move, CWS stockpiled sufficient piping, road crossings and miscellaneous supplies and contracted with Godwin Pumps to set up and operate the by-pass system.