Evaluation of Methods to Reduce Backflows from the Chicago Waterway System to Lake Michigan

A detailed study of seven historic backflow from the Chicago Waterway System (CWS) to Lake Michigan events (including the 2^nd and 4^th largest since 1965) was done to evaluate changes in waterway operation procedures to determine if backflows could be avoided or reduced in volume. The study applied a dynamic-wave simulation model to the CWS at a 15-minute time step combining measured inflows for the major tributaries, combined sewer overflow (CSO) pump stations, and water reclamation plants with simulated gravity CSO flows computed by the U.S. Army Corps of Engineers. The simulated CSO inflows were confirmed against a detailed mass balance approach for four storms and the peak flows from the two approaches were found to be remarkably similar. It was found that the hydraulic capacity limitations of the Chicago Sanitary and Ship Canal (CSSC) prevented changes in operations at the Lockport Powerhouse and Controlling Works from substantially reducing backflows. The maximum discharge through the CSSC is around 19,500 cfs whereas inflows may peak at 40,000 to 50,000 cfs and flows cannot drain away fast enough to avoid having to allow backflows to Lake Michigan. These backflows cannot be avoided until the reservoirs of the Tunnel and Reservoir Plan (TARP) are completed.