SIMULATION OF WATER QUALITY DURING UNSTEADY FLOW IN THE CHICAGO WATERWAY SYSTEM

The Chicago Waterway System (CWS) is composed of the Chicago Sanitary and Ship Canal (CSSC), Calumet-Sag Channel, North Shore Channel, lower portion of the North Branch Chicago River, South Branch Chicago River, Chicago River, and Little Calumet River. In total, the CWS is a 76.3 mi (122.8-km) branching network of navigable waterways controlled by hydraulic structures in which the majority of flow is treated sewage effluent. The Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) will soon be faced with a number of difficult water-quality management issues including the impact of reduced discretionary diversions from Lake Michigan for water-quality improvement in the summer, the outcome of a use attainability analysis for the CWS, and development of total maximum daily load allocations. To evaluate these management issues and their impact on water quality in the CWS, a model capable of simulating water-quality processes under unsteady-state flow is being developed to assist in water-quality management and planning decision making.The DUFLOW model developed in The Netherlands was selected for simulation of the CWS. The model hydraulics were calibrated using hourly stage data at three gages and verified at five gages operated by the MWRDGC within the CWS and at the downstream boundary at Romeoville. The water-quality model was calibrated using monthly grab sample data at 21 locations and hourly dissolved oxygen (DO) data at 27 locations all collected by the MWRDGC. The model was run at a 15- min. time step for the period of April 1 to May 4, 2002. The stage simulation agreed with the measured data nearly always within two percent relative to the flow depth. The simulated DO values agreed well with the observed values. The model then was applied to evaluate the effect of a proposed change in CWS navigational rules on the water quality in the CWS. Allowing lower water levels in the CWS immediately after storms that were improperly forecast could reduce the amount of water diverted from Lake Michigan. The reduced diversion was found to have a maximum average impact on DO of −2 mg/L on the South Branch of the Chicago River near the main Lake Michigan Diversion point, this was reduced to an average impact on DO of −0.4 mg/L on the downstream reaches of the CSSC. The effects on the North Branch Chicago River and Calumet-Sag Channel were far smaller.