Density currents in the Chicago River, Illinois Carlos M. García & Claudia Manríquez Graduate Research Assistants, Ven Te Chow Hydrosystems Laboratory, Dept. of Civil Engineering, University of Illinois at Urbana-Champaign Kevin Oberg U.S. Geological Survey, Office of Surface Water, Urbana, IL Marcelo H. García Chester and Helen Siess Professor, Ven Te Chow Hydrosystems Laboratory, Dept. of Civil Engineering, University of Illinois at Urbana-Champaign ABSTRACT: Bi-directional flow observed in the main branch of the Chicago River is due, in most cases, to density currents generated by density differences between the water in the North Branch Chicago River and the Chicago River. An upward-looking 600-KHz acoustic Doppler current profiler was installed by the U.S. Geological Survey (USGS) in the center line of the Chicago River at Columbus Drive at Chicago, IL, to char- acterize these flow conditions. Bi-directional flow was observed eight times in January 2004 at Columbus Drive. Three bi-directional flow events, with temperature stratification of approximately 4°C, were also ob- served on the North Branch Chicago River. Analysis of these data indicates that the plunging point of the density current moves upstream or downstream on the North Branch Chicago River, depending on the density difference. Complementary water-quality and meterologic data from January 2004 help confirm the mecha- nism causing the formation of the density currents. port, IL. The CSSC carries waste waters away from the city and Lake Michigan 1 INTRODUCTION The City of Chicago, Illinois (IL) and many of its suburbs lie within the glacial Lake Chicago Plain. The Lake Chicago Plain encompasses the Chicago, Des Plaines, and Calumet Rivers. Early explorers discovered and used the Chicago Portage, an area within Mud Lake that was only 4.6 meters (m) above the level of Lake Michigan and near the wa- tershed divide between the Mississippi River and the Great Lakes basins. Because of the low relief, the area was poorly drained. The level of Lake Michi- gan in the late 1800s was only 0.61 m below the river banks, making subsurface drainage ineffective (Juhl, 2005). Flow from the North Branch Chicago River (NB) and the South Branch Chicago River (SB) joined Figure 1. The Chicago River and the North and South Branches of the Chicago River at Chicago, IL. just north of present-day Lake Street (Fig. 1) and flowed eastward into Lake Michigan. Sewage dis- Today (2005), the Chicago River (CR) flows charged into the Chicago River caused serious west from Lake Michigan, through downtown Chi- health hazards during the late 1800s as this sewage cago, and joins flow coming from the NB where it affected the drinking water supply from Lake enters the SB/CSSC. Flow in the CR is controlled by Michigan. In 1900, a canal dug by the Sanitary Dis- the Lockport Powerhouse and Controlling Works trict of Chicago linking the Chicago River to the Des (near Joliet, IL) and by the Chicago River Control- Plaines River (Mississippi River basin) was com- ling Works (CRCW) and the Chicago Lock. During pleted and reversed the flow in the Chicago River. summer, water from Lake Michigan flows into the This canal, the Chicago Sanitary and Ship Canal CR through sluice gates in the CRCW and, because (CSSC) is 45 kilometers (km) from the SB to Lock- of lockages, through the Chicago Lock at CRCW. Flow of water from Lake Michigan into the CR dur- at the NB and to evaluate the boundary conditions in ing the summer months, called discretionary diver- the CR. sion; is used to preserve or improve the water qual- In this paper, we first present a description of the ity in the CR and CSSC. During winter, flow from available data. This presentation is followed by an Lake Michigan into the CR is small and typically re- analysis for the flow conditions observed during the sults from leakage through the gates and sea walls at entire month of January 2004. Density current CRCW and some lockages. Other contributions to events are identified based on the water-velocity re- the CR discharge include water from direct precipi- cords, and the main characteristics of each identified tation and discharges of water used for cooling pur- event are described (for example, duration, flow poses from neighboring buildings. The NB carries conditions in the NB, and aver-age air temperature). runoff from the watershed up-stream and treated Finally, one of the observed density current events municipal sewage effluent released by a water- in January 2004 is analyzed in detail, and includes treatment plant located 16 km upstream from the an estimation of the force driving the underflow and confluence of the branches. Most or all of this ef- the description of the time evolution of both the ver- fluent is transported down the SB into the CSSC and tical velocity profiles and the bed shear stress. then to the Des Plaines and Illinois Rivers. Causes for the observed flows are established based Discharge measurements made by the U.S. Geo- on analysis of the boundary conditions. logical Survey (USGS) beginning in 1998, indicated bi-directional flow in the CR. Although the duration of this bi-directional flow was not known, it indi- 2 DATA DESCRIPTION cated the possibility that water from the NB might be flowing into the CR and perhaps even into Lake Most of the data analyzed for this study are velocity, Michigan. The possibility of flow from the NB en- backscatter, and temperature data obtained from a tering the CR meant that water quality in the CR, 600-KHz acoustic Doppler current profiler (ADCP) and hence Lake Michigan, might be impaired. manufactured by RD Instruments (Fig. 2). The in- The Metropolitan Water Reclamation District of strument was installed in an upward-looking con- Greater Chicago (MWRDGC) manages the flow and figuration on the bottom of the CR at Columbus quality of the CR. Because of the possible effects Drive (CR_CD), in the center of the channel, ap- that bi-directional flow may have on water quality, proximately 0.8 km downstream from the Chicago the MWRDGC contacted researchers at the Ven Te River Lock. The CR is 55 m wide at this location. Chow Hydrosystems Laboratory (VTCHL) at the The water depth at CR_CD is held at a nearly con- University of Illinois at Urbana-Champaign to in- stant value of 7.0 m in the center of the channel vestigate the possible causes of these flows. The throughout the year. The center of the ADCP trans- staff of the VTCHL suggested that the bi-directional ducers were located about 0.30 m above the stream- flows could indicate the presence of density currents bed. The ADCP was connected by means of an un- in the CR. It was hypothesized that these density derwater cable to a computer in the USGS stream- currents developed because of differences in density flow gaging station located on the south side of the between waters from the NB and CR. Density dif- CR_CD. Data measured by the ADCP were trans- ferences might be caused by temperature differ- ferred from the computer to the USGS office in Ur- ences, or by the presence of salt or sediment in sus- bana, IL, by way of a dedicated high-speed Internet pension or some combination thereof. Density connection. currents are well known for having the capacity to transport contaminants, dissolved substances, and suspended particles for long distances (Garcia, 1994). The hypothesis of density currents in the CR was supported initially by the results from a three- dimensional hydrodynamic simulation conducted by Bombardelli and Garcia (2001). Field information is presented herein to support this hypothesis through the analysis of a unique set of water-velocity meas- urements (vertical profiles of three dimensional wa- ter-velocity components) collected continuously by the USGS near Columbus Drive on the CR (Fig. 1) during January 2004. In addition, hydrological, wa- ter-quality, and meteorological data collected by the Figure 2. A 600-KHz acoustic Doppler current profiler in- USGS and MWRDGC are used as complementary stalled at Chicago River at Columbus Drive at Chicago, IL. information to both characterize the flow conditions The ADCP provided continuous three- streamflow gaging station, NB at Grand Avenue at dimensional velocity profiles at a sampling fre- Chicago, IL (NB_GA), has a SonTek/YSI Argonaut- quency of 0.2 Hz (a complete water-velocity profile SL (Side-Looking) current meter that is located at is recorded every 5 seconds (s)). The 600-KHz about half of the depth (1.55 m above the streambed) ADCP was configured to collect velocity profiles on the right bank. This side-looking sensor is used to using a pulse-coherent technique known as mode 5 quantify the discharge by the index-velocity method. in RD Instruments profilers. Depth-cell size for the In addition, a string of six water-temperature sensors velocity measurements was 0.1 m and the blanking are used to measure water temperatures at various distance was set to 0.25 m. With this depth-cell size elevations along the right bank at Grand Avenue. and blanking distance, the deepest velocity meas- The water-temperature probes are located at 0.6-m urement was made in a depth cell centered approxi- increments in the vertical with sensor 6 (T6) being mately 0.65 m above the stream-bed. Therefore, no the lowest in the water column (located at 0.15 m velocity data were available for analysis in the first above the streambed at the wall) and sensor 1 (T1) 0.65 m above the stream-bed. Velocity measure- being the highest.