Kevin K. Johnson

Density currents in the Chicago River: Characterization, effects on water quality, and potential sources

Bidirectional flows in a river system can occur under stratified flow conditions and in addition to creating significant errors in discharge estimates, the upstream propagating currents are capable of transporting contaminants and affecting water quality. Detailed field observations of bidirectional flows were made in the Chicago River in Chicago, Illinois in the winter of 2005-06. Using multiple acoustic Doppler current profilers simultaneously with a water-quality profiler, the formation of upstream propagating density currents within the Chicago River both as an underflow and an overflow was observed on three occasions. Density differences driving the flow primarily arise from salinity differences between intersecting branches of the Chicago River, whereas water temperature is secondary in the creation of these currents. Deicing salts appear to be the primary source of salinity in the North Branch of the Chicago River, entering the waterway through direct runoff and effluent from a wastewater-treatment plant in a large metropolitan area primarily served by combined sewers. Water-quality assessments of the Chicago River may underestimate (or overestimate) the impairment of the river because standard water-quality monitoring practices do not account for density-driven underflows (or overflows). Chloride concentrations near the riverbed can significantly exceed concentrations at the river surface during underflows indicating that full-depth parameter profiles are necessary for accurate water-quality assessments in urban environments where application of deicing salt is common.

Measuring suspended sediment concentrations and flow velocities using multibeam sonar.

Modern data handling and storage technologies facilitate the logging of the large quantity of water‐column backscatter information received by multibeam sonars. Methods of using these data to derive estimates of the mass concentration and flow velocities of suspended sediment flow structures have been developed. The results obtained by the application of these methodologies to data collected at the confluence of the Parana and Paraguay rivers in Argentina and the confluence of the Mississippi and Missouri rivers in the United States will be presented. An analysis of those data in conjunction with a set of experimental data collected in a large‐scale test facility will be also given. The applicability and limitations of the use of multibeam sonar for deriving suspended sediment concentrations will be discussed. By enabling the simultaneous measurements of suspended sediment concentration, flow velocities, and bathymetric data, multibeam echo‐sounders are demonstrated to be a versatile tool for the surveyin...

Measuring Gravity Currents in the Chicago River, Chicago, Illinois

Recent studies of the Chicago River have determined that gravity currents are responsible for persistent bidirectional flows that have been observed in the river. A gravity current is the flow of one fluid within another caused by a density difference between the fluids. These studies demonstrated how acoustic Doppler current profilers (ADCPs) can be used to detect and characterize gravity currents in the field. In order to better understand the formation and evolution of these gravity currents, the U.S. Geological Survey (USGS) has installed ADCPs and other instruments to continuously measure gravity currents in the Chicago River and the North Branch Chicago River. These instruments include stage sensors, thermistor strings, and both upward-looking and horizontal ADCPs. Data loggers and computers installed at gaging stations along the river are used to collect data from these instruments and transmit them to USGS offices.

Characterizing a December 2005 Density Current Event in the Chicago River , Chicago, Illinois

During the winter months, the Chicago River in Chicago, Illinois is subject to bi dire ctional flows , and density currents are thought to be responsible for the se flow variations. This paper presents detailed field measurements using three acoustic Doppler current profiler instruments and simultaneous water -quality measurements made during December 2005 . Observations indicate that the formation of density currents within the Chicago Riv er and density differences are mostly due to salinity differences between the North Branch and the main stem o f the Chicago Rive r, wh ereas temperature difference does not appreciably affect the creation of density currents. Sources of higher water temperat ure, conductivity, and salinity values should be addressed in future studies.

Use of an Acoustic Doppler Current Profiler (ADCP) to Measure Hypersaline Bidirectional Discharge

The U.S. Geological Survey measures the exchange of flow between the north and south parts of Great Salt Lake, Utah, as part of a monitoring program. Turbidity and bidirectional flow through the breach in the causeway that divides the lake into two parts makes it difficult to measure discharge with conventional streamflow techniques. An acoustic Doppler current profiler (ADCP) can be used to more accurately define the angles of flow and the location of the interface between the layers of flow. Because of the high salinity levels measured in Great Salt Lake (60-280 parts per thousand), special methods had to be developed to adjust ADCP-computed discharges for the increased speed of sound in hypersaline waters and for water entrained at the interface between flow layers.