Holtschlag, David J. and John A. Koschik, 2003, An acoustic Doppler current profiler survey of flow velocities in St. Clair River, a connecting channel of the Great Lakes, Date Posted: March 28, 2003, US Geological Survey Open-File Report 03-119 [http://mi.water.usgs.gov/pubs/OF/OF03-119/index.php]

An acoustic Doppler current profiler survey of flow velocities in St. Clair River, a connecting channel of the Great Lakes

Prepared in cooperation with the
SOURCE WATER ASSESSMENT PROGRAM OF THE MICHIGAN DEPARTMENT OF ENVIRONMENTAL QUALITY, DETROIT WATER AND SEWERAGE DEPARTMENT, and the AMERICAN WATER WORKS ASSOCIATION RESEARCH FOUNDATION

Acoustic Doppler current profilers (ADCP) were used to measure flow velocities in St. Clair River during a survey in May and June of 2002, as part of a study to assess the susceptibility of public water intakes to contaminants on the St. Clair-Detroit River Waterway. The survey provides 2.7 million point velocity measurements at 104 cross sections. Sections are spaced about 1,630 ft apart along the river from Port Huron to Algonac, Michigan, a distance of 28.6 miles. Two transects were obtained at each cross section, one in each direction across the river. Along each transect, velocity profiles were obtained 2-4 ft apart. At each velocity profile, average water velocity data were obtained at 1.64 ft intervals of depth. The raw position and velocity data from the ADCP field survey were adjusted for local magnetic anomalies using global positioning system (GPS) measurements at the end points of the transects. The adjusted velocity and ancillary data can be retrieved through the internet and extracted to column-oriented data files.

Michigan Department of Environmental Quality (MDEQ) Source Water Assessment Program (SWAP), the Detroit Water and Sewerage Department (DWSD), and the American Water Works Association Research Foundation (AwwaRF) in cooperation with the U.S. Geological Survey (USGS), are assessing the vulnerability of public water intakes on the St. Clair-Detroit River Waterway to contamination. Intakes on this waterway provide a water supply to about six million residents of Michigan and Ontario. The assessments will identify likely sources of water to public intakes in Michigan and will provide a basis for preparing emergency responses to contaminant spills.

The primary purpose of this ADCP survey is to provide data to characterize local flow velocities within St. Clair River. The velocity data described in this report will be used to improve the calibration of a hydrodynamic model of the waterway (Holtschlag and Koschik, 2001), which is being developed to assess the susceptibility of public water intakes to contamination.

Water velocities were measured at 104 cross sections from the head of St. Clair River near Port Huron, Michigan, to Russell Island, near Algonac, Michigan, a distance of about 28.6 miles. The survey was conducted during two deployments. The first deployment was from May 20-23; 2002, and the second deployment was from June 24-25, 2002. This report documents the survey, provides necessary adjustments to the velocity and positional data to correct for local magnetic anomalies, reformats the data for analysis, and provides access to the velocity and ancillary data through the internet.

St. Clair River, Lake St. Clair, and Detroit River form a waterway that is part of the boundary between the United States and Canada (Figure 1). The waterway is a major navigational and recreational resource of the Great Lakes region that connects Lake Huron with Lake Erie. St. Clair River (the upper connecting channel) extends about 39 mi from its head at the outlet of Lake Huron near Port Huron, Michigan, to an extensive delta area. Throughout its length, water-surface elevations in St. Clair River decrease about 5 ft as it discharges an average of 182,000 ft³/s from a drainage area of about 222,400 mi². Water from St. Clair River discharges into Lake St. Clair, which then flows into Detroit River and then into Lake Erie.

This report was developed by U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers (USACE) in cooperation with Michigan Department of Environmental Quality, Source Water Assessment Program, directed by Bradley B. Brogren; Detroit Water and Sewerage Department's Water Quality Division, managed by Pamela Turner; and the American Water Works Association Research Foundation's Christopher Rayburn, who directs the Research Management Department. Two crews were needed to survey St. Clair River. Paul Taylor and Benjamin Harrison of the Detroit District operated the USACE boat and ADCP unit. Don James and Todd Dewitt of the Grayling, Michigan Field Office operated the USGS boat and ADCP unit. Marie Reynolds, of the USGS Michigan District, created the web pages that form this report. Andreanne Simard, of the USGS Michigan District, and Kevin Oberg, of the USGS Illinois District, reviewed the report.

Acoustic Doppler current profiler (ADCP) measurements of flow (discharge of water) in rivers began about 1982 (Simpson, 2001). An ADCP uses sound waves to measure water velocity. In this survey, the ADCP transmitted sound waves at an ultrasonic frequency of 600 kilohertz (kHz), where one kilohertz is one thousand waves or cycles per second. Water velocity is computed by use of the Doppler principle, which relates the observed change (shift) in the acoustic frequency to the relative velocities between the source and the observer. Mathematically, this principle can be written as

is the relative velocity between the source and the observer, in feet per second, and

An ADCP measures water velocities by transmitting acoustic water pulses and bottom pulses (pings) as it moves across the river in a boat (Figure 2). The water pulse reflects off small particles of sediment and other material in the water, and returns an echo to a transducer on the ADCP. The shift between measured and transmitted frequencies is used to compute the relative water velocity between the particles and the ADCP. To determine water velocity components and error estimates in three dimensions, four transducers are arrayed on an ADCP at 90 degrees of horizontal separation and at a 20-degree offset from a downward orientation into the water (Figure 3). This configuration provides the data necessary to resolve the northing, easting, and vertical components of local flow velocities relative to the ADCP.

After transmitting the acoustic water pulse, the reflected pulse is subdivided into a number of discrete time intervals to describe the variation of local flow velocities with depth below the water surface. The number of time intervals is based on the expected maximum channel depth. Frequency shifts and corresponding velocity components are computed for each time interval. The collection of flow velocities for the set of time intervals from the returning signal is referred to as an ensemble. The ensemble describes the velocity profile at discrete intervals of depth (depth cells) in the water column. Depth cells (bins) are described by their heights and by the centroid of their depths below the water surface. In this survey, all bin heights were 1.64 ft (0.5 meters).

Velocity measurements near the tops of the uppermost bins in the water column were below the water surface by a depth that included the transducer draft and the blanking distance (Figure 4). The transducer drafts, the depths of the transducers below the water surface, for the USACE and the USGS survey vessels were 0.94 ft and 0.60 ft, respectively. The blanking distance allows time for oscillations in the transducer head that follow each ping to die out before the transducer can detect reflected pulses. Length of the die out interval and speed of the acoustic signal in water determine the blanking distance. The blanking distance used in this survey was 0.82 ft (0.25 meters).

Velocity measurements near the channel bottom are limited by side lobe interference (Figure 4). Side lobes are secondary acoustic signals that are emitted 30-40 degrees off the main acoustic beam (Simpson, 2001). Reflection of the side lobe signals off the channel bottom interferes with the detection of the returning main acoustic signal. Shallow depths or navigational hazards, such as sunken pilings and littoral vegetation, sometimes limited velocity measurements near the shoreline.

Although the primary purpose of this survey was to obtain point velocity data, flows (water discharges) also were computed to help compare measurements made by the USACE and USGS, and to enable comparisons of transect measurements made at the same cross section. In channel areas of measured velocity (Figure 4), measured flows were computed using the measured velocity and corresponding flow-area data. In addition, total flows were computed by combing measured flows with estimated velocities and flow areas in marginal cross section areas. Simpson (2001) describes methods for estimating flow velocities and areas used in ADCP measurements. In some areas, total flows are highly uncertain because aquatic growth limited the shoreline approach and long distances from the boat to the shoreline were estimated visually.

In addition to the water pulse, a bottom-track pulse was used to measure the position and velocity of the boat with respect to the channel bottom. The bottom track (BT) is combined with the water velocities computed relative to the ADCP to determine absolute water velocities. Bottom tracking provides reference distances for computing flow areas and flow directions, and is essential when precision GPS measurements are unavailable. Bottom tracking, however, is reliant on a magnetic compass for directional information, which can be degraded by local magnetic anomalies. In addition, the accuracy of positions indicated by the bottom track can be degraded by a moving channel bed and by the pitch and roll of the ADCP during deployment. GPS measurements at the end points of all transects were used to reference bottom track measurements to Michigan SPCS 83 coordinates and to adjust the direction and scale the lengths measured using bottom tracking.

This ADCP survey of St. Clair River was split into two deployments because of preexisting scheduling commitments of the survey equipment and crew. USACE and USGS crews conducted most of the survey from May 20-23, 2002, starting from the head of St. Clair River near Port Huron, Michigan, and working downstream to Fawn Island, near Marine City, Michigan. The USGS crew surveyed the remaining sections from Fawn Island to Russell Island near Algonac, Michigan from June 24-25, 2002 (Figure 5).

The velocity measurement data for the two deployments are considered to be comparable, even though water levels and flows on St. Clair River increased slightly during the survey ( Figure 6). The increases in water levels were small and the effect of increasing flows on increasing velocities was somewhat offset by increases in water level and cross sectional flow area.

Both USACE and USGS crews used 600 kHz Workhorse Rio Grandeâ ADCP units by RD Instruments of San Diego, Calif., and deployed the profilers from moving boats. The USACE and USGS used similar types of boats to survey flow velocities (Figure 2 and Figure 7). The USACE ADCP, however, was rigidly mounted to their boat (Figure 3); while the USGS ADCP floated on a pontoon that was tethered to the side of their boat (Figure 8). Based on nearby transects surveyed at about the same time, the rigid mount limited the pitch (rotation along the fore/aft axis) and roll (rotation in the direction of the starboard/port axis) of the ADCP more effectively than the tethered pontoon (Figure 9).

ADCPs have a mechanism to adjust for the effects of minor pitch and roll. Horizontal water velocities, which are computed as the cosine of pitch and roll angles, are insensitive to angles of less than 5 degrees (Simpson, 2001). Because both USACE and USGS pitch and roll angles were usually less than 3 degrees, horizontal velocities are considered reliable. In contrast, vertical velocities are highly sensitive to small amounts of pitch and roll (Simpson, 2001). Thus, although vertical velocity components are reported for both USACE and USGS, the vertical velocity component for USGS data, in particular, should be considered of uncertain utility.

The boat operated by the USACE carried a precision GPS (Global Positioning System) receiver that was integrated with the ADCP unit. This integration provided precise (sub-meter) geographic system coordinates (latitudes and longitudes) for each velocity ensemble. The boat operated by the USGS did not have an integrated GPS unit, but relied on a Garminâ model GPS 76 handheld receiver for geographic coordinates measured at the first and last ensemble on each transect. The GPS data were combined with the bottom track data obtained by the ADCP to determine positions of the velocity profiles.

GPS 76 receivers typically (95 percent of the time) provide geographic coordinates that are within 49 ft of their true position (Garmin Corporation, 2001). When a GPS 76 receiver is Wide Area Augmentation System (WAAS) enabled, picking up satellite and land-based differential corrections, the geographic coordinates are typically within 10 ft of their true positions (Garmin Corporation, 2001). The GPS 76 receiver used by the USGS during the St. Clair River survey was WAAS enabled during most of the survey. The accuracy of the GPS receivers, however, was not field verified.

Cross sections of ADCP survey were traversed at approximately right angles to the average flow in both directions. The first traverse (transect) generally started from the left bank, referenced looking downstream, which was generally near the Canadian shoreline. A second traverse of the same cross section generally followed immediately after the first and was usually started from the right bank, which was generally near the shoreline of the United States. In this report, cross sections are referenced sequentially downstream from CS 001 near Port Huron, Michigan, through CS 094W near Algonac, Michigan (Appendix A). In areas where additional cross sections were surveyed because flow branched around an island, an "E" or "W" is appended to the cross section number to designate the east or west channel (Figure 5). Finally, the direction of the river traverse in encoded in the transect name by appending a "C" or a "U" after the cross section number to indicate that the transect started nearer the Canadian or United States shoreline.

The USACE crew surveyed 63 cross sections on St. Clair River. Along each transect, a velocity profile was acquired every 0.48 seconds. Operating with an average boat speed of 3.95 ft/s, this resulted in profiles that averaged 1.88 ft apart. In contrast, the USGS crew surveyed 41 cross sections and measured velocity profiles at about 1.2-second intervals. The USGS crew operated their boat at an average speed of 2.95 ft/s, which resulted in an average separation distance between profiles of 3.44 ft. USGS data acquisition rates were generally slower than USACE rates because of slower communication rates on the USGS computer. The average flow velocity in St. Clair River ranged from 1.84 ft/s at transect CS 093C to 5.92 ft/s at transect CS 006U (Appendix A).

During the ADCP field surveys, onboard data processing provided real time information on the velocity data being acquired. The processing uses WinRiver software, which was developed by RD Instruments (2001a), in cooperation with the USGS, as a part of a cooperative research and development agreement. In particular, the stick ship track (Figure 10) provided unadjusted average flow velocities at their relative east and north positions indicated by the ADCP's bottom track. Because of a bend in St. Clair River near transect CS 010C, which is about 2,500 ft downstream from the Blue Water Bridge, flows are downstream along the United States shoreline (right bank) and upstream along the Canadian shoreline. In addition to the depth-averaged velocity information, cross sectional plots show the speed (Figure 11) and direction (Figure 12) of flow for individual depth cells. Profiles show the maximum depth of measured velocities, as the lighter line at the lower limit of the measured depth cells, and the channel bottom, as the darker line at the bottom of the plot. These data are monitored continuously to ensure the integrity of the velocity survey. Missed ensembles are displayed as white vertical streaks in the profiles.

Many data acquisition characteristics of the ADCP were specified by commands in the configuration files (RD Instruments, 2001b). Typical ADCP configuration files for USACE and USGS are shown in Appendix B.

Post processing was used to correct errors in position and velocity information and to enhance the utility of the ADCP data. Errors are associated with local anomalies in magnetic north, which are not accounted for in the ADCP configuration files, and with difficulties associated with tracking the channel bottom by use of acoustic signals from a moving boat. In addition, errors in the distances indicated by bottom track measurements can occur if the channel bed is moving. Thus, east and north distances indicated by bottom track measurements are not necessarily consistent with precision GPS measurements of ensemble locations obtained by the USACE. For USGS data, GPS data were not available for individual ensembles. Post processing was necessary, therefore, to project the bottom track measurements, which reference distances from the beginning of the transect, to the Michigan SPCS 83.

In both USACE and USGS surveys, discrepancies between GPS and bottom track coordinates at the first and last ensembles were used to scale and rotate bottom track measurements of position and to adjust the azimuths of horizontal velocities. The following paragraphs detail the mathematical procedures involved in applying these adjustments. The precise GPS data are thought to provide a more accurate indication of the absolute position of the profiles than positions based on bottom track measurements, even after these adjustments. The relative positions of the profiles and the horizontal rotation of the velocity vectors, however, are more appropriately based on the bottom track information. Finally, post processing was used to reformat the ADCP data to facilitate its subsequent use.

Let and represent the easting coordinates in Michigan SPCS 83 (zone 2113) International feet at the beginning and ending ensembles derived from a GPS measurement, and let and be the corresponding northing coordinates. Also, let and be the change in beginning and ending eastings and northings indicated by GPS. Then, the distance between the endpoints of the transect is calculated as

Similarly, forrepresenting the easting and northing of the first ensemble on the transect indicated by the bottom track (BT) and and representing the easting and northing of the last ensemble on the transect, the corresponding distance between end points indicated by the BT is

The resulting scale factor, which represents the ratio of the length between the first and last ensembles measured by the GPS and ADCP bottom track respectively, is computed as

This ratio is multiplied by distances indicated by the bottom track to obtain distances consistent with GPS measurements.

The azimuth of the transect, as defined by the end points of the transect from GPS measurements is

where the azimuth is the angle computed by the arctangent (arctan) function from the specified changes in northing and easting. Here, the results of the arctangent function are positive when measured clockwise from zero degrees north (along the positive y-axis); (360 degrees) was added to negative values of the AzmGPS so that all reported azimuths would be positive. Similarly, the azimuth of the transect defined by BT measurements is

The corresponding angle between transects as defined by GPS and bottom track measurements is

Application of the scale and angle adjustments to positional data is described in the following paragraphs. For each velocity profile indexed by n, for , the adjusted easting and northing was computed as

where sin and cos are the sine and cosine functions, respectively. The adjusted easting and northing velocity components, and , were computed from the measured velocity magnitudes and azimuths, and as

USACE and USGS crews alternated measuring cross-sections from CS 023 to CS 035 on May 21, 2002 (Figure 13) to provide a basis for comparing flows obtained by the two agencies. For total flows, defined as the sum of the measured flows and estimated flows along marginal channel areas (Figure 4) at these sections, the average USGS flow was 2.7 percent lower than the average USACE flow. Part of the discrepancy between total flows measured by the USACE and the USGS may be related to differences in the way velocities in the top margin of the transect were estimated. In particular, the USACE used a constant extrapolation technique and the USGS used a power curve to estimate the top velocities. For the measured flow component alone, the average USGS flow was 1.9 percent lower than the average USACE flow. Again, on May 22, 2002, USACE and USGS alternated measurements of cross sections between CS 043 and CS 060 (Figure 14). In this case, USGS total flows averaged only 0.87 percent lower than the USACE average, and USGS measured flows averaged 1.4 percent lower than the USACE. In addition, there was no consistent difference between USACE and USGS flows on May 22, 2002.

Some of the discrepancy between USACE and USGS total flows may be related to the difficulty of estimating distances from the start of the measurement section to the shoreline. The discrepancies between USACE and USGS flows, however, are not considered to be sufficiently large to indicate a significant inconsistency. Thus, the velocity data also are considered to be comparable, given that the flows measured by the two crews are comparable.

Possible discrepancies between flows measured in transects starting near the Canadian shoreline (left bank) were compared with flows measured in transects starting near the shoreline of the United States (right bank). Results (Figure 15) indicate that flows measured in transects run from left-to-right were consistent with flows measured in transects run from right-to-left. In particular, the coefficient of determination (r²) between flows in these paired transects was 0.9869. The average and standard deviation of percent difference in flows measured by the USACE was -0.93 and 3.33 percent, respectively. Correspondingly, average and standard deviation of percent difference in flows measured by the USGS was -0.06 and 1.91 percent, respectively. Given this consistency in flows, the point velocity data in transects run from left-to-right are considered comparable with velocity measured in transects run from right-to-left.

The data obtained during the ADCP survey were initially stored in a binary file written by the WinRiver software. This software can also output an ASCII (American Standard Code for Information Interchange) formatted data file, the contents of which are described in detail by the help file in the WinRiver software. This ASCII format includes three sets of lines that describe the ADCP measurement. The first set of lines describes comments and properties common to the entire transect. The second set of lines describes summary properties of a particular ensemble (velocity profile), which include position, timing, and number of depth cells. The third set of lines describes velocity characteristics of individual bins (depths cells). The number of depth cells specified in the configuration file fixes the number of lines in the third set. The number "-32768" in the third set of lines indicates that no velocity data were obtained at that depth cell, often because the depth interval exceeded the maximum channel depth at the location of the ensemble. The second and third set of lines is repeated for each ensemble. A detailed description of the ASCII output format is provided in the help file of the WinRiver application and a brief description is provided in Table 1.

The ADCP position and velocity data were modified as discussed in the preceding sections and new ASCII files were created to facilitate the analysis of flow velocities in St. Clair River. The new format is a rectangular array of data, similar to that seen in a spreadsheet, with each valid velocity measurement at a depth cell contained in an individual row. The column contents of each row are described in Table 2. The new format includes all valid data from a cross section by combining data from both the left-to-right and the right-to-left transects in a single file. The column "L2R" identifies the direction the transect was surveyed. Together, the ADCP data files provide point velocity data at more than 2.7 million locations in St. Clair River.

The modified ADCP data obtained during this survey can be retrieved through the internet by use of Figure 5. To facilitate online storage and retrieval, the ASCII files were compressed using the WinZip® utility. The files themselves, however, are self-extracting, which implies the WinZip® utility does not need to be loaded on the local computer to decompress the files to an ASCII format. A vicinity map showing transect locations can be obtained by left clicking on the red rectangles of the index map in Figure 5. Clicking on the transects shown on the vicinity maps will initiate the retrieval of the cross section data. The file names of the cross sections are "cs###" where, "###" is the cross section number. The extension of the file names upon retrieval is "exe." Once downloaded, clicking on the file name will automatically decompress the files, which are extracted to the same file name, but with the extension "3d."

Crews from the U.S. Army Corps of Engineers (USACE) and the U.S. Geological Survey (USGS) used acoustic Doppler current profilers (ADCP) to measure flow velocities in St. Clair River. The survey was conducted to provide a basis for describing flow patterns in St. Clair River and for improving the calibration of a hydrodynamic model of the waterway. The model is being developed to assess the susceptibility of public water intakes to contamination.

The surveyed reach extends from the head of St. Clair River near Port Huron, Michigan, downstream to Russell Island near Algonac, Michigan, a distance of 28.6 miles. The survey was conducted in two deployments, the first deployment was from May 20-23, 2002, and the second was from June 23-24, 2002. Both crews used RD Instruments' Workhorse Rio Grandeâ ADCP units operating at 600 kHz, and were deployed from moving boats. Velocities were measured at 104 cross sections separated by an average distance of 1,630 ft. At each cross section, two transects were measured, one from left-to-right across the river (looking downstream) and a second running right-to-left. For each transect, ensembles describing flow velocities with depth were measured 2 to 4 ft apart as the boats traversed the river. Each ensemble contains a profile of flow velocities at 1.64 ft (0.5 m) intervals of depth, starting less than 3 ft below the water surface and extending nearly to the bottom of the river. More than 2.7 million point velocities were measured during the survey.

The ADCP position and velocity data have been adjusted for the effects of the local magnetic declination and differences between the channel positions indicated by the ADCP bottom track measurements and global positioning system (GPS) measurements at the ends of the transects. In addition, the data has been reformatted to facilitate analysis. Flows measured during the May and June deployments are considered comparable because of the small changes in water levels and flows occurring between these two months. Also, flow data obtained by USACE and USGS crews is shown to be comparable and flows from transects surveyed from left to right are comparable with flows in transects that were surveyed from right to left. This report provides a mechanism for downloading the adjusted and reformatted ADCP data through the internet.

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