SonTek 1.5-Mhz RiverCAT Narrowband ADP Discharge, Velocity, and Depth Comparison

Measurements on the Han River at the Huang Jia Gang Hydrological Station, Danjiangkou, China

Results and summary of comparison measurements of a 1.5-Mhz RiverCAT and a

traditional horizontal-axis mechanical current meter on the Han River, Huang Jia Gang Measurement Station, Danjiangkou, China, March 9th , 2004

Prepared for:

Han River Hydrology & Water Resources Survey Bureau, Yangtze River Water Conservancy Committee

Written By:

John V. Sloat, Principal Hydrologist, SonTek/YSI, Inc., USA Mr. Steven Chen, Senior Engineer, SonTek/YSI, Shanghai, China

Danjiangkou Reservoir Dam upstream of Huang Jia Gang Measurement Station on the Han River

Introduction: On March 9th, 2004 comparison measurements of discharge, velocity, and depth were made at the Huang Jia Gang Hydrological Station using a SonTek 1.5-Mhz RiverCAT narrowband ADP (Acoustic Doppler Profiler) and a traditional mechanical vertical-axis current meter. Measurements were performed by engineers from the Han River Hydrology & Water Resources Survey Bureau, Yangtze Water Conservancy Committee and engineers from SonTek/YSI China (Mr. Mark Tepper and Mr. Steven Chen) and SonTek/YSI’s Principle Hydrologist, John V. Sloat. The intention of the measurements were to compare the existing mechanical measurement technology with a newer acoustic Doppler technology in order to better understand the performance of the new acoustic technology as well as any benefits that come from using it in place of the mechanical technology. It should be noted that the use of the terminology “ADP” or acoustic Doppler profiler in this report is synonymous to the terminology “ADCP” or acoustic Doppler current profiler. “Total Discharge” was measured by the RiverCAT using the moving-boat method and compared to a concurrent “total discharge” measurement made using a mechanical vertical-axis current meter using the traditional ISO/USGS discharge measurement mid-section method (described in USGS WSP 2175 vol. 1 &2, Rantz, and others). In addition, “Depth” and “Mean Velocity” data was measured by the RiverCAT at identical locations and at the same time as the mechanical current meter at 14 independent stations equally spaced across the river at the stations traditional measurement location. This report will explain: the basic configuration of the 1.5-Mhz RiverCAT narrowband ADP used during the comparison; the mechanical current meter configuration; the types of measurements performed during the comparison including the technique used to collect the data; results (data) from each measurement technique, and a summary of results. It should be noted that the RiverCAT data was collected by engineers from SonTek and data from the mechanical current meter was collected by engineers from the Han River Hydrology & Water Resources Survey Bureau, Yangtze Water Conservancy Committee. In addition, the data presented in this report has not been modified or edited and represents the exact values measured during the comparison.

Instrument Configurations 1.5-Mhz-RiverCAT Narrowband ADP By definition, the “RiverCAT” is a unique configuration of a RiverSurveyor Acoustic Doppler Profiler (ADP). SonTek/YSI, Inc. manufactures three RiverSurveyor ADP configurations (Standard, Splash-proof, and RiverCAT) that are used to measure discharge in large and small rivers. At the request of the readers of this report, SonTek/YSI, Inc. will be pleased to provide additional information for each of the three (3) RiverSurveyor configurations that have been specially developed for the RiverSurveyor class of ADP instruments. For this comparison SonTek/YSI, Inc. selected the “RiverCAT” configuration primarily because of the flexibility it provides to use a variety of deployment configurations. The “RiverCAT” configuration is composed of a transducer housing, a watertight electronics compartment, and 2 sealed-aluminum hulls. This configuration is shown below in Figure 1. An optional DGPS antenna is shown in figure 1 enabling users to reference data from either the ADP or GPS to optimize measurement results based on the current measurement conditions. The DGPS signal is internally processed by the RiverCAT electronics and thereby removes the need for a second set of radio modems or a complex external wiring harness using a multiplexing device. SonTek/YSI, Inc. is the only company that provides this internally processed “dual reference” option for tethered-boat applications. Alternative to the aluminum hulls used for this demonstration are options for slightly larger fiberglass hulls in a trimaran configuration as shown in figure 2. The ADP transducer housing is specially designed to be small (10-cm diameter) and lightweight (3.5 kilograms). Inside the transducer housing is located the compass, tilt/roll sensor, temperature sensor, pressure sensor, and receiver electronics for each transducer. The transducer housing is connected to the main watertight housing via a high-frequency communications cable. The watertight housing contains the 24-volt power-supply, an internal RS-232 radio modem (for communication to a computer located on the shore), a communications antenna, and electronics boards for the Acoustic Doppler Profiler (ADP). Aluminum braces are also used to firmly attach each aluminum hull to the watertight electronics box and to the transducer housing. The total weight of the RiverCAT is approximately 12.5 kilograms. Discharge data are transmitted (via the RS-232 radio modem) continuously from the RiverCAT to a laptop computer located on the shore within sight of the measurement vessel. The laptop computer is also connected to a RS-232 radio-modem (base-station) and communicates with the RiverCAT continuously. Using the laptop computer and the base-station radio-modem located on shore the RiverCAT is programmed and discharge data collected using the SonTek “RiverSurveyor” acquisition/processing software.

Figure 3 shows the 1.5-Mhz RiverCAT ADP including the motorized boat used in the comparison study prior to deployment on the Han River.

Figure 1: 1.5-Mhz SonTek RiverCAT ADP configuration used in demonstration on the Han River.

Figure 2: Picture showing alternative configuration of the RiverCAT ADP using fiberglass trimaran.

Figure 3.-- SonTek 1.5-Mhz RiverCAT ADPand motor boat used during

comparison measurements on the Han River at the Huang Jia Gang Measurement Station.

SonTek ADP Narrowband Signal Processing A high-speed digital signal processor (DSP) and an 8-bit analogue to digital (A/D) converter are used by the RiverCat to rapidly process the digital narrowband signal. This is an advanced narrowband processing technique that is proven to provide an accurate, independent estimate of velocity for each depth cell, without the need for vertical filtering. This permits the RiverCat to measure in areas of high-shear (steep velocity gradients) without any of the errors documented in narrowband systems developed by RD Instruments between 1981 and 1989. Ultimately, the undeniable benefits of this signal processing to the operator are primarily the ease-of-use (as compared to broadband signal processing).

A single, robust mode of operation allows one instrument setting to be used successfully across the entire river cross-section. Broadband systems inherently require the operator to set specific modes for certain dynamic conditions of the river (depth/velocity). Broadband systems complicate the entire measurement process — both in the set up of the system and for a need to have highly trained personnel to operate the system who understand broadband processing, ambiguity velocities, error thresholds, etc.

The most significant operating advantage of a narrowband vs. broadband system is that narrowband processing allows the user to “continuously” measure discharge (at a constant accuracy) from shallow to deep water and from slow to fast (or conversely) water. Meaning, for the 1.5-Mhz RiverCAT as long as the instrument depth is between 0.9 and 25 m, and water-velocities are between 0 and 10 m/s, the RiverCAT will accurately compute discharge no matter the dynamic change of depth/velocity. In contrast, for a single given frequency it may appear that a broadband system has more operating range than a narrowband system. In reality, from an operational perspective it does not. Broadband signal processing requires the user to either operate the system in a

single dynamic range across a river (severely limiting the velocity and depth range for accurate measurements) or stop during the middle of a discharge measurement and change mode settings when the present operating mode thresholds have been exceeded. Mechanical Horizontal-Axis Current Meter A horizontal-axis current meter measures water velocity based on the proportionality between the water-velocity and the angular velocity of the current meters rotor. In the case of a “horizontal-axis” current meter, the rotor spins about the horizontal axis. The number of revolutions of the rotor is measured by an electrical circuit through a contact chamber (located in the current meter). The electrical pulse produces an audible “click” in a headphone or registers the “click” to an external counting device. Figure 4 shows the horizontal-axis mechanical current meter used in the comparison study attached to a sounding weight.

Figure 4. -- Mechanical horizontal-axis current meter and sounding weight used during

the comparison measurements on the Han River.

The mechanical current meter and sounding weight is attached to a “sounding-reel”. The sounding reel is composed of a reel (winch), steel cable, hand-crank, and boom. Upon positioning the boat in a stationary position in the measurement cross-section, the instrument and weight is lowered to the river bottom to measure depth. The instrument is then raised to specified measurement depth(s) (typically 0.2, 0.6, 0.8 depths) where a water-velocity measurement is made. The depth is know by graduation marks located on the sounding reel cable or by a counter used to measure the distance the cable travels. Based on the number of vertical locations measured for a given station, mean-velocity is computed as the average of the collected data for that particular station. Discharge is then computed at the particular station based on the depth, mean-velocity, and station width (formally called the mid-section method for computing discharge)1. Figure 5 shows the complete mechanical meter setup including the boat used in the comparison measurements on the Han River at the Huang Jia Gang Measurement Station.

1 USGS Water-Supply Paper 2175 vol. 1 & 2, by Rantz and others, 1982 provides a complete description of this measurement technique

Figure 5. – Mechanical horizontal-axis current meter equipment setup used during the comparison measurements on the Han River Huang Jia Gang Measurement Station.

Comparison Measurement Techniques Two measurement techniques were used during the comparison of the 1.5-Mhz RiverCAT and the mechanical horizontal-axis current meter. The two techniques were necessary in order to properly compare: Total Discharge, Depth, and Mean Velocity. In the case of comparing depth and mean velocity each instrument was located side-by-side on a boat and data was collected simultaneously for a specified measurement interval. In the case of “total discharge” the RiverCAT used the “moving-boat” by traversing across the river on a motorized boat and continuously collecting depth, position, and velocity data. Just prior to the RiverCAT discharge measurement the mechanical horizontal axis current meter was used to measure total discharge using the mid-section method. The following sections describe the measurement method and results for the stationary measurements and the moving boat measurements used during the comparison study. Stationary Measurements of Depth and Mean-Velocity Using the 1.5-Mhz RiverCAT ADP and the Mechanical Horizontal-Axis Current Meter “Mean-velocity” and “depth” data were collected using the mechanical horizontal-axis current meter and the 1.5-Mhz RiverCAT ADP at 14 independent stations along the normal measurement section on the Han River. The instruments were located directly next to each other and data was collected simultaneously. This was done in order to assure that a closer, more direct comparison of depth and mean-velocity was made. At each measurement station the mechanical current meter was used to measure the depth (using the sound weight and reel) and then positioned at either one (0.6 depth) or two (0.2 and 0.8 depth) verticals to compute a mean-velocity for the station. The averaging time for each vertical measurement was 100-seconds. If a single vertical (0.6 depth) was measured at a station it was used as the mean-velocity. If two verticals (0.2 and 0.8

depths) were measured the average of the two was computed to determine the mean-velocity for the station. The 1.5-Mhz RiverCAT ADP was sampled during the same time the mechanical current meter was measuring. A single 100-second sample was collect and stored to an independent measurement file using the RiverSurveyor software. The RiverCAT vertically integrated the velocity profile into 0.25-meter increments (cells) and averaged each depth cell to estimate the “mean-velocity”. The “depth” was determined from a 3-beam average depth measured during the sample period. RiverSurveyor data files are noted in Appendix I for each measurement station and can the electronic files containing the raw data be provided per the request of the Han River Hydrology & Water Resources Survey Bureau, Yangtze River Water Conservancy Committee. The results of the comparison of depth and mean-velocity were extremely close for all measurements of depth and mean-velocity. The average depth difference between the two technologies was 0.06-meters. Considering that the mechanical current meter depth soundings were made with a resolution of 0.1 meter this difference is insignificant. In addition, the maximum depth of the river was nearly 8-meters with an average depth of approximately 5 – 6 meters which results in less than a 1% difference in depth on average. The average mean-velocity difference between the two technologies was 0.02 m/s which for all intentions is well within the accuracy of both the mechanical current meter and the RiverCAT and can be considered insignificant. The complete results of the depth and mean-velocity measurements are tabulated and summarized in Appendix I. The discharge computed with the mechanical horizontal-axis current meter, the RiverCAT was 1460 and 1449 cubic meters per second, respectively. The percent difference between the mechanical current meter and the RiverCAT discharge was 0.75 percent. The results of the discharge comparison between the two sources: 1. 1.5-Mhz RiverCAT (moving-boat method); 2. Mechanical horizontal-axis current meter (mid-section method) are presented and summarized in Appendix II. Moving-Boat Discharge Measurements Using the 1.5-Mhz RiverCAT Two moving-boat discharge measurements were made using the 1.5-Mhz RiverCAT. The measurements were made immediately after the mid-section method stationary measurements made using the mechanical current meter. For each moving boat measurement, the RiverCAT was positioned alongside a riverbank and held nearly stationary within about 5 –6 meters of the rivers edge. Data collection was started (using the RiverSurveyor software) and the RiverCAT was allowed to collect between 5 – 10 seconds of data before starting its course to the opposite riverbank. The primary reason for this is that data collected at the start of the measurement is used to estimate the flow from the measurement starting point to the actual rivers edge. In a general sense, the more stationary the vessel is the better the true starting distance from the bank can be measured resulting in more representative data estimates of the unmeasured flow. In addition, by collecting 5 – 10 seconds worth of data a better estimate can be made of the unmeasured velocity section based on measured velocity data near that point.

Once the data collection was started, the RiverCAT was steadily transected across the Han River to the opposite riverbank. During the measurement, the RiverCAT crossing speed was typically slightly above the actual speed of the water. In addition, during each measurement, the towing speed as well as the course of the RiverCAT was held as steady as possible in order to obtain the most accurate measurement possible. Once the RiverCAT reached the opposite riverbank it was allowed to become stationary and collect data for 5 – 10 seconds. This is for similar reasons as described during the start of each measurement. The measurement duration for each measurement was 00:16:25 (hr:min:sec) and 00:22:50, respectively. In practice, measurement durations between 00:15:00 to 00:30:00 minutes are typical for a river of similar geometry. The maximum depth was nearly 8 meters with the average depth being closer to about 5-6-meters. This is well within the measurement range of the 1.5-Mhz RiverCAT which has a normal maximum range of about 22- 25 meters depending on water conditions. The boat speed to water speed ratio was typically between 2 -3 which is slightly high (less than 1 is preferable) but measurement results we not negatively affected in this case primarily because of the steady operation of the boat and the uniformity of the river flow. Figures 6 and 7 show a graphical display (from the RiverSurveyor software) of the measurement results of both passes (transects). In each case, the ship-track windows show the course of the vessel (blue line) and the green lines (velocity vectors) indicate that the river flow is relatively uniform in the selected cross-section.

Figure 6 – RiverSurveyor data display screen showing results of “first” moving-boat discharge measurement using a 1.5-Mhz RiverCAT on the Han River, China.

Figure 7 -- RiverSurveyor data display screen showing results of “second” moving-boat discharge measurement using a 1.5-Mhz RiverCAT on the Han River, China. A summary of the results (automatic output from RiverSurveyor) of the two measurements are provided in figure 8. It is important to note that the data presented in this report has not been edited or altered in anyway. The results are presented are complete and exactly as they were collected in the field.

Figure 8. -- RiverSurveyor Discharge Summary of moving boat measurements made on the Han River using a 1.5-Mhz RiverCAT tethered to a motorized boat. The discharge results from the 1.5-Mhz RiverCAT moving-boat discharge measurements show that the average discharge measured was 1449 cubic meters per second. The difference (coefficient of variance, COV) between the two successive discharge measurements was 0.7 percent. Typically, replicate measurements are considered valid if

they fall within 5% so the results clearly show that the RiverCAT performed exceptionally well and far beyond the minimum requirements. Summary On March 9th, 2004 comparison measurements of discharge, velocity, and depth were made at the Huang Jia Gang Hydrological Station using a SonTek 1.5-Mhz RiverCAT Narrowband ADP (Acoustic Doppler Profiler) and a traditional mechanical vertical-axis current meter. Measurements were performed by engineers from the Han River Hydrology & Water Resources Survey Bureau, Yangtze Water Conservancy Committee and engineers from SonTek/YSI China (Mr. Mark Tepper and Mr. Steven Chen) and SonTek/YSI’s Principle Hydrologist, John V. Sloat. “Total discharge” was measured by the RiverCAT using the moving-boat method and compared to a concurrent “total discharge” measurement made using a mechanical vertical-axis current meter using the traditional ISO/USGS discharge measurement mid-section method (described in USGS WSP 2175 vol. 1 &2, Rantz, and others). In addition, “Depth” and “Mean Velocity” data was measured by the RiverCAT at identical locations and at the same time as the mechanical current meter at 20 independent stations equally spaced across the river at the stations traditional measurement location. The results of the comparison of depth and mean-velocity were extremely close for all measurements of depth and mean-velocity. The average depth difference between the two technologies was 0.06-meters. Considering that the mechanical current meter depth soundings were made with a resolution of 0.1 meter this difference is insignificant. In addition, the maximum depth of the river was nearly 8-meters with an average depth of approximately 5 – 6 meters which results in less than a 1% difference in depth on average. The average mean-velocity difference between the two technologies was 0.02 m/s which for all intentions is well within the accuracy of both the mechanical current meter and the RiverCAT and can be considered insignificant. Thus it can be concluded that the comparison measurements of depth and velocity between the 1.5-Mhz RiverCAT Narrowband ADP and the traditional mechanical meter were identical. The discharge results from the 1.5-Mhz RiverCAT moving-boat discharge measurements show that the average discharge measured was 1449 cubic meters per second. The difference (coefficient of variance, COV) between the two successive discharge measurements was 0.7 percent. Typically, replicate measurements are considered valid if they fall within 5% so the results clearly show that the RiverCAT performed exceptionally well and far beyond the minimum requirements. The discharge computed with the mechanical current meter and the RiverCAT was 1460 and 1449 cubic meters per second, respectively. The percent difference between the mechanical current meter and the RiverCAT discharge was 0.75 percent.

Acknowledgements SonTek/YSI, Inc. would personally like to thank all the participants for attending and assisting with this comparison of the SonTek 1.5-Mhz RiverCAT and the traditional mechanical current meter technology. Very special thanks goes out specifically to the engineers with the Han River Hydrology & Water Resources Survey Bureau, Yangtze River Water Conservancy Committee that participated in the comparison measurements as well as organized and prepared equipment enabling a successful day in the field.

Figure 9: Engineers from SonTek/YSI and the Han River Hydrology & Water Resources Survey Bureau, Yangtze River Water Conservancy Committee at the Huang Jia Gang Measurement Station on the Han River in Danjiangkou , China.

Appendix I – Summary Results of “Stationary” Comparison Measurements of Depth and Mean-Velocity

Huang Jia Gang Measurement Station

Station Number

RiverCAT Data

Filename

RiverCAT measured "Depth"

(m)

Mechanical Current Meter

Measured Depth (m)

RiverCAT/Mechanical Meter Depth Difference (m)

RiverCAT Mean-

Velocity (m/s) Average Mechanical Velocity (m/s)

Point Velocity (m/s)

Mechanical method

Depth (m)

RiverCAT/Mechanical Meter Mean-velocity difference (m/s)

1 1805 3.09 3.15 -0.06 0.14 0.11 0.11 0.6 0.03 2 1817 4.1 4 0.10 0.24 0.25 0.25 0.6 -0.01 3 1829 3.48 3.3 0.18 0.39 0.38 0.43 0.2 0.02 0.32 0.8 4 1839 4.35 4.35 0.00 0.49 0.50 0.56 0.2 -0.01 0.43 0.8 5 1848 5.17 5.2 -0.03 0.68 0.67 0.73 0.2 0.01 0.61 0.8 6 1856 5.75 5.7 0.05 0.67 0.73 0.84 0.2 -0.05 0.61 0.8 7 1904 6.18 6.2 -0.02 0.68 0.68 0.79 0.2 0.01 0.56 0.8 8 1913 6.55 6.6 -0.05 0.65 0.64 0.68 0.2 0.01 0.6 0.8 9 1924 6.93 7 -0.07 0.65 0.69 0.75 0.2 -0.04 0.63 0.8

10 1933 7.37 7.4 -0.03 0.69 0.68 0.75 0.2 0.01 0.6 0.8

11 1944 7.82 7.9 -0.08 0.76 0.78 0.78 0.6 -0.02 12 1954 8.38 8.4 -0.02 0.65 0.63 0.63 0.6 0.02 13 2008 8.42 8.4 0.02 0.58 0.57 0.57 0.6 0.01

14 2015 7.61 7.7 -0.09 0.55 0.56 0.56 0.6 -0.01

Avg. Depth Difference (m) 0.06 Avg. Mean-Velocity Difference (m/s) 0.02

Net Depth Difference (meters): -0.007

Net "Mean-Velocity" Difference (m/s) -0.002

Max. Depth Difference (meters): 0.18

Max. "Mean-Velocity" Difference (m/s) 0.05

Min. Depth Difference (meters): 0

Min. "Mean-Velocity" Difference (m/s) 0.01

Appendix II – Summary Results of Comparison Measurements of Discharge

RiverCAT (Moving) Discharge: 1449 cubic meters per secondMechanical Discharge: 1460 cubic meters per second

Percent Difference (RiverCAT discharge versus Mechanical Meter Discharge) -0.76 %