Experimental Evaluation of the Q-Liner Measurement Performance

Transcription

1 Experimental Evaluation of the Q-Liner Measurement Performance by Andrew Craig and Marian Muste to: Nalley, G.M., McVay, J., and Goodrich, R.D. Iowa Water Science Center U.S. Geological Survey 400 S. Clinton St Iowa City, IA IIHR Technical Report No. 485 IIHR - Hydroscience & Engineering College of Engineering The University of Iowa Iowa City, Iowa June 22, 2009

2 Project Summary Several new hydrometric instruments and measurement techniques have been developed over the last few decades. Among them, hydro-acoustics have undoubtedly been a positive influence on the measurement of stream discharge within the United States Geological Survey (USGS). Over the past 15 years, the acoustic instruments brought in new capabilities that increased considerably the accuracy and efficiency of discharge data collection. The increased pace of the new equipment that has been offered by the market was paralleled by a considerable effort to the test the instrument capabilities and limitations. However, these tests have not witnessed an equal growth in knowledge and practices pertaining to the procedural manuals and standards for the use of the newer instruments and techniques nor for establishing the uncertainty associated with the estimate of the measurement. One of the newest introduced instruments is the Ott Q-Liner. Initial results from direct comparison of Q-Liner discharge data with approved discharge measurement devices have been promising and have lead to the need for further testing. The present report describes a laboratory study aimed at investigating several key Q-Liner performance issues including: a) required sampling time for capturing the mean flow characteristics; b) the contribution of individual acoustic beams to the accuracy of the overall measurement; and, c) estimation of the velocity and discharge measurement accuracies;. The study was conducted in a large-scale flume using the Acoustic Doppler Velocimeter (ADV) as a reference instrument. The main findings are that: a) Q-Liner measurements are generally biased low when compared to the ADV reference profile; b) Q-Liner is relatively noisy but can achieve acceptable accuracy if enough sampling (exposure) time is allowed (results demonstrating the required sampling time are presented); c) the near-transducer bias is not a concern for measurements taken with Beams 1 and 2 (as they are far from the region disturbed by the float), and, d) Beam 3 measurements are biased low near the transducer. The overall conclusion of the study is that, despite the limitations listed above, the Q-Liner is an adequate instrument for measuring the discharge with errors from the reference instrument not exceeding 6.5%. 2

3 TABLE OF CONTENTS Chapter Page Project Summary Introduction The Experimental Setup Experimental facility and instruments Verification of the flow and instrument operating conditions Establishment of the sampling time for ADV and Q-Liner measurements Field vs. flume test series Alternative Q-Liner comparison Q-Liner Measurement Capabilities Assessment In-axis test series In-axis test series In-beam test series Discharge estimation in laboratory flume Software resolution problem Findings and Conclusion References

4 LIST OF FIGURES Figure Page 1. Q-Liner Beam geometry CAD drawings of Environmental Flow Facility (EFF) The experimental flume: a) photo of the flume exterior; b) roughness array used for triggering the turbulent boundary layer in the flume Calibration of the Micro ADV vs. the LDV: a) LDV calibration curve; b) ADV-LDV calibration curve for the streamwise velocity Verification of uniform flow (maximum pump speed for all cases): a) Vertical velocity profiles measured with ADV in a 0.91 m flow; b) Velocity transects measured with ADV in a 0.91 m flow; c) Vertical velocity profiles measured with ADV and Q-Liner in a 1.52 m flow; d) Velocity transects measured with Q-Liner in a 1.52 m flow; e) Vertical velocity profiles measured with ADV and Q-Liner in a 1.98 m flow; f) Velocity transects measured with Q-Liner in a 1.98 m flow Estimation of the required sampling time using velocity cumulative averages: a) Q-Liner in 0.91 m flow at y/d = 0.37; b) MicroADV in 0.91 m flow at y/d = 0.35; c) Q-Liner in 1.52 m flow at y/d = 0.23; d) MicroADV in 1.52 m flow at y/d = 0.21; e) Q-Liner in 1.98 m flow at y/d = 0.25; f) MicroADV in 1.98 m flow at y/d = Q-Liner measurements in 1.13 m flow: a) 40 second flume measurements; b) 5 minute flume measurements; c) 40 second field measurements; d) 5 minute field measurements (solid squares represent Beam 1, 2 data; hollow squares represent Beam 3 data; black represents 50 minute composite measurement) Comparison of 2 Q-Liners: a) Q-Liner #1 40 second measurements; b) Q-Liner #1 5 minute measurements; c) Q-Liner #2 40 second measurements; d) Q-Liner #2 5 minute measurements (solid squares represent Beam 1, 2 data; hollow squares represent Beam 3 data; black represents 100 minute composite measurement) Photographs of in-axis setup: a) Measuring 5 cm from ADV probe tip to center of Q-Liner transducer head; b) Leveling the Q-Liner on the traverse Plan View of In-Axis test series measurement setup

5 11. In-axis test series velocity profiles: a) 0.91 m, 40 second measurements; b) 0.91 m, 5 minute measurements; c) 1.52 m, 40 second measurements; d) 1.52 m, 5 minute measurements; e) 1.98 m, 40 second measurements; f) 1.98 m, 5 minute measurements (blue line represents 5 minute ADV profile with Q-Liner deployed; black line represents 5 minute ADV profile without Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents 25 minute Q-Liner composite measurement) Agreement of Q-Liner Beams 1 and 2 with Beam 3: a) 0.91 m, 40 seconds; b) 0.91 m, 5 minutes; c) 0.91 m, 25 minute; d) 1.52 m, 40 seconds; e) 1.52 m, 5 minutes; f) m, 25 minutes; g) 1.98 m, 40 seconds; h) 1.98 m, 5 minutes; i) 1.98 m, 25 minutes (blue lines represent Beam 1, 2 data; Pink lines represent Beam 3 data) In-axis test series velocity profiles: a) 1.98 m, 40 second measurements; b) 1.98 m, 5 minute measurements (black line represents 5 minute ADV profile without Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents 25 minute Q-Liner composite measurement) In-beam test series setup pictures: a) frontal view of projected Q-Liner beams with ADV in beam 1; b) side view of projected Q-Liner beams Schematic of in=beam test series setup In-beam test series 1.98 m flow profiles: a) 40 second Q-Liner measurements; b) 5 minute Q-Liner measurements (black line represents 5 minute ADV profile with Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents composite Q-Liner measurement (35 minutes for Beams 1, 2; 40 minutes for Beam Time series outputted by the Q-Liner: a) before software fix cumulative average; b) after software fix actual time series of velocities

6 LIST OF TABLES Table Page 1. Main characteristics of the instruments used in the tudy Errors and scattering associated with Q-Liner in 1.13 m flow in the flume Errors and scattering associated with Q-Lineer in 1.13 m flow in the field Errors and scattering associated with Q-Liner #1 in 1.98 m flow Errors and scattering associated with Q-Liner #2 in 1.98 m flow Errors associated with Q-Liner in 0.91 m flow: in axis test series Errors associated with Q-Liner in 1.52 m flow: in axis test series Errors associated with Q-Liner in 1.98 m flow: in axis test series Errors associated with Q-Liner in 1.98 m flow: in axis test series Errors associated with Q-Liner in 1.98 m flow: in-beam test series Discharge estimates and associated errors from virtual river measurements Individual discharge estimates and associated errors from in-axis test series

7 1. Introduction Q-Liners measure velocities along three beams that are subsequently used to report velocity distribution in the vertical passing through the axis of the instrument (see Figure 1). Beams 1 and 2 point downward at 25 from vertical in the forward and aft directions. Measurements with Beams 1 and 2 are not reported in the upper 20 cm (for the Ott 1MHz Q-Liner) of the water column as the errors associated with ringing and the flow disturbance in this area generate a low bias in the measurements (Gartner & Ganju, 2002; Muste et al., 2006). Beam 3 measures away from the instrument at an angle of 20 below horizontal in the forward direction, in an area of the flow which is assumed to be undisturbed. This particular instrument configuration was designed with the intention to use the velocities measured by Beam 3 in the area unmeasured by Beams 1 and 2 immediately under the probe (a.k.a. blanking distance). Data collected by USGS and other studies (Frizell & Vermeyen, 2007) showed, however, that velocity measured with Beam 3 does not always provide a good match with the profile measured by Beams 1 and 2, questioning the overall utility of this additional beam for measurement of velocity distribution and discharge. The above mentioned uncertainties in the performance of the Q-Liner triggered a collaborative study between IIHR-Hydroscience & Engineering (IIHR) and the Iowa USGS Iowa Water Science Center. Presented herein are the main results of the study with emphasis on Q-Liner performance and essential operational aspects. The laboratory study was conducted in a largescale facility at IIHR. Preliminary measurements were conducted to verify the quality of the flow in the facility and to document the presence of any facility-induced interference with the Q- Liner or the reference instrument used for performance evaluation. Measurements were conducted in the field and replicated in the laboratory to confirm the absence of instrument interference with the experimental flume. An extensive set of data was acquired to establish the operational characteristics of the Q-Liner and Acoustic Doppler Velocimeter (ADV) used as reference for the study. The core measurements entailed in-axis (vertical) and in-beam measurements with the Q-Liner and ADV. The measurements were compared with the ADV measurements in the undisturbed channel flow. Analysis was focused on documenting the quality of the data by evaluating the scattering of the velocities around the reference velocity profile for various sampling (exposure) times. The utility of Beam 3 was investigated by comparison with the ADV measurements and assessment of the concurrence with the velocity profile provided by Beams 1 and 2. The measurements were repeated for a series of depths and a range of velocities to validate findings. Using the extensive velocity dataset, the Q-Liner s ability to accurately estimate discharge was also evaluated. 7

8 Figure 1. Q-Liner Beam geometry 2. The Experimental Setup 2.1. Experimental facility and instruments The tests were conducted in the Iowa Institute of Hydraulic Research (IIHR) Environmental Flow Facility (EFF). CAD drawings and photographs of the flume are provided in Figures 2 and 3. The re-circulating non-tilting flume is m long, 3.05 m wide and 2.29 m deep. The maximum flume flow capacity is 3.54 cubic meters per second re-circulated through two 0.91 m pipe lines. The EFF side walls are made of glass to facilitate flow visualization. The windtunnel based design of the flume (smooth cross-section contraction at the entrance of the flume) conditions the flow well in the channel close to the flume entrance. Given the large size of the experimental facility, however, the turbulent boundary layer has to be initiated. For this purpose, an arrangement of surface roughness elements (concrete masonry units of 19.4 cm x 19.4 cm x 38.8 cm) in the upstream section of the flume was installed. Reserving a 0.3 m free board, the 8

9 maximum depth of flow available for this study was 1.98 m. The test area for the present study was set 4.27 m downstream from the beginning of the raised floor section shown in Figure m 0.51 m 0.51 m typ 0.61 m 0.61 m typ 0.51 m 0.51 m typ 0.61 m 0.51 m PLAN VIEW TEST SECTION 1.23m 1.23m 1.98m 1.37m 0.91m 1.52m 0.91m 2.41m 2.56m 4.27m 7.87m W.S. RAMP Concrete Masonry Units 2.54cm raised floor section begins 1.98m LONGITUDINAL ALONG THE CENTERLINE Figure 2. CAD drawings of Environmental Flow Facility (EFF) a) b) Figure 3. The experimental flume: a) photo of the flume exterior; b) roughness array used for triggering the turbulent boundary layer in the flume 9

10 The Q-Liner utilized in the present study was a 1 MHz model. The high-resolution, highfrequency reference instrument for the study was a microadv (Sontek/YSI). The Q-Liner and microadv main characteristics are summarized in Table 1. The reference instrument was traced to a primary standard through calibrations against a Laser Doppler Velocimeter (LDV) as seen in Figure 4. Despite their widely different configurations, the Q-Liner and ADV are based on the same operating principle, i.e., acoustic waves, making their comparison simpler than in situations when the sensors are operating on different principles, leading to separate checking for their interaction with the facility, flow, and environment. a) b) Calibrator (m/s) Velocity ADV (m/s) LDV Velocity (m/s) Velocity LDV (m/s) Figure 4. Calibration of the Micro ADV vs. the LDV: a) LDV calibration curve; b) ADV-LDV calibration curve for the streamwise velocity Table 1. Main characteristics of the instruments used in the study 10

11 2.2. Verification of the flow and instrument operating conditions In preparation of the experimental program, a series of measurements were collected with the microadv reference instrument and the Q-Liner to verify the quality of the flume flow within the Q-Liner footprint. Results are shown in Figure Centerline 3 ft us centerline 3 ft ds centerline surface D 0.67D 0.915D 6 5 Vx (cm/s) Vx (cm/s) a) Position (m) b) 11

12 ADV mid center surface Qliner mid cent B1,2 Qliner mid cent B3 Qliner US cent B1,2 Qliner US cent B3 Qliner mid 3ft left B1,2 Qliner mid 3ft left B3 Qliner mid 4ft left B1,2 Qliner mid 4ft left B3 Qliner mid 3ft right B1,2 Qliner mid 3ft right B3 Qliner mid 4ft right B1,2 Qliner mid 4ft right B3 Qliner DS cent B1,2 Qliner DS cent B3 Vx (cm/s) D 0.62 D D Vx (cm/s) ADV mid center surface Qliner mid cent B1,2 Qliner mid cent B3 Qliner US cent B1,2 Qliner US cent B3 Qliner mid 3ft left B1,2 Qliner mid 3ft left B3 Qliner mid 4ft left B1,2 Qliner mid 4ft left B3 Qliner mid 3ft right B1,2 Qliner mid 3ft right B3 Qliner mid 4ft right B1,2 Qliner mid 4ft right B3 Qliner DS cent B1,2 Qliner DS cent B3 c) Vx (cm/s) Position (m) D 0.64 D 0.86 D d) Vx (cm/s) Position (m) e) f) Figure 5. Verification of uniform flow (maximum pump speed for all cases): a) Vertical velocity profiles measured with ADV in a 0.91 m flow; b) Velocity transects measured with ADV in a 0.91 m flow; c) Vertical velocity profiles measured with ADV and Q-Liner in a 1.52 m flow; d) Velocity transects measured with Q-Liner in a 1.52 m flow; e) Vertical velocity profiles measured with ADV and Q-Liner in a 1.98 m flow; f) Velocity transects measured with Q-Liner in a 1.98 m flow ADV and Q-Liner measurements were made at the test section in the center of the flume, at the test section 3 ft and 4ft right and left of the centerline and at the centerline 3 ft upstream and downstream of the test section. Given that the footprint of Q-Liner Beams 1 and 2 in a 1.98 m flow are projected approximately 0.40 m upstream and downstream, the tested intervals of 0.91 m upstream and downstream were sufficient to verify a uniform flow field within the Q-Liner footprint. The flow proved to be satisfactorily uniform throughout the Q-Liner footprint and ADV test section at the flume centerline Establishment of the sampling time for ADV and Q-Liner measurements The required sampling time for each measurement situation and location is a function of the flow characteristics and the instrument temporal resolution. Given that the present measurements are concerned just with time-interval independent flow quantities (i.e., mean velocity), the instrument temporal resolution is not affecting the measurement accuracy as long as the 12

13 sampling time is long enough to capture over an adequately large number of large-scale turbulence events. Despite that there are several analytical relationships for estimating the sampling time needed to adequately characterize turbulent flows (e.g., Muste et al., 2005; Abad et al., 2004), a more practical approach was used in the study, as illustrated in Figure 6. The sampling time estimation approach entailed collection of long-term velocity samples followed by the analysis of the stability of the average sample statistics for gradually increased sampling times up to the total collection time. Figure 6 displays the cumulative averages of the Q-Liner velocity time series (collected at 3.0 seconds interval) and ADV (collected at 2 seconds interval) at locations near the bed where turbulence is most prevalent. One can observe that the Q-Liner averaged samples take a constant value near 200 seconds (see plots in Figure 6.a, c, and e). ADV measurements taken at approximately the same location in the same flow display a similar stabilization for the mean velocity near 200 seconds (see plots in Figure 6.b, d, and f). For practical purposes, an equal sampling time of 5 minutes was chosen for all the measured flows for both the Q-Liner and ADV Field vs. flume test series Comparisons between field measurements and laboratory flume measurements were made in order to ensure that any scattering or noisy behavior observed in the subsequent tests were not artifacts of the flume itself (i.e. interference with flume walls and floor). Ten consecutive 5 minute stationary field measurements were made at a location in the Iowa River at Iowa City, Iowa with the Q-Liner tethered from a bridge (Benton St.) and the compass enabled. The Q- Liner s reported depth and mean velocity from the field measurements were then reproduced in the laboratory flume where ten consecutive 5 minute stationary flume measurements were collected. Figure 7 illustrates the similarity in behavior between the field and flume measurement sets for 40 second (extracted from 5 minute measurements) and 5 minute measurements. The differing profile shapes in the flume and in the field conditions are due to higher turbulence in the near bed region where, as expected, the roughness in the river is considerably larger than in the flume. Computed errors and scattering were comparable for the field and flume measurements (Tables 2 and 3), indicating no interference between the Q-liner and flume. The reference profiles used for computing errors and scattering in Tables 2 and 3 were the 50 minute composite measurements from each case (field or flume) computed by averaging the 10 measurements. 13

14 Velocity Cumulative Average (cm/s) Velocity Cumulative Average (cm/s) Measurement Time (s) Measurement Time (s) a) b) Velocity Cumulative Average (cm/s) Velocity Cumulative Average (cm/s) Measurement Time (s) Measurement Time (s) c) d) Velocity Cumulative Average (cm/s) Velocity Cumulative Average (cm/s) Measurement Time (s) Measurement Time (s) e) f) Figure 6. Estimation of the required sampling time using velocity cumulative averages: a) Q-Liner in 0.91 m flow at y/d = 0.37; b) MicroADV in 0.91 m flow at y/d = 0.35; c) Q-Liner in 1.52 m flow at y/d = 0.23; d) MicroADV in 1.52 m flow at y/d = 0.21; e) Q-Liner in 1.98 m flow at y/d = 0.25; f) MicroADV in 1.98 m flow at y/d =

15 V (cm/s) V (cm/s) a) b) V (cm/s) V (cm/s) c) d) Figure 7. Q-Liner measurements in 1.13 m flow: a) 40 second flume measurements; b) 5 minute flume measurements; c) 40 second field measurements; d) 5 minute field measurements (solid squares represent Beam 1, 2 data; hollow squares represent Beam 3 data; black represents 50 minute composite measurement) 15

16 Table 2. Errors and scattering associated with Q-Liner in 1.13 m flow in the flume 50 min Max % Difference from 50 min % Data Spread from 50 min Bin # Depth (cm) (cm/s) 5min 40sec 5min 40 sec Beam 1 and Beam Table 3. Errors and scattering associated with Q-Liner in 1.13 m flow in the field 50 min Max % Difference from 50 min % Data Spread from 50 min Bin # Depth (cm) (cm/s) 5min 40sec 5min 40 sec Beam 1 and Beam The Max % Difference from 50 min and % Data Spread from 50 min are presented in tables 2 and 3 for the flume and field measurements, respectively, and are defined as: Max % Difference = (1) where V Q-Liner is the velocity measurement that produces the largest deviation from the reference velocity, V Ref. % Data Spread = (2) where StDev Q-Liner is the standard deviation of the Q-Liner measured velocities Alternative Q-Liner comparison A second Q-Liner (Q-Liner #2) was obtained from the Iowa USGS Iowa Water Science Center to ensure reproducible measurement behavior among Q-Liners. In this test series 10 consecutive 5 minute stationary measurements were made in the flume with each Q-Liner (one at a time) in a 16

17 1.98 m flow. Profiles from 5 minute and extracted 40 second measurements from each Q-Liner were compared to the 100 minute composite measurement obtained by averaging all 20 of the 5 minute measurements made (10 from each Q-Liner). The 100 minute composite measurement was treated as the reference profile in this test series. Figure 8 shows the 40 second and 5 minute velocity profiles from each Q-Liner. Tables 4 and 5 summarize the errors and scattering around the reference profile. The two Q-Liners demonstrated comparable behavior in terms of deviations from the reference profile and data spread (scattering). It is then assumed that Q- Liner #1 (referred to as the Q-Liner hereafter) is equivalent to any Q-Liner used by the USGS or other agency and is representative of typical Q-Liner performance Vx (cm/s) Vx (cm/s) a) b) Vx (cm/s) Vx (cm/s) c) d) Figure 8. Comparison of 2 Q-Liners: a) Q-Liner #1 40 second measurements; b) Q-Liner #1 5 minute measurements; c) Q-Liner #2 40 second measurements; d) Q-Liner #2 5 17

18 minute measurements (solid squares represent Beam 1, 2 data; hollow squares represent Beam 3 data; black represents 100 minute composite measurement) Table 4. Errors and scattering associated with Q-Liner #1 in 1.98 m flow Q-Liner #1 100 min Max% Difference from 100min % Data Spread from 100min Bin # Depth (cm) (cm/s) 50min 5min 40sec 5min 40 sec Beam 1 and Beam Table 5. Errors and scattering associated with Q-Liner #2 in 1.98 m flow Q-Liner #2 100 min Max% Difference from 100min % Data Spread from 100min Bin # Depth (cm) (cm/s) 50min 5min 40sec 5min 40 sec Beam 1 and Beam

19 3. Q-Liner Measurement Capabilities Assessment 3.1 In-axis test series In this test series ADV and Q-Liner velocity profiles were measured non-concurrently at the centerline of the test section in 0.91 m, 1.52 m, and 1.98 m flows. ADV profiles were measured with and without the Q-Liner deployed to investigate the disturbance produced by the Q-Liner (float) itself. For each flow depth, 5, 5 minute measurements were made with the Q-Liner mounted to a rigid aluminum frame suspended from a traverse and the compass disabled. For all tests the minimum blanking distance (0.2m) and minimum cell size (0.3m) were selected. The maximum depth setting was set to the actual flow depth plus one cell (0.3m) for each measurement set. The immersion depth was measured at 6 m. 40 second measurements were extracted from the 5 minute measurements in post-processing. Photographs of the setup for these tests are illustrated in Figure 9 and a schematic is provided in Figure 10. Figure 11 shows plots of the profiles measured for 40 second (typical USGS measurement time) and 5 minute Q- Liner measurements. It is clear that the accuracy and scattering of the measurements are improved significantly with increased sampling time. Consistent with the findings of Dybvik (2008), the Q-Liner velocity profiles are generally biased low compared to the reference profile. Visual inspection suggests that the typical USGS measurement time of 40 seconds is insufficient for accurate measurement of the velocity distribution in a given flow. 19

20 a) b) Figure 9. Photographs of in-axis setup: a) Measuring 5 cm from ADV probe tip to center of Q-Liner transducer head; b) Leveling the Q-Liner on the traverse The deviations from and scattering around the reference profile increase with increasing depth and decreasing channel velocities as can be seen in Tables 6, 7, and 8. Comparisons between Beam 1 and 2 velocity data and Beam 3 velocity data for representative (3 rd measurement out of 5) 40 second and 5 minute measurements and the overall 25 minute composite measurement are made in Figure 12. Similar to Beams 1 and 2, Beam 3 performance is also improved substantially with increased sampling time. The addition of Beam 3 to the measurements with Beam 1 and 2, however, is of limited practical use, as near the transducer the measurements with Beam 3 are biased low, similarly to ADCPs. While Figures 10 and 11 indicate that the first two bins of Beam 3 are always biased, this is not a definite answer as the near-transducer bias is related to the velocity of the channel flow. More tests of the same nature as those reported in this study are needed to provide more practical information. 20

21 Figure 10. Plan View of In-Axis test series measurement setup V (cm/s) V (cm/s) a) b) 21

22 V (cm/s) c) V (cm/s) d) V (cm/s) V (cm/s) e) f) Figure 11. In-axis test series velocity profiles: a) 0.91 m, 40 second measurements; b) 0.91 m, 5 minute measurements; c) 1.52 m, 40 second measurements; d) 1.52 m, 5 minute measurements; e) 1.98 m, 40 second measurements; f) 1.98 m, 5 minute measurements (blue line represents 5 minute ADV profile with Q-Liner deployed; black line represents 5 minute ADV profile without Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents 25 minute Q-Liner composite measurement) 22

23 y/h 0.5 y/h 0.5 y/h a) V/V 0.6D b) V/V 0.6D c) V/V 0.6D y/h 0.5 y/h 0.5 y/h d) V/V 0.6D e) V/V 0.6D f) V/V 0.6D y/h 0.5 y/h 0.5 y/h V/V 0.6D g) h) V/V 0.6D i) V/V 0.6D Figure 12. Agreement of Q-Liner Beams 1 and 2 with Beam 3: a) 0.91 m, 40 seconds; b) 0.91 m, 5 minutes; c) 0.91 m, 25 minute; d) 1.52 m, 40 seconds; e) 1.52 m, 5 minutes; f) 23

24 1.523 m, 25 minutes; g) 1.98 m, 40 seconds; h) 1.98 m, 5 minutes; i) 1.98 m, 25 minutes (blue lines represent Beam 1, 2 data; Pink lines represent Beam 3 data) Table 6. Errors associated with Q-Liner in 0.91 m flow: in-axis test series Bin # Depth (cm) ADV reference Max% Difference from ADV % Data Spread from ADV (cm/s) 25min 5min 40sec 5min 40 sec Beam 1 and Beam Table 7. Errors associated with Q-Liner in 1.52 m flow: in-axis test series Bin # Depth (cm) ADV reference Max% Difference from ADV % Data Spread from ADV (cm/s) 25min 5min 40sec 5min 40 sec Beam 1 and Beam

25 Table 8. Errors associated with Q-Liner in 1.98 m flow: in-axis test series Bin # Depth (cm) ADV reference Max% Difference from ADV % Data Spread from ADV (cm/s) 25min 5min 40sec 5min 40 sec Beam 1 and Beam In-axis test series 2 The original pumps used in this study were destroyed during the flooding of the Iowa River in June, New 50 horsepower (HP) pumps have since been installed in the EFF. A second inaxis test series was conducted where Q-Liner and ADV measurements were directly compared as in the original test series. Measured velocity profiles are presented in Figure 13 and errors and scattering presented in Table Vx (cm/s) Vx (cm/s) 25

26 a) b) Figure 13. In-axis test series velocity profiles: a) 1.98 m, 40 second measurements; b) 1.98 m, 5 minute measurements (black line represents 5 minute ADV profile without Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents 25 minute Q-Liner composite measurement) Table 9. Errors associated with Q-Liner in 1.98 m flow: in-axis test series 2 ADV reference Max% Difference from ADV % Data Spread from ADV Bin # Depth (cm) (cm/s) 25min 5min 40sec 5min 40 sec Beam 1 and Beam n.a. n.a. n.a. n.a. n.a. n.a In-beam test series The in-beam test series differed from the in-axis test series in that ADV measurements were made concurrently and co-located with the Q-Liner measurement cells in beams 1, 2, and 3 in a 1.98 m flow instead of in the vertical axis below the transducer head. A total of 15 ADV measurements were made corresponding to the 15 Q-Liner measurement cell locations: 4 in beam 1, 4 in beam 2, and 7 in beam 3. The 3 Q-Liner beams were projected with survey string from the transducer head and fastened to the flume floor with staples (Figure 14). The measurement cell heights for a 1.98 m flow, pre-determined in the in-axis test series, were then measured vertically from the floor to the intersection of the projected beam and marked. ADV measurement locations were determined and marked on the traverse. An elevation view of the in-beam test series is provided in Figure 15. Each time an ADV measurement was made a concurrent stationary Q-Liner measurement was also made. All Q-Liner measurement settings were the same as in the in-axis test series. Due to potential interference and/or obstruction between the two instruments during co-located measurements, the profiles were created by plotting Q-Liner beam 3 measurements wherein the ADV was measuring in either beam 1 or 2 26

27 locations and Q-Liner Beam 1 and 2 measurements wherein the ADV was measuring in Beam 3 locations. ADV measurements made in Beams 1 and 2 were averaged since they are also averaged by the Q-Liner software (Only output individually as radial velocities). Figure 16 shows the velocity profiles measured during the in-beam test series. The Max % Difference from ADV and % Data Spread from ADV are presented in Table 10. a) b) Figure 14. In-beam test series setup pictures: a) frontal view of projected Q-Liner beams with ADV in beam 1; b) side view of projected Q-Liner beams Figure 15. Schematic of in-beam test series setup 27

28 V (cm/s) V (cm/s) a) b) Figure 16. In-beam test series 1.98 m flow profiles: a) 40 second Q-Liner measurements; b) 5 minute Q-Liner measurements (black line represents 5 minute ADV profile with Q-Liner deployed (reference); solid squares represent Q-Liner Beam 1, 2 data; empty squares represent Q-Liner Beam 3 data; yellow represents composite Q-Liner measurement (35 minutes for Beams 1, 2; 40 minutes for Beam 3) Table 10. Errors associated with Q-Liner in 1.98 m flow: in-beam test series 28

29 Bin # Depth (cm) ADV reference Max% Difference from ADV % Data Spread from ADV (cm/s) 35min 5min 40sec 5min 40 sec Beam 1 and 2 (7-5 minute Q-Liner measurements) Bin # Depth (cm) ADV reference Max% Difference from ADV % Data Spread from ADV (cm/s) 40min 5min 40sec 5min 40 sec Beam 3 (8-5 minute Q-Liner measurements) Discharge estimation in laboratory flume Discharges and associated errors were estimated using the 1/6 th power law for Q-Liner measurements made in the in-axis test series. For each measurement the velocity at height y/d = 0.6 generated by the power law was multiplied by the flow cross-sectional area to compute discharge. The ADV profile was used to compute reference discharges in the same fashion for each flow depth. The Q-Liner measurements were summed to create a virtual river in which 5 stationary measurements were made in a hypothetical m wide rectangular channel. The ADV discharge was multiplied by 5 to establish the total reference discharge of the virtual river. Table 11 summarizes the errors from the virtual river measurement set. In all cases but one the discharge measured by the Q-Liner was less than the reference discharge, again consistent with Dybvik (2008). In 4 of the 6 cases the discharges computed by incorporating Beams 1 and 2 data only were more accurate than when Beam 3 was included in the measurement. Table 12 provides the errors and standard deviations of discharge estimates from individual profile measurements. It is again seen that in 4 of the 6 cases the mean error from the discharge estimates is worse when Beam 3 is used. Interestingly, the standard deviations from discharge estimates where only Beams 1 and 2 are used is always greater than when Beam 3 is used. Beam 3, while more precise, is less accurate and contributes an additional low bias to the discharge estimates. 29

30 Table 11. Discharge estimates and associated errors from virtual river measurements Beams 1,2 only Beams 1,2,3 Flow Depth (ft) Q ref (cfs) Q 40sec (cfs) Q 5min (cfs) Error 40sec (%) Error 5min (%) Q 40sec (cfs) Q 5min (cfs) Error 40sec (%) Error 5min (%) Table 12. Individual discharge estimates and associated errors from in-axis test series 30

31 Measurement Nominal Flow Q1 (cfs) Q2 (cfs) Error Error Error 1 (%) Error 2 (%) StDev 1 Depth (ft) (beams 1, 2 only) (beams 1, 2, 3) 1 (%) 2 (%) StDev 2 ADV 5 min Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 5min Q-Liner 5min Q-Liner 5min Q-Liner 5min Q-Liner 5 min ADV 5 min Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min ADV 5 min Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 40 sec Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min Q-Liner 5 min Software resolution problem During the course of this study it became apparent that what was thought to be the time series of individual velocity measurements output by Q-Liner review was actually a cumulative average of the velocity time series and the output mean was the mean of the cumulative average. All data presented in this report were extracted from the original measurements by back-calculating the time series and then computing the true mean for each measurement cell. Since this problem came to light the USGS requested that Qmetrix fix the problem. Qmetrix responded by releasing QLiner v3.19 PDA software which has proven to function correctly. The time series output by the new version of PDA software has been verified to be the time series and not a cumulative average. Figure 17 presents the output time series before and after software fix (PDA software v3.19). 31

32 1.5 Bin 1 Bin 2 Bin Bin 1 Bin 2 Bin V (m/s) 0 V (m/s) time (s) time (s) Figure 17. Time series outputted by the Q-Liner: a) before software fix cumulative average; b) after software fix actual time series of velocities 4. Findings and Conclusion The required sampling time for a stable Q-Liner measurement was estimated by observing the cumulative averages of the velocity time series until stabilization of the mean velocity occurred. For 0.91 m, 1.52 m, and 1.98 m flows in the laboratory flume the measurement time required for the Q-Liner to obtain a stable velocity measurement was approximately 200 seconds, 5 times the typical 40 second measurement time currently used by the USGS. The minimum blanking distance of 0.2 meters that a user can choose is sufficient for making the measurements with Beams 1 and 2 free of errors as the first measurement cell is reported at a depth of 0.58 m (with a cell size of 0.3 m and immersion depth of 6 cm). At this depth the flow is not disturbed by the immersed part of the boat and probe as is seen closer to the instrument (Figure 11). With these settings and for the range of the tested experimental conditions, however, the shallowest measured Beam 3 velocities are still biased low and should be discarded. The low bias is due to Beam 3 measuring in front of the boat in the stagnation zone created by the boat itself. Overall, it can be concluded that the innovative design of the Q- Liner mitigates the bias error in the near-free surface region but the error is not completely removed. More studies are needed to establish the magnitude of the near-transducer bias for a wider range of flow velocities, commensurate with the range encountered in practical measurement situations. Direct comparisons were made between ADV and Q-Liner velocity measurements with the instruments set at the same locations in the same flows for the in-axis. Results show that an operator can expect the following velocity errors to occur under similar flow conditions: 32

33 0.91 m flow, 40 second measurement: 18% error for Beams 1 and 2 and -16% for Beam m flow, 5 minute measurement: 10% error for Beams 1 and 2 and 9% for Beam m flow, 25 minute measurement: -7% for Beams 1 and 2 and 7% for Beam m flow, 40 second measurement: 28% error for Beams 1 and 2 and -24% for Beam m flow, 5 minute measurement: -11% error for Beams 1 and 2 and -11% for Beam m flow, 25 minute measurement: -4% error for Beams 1 and 2 and -9% for Beam m flow, 40 second measurement: -47% for Beams 1 and 2 and -22% for Beam m flow, 5 minute measurement: -18% for Beams 1 and 2 and -15% for Beam m flow, 25 minute measurement: -6% for Beams 1 and 2 and -14% for Beam 3 The experiments show that, as expected the sampling time plays a critical role in accurately capturing the mean flow characteristics. Field and flume measurements were found to be quite comparable which verified that the Q-Liner s behavior in the flume study was not induced by the flume itself. Discharge estimations were made using the virtual river concept by summing 5 consecutive measurements and comparing to the ADV reference discharge. In 4 of the 6 cases, the discharge computed by using Beams 1 and 2 only was closer to the reference discharge than that computed with inclusion of Beam 3 velocities (Table 10). However, in half the cases the computed error was greater than 5 % regardless of what beams were used for the measurement. 33

34 5. References Abad, J.D., Musalemi, R.A., Garcia, C.M., Cantero, M.I., and Garcia, M.H. (2004). Exploratory Study of the Influence of the Wake Produced by Acoustic Doppler Velocimeter Probes on the Water Velocities within Measurement Volume, Proceedings EWRI World Water Congress, Salt Lake City, UT. Dybvik, Kristoffer (2008). Personal Communication on Hydroacoustics mailing list ( Frizell, K.W. and Vedrmeyen, T.B. (2007). Comparing Apples and Oranges: Teledyne/RDI StreamPro ADCP and the OTT, EWRI-IAHR Hydraulics Measurement & Experimental Methods Conference, Lake Placid, NY Gartner, J.W., and Ganju, N.K. (2002). A Preliminary Evaluation of Near-Transducer Velocities Collected with Low-Blank Acoustic Doppler Current Profiler, Proc., Hydraulic Measurements & Experimental Methods 2002 (CD-ROM), ASCE, Reston, Va. Muste, M., Gonzalez-Castro, J., Yu, K., and Kim. D. (2005). ADCP Discharge Measurement Accuracy Assessment, Limited Distribution Report, IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, IA. Muste, M., Kim, D., Burkhardt, A. and Brownson, Z. (2006). Drag-Induced Bias of ADCPs Measurements, Technical Report, IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, IA. Qmetrix (2005). Q-Liner User Manual Version 3, Sept PL Badhoevedorp, The Netherlands 34