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  • To estimate the stationing of the y-axis, rely on the location of the maximum-surface water velocity; it generally coincides at the same vertical as the maximum-instream velocity

  • Develop a stage-area rating using AreaComp (https://hydroacoustics.usgs.gov/indexvelocity/AreaComp.shtml)

  • Generally, data collection and radar deployments point should be upstream of bridges or structures to avoid wind-dominated reaches, eddies, secondary flows, and macro turbulence

  • Wind dominated reaches complicate data collection by requiring a way to remove wind effects from recorded surface velocities

  • Velocity radars can be deployed by hand or fixed on bridges, light cableways, or cable stays

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This document was designed to (1) evaluate whether radars can accurately measure surface-water velocities, (2) compute the mean velocity and discharge at a channel cross-section, (3) identify the environmental and hydraulic factors that influence surface-water velocity measurements, and (4) establish a protocol for transitioning the this proof-of-concept to an operational streamgage platform.

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Channel, velocity and discharge data measured at USGS streamgages will be used to validate the radar-derived data; however, when siting a surface -water velocity radar, the same hydrodynamic conditions used to site a conventional streamgage (Site Selection, p. 9; Turnipseed and Sauer, 2010) should be followed, which include:

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It is important to note the stationing or location of the vertical (“termed the y-axis”) in a cross section where the maximum-instream or maximum-surface water velocity occurs. Generally, the maximum-surface-water velocity occurs at the same vertical as the maximum-instream velocity.  The y-axis is also where all velocity and depth data should be collected to translate surface -water velocities into a mean-channel velocity. Velocity data can be collected using current meters, ADCPs, and ADVs.  The The following data will be recorded at each measured cross section:

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If the channel has not been surveyed, it is recommended that the cross section be surveyed and include the wetted perimeter and above-water points-of-interest in the floodplain.   The stage-area rating is used to estimate compute area, which is required to compute discharge. Horizontal and vertical control should be surveyed relative to the gage datum using a total station survey, GPS receiver, or an equivalent. The program AreaComp (Lant and Mueller, 2012) can be used to generate a synthetic stage-area rating, when estimating areas above the water surface during the day of the siting.

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When wading is possible, measure velocities and compute discharge in accordance with Turnipseed and Sauer (2010).  Select the y-axis from the 25 - 30 verticals comprising the measurement that exhibits the greatest velocity value based on either the maximum-surface water velocity; 0.2D and 0.8D velocities; or 0.6D velocity. At the y-axis, measure the surface -water velocity using handheld or portable velocity radar concurrently with point velocities immediately below the water surface to the channel bottom at an interval that can be used to adequately define the velocity distribution along the selected vertical. Depending on water depth, this should include a minimum of 6 point velocities (near the channel bottom, 0.2D, 0.4D, 0.6D, 0.8D, close to the water surface while minimizing air entrainment); however, it is preferred that the surface -water velocity, point velocities near the water surface and close to the channel bottom, 0.2D, 0.3D, 0.4D, 0.5D, 0.6D, 0.7D, 0.8D, 0.9D be collected. Repeat this procedure to the left and right of the y-axis.

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When wading is not possible, velocity data should be collected at the y-axis using a Stationary Moving Bed Analysis (SMBA) either by boat, light cableway, tethered from a bridge, or river banks. Surface -water velocities should be collected concurrently with the ADCP measurement using handheld or portable velocity radar. Repeat this procedure to the left and right of the y-axis. Discharge should be computed using QRev or an equivalent. When coupled with a GPS receiver, the lat/long of the y-axis should be recorded or established using VMT (Parsons, 2012). Process the velocity distribution in WinRiver II by choosing:

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The depth and velocity data can be copied to a text file.

Surface

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velocity Radars

Velocity radars are used to measure surface -water velocitiesvelocities and do not penetrate the water surface. Typically, the vertical containing the maximum-surface water velocity will contain the maximum-instream velocity. Velocities should be measured relative to bridge stationing or geo-referenced using a GPS receiver. 20 to 25 surface-water velocities are needed to adequately identify the maximum-surface water velocity and y-axis.  The velocity radar can be pointed upstream (preferred) or downstream from a bridge or walkway. It should be oriented parallel to flow lines and tilted (from horizontal) at a nominal 45-degree incidence angle.  It should be noted that different radar units operate at different incidence angles. It’s important to note when collecting velocity data to avoid wind-dominated reaches, eddies, secondary flows, and macro turbulence.

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Table 1 Method summary.

Field Methods

 

Computation Methods

Conventional

  

ADCP

 

QRev, WinRiver II or RiverSurveyor

ADV

 

Mid-section Method

Current-meter

 

Mid-section Method

Stage measurement

 

Rating table

Surface-water velocity

 

Surface Method

Radar

  

Surface-water velocity radar

 

Probability Concept (see https://my.usgs.gov/confluence/pages/viewpage.action?pageId=552933693

 

Challenges

Surface -water velocity radars  will will not work at every site, particularly where wind drift is dominant and surface-water velocities are less than 0.5 fps and the surface-water scatterers (small wave forms) are ill-defined.

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When using ADCPs it it important to minimize noisy data related to ADCPs, it’s recommended that  an this is achieved by collecting an even number of (reciprocal) transects with a minimum of 720 seconds total exposure time be made during steady-flow conditions. The measured discharge will be the average of the discharges from all valid reciprocal transects. Reciprocal transects should always be made to reduce potential directional biases.  For policy detail, see OSW Technical Memorandum 2011.08. If using a TRDI product, the Correlation Profile should be reviewed to ensure the quality of the measurement is sufficient (reviewing the signal-to-noise ratio, SNR)  to ensure the SNR is greater than 128. After locating the y-axis and to provide for redundancy, velocity data should be collected at the y-axis using a an SMBA either by boat or tethered from a bridge or river banks.

When using surface -water velocity radars and depending on variations in surface-water velocities with time, each vertical should be sampled for 40 seconds to 2 minutes. Please note if measurements are collected downstream of a bridge and depending on stage, piering can create secondary flow patterns that can influence the velocity distribution at the y-axis and ultimately the parameters used to compute discharge. It’s preferred that all radar and hydroacoustic measurements be collected upstream of bridges. Using ADCPs downstream of a bridge to compare discharge rates is acceptable. Diagnostic tests, which are run for current meters or acoustic instruments, are not available for velocity radars. However, tuning forks can be used to validate the velocities reported by a radar (equations 2 and 3).  By striking the tuning fork and placing the tuning fork in front of the radar antenna, the recorded velocity is measured using the Doppler shift.

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Figure 1

Handheld radar (Stalker Pro II SVR) deployed from a bridge used to measure surface-water velocities concurrently while measuring point velocities with a FlowTracker at the y-axis in a cross section.

 

 

Figure 2

Fixed-mount radar (Sommer RQ30 and Stalker Surface Velocity Sensor) deployed from a bridge near upstream of USGS streamgage 05082500 Red River of the North at Grand Forks, ND and used to measure surface-water velocities and compute discharge. Courtesy of Chris Laveau.

 

 

Figure 3

 

Cable stay deployment (Sommer RQ30), where little or no infrastructure exists at USGS streamgages 385309104561101 Middle Waldo and 385254104560401 Lower Waldo. The radars are used to measure surface-water velocities and compute discharge. Data is transmitted via 3G and Iridium modems.

 

 

Figure 4

1.PNG

Good surface scatterers (Green for go!) and minimal wind drift. Poor surface scatterers (Red for stop!) downstream of piering. Fixed-mount radars (Sommer RQ30 and Stalker Surface Velocity Sensor) deployed from a bridge upstream of the USGS streamgage 08279500 Rio Grande at Embudo, NM. The radars are installed at the y-axis of the cross section and are used to measure surface-water velocities and compute discharge. Courtesy Jay Cederburg.

 

 

Figure 5

2.PNG

Good surface scatterers (Green for go!) and minimal wind drift. Poor surface scatterers (Red for stop!) caused by wind drift. Wind drift creates noise in the radar returns and must be corrected prior to transmitting surface-water velocities. Upstream of the USGS streamgage 15515500 Tanana River at Nenana, AK. Courtesy Heather Best.

 

 

Figure 6

3.PNG

Good surface scatterers (Green for go!) and minimal wind drift upstream of the pier, where the radar is pointing. Poor surface scatterers (Red for stop!) located downstream and adjacent to the pier caused by secondary flows and eddies. Fixed-mount radar (Sommer RQ30) deployed from a bridge upstream of the USGS streamgage 06192500 Yellowstone River near Livingston, MT. The radars are installed at the y-axis of the cross section and are used to measure surface-water velocities and compute discharge. Courtesy Steve Holnbeck.

 

 

Figure 7

4.PNG

Good surface scatterers (Green for go!) associated with pancake ice. Fixed-mount heterodyne radar (APL-UW RiverScat) is pointing upstream near USGS streamgage 01538700 Susquehanna River at Bloomsburg, PA.

 

 

Figure 8

5.PNG

Good surface scatterers (Green for go!) near the USGS streamgage 05056678 Tolna Coulee near Tolna, ND. Courtesy Chris Laveau.

 

 

Figure 9

6.PNG

Poor surface scatterers (Red for stop!) and significant wind drift near USGS 06751490 North Fork Cache La Poudre River at Livermore, CO.

 

 

Figure 10

7.PNG

Raw discharge data, Red River of the North, Grand Forks, ND (PROVISIONAL).

 

 

Figure 11

8.PNG

Wind-drift corrected discharge data, Red River of the North, Grand Forks, ND (PROVISIONAL).

 

 

Figure 12

9.PNG

Stage-discharge rating and instantaneous radar-derived discharge, Tanana River at Nenana, AK (PROVISIONAL). Noise is caused by wind driftUnsure if noise

is from wind or hydraulic features such as boils.

 

 

Figure 13

10.PNG

Stage-discharge rating, radar-derived discharge and measured discharge, Rio Grande at Embudo, NM (PROVISIONAL).

 

 

Figure 14

11.PNG

Good spectra with a sharp peak and no velocities from opposite directions. Courtesy Wolfram Sommer (Sommer Messtechnik).

 

 

Figure 15

12.PNG

Fair spectra with small velocities from opposing directions, yet a significant peak in the direction of flow. Courtesy Wolfram Sommer (Sommer Messtechnik).

 

 

Figure 16

13.PNG

Poor spectra with velocities from a variety of directions and multiple peak velocities. The radar will recognize a velocity, but it will not be very accurate. Courtesy Wolfram Sommer (Sommer Messtechnik).

 

References

Chiu, C.-L., Tung, N.C., Hsu, S.M., and Fulton, J.W., 2001, Comparison and assessment of methods of measuring discharge in rivers and streams, Research Report No. CEEWR-4, Dept. of Civil & Environmental Engineering, University of Pittsburgh, Pittsburgh, PA.

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