CHRISTOPHER J. WARD
SonTek/YSI Inc, 6837 Nancy Ridge Drive, Suite A San Diego, CA 92124 USA
+1 858.546.8327+1 858.546.8327 (fax), chris@sontek.com
MARK TEPPER
SonTek/YSI Inc. Asian Hydroacoustics Centre of Excellence, East China Normal
University, Shanghai, China
86-21-62223238, 86-21-62227213 (fax), mtepper@sontek.com
While the use of acoustic Doppler technology for rivers up to 50 m deep has become commonplace over the past decade, the application of this technology to the measurement of flow in shallow streams and canals has only recently been considered. In regions around the world, the majority of hydrometric operations are conducted in very shallow streams where technicians can safely wade into the water. In addition, the water supply and irrigation industries have an increasing demand for robust and accurate measurement of flow in shallow open channels. This includes both instantaneous measurements for rating flow against stage and or/velocity as well as for the continuous monitor of flow in real time.
In this paper, we describe the capabilities and limitations of some presently available
SonTek acoustic Doppler instrumentation specifically designed for use in shallow open channels. These products operate at acoustic frequencies above 1.5 MHz and use both incoherent and coherent Doppler processing methods.
INTRODUCTION
Traditionally, professionals in surface water hydrology have relied on stage-discharge relationships to monitor discharge in shallow streams of all depths. Hydrologists would typically embark on a time-intensive program to rate a stream using a mechanical
(usually a propeller-type) current meter to measure instantaneous discharge against several different stage events and a rating curve was developed. Complicated flow regimes such as are found in tidal areas or downstream of dams were typically left unrated as suitable means to monitor the flow did not exist until recent days. In addition ultra-shallow (less than 10 cm) and ultra-low flow (less than 1cm/s) were not measurable by conventional techniques.
Mechanical instruments have proven capability dating back hundreds of years.
Commonly, these mechanical instruments have been used from a graduated pole
(commonly referred to as a “wading rod”) at depths less than 1.5m. Their limitations have been frequent calibrations, mechanical wear and tear, and high threshold velocities. As

a modern method that works well in all flow regimes and depths down to a few cm,
SonTek offers the FlowTracker Handheld ADV. The FlowTracker was designed with user-input from the USGS and it works from a wading rod much as the mechanical meter does. However, its capabilities far exceed mechanical equipment and built-in discharge algorithms are part of the basic package. A basic description of the FlowTracker and some sample data will be shown here
To support the traditional rating method in a highly efficient manner, Acoustic
Doppler profilers (ADPs or ADCPs) with bottom-tracking capability have become popular to make instantaneous discharge measurements in rivers up to 50 m deep; however, their use in shallow channels less than 1.5 m deep has been limited because of design considerations of acoustic frequency, size, processing technique employed, etc. In an effort to better support shallow water measurements, SonTek has developed higher frequency systems at or above 3.0 MHZ to specifically measure in these instances; however, these will not be covered in this paper.
For continuous monitoring of flow, Doppler sensors have the distinct advantage of measuring velocity in addition to stage. This makes them particularly useful for complicated flow regimes and non-steady flow conditions where stage-area relationships do not hold up. Again, whereas the use of these sensors has become quite popular in recent years, their specific use in depths under 1.5 m has been limited to the design considerations of the instruments relative to the natural environment. These sensors can be mounted either on the bank of an open channel (side-looking) or on the bottom of a channel (up-looking). In this section we will look at both types with an emphasis on a newly released product, the Argonaut-SW.
INSTANTANEOUS MEASUREMENTS REQUIRING WADING RODS
The ADV
® has been well-established as a leading instrument for precision water velocity measurements both in the laboratory and the field. ADVs use the pulse coherent Doppler technique to make the highest precision Doppler measurements possible over a short range (typically 5 to 10 cm). The ADV’s sampling volume occupies a small cylinder (in space) approximately 0.25 cc in volume. For the practical purposes of practical stream gaging, we call this a “point measurement” even though in technical terms, it is not.
Among the advancements the ADV has over mechanical Rotating Element (RE) instruments for hydrometry are:

It can measure precise velocities from about 4.5 m/s down to about .001 m/s without the user having to change out any hardware or settings

It can measure in depths down to about 3 cm (2D probe option)

Calibrations are not required (unless physically damaged)

It measures flow that is undisturbed by the instrument’s own wake field

Its one-second velocity average is accurate to 1 % . This one second data is recoverable for looking at velocity fluctuations throughout the averaging period


It measures either two or three dimensional water velocity, thus it can compute both the velocity magnitude and direction. Unlike a RE device there is no need for a cosine of the angle correction for skew flow measurements.
Acoustic
Receiver
Acoustic
Transmitter
Acoustic
Receiver
Sampling Volume
6 mm Diameter
9 mm Height
Fixed Distance to
Sampling Volume
10 cm (nominal)
Figure 1. 2-D ADV Probe and sampling volume as typically configured on a
FlowTracker
Previous to 2001, the ADV’s limitations for use in wading applications for discharge measurement were that it consumed too much power, it required the user to pre-select velocity ranges, and it did not have an easy way to control the unit nor calculate the subsequent discharge.
Working with the USGS Indiana and Maryland/DE/DC Districts, SonTek embarked upon a program to develop the ADV into a suitable device for use by hydrometric technicians in wadeable streams (less than 1.5 m deep). Most of these innovations were centered around improving the usability of the existing ADV technology for the specific hydrometric application – taking into account the needs of the user group. Among the new features added to the ADV included:

An automatic algorithm for determining the appropriate velocity range

A low power CMOS-based circuitry based on the SonTek Argonaut platform

A keypad/display for controlling the unit and displaying data in real time

An ISO discharge calculation algorithm (mid-section method)

A 2 m flexible probe cable and mounting apparatus for wading rods
As previously mentioned, the FlowTracker uses the mid-section method to calculate the total flow. The FlowTracker, mounted onto the wading rod, is used in the traditional current meter fashion. The user types the observed depth from the graduated wading rod onto the FlowTracker keypad, and then the velocity reading is displayed second by second to the user, while also recorded directly by the instrument. The velocity averaging time is user programmable between 10 to 1000 seconds. As the user wades across the stream, the subsequent discharge is calculated station by station. The data are

recorded on the FlowTracker’s internal recorder, and can be downloaded via PC or PDA in several different formats. The non-volatile internal memory has enough capacity for approximately 64 independent discharge measurements (depending on how many individual velocity measurements make up the discharge measurement).
Figure 3. FlowTracker Handheld ADV
From observations using the FlowTracker’s one-second velocity updates, we have noticed just how critical it is to adopt a correct standing position in the river when undertaking a flow gauging. The Environment Agency in the UK undertook research about 6 years ago and came up with a number of recommendations as to the way their field staff approached a gauging - particularly in narrow, slow moving or shallow streams. The effects of standing ‘in the wrong place’ are not so obvious when you are stood there with a traditional RE device, but when you are there continuously being given an update of flow velocity, gaugers see the effects of positioning on the flow field.
The FlowTracker’s velocity accuracy had been verified in 2001 during tests at the
USGS HIF tow tank in Mississippi. The basic premise of the test was to compare the velocity of the carriage (to which the FlowTracker rigidly mounted) against the velocity recorded by the FlowTracker. Figure 5 below shows plots the results of all runs as tow speed versus measured speed.
A linear regression of all runs, comparing cart speed to measured speed, indicated a net bias of less than 0.2%-well within instrument specifications. Note that this test was carried at various cart speeds and instrument angles relative to the flow. There were a

number of individual runs where the difference between cart speed and measured speed were greater than expected. Most of these runs occurred at mounting angles of 20° and higher where errors in speed of 1-2% are typical however; speed was not consistently biased either high or low. For a full description of the test, please refer to SonTek
Technical Note “Tow Tank Testing of the Handheld ADV” (February 2001).
Figure 4a (left) shows a FlowTracker being used in an ultra-shallow stream (3-4 cm) at
Yellowstone National Park, USA. Figure 4b (right) shows the FlowTracker being used with a wading rod in a typical Chinese stream.
Tow Carriage Testing
160
140
120
100
40
20
80
60
0
0 20 40 60 80 100
Tow Carriage Speed (cm/s)
120 140 160
Figure 5. FlowTracker measured Speed versus cart speed for all runs
Subsequent use of the instrument by the USGS and other agencies has validated its use for discharge measurement. In particular, the FlowTracker has shown its advantages over mechanical equipment in shallow, low-flow environments of urban streams (Fisher,
2002).
Because the FlowTracker is typically used in the conventional wading rod manner, it lends it self to being used under a variety of channel geometries and flow velocity.
Basically, it can be used nearly everywhere traditional current meter measurements are made. This includes narrow and wide channels, and irregular, braided streams. A

present limitation is a standard cable length of two meters and thus it cannot be suspended from a high bridge.
CONTINUOUS MEASUREMENTS USING SIDE-LOOKING INSTRUMENTS
Over the past four years the use of horizontal (side looking) Doppler systems has become increasingly prevalent for monitoring velocity and/or stage in streams and canals.
Typically these are used in environments where reversing flow or other complex conditions exist, rendering the traditional stage-discharge relationship unworkable. At these sites, an index velocity relationship may be used. A channel survey (using conventional or hydroacoustic means) provides a relationship between stage and cross sectional area. A Doppler velocity sensor is installed and a relationship is developed between this reference velocity and the mean velocity in the channel. The combination of the stage-area and measured-mean velocity relationships provides the ability to continuously monitor discharge. As with stage-discharge relationships, periodic instantaneous discharge measurements are made to validate the rating. A full description of the velocity index method is outside the scope of this paper but may be found in the
USGS Water-Resources Investigation Report 01-4157 (2002, Morlock, Nguyen, and
Ross).
The side-looking Argonaut sensor is mounted on a vertical structure, and measures velocity in a programmable cell some distance into the channel. Simple installation, low maintenance and the ability to monitor velocity away from flow interference generated by underwater structures are advantages of these sensors.
Figure 6. Cross-sectional channel view of a typical Argonaut-SL installation
Table 1. Specifications on channel widths for Argonaut-SL Doppler velocity meters.
Given mounting apparatus and environmental conditions, conservative estimate on minimum channel width is more likely about 2 m for 1.5 MHz and 1 m for 3.0 MHz
Minimum Blanking Distance
1.5 MHz Argonaut-SL 3.0 MHz Argonaut-SL
50 cm 20 cm
Minimum Cell size 40 cm
Instrument cross section (M/X model) 10/18 cm
Minimum measureable channel width 140/148 cm
20 cm
10/18 cm
70/78 cm

As with the Doppler profiling equipment where higher frequencies are preferred for smaller channel geometries, the same is true with horizontal measurements. Thus,
SonTek offers the Argonaut-SL in frequencies of 1.5 and 3.0 MHz for this purpose.
Some specifications are shown in Table 1.
In typical installations, side-looking units are mounted at approximate mid-channel depth to achieve the maximum horizontal range. Thus in channel depths of 1.5 meters, the instrument should be located 0.6 to 0.9 m above the bottom for optimal performance.
This also ensures that units equipped with a vertical acoustic beam have adequate clearance to make a stage measurement. For general practice, the user should allow about 0.3 m above the instrument head.
Thus, side-looking sensors are very useful for shallow streams or channels where the water level does not come near to or below the level of the sensor itself. Recent innovations include the ability to program the unit with the channel’s cross sectional area in order for the Argonaut to internally calculate the flow based on its measured stage and velocity. Its limitations include areas of highly variable water level (which would leave the unit dry) or highly stratified flow requiring measurement at more than one level
(which would require additional instruments).
CONTINUOUS MEASUREMENTS USING AN UP-LOOKING ARGONAUT-SW
In a number of small, low head-loss channels (often used for in irrigation, though there are many others) hydrometrists have tried a number of methods to obtain continuous or near-continuous flow measurements. This includes the use of flumes and weirs often being installed in situations where discharges are very low. Flumes are specially manufactured a for the better accuracy of a trapezoidal profile. Weirs have often been of the thin plate variety (v-notch and horizontal). The trapezoidal flumes are more difficult to build and costly to install but allow better measurements throughout the range of flows without having to introduce large head losses. The thin-plates weirs, though offering good accuracy, can create large areas of backwater and have the ever present ‘threat’ of lost accuracy (and further backwater effects) by becoming clogged with debris, not to mention the issues of drowning when the downstream levels rises because of ‘unforeseen reasons’. This seems to point well to the use of the side-mounted Doppler velocity sensors, where there is not the need to concern ourselves with head-loss or backwater effects. However, the difficulty of maintaining water depth or mounting a relatively large sensor in a small channel motivated the development of a new Doppler velocity sensor, the Argonaut-SW (SW for “Shallow Water”). The Argonaut-SW is a bottom-mounted system that is intended for complex index velocity sites (those with large stage variation or stratified flow), and for sites where purely theoretical discharge calculations are desired.

Figure 7a (left) shows the Argonaut-SW. Figure 7b (right) shows an installation site at an irrigation channel
The Argonaut-SW was designed with the following basic considerations:

Operation in a wide range of water depths, with the minimum depth less than 0.3 m.

A vertically-integrated velocity cell covering most of the water column.

Accurate water level measurement
The Argonaut-SW uses two acoustic beams for velocity: one pointed upstream and one pointed downstream. The instrument is aligned with the axis of the channel. Small errors in alignment have negligible effect on velocity data since the velocity error is proportional to (1-cos(q)) where q is the error in alignment angle. Using two beams for velocity, instead of a single beam aimed forward, greatly reduces sensitivity to tilt angles in the installation. A third acoustic beam is aimed vertically up and is used to measure water level based on the timing of the reflection from the surface. The Argonaut-SW beam configuration is illustrated in fig. 8.
Cell End
Beam 1
Velocity
Vertically
Integrated
Velocity
Cell
Beam 3
Water
Level
Beam 2
Velocity
Flow
Cell Begin
Argonaut-SW
Figure 8. Argonaut-SW Beam configuration
To adapt to changing water level, the Argonaut-SW uses water level data measured by the vertical acoustic beam. The size of the sampling volume is automatically adjusted in real time to allow the Argonaut-SW to measure the greatest possible portion of the water column. The velocity measurement starts 7 cm above the sensor head (the acoustic

blanking distance of the Argonaut-SW), and continues to the water surface. The instrument returns a single integrated velocity value representing an average over this portion of the water column. An optional feature is also available that returns velocity in up to 5 user programmable cells through the water column.
Because its intended use was geared towards small open channels, irrigation ditches, and culverts, the SW was designed to be as compact and low-profile as possible. Using an acoustic frequency of 3.0 MHz, its housing size is 24.6 x 10.2 x 6.4 cm.
0.8
Argonaut-SW / FlowTracker Velocity Comparison
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Argonaut-SW
FlowTracker ADV
0
0 5 10 15 20 25 30
Velocity (cm/s)
35 40 45 50
Figure 9. Velocity Profile Comparison
A key technical innovation in the Argonaut-SW that separates it from other Doppler sensors is that measurements are made all the way to the water surface without any contamination normally associated with side-lobe interference. This allows the SW to take full advantage of the vertically-integrated velocity in its internal flow calculations based on the channel cross-sectional area. Test data demonstrating this effect are shown in Figure 9.
These data were collected in a re-circulating flume with water depth of about 0.7 m.
The Argonaut-SW was mounted on the bottom, slightly off center. A FlowTracker ADV was manually raised and lowered to measure the current profile at the same location along the length of the flume; the ADV measurement location was 0.2 m to the side of the Argonaut-SW. The data shown in fig. 9 represent the average profile over a period of more than 1 hour.
The offset between Argonaut-SW and ADV velocity data (~2 cm/s) is attributed to variations in the velocity field across the width of the flume, and is consistent with other flume data. The important comparison is the consistent shape of the velocity profile, particularly in the top half of the water column. The Argonaut-SW is able to accurately measure water velocity all the way to the water surface, with no evidence of sidelobe interference. Tests in other flow conditions have shown similar results, with no evidence of sidelobe interference.

The Argonaut-SW has been designed to provide a robust, accurate velocity measurement for complex index velocity sites, and to give the most accurate theoretical flow data possible at sites where no index calibration is possible. The instrument is suited for a wide range of water depths from 0.2 to 5.0 meters, and can be operated with minimal user training. Its limitations include all those associated with bottom-mounting instruments, which include loss due to burial or flood, more difficult installation
(compared to side-mounting) and potentially cumbersome cable runs.
CONCLUSION
There have been a number of developments in recent years that have resulted in new and better Doppler equipment for both instantaneous measurements of flow, and full-time monitoring in channels less than 1.5m. Because of the wide variety of natural and manmade channel conditions, the user must carefully consider all aspects of an instrument’s capabilities and limitations before selecting which one to use. Fortunately, due to the advances in high frequency acoustics, there are now several alternatives to consider for flow situations previous thought to be immeasurable. This has resulted in higher data confidence, increased personnel efficiency and better service to the public for real-time water supply information.
REFERENCES
[1] Fisher, G.T., and Morlock, S.M., 2002, Discharge Measurements in Shallow Urban
Streams: in Hydraulic Measurements and Experimental Methods, Environmental and
Water Resources Institute of the American Society of Civil Engineers, Estes Park,
Colorado, July 28-August 1, 2002, Proceedings.
[2] Huhta, C., 2002, Handheld Acoustic Doppler Velocimeter (ADV) for Water Velocity
Surveys: in Hydraulic Measurements and Experimental Methods, Environmental and
Water Resources Institute of the American Society of Civil Engineers, Estes Park,
Colorado, July 28-August 1, 2002, Proceedings.
[3] Huhta, C., and Ward, C.J., 2003, Flow Measurements using an Upward-Looking
Argonaut-SW Doppler Current Meter: in IEEE/OES Seventh Working Conference on Current Measurement Technology, San Diego, CA, March 10-13, 2003,
Proceedings.
[4] SonTek, ADP, Argonaut, and ADV are registered trademarks of SonTek/YSI Inc.
San Diego, CA, USA