River Discharge and Water Supply Canal Monitoring with Argonaut-SL

The SonTek/YSI Argonaut current meters are world-renown for their compact size, high accuracy, ease of use, and low power consumption. These instruments can be classified as range-integrating current meters that use the acoustic Doppler technique to precisely measure water currents. The Argonaut is presently available in three models:

• Mooring-Deployed Argonaut-MD - Designed for offshore mooring applications
• Extended Range Argonaut-XR - Intended for shallow-water current measurement
• Side-Looking Argonaut-SL - Designed to be used where beams need to be directed away from boundaries or objects that might cause flow interference

This article describes a practical approach to carrying out river gauging using an Argonaut-SL, which provides considerable improvement in velocity accuracy over travel-time systems, while reducing operational costs. Also discussed are some advantages as compared with more traditional practices.

Previously Used Techniques

Most commonly, river flow is estimated by measuring water velocities at several locations across a channel, and then integrating these velocities over the cross-sectional area of the river. This approach involves extensive spatial mapping of the flow (usually several times a year) which produces what is known as a calibration curve; that is, the total discharge as a function of the water level and velocity at one particular location (e.g., center of the channel). This calibration information is later combined with real-time velocity measurements, yielding an estimate of the discharge between the calibrations.

Alternatively, in the case of a small canal or waterway, an electromagnetic coil-based system can be implemented that provides true discharge without relying upon any assumptions (although the exact channel cross-section needs to be known). However, this method requires significant installation efforts, is rather costly, and is not very flexible when it comes to changing the location of the measurements.

When the channel cross-section is regular and of a known shape, an estimate of the discharge can be obtained by simply measuring the averaged flow either across the channel at some depth or over depth in the center of the channel.

Acoustical travel-time systems have been previously deployed to provide a path-average velocity across a river or a channel. Their installation involves an extensive site survey, accurate positioning, and careful alignment of both the transmitter and the receiver along with a cable connection between the two sides. This is rather expensive, time consuming, and can be hazardous. Travel-time systems are also susceptible to fouling and require continuous maintenance and periodic calibration. When the water level changes significantly, the acoustic path traverses at a different depth, relative to the water level, which may affect discharge calculations. In addition, in order to take measurements at a different location the whole procedure needs to be repeated each time.

Using the Argonaut-SL for River Discharge

The Argonaut-SL range-integrating acoustic Doppler current meter provides an attractive alternative to traditional methods of measuring river discharge. The Argonaut-SL features state-of-the-art Doppler technology, remote sampling, resistance to fouling, and requires no calibration. The most unique feature of the Argonaut-SL is adjustable location and size of the sampling volume (i.e., the volume from which the velocity measurements are made). The sampling volume can be pre-programmed by the user to be as close as 0.25 m away from the sensor and can extend as far as 15 m. The Argonaut SL will then compute a mean velocity of the water within this volume. While mechanical or electromagnetic current meters are limited to measuring flows in the immediate vicinity from the sensor, the Argonaut-SL allows a user to extend the measurement area away from the instrument and avoid signal contamination due to boundaries or mounting structure (Figure 1a).

(a)

(b)

Figure 1.
(a) Monitoring channel flow with an Argonaut-SL pointing across
(b) Flow monitoring with a bottom-mounted Argonaut-SL

Another unique feature of the Argonaut-SL is AutoTide^TM. A bottom-mounted Argonaut-SL (Figure 1b) detects the location of the surface for each sample using an integrated pressure sensor and automatically adjusts the extent of the sampling volume accordingly. This is designed to avoid data contamination from acoustic signals reflecting from the surface when the water level changes with the tide or river stage. In addition to measuring velocity, the Argonaut simultaneously records the water level.

A bottom-mounted Argonaut-SL with the beams oriented in the along-flow direction will provide a depth integrated velocity (Figure 1b) while the Argonaut-SL mounted on a bridge piling or a riverbank (Figure 1a) gives a range-averaged velocity that can be incorporated into the discharge calculation scheme in real time. In both configurations, the Argonaut-SL's integrated pressure sensor also provides the water level required for discharge calculations. If the channel cross-section has a known regular shape, this velocity/water level can be combined with the existing analytical or numerical model to estimate discharge. In cases where the channel has an irregular shape, the Argonaut-SL can provide an index velocity for real-time discharge monitoring in between cross-sectional calibrations of the channel.

Given the small size and weight, the installation of the Argonaut current meter is fairly straightforward and can be done without extensive surveys or special equipment. The Argonaut-SL can report data in real time when connected to a shore computer via cable, or it can record data internally when outfitted with external batteries. When compared to cumbersome, traditionally deployed travel-time systems, the easy-to-use Argonaut-SL saves time, money, and headaches.

• Argonaut-SL