An alternative to mechanical current meters for making wading discharge measurements is hydroacoustic current meter technology. Hydroacoustic current meters use sound transmitted in the water to measure water velocities. The basic principles of hydroacoustic water-velocity measurements are
The goal of this project is to adapt existing commercially available hydroacoustic current meters to the task of making wading discharge measurements. The scope of the project is to adapt two different types of instruments:
Advantages of these hydroacoustic current meters over mechanical current meters include
Disadvantages of these hydroacoustic current meters include
ADV's (acoustic Doppler velocimeters) are high-precision, point-velocity current meters. They are "point-velocity" meters because they measure velocities within a very small volume. ADV's use a complex "pulse-to-pulse coherent" sampling scheme that gives the instruments high precision in velocity measurements. Because of the precision with which they measure velocities, ADV's often are used to measure fluid velocities in laboratory settings. An ADV consists of a probe head connected by a stem to a cylindrical electronics housing. The probe head houses bistatic transducers ("bistatic" refers to the fact that the transducers either only transmit sound pulses or only receive sound pulses).
ADV
Photograph courtesy of SonTek Corporation.
The probe head consists of a transmitting transducer in the center and two or three probes containing receiving transducers. The center transducer transmits sound pulses, which reflect back to the receiving probe transducers. Velocities are measured within a sampling volume located at a fixed distance from the transmitting transducer, as illustrated below.
Probe head graphic courtesy of SonTek Corporation.
With three probes, an ADV measures velocities in three dimensions. A two-probe head measures velocities in two dimensions. An ADV would need to have a specially configured probe head for discharge measurements in a stream. Such a probe head is pictured below.
Custom ADV probe head for discharge measurements.
Photograph
courtesy of SonTek Corporation.
The probe head would be submerged with the center transmitting transducer pointed across the stream, perpendicular to the flow. The bottom probe with the red band would be positioned on the downstream side of the probe head:
The bottom two probe heads would measure two-dimensional velocity components (an across stream or y-component and a downstream or x-component). The third vertical probe would measure the vertical or z-component of velocity (this component would not be necessary to measure discharge).
The x-component of velocity would be used to compute discharge; so if only the bottom two probes were submerged, discharge could still be measured with the ADV.
This custom probe head is called a "2D/3D" head. The 2D/3D head gives the ADV the capability of measuring two-dimensional velocities in depths of under 1 inch.
Commercially available ADV's are configured for data collection with a personal computer (PC) or for internal data collection and storage. For the wading-discharge-measurement application, a custom interface would need to be designed. The interface would need to be small for portability and would need to take depth and subsection distance inputs to compute discharge. It would need to output pertinent data from the discharge measurement.
The potential for an ADV to measure velocities with high precision, coupled with the potential for shallow depth operation, suits the instrument for discharge measurements in streams under low-flow conditions.
Acoustic Doppler profiling current meters can measure vertical velocities over most of the distance between the water surface and streambed. A profiler is typically a tube-shaped instrument with a transducer head containing two or three monostatic transducers. "Monostatic" refers to the capability of the transducers to transmit and receive sound pulses.
An acoustic Doppler profiler current meter with three monostatic
transducers in the transducer head.
Photograph courtesy of Nortek Corporation.
A profiler transmits sound pulses down through the water column and receives sound pulses back from particles in the water. The profiler measures velocities in discreet, evenly spaced depth cells. After transducers transmit a sound pulse, for short time they "ring," and cannot receive any reflected pulses. This creates a "zone," called the "blanking distance," under the transducers in which velocities cannot be sampled. There is a second, bottom unmeasured zone near the streambed caused by a phenomena called "side-lobe interference." Side-lobe interference occurs when some of the energy from a sound pulse strays outside the main path of the sound pulse. This stray energy can strike the bottom before the main sound pulse and then can reflect from the bottom into the main sound pulse. Because the reflection from the bottom is a very strong pulse compared to reflections from particles in the water, the bottom reflection tends to overwhelm particle reflections within the bottom zone. Because velocity measurements depend upon particle reflections, velocity measurements are not reliable where the bottom reflections have overwhelmed the particle reflections.
Sketch of profiler operational features.
A profiler, like an ADV, can measure velocity components in three dimensions with three transducers and in two dimensions with two transducers.
To be adapted to wading-discharge-measurement application, a profiler could be mounted in a small flotation device. The profiler would be moved across a stream and held at each subsection while velocities were measured. To compute discharge within a subsection, velocities in the unmeasured zones would need to be estimated by extrapolation from the measured velocities. Velocities can be estimated by fitting theoretical vertical velocity curves to the measured velocities. Because the sound transmitted by a profiler strikes the streambed and returns a strong echo, the profiler can measure depths as well as velocities.
Because a profiler measures velocity profiles, it would not need to be set at a particular depth. With a custom transducer head for the wading-discharge-measurement application, a profiler potentially could be used in depths as low as 6 inches.
Like ADV's, commercially available profilers are configured for data collection with a PC or for internal data collection and storage. For the wading discharge measurement application, a custom interface would need to be designed. The interface would need to be small for portability and would need subsection-distance inputs to compute discharge. It would need to output discharge data.
It should be noted that this profiler does not have any type of bottom-tracking feature. Thus, the instrument is for the stationary measurement of velocities only. It is not intended for deployment from a moving platform.
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| Introduction |
| Wading Discharge Measurements |
| Hydroacoustic Technologies for Wading Discharge Measurements |
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| Project Plans |
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| References |
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