How to Measure the Water Current of the Saint Louis River

1. Where is the Saint Louis River

The Saint Louis River is located in the upper Midwest of the United States. It flows from its headwaters in Minnesota into Lake Superior.

The river basin encompasses different stretches of landscape, such as forests, wetlands, and urban areas. Downstream, it provides significant living environments for a large and diverse array of fish-the trout and walleye-waterfowl, and mammals. The river, therefore, has immense cultural, historical, and economic relevance. There are ports and industrial works along its banks, although in the past, this would have been used as part of transportation and trade routes.

2. Flow Rate Characteristics

The Saint Louis River undergoes several factors that take into consideration its flow rate. One of the obvious reasons includes seasonal changes. In spring, for example, there is a vast increase in flow due to the melting of snow at the upstream parts. It results in a rise of water level and strength in current. This period is most favorable for sediments and nutrient transportation downstream.

Through summertime and into the autumn period, it stabilizes somewhat by reason of rainfall. Conversely, in winter, its rate of flow may be quite reduced, though it almost never dries up in general because of groundwater supply and the release of previously stored water in the course of the river. The topography of the river basin also influences the average flow rate since it determines the way water is collected and drained. Besides, human -induced factors, such as water withdrawals for industrial and domestic uses.

3. Methods to Measure Water Current

Velocity Meter Method

Traditional methods of measurement involve the use of mechanical or electronic velocity meters. These meters are placed at certain points in water to measure the speed of water passing by a given point. However, for a proper understanding of the current, several placements at different locations and depths will be required. This may be very time-consuming and may not give a continuous profile of the current throughout the water column.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current meter method is a more current and efficient means of water current measurement. It detects the simultaneous velocity of water in different depths using sound waves. Thus, by emitting acoustic signals, it analyzes the Doppler shift of the reflected signals toward building a detailed profile of the current from the surface to the riverbed. This provides a much more accurate and wider measurement of the water current across sections of the river.

Buoy Method

The principle of this method is to set buoys in the river and monitor their movements over a certain period. The displacement of the buoys by water current gives an indication of the speed and direction of flow. However, this method has its limitations. It may not provide very accurate velocity measurements at different depths and can be affected by external factors such as wind and waves, which influence the movement of the buoys.

Among the mentioned methods, the ADCP current profiler is normally more advanced and serviceable in the measurement of the water current of the Saint Louis River.

4. How ADCPs Using the Doppler Principle Work

The basic operating principle of ADCPs is based on the Doppler effect. When an acoustic signal from the ADCP flow meter transducer is emitted into the water, the sound waves come into contact with the moving particles of water. In case of flowing water, the frequency of the reflected sound waves back to the transducer changes, commonly referred to as the Doppler shift.

If the water is moving towards the transducer, then the frequency of the reflected waves will be higher than the emitted frequency; if the water is moving away from the transducer, the frequency will be lower. By precisely measuring this frequency shift at multiple angles and depths, the ADCP can calculate the velocity of the water in different directions and at different levels within the water column. This allows it to produce a detailed profile of the water current, hence delivering useful information on the speed and direction of the flow from the surface to the riverbed.

5. Requirements for High-Quality Measurement

The measuring equipment used to measure the currents in the Saint Louis River should possess some characteristics for high-quality measurement.

The materials used to build it must be such that the results obtained from it are accurate and consistent. A small size is beneficial as it allows for easier deployment and retrieval in the river, especially in areas with difficult access.

The equipment should also be lightweight to simplify the installation process and reduce the effort required for handling.

Low power consumption is essential to ensure that the equipment can operate for extended periods without the need for frequent battery replacements or a complex power supply setup.

Cost-effectiveness is another important factor as it enables more widespread use and larger - scale measurement campaigns.

When it comes to the casing of the ADCP profiler, titanium alloy is an excellent choice. Titanium alloy has many advantages. It is highly resistant to corrosion, which is very important because the equipment is always in contact with water that may contain different minerals, sediments, and pollutants. It also has a good strength-to-weight ratio, which allows for a durable yet lightweight casing. Besides, it is resistant to the mechanical stresses occurring at the time of its deployment and operation in the flowing water of the Saint Louis River.

6.Selection of Appropriate Equipment for Measurement of Current

While selecting appropriate equipment for the measurement of Saint Louis River current, two aspects must be considered.

Based on the purpose of usage, in the case of the horizontal cross-section measurement, HADCP is suitable. It is designed to measure the flow velocities across a horizontal plane in the river with accuracy for understanding the lateral distribution of the current. In the case of vertical cross-section measurement, the Vertical ADCP is more appropriate since it focuses on profiling from the surface to the bottom along a vertical line in the water column.

Different frequencies of ADCPs are suitable for different water depths. As an example, an ADCP meter of 600 kHz will suit water within 70 m. It can hence be used to give meaningful current measurements in the fairly shallow parts of the Saint Louis River. An appropriate choice for deeper waters up to 110 m is an ADCP of 300 kHz and will be useful in areas with a greater depth of river.

There are well-known brands of ADCPs such as Teledyne RDI, Nortek, and Sontek. However, for those looking for a cost-effective option with good quality, the China Sonar PandaADCP is highly recommended. It is made of all-titanium alloy material and offers an incredible price-performance ratio. You can find out more about it on its website: https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.