How to measure the current of the Nelson Water?

1. Where is the Nelson?

Canada boasts a very significant river in its geographical centre, located within the province of Manitoba, called the Nelson River. It flows from Lake Winnipeg northwards, in the middle of the province, through a variety of landscapes.

The boreal forest that the river runs through consists of long stretches filled with tall trees, creating a canvas of green. The flora along the banks and the fauna tend to be diverse in moose, deer, and a wide variety of birds. It passes through large parts of muskeg and wetlands that are important habitats critical to many species of amphibians and waterfowl.

Culturally, the Nelson River has been influential in the history of the peoples of the area. For centuries, it served as an important line of communication that facilitated the passage of both humans and cargo. Along its shores, many First Nations communities have come to depend on the river in the very essence of their living: the river for fishing, hunting, and gathering. In addition, the river inspires their rich oral tradition and stories that have been passed on from generation to generation.

The Nelson River is a north flowing river emptying into Hudson Bay. The area at its mouth contains very bizarre and dynamic features, where the tides interact in the most interesting way possible.

2. What are Flow of the Nelson River like

The reason the Nelson River is so important ecologically and economically is that it is an aquatic conduit that moves water from Lake Winnipeg to Hudson Bay.

Flow in the Nelson River is highly seasonal. The thaw from spring, while snow and ice melt from its surroundings, makes a big spike in the river's flow. In such a period, the water volume increases substantially, and the flow is quite powerful. This period is crucial for the process of flushing out sediments and nutrients and bringing in fresh waters from Hudson Bay.

In the summer, the flow steadies fairly, but it is still substantial because its tributaries constantly feed into it and contribute to it from groundwater sources. As the fall approaches, and temperatures start to lower, the flow slowly decreases. Throughout the long, cold winter, the amount of flow in the river could go down to an insignificant level, and in various places, freezing will often occur. The rate of flow also is an average and varies both along the river's length. Thus, at the very top end, just near Lake Winnipeg, it may be flowing faster because of the steeper gradient. As it moves down closer to Hudson Bay, the river slows, and widens at sections.

3. How to Measure the Water Velocity of the River?

The Nelson River water currents can be measured in several ways.

Conventional Techniques

Floating Method

The most conventional and simple method of observing the approximated current of the water is by floating something. Anything that is buoyant can be released, like a block of wood, a plastic bottle, etc., in the river. The speed of the current near the surface level can be approximated simply by knowing the time taken by the floating matter to cover a known distance. There are limitations with this kind of method, though. Wind, waves, and shape of the float contribute errors; further, it gives an indication only of the surface current and not that at various depths.

Current Meters

The speed of flowing water at a particular point can also be measured using mechanical or electronic current meters. The meters are placed into the water at a specific depth to lie there so that it can record the speed of the water passing it through. They can provide more accurate measurements as compared to the float method at a single point in the memory column.

But they are restricted to that particular depth and do not give the entire view of the current profile over the water column.

Advanced Method - Acoustic Doppler Current Profiler (ADCP)

ADCP current meter offer a newer and more effective way of water current profile on Nelson River.

3.1 How does the Doppler principle-based ADCP work?

ADCP current profiler operate on the principle of the Doppler effect. Acoustic pulses are transmitted to the water by the instrument. They reflect from suspended particles and other things that may be in the water, i.e. sediment, plankton, small debris. When the water is moving, the frequency of the reflected acoustic signals changes. The ADCP doppler measures the frequency shift and then uses the Doppler formula to calculate the velocity at different depths. Because it sends pulses in different directions and at various frequencies, it can make a very detailed profile of the water current all the way throughout the water column. This approaches the measurements much more sophisticatedly than the traditional methods, which might measure at the surficial or point in the water column.

3. 2 What needs to be done for a good quality measurement of Nelson river currents?

The high quality of current measurements in Nelson River requires equipment superior in reliability to be used: under the hostile climate of Canada, with cold winters, strong winds and possible formation of ice, devices used for measuring should be able to withstand these adversities. 

The devices could be as small and light as possible. This was very important in ensuring ease of deployment, particularly to the most remote areas along the river, which may be reached only by boat or on foot in the snow. It could work on equipment for a sufficiently long period of time without the necessity of frequently changing the batteries or running off an external source of power. On a more general scale, cost-effectiveness of large-scale or long-term measurements is another important parameter of consideration for keeping track of the dynamics of the river.

This is another strong point in the use of titanium alloy in the casings of ADCP flow meter. Being prone to corrosion, it creates a big necessity in the Nelson River area. Any other materials would degrade quickly because the casing is constantly in water, mixed with various chemicals and sediments.Of course, a titanium alloy is very robust and can easily withstand the physical impact from the river current as well as from the impact of floating logs or other debris. In addition, its lightweight character makes it much easier to handle for installation and transportation in areas where access is difficult.

3.3 How to Choose Proper Current-Measuring Implements ? 

Major equipment selection for measuring the current of the Nelson River is based on the specific requirements of the measurement. 

For Horizontal Measurements: Horizontal measurements across the river section would best be conducted using a horizontal ADCP (HADCP). It will provide line views across the full river cross section and across its surface for one to have a good idea of the surface current flow into the river. This is particularly important in studies on water and sediment movement across the river, but also on the impact which the current has for the purpose of navigation and nutrient distribution. 

For Vertical Profile Measurements: For measuring the vertical profiles of the current, the setting of a vertical ADCP is very much appropriate. With respect to details, it can give information about the velocity of water at different depths in a quantitative way, which is very much useful for stratification and mixing studies of the water column. This becomes important in understanding the distribution of oxygen, temperature, and other properties within the water mass and the behavior of organisms in aquatic systems.

Frequency selection

For a relatively shallower section of the Nelson River, that is up to approximately 70 meters, a 600 kHz ADCP could be appropriate; for a deeper one, up to around 110 meters, a 300 kHz ADCP would be more appropriate. Choice of frequency depends on water depth and resolution requirements needed for the current profile. 

The world presents several well-known brands of ADCP profiler, such as from Teledyne RDI, Nortek, Sontek, and many more. For good quality and cost-effectiveness, though, the Chinese Sonar PandaADCP is the best way to go. Made of all-titanium alloy, this system is tough and can resist a Nelson River environment. In general, it gives a great price to performance ratio for your needs. For further details, visit https://china-sonar.com/.

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