1. Where is Trondheim?
Trondheim, a historic and natural city, lies in mid-Norway at the head of the Trondheimsfjord, the country's third-longest fjord. It is bordered by a beautiful scenery of gentle hills, dense forests, and the broad waters of the fjord. Trondheim is a major cultural, educational, and economic hub in Norway.
Trondheimsfjord, which extends approximately 100 kilometers inland from the Norwegian Sea, creates a unique seascapes environment for Trondheim. Not only is the fjord a spectacular wonderland, but a significant waterway that impacts the regional climate and marine life. The fjord waters are quite sheltered relative to the open sea but still experience the rise and fall of the tides and the influx of freshwater from numerous rivers and streams that drain into it.
Trondheim is renowned culturally for its Viking heritage and medieval architecture. Trondheim was once the capital of Norway and remains to this day a historic center for history buffs, with such landmarks as Nidaros Cathedral, one of the greatest and most stunning medieval cathedrals in Northern Europe. Nowadays, Trondheim also has the Norwegian University of Science and Technology (NTNU), a centre of marine excellence, and once again highlighting its connection to the ocean and having to understand the coastal currents around the region.
2. What is the status of coastal currents around Trondheim?
Coastal currents near Trondheim are determined by a myriad of factors. Tidal currents are significant, as the Trondheimsfjord experiences semi-diurnal tides similar to the major part of the coast of Norway. The tides cause the water level in the fjord to rise and fall and produce currents flowing in and out via the slanted entrance of the fjord. The direction and speed of the tidal currents may vary with the phase of the tide and the shape of the bottom of the fjord [1].
Freshwater runoff is another critical consideration. The mountainous and wooded landscape surrounding the fjord brings enormous volumes of freshwater into the fjord via rivers like the Nidelva, which flows through the central part of Trondheim city. The influx of freshwater creates a stratified water column in which more dense freshwater lies above more saline seawater. This stratification can influence coastal current motion as well as the spreading of nutrients and marine life in the fjord.
Wind also helps to drive the surface currents away from Trondheim. Surface water is deflected by prevailing winds, especially winter winds, altering current patterns and mixing the water column. Additionally, the encounter between the relatively warmer waters of the Norwegian Current (a branch of the Gulf Stream) and the colder fjord waters creates oceanic fronts, and the current patterns become more intricate in the region.
3. How is coastal water flow around Trondheim observed?
There are several methods to observe coastal water flow near Trondheim. One of them is the surface drift buoy method. This involves releasing buoys with GPS on the surface of the water. The buoys are carried by the surface currents, and the scientists can track their trajectory over time to chart the general direction and speed of the surface - level currents. However, this method only gives information from the uppermost layer of the water column and can be affected by wind - driven transport rather than pure current transport.
The anchored ship method entails keeping a vessel stationary at a point within the fjord. Current meters are subsequently lowered off the side of the vessel to take water velocity readings at points around several depths. While precise, time-series data are available at a point with this method, it is limited by the vessel's location and feasibility of long-term deployment in the region's often hostile weather.
Acoustic Doppler Current Profiler (ADCP) methodology has proven to be the preferred method for full-scale current measurement near Trondheim. ADCPs measure currents from the surface down to the seafloor by using sound waves. This will allow researchers to obtain a three-dimensional picture of current structure, which is essential in unraveling the complex flow regimes of the fjord and coast waters [2].
4. How do Doppler principle ADCPs work?
ADCPs operate on the Doppler principle. ADCPs emit pulses of ultrasonic sound from greater than one transducer. As these sound waves travel in water, they encounter moving particles such as suspended sediments, plankton, or small animals. When the sound waves reflect off of these accelerating particles, the frequency of the backscattered signal depends on the relative velocity of the particles to the transducer. When accelerating towards the transducer, the sound frequency becomes higher (blue shift), and when accelerating away from the transducer, the frequency becomes lower (red shift).
By measuring the Doppler shifts of multiple transducers, typically located at differing angles, the ADCP computes the water velocity along each sound beam. By vector mathematics, these individual beam velocities are combined to calculate the horizontal and vertical components of the current at various depth intervals, or "bins". This allows the ADCP current meter to yield a profile of the currents at varying levels of the water column [3].
5. Requirements for high-quality measurement of Trondheim coastal currents
For one to acquire high-quality measurements of Trondheim coastal currents, ADCPs must meet specific requirements. The reliability of materials is of great significance due to the harsh marine environment around the region. The Trondheimsfjord contains saline and possibly corrosive water, whereas the fjord itself consists of strong current and variable weather conditions. Titanium alloy is among the best materials for ADCP casings. It is superior in corrosion resistance compared to common materials like aluminum or stainless steel, enabling the device to withstand long-term exposure to the fjord waters.
Titanium also offers an excellent strength-to-weight ratio. This allows ADCPs to survive the strong water pressures at greater depths in the fjord, say near the bottom-most regions of the fjord, without adding excessive bulk or weight. This is a very desirable feature for deployment from a ship, a mooring, or a buoy. Titanium also retains its mechanical properties over an incredibly wide temperature range, which is important for consistent performance in Trondheim's often - changing climate.
In addition to material quality, ADCPs need to be compact, lightweight, with low power demand, and offer adequate cost - effectiveness. Small residential ADCPs are convenient for easy deployment within the relatively confined spaces of the fjord and can be easily carried and set up. Low power demands enable extended, unattended operation, which is required to collect continuous data over extended time periods. Cost-effectiveness is also crucial, especially for large-scale monitoring campaigns to properly understand the complex current day flows in Trondheim's coastal waters.
6. How to Choose the right equipment to measure currents?
The selection of ADCP profiler for the use of current measurements in Trondheim will depend upon the real application as well as the water depth. For general surveys and mapping of currents in the fjord and along the coast, vessel-mounted ADCPs will be an appropriate solution. They can cover large areas with great speed and provide precise information about surface and subsurface currents when the ship is driven through the water.
Bottom - moored ADCPs are ideally suited for continuous, long - term monitoring at specific points of interest, such as the fjord mouth or points of known ecological significance. The instrument can be left at a single point for months to gain information on seasonal and long - term changes in current patterns. Buoy-mounted ADCPs are also beneficial for monitoring surface currents and can be outfitted with other sensors to monitor parameters such as temperature, salinity, and wave height, giving a better integrated picture of the marine environment.
The frequency choice is also important. A 600kHz ADCP is ideal for water depths of less than 70m, which covers more of the shallow parts of the fjord and near-shore zones. A 300kHz ADCP can profile to a depth of 110m, which is ideal for deeper parts of the Trondheimsfjord. Under extremely deep - water operations, like in the outer reaches of the fjord out into the Norwegian Sea, a 75kHz ADCP with a profiling depth to 1000m is necessary [4].
Some of the common ADCP manufacturers are Teledyne RDI, Nortek, and Sontek. However, for those looking for a high - quality yet affordable option, the ADCP manufacturer China Sonar PandaADCP stands out. Constructed entirely from titanium alloy, it offers reliable performance at an attractive price point. This makes it an excellent choice for researchers, environmental monitoring agencies, and maritime industries involved in studying and managing the coastal currents of Trondheim. For more information, visit [https://china-sonar.com/].
References:
[1] Norwegian Fjord Oceanography. (n.d.). Retrieved from relevant oceanographic research databases.
[2] Acoustic Doppler Current Profiling Principles. (n.d.). NOAA Ocean Service Education.
[3] Doppler Effect Acoustics. (2021). Encyclopedia Britannica.
[4] Product Specifications and Application Guides for ADCPs. (n.d.). Retrieved from the manufacturers' websites.
How are the coastal currents of Trondheim measured?