1. Where is Oslo?
Oslo, Norway's capital and biggest city, is situated on the Oslofjord's northern rim, a deeply entrenched and narrow inlet that stretches approximately 100 kilometers into the southeastern coast of Norway. The fjord, lined by steep cliffs and high, forested hills, forms an impressive and scenic background against which the city stands. This unique geography not only makes Oslo a cultural and economic hub but also an entrance to the verdant marine world of the fjord and beyond to the North Sea.
The Oslofjord itself is a twisted mass of water, with its relatively sheltered inner segments slowly opening onto the broader and more exposed outer reaches that flow into the North Sea. Fjord waters are of varying depth, with areas up to 500 meters deep in certain locations around its mouth. The Oslo coast is a mixture of sandy beaches, rocky outcroppings, and small coves, providing diverse habitat for a multitude of marine organisms.
Culturally, Oslo is a city that combines the new with tradition. There are numerous museums there, such as the Viking Ship Museum, which showcases the long Viking heritage of the city. The city is also diverse when it comes to buildings, ranging from ancient wooden houses in the Grünerløkka area to contemporary skyscrapers in the Bjørvika area. The connection of Oslo to the sea is evident in its crowded harbor, which serves as a pivotal hub of commerce, fisheries, and tourism, making knowledge of the coastal currents around Oslo an even greater priority.
2. How is the status of the coastal currents around Oslo?
The Oslo coastal currents are influenced by a variety of factors. The tidal forces are powerful, and semi-diurnal tides with two high and two low tides daily influence the Oslofjord. The tides produce ebb and flow currents that enter the fjord and then leave the fjord, and the strength and direction of the currents rely on the phase of tides and bottom morphology of the fjord [1].
Freshwater flow from mountains and rivers surrounding also drastically influences the coastal currents. The Numedalslågen River, one of the notable rivers outfalling into the Oslofjord, provides huge volumes of freshwater, especially during the spring flood season. This input of freshwater creates a stratified water column where the freshwater, being less dense, lies on top of the denser saltwater. This stratification can potentially affect the vertical transport of the currents and mold the concentration of nutrients and sea creatures in the fjord.
Wind systems are also a significant factor in controlling the coastal currents. The region experiences a variety of winds throughout the year, and strong wind tends to push the surface waters, altering the direction and speed of the currents. In winter, storms generate high wind-driven currents, while in summer, light breezes have a subtle but still important effect on the surface-level flow.
3. How to track the Oslo coastal water flow?
There are many ways of tracking the coastal water flow around Oslo. One of them is the surface drift buoy technique in which surface drift buoys containing GPS units are released onto the surface of the water. The surface currents propel them forward, and by tracking their travel over a course of time, scientists can map the general direction and speed of the surface-level currents. This method, though, only provides measurements for the near-surface layer of the water column and is prone to wind-driven drift, possibly not representative of true current direction.
The ship-anchoring technique uses a ship to be anchored at a fixed location within the fjord. Current meters are then lowered off the side of the ship to measure velocity at different depths. While such a strategy could provide high, time-series data at any one point, it is limited by ship position and the practicality of long deployment, especially given the high shipping density and variable weather patterns of the Oslofjord.
Acoustic Doppler Current Profiler (ADCP) method has been the most advanced and efficient technique used to measure coastal currents near Oslo. ADCPs map currents from the surface waters down to the seafloor by emitting sound waves. This allows researchers to obtain an integrated, three-dimensional portrayal of the current structure, a requirement for identifying the subtle flow patterns in the fjord and along its coastlines [2].
4. How does the mechanism of ADCPs operate on the Doppler principle?
ADCPs operate on the Doppler effect principle. They emit pulses of ultrasonic sound from several transducers. When these sound waves pass through the water, they encounter moving particles, for instance, suspended sediment, plankton, or small sea creatures. When these moving particles are reflected by sound waves, the frequency of the returning signal depends upon the velocity of particles relative to the transducer. When the particles are moving toward the transducer, the sound frequency increases (blue shift), and when the particles are moving away from the transducer, the sound frequency decreases (red shift).
By comparing Doppler shifts from greater than a single transducer, typically at non-parallel angles, the ADCP current profiler can calculate water velocity along each sound beam. Through vector mathematics, these beam velocities are added to calculate the horizontal and vertical component of the current in different depth intervals, or "bins.". This enables the ADCP to produce a high-resolution current profile of currents at different depths of the water column, informative about the flow characteristics of the water [3].
5. What are the requirements for high-quality measurement of Oslo's coastal currents?
For high-quality measurement of Oslo's coastal currents, ADCPs need several important attributes. Material reliability is most critical due to the harsh marine environment of the Oslofjord. Water is corrosive and saline, and the fjord is also subjected to strong currents and variable weather. Titanium alloy is most ideal for ADCP casings. It offers better corrosion resistance, much better than in common materials like stainless steel or aluminum, so that the device may be subjected to the fjord waters over the long term without deterioration.
Titanium also offers an excellent strength - to - weight ratio, allowing ADCPs to withstand the high water pressures in deeper depths within the fjord, e.g., around the deepest channels of the fjord, without adding excessive bulk or weight. This makes the deployment of ADCPs easier, whether from a ship, a moored platform, or a buoy. And since titanium will maintain its mechanical characteristics throughout a wide temperature range, this is valuable for consistent performance under Oslo's varying climate.
In addition to high quality material, ADCPs need to be small, lightweight, with low power consumption, and with good cost-effectiveness. Smaller and lighter ADCPs are more maneuverable and can be mounted in the narrower and often - inaccessible locations within the fjord. Low power consumption makes it possible for extended - term, unattended operation, which is essential in collecting continuous data over extended periods of time. Cost - effectiveness is also important, especially when it comes to large - scale monitoring studies intended to acquire a full understanding of the complex current dynamics in Oslo's coastal waters.
6. How to Choose the right equipment for current measurement?
Selecting the proper ADCP for the measurement of Oslo's currents depends on two main factors: the application intended and the water depth. For the mapping and general surveys of the currents along the coast and within the fjord, vessel-mounted ADCPs are the best bet. They can cover large areas relatively quickly and are effective in providing wide-ranging information on the surface and subsurface currents as the boat travels across the water.
Bottom - moored ADCPs are optimally adapted to long - term, continuous monitoring at specific points of interest, such as close to critical fishing areas, industrial sites, or areas with special ecological features. Bottom - moored ADCPs can be left in the field for long periods to quantify seasonal and long - term changes in current regimes. Buoy-mounted ADCPs are versatile for observing surface currents and can be equipped with sensors to observe parameters such as temperature, salinity, and wave height, providing a richer description of the marine environment.
The frequency selection is also critical. A 600kHz ADCP can be utilized to sample up to 70m of water depth, which is most suited for Oslofjord's shallows and near - shore locations off Oslo. A 300kHz ADCP measures up to 110m in depth, which is most appropriate for deeper sections of the Oslofjord. For very deep-water applications, such as in the outer part of the fjord where it is subjected to the North Sea, a 75kHz ADCP, profiling up to 1000m depth, is needed [4].
Teledyne RDI, Nortek, and Sontek are some of the popular brands of ADCPs. But for anyone who is searching for a quality but not too expensive option, the ADCP supplier China Sonar PandaADCP comes highly rated. Made of titanium alloy completely, it is a reliable performer at a reasonable price. This makes it an excellent choice for researchers, environmental protection departments, and oceanic departments handling the ocean currents of Oslo on a study and management level. Contact [https://china-sonar.com/]for further details.
References
[1] Oceanography of the Oslofjord. (n.d.). Retrieved from relevant oceanographic research databases.
[2] Principles of Acoustic Doppler Current Profiling. (n.d.). NOAA Ocean Service Education.
[3] Doppler Effect in Acoustics. (2021). Encyclopedia Britannica.
[4] Product Specifications and Application Guides for ADCPs. (n.d.). Retrieved from manufacturer websites.
How can we measure the Oslo coastal currents?