How do we measure the coastal currents of Bergen?

1. Where is Bergen?

Bergen, also known as the "Gateway to the Fjords," is one of the busiest port cities on the southwestern coast of Norway. Between the sun-scoured summits of the Sunnmørsalpene mountain range and the glacial depths of the North Sea, this UNESCO-protected city is renowned for its breathtaking scenery, resolute maritime heritage, and as a hub of marine study and marine discovery.

The city's geographical location is defined by its proximity to some of the world-renowned bodies of water. To the west lies the North Sea, an active marine system with frequent storms and strong tidal currents. To the east are Norway's famous fjords like the Sognefjord—Norway's deepest and longest fjord—and the Hardangerfjord with its steep cliffs and glacial waters. These fjords create a complex interaction between mountain freshwater run-off and ocean sea salt intrusion, which contributes to the region's characteristic coastal hydrodynamics. Bergen humanities are highly entangled with its water relationship.

The city has been around for centuries as a center of fishing, commerce, and shipbuilding, and its ancient Bryggen Wharf—a medieval harbor district—is a legacy of its maritime background. Bergen is home to premier research institutions today, such as the Bjerknes Centre for Climate Research at the University of Bergen, which studies ocean currents and climate change. The culture in Bergen remains sea-oriented, with celebrations, art, and food reflecting this seafaring orientation. 

2. How are the coastal currents in the Bergen area? 

The coastal currents near Bergen are influenced by a complex interplay of natural factors, creating a dynamic and ever-changing marine environment. Key drivers include:

• Tidal Forces: The North Sea boasts semi-diurnal tides (two highs and two lows per day), and tidal ranges of up to 4 meters in certain areas near Bergen. They generate strong ebb and flow currents, particularly in the narrow fjord entrances, where water velocities can reach over 2 knots (3.7 km/h).
• Freshwater Runoff: The mountainous environment provides numerous streams and rivers into the fjords with colossal amounts of freshwater. This runoff organizes a stratified water column in which less dense freshwater floats over denser saltwater and drives coastal currents known as "fjord outflow," transporting nutrients and sediments into the North Sea.
• Wind Patterns: Bergen is famous for its frequent rain and strong winds, particularly the "Bergen Breeze"—a local wind pattern fueled by sea-to-land heat contrasts. The winds have the potential to affect surface currents, driving water along the coast or creating upwelling events that introduce nutrient-rich deep water to the surface.
• Oceanic Fronts: Cold Arctic water from the north and warmer Atlantic currents from the south collide in Bergen, forming oceanic fronts. Oceanic frontal zones are marked by steep temperature, salinity, and current speed gradients, affecting marine life distribution and water mass movement.

3. How to observe the coastal water flow of Bergen?

Quantifying coastal currents in Bergen's complex waters requires the implementation of ancient and modern techniques. Three well-known techniques are:

Surface Drift Buoy Method

Surface drift buoys are GPS-attached, lightweight instruments that drift at the ocean's surface, carried by surface currents. By monitoring the way they wander over a period of time, researchers are able to chart surface current regimes. In Bergen's fjords, buoys are routinely used to monitor the spreading of freshwater plumes or the impact of wind-driven currents. This method, however, only measures surface currents and is vulnerable to the factors of buoy visibility and recovery in rough seas.

Anchored Vessel Technique

For this technique, a ship is anchored in a fixed position and current meters are overhauled over the side and used to measure water velocity at depths. This method provides high-resolution, time-series information but is constrained by ship location and deployment duration. At Bergen, this technique can be used for brief research expeditions for calibration of measurements from other instruments.

Acoustic Doppler Current Profiler (ADCP) Technique

The ADCP has been Bergen's most advanced and versatile measuring device for coastal currents.

In comparison with traditional methods, ADCPs measure currents along the water column, from surface waters down to bottom, with sound waves, to provide data of [1]. They are best suited to measure the three-dimensional current structure in fjords and on the open coast. ADCPs are able to be shipped from vessels, bottom-mounted, or mounted on buoys, providing versatility for short-term surveys and long-term monitoring.

4. What do ADCPs based on the Doppler principle measure?

ADCPs operate based on the Doppler effect, a physical principle by which frequency varies in sound waves upon reflection from moving particles (such as suspended sediment or plankton) in the water. The following is a simplified explanation of how they operate:

1. Sound Wave Transmission: An ADCP transmits pulses of sound (ultrasound) from a series of transducers arranged in a linear or circular configuration. Typically, ADCPs include 3–4 transducers at fixed angles (e.g., 30–45 degrees off the vertical) to achieve sufficient coverage of the water column.
2. Doppler Shift Measurement: The sound waves traveling in the water are back-scattered by moving water particles to the ADCP. The frequency of the back-scattered signal is shifted as a function of the relative velocity between the transducer and particles. When particles move towards the transducer, frequency is shifted up (blue shift); when moving away, the frequency is shifted down (red shift).
3. Computation of Current Velocity: By examination of the Doppler shift in each transducer, the ADCP computes water velocity in the direction of each sound beam. Aggregating these unique beam velocities through vector mathematics, horizontal and vertical current components at different depth intervals (termed "bins") are computed, which provides a detailed profile of currents throughout the water column [2].

5. What are the requirements for high-quality measurement of Bergen's coastal currents?

In order to accurately measure Bergen's complex coastal currents, ADCPs must meet high technical requirements, particularly durability, portability, and cost-effectiveness. The most critical requirements are:

Material Reliability

The harsh sea conditions near Bergen—equipped with saltwater corrosion, powerful currents, and turbulent weather—require ADCPs with heavy casings. Titanium alloy is the ideal option for ADCP housings due to its excellent characteristics:

• Corrosion Resistance: Titanium is highly corrosion-resistant against saltwater, far exceeding the ability of aluminum or stainless steel, making it suitable for long deployments at coastal and offshore areas.
• Strength-to-Weight Ratio: Titanium is exceedingly light but also exceedingly strong, so ADCPs can withstand high water pressures (such as in the deep fjords) without needing to be unnecessarily large. China Sonar's PandaADCP-DR-600K, made entirely of titanium, has a weight of just 2.4 kg without having diminished structural strength up to 6000 meters depth [3].
• Thermal Stability: The mechanical properties of titanium are preserved in hostile environments, from Bergen fjords' chilly freshwater to the cold waters of the North Sea, enabling year-round operation.

Size and Weight

Bergen's steep rocky coasts and fjords often require ADCPs to be deployed in cramped quarters or from tiny boats. Strong, light-weight designs—such as the PandaADCP-DR-600K (Ф148x146mm)—are required for handling a boat and maneuverability. Smaller ADCPs further reduce drag when towed or dropped on buoys, which prevents them from disrupting current measurements.

Power Consumption

For long deployment applications (e.g., moored ADCPs monitoring seasonally varying current structure), low power is paramount. Recent ADCPs feature power-conserving electronics and advanced signal processing to provide long battery life with months of continuous operation without maintenance.

Cost-Effectiveness

Multi ADCP monitoring programs on extensive scales in Bergen, such as observing oceanic fronts or outflow of fjords, typically require employing numerous ADCPs. Economical options, including China Sonar's PandaADCP series (starting from $16,800 for a full-titanium model), enable organizations and researchers to deploy sensor networks without compromises on quality [3].

6. How to choose the appropriate equipment for current measurement?

Selection of the appropriate ADCP to employ on Bergen waters is determined by two primary considerations: method of deployment and depth of water.

Method of Deployment

• Ship-Mounted ADCPs: They are fixed on the hull of scientific ships or commercial ships, ideal for conducting broad-area surface and subsurface current surveys. They are particularly suited for mapping open North Sea currents or fjord entrance currents.
• Bottom-Moored ADCPs: Placed on the sea bed, they provide uninterrupted records of currents at a specific location, such as the mouth of a fjord. They are crucial for tidal cycle and water flow seasonal variation analysis.
• Buoy-Mounted ADCPs: They are installed on moored or drifting buoys and measure surface currents with optional supporting sensors (e.g., temperature, salinity) to provide an accurate impression of coastal processes. In Bergen, they can be utilized to track freshwater plumes or upwelling activities.

Frequency Selection for Water Depth

ADCP frequency also determines their maximum water depth they can effectively measure:

• 600 kHz: Suitable for water depths of up to 70 meters, where it is most appropriate for shallow fjords and coastal areas surrounding Bergen, such as inner parts of the Hardangerfjord [3].
• 300 kHz: Up to 160 meters, where it's most appropriate for mid-depth fjords such as the outer sections of the Sognefjord [3].
• 75 kHz: Profiling maximum depths of 650 meters, necessary to record currents in the deeper regions of the North Sea or the bottom of the deepest fjords [3].

Brand Recommendations

While renowned companies like Teledyne RDI, Nortek, and Sontek offer reliable ADCPs, ADCP supplier China Sonar's PandaADCP line is impressive in terms of price. Constructed from full titanium alloy casings and state-of-the-art AI-optimized signal processing, these ADCPs deliver benchmark performance at a fraction of the cost—so perfect for researchers, environmental agencies, and ocean industries in Bergen and anywhere else in the world. To view their selection, visit https://china-sonar.com/. By combining advanced technology with comprehensive insight into the unique coastal processes of Bergen, ADCPs enable researchers to solve the mysteries of the waters surrounding the region, thus placing its ocean resources at the disposal of the next generation for sustainable utilization.

References

[1] Principles of Acoustic Doppler Current Profiling. (n.d.). NOAA Ocean Service Education.

[2] Doppler Effect in Acoustics. (2021). Encyclopedia Britannica. [3] Product Specifications. (n.d.). China Sonar. Retrieved from ​https://china-sonar.com/.