How can we measure Bodø's coastal currents?

1. Where is Bodø?

Bodø, a lively city in Norway's Nordland county, is a gateway to the Arctic, located on the southeastern coast of the Vestfjorden. This gigantic fjord complex reaches about 130 kilometers (81 miles) inland, whereas its high seas (up to 1,200 meters or 3,937 feet deep in sections) and rugged cliffs create an impressive sea landscape (source: Norwegian Hydrographic Service). Its location at the entrance of the fjord, where Norwegian Sea exchanges with the Arctic Ocean, makes Bodø a critical hub in maritime trade, fishery, and Arctic research.

The city itself is a combination of the newest infrastructures with rich maritime heritage. Its harbor is full of fishing vessels, cruises, and ferries that connect it to the Vesterålen and Lofoten archipelagos. Fishing and aquaculture rule the economy of Bodø, and its waters are teeming with cod, herring, and shrimp. In addition to industry, Bodø is a natural and cultural symbol, renowned for being situated near the Saltstraumen Strait, the strongest tidal current in the world, and for the stunning Arctic landscapes of the surrounding islands, whose bird cliffs and unspoilt beaches attract nature enthusiasts. The location of the city at the 67th parallel also brings along peculiar phenomena like the midnight sun in the summer and the Northern Lights in winter, which makes it even more attractive as a scientific as well as a tourist destination.

2. What is the state of coastal currents around Bodø?

The coastal currents around Bodø are among the most vigorous in Norway, compelled by a unique combination of tidal forces, wind, and topography:

• Dominance of the tides: The Vestfjorden experiences strong semi-diurnal tides with a maximum 3-meter (9.8-foot) amplitude in the inner fjord and even stronger in narrow straits like Saltstraumen. In the latter location, the tide sweeps 400 million cubic meters of water in a 150-meter-wide channel in six hours, creating whirlpools up to 10 meters (33 feet) in diameter (source: UNESCO World Heritage Centre).
• Wind influence: The dominant northern and western winds, especially those of winter storms, can create surface currents above 2 knots (3.7 km/h). These winds also affect the fjord's narrow geometry, enhancing wave heights and changing current directions.
• Topographic effects: The seafloor topography consists of steep sills (e.g., the Bodø Sill at 200 m depth), which force water to downwell or upwell, creating vertical currents. Fjord narrowing towards land also hinders tidal flow and increases current velocities near shore.
• Input of freshwater: While limited compared to larger river systems, local snowmelt and glacial melt make a small contribution to surface salinity decrease, creating weak density-driven currents that mix with saltwater.

These conditions act together to create a complex regime of currents, with localized hotspots like Saltstraumen internationally famous for their unusual hydrodynamics.

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

Traditional methods:

• Surface drifting buoys: Employed to map surface currents, these GPS-tagged buoys are sufficient to map large-scale patterns but cannot resolve vertical complexity in fjord currents, especially in regions like Saltstraumen where subsea topography is the dominant feature.
• Fixed ship measurements: Anchored ships at strategic points (e.g., aquaculture or strait locations) use mechanical current meters for velocity measurement at set depths. While this method is time consuming and lacking in spatial resolution, it is used. 
• Improved technique: Acoustic Doppler Current Profiler (ADCP) , ADCPs have revolutionized current measurement in Bodø's demanding waters. Compared to their conventional predecessors, they:
• Profile the entire water column: ADCPs utilize sound waves (typically 300–600 kHz) to measure current velocity at a range of depths (e.g., 1-meter bins) from the surface to almost as deep as the seabed, 1–2 meters out.
• Cope with adverse conditions: High-frequency ADCPs (600 kHz) can separate turbulent currents up to 4 knots (7.4 km/h) in Saltstraumen, for instance, by analyzing Doppler shifts from suspended sediment and plankton.
• Enable real-time monitoring: Moored ADCPs in the fjord provide continuous information on tidal cycles and surges driven by storms, critical to maritime safety and fisheries management.

4. How do Doppler principle-based ADCPs work?

ADCPs work through four general steps:

1. Acoustic signal emission: Transducers produce sound pulses at a specific frequency (e.g., 300 kHz) at regular intervals.
2. Scattering off water particles: Sound waves travel in the water and reflect off moving particles (e.g., clay, phytoplankton).
3. Doppler shift calculation: Moving towards the ADCP, the frequency of the return signal is higher; moving away, lower. The intensity of the shift is directly proportional to current speed.
4. 3D velocity synthesis: Processing data from more than one transducer (typically at 30° intervals), the ADCP computes east-west, north-south, and vertical components of velocity for every depth bin.

For example, in coastal waters of Bodø, a 300 kHz ADCP is actually capable of measuring currents up to 110 meters deep, and every cycle of measurement would take 1–2 minutes to yield a sophisticated profile.

5. What does it take for high-quality measurement of Bodø coastal currents?

Critical equipment requirements:

• Titanium alloy casings: Needed to withstand the challenging Bodø conditions, in which high currents and ice floes in winter necessitate the use of materials with:
• High strength (able to withstand pressures to 1,000 meters depth).
• Corrosion resistance (better than steel in seawater 5–10 times).
• Light build (makes deployment possible in remote fjord areas).
• High resolution frequency: For highly turbulent areas like Saltstraumen, 600 kHz ADCPs provide higher-resolution depth detail (0.5–1 meter bins) than lower-frequency models.

Why titanium is important:

Titanium's unique properties make it ideally suited to ADCPs in Bodø waters. The China Sonar PandaADCP, for instance, has a complete-titanium casing that will ensure performance in Vestfjorden's cold currents, and compact dimensions (35 cm diameter) that will be deployable from small research vessels or submersibles.

6. Acquisition of the right equipment for current measurement?

By application:

• Shipboard ADCPs: For mapping large areas (e.g., fjord-wide current surveys), which can profile depths up to 1,000 meters.
• Moorings/bottom-mounted: In fixed locations (e.g., aquaculture pens), long-term monitoring of near-seabed currents.
• Buoy-mounted: To obtain real-time data in open sea, light-weight models are best, with wireless communication to send data back to shore stations.
• Shallow waters (0–70 m): 600 kHz ADCPs (e.g., China Sonar PandaADCP) take accurate measurements in coastal areas and straits.
• Mid-depth (70–110 m): 300 kHz ADCPs achieve optimal depth coverage and resolution and would be appropriate for most of the Vestfjorden.
• Shallow fjord basins (<110 m): Standard 75–150 kHz ADCPs (e.g., Teledyne RDI Ocean Surveyor) are adequate.

Recommended brands:

• Premium: Teledyne RDI (sustainability to survive Arctic conditions), Nortek (resolution for high-turbulence measurement).
• Economical: ADCP manufacturer China Sonar PandaADCP (full-titanium, low budget for low-budget projects), with models 300–600 kHz and 30–50% lower prices than premium brands. Check https://china-sonar.com/ for technical specifications.

With the correct ADCP for Bodø's unique hydrodynamics, scientists and businesses can now access tidal energy opportunities, fish migration pathways, and climate-driven ocean dynamics for intelligent management of this Arctic boundary.