How will we measure the Eureka coastal currents?

1. Where is Eureka?

Eureka, which is the Humboldt County capital of California, is strategically positioned along the north coast of California. It's approximately 275 miles north of San Francisco, making it the principal economic as well as a cultural hub for the region. It's placed at the south edge of the Humboldt Bay, which ranks as one of the largest West Coast estuaries in the United States.

The area has a solid indigenous heritage. The Wiyot, the earliest inhabitants, have a deep connection with land and sea. Their way of life is still common in the area today through ancient shell middens, ceremonial places, and traditional ways. European settlers came during the mid - 19th century, attracted to the area by its dense timber resource. The city's architecture is a demonstration of its early history, with many Victorian - era buildings still present in the downtown business district.

Humboldt Bay, which Eureka borders, is a complex ecosystem. The bay has extensive mudflats, salt marshes, and tidal sloughs bordering it, and these are crucial habitats for numerous species of birds, fish, and invertebrates. The underwater topography of the bay is complex with shallow water near the shore and deeper channels in the center. The North Jetty and South Jetty bound the entrance to the bay, which regulates the amount of water passing between the bay and the Pacific Ocean.

2. What is the state of the coastal currents near Eureka?

The coastal currents near Eureka are governed by a combination of natural and man-induced forces. Tides are a dominant force. The region has a semi-diurnal tidal regime with two high waters and two low waters every day. The tidal range within Humboldt Bay may be large, as much as 7 feet along certain shores. At high tide, water pours into the bay, creating strong flood currents. At low tide, water flows out, creating ebb currents. These tidal currents are very strong close to the entrance of the bay and in the thin channels.

The California Current, which is cold and flows to the south, impacts the water in the region very much. It brings richly nourished water from the north, maintaining a healthy marine environment. Episodes of upwelling, commonly produced by winds flowing to the northwest, bring nutrient-rich cold water to the surface. Phytoplankton thrive in such waters, making them the source of the food chain and keeping vast amounts of marine life healthy.

Both the offshore and onshore topography also affect the current patterns. The presence of Trinidad Canyon, which is nearby, has the ability to guide deep - sea currents, and as a result, influence the overall circulation in the area. Human - made structures such as jetties, breakwaters, and the port infrastructure in Humboldt Bay are able to also disrupt the natural flow of currents. These characteristics can create local eddies, change the direction of the flow of water, and impact sediment and nutrient distribution.

3. How to measure the coastal water flow of Eureka?

Surface Drifting Buoy Method

One way of measuring the coastal water flow near Eureka is the surface drifting buoys. They are floating buoys which can drift along with the current. GEOCEPHALOGRAPHS are outfitted with GPS tracking devices that provide real-time location data. The information is interpreted by scientists to determine the direction and velocity of the surface currents. The method has some drawbacks. Wind may drive the buoys off course from the actual current, and the subsurface flow measurements become erroneous. Also, surface drifting buoys provide information only for the uppermost portion of the water column and give a restricted view of the overall current structure.

Anchor Moored Ship Method

The anchor moored ship method involves mooring a ship in one location. Researchers lower current meters over the ship's side at different depths to measure the current velocity. This method provides depth - specific information on the currents. But it is laborious and costly, as it involves having to keep a research ship tied in position. The readings are also only representative of the region around the vessel and are not easy to get a total impression of the coastal currents in a large region.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is an increasingly advanced and easy-to-use method for coastal current measurement. ADCPs utilize the Doppler shift of acoustic waves to estimate water current speeds at a number of depths. ADCPs emit sound signals into the water column. When sound signals bounce off water particles, the frequency shift of the backscattered signals is used to calculate the speed of the water. ADCPs can provide a full record of the current structure, from the surface down to near the seabed. This gives them an ideal application for studying the complex coastal currents along Eureka.

4. How do Doppler principle ADCPs work?

ADCPs operate on the Doppler effect. They are equipped with piezoelectric transducers that emit sound waves into the water. When these sound waves encounter particles such as plankton, sediment, or bubbles in the water, some of the sound energy is bounced back to the ADCP flow meter. The time it takes for the sound waves to travel to the particles and back provides an estimate of the distance to the particles.

The Doppler shift principle is the method of measuring the current velocity. If the particles move with the water current, the frequency of the backscattered sound waves received by the ADCP will be other than the frequency of the sending waves. The magnitude of this frequency difference is proportional to the water's velocity along the acoustic path. To calculate three - dimensional velocities, most ADCPs use a minimum of three beams. Modern ADCPs also possess various sensors, including temperature sensors for correcting the effect of water temperature on sound velocity, compasses for determining the heading of the instrument, and pitch/roll sensors for accurate measurements even in turbulent seas. The signals obtained are amplified, converted to digital, and processed to obtain the current velocity at different depths.

5. What is needed for high-quality measurement of Eureka coastal currents?

To perform high-quality measurement of Eureka's coastal currents, the equipment employed must meet several demands. Material reliability is extremely crucial. The ADCP casing should be made of a material that can withstand the aggressive marine environment. Titanium alloy is the best choice. It is also highly resistant to corrosion, making it suitable for long-term saltwater deployment. The titanium alloy is also very strong and lightweight, making its ease of use and deployment a possibility. Its strength shields the ADCP profiler from deformation by mechanical water motion and from being affected by any impact from waterborne debris.

Equally important are size, weight, and power consumption. A lower-profile ADCP is more versatile, as it can be installed on various platforms, from small research vessels to buoys or underwater autonomous vehicles. Less power consumption means longer-term deployments, especially when using battery power. Cost is also an issue. A lower-cost ADCP allows for large-scale measurement, thus enhancing the spatial and temporal resolution of the data collected.

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

Mounting Types

• Ship-mounted ADCP: It is mounted on a moving ship. It can be employed to conduct large-scale surveys of the coastal water off Eureka. As the ship is in motion, the ADCP can record the currents continuously and provide a broad-scale impression of the patterns of currents.
• Bottom-mounted ADCP: Placed at the sea bottom, this type is optimum for long - term, point measurements. It can provide valuable data on the trends and variability over the long - term at any location.
• Buoy - mounted ADCP: On a buoy, these ADCPs will travel with the water, and data can be acquired where fixed-point measurements are impossible. They are particularly useful where there are large tidal currents or where a more portable measuring platform is desired.

Frequency Selection

The frequency of the ADCP depends upon the depth of the water. A 600kHz ADCP is best suited for water depths of up to 70m. In the relatively shallow coastal waters off Eureka, a 600kHz ADCP can provide high-resolution current profiles. For water depths of up to 110m, a 300kHz ADCP is more suitable. It has a greater range but still offers a decent level of accuracy. When working with deeper waters in the outer regions of Humboldt Bay or off-shore, a 75kHz ADCP is the most suitable option since it can penetrate deeper into the water column.

There are several well-established ADCP manufacturers in the market such as Teledyne RDI, Nortek, and Sontek. But for those who desire an affordable yet high-quality product, the ADCP supplier China Sonar's PandaADCP is the most suitable. Made of all-titanium alloy, it is more durable in the marine environment. Having a very good cost - performance ratio, it is ideal for researchers, coastal managers, and anyone who needs reliable current measurement data. For more information, see https://china-sonar.com/.

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