How to measure the coast currents of Fortuna?

1. Where is Fortuna?

Fortuna, a small town in Humboldt County, California, is a key location in northwestern California. While not directly on the coast itself, it is approximately 16 miles inland of the Pacific Ocean, along the shores of the Eel River. The Eel River is a significant passageway between Fortuna and the ocean, acting to be a powerful force in hydrology and ecology within the region.

The area was originally inhabited by the Tolowa and Wiyot natives whose way of life was naturally tied with the ocean, rivers, and earth. The initial European inhabitants of the area immigrated to the area in the mid - 19th century because of the profitable wood products the area had to offer. The city was closely tied with the lumbering industry, and the historic buildings in the area are still symbolic of their time building themes.

Coastal waters within the vicinity of Fortuna fall within the Humboldt Bay complex, which is bigger. One of the United States West Coast's largest natural estuaries is Humboldt Bay, which has a very complex ecosystem. There are wide mudflats, salt marshes, and tidal sloughs bordering the bay. It has a very diverse community of bird life, fish, and invertebrates. The underwater topography of the bay is irregular, with shallow near-shore waters and deeper channels in the middle. The entrance to the bay, between North Jetty and South Jetty, regulates water exchange between the Pacific Ocean and the bay.

2. What is the status of the coastal currents at Fortuna?

Currents offshore near Fortuna are controlled by a complex combination of natural and man - caused factors. Tides are dominant. The region experiences a semi - diurnal tidal regime, in which two highs and two lows per day take place. Range of tides within Humboldt Bay may be extreme, with ranges exceeding 7 feet or more in some areas. During high tides, water flows into the bay, creating strong flood currents. Conversely, water is expelled during low tides, resulting in ebb currents. These tidal currents are especially strong near the entrance of the bay and through the confined channels.

The California Current, a cold south-flowing current, is significant in the local waters. The current brings nutrient-rich water from the north, which nourishes a thriving marine ecosystem. Upwelling events, typically brought on by northwest winds, bring cold, nutrient-rich water to the surface. Phytoplankton bloom in the nutrient-rich waters, forming the basis of the food chain and supporting a wide variety of marine life.

The Eel River also plays a very significant role in the coastal currents. The river's outflow can affect the temperature, salinity, and current patterns of the surrounding coastal waters. In the rainy or snowmelt season, the river can bring a high concentration of freshwater and sediment into the bay and alter the local current patterns. In addition, man-made structures such as jetties, breakwaters and Humboldt Bay harbor facilities may obstruct the natural flow of currents. These structures can cause local eddies, change the direction of water flow and affect sediment and nutrient distribution.

3. How to study coastal water flow around Fortuna?

Surface Drifting Buoy Method

Surface drifting buoys are also employed to monitor coastal water current around Fortuna. They are tiny buoys that remain on the water surface and drift based on the movement of currents. They have GPS tracking devices with them and provide real-time location in space. These readings are gathered and processed to infer the velocity and direction of surface currents. However, the constraint is there. Wind may cause the buoys to move away from the actual current and thus provide an incorrect evaluation of the subsurface flow. Furthermore, surface drifting buoys can only account for the upper surface of the water column and only provide part of the world current structure.

Anchor Moored Ship Method

Keeping a ship at a specified point is anchor moored ship method. Researchers deploy current meters over the side of the ship at different depths to measure the velocity of the currents. The process provides depth - specific information about the currents. It is time - consuming and expensive, though, as it entails a research vessel that must be kept in place. The measurements are also only representative of the local area surrounding the ship and thus it is difficult to make a general impression of the coastal current across a wide distance.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has evolved as a newer, more effective method of coastal current measurement. ADCPs utilize the Doppler shift in sound waves to measure water current velocities at different depths. They emit sound signals into the water column. When the signals bounce off water particles, the frequency shift of the bounced signals is used to calculate the water velocity. ADCPs can provide a full picture of the current structure, from the surface to near the seabed. This makes them extremely suitable for examining the complex coastal currents near Fortuna.

4. How do ADCPs based on the Doppler principle operate?

ADCPs operate on the Doppler effect principle. They use piezoelectric transducers to transmit sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, they bounce back some of the sound energy to the ADCP flow meter. The time taken for the sound waves to travel to the particles and back provides an estimate of the distance to the particles.

The velocity of current is a function of Doppler shift. When particles are transported along with the water flow, the scattered sound waves arriving at the ADCP will differ in frequency from transmitted waves. The magnitude of the frequency difference is proportional linearly to the speed of water in the direction of the path of sound. In order to measure velocities in three dimensions, at least three beams are utilized in the majority of ADCPs. Contemporary ADCPs are also equipped with an array of sensors, including temperature sensors to account for the effect of water temperature upon sound speed, compasses to determine the heading of the device, and pitch/roll sensors for correction of accurate measurement under rough seas. These arriving signals are amplified, digitized, and processed to determine the current velocity at different depths.

5. What does it take to accomplish high-quality measurement of Fortuna coastal currents?

In order to accomplish high-quality measurement of coastal currents of Fortuna, equipment must meet various requirements. Foremost among these is material reliability. The material of the ADCP meter casing ought to be made of something capable of withstanding the harsh marine environment. Such a material includes titanium alloy. It has good corrosion resistance, which is necessary for long deployment in seawater. It is also tough and light, making it easy to handle and deploy. The strength ensures that the ADCP can handle mechanical strain in water flow and impact from floating debris.

Size, weight, and power consumption are also critical. Lighter and smaller ADCP are easier to deploy on any number of various platforms, ranging from small research vessels to buoys or submersible robots. Lower power consumption is even feasible to allow longer - duration deployments when power is supplied by batteries. Cost is also a factor. A lower - cost ADCP makes it economically feasible for measurements on large - scale scales to offer higher spatial and temporal resolutions to the data acquired.

6. How to Choose the right equipment for current measurement?

Types Based on Mounting

• Ship-mounted ADCP: Installed on a traveling ship, this is ideal for large-scale surveys of the coastal waters off Fortuna. As the ship travels, the ADCP can continuously measure the currents, providing a broad-scale view of the current pattern.
• Bottom-mounted ADCP: Placed on the ocean floor, this version is best for fixed-point, long-term measurement. It has the capability of providing valuable data on the long-term trend and variability of currents at a specific point.
• Buoy - mounted ADCP: Mounted on a buoy, these ADCPs can ride with the water, allowing for measurements where fixed - point measurements would be inconvenient. They are particularly useful in areas of strong tidal currents or where a more mobile system of measurement is required.

Frequency Selection

ADCP frequency selection is a function of water depth. 600kHz ADCP will be suitable for up to 70m water depths. For Fortuna's relatively shallow coastal water, a 600kHz ADCP can provide detailed current profiles. For waters up to 110m water depth, 300kHz ADCP is better used. It provides a greater range with an appropriate degree of accuracy. Wherever the water is deeper in the outer parts of Humboldt Bay or off-shore, a 75kHz ADCP is utilized instead, as it can penetrate deeper into the water column.

Several reputable ADCP brands are found in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those who prefer an inexpensive yet quality product, the ADCP manufacturer China Sonar's PandaADCP is highly recommended. Made of all-titanium alloy, it offers excellent toughness in the sea environment. Having a better cost - performance ratio, it is the optimal choice for researchers, coastal managers, and anyone who needs precise current measurement data. For more details, visit https://china-sonar.com/.

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