How do we measure the coastal currents of Arcata?

1. Where is Arcata?

Arcata is a vibrant city of Humboldt County, California, situated on the northern edge of California. Some 270 miles north of San Francisco, it enjoys a strategic location at the hub of Humboldt Bay, a natural estuary that is amongst the largest estuaries in the United States West Coast. Its setting is taken from the setting in the vicinity of the bay and consists of an attractive central business district with amenities of the modern era but retaining a lingering - anchored penchant for the outdoors.

The history of Arcata stretches thousands of years back and was initially occupied by the Wiyot. They possess a rich culture which is greatly connected with the land and sea. Europeans came during the mid-19th century when the wood resources in the region attracted them there. The city evolved progressively as a lumbering, fishing community and presently it is concerned with environmental education and sustainable life.

Arcata's coastal waters are part of Humboldt Bay, a bay that opens onto the Pacific Ocean. The topography of the seafloor in the bay is diverse. Large mudflats and salt marshes line the shore. These are critical nesting sites for flocks of birds, fish, and invertebrates. The floor of the bay is deeper toward the center, and troughs and channels are present, some up to 30 meters deep. North Jetty and South Jetty comprise the bay mouth and significantly contribute to exchange of water in and out of the bay and to coastal currents in the vicinity of the locality.

2. What are the coastal currents around and in Arcata?

Arcata coastal currents result from a complex interplay between natural and man-related forces. Tides are a controlling factor. The area has a semi-diurnal tidal cycle, two high tides, and two low tides daily. Humboldt Bay's tidal range will be extremely variable, up to 7 feet in some areas. During high tide, water flows into the bay and creates strong flood currents. During low tide, water flows out of the bay, creating ebb currents. Both tidal currents are most intense in the constricted channels and the entrance to the bay.

The cold south-flowing California Current directly affects the region's waters. The current conveys nutrient-fertilized water from the north, which nurtures a dynamic marine ecosystem. Upwelling occurrences, generally initiated by northwest winds, allow cold, fertile water to resurface. The phytoplankton in fertile waters thrive and provide the support base of the food web to nourish an abundant variety of marine life.

The offshore and onshore topography also has an important contribution towards the shaping of the patterns of the currents. The existence of the Trinidad Canyon, which is close by, can potentially deflect deep-sea currents, thus affecting the overall circulation within the area. Artificial structures like jetties, breakwaters, and port infrastructure in Humboldt Bay also potentially disrupt the natural flow of currents. These formations can form eddies locally, reverse the movement direction of water, and affect the deposition and transport of sediment and nutrients.

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

Surface Drifting Buoy Method

One way to monitor the coastal water flow around Arcata is by using surface drifting buoys. Surface drifting buoys are meant to drift on the water surface and follow the currents. They are equipped with GPS tracking instruments and transmit real-time position data. Specialists translate this data to observe where the surface currents are going and how quickly they're traveling. This isn't a flawless process, however. Wind can push the buoys out of position and lead to measurements that don't accurately depict the subsurface flow. Moreover, surface drifting buoys provide information regarding only the upper part of the water column and therefore give an incomplete view of the overall current structure.

Anchor Moored Ship Method

The anchor moored ship method involves mooring a ship at a fixed location. Scientists drop current meters off the ship's side at different depths to measure the current velocity. This method provides depth-specific information regarding the currents. But it is expensive and time-consuming as a research ship has to be anchored. The readings are representative only of the area near the ship and therefore hard to obtain a broad picture of the coastal currents over a large area.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler, or ADCP, has become a newer, more technologically sophisticated and convenient method of measuring coastal currents. ADCPs detect water current velocity at multiple depths levels using the Doppler effect on the sound waves. The ADCPs emit sound waves into the column of water. Upon contact with particles in the column of water, the Doppler frequency shift in the waves is then used to calculate the water's velocity. ADCPs can provide a general image of the structure that already exists, from the surface to near the seabed. This makes them highly suitable for the study of the complex coastal currents near Arcata.

4. What is the working principle of ADCPs based on the Doppler principle?

ADCPs work on the basis of the Doppler effect. They are equipped with piezoelectric transducers that emit sound waves into the water. When these sound waves reflect off particles such as plankton, sediment, or bubbles in the water, some of the sound energy is reflected back to the ADCP profiler. The time it takes for the sound waves to travel to the particles and return provides an estimate of the particle distance.

The velocity of current is measured based on the Doppler shift principle. If particles carrying the flow of the current are moving, then frequency for the back-scattered sound wave can change as recorded by the ADCP, and that for the emitted sound will also be different. The higher this variation in the frequencies, the higher will be the velocity of the water current in the direction of the acoustic beam. To record three-dimensional velocities, most ADCPs employ a minimum of three beams. Modern ADCPs also carry sensors like temperature sensors for accounting for water temperature on sound velocity, compasses to record the direction of the instrument, and pitch/roll sensors to offer accurate measurements even in turbulent seas. The returned signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What are the requirements for high-quality measurement of Arcata coastal currents?

To achieve high-quality measurement of coastal currents in Arcata, the equipment used should meet a series of requirements. Material reliability is paramount. The ADCP flow meter casing should be made of a material that is appropriate for the hostilities of the marine environment. Titanium alloy is an ideal choice. It has high corrosion resistance, which is essential for extended deployment in seawater. The titanium alloy is also light in weight and highly strong, and hence easier to deploy and handle. Its strength will enable the ADCP to endure the mechanical load of water movement and potential impacts by debris.

Size, weight, and power consumption are also of prime importance. A light and small ADCP is more beneficial, as it can be mounted on a variety of platforms, including small research vessels, buoys, or underwater vehicles. Lower power consumption allows long - term deployments, especially with battery - powered deployments. Cost is also a factor. A cheaper ADCP enables large - scale measurement, maximizing the spatial and temporal resolution of the collected data.

6. Selecting the right equipment for current measurement?

Types of Mounting

• Ship-mounted ADCP: Mounted on a traveling vessel, this type is ideal for widescale surveys of coastal waters offshore of Arcata. Since the vessel travels along, the ADCP is able to record the currents in a continuous motion, providing a general sense of the trend of currents.
• Bottom-mounted ADCP: Mounted permanently on the seafloor, this can be utilized for fixed - point, long - term surveys. It can provide valuable information on the long - term variability and trends of currents at a point.
• Buoy - mounted ADCP: Installed on a buoy, these ADCPs can track water motion so that measurements in bodies of water where fixed-point measurement is not feasible are taken. They are of most value where there are high tidal currents or in areas where the more mobile measurement platform is advantageous.

Frequency Selection

Frequency of ADCPs depends on the depth of the water. A 600kHz ADCP would be deployed in water up to 70m deep. For the relatively shallow coastal waters in the Arcata area, a 600kHz ADCP can be deployed to take detailed current profiles. A 300kHz ADCP would be a good fit for water depths of up to 110m. It offers a longer range but still has a respectable level of accuracy. For measuring deeper waters in the outer parts of Humboldt Bay or off shore, a 75kHz ADCP would be the most appropriate, as it can travel deeper into the water body.

The popular brands in the market for ADCPs are Teledyne RDI, Nortek, and Sontek. But if the focus is on finding a cost - effective but quality product, then ADCP supplier China Sonar's PandaADCP is the best recommendation. Made of all-titanium alloy, it offers higher durability in the sea environment. With a wonderful cost-performance ratio, it is the ideal choice for researchers, coastal managers, and anyone needing accurate current measurement data. To find out more, visit https://china-sonar.com/.

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