How do we measure the coastal currents of Fremont?

1. Fremont's location

Fremont, an energetic city in California's Alameda County, is around 37 miles south of San Francisco. Not located on the seashore, but part of the San Francisco Bay Area, Fremont stretches across around 90.5 square miles of urban terrain as well as natural land, comprising varied landscape with flat plains, rolling hills, and swampy marshes.

The city has a rich past. The Ohlone Native Americans initially settled in the city, and afterward, it was established by Spanish colonists in the 18th century. It grew from an agricultural town to a prominent industrial and technological hub over time. Fremont is inhabited by individuals of various races, so it is a culturally diverse city, with various festivals, art museums, and community events during the year.

San Francisco Bay, nearby Fremont, exerts significant influence over the city. The bay, which is one of the West Coast of North America's largest estuaries, is a massive watercourse connecting Fremont with the Pacific Ocean. The bay waters possess abundant marine biodiversity from tiny invertebrates to great migratory birds. Guadalupe River, passing through Fremont, drains into the bay again impacting the surrounding hydrology and coastal wetlands.

2. What is the state of coastal currents in Fremont?

Currents in Fremont's coastal area are driven by the impacts of a huge array of factors. Included among these is the California Current, a significant oceanic force in the area. The cold, south-moving current, powered by westerly winds and Earth's rotation, brings nutrient-rich waters in from the north. The presence of a California Current affects Fremont's temperature and water salinity and affects marine ecosystems in the region.

The tidal forces must also be accounted for. San Francisco Bay is semi-diurnal in its tides, and the water level rises and falls around twice daily. During high tide, seawater overflows the bay, and during low tide, it is forced seaward. The tidal current creates strong ebb and flood currents, particularly within the closed channels and estuaries that border Fremont. The flood and ebb tides carry sediments, nutrients, and pollutants, and this affects the ecological balance and water quality in the bay.

The coastal currents also are directly affected by the wind patterns. Persistent northwesterly winds drive the surface waters offshore and induce upwelling. Upwelling pushes cold nutrient-rich deep ocean water into the upper levels, encouraging phytoplankton growth. On the other hand, southerly winds force surface waters onshore, shifting the current streams. Local water and land topography modify the current flow even further. Current splitting into two halves, convergence, or current reversal is controlled by submarine ridges, channels, and coastline geometry.

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

Surface Drifting Buoy Method

One of the ways to monitor the coastal water current along Fremont is by using surface drifting buoys. The buoys are designed to drift on the water surface and track the currents. They are equipped with GPS tracking devices and telemetry systems, which transmit real-time location data. Scientists decode this data to determine the direction and speed of the surface currents. Scientists in a recent Fremont water study dropped a line of buoys. The brightly colored surface float and the drogue at a set depth per buoy allowed accurate tracking of the surface current. This is a good technique to measure the water surface layer. Wind can sometimes cause the buoys to stray from the true current, thus inducing discrepancies in the readings of subsurface flow.

Anchor Moored Ship Method

Anchor moored ship method involves mooring a ship at a fixed point and employing equipment on board to measure the currents. Researchers allow current meters to slide down the side of the ship for different depths to obtain the profile of the current's velocity. Although this technique offers more precise depth - specific data than surface drifting buoys, it is not without its disadvantages. The readings are representative only of the vicinity of the ship. Transposing the ship to various positions for readings can be time - consuming and expensive, particularly in stormy seas.

Acoustic Doppler Current Profiler (ADCP) Method

Acoustic Doppler Current Profiler (ADCP) is a more advanced and simple method of coastal current measurement. ADCPs rely on the Doppler shift of sound waves to estimate water current speeds at a range of depths. ADCPs send out sound pulses into the water column. When it bounces back from suspended particles in the water, the Doppler shift in the backscattered sound is used to measure the water velocity. ADCPs can provide a worldwide view of current structure, from the surface to near the seabed. This provides them with a special capability to address the complex coast currents off Fremont.

4. How do Doppler principle-based ADCPs work?

ADCPs operate on the principle of the Doppler effect. ADCPs have piezoelectric transducers that transmit sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, they reflect some of the energy back to the ADCP. The time it takes for the sound waves to travel to the particles and back provides an estimation of the particles' distance.

The Doppler shift is the method of measurement of velocity of current. If the particles are transported along with the water current, the scattered sound waves from them and recorded by the ADCP will have a frequency that is not equal to the emitted frequency. The magnitude of the frequency difference depends directly on the speed of the water in the direction of the acoustics. To measure three - dimensional velocities, most ADCPs use at least three beams. Modern ADCPs are also equipped with various sensors, including temperature sensors to account for the effect of water temperature on sound velocity, compasses to determine the instrument's heading, and pitch/roll sensors to ensure accurate measurements even in rough seas. The received signals are amplified, digitized, and processed to calculate the current velocity at different depths.

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

The equipment used to measure Fremont's coastal currents must meet a set of requirements to deliver high-quality measurement. Material reliability is the initial requirement. The casing of the ADCP, for example, should be made of a material that is capable of withstanding the corrosive sea environment. Titanium alloy is an ideal choice. It has a high corrosion resistance, which is very important for long-term immersion in seawater. The titanium alloy is also strong yet light in weight, and therefore it is easier to handle and deploy. Its strength enables the ADCP to resist the mechanical pressure of water movement and potential impacts from trash.

Size and weight and power needs are also significant. A lighter, smaller ADCP is more useful, as it can be deployed on numerous different platforms, including small research vessels, buoys, or underwater robots. Low power enables longer deployments, particularly if one is battery-powered. Expense is also a consideration. A less expensive ADCP enables the measurement over larger scales, and increases the spatial and temporal resolution of the data collected.

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

Types Based on Mounting

• Ship-mounted ADCP: Placed on a mobile ship, this is the most suitable for large-scale surveys of Fremont's coastal waters. While the ship moves, the ADCP can measure the currents continuously and offer a broad-scale image of the patterns of currents.
• Bottom - mounted ADCP It should be installed at the bottom. It is more suitable for fixed - point long - term observation. It provides useful information regarding long - term trends and variability of currents at a location.
• Buoy - mounted ADCP: The ADCP mounted on a buoy can ride along with the water, thereby facilitating measurements in areas where fixed - point measurements are impossible. They are of great value in areas where tidal currents are high or where a more mobile measuring device is required.

Frequency Selection

The ADCP frequency depends on the depth of the water. A 600kHz ADCP may be operated in water at depths of 70m or less. To achieve accurate profiles for the relatively shallow waters in the coast outside Fremont, a 600kHz ADCP would be used. A 300kHz ADCP would apply in water of depth up to 110m. It enjoys greater range for an acceptable measure of accuracy. In the face of deeper waters in the outer areas of San Francisco Bay, a 75kHz ADCP is most suitable as it goes deeper into the water column.

There are several well - known brands of ADCPs in the market, such as Teledyne RDI, Nortek, and Sontek. But in the case of somebody who wants to make an economical yet high - quality purchase, the ADCP supplier China Sonar's PandaADCP is suggested. Made of all - titanium alloy, it provides superior durability under marine conditions. With a superior cost - performance ratio, it is the perfect option for researchers, coastal managers, and anyone who requires stable current measurement data. To learn more, visit https://china-sonar.com/.

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