How do we measure San Jose's coastal currents?

1. Where is San Jose?

San Jose, the largest city of Silicon Valley and California's third largest city, is located in the south part of the San Francisco Bay Area. It occupies around 180.6 square miles and is strategically positioned in the Santa Clara Valley. The area of the region includes fiat valley bottoms and surrounding hillsides, resulting in an anomalous environment.

San Jose has a marvelous history. It was once a Spanish pueblo that was established in 1777 and was the first civilian settlement of the California province. With the passage of time, it evolved from an agricultural society into a technological innovation center in the world. San Jose is home to numerous high-tech companies, research centers, and a vibrant startup culture today.

Although San Jose is not a coast city, the adjacent San Francisco Bay plays an influential role in determining the area. The behemoth estuary of the bay is an important waterway that connects the city to the Pacific Ocean. The Guadalupe River in San Jose flows into the bay and is part of the complex hydrologic network. The bay beaches are ringed by tidal creeks, salt marshes, and mudflats that provide a rich variety of flora and fauna including migratory birds, fish, and invertebrates.

2. What is the state of San Jose coastal currents?

San Jose coastal currents, especially along the south rim of San Francisco Bay, are ascertained by a host of determinants. Tidal forces are a highly important force. The bay experiences semi - diurnal tides, where water level rises and drops roughly twice a day. Sea water enters the bay at high tide and returns to the sea at low tide. The tidal action generates powerful ebb and flood currents, which are very characteristic in the narrow channel and estuaries.

The California Current, which is one of the large west coast ocean currents off the coast of North America, is also implicated. The cold south-moving current affects the salinity and temperature of the water in the bay. The effect of the current on the water mass of the bay is felt through the modification of the coastal currents.

Wind patterns also have a significant role in the formation of the coastal currents. Northwest winds dominate and can push the surface waters away from the coast, leading to upwelling. Upwelling brings the cold, nutrient-rich water from the bottom layers of the oceans to the surface, which feeds the phytoplankton growth. It also forms the foundation of the sea food web. South winds can push the surface waters towards the coast, altering the currents.

The topography of the near-shore environment, sub- and super-jacent to the water surface, also alters the currents. Sub-surface topography such as ridges, channels, and shoals deflect the current direction, speed up or slow down. The coastal topography and artificial structures, such as piers and breakwaters, also alter the coastal currents.

3. How to observe the coastal water flow of San Jose?

Surface drifting buoy technique involves

It is possible to observe the coastal water current along the San Jose coast using surface drifting buoys provided. Surface drifting buoys float on the water surface and travel at the direction of currents. Surface drifting buoys are equipped with GPS tracking devices and telemetry systems that enable them to send real-time location information. Scientists analyze this information in order to identify the direction and magnitude of surface currents. A group of scientists deployed a series of buoys in an experiment in San Francisco Bay's south. Each buoy had a surface float and a drogue at a designated depth so that it followed the surface current closely. Although this method is effective to measure only the surface water layer, however, it is not effective to measure the other layers. Wind can sometimes push the buoys off the actual current, creating false measurements of subsurface flow.

Anchor Moored Ship Method

Anchor moored ship method is fixing a ship in position and measuring the current with instruments aboard. Current meters are lowered over the side of the ship at different depths by scientists to obtain a profile of current speed. Though this method provides more accurate depth - specific information than surface drifting buoys, it suffers from some drawbacks. The measurement is only indicative of the area near the ship. It is labor - intensive and costly to shift the ship to different points for readings, especially in rough seas.

Acoustic Doppler Current Profiler (ADCP) Method

Acoustic Doppler Current Profiler (ADCP) is a recent, advanced, and affordable instrument to measure coastal current. ADCPs use the Doppler shift in sound waves to measure water current velocity at numerous depths. ADCPs fire sound pulses into the water column. When they move past suspended matter in the water, the change in frequency of the reflected signals is used in calculating the velocity of the water. ADCPs can provide a comprehensive view of the structure that exists, from the surface to nearly the seabed. This renders them highly suitable for examining the complex coastal currents off San Jose.

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

ADCPs operate on the Doppler effect principle. They contain piezoelectric transducers that emit sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, part of the sound energy bounces back to the ADCP profiler. 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 key to measuring current velocity is the Doppler shift. In the event that the particles travel along the current of water, the frequency of the backscattered sound waves that are received by the ADCP will be altered from the frequency of the sent waves. The magnitude of the frequency shift is higher in proportion to the velocity of the water along the acoustic path. In order to get three - dimensional velocities, all except a few ADCPs use at least three beams. Modern ADCPs also have other sensors like 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 make measurements in rough seas accurate. The received signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What is it that is necessary to measure high-quality San Jose coastal currents?

In order to measure San Jose's high-quality coastal currents, the equipment utilized must meet some criteria. Chief among these requirements is material reliability. The material used for ADCP casing, for example, must be corrosion-resistant to fit the corrosive marine environment. Titanium alloy fits the bill to a T. It has good corrosion resistance, which is required for long-term operation in seawater. Titanium alloy is also strong and light, simple to handle and deploy. Its strength ensures that the ADCP current profiler can endure the mechanical loading of water movement and potential collisions with trash.

Size, weight, and power consumption are also significant factors. A lightweight and compact ADCP is more general, deployable on numerous platforms, from small research vessels to buoys or underwater robots. Lower power consumption allows longer - term deployment, especially in the case of battery power. Cost is significant as well. A lower - cost ADCP allows large - scale measurements to be made, increasing the spatial and temporal resolution of the collected data.

6. How to choose the suitable equipment for the measurement of the currents?

Types of mounting

• Ship-mounted ADCP: Attached on a journeying ship, it is convenient to use this type for comprehensive measurement of large scales in San Jose coastal seas. While moving on the ship, the ADCP can simultaneously make measurements of currents, giving the wide-scale measurement of current regime.
• Bottom - mounted ADCP: Placed on the sea floor, this type is suitable for fixed - point long - term observations. It is capable of providing useful data on the long - term trend and variability of the currents in an area.
• Buoy - mounted ADCP: Mounted on a buoy, these ADCPs can track the water and measure where fixed - point measurements are not possible. They are of greatest use in high tidal current locations or where a more portable platform is desired for measurements.

Frequency Selection

ADC frequency is chosen according to the depth of the water. A 600kHz ADCP can be deployed to depths of 70m. On the relatively shallow water of the southern San Francisco Bay coast, a 600kHz ADCP can be deployed to provide thorough current profiles. A 300kHz ADCP would be preferable for water depths of up to 110m. It offers a greater range with a comparable degree of accuracy. For the processing of the deeper waters on the outer part of the bay, a 75kHz ADCP would be the best option, as it is able to penetrate deeper into the water column.

There are some of the most popular ADCP brands out there in the market, such as Teledyne RDI, Nortek, and Sontek. However, for people who desire an affordable but quality ADCP, the ADCP manufacturer China Sonar's PandaADCP is suggested. All - titanium alloy build, it offers higher durability in marine environments. With high cost - performance ratio, it is perfectly suitable for researchers, coastal managers, and users who need reliable current measurement information. To find out more, visit https://china-sonar.com/.

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