How do we measure San Francisco's coastal currents?

1. Where is San Francisco?

San Francisco, on the west coast of the United States, enjoys a unique and strategic position at the tip of a peninsula flanked by the Pacific Ocean on the west and San Francisco Bay on the east. It sits on approximately 46.9 square miles and combines land and water in an interesting way, with its hilly landscape creating dramatic seascapes.

The city has a colorful and rich history. It was discovered for the first time by Spanish colonists in the 18th century and afterward grew as a top center during the California Gold Rush of 1849. This wave of treasure - hunters transformed San Francisco into a bustling city from a tiny colony. The city is proud of its iconic sites such as the Golden Gate Bridge, Alcatraz Island, and the cable cars.

San Francisco Bay, which borders the city of San Francisco, is the largest estuary on the west coast of North America. It is a crucial habitat for an immense diversity of marine life, ranging from tiny plankton to whales. The bay is connected to the ocean through the Golden Gate Strait, which is a narrow but deep-water passage that plays an important part in the movement of water from the bay to the ocean and in the coastal currents in the area.

2. How are the coastal currents off San Francisco?

San Francisco coastal currents are affected by a complex interplay of numerous variables. The California Current, a cold south-flowing current, governs the coastal waters of the region. The California Current, powered by the westerly prevailing winds and Earth rotation, transports nutrient-rich water from the north. The California Current's water supports an enormous diversity of marine ecosystems, from kelp forests to commercial fisheries.

Tidal forces then directly influence coastal currents. Between semi - diurnal tides in San Francisco Bay, the water level fluctuates approximately twice a day. During the high tide, seawater flows into the bay through the Golden Gate Strait, and during the low tides, it returns outside. These tidal variations have the potential of generating strong flood and ebb currents, often reaching several knots in the strait's tightest parts.

Wind patterns play an important role in reversing the coastal flows. Preponderant northwesterly winds tend to force surface waters out of the coastal area, with the consequence being upwelling. Upwelling provides nutrient-rich, cold water from lower in the sea to the upper levels, enabling the growth of phytoplankton. Conversely, southerly winds tend to drive surface waters onto the beach, reversing the current directions. The surrounding topography both on and beneath the water surface influences the current directions. Ridges beneath the water, channels, and even the topography of the shoreline, may cause the currents to bifurcate, converge, or reverse.

3. How to look at the San Francisco coastal water flow?

Surface Drifting Buoy Method

To monitor coastal water currents around San Francisco, surface drifting buoys may be used. They are constructed to float along the surface of the water and follow the movement of the currents. The buoys contain GPS tracking technology and telemetry devices, which provide real - time location. This data is read by the scientists to determine the direction and speed of the surface currents. In a recent test in San Francisco coastal waters, researchers set out an array of buoys. Each buoy had a very visible surface float and a drogue at a set depth to measure the surface current exactly. This method, however, can only be used to measure the surface layer of water. Wind sometimes causes the buoys to move away from the actual current, leading to incorrect measurements of the subsurface flow.

Anchor Moored Ship Method

The anchor moored ship method involves anchoring a ship at one location and taking readings of the currents on instruments on board the ship. Researchers drop current meters over the ship's side at different levels to obtain a profile of the velocity of the current. Though this technique gives more precise depth - specific data than surface drifting buoys, it has limitations. The measurements are representative of the vicinity around the ship. Transposing the ship to other positions for measurements can be expensive and time - consuming, particularly in bad seas.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more recent and more convenient method of measuring coastal currents. ADCPs detect water current velocities at multiple depths by using the Doppler shift of sound waves. ADCPs emit sound signals into the water column. When these signals bounce off water particles, the frequency shift of the backscattered signals is used to calculate the velocity of the water. ADCPs are capable of giving an overall picture of the present configuration, from the surface to close to the seabed. They are thus particularly well-suited to investigating the intricate coastal currents near San Francisco.

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

ADCPs operate based on the Doppler effect. ADCPs possess piezoelectric transducers that force sound waves into the water. When the sound waves bounce off particles such as plankton, sediment, or air bubbles in the water, part of the energy in the sound gets reflected back to the ADCP. The time it takes for the sound waves to travel to the particles and back provides an estimate of how far away the particles are.

The key to measuring current velocity is the Doppler shift. If particles are moving with the water current, the frequency of backscattered sound waves that arrive at the ADCP will be different from the transmitted waves. The magnitude of this frequency difference is proportional to the water velocity in the acoustic path. For measuring three-dimensional velocities, a minimum of three beams are used in most ADCPs. Modern ADCPs also have other sensors like temperature sensors to adjust for water temperature effects on sound velocity, compasses to determine instrument orientation, and pitch/roll sensors for accurate measurement in stormy waters. Signals to the instrument are amplified, digitized, and processed to calculate current velocity at different depths.

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

Equipment for high-quality measurement of the San Francisco coastal currents has several requirements that need to be fulfilled. The most important requirement is high material reliability. The ADCP casing, for example, must be constructed from a material that is capable of withstanding the corrosive marine environment. Titanium alloy is just what the doctor ordered. It is also highly corrosion-resistant, which is necessary for long-term deployment into seawater. The titanium alloy is likewise incredibly durable and light in weight, making it more controllable and deployable. Its durability guarantees the ADCP to be able to withstand water movement's mechanical load and potential impact from objects.

Size, weight, and power consumption matter too. A smaller and lighter ADCP is more versatile, since it can be used on a wide range of platforms from small research boats to buoys, or underwater autonomous devices. The reduced power consumption allows for longer-term deployments, especially with batteries. Cost factors. A lower-cost ADCP makes mass measurements possible and increases the spatial and temporal resolution of the data obtained.

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

Types Based on Mounting

• Ship-mounted ADCP: It is mounted on a moving vessel. This is the most appropriate equipment for large-scale surveys of the coastal waters off San Francisco. As the vessel moves, the ADCP continuously measures the currents, providing a broad-scale view of the pattern of currents.
• Bottom - mounted ADCP: Mounted on the ocean floor, it is suitable for long - term, stationary point observation. It can provide useful insights into the long - term trends and variability of the flow at a point.
• Buoy - mounted ADCP: ADCPs are installed on a buoy that can float with the water, enabling measurement in areas where fixed - point measurement is not practical. They are best applied in areas with high tidal currents or in areas where a more mobile measurement platform is required.

Frequency Selection

The frequency of the ADCP depends on the depth of the water. A 600kHz ADCP is sufficient for water depth levels of 70m or less. Considering the relatively shallow coastal waters off San Francisco, where the water depth is generally within this range, a 600kHz ADCP can provide accurate current profiles. A 300kHz ADCP is sufficient for water depth levels of 110m or less. It covers a greater range but is acceptable in terms of accuracy. For the situation of dealing with deeper waters in the outer parts of San Francisco Bay or further offshore, a 75kHz ADCP is the ideal choice as it will penetrate deeper into the water column.

There are quite a few popular ADCP brands available in the market, such as Teledyne RDI, Nortek, and Sontek. But for whoever is looking for a cost - effective but high - quality product, the ADCP manufacturer China Sonar's PandaADCP is an excellent option. Made of all-titanium alloy, it possesses greater durability in the marine environment. With a higher cost - performance ratio, it is ideally suited for use by researchers, coastal planners, and anyone who needs reliable 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.