How do we measure the coastal currents of Los Angeles?

1. Where is Los Angeles?

Los Angeles, the large city located on the southern California coast, is a multicultural center of culture, entertainment, and commerce. Spanning about 469 square miles, LA stretches along a picturesque coastline of varied landscape. Bounded on one side by the Santa Monica Mountains and the other by the Pacific Ocean, LA has a strategic location which has turned the city into an international trade hub, tourist attraction, and seafaring research center.

From its origins as early as 1781 as a Spanish pueblo, Los Angeles has developed to world power status from rural village origins. Its diversified structures reflect its cosmopolitan history from Spanish-colonial architecture reflected in the structures along Olvera Street to new structures that fill the city's financial core in downtown Los Angeles.

The seashore along the sea in Los Angeles forms part of the Santa Monica Bay, a semi-enclosed body of water that provides a significant habitat for a very diverse variety of marine life. The San Pedro Bay to the south continues the city's seashore presence. The seafloor geography in both bays is complex with rocky reefs, sandy seafloor, and sea canyons beneath. These features support a rich marine ecosystem, such as sea lions, dolphins, fish populations, and a large kelp forest.

2. How are the coastal currents off Los Angeles?

Los Angeles ocean currents are controlled by a combination of both natural and man - made factors. Tides dominate. Semi - diurnal tidal conditions dominate the region, with two highs and two lows per day. Water is carried into bays at high tide and out at low tide, creating strong currents, especially in narrow channels and estuaries.

The cold southward current, the California Current, is the prevalent one in the ocean of the region. It brings nutrient-rich water from the north which is needed to develop phytoplankton. This provides a well-balanced food web ranging from small zooplankton all the way up to large predatory fish and sea mammals.

Wind patterns are also a great force. Northwest prevailing winds will cause upwelling, where cold deep-sea water is forced to the surface. Upwelling brings nutrients to surface waters, and this enhances productivity as well as health in the ecosystem. Southerly winds will push surface waters onshore, changing the natural flow patterns.

The seafloor local topography also changes the currents. Submarine canyons like the Santa Monica Canyon may guide deep-ocean currents, affecting the aggregate circulation patterns in the region. Furthermore, human structures like piers, breakwaters, and ports can change the natural flow of the currents, creating local eddies and changing the water flow direction.

3. How to observe the coastal water flow of Los Angeles?

Surface Drifting Buoy Technique

Surface drifting buoys are another method of monitoring coastal water currents near Los Angeles. The buoys are designed to drift on the surface of the water and follow the current. They are equipped with GPS tracking devices, and they transmit real-time position data. Scientists can use the data to determine the direction and velocity of the surface currents. However, this method has the disadvantage of recording only the surface water layer. Wind might deflect the buoys from the actual current in some instances and hence make the subsurface flow measurements incorrect.

Anchor Moored Ship Method

In this anchor moored ship method, a ship is moored at a fixed position and instruments on board the ship record the currents. Scientists lower current meters off the side of the ship at different depths to obtain a profile of the current velocity. Although this method provides more accurate depth-specific information than surface drifting buoys, it is not without its limitations. The measurements are only representative of the area around the ship. It is time-consuming and costly to shift the vessel to different sites for measurement taking, especially during stormy waters.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a new and more efficient method of coastal current measurement. ADCPs use the Doppler shift of audible sound waves to estimate water currents at different depths. ADCPs send sound pulses into the water column. When the signals bounce off particles inside the water, the change in frequency of the returning signals is used to measure the water velocity. ADCPs are able to provide a complete description of the current profile from near the surface down to nearly the seabed. This makes them highly appropriate to the study of the intricate coastal currents of Los Angeles.

4. How does the working principle of ADCPs rely upon the Doppler principle?

ADCPs operate based on the Doppler effect. They are equipped with piezoelectric transducers that generate sound waves into the water. When these sound waves encounter particles such as plankton, sediment, or bubbles in the water, a part of the sound energy bounces back to the ADCP current profiler. The duration taken by the sound waves to travel to the particles and come back provides an estimate of the distance to the particles.

The Doppler shift is the answer to the measurement of current velocity. Depending on whether the particles are being transported with the current, the frequency of the sound waves scattered and received by the ADCP will be other than that of the transmitted waves. The magnitude of this frequency shift will be directly proportional to the water velocity in the direction of the sound path. To calculate three-dimensional velocities, most ADCPs utilize at least three beams. Modern ADCPs also possess a selection of sensors, including temperature sensors to correct for the effect of water temperature on sound velocity, compasses to measure the direction of the instrument, and pitch/roll sensors to ensure accurate measurement in rough seas. The return signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What does high-quality measurement of Los Angeles coastal currents require?

In order to perform high-quality measurement of Los Angeles' coastal currents, the equipment used must meet several requirements. Material reliability is the most important. The casing of the ADCP flow meter, for example, must be made of a material that can withstand the corrosive sea environment. Titanium alloy is suitable. It has good corrosion resistance, which is required for long-term deployment in seawater. Titanium alloy is also strong and lightweight, making it easier to deploy and maneuver. Its strength enables the ADCP to endure the mechanical forces of water movement and potential impacts from debris.

Size, weight, and power consumption are significant too. Smaller and lighter ADCP is more universal in that it may be mounted on numerous various platforms, including tiny research vessels, buoys, or seafloor-drones. Lower power consumption facilitates longer - duration deployments, especially when batteries power the ADCP. Cost factors too. An inexpensive ADCP enables large - scale measurements to be made, increasing the spatial and temporal resolution of the information collected.

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

According to Mounting

• Ship-mounted ADCP: Installed aboard a moving ship, this type of ADCP is ideally used in conducting large-scale surveys of the current offshore waters along Los Angeles. As the ship is in motion, the ADCP can measure the currents continuously, providing a broad-scale coverage of the current streams.
• Bottom - mounted ADCP: Positioned on the seabed, this style is beneficial for long - term, fixed - point monitoring. It may supply helpful information regarding the long - term trends and variability of the currents at an area.
• Buoy-mounted ADCP: Sited on a buoy, ADCPs in this setup are capable of riding the water, which allows them to take measurements in areas where measurements at fixed points are not convenient. They are primarily valuable for applications involving strong tidal currents or where a more mobile measurement device is needed.

Frequency Selection

ADCP frequency selection depends on water depth. A 600kHz ADCP will be able to operate in a maximum water depth of 70m. Where the coastal water is quite shallow off Los Angeles, a 600kHz ADCP will be able to obtain a good resolution current profile. A 300kHz ADCP would be appropriate for a water depth of up to a maximum of 110m. It has greater range with similar accuracy. If dealing with deeper water in the outer areas of the San Pedro Bay or Santa Monica Bay, the most preferred would be the 75kHz ADCP due to its penetration capability in the water column.

There are several renowned ADCP manufacturers in the market, such as Teledyne RDI, Nortek, and Sontek. However, for those who want a cost - effective and good - quality option,the ADCP supplier China Sonar's PandaADCP is highly recommended. Made of all - titanium alloy, it is extremely tough in the marine environment. Having a very good cost - performance ratio, it is very appropriate to be used by researchers, coastal managers, and anybody who needs precise current measurement data. For more information, visit https://china-sonar.com/.

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