How are we measuring Long Beach's coastal currents?

1. Where is Long Beach?

Long Beach, located in southern California, is a successful seashore city renowned for its broad beach, active ports, and multiracial history. The city extends over 50 square miles and outlines about 22 miles of the Pacific Ocean coast, offering a diverse blend of coastal views.

The city has a history of more than thousands of years, with the Tongva people being the native city inhabitants. The city was then visited by Spanish explorers who left their profound influence on the region's architecture and culture. Long Beach has various locations of historical significance, such as the Queen Mary, which was once an ocean liner and now serves as a hotel, museum, and venue.

Shores off Long Beach form the San Pedro Bay, a huge semi-closed sea water body. Seabed morphology underwater at the bottom of the bay is complex with its mixture of sand bottoms, rocks, and man-made reefs on the basis of shipwreck or debris. The varied ecosystems provide high biological diversity comprising numerous species of fish, birds such as sea birds, and mammals such as dolphins and seals. Long Beach Port, the nation's largest port, is of serious economic significance to the area. But its extensive infrastructure made up of piers, breakwaters, and channels exerts a strong influence on the coastal currents of the area as well.

2. What is the status of coastal currents off Long Beach?

Along the Long Beach coast, coastal currents are controlled by natural and anthropogenic processes. Tides are among the controlling processes. The region is characterized by a semi-diurnal tidal regime with two high tides and two low tides daily. At high tide, enormous amounts of water flood in and out of the bay, generating strong currents, especially in the narrow channels and estuaries. At low tide, water flows out in obvious ebb currents.

The area is controlled by the California Current, a cold, south-flowing current. The current delivers nutrient-rich water from the north, which nourishes the phytoplankton. Phytoplankton, in turn, forms the basis of the marine food web, supporting an immense range of organisms. The upwelling conditions, often triggered by northwest winds, deliver cold, nutrient-rich water towards the surface, which contributes to the productivity of the marine community.

The occurrence of this San Pedro Canyon, an underwater phenomenon along the coast, is the cause of the current patterns within the locality. The canyon acts as a channel for deep-sea currents, by means of which water can flow onto the shore and be an agent of controlling the spatial distribution of this temperature, salinity, and nutrients. Moreover, the massive port facilities in Long Beach break up this natural current flow. Channels and breakwaters result in local eddies, reversing water circulation direction, as well as reversing the overall flow pattern of circulation in the bay.

3. How to observe the coastal water flow of Long Beach?

Surface Drifting Buoy Method

One method to observe the coastal water flow surrounding Long Beach is through the use of surface drifting buoys. These buoys are designed to drift on the surface of the water and track the currents. Equipped with GPS position devices, they transmit real-time location. Researchers decode this data to determine the direction and speed of the surface currents. There are, however, constraints in this method. Wind can alter the direction of the buoys from the actual current, leading to faulty estimations of the subsurface flow. Secondly, surface drifting buoys provide only details of the topmost water layer and a partial description of the overall structure of the currents.

Anchor Moored Ship Method

Anchor moored ship method includes fixing a ship in a spot. Scientists cast current meters at several depths along the side of the ship and record the speed of the currents. This method provides depth - specific information of the currents. But it is laborious and expensive, for it requires a research vessel anchored in place. Furthermore, the readings are representative only of the area around the ship, and it is not simple to obtain an overview of the coastal currents over a large distance.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more advanced and convenient method of coastal current measurement. ADCPs use the Doppler shift in sound waves to measure water current velocities at various depths. ADCPs emit sound signals into the water column. When the sound signals bounce back from particles within the water, the frequency shift of the reflected signals is used to calculate the water velocity. ADCPs are capable of providing a full picture of the current structure from the surface of the sea to near the seabed and therefore are very well suited for studying the complex coastal currents along Long Beach.

4. How do ADCPs working on the Doppler principle function?

ADCPs operate on the Doppler effect. They have piezoelectric transducers that emit sound waves into water. When particles such as plankton, sediment, or bubbles in the water encounter the sound waves, a portion of the sound energy bounces back to the ADCP. The amount of time that the sound waves take to go to the particles and return is an estimate of the distance of the particles.

The key to the measurement of current velocity lies in the Doppler shift. As the particles are transported by the water current, the frequency of the sound waves backscattered and heard by the ADCP will not be the same as the emitted waves. The magnitude of the difference in this frequency is equal to the speed of the water along the sound path. To obtain three-dimensional velocities, most ADCPs employ at least three beams. Modern ADCPs also include a variety of sensors, including temperature sensors to account for the effect of water temperature on sound velocity, compasses to measure the instrument's heading, and pitch/roll sensors to ensure accurate measurements in poor seas. The received signals are amplified, digitized, and processed to calculate the current velocity at different depths.

5. What are the requirements to get high-quality measurements of Long Beach coastal currents?

To get high-quality measurement of Long Beach coastal currents, equipment has to satisfy several requirements. Foremost is the reliability of the material. The ADCP casing needs to be made of material that is capable of withstanding the harsh marine environment. The application of titanium alloy is excellent. It is also corrosion resistant to a high degree, which is essential for long-term use in seawater. Titanium alloy is also light and durable, hence more deployable and manageable. Its durability ensures that the ADCP will be able to endure water flow mechanical stress and potential hits from trash.

Size, weight, and power consumption are also relevant. A light and compact ADCP is more versatile, being deployable on a variety of platforms from small research vessels to buoys or underwater vehicles. Reduced power consumption allows long - term deployments, especially if battery power is used. Cost is also significant. A less expensive ADCP enables large - scale measurements, increasing the spatial and temporal resolution of the acquired data.

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

Types According to Mounting

• Ship-mounted ADCP: It is mounted on a traveling vessel, and this kind of system is adequate for large-scale coverage of the coastal waters off Long Beach. As the vessel travels, the ADCP can continuously monitor the currents so that a large-scale representation of the current patterns can be obtained.
• Bottom - mounted ADCP: Installed on the seafloor, this is best suited for fixed - point, long - term monitoring. It can provide valuable information on the long - term trends and variability of the current at a location.
• Buoy - mounted ADCP: Deployed on a buoy, these ADCPs can travel with the water, enabling measurements where fixed - point measurements are impractical. They are especially valuable in tidal current areas of strong currents or areas where more mobile measurement is needed.

Frequency Selection

The ADCP frequency is determined by the water depth. A 600kHz ADCP is applicable to water depths of as much as 70m. In the comparatively shallow coastal waters of Long Beach, a 600kHz ADCP can provide high-resolution current profiles. A 300kHz ADCP is better suited to water depths of as much as 110m. It is longer ranging but still retains a fair degree of accuracy. When operating at greater depths within the outer area of the San Pedro Bay, a 75kHz ADCP is the optimal choice, as it travels more deeply into 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.