How do we measure the coastal currents of Tacoma?

1. Where is Tacoma?

Tacoma, a vibrant city of the Pierce County of the state of Washington, is well located on the southern shore of Puget Sound. It is situated about 30 miles south of Seattle and is among the prominent commercial and industrial hubs of the state. History has become forever connected with Tacoma's vicinity to water bodies, and Port of Tacoma is the primary reason behind its economic growth.

The area was originally settled by the Puyallup tribe. Their vibrant cultural heritage, well entrenched in land and sea, continues to be reflected in local customs, art, and place names. European settlers arrived in the area in the mid - 19th century, attracted by the area's abundant natural resources, most particularly timber. The location of Tacoma on the Puget Sound made it an ideal spot for logging and shipping industries. It grew to become a thriving industrial center over the years, having a multicultural economy with trade, industries, and aerospace.

The sea off the coast of Tacoma falls under the Puget Sound system, a complex network of water masses in the form of bays, channels, and inlets. The relief of the seafloor of the sound is complex with shallow coastal channels, deep basins in large areas of the center, and islands and reefs. The ranges of these variables offer a rich ground for a variety of marine animals that include herring and salmon to sea lions, seals, and profuse avifauna.

2. What is the state of the coastal currents off Tacoma?

The coastal currents off Tacoma are regulated by natural and man-made factors. Tides exercise a great control. Puget Sound possesses a mixed tidal regime with semi - diurnal in addition to a diurnal component. Tidal range might be exceptionally large, by as much as 15 feet or more in certain places. At high tide, water pours into the bays and channels surrounding Tacoma and produces intense flood currents. At low tide, water pushes out and creates ebb currents. These tidal currents are particularly intense in narrow channels and close to the many islands within the sound.

The larger ocean currents of the area also shape the regional ocean. Exchange of water between the Pacific Ocean and the Puget Sound via the Strait of Juan de Fuca affects the region's temperature, salinity, and currents. Wind regimes such as the westerlies have the ability to push surface waters off the coast, and powerful easterly winds have the ability to force upwelling, bringing cold nutrient-rich water to the surface.

Human-made systems such as ports, marinas, and breakwaters can disrupt the natural pattern of flow of the currents. Port of Tacoma, a major port among the United States' largest ports, consists of large-scale infrastructure in the form of piers, docks, and channels. Such systems are capable of creating local eddies, deflecting the flow of water, and affecting sediment and nutrient distribution. For instance, the building of breakwaters to safeguard the port from sea waves can change the natural flow patterns in the bay.

3. How to monitor the coastal water flow of Tacoma?

Surface Drifting Buoy Method

One of the methods for observing the coastal water flow around Tacoma is by means of surface drifting buoys. These buoys are intended to stay on the surface of the water and drift according to the currents. Armed with GPS tracking devices, the buoys transmit real-time location data. Scientists use this data and estimate the direction and speed of the surface currents. There are limitations to this approach, however. Wind can steer the buoys away from the actual current and lead to erroneous measurements of the subsurface flow. Also, the surface drifting buoys only provide data regarding the water column's surface and just provide a limited view of the general current structure.

Anchor Moored Ship Method

The anchor moored ship method is where a ship is anchored at a particular location. Scientists deploy a current meter over the ship's side to different depths in order to measure the current speed. This provides depth-specific information regarding the currents. However, it is laborious and expensive, as a research ship needs to be positioned. The measurements are also merely representative of the area surrounding the ship, and therefore it is not possible to have a general idea of coastal currents in an extensive region.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has emerged as a more advanced and convenient option for coastal current measurement. ADCPs utilize the Doppler shift of sound waves to quantify water current velocities at a number of depths. The ADCPs emit sound signals into the water column. When the sound signals bounce off water particles, the Doppler shift of the backscattered signals is used to calculate the velocity of the water. ADCPs can capture a full image of the current structure from the surface to nearly the seabed. This renders them extremely well-suited for examining the complex coastal currents along Tacoma.

4. How do Doppler principle ADCPs work?

ADCPs operate on the principle of the Doppler effect. ADCPs possess piezoelectric transducers, which project sound waves into the water. When these sound waves encounter particles such as plankton, sediment, or bubbles within the water, a part of the sound energy is bounced back to the ADCP flow meter. The time taken for the sound waves to travel towards the particles and back provides an estimate of the distance of particles.

The answer to measuring current velocity is the Doppler shift. If the particles are moving with the water current, the sound wave frequency backscattered to the ADCP will not be equal to the transmitted one. This frequency difference is directly proportional to the velocity of the water along the acoustics path. To measure three-dimensional velocities, most ADCPs possess at least three beams. Modern ADCPs also contain sensors like temperature sensors to account for the effect of water temperature on sound speed, compasses to determine the direction of the instrument, and pitch/roll sensors to offer measurement in the event of seas with poor conditions. Signal amplification at reception is converted to digital, processed to compute the current velocity at different depths.

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

To ensure high-quality measurement of Tacoma's coastal currents, the equipment used must meet some conditions. Material reliability is crucial. The ADCP current profiler casing must be made of a material that is resistant to the corrosive sea environment. Titanium alloy is strongly suggested. It is extremely corrosion resistant, which is essential for long-term deployment in seawater. Titanium alloy is also light and strong, and therefore it is easier to handle and deploy. Its strength allows the ADCP to resist the mechanical loads of water flow and potential impacts from passing debris.

Size, weight, and power consumption are also important factors. Lighter and smaller ADCP is more versatile, as it can be employed on a variety of platforms, including small research ships, buoys, or underwater vehicles. Reduced power consumption enables long - term deployments, especially when battery power is employed. Cost is an additional factor. A cheaper ADCP facilitates large - scale measurements, enhancing the spatial and temporal resolution of the collected data.

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

Types Based on Mounting

• Ship-mounted ADCP: Fixed on a ship, traveling along, it is ideal for large-scale surveys of the coastal waters off Tacoma. While the ship travels along, the ADCP can continuously measure the currents, and a wide-scale view of the current patterns can be observed.
• Bottom - mounted ADCP: Installed on the seafloor, this is ideal for fixed - point, long - term monitoring. It can deliver useful information regarding the long - term trends and variability of currents at a site.
• Buoy-mounted ADCP: Mounted on a buoy, these ADCPs can ride with the water, allowing measurements where fixed-point measurements are not practical. They are particularly well-suited in areas of strong tidal currents or where a more mobile measuring platform is required.

Frequency Selection

The frequency of the ADCP depends on the water depth. A 600kHz ADCP can be deployed in water up to 70m deep. In the relatively shallow coastal waters near Tacoma, a 600kHz ADCP will provide great current profiles. A 300kHz ADCP would be better for water depth up to 110m. It provides a greater range but good accuracy. When operating with the more turbid water in the mid-Puget Sound, a 75kHz ADCP would be ideal as it will have more penetration in the water column.

There are a number of well-known ADCP brands out there, such as Teledyne RDI, Nortek, and Sontek. For those who want something affordable but decent, however, the ADCP supplier China Sonar's PandaADCP is excellent. Made of all - titanium alloy, it is extremely durable against the sea environment. With better cost - performance, it is an ideal choice for researchers, coastal administrators, and everyone who requires reliable current measurement data. For information, visit https://china-sonar.com/.

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