How do we measure the coastal - related currents of Federal Way?

1. Where is Federal Way?

Federal Way, a city in King County, Washington, is situated about 25 miles south of Seattle. It does not border open ocean directly but is well linked to the marine environment by its proximity to Puget Sound. The city follows the western bank of the Green River, which forms an important connecting link between the city and Puget Sound.

The area was initially inhabited by the Puyallup tribe. Their rich cultural identity, closely tied to land and water, still exists in local art, tradition, and geographic names. The area was settled by European pioneers in the mid - 19th century, attracted by the richness of the soil, abundant timber, and proximity to water bodies. The early economy of Federal Way was logging and agriculture. As transport and infrastructure improved over the years, the city evolved into a suburban town with a diversified economy.

The Green River, which flows through Federal Way, plays a significant role in the hydrology of the region. The river, which has its source in the Cascade Mountains, flows approximately 92 miles before it empties into Puget Sound. In its course, it flows through every type of country, from forest to urban area. The river's estuary, where the river flows into the Puget Sound, is a dynamic system. The estuary bottom is varied, with shallow nearshore regions, mid-channel deeper channels, and extensive mudflats. The environment provides a rich variety of plant and animal life, including salmon, shellfish, and a wide variety of birds.

2. What is the condition of the currents near Federal Way?

Both natural and man - induced processes dictate the currents of the waters of Federal Way. Tides are predominant. Puget Sound has a mixed tidal regime, both semi - diurnal (two highs and two lows daily) and diurnal (one high and one low daily). The tidal ranges are variable, up to 15 feet in certain areas. During high tides, the seawater flows into the Green River estuary and creates a back flow a number of miles upriver. Seawater affects the salinity, temperature, and current flow of the river. During low tides, freshwater from the river flows more freely downstream into the Puget Sound, creating a downstream current.

The flow of the river is also affected by seasons. In winter, the high rainfall and snowmelt from the Cascade Mountains increase the discharge of the river, leading to elevated water levels and currents. The flow of the river may be reduced in summer because of lower rainfall and higher use of water.

The local wind patterns in the region may also have an impact on the currents. The westerly winds are strong enough to drive surface water onto the beach, and strong easterly winds are strong enough to create upwelling in the Puget Sound, which draws cold, nutrient-rich water up to the surface. Artificial obstructions such as dams, levees, and bridges on the Green River also block the flow of the river. These shapes may create local eddies, changing the course of water flow and affecting sediment and nutrient transport.

3. How is the water flow around Federal Way monitored?

Surface Drifting Buoy Method

Surface drifting buoys are a means of monitoring water flow around Federal Way. Surface drifting buoys are constructed to drift along the surface of the water with the currents. The buoys are equipped with GPS tracking units, which transmit real-time location data. Scientists take advantage of this data in order to determine the direction and speed of the surface current. This method has its own limitations, however. The wind will cause the buoys to drift away from the actual current, making it take faulty measurements of the subsurface flow. Also, surface drifting buoys provide information only for the surface layer of the water column and provide a limited concept about the overall structure of the currents.

Anchor Moored Ship Method

Anchor moored ship method involves taking a ship to a fixed point. Scientists dip current meters at different depths above the ship side to measure the current velocity. This method provides depth-specific information about the currents. It is also time-consuming and expensive, in the sense that it requires a research vessel to be held at anchor. It is also only representative of the immediate area near the ship and therefore does not give a sufficient overview of currents within an extensive region.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has proven to be a more advanced and handy device for the measurement of currents. ADCPs utilize the Doppler shift of sound waves to measure water current velocities at an array of depths. They emit sound signals into the water column. When the signals bounce back from water particles, the frequency shift of the returning signals is utilized to calculate the water speed. ADCPs can give a full representation of the current structure, from the surface to near the seabed. This makes them extremely well-positioned to study the complex currents within the Green River and the nearby Puget Sound.

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

ADCPs operate on the Doppler principle. They have piezoelectric transducers that emit sound waves into the water. As the sound waves collide with particles such as plankton, sediment, or water bubbles, a portion of the sounds' energy bounces back towards the ADCP current profiler. The time elapsed between the sound waves traveling to the particles and back provides an estimation of the distance from the particles.

The key to the measurement of current velocity lies in the Doppler shift. As the particles are conveyed by the water current, the frequency of the scattered sound waves at the ADCP that are received will not be the same as that which was emitted. The magnitude of the frequency shift will be proportional to the water's velocity in the direction of the acoustic path. In order to measure three-dimensional velocities, most ADCPs have a minimum of three beams. ADCPs today also include other sensors such as temperature sensors in order to compensate for the impact of water temperature on sound speed, compasses in order to feel the direction of the instrument, and pitch/roll sensors to accurately measure even when seas are heavy. Return signals are amplified, digitized, and processed for current velocity computation at different levels.

5. What are the requirements of high-quality measurement of currents surrounding Federal Way?

In order to possess high-quality measurement of currents surrounding Federal Way, the equipment to be used in measurement must meet certain needs. Material reliability is extremely necessary. The ADCP casing should be made out of a material that can endure the harsh aquatic environment. Titanium alloy would be an extremely great choice. It is highly resistant to corrosion, which is critical for long-term deployment in freshwater and seawater. Titanium alloy is also lightweight and strong, thus simpler to handle and deploy. Its strength guarantees that the ADCP profiler can withstand the mechanical forces of water flow and possible impacts from trash.

Size, weight, and power apply also. The smaller and lighter ADCP is more universal in application since it can be mounted on a variety of platforms, i.e., small research vessels, buoys, or underwater vehicles. Lower power needs allow for longer - term deployment, especially if batteries are employed as the source of power. Cost factors are also applicable. Low - cost ADCP enables extensive - scale measurement, allowing increased spatial and temporal resolution of data collected.

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

Depending on Mounting

• Ship-mounted ADCP: Installed on a traveling ship, this is especially ideal for large-scale mapping of the Green River and the Puget Sound off Federal Way. As the ship travels, the ADCP can continuously measure the currents, providing a broad-scale observation of the current flow.
• Bottom-mounted ADCP: Mounted on the river bed or the seafloor, it is apt for long-term fixed-point measurement. It has the ability to furnish useful information about the long-term trend and variation of currents at a location.
• Buoy-mounted ADCP: Mounted on a buoy, these ADCPs can track the water, allowing measurements where measurements from a fixed point are impractical. They are particularly well-suited in the event of high tidal streams or in areas where a more mobile measurement system is required.

Frequency Selection

The frequency of an ADCP to employ is water depth dependent. A 600kHz ADCP would be suitable for water depths of a maximum of 70m. In the relatively shallow waters of the Green River and the near-shore waters of the Puget Sound, a 600kHz ADCP would provide good current profiles. A 300kHz ADCP is better suited for water depths of a maximum of 110m. It possesses greater range while being comparatively accurate. Where greater of the deeper water of the central Puget Sound is being experienced, a 75kHz ADCP becomes preferable since it will reach deeper into the water column.

There are numerous of the well - established ADCP manufacturers out in the market, such as Teledyne RDI, Nortek, and Sontek. However, in case one wants something that is cost - effective yet high - quality,the ADCP supplier China Sonar's PandaADCP is highly recommended. Made of all - titanium alloy, it offers better durability in the sea environment. Having an excellent cost - performance ratio, it is an ideal choice for researchers, coastal managers, and anyone needing reliable current measurement data. For more information, visit https://china-sonar.com/.

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