How do we measure the coastal currents of Port Townsend?

1. Where is Port Townsend?

Port Townsend, an old town in Jefferson County, Washington, has a fantastic position on the northeastern tip of the Olympic Peninsula. Between the Strait of Juan de Fuca and the Puget Sound, it is a vital coastal hub. The city lies approximately 50 miles northwest of Seattle, and it offers breathtaking scenery of the surrounding waterways and Olympic Mountains.

The area has an indigenous history. The S'Klallam and Chimacum were the native people, whose lives were intricately entwined with the sea and land. Their legacy is found in place names, artwork, and traditional practices. The European settlers arrived in the mid - 19th century, attracted first by the natural resources of the region, especially timber. Port Townsend's location became beneficial as a trading port, and the city itself betrays its prosperity through its architecture in the form of many intact Victorian-style buildings.

Port Townsend waters exist within a complex marine ecosystem along the coast. The Strait of Juan de Fuca, which connects the Pacific Ocean with the Puget Sound, is the focal point of local hydrology. The seafloor topography at the bottom of the waters is diverse, featuring shallow nearshore areas, deeper channels in the middle, and a number of reefs and shoals. All these features combine to form a dynamic habitat that supports a wide variety of marine life including salmon, orcas, seals, and a large concentration of birds.

2. How is the condition of the coastal currents surrounding Port Townsend?

Coastal currents along the waters off Port Townsend are characterized by a combination of natural and human - induced factors. Tides are actually a key. The region possesses a mixed tidal regime with semi - diurnal and diurnal components. The tidal ranges can also be quite extensive and can be as much as 10 feet in some areas. At high tide, water rushes into the bays and channels and establishes strong flood currents. Conversely, at low tides, water drains out, forming ebb currents. The tidal currents are particularly strong in narrow channels, where the water flow is extremely rapid.

The larger oceanic currents in the region also impact the waters locally. The Pacific Ocean's influence is felt through the Strait of Juan de Fuca. The currents' strength and direction within the strait are subject to seasonal variation, impacting the temperature of the water, salinity, and amount of nutrients within the area. Prevailing wind regimes, such as the westerlies, are capable of forcing surface water onshore, whereas strong easterly winds are capable of forcing upwelling, entraining cold, nutrient - dense water to the surface.

Man-made facilities such as marinas, breakwaters, and ports will disrupt the natural flow of the currents. Local eddies are created, the flow of water reversed, and distribution of sediment and nutrients altered during the construction of these facilities. For example, the Port of Port Townsend and its complex have altered the natural flow in the area.

3. How to measure the coastal water flow of Port Townsend?

Surface Drifting Buoy Method

One of the methods used to monitor coastal water flow in the vicinity of Port Townsend is by using surface drifting buoys. They are meant to drift on the surface of water and follow the currents. GPS tracking devices are installed on them, and they send real - time location signals. Researchers utilize this data to determine the direction and speed of the surface currents. The method is not flawless, however. Wind may deflect the buoys from their course and create erroneous measurements of the subsurface current. Besides, surface drifting buoys monitor only the uppermost portion of the water column, showing a partial view of the current structure.

Anchor Moored Ship Method

The anchor technique for mooring a ship involves dropping a ship to a point. Researchers lower current meters over the ship side at different depths to record readings of the velocity of the current. The method provides depth - specific information on the currents. It is expensive and labor - intensive since it requires a research vessel anchored in position. The measurements are also indicative of the region near the ship and not of the extensive region of the coastal currents.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a recent and more accessible method of coastal current measurement. ADCPs use the Doppler shift of the sound waves to estimate water current velocities at various depths. They send sound pulses radiating into the water column. Upon reflection with water particles, the frequency change in the returned pulses is then used to estimate the water's velocity. ADCPs are capable of providing an overview of current structure, both near the surface and down near the seabed. This would be extremely conducive for studying complex coastal currents such as those existing in Port Townsend.

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

ADCPs operate on the Doppler principle. They possess piezoelectric transducers that transmit sound waves into the water. When the sound waves encounter particles such as plankton, sediment, or bubbles in the water, part of the sound energy bounces back to the ADCP flow meter. The time that it takes for the sound waves to travel to the particles and back provides an estimate of the distance to the particles.

The most significant aspect of determining the current velocity is the Doppler shift. If the particles move with the water current, the frequency of the sound waves that are backscattered into the ADCP will differ from the transmitted waves. The magnitude of this difference in frequency will be directly proportional to the water velocity along the direction of the sound waves. To measure three-dimensional velocities, most ADCPs use at least three beams. Modern ADCPs also include other sensors, like temperature sensors to account for the effect of water temperature on sound velocity, compasses to determine the orientation of the instrument, and pitch/roll sensors to take the readings correctly even in rough seas. The received signals are amplified, digitized, and processed to derive the present velocity at various depths.

5. What are the requirements for high-quality measurement of Port Townsend coastal currents?

For high-quality measurement of Port Townsend's coastal currents, the equipment used must meet several requirements. Material reliability is crucial. The ADCP casing must be made of a material that can withstand the corrosive marine environment. Titanium alloy is a suitable choice. It is also heavily resistant to corrosion, which is required for prolonged deployment in sea water. Titanium alloy is lightweight as well as strong and hence easier to handle and deploy. Its strength ensures that the ADCP profiler will endure the mechanical stress of water flow and potential impacts from trash.

Size, weight, and power requirements are equally important. A small and light ADCP is more universal in use, since it can be deployed on several platforms like small research vessels, buoys, or underwater autonomous vehicles. Reduced power requirement allows for long - term deployment, especially when powered by batteries. Cost is equally important. A less expensive ADCP allows large - scale measurement, increasing the spatial and temporal resolution of the data obtained.

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

Mounting Types

• Ship-mounted ADCP: Mounted on a traveling ship, this mounting type is suitable for large-scale surveys of Port Townsend's coastal waters. As the ship moves, the ADCP can take continuous measurements of the currents, giving a broad-scale overview of the current flows.
• Bottom-mounted ADCP: Mounted on the ocean floor, it is a suitable kind for fixed - point, long - term monitoring. It can provide useful information about the long - term variability and trends of the currents at a point.
• Buoy-mounted ADCP: These ADCPs are mounted on a buoy, and they are able to travel with the water, making them useful where fixed-point measurements are not practical. They are particularly useful in high tidal current regions or where an even more mobile measurement platform is desired.

Frequency Selection

The ADCP frequency is dictated by the depth of the water. A 600kHz ADCP can be used in water depths of up to 70m. In the relatively shallow coastal waters around Port Townsend, a 600kHz ADCP can deliver detailed current profiles. For water depths of up to 110m, a 300kHz ADCP is better. It has a higher range while retaining a reasonable level of accuracy. When operating within deeper water of the Strait of Juan de Fuca, a 75kHz ADCP will be best suited as it will penetrate deeper into the water column.

There are numerous well - known brands available in the industry for ADCP, such as Teledyne RDI, Nortek, and Sontek. However, for individuals who prefer something affordable yet of high quality,the ADCP supplier China Sonar's PandaADCP is highly recommended. Made of all - titanium alloy, it offers better durability in the marine environment. With a better cost - performance ratio, it is the ideal choice for researchers, coastal managers, and anybody needing precise current measurement data. For further information, visit https://china-sonar.com/.

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