How do we measure Shoreline's coastal currents?

1. Where is Shoreline?

Shoreline, a vibrant city in King County, Washington, is positioned in a prime location on the shores of Puget Sound. North of Seattle, the city is a suburban municipality with close ties to the marine environment of the area. The city's relatively small size belies its importance, however, as it provides access to a varied set of coastal habitats.

The area was first settled by the Duwamish tribe. Their rich cultural heritage is strongly tied to Puget Sound, and it can still be seen in neighborhood history and geographic names. European-Americans arrived in the 19th century, attracted by the natural resources and access to waterways in the area. Shoreline slowly developed from a rural town into a residential and commercial hub, its course guided by its location on the beach.

The waters off Shoreline along the coast are a part of the Puget Sound ecosystem. Puget Sound is a highly complex estuary consisting of a set of bays, inlets, and channels. The seashore topography off Shoreline changes from shallow sand beaches close to shore to deeper channels and troughs as one advances towards the ocean. Sub-sea reefs and rocky outcroppings generate an environment laden with a wealth of marine species. From crabs and salmon to seabirds of a number of species as well as marine mammals like seals and sea lions, the area is a biodiversity hotspot.

2. What is the status of coastal currents off Shoreline?

The coastal currents off Shoreline are the result of an intricate mix of natural as well as man - induced factors. Tides play a crucial role. Puget Sound has a mixed tidal regime. Semi-diurnal and diurnal tides occur, with tidal ranges reaching as much as 15 feet in some areas. Water floods into the bays and inlets near Shoreline during high tide, creating strong flood currents. During the receding tide, ebb currents sweep water out of the sound into open sound. These tidal currents are particularly strong in the narrow, small channels and around the numerous small islands and peninsulas of the area.

The larger ocean currents also impact the water locally. The flow of water through the Strait of Juan de Fuca, which connects the Pacific Ocean to Puget Sound, impacts the temperature, salinity, and current flows in the coastal waters of Shoreline. Prevailing patterns of wind can also impact the currents. Westerlies have the ability to push surface waters onshore, while easterly winds of high intensities can cause upwelling. Upwelling events bring cold, nutrient-rich water to the surface, causing phytoplankton growth and sustaining the entire marine food chain.

Human activities have also disrupted the natural circulation of currents. The marinas, piers, and seawalls on the Shoreline have fragmented the natural circulation patterns. They can create local eddies, change the water movement direction, and affect sediment and nutrient transport. For example, major marinas can slow down water flow, leading to the change of the local current regime.

3. Measurement of the coastal water flow of Shoreline?

Surface Drifting Buoy Method

One of the techniques employed to track the coastal water flow around Shoreline is through the use of surface drifting buoys. Surface drifting buoys are designed to float on the water surface and drift with the currents. They are equipped with GPS tracking devices that transmit real - time location data. Scientists analyze this data to determine the direction and speed of the surface currents. However, this method has its limitations. Wind will cause the buoys to drift away from the actual current, and hence, the subsurface flow is determined inaccurately. Surface drifting buoys provide information from no more than the top layer of the water column, hence describing a limited scenario of the complete current structure.

Anchor Moored Ship Method

The anchor moored ship method involves leaving a ship at a certain position. Researchers deploy current meters off the side of a ship at different depths to measure the speed of the current. The method provides depth - specific information about the currents. However, it is time-consuming and expensive as a research vessel must be held stationary. The readings are also representative only of the area near the ship, and so it is impossible to obtain a total view of the coastal currents over a large distance.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) has grown into a more advanced and convenient means of measuring coastal currents. ADCPs use the Doppler effect of sound waves to estimate water current velocities at different depths. They pass on sound waves into the water column. When they meet particles within the water, they utilize the frequency shift of the reflected wave to calculate the water's speed. ADCPs can provide an unambiguous depiction of the structure of the currents from the surface to near the seabed. They are therefore well placed to be utilized in order to study the complex shore currents along Shoreline.

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

ADCPs employ the Doppler effect. They employ piezoelectric transducers which emit sound waves into the water. When the sound waves meet particles such as plankton, sediment, or bubbles in the water, they scatter some sound energy back towards the ADCP current meter. The amount of time for the sound waves to travel to the particles and back provides an estimate of how far away the particles are.

The Doppler shift is the solution to the measurement of current velocity. Because the particles are moving with the water current, the frequency of the sound waves which have been scattered and also picked up by the ADCP will differ from the transmitting wave's frequency. The magnitude of the frequency shift is a direct function of the water's velocity down the acoustic wave path. To measure three-dimensional velocities, most ADCPs have at least three beams. Modern ADCPs also include sensors such as temperature sensors to account for water temperature effect on sound velocity, compasses for instrument orientation determination, and pitch/roll sensors to get accurate measurements in the presence of wavey seas. The incoming signals are amplified, digitized, and processed to estimate the prevailing velocity at different depths.

5. What should be needed for good quality measurement of Shoreline coastal currents?

To enable good quality measurement of the coastal currents of Shoreline, the equipment used must meet several needs. The reliability of the material is critical. The ADCP casing must be made of a material that is robust and resistant to withstand the harsh marine environment. A good material is the titanium alloy. It is very corrosion-resistant, which is essential for long-term operation in seawater. Titanium alloy is also strong and lightweight, and therefore easier to deploy and maneuver. Its strength ensures that the ADCP current profiler can endure the mechanical stress of water flow and potential impact from garbage.

Size, weight, and power consumption are also important. A smaller and lighter ADCP is more versatile since it can be mounted on a wide variety of platforms, from small research ships to buoys or underwater robots. Lower power consumption translates into longer - term deployment, especially when battery power is used. Cost is also an issue. A lower - cost ADCP enables high - density measurements, increasing the spatial and temporal resolution of the data obtained.

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

Mounting Based on Types

• Ship-mounted ADCP: Installed on a traversing ship, this type can be utilized effectively to conduct large-scale surveys of coastal waters near Shoreline. As the ship proceeds, the ADCP can repeatedly measure the currents at intervals, taking a broad-scale perspective of the current flow.
• Bottom - mounted ADCP: Fixed on the seafloor, this is suitable for fixed - point, long - term monitoring. It can also provide valuable information on the variability and long - term trends in the currents at a given location.
• Buoy - mounted ADCP: Placed on a buoy, the ADCPs can ride with the water, allowing for measurement where fixed - point measurement is not possible. It is particularly useful in areas of strong tidal currents or where a more mobile platform for measurement is required.

Frequency Selection

The frequency of the ADCP depends on the depth of the water. A 600kHz ADCP is sufficient for a depth of 70m. In the relatively shallow coastal waters along Shoreline, a 600kHz ADCP can provide good current profiles. For depths of a maximum of 110m, a 300kHz ADCP is preferable. It provides a longer range and yet is similar in terms of accuracy. When operating in the deeper waters of the central Puget Sound, a 75kHz ADCP is best as it goes deeper into the water column.

Various brands of ADCPs that are well - known in the market are Teledyne RDI, Nortek, and Sontek. But for anyone looking for a cost - efficient but high - quality option,the ADCP supplier China Sonar's PandaADCP would be the best to go for. Made of all - titanium alloy, it possesses better durability in the marine environment. With a very good cost - performance ratio, it is appropriate for researchers, coastal managers, and anyone who needs stable current measurement data. For more information, see https://china-sonar.com/.

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