How are we going to measure the Bremerton coastal currents?

1. Where is Bremerton?

Bremerton, Washington's Kitsap County city, occupies the strategic location on the Kitsap Peninsula across from Puget Sound. Located some 17 miles west of Seattle and within the reach of a ferry, it is an important naval and industrial base. The urban development of the city has been inextricably tied to its position on water courses and as part of national defense.

Originally occupied by the Suquamish tribe. Their original culture, far into the land and sea, is perpetuated in local customs, art, and place names. European settlement occurred in the mid - 19th century, drawn by the natural resources of the region, specifically timber. Bremerton's position on Puget Sound made shipbuilding and naval operations a natural fit. Over time, the city has become an important hub for the United States Navy, and the Bremerton Naval Shipyard became an important provider of naval construction and ship maintenance to the nation.

Seas off Bremerton are an integral part of the Puget Sound community. Puget Sound is an extremely complex estuary that is a system made up of an inlets-branched channel-bay. The seafloor off Bremerton varies from shallow sandy beaches along the coast to deeper basins and channels offshore. Underwater reefs and rocky outcroppings provide a unique environment that is populated by a great variety of marine life. From salmon and shellfish to seabirds and marine mammals like seals and sea lions, the area teems with biodiversity.

2. What is the character of the coastal currents off Bremerton?

The coastal currents off Bremerton are governed by natural and man-imposed conditions. Tides are a major control. Puget Sound is a mixed tidal regime with semi-diurnal and diurnal tides. The tidal ranges are large, as much as 15 feet in some areas. During high tide, water overflows into the channels and bays near Bremerton, creating strong flood currents. When the tide recedes, ebb currents force water back out into the open body of the sound. The tidal currents are most powerful in narrow channels, such as Sinclair Inlet, where the water flow can be extremely rapid.

The larger ocean currents in the region also influence the local water. Inflow of water from the Strait of Juan de Fuca, which connects Puget Sound with the Pacific, influences coastal salinity, temperature, and current regimes in Bremerton coastal waters. The persistent winds such as the westerlies push surface waters towards the coastline, while strong easterly winds result in upwelling. Upwelling events force cold, nutrient-rich water to the surface, initiate phytoplankton blooms, and sustain the entire marine food web.

Human activities have been the cause of a very considerable modification of the natural circulation of currents. Artificial shipyard structures, marinas, and seawalls along the Bremerton waterfront zone have changed natural circulation patterns. The structures create local eddies, modify the direction of water flow, and affect the sediment and nutrient distribution. For instance, the extensive development of the infrastructure along Bremerton Naval Shipyard has changed the local currents regime and has the potential to affect the neighboring marine ecosystems.

3. How to observe Bremerton coastal water flow?

Surface Drifting Buoy Method

One concept to observe the coastal water flow near Bremerton is to use surface drifting buoys. Surface drifting buoys are designed so that they float on the water surface and drift with the flows. They have GPS monitoring equipment so that in real time they are capable of sending positional data. Scientists study this data in order to determine the direction and strength of the surface currents. This method, however, is not without its limitations. Wind may shift the buoys from the actual current, leading to misleading measurements of the subsurface flow. Secondly, surface drifting buoys provide data relative only to the surface section of the water column, providing limited knowledge regarding the overall current structure.

Anchor Moored Ship Method

The anchor moored ship method involves mooring a vessel at a point with a defined position. Scientists send current meters along the side of a ship at different depths to measure current velocity. It is depth - specific information on the currents, but it is time - consuming and expensive since it requires a research ship to be moored in place. The readings are again typical only of the immediate locality of the ship and, as such, cannot be applied to obtain a general impression of the coastal currents in an extensive area.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more advanced and more handy method for measurement of coastal currents. ADCPs work on the Doppler shift of sound waves to measure water current velocities at greater than one depth. They transmit sound pulses into the water column. Upon reflection off water particles, the change in frequency of the reflected pulses is used to calculate the water velocity. ADCPs can also provide a clear description of the structure of the currents from near the surface to close to the seafloor. This makes them highly suited to investigating the complex coastal currents off Bremerton.

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

ADCPs operate on the Doppler effect principle. ADCPs possess piezoelectric transducers which emit sound waves into water. Upon collision with particles such as plankton, sediment, or bubbles within the water, some of the sound energy is bounced back to the ADCP. The time that took the sound waves to reach the particles and bounce back is an estimation of the distance to the particles.

The key to measuring current velocity lies in the Doppler shift. Should particles be brought along with the water current, the frequency of sound waves bounced back to the ADCP will differ from the transmitted frequency. The amount of the frequency shift is proportional to the velocity of the water along the sound path. To answer the question of how three-dimensional velocities are measured when the ADCPs mostly use at least three beams, the new ADCPs also include additional sensors like temperature sensors to adjust for the effect of water temperature on sound velocity, compasses to indicate the instrument heading, and pitch/roll sensors to give accurate measurements even in turbulent seas. The received signals are amplified, digitized, and processed so that the present velocity at different depths can be determined.

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

To conduct high-quality measurement of Bremerton's coastal currents, the equipment used must possess a number of characteristics. Material reliability is paramount. The ADCP case must be made of material that can withstand the corrosive marine environment. Titanium alloy is highly appropriate. It is extremely corrosion resistant, which is needed for long-term deployment in saltwater. The titanium alloy is also strong and lightweight, rendering it more manageable and deployable. Its strength also ensures that the ADCP can resist mechanical stress due to its movement through water and potential impacts from detritus.

Size, weight, and power consumption are also important. The lighter and more compact ADCP is more generic, as it can be employed on a large variety of platforms, including small research vessels, buoys, or underwater vehicles. Less power consumption results in longer-term deployments, especially when batteries are employed. Another factor is cost. A lower - cost ADCP enables large-scale measurements, so the spatial and temporal resolution of the acquired data is greater.

6. What to Choose the appropriate equipment for measurement of currents?

Types of Mounting

• Ship-mounted ADCP: It is installed in a moving ship, and thus most suited to carry out large-scale surveys in the sea off Bremerton's coast. As the ship moves, the ADCP can make regular measurements of currents, and a general broad-scale observation regarding the trends in currents.
• Bottom - mounted ADCP: Mounted on the seabed, this type is best suited for fixed - point, long - term monitoring. It can provide useful information on the variability and long - term trends of the currents at a specific location.
• Buoy - mounted ADCP: These ADCPs, mounted on a buoy, are able to move with the water, allowing for measurements in areas where fixed - point measurement is not practical. They are particularly valuable in areas where there are high tidal currents or where a more mobile measurement platform is desirable.

Frequency Selection

The depth of the water dictates the frequency of the ADCP. A 600kHz ADCP can be employed in waters of up to 70m in depth. In the relatively shallow water around Bremerton, a 600kHz ADCP will provide detailed current profiles. A 300kHz ADCP is more suitable for water depths of up to 110m. It offers greater range with minimal loss of accuracy. For use in the deeper waters of the central Puget Sound, the best is a 75kHz ADCP as it travels deeper into the water column.

There are very many established brands of ADCPs in the market, such as Teledyne RDI, Nortek, and Sontek. But for anyone seeking a cost-effective yet quality ADCP, the ADCP supplier China Sonar's PandaADCP is the best to opt for. Constructed with all-titanium alloy, it boasts excellent durability in the marine setting. With an excellent cost-performance ratio, it is a fine tool for scientists, coastal managers, and any other user in need of reliable current measuring data. For more information, visit https://china-sonar.com/.

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