How do we measure Provincetown's coastal currents?

1. Where is Provincetown?

Provincetown, sitting at the tip of Cape Cod in Massachusetts, is a special and beautiful spot in America. The long, hook-shaped peninsula extends out into the vast sea of the Atlantic Ocean, and Provincetown therefore becomes a sea town and a natural haven. Geologically, it sits between the comparatively tranquil waters of Cape Cod Bay to the north and more energetic waters of the Atlantic Ocean to the south.

Provincetown has been a significant port for centuries. It was the site of the Pilgrims' initial landing in 1620, even prior to their settling at Plymouth. The town's most iconic monument, the Pilgrim Monument, is a massive testament to this momentous milestone in American history, attracting millions of visitors every year. Provincetown's active artistic community also helps make it so well-visited. It has been a retreat for artists, writers, and performers since the early 20th century, and there are various galleries, theaters, and annual art festivals along the town.

The beaches of Provincetown are a beautiful blend of sandy coastlines, salt marshes, and rocky cliffs. Sandy shores such as Herring Cove Beach and Race Point Beach offer picturesque spots for beachcombing, sunbathing, and swimming. The beaches also represent essential habitat for a variety of shorebirds, including the threatened piping plover. The salt marshes, in the inner-harbor position, provide a diverse environment for sea creatures, from clams and crabs to multiple species of fish. The marshes also serve as natural shorelines to shield against beach erosion and filter out impurities, promoting the overall well-being of the coastal ecosystem. Rocky coastlines, like those in the Province Lands, produce dramatic landscapes behind the ocean and provide breeding areas for seabirds such as cormorants and gulls.

Off the coast of Provincetown is an active convergence of oceanic systems. To the north is Cape Cod Bay with relatively calm conditions due to the protected effect of the Cape. Nevertheless, in virtue of its proximity to open Atlantic, Provincetown still has full exposure to the force of swells, storms, and strong currents. The convergence of such forces makes an understanding of Provincetown's coastal currents highly crucial to sail safely, to fish safely, and for conservation purposes.

2. What are coastal currents off Provincetown?

Coastal currents off Provincetown are characterized by a complex interaction of numerous forces. The dominant force is tidal forces. The local large-scale tidal regimes impose a periodic ebbing and flowing of the water. Water flows into Cape Cod Bay during high tide, affecting currents off Provincetown. In the narrow channels, such as the entrance to Provincetown Harbor, the tidal currents tend to be extremely powerful, several knots in velocity. The reversing tides also combine with the curvature of the coast, which reverses the direction and intensity of the currents.

Wind patterns also significantly determine the currents along the coastline. Southwest prevailing winds are capable of driving surface waters onto the coast and forming onshore currents. The stronger the winds, the larger the waves and the stronger the currents, which significantly impact local navigation and fishing industries. Northeast winds, on the other hand, are capable of forming offshore currents that sweep surface waters off the coast.

Bathymetry in the Provincetown area also helps form the currents. The deep channels and shallow banks of the area control the current. Shallow water slows down the currents, while deep channels make the water converge, so it travels at a higher speed. The Provincetown coast's configuration of many bays, inlets, and headlands also contributes to the intricacy of the current patterns. Headlands, like Long Point, induce the currents to split and accelerate around them, and bays trap and recycle water, creating local current systems specific to the region.

3. How to measure the Provincetown coastal water flow?

Surface Drift Buoy Method

One other perspective on the coastal water flow is with surface drift buoys. The buoys are fixed on the water surface and carried along with the currents. With the help of GPS tracking devices, they monitor their path over time. The path of these buoys can be examined to provide data on surface current patterns. The process has its limitations. Surface drift buoys are affected by wind-sustained surface waves and currents. It can be challenging to distinguish them, especially in high-wind areas. As a result, the readings may be skewed in relation to the true underlying current structure. Secondly, surface drift buoys are providing information from the surface of the water column and cannot see the fine vertical profile of the currents.

Anchor - Moored Ship Method

The anchor - moored ship method involves using a moored ship which is anchored at a known location. Current meters are lowered from the shipboard gear to measure at various depths for velocity and direction of the currents. This technique provides measurements at various depths, providing more information about the vertical structure of the currents than the surface drift buoy technique. But since the ship is anchored, it will only be able to sample the currents in a relatively limited region. It might not be enough to capture the large-scale variability of the coastal currents over a broad region, particularly in an active one like Provincetown. Moreover, the ship's presence can affect the natural course of the water and contaminate the accuracy of the measurements.

Acoustic Doppler Current Profiler (ADCP) Technique

The Acoustic Doppler Current Profiler (ADCP) has been introduced as a more advanced and simpler-to-use measuring method in recent years. ADCPs are commonly used for coastal surveys and research studies in oceanic science. ADCPs are capable of measuring water velocity at numerous depths over an extensive vertical range. By radiating sound waves into the water and detecting the Doppler shift of returned signals backscattered from suspended solids in the water, such as plankton or sediment, the ADCP will ascertain the velocity of the water. This provides a more complete and accurate description of the currents configuration in the water column and is properly geared to investigate the complex coastal currents around Provincetown. ADCPs are readily deployed in several manners, i.e., aboard a ship, on a mooring, or even on unmanned underwater vehicles, hence allowing measurements under dynamic conditions easily.

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

ADCPs function on the Doppler principle. They send acoustic pulses into the water at a specific frequency. When these pulses pass through moving particles in the water, such as suspended sediment or plankton, the frequency of the backscattered pulses changes. If the particles are approaching the ADCP, the frequency of the backscattered pulse will rise, and if they are moving away from it, the frequency will fall. The magnitude of the change in this frequency, the Doppler shift, is proportional to the velocity of the particles and therefore the velocity of water upon which particles are in suspension.

The ADCP is typically mounted on a series of transducers, each of which can transmit and receive signals at variable angles. It can thereby measure the three-component velocity of water flow (east-west, north-south, and vertical). By measuring the Doppler shift in several directions, the ADCP can calculate the velocity vectors of the water in various depths. Data is decoded to create a profile of the current velocity on a vertical cross-section of the water column. This profile helps in giving good information regarding the direction and intensity of currents at different levels of depth, and this is helpful in making the scientists advance more knowledge regarding the complex movement tendencies of the Provincetown coastal waters.

5. What's necessary for high-quality measurement of Provincetown coastal currents?

To have precise measurement of Provincetown coastal currents, there are some characteristics that are beneficial to the measuring instrument. The device should be made of very dependable materials. Due to harsh sea conditions, the device needs to be corrosion-proof against saltwater, should have sufficient strength for operation in high water pressure, especially at greater depths, and should be resilient for extended application.

The equipment must be compact in size. This renders it easier to transport and deploy to different coastal locations, even in areas with intricate topography or sites with limited access, such as narrow inlets or shallow bays around Provincetown. Lightweight equipment is also preferable since it reduces the problem of handling and transportation, be it from a ship or a dinghy.

Low power consumption is essential. This ensures that the device will be able to operate for extended periods without the necessity for frequent battery replacement or the use of a large, cumbersome power supply, which is not practical in remote coastal regions or in extended monitoring campaigns. Cost - effectiveness is also a major factor. A low - cost device enables it to be deployed more extensively, and large - scale measurement of the coastal currents can be attained. This becomes especially critical for broad-range studies that require data from greater than one site in the waters outside Provincetown.

With respect to ADCPs, the housing is a critical consideration. Titanium alloy is an excellent choice for the ADCP housing. Titanium alloy is strongly resistant to corrosion, which is important for long-term use in the corrosive seawater environment. It is also light, yet strong enough to protect the ADCP's internal components from physical damage, such as wave impact or debris. The corrosion-resistant nature of titanium alloy ensures that the ADCP will be capable of withstanding the brutality of the marine environment for a considerable duration, providing precise measurements for an extended period, which is vital in identifying the dynamic coastal currents surrounding Provincetown.

6. Selecting the Optimum Equipment for Measuring Current?

Classification of ADCPs Based on Application

ADC selection depends on the application. If the ship is in motion to be observed, then a ship-mounted ADCP would be the choice. It can record the currents during vessel passage and provide real-time information along the vessel track. This is useful when there is a need to cover a big area at speed during surveys or studies on how the currents change within a wide geographical extent.

A bottom - mounted (or moored) ADCP, or sit - on - the - bottom ADCP, is especially well - suited for long - term monitoring in the same location. It can be placed on the ocean floor and left to take current measurements for a long time. This will be valuable for examining the long - term trends and variability of the currents in an area, such as a particular bay or channel off Provincetown.

Buoy - mounted ADCPs, or floating ADCPs, are well suited to monitor large - scale oceanic or coastal processes. They may be carried by the currents, sending data from different points as they move. This is a highly suitable ADCP for monitoring the general circulation currents in the seas off Provincetown and how coastal currents interact with them.

Frequency Selection

The frequency of the ADCP is also something to consider. Different frequencies are best for different water depths. For shallow water depths of approximately 70m or less, an ADCP with a frequency of 600kHz is suitable. This frequency provides relatively high-resolution measurements in shallow water and allows for good analysis of the current structure near the surface and in the upper part of the water column.

For depths of around 110m, a 300kHz ADCP is preferable. It will probe deeper into the water column but still be sufficiently accurate, and it will be appropriate for mid-depth measurements in areas of moderate water depths, such as portions of the waters off Provincetown.

For very deep waters, up to 1000m, a 75kHz ADCP is recommended. Lower frequency ADCPs can penetrate deeper into the water and thus can measure deeper down. This is required to study the deep - water currents that may influence the overall coastal circulation off Provincetown.

Recommended Brands

There are several well-known brands in ADCP, some of which are Teledyne RDI, Nortek, and Sontek. But for a low-cost but good one, the ADCP manufacturer China Sonar's PandaADCP is great. More information can be obtained on its official website: https://china-sonar.com/.

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