How do we measure the coastal currents of St. Augustine?

1. Where is St. Augustine ?

St. Augustine, on the northeast coast of Florida, USA, is the nation's oldest continuously occupied European - founded city. It lies between the Matanzas River and the Atlantic Ocean, and this geographical location is a typical one. The Matanzas River, a small stream, separates the old downtown area, or the "Old City," from Anastasia Island. The island is a natural barrier, protecting the mainland from the full force of the ocean waves and storms.

The city coastline is made up of sandy beaches, salt marshes, and mangrove swamps. The white sandy and soft beaches are the finest locations of tourist interest, where one travels because of heat and terrain provided. The salt marshes and mangroves are made up of diversified wildlife that support a variety of bird, fish, and crustacean life. These coastlines stabilize the coast along with filtering out contaminants, thereby affecting the local regime of currents.

St. Augustine's own historic charm is buried deep in its seaside setting. Founded by the Spanish in 1565, it has a storied history of sea trade and combat. The Castillo de San Marcos, a monumental rock citadel with a view of the Matanzas River, was built in the 17th century and protects the city from invaders. The fort and other buildings in the town are not just cultural symbols, but tangible property that could change water current in the area.

2. What are the coastal currents off St. Augustine?

There are a variety of factors affecting the coastal currents off St. Augustine. Tides are a significant factor, and it is a semi-diurnal tidal regime there. It has two high tides and two low tides daily. Tidal range, i.e., difference in level at high and low tide, is great, particularly at spring tide when the two gravitational pulls go together. On approaching high tide, water surges into the Matanzas River and causes flood currents which, when there is ebb tide, get reversed and are carried outward toward the sea.

Geometry of the coast as well as the presence of the Matanzas River estuary also play their part. Narrow river channels as well as the non-circular shape of the estuary channel the flow of water in a particular direction. For example, the Matanzas River mouth when flowing into the Atlantic Ocean might have strong tidal currents as water flows in and out with great force. Moreover, the presence of jetties and other artificial constructions, built to protect the harbor and maintain the navigation channels open, can affect the flow of natural currents.

Wind patterns are equally important. Predominant northeasterly winds in the area can cause surface waters near the coast to support or oppose tidal currents. During storms, e.g., hurricanes, high winds can form storm surges, which may have a powerful effect on local current patterns. Storm surges cause sudden rise in water level, flooding low-lying areas and altering the normal path of the currents.

3. How to track the coastal water current off St. Augustine?

Surface Drift Buoy Method

Surface drift buoys are an inexpensive and simple way of tracking the coastal water current off St. Augustine. GPS tracking devices are fitted to them, and they are deployed onto the surface of the water. While moving with the surface currents, their locations are recorded at intervals. This method provides valuable information about the drift of the surface layer of the water column. But this method is not without its limitations. Wind currents can cause the buoys to get displaced from the actual path of the current and therefore provide false readings. Surface drift buoys also provide data about the currents only at the surface level and cannot provide data about deeper layers of water.

Anchored Ship Method

The anchored ship method involves mooring a ship at a stationary point off the coast of St. Augustine. Current meters are then lowered from the ship at different depths. The meters measure the velocity and direction of the water flow at each depth, providing a vertical profile of the currents at a point. While this method has the potential to offer minute detail about the present at a single location, the very presence of the ship may cause an interruption to the natural flow of water movement. Additionally, the measurement at a single point drastically restricts the scope of data acquisition about the currents over the extensive stretch of coast.

Acoustic Doppler Current Profiler (ADCP) Method

The Acoustic Doppler Current Profiler (ADCP) is a more efficient and newer instrument for coastal current measurement in the area around St. Augustine. ADCPs have the ability to provide real-time, high-resolution data over a large vertical range in the water column. They can simultaneously measure currents at different depths, and this enables one to characterize in detail the three-dimensional structure of the coastal current system. ADCPs utilize the application of acoustic pulses to determine the flow of particles in the water, and from this, they can calculate the velocity and direction of the currents. This makes them highly effective at determining the complex current patterns in the waters off St. Augustine.

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

ADCPs operate on the Doppler principle. They transmit acoustic waves into the water. When these signals encounter suspended particles within the water, such as sediment or small marine organisms, the signals are redirected back towards the ADCP profiler. The frequency of the redirected signals is not the same as the emitted frequency. This difference, known as the Doppler shift, is linearly proportional to the particles' velocity and thus the velocity of the water current. By measuring the Doppler shift at different depths, the ADCP can calculate the water current speed and direction at different levels of the water column. This enables a complete and accurate measurement of the St. Augustine coastal current structure.

5. What is required to achieve high - quality measurement of St. Augustine coastal currents?

For high-quality measurement of coastal currents off St. Augustine, the measuring gear must meet several key requirements. The corrosive properties of seawater and the presence of sediment in the water make material reliability paramount. The gear has to be constructed from materials that can withstand long-term exposure to the harsh marine environment. Small size is beneficial in that it will not disrupt the natural water flow. Light structure is also essential for ease of deployment at different coastal locations, e.g., shallow water of the Matanzas River and beach. Low power consumption is critical, especially for long - term, unattended operation in remote coastal areas. Cost - effectiveness is also something to be taken into account as it makes for the large - scale deployment of measurement equipment so that it would cover the extremely long coastline from St. Augustine.

In ADCPs, an appropriate casing material selection is pertinent. A better option for casing in ADCP is titanium alloy. Titanium alloy possesses outstanding resistance to corrosion and is critical where long - term use in the brackish waters along the coast of St. Augustine. It also has a high strength - to - weight ratio, indicating that it is light but strong. This allows the ADCP current meter to withstand the physical forces and pressure variations within the ocean without sacrificing proper and accurate measurement for extended periods.

6. How to choose the right equipment for measuring currents?

Based on Usage

• Ship-based ADCP: This is ideally suited for large-scale coastal water surveys off the coast of St. Augustine. Fitted on research vessels, it can sample large areas as the vessel moves, providing valuable information regarding the spatial variation of coastal currents. Ideally suited for delineating the general current patterns over a broad section of the coast.
• Bottom-mounted ADCP: Located on the ocean floor, it is appropriate for long-term continuous monitoring at a station. It can measure current data continuously over extended periods, beneficial when investigating long-term behavior and trends in the coastal current in the vicinity of St. Augustine.
• Buoy-mounted ADCP: Mounted on floating buoys, it is employed for the measurement of surface and near-surface currents. It is a convenient form for deployment where ship access is problematic or unattended, long-term monitoring of surface-level currents is needed.

Dependent on Frequency

The selection of frequency for ADCPs is a critical consideration. A 600kHz ADCP would be well suited for water depth of about 70m. It provides high-resolution data and is therefore best suited to the relatively shallow waters off the coast of St. Augustine, where water depth is often in this range. A 300kHz ADCP can be used to depths of 110m, providing a good tradeoff between resolution and depth of penetration. For the deeper waters, up to a depth of 1000m, a 75kHz ADCP is preferable as it penetrates deeper but with lower resolution compared to the higher-frequency models.

Some of the common ADCP brands in the market include Teledyne RDI, Nortek, and Sontek. But for those seeking a cost - effective alternative without sacrificing the quality, then the ADCP supplier China Sonar's PandaADCP is a great choice. It's an affordable ADCP utilizing all - titanium alloy materials to ensure high corrosion resistance and durability. With its high cost - performance ratio, it's a great option for researchers and institutions in need of large - scale coastal current measurements in the proximity of St. Augustine. For further details, visit them on https://china-sonar.com/.

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