How can we quantify Charleston's coastal currents?

1. Where is Charleston?

Charleston, South Carolina, is a beautiful historic city of natural beauty. It is situated on the southeastern Atlantic coast of the United States, at the mouth of the Ashley and Cooper Rivers, where they empty into the Atlantic Ocean. Charleston was an important port city for centuries due to its location.

Charleston's historic district is a charming combination of preserved antebellum buildings, cobblestone streets, and gardens. The French Quarter, with wrought-iron balconies and narrow alleys, is reminiscent of Europe. Charleston's cultural scene is lively, with numerous art galleries, theaters, and a robust food culture that gives Southern cuisine a coastal twist.

Charleston is surrounded by a chain of barrier islands, such as Folly Beach, Sullivan's Island, and Kiawah Island. They are natural barriers that protect the mainland from the force of ocean storms. Charleston waters are part of the Atlantic Intracoastal Waterway, a 3,000 - mile - long inland navigable waterway from Massachusetts to Florida. It's a vital lifeblood for boating, shipping, and fishing in the region. The estuaries that the rivers form with the sea are teeming with life, full of all sorts of fish, shrimp, and crabs. The salt marshes along the riverbanks and on islands are not just lovely but also great places for sea animals to raise their families.

2. How is the condition of the coastal currents in and around Charleston?

There are several factors that condition the coastal currents in and around Charleston. The major contributory factor is tides. Charleston experiences semi-diurnal tides with two high and two low tides daily. The tidal range may be oscillating, but averaging 5 - 6 feet. The seawater floods the estuaries during high tide, including water-borne nutrients from the sea. With its nutrient supplementing stimulation to phytoplankton and the other sea crops, the nutrient-watery stimulating the growth in the estuaries is preferred. During low tide, the water is drained back into the sea along with sediment and organic matter accumulated in the estuaries.

Wind patterns also contribute significantly to the coastal currents. Regional season winds can drive water onto or off the coastline. Southeasterly winds dominate in summer. The southeasterly winds tend to drive surface water onto the coastline and create onshore currents. The onshore currents tend to enhance water mixing within the estuaries and bring warmer water from more off-shore areas. Northerly winds dominate in winter. They create offshore currents, and these have the potential to produce upwelling in some places. Upwelling results in cold, nutrient - rich water from deeper parts rising to the surface, which is beneficial for the development of some fish and other marine species.

The shape of the coastline and the bathymetry of the seafloor also determine the patterns of currents. The complex network of rivers, estuaries, and barrier islands can create a jigsaw puzzle of channels and inlets. The narrow inlets, such as those between the barrier islands, can limit water flow, accelerating currents. Water depth is also a factor. Shallower areas of nearshore water exhibit varying current patterns compared to deeper offshore waters. The subsurface sandbars and reefs likewise have the effect of deflecting the movement of water, creating eddies and other complex patterns of flow.

3. Observation of the Charleston coastal water flow

A way to see the coastal water flow off Charleston is by means of surface drift buoys. These are small, floatable devices equipped with GPS tracking instruments. Upon dropping them in the water, they are carried away by the surface currents. By observing the movement of the buoys over time, the direction and velocity of the surface - level currents can be estimated. However, this method only provides a view of the upper few meters of the water column and could be affected by wind - driven waves.

Another technique is the use of moored current meters. These devices are anchored to the bottom or seafloor of estuaries at permanent points around Charleston. They are able to record current velocity and direction at different depths. Moored current meters are capable of taking data continuously over long periods of time, which can be useful in comprehending the long-term character of the currents. But they are limited to where they are installed, and their installation and maintenance can be troublesome.

The Acoustic Doppler Current Profiler (ADCP) has proven to be a valuable instrument for measuring coastal currents in the Charleston area. ADCPs are able to measure current speeds at multiple depths simultaneously, providing a more representative picture of water flow. They are well adapted to Charleston's complicated estuarine and coastal environment.

4. On which principle do ADCPs work?

ADCPs work on the principle of the Doppler effect. They emit acoustic pulses (sound waves) into the water. When these sound waves encounter small particles in the water, say plankton, sediment, or small air bubbles, these particles scatter the waves. The ADCP measures the change in the frequency of the scattered waves.

If particles are moving towards the ADCP profiler, the scattered waves' frequency will be greater than the frequency of the original waves. If particles are receding, the frequency of scattered waves will be less. By measuring the frequency changes correctly, the ADCP is able to calculate water velocity at different depths.

Most ADCPs utilize multiple beams in order to measure three-dimensional flow of the water. For example, four-beam ADCP can calculate both the horizontal and vertical velocity of the current. This allows one to obtain a clear picture of the complex motion of the water, including the presence of vertical shear (where the velocity of the current changes with depth) and eddies (whirlpool-shaped patterns of currents).

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

There are several requirements for high-quality measurement of coastal currents off Charleston. The material should be reliable. Due to the corrosive environment and the presence of saltwater, the ADCPs and other equipment should be made with materials that do not corrode. Titanium alloy is an excellent choice to use for ADCP housings. Titanium is highly resistant to corrosion, and this guarantees the longevity of the equipment. This reduces the instances of frequent replacement, which is especially important for long-term monitoring operations.

Size and weight are also considerations. Lighter and smaller ADCPs are easier to deploy and transport, especially into the shallow and oftentimes hard - to - reach estuarine waters of Charleston. This is beneficial in the field because the researchers are able to move the equipment more conveniently to different points of measurement.

There is a need for minimum power consumption, particularly for extended monitoring. Because ADCPs are commonly battery-powered, components with minimal power requirements guarantee that the device can last for a significant period of time without requiring replacement batteries from time to time. This is very vital in remote coastal areas where power sources can be difficult to obtain.

Cost effectiveness is a principal issue, especially for large-scale monitoring programs. Low-cost but reliable ADCPs are highly necessary. China Sonar PandaADCP is a case of good cost-performance. Made from all-titanium alloy, it provides durability under the marine environment that is tough and yet remains cost-effective.

6. Selection of proper equipment for current measurement

The choice of equipment to measure currents off Charleston depends on the purpose. Ship-mounted ADCPs are ideal for large-scale survey work. They can be installed on research vessels and can measure the currents continuously as the ship moves along the coast, estuaries, and Intracoastal Waterway. This provides a wide-scale overview of the current flows in the area.

Bottom - mounted or moored ADCPs are ideally suited for long - term fixed - point observations. They may be placed on the seafloor or the bottom of estuaries at strategic locations around Charleston and take current measurements for months or years. This helps to study long - term trends in coastal currents.

Buoy-mounted ADCPs are best suited for surface-level current measurement. They are relatively easy to deploy and retrieve and are hence suitable for short-term studies or for quick checks of surface-current conditions.

In the choice of ADCP current meter, frequency is an important consideration. For water depths to 70m, a 600kHz ADCP is typically satisfactory. It has suitable resolution for measurement of currents in Charleston's relatively shallow estuarine and coastal waters. A 300kHz ADCP will suffice for water depths to 110m, and a 75kHz ADCP for greater depth to 1000m. Lower frequencies travel farther but are less resolved than higher frequencies.

There are some well-known ADCP brands, i.e., Teledyne RDI, Nortek, and Sontek. However, for cheaper alternatives without compromising quality, the ADCP manufacturer China Sonar's PandaADCP is another option to look into. Information can be obtained on its website: https://china-sonar.com/.

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