How can We Measure the Coastal Currents of Swanage?

1. Where is Swanage?

Swanage is a charming small coastal town on the south coast of England, in the county of Dorset. It lies on the Isle of Purbeck, bounded by the waters of the English Channel. Its ideal location, with its sandy beaches and dramatic cliffs from the Jurassic era, has made it a favored destination among tourists and a place with strong maritime traditions.

It has a history from medieval times, and the first records of settlement here date back to the early Middle Ages, orienting the life of its inhabitants towards fishing and trade. Architecture in Swanage represents a mix of traditional Dorset charm with modern convenience: stone-built cottages with thatched roofs standing alongside contemporary holiday homes and shops. The sea has an attached factor to the local population because it is their traditional occupation for generations, even though in reduced scale. Locals have their small harbor, where fishing fleet operates. This port also caters to a lot of pleasure boats, and boating trips along the coast are pretty common among tourists and locals.

The beaches in Swanage are not only the allure for the visitors but also the vital link in the chain of the local ecology. The waters of the English Channel that run along it have their unique life. The seals are quite commonly seen lazing around on the rocks lying offshore, and the seabed is populated by different fish species like pollock, wrasse, and crabs. The topography is made up of sand areas, rocky reefs, and submerged caves; the richness of these different elements underlines a variety of biological diversity but, at the same time, controls water movements.

2. What is the situation regarding coastal currents around Swanage?

In the surroundings of Swanage, coastal currents depend on several elements interacting with each other. Among the prevailing factors is tidal force. The English Channel has semi-diurnal tides, with two high and two low tides each day. The tidal range is subject to variation, and the consequential tidal streams can attain a speed of up to 2-3 knots in some areas, particularly during entrance into the small bays or around the constrictions of the coastline. At high tide, water rushes into the bays and carries with it nutrients and sediments from the open sea. At low tide, the water pulls back, exposing the intertidal zones and their host of shore-dwelling organisms.

The second important factor involves wind. Dominant south-westerly winds can push the surface waters toward the coast and create a prospering net coastal current. These winds also generate waves that interact with the currents to provide a more complicated flow pattern. The northerly winds will push the water away from the shore. The shape of the coastline, with bays, headlands, and the specific geology around the Isle of Purbeck, causes the currents to converge and diverge. The presence of underwater rocky reefs and sandbars further complicates the current patterns. These features may serve both as obstacles and conduits, therefore impacting the flow and creating turbulence or patches of slack water in certain areas. In addition, the input of freshwater from streams off the Isle of Purbeck also impacts the current patterns locally. The freshwater has a different density than the seawater, creating density - driven currents that interact with the tidal and wind-induced currents.

3. How to observe the coastal water flow of Swanage?

Surface Drifting Buoy Method

Surface drifting buoys are one of the simpler and low-cost methods to observe the surface-level currents. These buoys are generally tagged with GPS or satellite-based tracking systems. Once these buoys are let into the water, they will be carried by the surface currents. By analyzing the buoy's trajectory, scientists can estimate the speed and direction of the surface waters over time. However, this method is restricted to the upper few meters of the water column and may not be representative of the deeper currents.

Moored Ship Method

In the moored ship method, a ship is anchored at a specific location near Swanage's coast. Current measuring instruments, such as electromagnetic current meters, are then deployed. These instruments can measure the flow of water at different depths, providing a detailed vertical profile of the current velocity and direction. However, this method is limited to only one location, and the ship itself may disturb the natural water flow and, consequently, affect the measurement.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCPs introduced a whole new change in the measurement of coastal currents around Swanage. They are able to acquire the velocity profile of the whole water column from surface to seabed. The ADCPs use sound waves to measure the water flow non-intrusively. By emitting acoustic pulses and measuring the Doppler shift of the reflected signals from suspended particles in the water, they are able to calculate the current velocity at different depths. This provides a full picture of the current structure and is hence very suitable for the study of the complicated current patterns in waters around Swanage.

4. How do ADCPs using the principle of Doppler work?

ADCPs using the principle of Doppler basically work on the principle of the Doppler effect. They transmit high-frequency acoustic pulses into the water. These pulses, once they encounter minute particles suspended in the water-such as sediment, plankton, or even bubbles-the same are reflected back to the ADCP profiler by such particles. Provided these particles move with water currents, a reflected signal shows frequency shift related to the transmitted signal. The latter is what defines the so-called Doppler shift, and is directly proportional to the velocity of the particles relative to the ADCP. The ADCP current meter determines the velocity of the water at different depths by measuring the Doppler shift at those different depths in the water column. A number of transducers on the ADCP measure the components of velocity in various directions, thus determining the three-dimensional velocity vector of the flow of water.

5. What's needed for high-quality measurement of Swanage coastal currents?

Equipment Material Reliability

Good-quality measurement of coastal currents near Swanage depends basically on the dependability of materials used for that equipment. Titanium alloy would work well in composing the casing that will hold all the vital organs of the ADCP meter in light of the mean conditions presented previously around the bottom of the sea floor in the English Channel area. The high strength can also help withstand strong water current impact, impact with floating debris, and even corrosive properties of seawater. Low elastic modulus imparts the necessary flexibility so that a chance of damage by mechanical stresses can be minimum. Moreover, excellent corrosion resistance enables ADCP flow meter to stay deployed in seawater for longtime monitoring without deteriorating much.

Compact Size, Light Weight, Low Power Consumption, and Low Cost

Compact and lightweight, the ADCP current profiler is easier to install and operate on a small research vessel attached to a buoy or placed on the seabed. It will affect the natural water flow much less, yielding better measurements. Low power consumption is important in view of long-term autonomous monitoring, especially with battery-powered systems. The possibility of large-scale deployment because it is a low-cost ADCP is a requirement to fully understand the complex coastal current patterns around Swanage.

6. How to Choose the Right Equipment for Current Measurement?

Based on Usage

• Shipborne ADCP: Suitable for real-time current data along the path of a ship in the vicinity of Swanage, it is used in oceanographic surveys to study the general circulation in the area and also for the optimization of shipping routes.
• Bottom-mounted ADCP: Suitable for long-term fixed-point monitoring of the currents at the seabed. This will be useful for studying the long-term trends in the coastal currents, such as the impact due to climate change on the local marine ecosystem.
• Buoy-mounted ADCP: Useful to monitor surface-level currents within large areas as the buoy moves with the water. It allows for an understanding of spatial variability in the surface currents and how the coastal waters interact with the open English Channel.

Based on Water Depth

• 600kHz ADCP: For water depths up to about 70m, it provides high-resolution measurements in the relatively shallow waters off Swanage.
• 300kHz ADCP: For water depths around 110m, which may be useful in some areas of deeper sections of the English Channel off the town.
• 75kHz ADCP: For deep-water applications. However, it can be applied for the Swanage waters in studies related to the deeper-layer currents or for those areas with deeper channels, and is capable of measuring current up to depths of 1000m.

There are some famous brands of ADCP in the world market, such as Teledyne RDI, Nortek, and Sontek. For those who want to find a cheaper alternative, the China Sonar PandaADCP made of all-titanium alloy has given quite a good performance based on the price. You can learn more about it at (https://china-sonar.com/).

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