How can we measure the coastal currents of Southend-on-Sea

1. Where is Southend-on-Sea

Southend-on-Sea is a lively seaside town in Essex, a county in the southeast of England. It covers the northern bank of the Thames Estuary and enjoys a privileged position at the mouth of the North Sea. Its favorable geographic position has made it an important center of maritime activities, tourism, and trade in this area for many centuries.

With its steepness in history, Southend has a rich heritage tracing back to the Roman times. The town features a mix of historical and modern architecture. The iconic Southend Pier is one of the longest pleasure piers in the world and acts as a symbol of the town's maritime past, while being a popular tourist attraction. Along the seafront, Victorian-era buildings stand alongside contemporary developments. The sea is inextricably linked with the local community, whose traditional source of employment, fishing, has diversified into a broad range of other industries linked to the marine environment. Vessels from inshore fishing boats to pleasure craft use the port at Southend, and it is also significant to the local economy. The nearby waters of the Thames Estuary constitute a unique ecology, which acts as a dwelling place for manifold marine species that include migratory fish, wading birds, and seals. The shallow waters and intricately interlocked tidal flats of this estuary provide it with great ecological importance.

2. What are the coastal currents near Southend-on-Sea like?

Offshore currents along the coast at Southend-on-Sea represent a complicated blend of several interacting factors. Tidal forces are dominant here, with large-scale tidal variation in the Thames Estuary. These tides are semi-diurnal; every day, two high tides and two low tides come in. Such a strong tide could flow along the estuary with speeds up to several knots, especially where there are constrictions or narrower channels within it. Water flows into the estuary at high tide with nutrients and sediment and, at low tide, recedes further from the shore, exposing the tidal flats.

Wind is the other factor. South - westerly prevailing winds push surface waters towards the coast, increasing the tidal currents. North winds cause the water to flow away from shore. The shape of the coastline, featuring so many bends and inlets, makes the currents converge and diverge. The presence of sandbanks and shoals within the estuary further complicates the current pattern. These features underwater may either act as barriers or channels for the flow of water, which creates areas of turbulence and calm. In addition, the freshwater input from the River Thames itself has a great effect on the coastal currents. The river outflow will mix with the seawater and this will develop a salinity gradient that may induce density -driven currents.

3. How to observe the coastal water flow of Southend-on-Sea?

Surface Drifting Buoy Method

The surface drifting buoys are one of the simplest means to observe surface level currents. The buoys are prepared with GPS tracking devices and released in the water. Because they are travelling on the surface currents their motion can be monitored in real-time. By determining the position of a buoy in successive order, a researcher can estimate the speed and direction of the surface waters in that location. This is quite limited because it represents only the top few meters of the water column, and may or may not accurately represent the deeper currents.

Moored Ship Method

In the moored ship method, a ship is anchored at a certain location in the estuary and deploys current-measuring instruments. Such instruments, like mechanical current meters, are capable of measuring the flow of water at different depths. The data collected gives a detailed vertical profile of the current velocity and direction. But this technique is site-limited and can be affected by the presence of the vessel, which might perturb the natural circulation of the water.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCPs have transformed the monitoring of coastal currents around Southend-on-Sea. They are able to measure the velocity profile of the whole water column from the surface to the seabed. The ADCPs measure the water flow non-invasively using sound waves. They can measure the actual velocity at different depths by emitting an acoustic pulse and measuring the Doppler shift of the reflected signal from suspended particles in the water. This provides a complete profile of the current structure's velocity and is, therefore, well suited to the complex coastal patterns being studied in the Thames Estuary.

4. How do the ADCPs employing the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. They send high-frequency acoustic pulses through the water. These pulses are reflected back to the ADCP by the small particles in the water, such as sediment, plankton, or even bubbles. If the particles are moving with the current, then the frequency of the reflected signal will be shifted compared to that which was transmitted. This Doppler shift is directly proportional to the velocity of the particles relative to the ADCP. By measuring the Doppler shift at different depths in the water column, the ADCP calculates the velocity of the water at those depths. Multiple transducers on the ADCP are used to measure the velocity components in different directions, allowing for the determination of the three - dimensional velocity vector of the water flow.

5. What’s needed for high - quality measurement of Southend-on-Sea coastal currents?

Equipment Material Reliability

For high - quality measurement of the coastal currents near Southend-on-Sea, the reliability of the equipment materials is crucial. The casing of the ADCP should be fabricated with a material able to resist the hostile estuarine conditions comprising high-velocity tidal streams, high salinity, and possible impacts from floating debris. Titanium alloy is an excellent choice because it has very good strength, with a low elastic modulus, and is outstanding in corrosion resistance. It has good resistance to mechanical stress and corrosive effects of seawater, hence making long-term and stable operation possible.

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

A compact and lightweight ADCP is easy to install and operate on a small research boat, attached to a buoy, or placed on the seabed. It also minimizes disturbance in the natural flow of water, thus giving highly accurate results. Long-term autonomous monitoring, especially with battery-powered systems, requires low power consumption. The potential to deploy many instruments is a key factor in gaining a comprehensive understanding of the complex coastal current patterns in the Thames Estuary, made possible by a low-cost ADCP.

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

Based on Usage

• Shipborne ADCP: Suitable for real-time current data along the ship's route in the estuary. It can be used for oceanographic surveys, studying the general circulation in the Thames Estuary near Southend-on-Sea, and for optimizing shipping routes.
• Bottom-mounted ADCP: Suitable for long-term, fixed-point monitoring of the currents at the seabed. This is valuable for studying long-term trends in the coastal currents, such as the impact of climate change on the local marine ecosystem.
• Buoy-mounted ADCP: Suited for monitoring the surface-level currents over a large area while the buoy drifts with the water. It enables understanding of the spatial variability of the surface currents and how the coastal waters interact with the open North Sea.

Based on Water Depth

• 600kHz ADCP: Suitable for water depths up to about 70m, which provides high-resolution measurements in the relatively shallow waters of the Thames Estuary near Southend-on-Sea.
• 300kHz ADCP: Suitable for water depths around 110m, which may be useful for areas with slightly deeper channels in the estuary.
• 75kHz ADCP: For deep - water applications, although the Thames Estuary is generally shallow, it can be used in some deeper sections or for studies related to the connection with the North Sea. It is capable of measuring currents to depths of up to 1000m.

Among several well-known brands in the global market for ADCP, Teledyne RDI, Nortek, and Sontek emerge as leading ones. In case one is searching for a budget option, then a China Sonar PandaADCP made of all titanium alloy may prove to be the most apt, in terms of value for money, quality, and price. Learn more at https://china-sonar.com/].

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