How can we measure the coastal current of Poole

1. Where is Poole?

Poole is a sea-side town that is located on the south coast of England in the county of Dorset. It has one of the exceptional natural harbors called Poole Harbour, regarded as one of the largest in the world. This wide and sheltered sheet of water has been of quite great value to the town as it has become a focal development point over the years for all kinds of activities related to maritime.

Poole is a very historical city, which goes back into the Iron Age. This centuries-old past mirrors its architecture that contains both ancient edifices and modern construction. On the quay, handsome-looking Georgian-type houses and old fishing cottages stand alongside ultra-modern waterfront apartments. For centuries, this has been a community of the sea. Fishing has long been a profession, and the knowledge and skills are passed on through successive generations of local fishermen. Nowadays, though the fishing industry has entered the modern age, an active fleet remains based in the harbour.

Poole Harbour is not only vital to fishing but also to boating and tourism. This harbour is a favourite venue for yachtsmen and sailors with its abundance of marinas and other boating facilities. It caters to vessels of all kinds, from small fishing boats to large luxury yachts. The adjacent waters of the English Channel, connected with the harbor, form a dynamic ecosystem in itself. They are a habitat for almost all types of aquatic life, including seahorses, different fish species such as bass, mullet, and mackerel, and seabirds. The seafloor topography off the coastline comprises sandbanks, mudflats, and rocky outcrops that distinctly define the submarine morphology and affect the flow of water.

2. What is the situation of the coastal currents near Poole?

The coastal currents around Poole result from a very complex interaction. Tidal forces are the major determinant. Poole Harbour has semi-diurnal tides with two high tides and two low tides each day. The tidal range can be considerable, with resulting tidal currents in some areas of 3 to 4 knots, particularly in the narrower channels within the harbour and at the entrance to the English Channel. At high tide, the water enters the harbor with nutrient and sediment supplies, while at low tide, the waters pull back to expose the intertidal areas.

Another major determining factor involves the wind conditions. Dominant southwest winds can also push surface waters towards the coasts, thereby strengthening the tidal currents. Prevailing winds can be of a sufficient strength to create wave motions interacting with the currents and give a more complex flow pattern. The northerly winds can push the water away from the shore. The shape of the coastline, with its bays, headlands, and the big span of the harbour, creates convergences and divergences in the currents. The sandbanks and rocky outcrops in the sea floor make the patterns even more complicated. These underwater features can be barriers or channels, which also deflect and reorganize the water flow so as to create turbulence and areas of calmness. Moreover, the inflow of freshwater from the local rivers, such as the River Frome, may be relevant for the local current pattern. The freshwater has a different density than the seawater, and thus density-driven currents form that interact with the tidal- and wind-induced currents.

3. How to Observe the Coastal Water Flow of Poole?

Surface Drifting Buoy Method

Surface drifting buoys are one of the easiest but most practical tools for observing surface-level currents. The buoys are fitted with GPS or satellite-based tracking systems. Once the buoys are released into the water, they are taken by the surface currents. By analyzing the buoy's trajectory, scientists can estimate the speed and direction of the surface waters. However, this method is limited to the upper few meters of the water column and may not accurately represent the deeper currents.

Moored Ship Method

In the moored ship method, a ship is anchored at a specific location near Poole's coast or within the harbor. The present measuring instruments, such as electromagnetic current meters, are then deployed. These instruments can measure the flow of water at different depths, thus giving a detailed vertical profile of the current velocity and direction. This method is restricted to a single location, with the possibility of interference from the ship itself disturbing the natural water flow and thus affecting the accuracy of the measurement.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCPs revolutionized the measurement of coastal currents around Poole because they were capable of determining the velocity profile in the whole water column, from surface to seabed. In principle, an ADCP non-invasively measures water flow with sound. It can find the current velocity at several different depths by transmitting acoustic pulses and measuring the Doppler shift in the signals that bounce back from suspended particles in the water. This provides an integrated view of the present morphology and makes it very suitable for studying the complex coastal current patterns in Poole Harbour and in the adjacent English Channel.

4. How do the ADCPs with the principle of Doppler function?

The working of ADCPs is based on the Doppler effect. The devices emit high-frequency acoustic pulses into the water. These pulses would be reflected by the small particles in the water, such as sediment, plankton, or bubbles, and return as an acoustic signal back to the ADCP profiler. Since these particles have a movement connected with water currents, the reflected signal frequency shifts compared to that of the signal transmitted. In this respect, the Doppler shift is linearly 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 meter is able to calculate the velocity of the water at those depths. Several transducers on the ADCP measure the velocity components in various directions to allow for determination of the three-dimensional velocity vector of the water flow.

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

Equipment Material Reliability

For high measurement quality of coastal currents near Poole, several conditions should be foreseen considering equipment materials. Its casing should be manufactured from material able to bear extreme marine conditions in Poole Harbour and in general in the English Channel, such as made of titanium alloy. Its high strength enables it to withstand the impact of strong water currents, possible collision with floating debris, and the corrosive nature of seawater. The low elastic modulus imparts flexibility, reducing the possibility of damage due to mechanical stress. Furthermore, its excellent corrosion resistance allows the ADCP flow meter to operate in seawater for extended periods without significant deterioration.

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

Being compact and lightweight, an ADCP current profiler is much easier to install and operate, whether on a small research vessel attached to a buoy or on the seabed. It interacts much less with natural water flow and provides much better accuracy of measurement. Low power consumption allows operation in the mode of long-term autonomous monitoring when battery-powered systems are involved. The low-cost ADCP can provide a means of large-scale deployment that will be essential to gain a full understanding of the complex coastal current patterns around Poole.

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

Based on Usage

• Shipborne-ADCP: for real-time current data along a ship route in Poole Harbour and adjacent English Channel; oceanographic surveys may be conducted with the general circulation studies in the area and also shipping route optimization.
• Bottom-mounted ADCP: suited for the long-term monitoring of currents at the seabed in a fixed point. It is, therefore, relevant for studying the long-term tendencies of the currents near the coast, for example, how changes in climate might be affecting the local marine ecosystem.
• Buoy-mounted ADCP: Suited for surface-level currents over a large area, with the buoy drifting with the water. It enables understanding of the spatial variability of the surface currents and how the coastal waters interact with the open English Channel.

Based on Water Depth

• 600kHz ADCP: Conceived of for water up to about 70m deep, it gives high-resolution measurements in the relatively shallow waters near Poole.
• 300kHz ADCP: Suitable for water of depth around 110m, which can be useful for spots with slightly deeper sections within the harbor or the adjacent channel.
• 75kHz ADCP: For deep-water applications. Although waters around Poole are generally not that deep, it can be used for studies related to the deeper - layer currents or for areas with deeper channels. Can measure currents at depths to 1000m.

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