How can we determine the coastal current of Le Havre?

1. Where is Le Havre?

Le Havre is a busy port city situated on the northern bank of the Seine River estuary in Normandy, northwestern France. Its privileged position places it at the center of a diverse and dynamic coastal ecosystem.

The city itself is an interesting combination of historic and modern aspects. With a population of approximately 180,000, Le Havre has its own unique feel. Having been destroyed to a large extent during World War II, it was laboriously rebuilt, with its architecture today reflecting a mix of post - war modernist design and what is left from its earlier maritime-inspired buildings. It is a city known by its animated port-side promenades lined with cafes, restaurants, and shops where locals and visitors can be found anytime, taking in the atmosphere of the sea.

Le Havre borders the Seine Bay, one of the important bodies of water in the hydrography of the region. The Seine Bay is thus an estuary embayment area where the fresh waters of the Seine River meet the salinity of the English Channel; hence, in this bay the aquatic environment continuously changes. Here, from species of small-sized fish thriving within the nutrient-rich waters of the estuary to quite large migratory fish, assemblages used the bay either as a nursery or feeding site. It is an essential stopover for many seabird species and, therefore, an area that is attractive to birdwatchers. The coasts around Le Havre are lined with sandy beaches in places and rocky outcrops in others, lending it visual value as well.

2. What is the situation of the coastal currents close to Le Havre?

Of all the driving factors present, there exists great variety in the influences upon the coastal currents off Le Havre. Some of the most evident of these driving factors are tidal forces. The Seine Bay is a semi-diurnal tide; that is, it exhibits two high and two low tides daily. This rise and fall in tidal action drives very substantial water input and output to and from the Seine Bay. At the high tide, water pours into the bay and forms an incoming, or flood, current, and at low tide an ebb current is formed as the water flows out towards the open sea.

Another important factor is wind. The prevailing winds in the area, such as the south-westerlies, have a great effect upon the surface currents. Long, strong winds will push the surface waters either increasing the speed of the currents already there or changing their direction. For example, the consistent onshore wind will push up the water mass near the shore; hence, increase the water surface level and turn the direction of the currents' flow. In contrast, the offshore wind enhances the ebb current with water now leaving the bay much faster.

The other dominant controller of the current within the coastline is the discharge of freshwater resulting from the Seine River. The Seine River runs a good amount of fresh water into the bay. This fresh water plume is less dense than the salt water coming in from the English Channel and thus always spreads out onto the top of the denser seawater, making all sorts of complicated patterns in its flow. This can also create fronts and mixing zones, which may influence the movement of the coastal currents.

3. How to observe the coastal water flow of Le Havre?

Surface Drifting Buoy Method

The surface drifting buoy method is a rather simple way to observe the coastal water flow. Special buoys are set into the water. These buoys were to drift freely with the currents on the surface. They would contain tracking equipment, often GPS receivers. As the buoys are transported by the current, the position of the buoys' GPS receivers at regular time intervals is recorded. In analyzing the path these buoys have traveled over time, the direction and speed of surface currents can be obtained. There are a couple of drawbacks associated with this method. It gives information on the top portion of the water column, usually within the first few meters of it. Besides, the buoys can be affected by wind-induced waves, which can make them move out of the actual path of the current.

Moored Ship Method

In the moored ship method, a stationary ship is used as a platform for current measurements. Current meters attached to the ship at different depths detect the passage of water. The water flow in one form or another rotates or vibrates components within the current meter, which is translated into measurement of current velocity. This allows data on the vertical structure of the currents at that one location to be collected. However, this means a limitation with regard to the spatial coverage. Since the ship remains moored in one place, it can only measure the currents of that particular location; this makes it somewhat difficult to obtain an overview of the greater coastal current patterns.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current profiler is a more sophisticated and convenient solution for measuring coastal currents off Le Havre. ADCPs can measure simultaneously the water velocity at more than one depth. They can be deployed in a variety of ways, including being mounted on a moving ship, moored to the seafloor, or attached to a floating buoy. This provides a wider range of spatial and temporal measurements. It can be mounted on a vessel and produce maps of the current's present patterns over wide areas as the vessel is underway. Mated to the ocean floor or a buoy, it can collect long-term continuous data at a single location. In general, the ADCP flow meter can be used to provide a detailed view of coastal currents that is not possible with the surface drifting buoy and moored ship methods.

4. How do ADCPs using the Doppler principle work?

ADCPs operate on the well-known Doppler principle. First, an ADCP sends acoustic signals into the water. These become directed to small particles suspended in the water-sediment, plankton, or small air bubbles. When such particles are in movement-that is to say, carried by the current of water-the frequency of the reflected acoustic signals will change.

The Doppler shift in frequency is directly proportional to the velocity of the particles and hence the water relative to the ADCP. If the water is moving towards the ADCP, the frequency of the reflected signal will be higher than the frequency of the emitted signal. If the water is moving away from the ADCP, the reflected signal will have a lower frequency. By accurately measuring this Doppler shift for signals returned from various depths, the ADCP can determine the velocity of the water at those specific depths.

Most ADCPs are fitted with multiple acoustic beams, typically four or more. The beams are oriented in various directions. This ADCP current meter combines these velocity measurements, one from each beam, to obtain the three - dimensional velocity. For example, it is possible to derive the horizontal components of the velocity, i.e., east-west and north-south, from measurements in the horizontally angled beams, and the vertical component of the velocity from the vertically directed beam. This multi-beam setup thus offers a comprehensive overview of the movement of water across the area where measurement is to be effected.

5. What is required for high-quality measurement of coastal currents off Le Havre?

Equipment Reliability

Equipment reliability is an overriding constraint for high-quality measurement of coastal currents near Le Havre. The marine environment off Le Havre is severe: saltwater corrosion is important, tidal forces are strong, and weather is variably rough. An ADCP that is reliable must be able to operate for long periods without significant degradation in performance under such conditions. It needs to be run continuously, deliver data with accuracy and consistent results in unmoving and unstable rough seas and high winds in extreme temperature changes.

Small Size, Light Weight, and Low Power Consumption

A high premium is desired with a very compact and light weight ADCP device. A smaller gadget is pretty handy to place upon a smaller-sized research vessel on a buoy and moor underwater. A smaller device minimizes the degree to which the external natural flow is altered, having a smaller likelihood for measurement errors. Another very important design aspect is to have low power usage. Most practical deployments will want this because, if anything, long-term deployment ADCPs may be required to run just on batteries and small-scale renewable resources, like small solar panels attached to them. A low power-consuming ADCP can run for longer times without frequent battery replacement or recharging to allow for continuous data collections.

Low Cost

Cost is also another determining factor, especially for large measurement campaigns. If the price of the ADCP meter is too high, it may prevent how many ADCPs can be deployed. This would have the consequence of reducing the spatial and temporal coverage of the measurements; thus, a thorough understanding of coastal current patterns could hardly be obtained. An ADCP at a cost-effective level creates the possibility for more deployments. In turn, this enables scientists to measure data from multiple locations simultaneously for longer periods.

Titanium Alloy Casing

For an ADCP profiler applied to the coastal waters around Le Havre, casing made up of titanium alloy can be excellent. Titanium alloy has several advantages. Firstly, it possesses great corrosion resistance. Seawater in Seine Bay corrodes many ordinary materials within a very short period, while it can resist such corrosion for quite a long time. Secondly, it is lighter compared to the corrosion-resistant material such as stainless steel. The consequence is that this is easier to handle and deploy, especially for places where the constraint of weight needs to be observed, like on a buoy or a small boat. Thirdly, the strength in a titanium alloy will protect the internal components from mechanical stress, caused by waves and currents or any other impacts.

6. How to Select Proper Equipment for Current Measurement?

According to Application

• Shipborne ADCP: It is an ADCP that is designed to be installed on a ship. It does best in large-scale surveys of coastal currents. While the ship is underway, ship-borne ADCP can measure the currents along the ship's track. This provides a wide overview of the present patterns over a big area. Ship - borne ADCPs are often used for initial mapping of coastal current systems because they can cover long distances in a comparatively short time.
• Bottom-mounted (Sit-on-bottom) ADCP: A bottom - mounted ADCP is moored to the seafloor. It is well-suited for long-term monitoring of currents at a fixed location. This type of ADCP is able to give continuous data of the vertical structure of the currents at a particular site. Using these data, the scientists are enabled to study local current patterns-like how the currents change with tides, seasons, or over the long term.
• Float-buoy-type ADCP: These ADCPs are attached to buoys. It is good to go with measurement of currents in a more mobile yet still relatively local area. The buoy-mounted ADCP is able to move with the currents and provides information on the water movement in the vicinity of the buoy. This is very useful for research into the behaviour of currents around areas where flow might be affected by local features such as near-shore structures or underwater topography.

Based on Frequency

An ADCP is chosen based on the intended depth of measurement.

• A 600kHz ADCP is suitable for water depths less than 70m. In this frequency, the acoustic signals have a relatively high resolution, hence giving good and detailed measurements of the currents in shallow waters. The higher frequency signals are better at detecting small-scale variations in the current velocity near the surface and in the upper layers of the water column.
• A 300kHz ADCP is appropriate for water depths of around 110m. It offers a good balance between depth penetration and measurement resolution. This frequency can reach deeper into the water column compared to 600kHz, while still providing sufficient resolution to accurately measure the current velocity at different depths.
• Up to 1000 m water depths, a very adequate ADCP uses a frequency of 75kHz. Due to the lower frequency of acoustic signals, the diving operation extends deeper in water column length. With lowered frequency, as the resolution measurements are somewhat comparable to those produced by higher frequency ADCPs, measurements have relatively insufficient resolution; howsoever, with it, they are sufficient in getting an average view of general current patterns.
• Several brands have a great reputation in the market for ADCP, including Teledyne RDI, Nortek, and Sontek. However, for a good cost-effective and quality alternative, there is one Chinese brand highly recommended, namely China Sonar PandaADCP. It uses all-titanium alloy material construction for its casing and can boast good endurance. With its remarkable cost - effectiveness, it is an ideal choice for various current measurement needs, especially for large - scale and long-term monitoring projects. You can find more information on their website: https://china-sonar.com/.

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