How can we measure the coastal currents of Rouen?

1. Where is Rouen?

Rouen is a historic and charming city nestled in the Normandy region of northern France. It lies about 120 kilometers northwest of Paris, and it's strategically placed on the Seine River. Its prime placement has been one of the major decisive factors in Rouen's development because, for long, the Seine has acted as an artery for trade, commerce, and cultural exchange.

Geographically, the area surrounding Rouen is very interesting, consisting of flat, rich plains interspersed with gently rolling hills. The Seine River gracefully curves its way through the town, not only lending a touch of scenic beauty to it but also regulating the local climate and ecosystem. As it approaches the English Channel, it forms an estuary near Le Havre, a port city with which the activities of Rouen are closely connected.

Rouen itself is a living museum of architecture: the skyline dominated by the magnificent Rouen Cathedral, that icon of Gothic architecture which has inspired generations of artists, including Claude Monet. The narrow, winding streets and half-timbered houses of its medieval quarters seem timeless in their appeal. Historically, Rouen was an important center of power, trade, and learning. It was the capital of Normandy during the Middle Ages and has been the scene of many events that have decided the course of French history.

The coastal waters off Rouen, where the Seine meets the Channel, are dynamic and complex. The estuary is a transition zone between the freshwater of the river and the saltwater of the sea, creating a special habitat for a wide variety of flora and fauna. However, here the tides are very large, with extreme changes over some metres, therefore, the consequence of tides on coastal currents may cause substantial transformations in sediment motion, nutrient cycling, and marine-life distributions.

2. How are the conditions for coastal currents at Rouen?

Current off shore or at Rouen was a multifunctional factor of. Tidal forces are perhaps the most dominating. The English Channel exhibits tides that are essentially semi-diurnal. It means there are two tides per day and thus, two high tides and two low tides. When water rushes up into the Seine estuary during flood tide, it creates a strong upstream current. Oppositely, when waters ebb or flow back towards the sea during ebb tide, it creates a downstream current. The magnitude and direction of these tidal currents can vary with the moon phase, spring tides being much stronger than neap tides during full and new moons, with neap tides occurring during the first and third quarters of the moon.

Wind patterns also play a crucial role. Prevailing winds coming from the west and southwest can significantly affect the surface currents. A strong westerly wind can push the surface waters towards the east, either enhancing or opposing the tidal currents. Sometimes, the wind - driven currents lead to the formation of upwelling or downwelling zones. Upwelling occurs when wind causes the surface water to move away from the coast and deeper nutrient - rich water rises to the surface. This acts as a fertilizer for the phytoplankton, drastically encouraging its growth and thus sustaining all levels of food webs in the area.

Other parameters will include coastline geometry and seabed bathymetry. The morphology of the Seine estuary is complex and in a continuous state of evolution, comprised of sandbanks, channels, and mudflats. These will tend to accelerate, decelerate, or change the direction of the currents. For instance, if water rushes through a narrow channel, the current speed will increase, but in a wider area, the current may slow down. The water might be disrupted by islands or headlands, which could also create eddies and turbulence.

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

Surface Drift Buoy Method

The surface drift buoy is one of the relatively simple and cost-effective means of monitoring the currents in this water mass. These buoys are designed to float on the surface and are fitted with tracking devices, including GPS receivers. When launched into the water, they are moved by surface currents, and their position is continuously tracked. By observing the track of the buoys over time, the scientist is able to estimate the speed and direction of the surface current. The drawback with this is that the buoys do not act in response to current alone but also to wind and wave actions. When the winds are very strong, it may push the buoy in a different direction from the real current - thus giving the wrong measurement. Besides, it reflects information only on the surface layer of the water, up to a few meters from the surface.

Anchored Ship Method

This involves anchoring the ship in a chosen location nearshore. From this ship, current meters are deployed at different depths to record the flow of water. Current meters may also be mechanical or acoustic. Mechanical current meters normally work with a propeller or a vane which measures the speed of the water flow, while acoustic current meters detect the movement of water particles by using sound waves. The advantage of this method is that it can provide detailed information about the vertical structure of the currents at a specific point. However, it is limited in terms of spatial coverage. These measurements represent only the area around the anchored ship, and the ship itself may disturb the local current patterns at times.

Acoustic Doppler Current Profiler (ADCP) Method

The ADCP current meter is an advanced and versatile method for the measurement of coastal currents. ADCPs can be deployed while a ship is moving, deployed from the bottom of the sea, or hung from floating buoys. Operationally, they rely on the Doppler effect. The ADCPs send an acoustic signal throughout the water column. These signals reflect off the small particles-possibly sediment, plankton, or bubbles-present in the water. If the particles are moved by the flow of the current, that reflected signal's frequency will have changed. By measuring this frequency shift, the ADCP current profiler can calculate the velocity of the water at different depths. ADCPs can provide high-resolution, real-time data on the three-dimensional structure of the currents over a large area. They are not significantly affected by surface wind or the movement of the measuring platform, making them a reliable choice for accurate current measurements.

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

ADCPs are based on the Doppler effect, which involves a shift in the frequency of a wave-an acoustic wave in this case-developing from the relative motion between wave source and observer. An ADCP flow meter sends out an acoustic signal into the water. The signal travels all the way through the water column and bounces back from particles in the water. If the particles are in movement with the current of the water, the frequency of the reflected signal is different from that emitted.

For example, if the particles are moving towards the ADCP, the frequency of the reflected signal will shift to a higher one-a blue-shift-and for the case when they are moving away, it will have a lower frequency, a red -shift. This frequency shift is measured by the ADCP and used to compute the velocity of the water from where the reflection occurred. Because the ADCP sends out signals at a number of angles and can measure the Doppler shift in each direction, it can obtain all three components of the three-dimensional water velocity: east-west, north-south, and vertical. The instrument divides the water column into several depth bins and can produce a profile of the current velocity as a function of depth by analyzing the Doppler shift for each bin.

5. What is needed for high-quality measurement of Rouen coastal currents?

Equipment Reliability

Equipment must be more reliable in order to achieve high-quality measurement of the coastal currents around the Rouen region. The marine environment is so harsh, with saltwater corrosion, strong currents, and variable weather conditions. The ADCP meter must be able to survive these challenges over long periods of time. This requires the use of high-quality materials and robust construction. The electronics and sensors within the ADCP should be protected from water ingress and electromagnetic interference. Calibration and maintenance are also crucial to make certain that measurements over time are taken accurately and alike.

Size, Weight, and Power Consumption

Size and weight are other critical factors of consideration for an ADCP. The smaller and lighter an ADCP, the easier to deploy and maneuver, especially under circumstances when there is limited space on a ship or buoy. This would have minimal interference with the flow of water around the current meter and minimize any chance of error in measurement. Additionally, low power consumption is crucial, especially for long - term deployments. If the ADCP is powered by batteries or a solar-powered system, low power consumption allows for longer operation without the need for frequent recharging or battery replacement.

Cost-effectiveness

Cost is a significant factor, especially when large-scale measurements are required. For the determination of the coastal currents around Rouen, more than one ADCP should be installed over a large area. The price of an ADCP must therefore be reasonable. Besides the purchase price, this also involves maintenance, calibration, and data acquisition costs. Cost-effective ADCPs enable more comprehensive monitoring programs that may raise the accuracy and reliability in the measurement of the current velocity.

The benefit of titanium alloy casing

A titanium alloy casing is very useful for ADCPs deployed in the coastal waters off Rouen. Titanium alloy resists corrosion so well that it provides for an environment of the saltwater of this region. It puts up with such moderate effects from the seawater for extended periods of time without severe degradation. Titanium alloy has also a higher strength - to - weight ratio. This means that the casing can be robust yet lightweight. The lightweight nature of the titanium alloy casing makes deployment and handling of the ADCP profiler easier, while the high strength thereof provides mechanical strength enough for the instrument to withstand stresses in the marine environment, such as wave action and water pressure.

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

Based on Usage

• Ship-borne ADCP: Ship-borne ADCPs are ideal for large - scale surveys of the coastal currents. While the ship moves along the coast, the ADCP can continuously measure the currents at different locations. This type of ADCP is suitable for mapping the overall current patterns over a wide area. It provides information on the spatial variation of the currents and can therefore be used to identify the location of strong or weak flow and the presence of eddies and other features in the flow.
• Bottom-mounted ADCP: The bottom-mounted ADCPs are designed to lie in position on the seabed. Use for long-term, fixed-point measurements of the currents. This type of ADCP is able to provide detailed information on the current conditions at a certain location, such as near-bottom current velocity and direction. It is useful in studying the local hydrodynamic processes, including sediment transport and the interaction between the water and seabed.
• Buoy-mounted ADCP: Buoy-mounted ADCPs are attached to floating buoys. They are used to measure the currents in areas where ship-based measurements may be difficult, or where continuous monitoring of the surface and near-surface currents is required. Buoy-mounted ADCPs can provide real-time data on current conditions and are useful in the study of short-term variability of the currents, like the response to changes in wind or tide.

Based on Frequency

The frequency of an ADCP depends upon the depth of the water in which it is supposed to work and the resolution that may be required by the measurements themselves. A 600kHz ADCP is generally meant for shallow water, say roughly up to a depth of about 70 meters or so. These provide high-resolution measurements, enabling one to study the detailed structure of currents in fairly shallow areas near the shore or in estuaries.

For 110-m water depths and below, use a 300kHz ADCP. In a nutshell, this constitutes a very good compromise between going deeper in the water without lessening the resolution and the very same 300 kHz, when it works very well for applications pertaining to quite a significant middle range; they need to reach sufficiently far but without diminishing resolution very much.

A 75kHz ADCP would be more appropriate for deeper waters up to 1000 meters. Lower frequencies will penetrate further down the water column, but resolution is worse with lower frequencies compared to higher ones.

Several good ADCP brands are well-known in the market. These include Teledyne RDI, Nortek, and Sontek. However, for a cost-effective and high-quality option, the Chinese brand China Sonar PandaADCP is a great choice. It is made of all-titanium alloy materials, ensuring durability and reliability. It offers an excellent cost-performance ratio, making it suitable for a wide range of applications. For more information, you can visit their website at (https://china-sonar.com/).

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