How do we measure the Valdivia coastal currents?

1. Where is Valdivia?

Valdivia, which is a highly attractive city in the Region of Los Ríos in the south of Chile, Chile, is located in a special place where the Valdivia River flows into the Pacific Ocean. Valdivia is located approximately 670 kilometers southwest of Santiago, the capital city of Chile. Valdivia is also widely renowned for historic beauty, pluralistic culture, and natural beauty. Valdivia was established in 1544 by Spanish conquistador Pedro de Valdivia and has since developed as a key economic and cultural hub in the region.

The coastal ecosystem of Valdivia is a mix of riverine and marine ecosystems. The broad mouth of the Valdivia River is a zone of transition between the river's freshwater and the ocean's saltwater. The region contains extensive tracts of mangrove forests, which create significant habitat for many marine and terrestrial organisms. Along the coastline, there exist miles of sandy beaches, sea-jutting cliffs, and shallow coves that create a varied and scenic seascape. The city itself is located on the river, and its system of canals and bridges creates a picturesque habitat. Valdivia harbor is a significant part of the regional economy, which traps a variety of cargo that ranges from forest products to produce and seafood. The conjunction of its location, past, and economic roles guarantees that investigation of the coastal currents off Valdivia is of fundamental importance to regional marine environmental knowledge, secure navigation, and sustainable development.

2. What are the coastal currents off Valdivia like?

The sea waters off Valdivia are also under the influence of a complex array of forces. Large-scale Pacific Ocean circulation, and more specifically the Humboldt Current, or Peru-Chile Current, plays a very significant role. The Humboldt Current is a cold, nutrient-rich current flowing north along South America's west coast and affecting waters off Valdivia. The Humboldt Current brings cooler water temperatures, increased levels of productivity, and a unique mixture of marine organisms to the region. Its presence can alter the direction and velocity of the local coastal currents and influence the distribution of plankton and nutrients, and thereby impact the entire marine food chain.

The local wind patterns also play an important role in the coastal current situation. Valdivia experiences a variation in wind conditions throughout the year. The winds over the region can generate surface water movement, forming wind - driven currents. Offshore or onshore winds that are strong can radically change the coastal currents in some cases. For example, strong offshore winds can lead to upwelling events, where cold, nutrient-rich deep sea water is transported to the surface. This upwelling not only affects the temperature and nutritional content of the surface waters but also the direction and strength of the currents. Additionally, the shape of the surrounding land, i.e., the surrounding hills and mountains, can guide and modify the direction of the wind flow, and the current patterns get further complicated.

The flow of the Valdivia River itself is another factor that affects the coastal currents. The river's fresh water discharge can create a plume of low-salinity water that disperses into the ocean. The plume of fresh water will be able to mix with the salt water of the ocean, affecting the density and currents of the water masses. Mixing of seawater and freshwater can create different current patterns like estuarine circulation where the water enters and leaves the estuary in a complex pattern. The tidal currents also account for the change of coastal currents along Valdivia. The semi-diurnal tides of the region produce repeated fluctuations in the water level, generating ebb and flow currents that mix with the remainder of the current-generating forces, resulting in a changing current regime.

3. How can the coastal water flow of Valdivia be observed?

There are a few means by which the coastal water flow of Valdivia can be observed. Surface drifting buoy method is the traditional method. They release special buoys, which carry GPS, into the ocean. The surface currents carry these buoys, and the trajectory of the buoys is monitored over a period of time. The direction and speed of the surface-layer currents can be determined from the path traveled by the buoys. However, this method can only provide data regarding the surface flow and not necessarily the current situation at larger depths in the water column.

The anchored ship method involves a ship anchoring at some point along the coast or estuary. Measurement equipment on board, such as current meters, record the water flow at various depths near the ship. While this method can potentially yield more depth-specific information, it is limited to where the ship can be anchored and can be affected by the presence of the ship, which can introduce unnatural water flow patterns.

In comparison, the Acoustic Doppler Current Profiler (ADCP) method has emerged as a highly advanced and efficient tool for coastal current measurement. ADCPs can measure the velocity of water at numerous depths simultaneously and thus obtain a complete understanding of the current structure in the water column. ADCPs are an ideal tool for studying the complex and dynamic coastal current systems offshore of Valdivia. With their ability to record three - dimensional water flow data, ADCPs have the potential to bring out the subtle patterns of the currents, like the interaction of surface, mid - water, and near - bottom flows, that are crucial in making a complete picture of the surrounding marine environment.

4. How do ADCPs based on the Doppler principle work?

ADCPs operate on the principle of Doppler. ADCPs emit acoustic pulses into the water column at a fixed frequency. The acoustic pulses move in the water and encounter particles suspended in the water, i.e., sediment, plankton, and small animals. The particles are carried along by the water when it flows, and this changes the frequency of the backscattered acoustic pulses on their return to the ADCP.

By correctly measuring the shift in frequency, the ADCP flow meter can calculate the velocity of water at different depths. The device is usually composed of a series of transmitters emitting signals at different angles. The ADCP, through this provision, can measure the three - dimensional components of the velocity of water, the horizontal as well as the vertical direction. The information gathered is subsequently processed using onboard or shore-based computers to create detailed visualizations and archives of the conditions at any moment. The scientists can then study the complex flow patterns in the coastal waters off Valdivia, thereby obtaining valuable insights into water mass movement and overall marine dynamics.

5. What's required for high-quality measurement of Valdivia coastal currents

In order to conduct high-quality measurement of Valdivia coastal currents, certain essential characteristics need to be in the measurement equipment. Material reliability is of utmost importance. The oceanic conditions around Valdivia, where there is intense saltwater corrosion, high-power waves, and the additional problem of freshwater - saltwater mixture, pose a serious challenge to the equipment. The measuring devices must be strong enough to withstand these harsh conditions for a significant amount of time to yield consistent and reliable data gathering.

Small size and light weight are also critical. This facilitates the equipment's ease of deployment and movement, especially in the often - narrow and shallow estuary and coastal regions. Low power needs are also a critical factor, as this makes it possible to operate continuously long - term, especially in areas where power supplies may not be ample. Additionally, cost - effective design is very much sought after as it will facilitate a greater use and deployment base, thus making possible holistic monitoring of the coastal currents in different areas.

With regard to the ADCP casing, titanium alloy is a suitable choice. Titanium alloy possesses superior corrosion resistance, which effectively protects against the corrosive action of the saltwater and the resultant chemical reactions because of the mixing of freshwater and saltwater. It is equally very strong and light, providing the necessary strength without increasing the overall weight of the ADCP unnecessarily. All these properties make the titanium - alloy - cased ADCPs uniquely suited to operating in the rugged marine conditions of Valdivia, with an assurance of reliable and long - term measurement of the coastal currents.

6. How to Choose the right equipment for measuring currents?

The selection of equipment for measuring current in Valdivia depends on several conditions. The intended use of the equipment is one major consideration. Where continuous measurement is required while on board a moving ship, such as during fishing cruises, shipping of cargo, or marine research trips, a ship-mounted ADCP is the best option. It can measure currents in real time as the ship travels through the waters of the estuary and along the coast, providing useful data on currents along the ship's route.

Fixed - location, long - term monitoring of near - bottom currents is best done with a bottom - mounted ADCP. It can be placed on the ocean floor or the floor of the river estuary and gather current data over extended times without ship presence. A buoy-mounted ADCP is best suited to monitor surface and upper-water-column currents in a flexible manner because the buoy can be located at different places according to research requirements to obtain extensive coverage of the coastal water flow.

The frequency of the ADCP should also be well-selected according to the depth of water. In the relatively shallow estuary and in-shore waters of Valdivia, a 600kHz ADCP is well-placed for water depths up to 70m, offering high-resolution measurement. A 300kHz ADCP would be appropriate for depths up to 110m, offering a good compromise between penetration depth and resolution of measurement. For deeper waters deeper in the water column, past 1000m, a 75kHz ADCP would be the preferred choice since its lower frequency allows it to travel deeper in the water column.

There are only a couple of popular ADCP brands that have presence in the market, viz. Teledyne RDI, Nortek, and Sontek. However, for those budget-conscious, there is the ADCP manufacturer China Sonar's PandaADCP. It is made wholly of titanium alloy and performs pretty well without much expense. For price-sensitive individuals who want reliable ADCPs still, it makes a fine alternative. You may know more about them on their website: https://china-sonar.com/.

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