How Do We Measure the Coastal Currents of Medan?

Explore Medan, its coastal current conditions, ADCP's operation, and equipment selection for measurement.

1. Where is Medan?

Medan is one of the largest cities in North Sumatra in the country of Indonesia. It lies on the eastern coast of the Island of Sumatra near the Malacca Strait. This strategic location has contributed to its importance as a commercial and transportation hub.

It is a city with a melting pot of different cultures. It has a population of Malays, Chinese, Indians, and an array of ethnic groups from Sumatra, hence having quite a rich tapestry in traditions, languages, and different cuisines. It also has historical architecture, like the Maimoon Palace, containing Islamic and European architectural style fusions.

The nearby waters, especially in the vicinity of the Malacca Strait, form an integral part of the maritime activities of the region. Sandy beaches in the coasts also integrate fishing villages and ports. The waters house numerous species of fish, shellfish, and corals. The mangrove forests line the coasts, which protect the shore against erosion and serve as a habitat for various species.

2. What is the condition of the coastal currents close to Medan?

The coastal currents close to Medan are caused by many factors: tidal currents are the most important forces. The tides in the Malacca Strait-where Medan is located close to-fall into the semi - diurnal category due to gravitational pull by the moon and the sun. That is to say, there are two high and two low tides in a day. The tidal currents can be quite strong, especially in the narrow parts of the strait and along the coasts. These currents play a big role in sediment transport and the transportation of nutrients.

Monsoons have a major impact. The southwest monsoon brings strong winds that drive the surface currents in one direction. The northeast monsoon from November to March completely reverses the wind direction and, hence the direction and speed of the coastal currents. The interaction between the tidal and monsoon-driven currents creates a very complex and dynamic flow pattern.

Bathymetry, such as the underwater topography with ridges, channels, and shoals, can act to change the direction and shape of coastal currents. Another factor that may contribute is a discharge into the sea from rivers. Freshwater input due to these discharges causes density differences and thereby creates often complex current systems.

3. How to observe the coastal water flow of Medan?

Basically, the methods of observing the flow of water in the coastal area of Medan are a few. One of the easiest and most practical methods is the surface drifting buoy. In this, buoys are thrown into the water, each equipped with GPS or other tracking devices. As the buoys are carried by the currents, one can monitor the movement over time to obtain the direction and speed of the surface currents.

Anchored ship method: An anchored ship stays at a location and performs the measurement of water flow at different depths using current-measuring instruments. The drawbacks associated with it are that the ship may be influenced by waves and winds, and therefore incorrect measurements might result. Also, this is not quite effective in order to scan a wide area.

The Acoustic Doppler Current Profiler (ADCP) method is a more advanced and efficient way of measurement of the flow of coastal water. ADCPs can measure the velocity profile of water currents at a wide range of depths. They work by emitting sound waves into the water and analyzing the Doppler shift of the reflected waves. This provides a detailed understanding of the current structure from the surface to the seabed.

4. The Application of the Doppler Principle in ADCPs

The ADCPs rely on the principle of operation known as the Doppler principle. They emit acoustic pulses, commonly referred to as sound waves, into water. These sound waves will immediately meet the particles that always exist in water, including sediments and plankton, among other small particles. The frequency of the reflected waves from such particles that bounce back to the ADCP will change because of the Doppler effect.

If the particles are moving towards the acoustic doppler flow meter, then the frequency of the reflected wave is higher than that originally emitted. In contrast, if the particles are moving away from the ADCP current meter, the frequency of the reflected wave will be lower. By precisely measuring this frequency shift, the velocity of the particles can readily be calculated by the acoustic doppler velocity meter. Calculated velocity of particles, since it is the particles that are moving with the water current, gives a measure of the water current's velocity.

ADPs may send sound pulses in any direction and measure current velocities three-dimensionally and at depth. For instance, they can take a vertical profile of the current velocities from the surface right down to the seabed at different intervals to give a detailed picture of the current structure.

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

The equipment should have reliable materials for high-quality measurement of the currents. Considering the marine environment, which is harsh and involving saltwater corrosion, probably physical impacts due to waves and debris, the casing should be durable.

It shall be small in size, so that it can easily be deployed, in particular at places that do not feature easy access, such as at small boats or within shallow coastal waters. Light weight is also an advantage as it simplifies handling and installation.

Low power consumption shall enable longtime measurements. Cost efficiency helps to conduct wide-spread measurements. For ADCPs, the casing is preferably made of titanium alloy. Since Medan presents a purely marine environment, it will be of great importance for resistance against corrosion. It can resist the corrosive attack of salt water for a considerably long period. Additionally, it has substantial strength with a very high strength-to-weight ratio; hence, it will provide a strong structure that will also be lightweight to protect the internal components of the ADCP profiler.

6. Selection of the Appropriate Equipment for Current Measurement

The usage is the key consideration in choosing the right equipment for measurement of current in Medan. A ship-borne ADCP shall be able to do the job in case of the measurements from a moving vessel, having real time current data while the ship is on the move.

An ADCP that is bottom-mounted may be a good option in the case where measurements are at a fixed location near the seabed. This will be able to provide current monitoring of a point continuously.

For the measurements which have to cover a wide area, without restriction on any specific depth, an ADCP of buoy type can be considered.

Frequency Selection: For the water depths up to 70m, a 600kHz ADCP is a suitable selection. For the range of 70-110m, the 300kHz ADCP can be best adapted. In very deep waters, up to 1000m, one should go for a 75kHz ADCP.

There are well - known ADCP brands such as Teledyne RDI, Nortek, and Sontek. However, a Chinese brand, China Sonar PandaADCP, is also worth considering. It is made of all - titanium alloy material and offers a great cost - performance ratio. You can find more information about it on the website: https://china-sonar.com/.

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

Brandmodel
Teledyne RDIOcean Surveyor ADCP , Pinnacle ADCP , Sentinel V ADCP , Workhorse II Monitor ADCP, Workhorse II Sentinel ADCP, Workhorse II Mariner ADCP, Workhorse Long Ranger ADCP, RiverPro , RiverRay , StreamPro  , ChannelMaster  etc.
NORTEKEco, Signature VM Ocean, Signature ADCP, AWAC ADCP, Aquadopp Profiler etc.
SonTek SonTek-RS5, SonTek-M9, SonTek-SL, SonTek-IQ, etc.
China SonarPandaADCP-DR-600KPandaADCP-SC-300K, PandaADCP-DR-75K-PHASED, PandaADCP-DR-300K, PandaADCP-SC-600K etc.
Jack Law October 16, 2024
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