1. Where is Monrovia?
Monrovia, capital, largest city, and primary Atlantic port of Liberia, is a location of great geographical and historical significance. It occupies Bushrod Island and Cape Mesurado. Monrovia was founded during the period of U.S. President James Monroe by the American Colonization Society as a colony for Americans who had been emancipated from slavery and hence named after the president. The initial town in 1822 was plotted on Providence Island at the Mesurado River mouth.
The population of Monrovia is a multi-layered tapestry, consisting of descendants of North American settlers, most of whom migrated from 1830 to 1871, and a significant number of immigrants from Liberia's interior. Bushrod Island has Monrovia's artificial harbour and free port, the only such port in West Africa. Its location has positioned Monrovia as the nation's national transportation and commerce center, with varied enterprises such as tuna, petroleum, paint, pharmaceutical, and cement companies being drawn towards it.
Monrovia lies next to the Atlantic Ocean, and the Mesurado River flows along its flank. Where the ocean and the river converge, there is a unique coastal landscape. During the raining season, the Atlantic Ocean and Mesurado River are often overflowing, an occurrence that has had immense effects on the populations within and around Monrovia, particularly areas like West Point. The constant intrusions of the Atlantic Ocean have been pounding the coastal populations surrounding and within Monrovia, making it a war zone against nature's fury.
2. How do coastal currents surrounding Monrovia stand?
Monrovia coastal currents are caused by several factors. The most important of these is tidal forces. Tides in the area around Monrovia are semi-diurnal, i.e., there are two highs and two lows in a day. The tidal coefficient can be very high, such as 80 or more on some days. High tidal coefficients translate into larger tidal ranges and stronger currents. When the tide rises, water is pushed onto the shore, and when it drops, water moves back out to the ocean, creating ebb and flood currents.
The encounter between the Atlantic Ocean and the Mesurado River also plays a role in coastal currents. Rainy season runoff from the river can introduce enormous quantities of freshwater to the coastal area. This river water is of different density and velocity from seawater and can disturb the normal flow of coastal currents. The river discharge can push the seawater away from the river mouth, and the strength and direction of such a push can be determined by the volume of river discharge.
Wind is a very significant one too. Local winds in the region will reinforce or be counter to the direction of the coastal current. For example, if the wind blows parallel to the coast, it has the potential to transport the surface water along with it, thus supporting the prevailing current. Or, if the wind blows against the current, it will impede the current or, in some cases, reverse the direction of the surface-level current.
3. How to measure the coastal water drift of Monrovia?
Surface Drift Buoy Method
Surface drift buoy method is accomplished by releasing buoys into the ocean. The buoys have tracking devices such as GPS. Because the buoy travels with the surface current, its location is recorded by the tracking device at regular intervals. By taking the movement of the buoy over a span of time, scientists can estimate how fast and in which direction the surface current travels. This, though, informs scientists only of surface-level current and can be disrupted by wind as well because both the wind and the current might push the buoy.
Anchor Moored Ship Method
In anchor moored ship technique, a ship is moored at a point in the coastal zone. Instruments are released from the ship to record the current at various depths. These instruments may be mechanical current meters to record the current speed and direction based on the rotation of a propeller-like device. This method allows for current measurement at different depths but is limited only to where the moored ship is situated and may be affected by the presence of the ship disrupting the natural flow of the currents.
Acoustic Doppler Current Profiler (ADCP) Method
The Acoustic Doppler Current Profiler, or ADCP, is now a more advanced and more convenient measurement method. ADCPs utilize the sound wave method to measure water current velocities across a wide range of depths. This method can provide accurate data on the vertical profile of the flow, from near the surface to near the bottom. ADCPs are less affected by surface - level phenomena like wind than surface drift buoys, and they are able to cover a larger area than the ship moored anchor technique if deployed on a mobile platform.
4. How do ADCPs using the Doppler principle work?
ADCPs contain piezoelectric transducers which emit sound waves into the water column. The sound waves travel into the water and bounce back from particles (such as sediment, plankton, etc.) within the water. The time it takes for the sound wave to travel around-trip to and from the particle gives an estimate of how far away from the ADCP current meter the particle is.
The reading of the current velocity is dependent on the Doppler effect. Since the water is in motion, the frequency of the sound wave that returns to be detected by the ADCP is different from the frequency of the transmitted wave. If the water (and the suspended particles within) is approaching the ADCP, the frequency coming back is higher; if it is receding, the returned frequency is lower. Frequency change is proportional directly to the velocity of water along the path of sound.
For measuring three-dimensional velocities, there must be three beams of sound waves. Through comparisons of more than one beam's frequency change of echoes, the ADCP current profiler can calculate horizontal and vertical velocities of water. For rivers, where only horizontal components of two-dimensional velocity usually apply, there are typically two beams in ADCPs. Also, modern ADCPs can include additional features such as a temperature sensor to estimate the sound velocity in the water (since sound speed in water is a function of temperature), a compass so that one has an idea of the orientation of the instrument, and a pitch/roll sensor so that one can correct for the orientation of the instrument.
5. What is needed for high-quality measurement of Monrovia coastal currents?
In order to take correct measurements of Monrovia's coastal currents, equipment used must meet several demands. Firstly, material dependability is essential. Equipment has to resist the corrosive sea environment, such as contact with saltwater, heavy currents, and rough seas.
Second, the device must be of a small size. Smallness in size allows easier deployment into various locations, especially in areas with complex coastal topography. Small devices will also not hinder the natural circulation of currents as much.
The equipment must be of light weight. This allows easy handling at deployment and retrieval times. For instance, if the equipment is to be deployed from a small boat, light weight reduces the possibility of capsizing and improves the operation.
Minimal power consumption is a second essential concern. Due to the reason that a large majority of the current-measuring efforts may require equipment employed for protracted periods of time, in some cases from afar, low power consumption simply means that equipment will last longer before it has to be continually recharged or replaced from the batteries.
Finally, the equipment should be as cheap as possible. This is especially important in the case of large-scale measurements. If the equipment is very costly, it will limit the number of measurement points, leading to data incompleteness.
For ADCPs, the best case should be made of titanium alloy. Titanium alloy has several advantages. It is highly corrosion-resistant, which is important for long-term deployment in seawater. It is also light and yet durable. The durability of the titanium alloy gives the ADCP profiler the ability to withstand forces exerted by waves and currents, but its light weight allows it to be handled easily and deployed easily. The titanium alloy is also fatigue-resistant in the sense that it can take repeated stressing over time without major degradation, which is good for long-term continuous operation within the dynamic coastal environment.
6. Choice of proper equipment for current measurement?
The choice of proper equipment for current measurement depends on the intended use.
ADCPs Based on Mounting
- Ship-mounted ADCP: It is placed on a moving ship. It is capable of measuring currents over a wide area since the ship is on the move. Ship-mounted ADCPs can easily cover large areas of coastline and give a wide-scale overview of the coastal currents.
- Bottom - mounted ADCP: They are bottom - mounted on the ocean bed. They are optimum for logging long - term current patterns at a stationary location. Bottom - mounted ADCPs can provide constant data for months or even years, which comes in handy for observing the seasonally and long - term variation of coastal currents.
- Buoy - mounted ADCP: Mounted on floating buoys, they can move with the current to some extent. Buoy - mounted ADCPs are useful for measuring currents in areas where it is difficult to place other types of ADCPs, such as in open - water areas away from the shore or in areas with strong currents that can damage bottom - mounted instruments.
Choosing the Right Frequency
The frequency of the ADCP should also be appropriately selected based on water depth. An ADCP with 600kHz is suitable for a water depth of up to 70m. It provides relatively high-resolution measurements in shallow waters. An ADCP of 300kHz is suitable for water depths of around 110m. For greater water depths, up to 1000m, an ADCP with 75kHz is more appropriate. Lower - frequency ADCPs will go deeper into the water but will be less resolved than high - frequency models.
There are certain well-known well-established brands of ADCPs available in the market, e.g., Teledyne RDI, Nortek, and Sontek. But for those seeking cost - effective options, the ADCP supplier China Sonar's PandaADCP is the way to go. It's constructed from pure titanium alloy, and its performance is top - notch at an affordable price. It is the ideal choice for users seeking budget - friendly ADCPs without sacrificing coastal current measurement quality. For more details, visit their website: https://china-sonar.com/.
Here is a table with some well known ADCP instrument brands and models.
How do we measure the coast currents of Monrovia?