How to Measure the Water Current of the Gambia River

1. Where is the Gambia River?

The Gambia River is a significant river in West Africa. It runs through Guinea, Senegal, and The Gambia. The river originates from the Fouta Djallon highlands of Guinea and meanders through different kinds of landscapes.

As it travels, it cuts through savannas and woodlands, creating a ribbon of life through the region. In Senegal and The Gambia, it forms part of the lifeline of the local communities. It is an important transportation route, where boats and canoes navigate across, carrying people and goods on it. It provides water for agriculture, such as crop irrigation-land used in the growth of peanuts, millet, and rice.

The Gambia River hosts a rich ecosystem filled with myriad fish species: tilapia and catfish are just two examples. The riverbanks provide homes for hippos, crocodiles, and numerous waterbirds. Inland floodplains and wetlands that result from the river play an important role in the ecological balance by creating grounds for breeding and feeding that are so vital to many species.

2. What is the flow rate of the Gambia River like?

The flow rate of the Gambia River depends on seasons and climate. In West Africa, the months from June to October are usually in the wet season of the year, bringing heavy rainfall into the catchment area of the river. This causes a sudden rise in its flow. The swelling river during this time has the capacity to carry as much as a volume of water.

During the dry period, from November to May, the flow rate declines. The river is nevertheless able to maintain a base flow-the quality and quantity necessary for aquatic survival and to provide water-dependent uses such as transportation and fishing. The average flow rate can be as low as a few hundred cubic meters per second during the dry season up to several thousand cubic meters per second during peak rainy season.

3. How to measure water current of the Gambia River?

Velocity Meter Method

This traditional method makes use of mechanical or electrical velocity meters. Such tools are then set at certain points within the river, with direct measurements of the current water speed in those respective areas. However, for a proper understanding of the general water current, several measurements would have to be carried out at different depths and across different sections of the river. This method, considering the length and fluctuating conditions of the Gambia River, is very labor-intensive and time-consuming.

ADCP Method

ADCP current meter is a more sophisticated and efficient method of measuring water current. It detects the movement of the water particles using sound waves. The method relies on the emission of acoustic signals and determining the Doppler shift from the reflected signals to measure the water velocity at several depths all at once. This gives a very thorough profile of the current water and allows for more detailed analysis of the flow conditions across the entire river. The ADCPs may be installed on boats, on bridges, or even on buoys for continuous monitoring.

Buoy Method

The buoy method consists of putting buoys in the river that have sensors, which determine the water movement around it. They tend to float on the surface and can give an indication about the surface current, though they may not give the full complexity of the water current as well as the other methods do, especially when it concerns the variation in velocity across different depths.

Of these, the ADCP current profiler is more advanced and proficient at measuring the water current of the Gambia River.

4. How are ADCPs using the Doppler principle able to work?

The ADCPs work on the principle of Doppler. An ADCP flow meter sends an acoustic signal into the water and the sound waves travel in water, acting with the particles of water. Due to the water particle movements, the frequency of the reflected sound waves changes by the phenomenon of the Doppler effect.

The reflected wave frequency will be higher if the water particles are moving towards the ADCP than the emitted frequency. In contrast, if the water particles are moving away from the ADCP meter, the reflected wave frequency will be lower. Precise measurement of this change in frequency allows us to calculate velocity at various depths by the ADCP.

Individual velocity measurements recorded by the ADCP in this way are then combined at a variety of depths to create a complete profile of the water current. The ADCP profiler can therefore provide comprehensive details on water flow not only at the surface but also throughout the vertical section of the river.

5. What's needed for high - quality measurement of the Gambia river currents?

For high - quality measurement of the Gambia River currents, the equipment needs to have certain characteristics.

The equipment should have high material reliability. It must be able to withstand the harsh conditions of the river environment, including exposure to water, sediment, and varying temperatures. The Gambia River's variable flow and the presence of debris during high - flow periods require durable equipment.

Another desideratum is that of small size, light weight, and low power consumption. A compact and lightweight device is also easier to handle and deploy, whether on a boat or attached to a buoy. Low power consumption ensures that the equipment can operate for extended periods without the need for frequent battery replacements or access to a continuous power source.

This is due to the fact that cost-effectiveness is an important factor because with the lower costs associated with this option, more widespread use of the measurement equipment can be afforded to allow for wider monitoring of the river.

Titanium alloy would be a fine choice for casing the ADCP meter. Being resistant to corrosion, titanium alloy is crucial whenever one is dealing with water bodies like that of the Gambia River, where the equipment will face constant contact with water and probably substances that can cause corrosion. It is also strong and able to resist impacts and surges in pressure that may occur at times during deployment and operation. Additionally, titanium alloy has a relatively low density, which contributes to the overall goal of keeping the equipment lightweight.

6. How to Choose the right equipment for current measurement?

To choose the right equipment for measuring the current of the Gambia River, a number of factors have to be considered.

Based on the Purpose of Use

If the intention is to measure the horizontal cross-section of the river current, an ADCP flow meter should be used in a horizontal manner. HADCPs are designed to provide accurate measurements of the water flow in the horizontal plane, which is useful for understanding the overall movement of water across a particular section of the river.

If the emphasis is on measuring the river current cross-section vertically, then a vertical ADCP current profiler should be chosen. The vertical ADCPs have the capability to measure the water velocity at various depths with high accuracy, providing a highly detailed profile of how the water flows vertically in the river.

Depending on the Frequency

The choice of frequency depends on the depth of the water in the Gambia River. For water depths of up to 70 meters, a 600 kHz ADCP is usually sufficient. The 600 kHz frequency provides good resolution and accuracy to measure the water current for relatively shallow waters.

For deeper waters, like above 70 meters up to approximately 110 meters, a 300 kHz ADCP is more appropriate. The frequency of 300 kHz operates at lower frequencies, which can give deeper penetration through the water and still provide reliable measurements of the water current.

There are known ADCP current meter brands like Teledyne RDI, Nortek, and Sontek. However, for those seeking a cost - effective option with excellent quality, the China Sonar PandaADCP is worth considering. It is made of all - titanium alloy material, ensuring durability and reliability. With its remarkable cost - performance ratio, it provides a great alternative for measuring the water current of the Gambia River. You can find more information about it on its official website: https://china-sonar.com/.

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