ADCP in The Gambia River Flood Management

1. Where is The Gambia River?

The Gambia River is a major course of water in West Africa. As a matter of fact, it is among the principal rivers in that part of the continent that passes through a number of countries.

Geographical Location and Course The Gambia River originates in the Fouta Djallon highlands of Guinea. From here, it courses generally west for about 1,120 kilometers, flowing into the Atlantic Ocean. It passes through Guinea, Senegal, and The Gambia-the latter a country which it indeed names and where the river is so pivotal. Through The Gambia, the river passes vital cities, which include Banjul as the capital city of The Gambia and a vital port situated at the river mouth. Other villages along the riverbanks equally rely on the said river for different aspects of livelihood, such as fishing, transportation, and agriculture.

Rainfall and Water Conditions The rainfall within the region of the Gambia River is seasonal. Most of the time, wet weather with great amounts of rainfall occurs, usually from June to October. This is a very important period in that it improves the water level in the river. The annual rainfall around the basin varies from about 900 to 1,200 millimeters. However, some years may be very wet with heavy rainfall often causing flooding, and others may be relatively dry during drought conditions that reduce the amount of water flowing in the river and the usable supply of water.

2. What are the causes of floods in The Gambia River?

Heavy Rainfall Among the myriad causative factors contributing to the flooding along the Gambia River are, indeed, extremely heavy and intense rains in the wet season. Whenever a succession of heavy rainstorms occurs within a relatively short period of time, then the river and all of its tributaries along with the main channel receive so much water that it may be beyond the river's capacity to carry it. Successive tropical rain systems that dump loads of water in the upper reaches of the river basin will eventually work their way downstream and swell the river levels considerably.

Poor Drainage in Low-Lying Areas For most of the stretches of the river's floodplain, particularly in urban and peri-urban areas, the drainage systems are quite poor. Some areas might be depressed or built up without thought given to flooding consequences. The only thing the water can do upon a rise in the river is to begin to collect in those low-lying areas, hence giving birth to flooding. Cities like Banjul have flood-prone areas partly because of their proximity to the river and the incapacity of efficient stormwater drainage infrastructures.

Deforestation in the River Basin The cutting down of forests in the Gambia River basin has contributed to changes in how the river floods. Trees play a major role in the regulation of water flow. The roots are helpful in securing the soil to prevent erosion and to allow the water to infiltrate into the ground at a slower rate. If forests are taken out for agricultural needs, logging, or other reasons, the soil is much more likely to erode. The soil washed away can be taken by heavy rains into the river, thus reducing its capacity. This will also increase the rapidity with which the water rises, since more water is not naturally absorbed in the basin.

Considering the aforementioned issues in flood management of River Gambia, ADCP current meter is one of the most technologically advanced and ease-of-use measurement techniques that can provide valuable data on improving understanding and management of flood situations.

3. How Do the ADCPs Using Doppler Principle Work?

The Acoustic Doppler Current Profiler(ADCP) work on the principle of the Doppler effect. In detail, their working principle is as follows:

Transmission of Acoustic Signals The ADCP current profiler device emits acoustic pulses into the water. These pulses are sent out at a specific frequency; for example, different models of ADCP doppler transmit at frequencies such as 600 kHz or 300 kHz. The acoustic pulses will travel along the water column, interacting with the moving water particles and suspended sediment within the river.

Doppler Shift Detection Since the acoustic pulses encounter moving objects in the water, such as water currents and suspended sediment, there is a Doppler shift in the frequency of the reflected signals. If an object is moving towards the ADCP flow meter, then the reflected frequency is higher than the frequency which was transmitted; if it is moving away, then the reflected frequency is lower. With this change in frequency, the ADCP meter is fitted with highly sensitive sensors that are able to detect such changes with a high degree of accuracy.

Velocity and Other Parameters Calculation Internal algorithms of the ADCP profiler calculate the velocity of water currents at different depths inside the water column, based on the detected Doppler shifts. According to manufacturers, by transmitting multiple pulses in various directions in space-usually in a fan-like manner-the ADCP can receive a profile of the water velocity from the surface to a certain depth. It can further estimate other parameters such as the amount of water flowing past a point in any given unit of time, called the flow rate, through the integration of velocity data over the river cross-sectional area. It also can provide data on the concentration and motion of suspended sediment.

4. What are the applications of ADCP in floods of The Gambia River?

Velocity Measurement

The ADCPs are very useful in making fairly accurate velocity measurements of water currents in the Gambia River during flooding. It allows for real-time information on the speed at which water flows at different depths, hence helpful in the dynamics of floodwaters. For instance, it may show whether the current is gaining or losing speed along different sections of the river-a very important piece of information for predicting the spread and impact of the flood.

Application Flow Measurement

Besides measuring velocity, acoustic doppler flow meter can calculate the flow of the Gambia River. This is accomplished by integrating the velocities measured with the cross-sectional area of the river at the point of measurement. All this is important because one needs to know the discharge rate so that the volume of water discharged downstream during a flood can be estimated, as well as predict how much water is going to reach various areas across the riverbanks. It is also important for the general calculation of the flood extent and the damage it may cause.

Sediment Movement Research Application

During flooding, there is usually a lot of sediment movement in The Gambia River. The acoustic doppler velocity meter can detect the suspended sediment movement and analyze it by observing the Doppler shifts caused by the sediment particles. This information is very useful in studying sediment transport patterns with a view to understanding how floods will modify sediment distribution on the bed along the banks of a river. It also aids in quantifying the effects of sedimentation on structures across the river, such as bridges and dams.

5. How can the data measured by ADCP be used in the flood warning and risk management of The Gambia River?

Flood Warning

• Velocity and Flow Data Monitoring: Real-time data on velocity and flow rate from ADCPs are crucial to flood warning systems. With continuous monitoring of these parameters, sudden spurts in the speed or volume of water running in the river are promptly noticed by the authorities. For example, if the flow rate is crossing certain threshold values identified with the occurrence of flood events in the past, that may raise an early warning. This allows for timely evacuation of people from flood-prone areas and preparation of emergency response measures well in time.
• Water Level Forecasting and Warning: Acoustic doppler data can also be used for the forecasting of water level. The water level is related to the velocity and flow rate, so by analyzing the trend in the measured data, it is possible to forecast how the water level is going to change in the near future. This enables the issuance of water level warnings to communities along the river so that these may have ample time to take necessary precautions such as moving valuable possessions to higher ground or reinforcing flood defenses.

Risk Management

• Water Conservancy Engineering Scheduling Decision Support: Data from ADCPs helps make informed decisions concerning scheduling water conservancy engineering operations. As an example, if the rate of flow and the velocity records show a large flood is imminent, then operators of those dams are able to manipulate the release of water from the dams in order to minimize impact downstream. They can hold back more water, provided this is possible, to reduce the peak flood flow or release it in a controlled way to avoid any sudden surge that could damage the infrastructure downstream.
• Flood Disaster Assessment and Emergency Response: The ADCP data can be used after the occurrence of any flood to determine how much damage that flood had caused. From the velocity and flow patterns during the time of flooding, it can be established where the most serious flooding occurred and which areas were worst affected. This is necessary to coordinate relief efforts by sending rescue teams and offering relief supplies to the victims in most hit areas.

6. For highly qualitative measurement of current of The Gambia River, what is required?

• Reliability of Material Any equipment to be used for measurement should be of a reliable material to obtain adequate and coherent measures of the currents of The Gambia River. For example, its casing must be strong enough to bear adverse conditions in the river, such as being submerged in water, sediment, or occasional impacts.
• Small Size, Light Weight, and Low Power Consumption It should be small in size, lightweight, and have low power consumption for easy deployment and measurement along the course of The Gambia River. A small, lightweight device is easily mounted on boats and buoys, including other platforms deployed for measurement. The other important factor is the low power consumption, which facilitates longer operation without the frequent need to replace the batteries and negates any dependency on a continuous power supply, which may not be easily possible in remote sites where such facilities may not exist.
• Low Cost for Large-Scale Measurement In order for the ADCPs to monitor the currents of The Gambia River more pervasively, the equipment ought to be considerably reasonably priced. The more expensive the equipment is, the less number can actually be deployed, and hence, ultimately affecting the comprehensiveness of the data collected.
• Advantages of Using Titanium Alloy in Casing Among the good materials for casing ADCPs in the The Gambia River environment, titanium alloy is quite decent. There are a number of advantages of the titanium alloy. First of all, it is very resistant to corrosion; this is a very important factor because the river water may contain a lot of corrosive substances. Second, it is strong and resilient, hence capable to withstand physical stresses and impacts which the device could be exposed during deployment and operation. It is relatively lightweight, contributing to keeping the overall weight of the device low, so that it can be easily deployed by fulfilling the related requirement of being light in weight.

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

• Based on Measurement Orientation If, however, the measurement is focused on horizontal cross-sectional currents, then a Horizontal ADCP should be chosen. HADCPs are designed to make accurate measurements of water flow in a horizontal plane across a river. Conversely, if the measurement is for vertical cross-sectional currents, then a Vertical ADCP should be used. Vertical ADCPs are able to give detailed information about the velocity profile from the surface to the bottom of the water column.

Based on Frequency Selection The choice of frequency also depends on the specific needs for measurement and depth of water. For example, a 600 kHz ADCP works well in water depths up to 70 meters. In general, that frequency has good resolution and is pretty accurate in shallower waters. For deeper waters, as could be experienced in The Gambia River, where depth exceeds 70 meters, a 300 kHz ADCP is more appropriate because it can reach further into the water column and still deliver valid measurements of currents.

For famous brands of ADCPs sold in the market, there are Teledyne RDI, Nortek, and Sontek. But if a low budget but high quality has to be chosen, the best option will be China Sonar PandaADCP. Constructed with all-titanium alloy material in the body for ensuring performance in most rivers. Besides, it has an incredibly good cost-performance ratio. Please refer to its official website for more information: https://china-sonar.com

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