ADCP in The Rukarara River Flood Management

1. Location and Course

The Rukarara River is located in the state of Rwanda. It constitutes a significant part of the local river system, flowing through a landscape dominated by hills, valleys, and intensive farmland.

The source of this river is within the hilly regions of Rwanda. At the lower courses, the terrain now becomes quite modified, cutting through small valleys, plains, and seeming to follow naturally topographical directions. The Rukarara River is fairly close to human settlements and farmlands; it plays an important role in the local water supply and irrigation.

Climate and Rainfall Influence

Rwanda experiences a tropical highland climate, in which the Rukarara River falls. There are two marked rainy seasons: from February to May and from September to November.

During these rainy seasons, the volume of the river increases enormously during the rains. Rainfall in the hilly catchment area round the source of the Rukarara River runs off into the river. Its high elevation parts receive fairly good precipitation; this water finds its way into the river channel very fast. These rains cause great variations in the water level, and when heavy and persistent rain occurs, this river has the potential to flood.

Causes of Flooding

Heavy rainfall: Heavy rains, especially during the wet seasons, constitute the single most influential factor in flooding along the Rukarara River. In such a short period, the volume of water falling can easily fill the river and its tributaries. The runoff is accelerated by the hilly upper reaches, sending a big volume of water downstream.

Topography: The valleys and low-lying areas present along the course of the river let the water accumulate in them. Where the river is wide, the reasonably flat regions result in slowing the flow of water, hence being more prone to flooding. Wherever the confluence of tributaries takes place, in case of heavy rainfall, extra water is contributed which aggravates the flood-like situation.

Human Induced Land Use Changes: Deforestation and expansion of agriculture in the catchment area may weaken the flood-resisting capabilities of the river. Deforestation reduces the capability of the land to intercept and retain rainwater. Agricultural activities may lead to soil erosion, and the eroded soil could be deposited in the river, reducing its carrying capacity.

4. Role of Acoustic Doppler Current Profiler (ADCP)

ADCP profiler is valuable tools for understanding and managing the flow of the Rukarara River, especially during flood events.

How ADCPs Work

The ADCP meter work on the principle of Doppler. The instrument sends an acoustic signal into the water. Due to the flow of water, the signal scatters by the moving particles of water. Since the instrument will receive the scattered acoustic signal, the frequency of the returning signal shifts through the Doppler effect.

The ADCP measures the difference in frequency between the emitted and received signal. From this frequency shift, it can calculate the water velocity at all different depths. These devices normally possess multiple transducers, which are able to send and receive acoustic signals in various directions. This enables them to build a profile of the water velocity across a section of the river.

For instance, if the water is moving toward an ADCP flow meter, then the frequency of the reflected signal will be higher than the one emitted. On the other hand, if water moves in the opposite direction, the frequency of the reflected wave will be smaller. Precise measurements of these changes in frequency can allow the ADCP to determine, through the use of appropriate algorithms, the exact velocity of the water at different points within its range.

Applications of the ADCP along the Rukarara River

Velocity Measurement: The velocity of the water flow in the Rukarara River during flood events has to be measured as accurately as possible. The ADCP current profiler shall be able to view continuously the water flow rate at different depths and places for real-time insight into the speed at which water moves. The information shall be used to contextualize the dynamics of the flood, such as the direction and intensity of the movement of the floodwaters.

Flow Measurement: The ADCPs could measure the Rukarara River flow rate. In concert, it can calculate the total volume of water that flows through a section per unit time-the flow rate-by applying the measured water velocities at various points across a section of the river, and with a known cross-sectional area of the river. This is vital information to understand the amount of water in a general flood event for the purpose of flood control decisions and water resource management.

Sediment Transport Research: Other than flow and velocity measurements, ADCPs are also helpful in sediment transport studies during floods. The moving water carries sediments along its path. An ADCP current meter can detect changes in the backscattered acoustic signal caused by sediments. Studying these changes will help estimate the quantity and the movement of sediments, which is one of the key components in understanding the long-term developments in a riverbed and changes imposed by floods upon the sedimentary environment of the river.

5. The Use of ADCP Data in Flood Warning and Risk Management

Flood Warning

Velocity and Flow Data Monitoring: In real time, ADCPs monitor the velocity and flow data continuously. If the measured water velocity is greater than the threshold value or the flow rate increases considerably, then the situation can indicate the approaching flood peak or a possible flood situation. This early warning enables the concerned authorities to take necessary precautions based on it, such as evacuation of the people who are staying in low-lying areas or strengthening of the flood defenses.

Water Level Forecasting: The measured flow data, correlated with historical data on water level and appropriate hydrological models, can also be used to predict future water levels from ADCP data. In case the forecasted water level is going to exceed the flood warning level, timely warnings can be issued to the public to get ready to face the flood.

Risk Management

Water Conservancy Project Decisions: The correct flow and velocity information obtained using acoustic doppler velocity meter can be used in making decisions regarding water conservancy projects that include the operation of structures like dams and sluices. For instance, the measured water flow may permit the adjusted release of water from the dam to regulate the level of water within the Rukarara River and reduce the flood impacts.

Assessment of Flood Disaster: The data from ADCPs after a flood event helps to assess the intensity of the flood in terms of area covered by inundation, speed of floodwater, and quantum of silt deposition. These are very useful data for formulating emergency response plans and for carrying out post-flood reconstruction and rehabilitation work.

6. Requirements for High - Quality Current Measurement

Reliable Equipment Materials: The equipment, particularly the casing of acoustic doppler flow meter for current measurement of the Rukarara River, must be made from the most reliable materials. In regard to water quality and possible debris in the flow of water, a corrosion-resistant metal like titanium alloy will be recommended for this purpose. Titanium alloy has the high strength that will enable the equipment to bear such pressure and impact from the flowing water.

Appropriate Size and Weight: The size and weight of the instrument shall be as small and light as possible. This provides an easy operation of ADCP installation and operating at various sites of the river, even in remote areas with poor accessibility.

Low Power Consumption: Low power consumption will be advantageous to maintain continuous operation for longer periods without interruptions, and thereby frequent replacement of batteries or hookups to a power source is avoided. Cost-Effectiveness: The cost of the equipment should not be too high to allow for large-scale measurement. Lower cost allows the deployment of more ADCPs in the Rukarara River for better and detailed coverage in flood management.

7. Selection of Appropriate Equipment for Measuring Currents

Type of Measurement: Depending on the type required, if it is for horizontal cross-section measurement, the HADCP should be chosen. If it is for vertical cross-section measurement, then a Vertical ADCP should be the choice.

Water Depth Consideration: Depending on water depth, different frequencies are suitable. An example is that a 600 kHz ADCP is suitable in water depths up to 70 m. If the Rukarara River's water depth is within this range, in addition to the requirements of the measurement, a frequency of 600 kHz ADCP can be considered. In the case of waters with depths greater than 70 m up to 110 m, a 300 kHz ADCP can be suitable for more precise measurements in such waters.

Presently, there are several well-known ADCP brands available on the market including Teledyne RDI, Nortek, and Sontek. In the recent years, however, China Sonar PandaADCP is an economical-class option. The all-titanium alloy body guarantees that it will be resilient and can work dependably in an aquatic environment. Moreover, it has an incredibly cost-effective performance. More about it can be viewed on its official website: (https://china-sonar.com/).

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