ADCP in The Nyabarongo River Flood Management

1. Location and Course

The Nyabarongo River is located in Rwanda and forms part of the waterways. It forms a tributary of the Kagera River.

It originates in the Nyungwe Forest in southwestern Rwanda and flows northward through a mix of hilly and valley regions. The Nyabarongo River weaves its course through the variation in the topography of Rwanda, which varies from mountainous to gentle slopes and plains.

2. Influence of Climate and Rainfall

In Rwanda, the climatic conditions are typically tropical highland. The volume of water in the Nyabarongo River is contributed much by rainfall in the catchment area. Normally, its rainy seasons fall between February and May and September and November.

During these wet periods, heavy rainfalls are experienced; this rain falling in the Nyungwe Forest and the surrounding catchment areas feeds into the Nyabarongo River. The high-elevation areas in the head are receiving a considerable amount of rainfall, which then runs off into the river. Heavy rain and natural land drainage can sometimes result in level changes. During heavy rainfall or rapid snowmelt from surrounding mountains-mostly not snow, the river may overflow its banks and flood.

3. Causes of Flooding

Intense Rainfall: Probably the most self-evident cause of flooding is the heavy and continuous rainfall in the wet seasons. The tremendous amounts of water falling in a very short duration can fill rapidly both the main course of the river and its tributaries. The hilly and mountainous regions in its upper reaches contribute to rapid runoff, which channels a significant volume of water downstream.

Topography: The occurrence of valleys and low-lying areas along the course of the river contributes to the accumulation of water. This could further be compounded by the relatively flat plains that the water may face further downstream, which slows the flow of water, hence being highly prone to flooding. Other tributaries of the Nyabarongo River contribute to the general volume of water during periods of high flow, hence aggravating the flood situation.

Changes in Land Use and Vegetation: Deforestation in the upper part of the catchment, for example, in Nyungwe Forest, decreases the interception of rain by the forest and its retaining ability. This also increases soil erosion, whereby the eroded soil settles in the river, reducing the carrying capacity. Besides, agricultural expansion and other changes in land use may disrupt the natural flow of water and drainage patterns, thereby increasing flood risk.

4. Role of ADCP (Acoustic Doppler Current Profiler)

Acoustic Doppler Current Profiler (ADCP) provide good insight into the Nyabarongo River flow and its management in case of flooding conditions.

How ADCPs Work

Acoustic Doppler Current Profiler (ADCP) operate on the principle of the Doppler effect. A device sends an acoustic signal into the water; if the water is in flow, the signal collides with the moving particles in the water and sends back the frequency of the reflected signal changed by the Doppler effect.

The ADCP measures the differential frequency between the emitted signal and the received signal. It can then use this frequency shift to determine the water velocity at each depth. Many of these instruments have multiple transducers that can project and receive acoustic signals in multiple directions. This enables them to provide a profile of the water velocity across a section of the river.

For example, the reflected signal will have a higher frequency if the water is flowing towards the ADCP current profiler, while if the water is flowing away from it, the reflected signal frequency is lower. By precisely measuring these changes with proper mathematical algorithms, the ADCP flow meter can estimate the velocity of the water at different points within its measurement range with good accuracy.

Application of ADCP to the Nyabarongo River

Velocity Measurement: The Nyabarongo River, when flooding, needs very accurate measurement of water velocity. ADCPs can continuously monitor water velocity at different depths and locations with real-time data on how fast the water is moving. The information obtained is crucial to understanding the dynamics of a flood, for example, predicting the direction and intensity of the movement of floodwaters.

Flow Measurement: ADCPs can also be used to calculate the flow rate of the Nyabarongo River. If the cross-sectional area of the river is known, then these measured water velocities at various points across a section of the river can be used to determine the total volume of water flowing through the section per unit time: the flow rate. This information becomes vital in assessing the total volume of water in a flood for decisions on flood control and management of water resources.

Sediment Transport Research: Other than flow and velocity measurements, ADCPs are employed in sediment transport research during floods. While the water flows, it carries sediments. The ADCP profiler can detect changes in the backscattered acoustic signal caused by the presence of sediments. From these variations, researchers can estimate the quantity and transportation of sediments, which becomes relevant to understand the long-term developments of the riverbed, considering the influence of floods on the river's sedimentary environment.

5. Use of ADCP Data for Flood Warning and Risk Management

Flood Warning

Real-time Velocity and Flow Data Monitoring: The ADCP-derived velocity and flow data are continuously monitored in real time. If the measured water velocity is higher than the threshold value or if the flow rate begins to increase substantially, then it could be indicative of an impending flood peak or flooding situation. The warning given by this method in advance allows the concerned authorities to adopt necessary precautions such as evacuating people from low-lying areas or consolidating flood defenses.

Water Level Forecasting: Relating the measured flow data with historical data of water level and using appropriate hydrological models will help in forecasting the future water level using ADCP data. If the forecasted water level is above the flood warning level, timely warnings could be issued to the public for getting ready to face the flood.

Risk Management

Water Conservancy Project Decisions: Accurate flow and velocity data by ADCP meter can be helpful in the decision-making process for operating strategies of water conservancy projects like dams and sluices. For example, depending on the amount of measured water flow, release from a certain dam is changed with the purpose of controlling the water level in the Nyabarongo River and thus preventing floods.

Assessment of a flood disaster: Immediately after the occurrence of a flood event, the data that would have been gathered by an ADCP would be useful in assessing the intensity of a flood. This would comprise information like the extent of the flood, the real velocity of flood waters, and sediment deposition. This kind of information is helpful and crucial in formulating emergency response plans, as well as for post-flood reconstruction and rehabilitation work.

5. Requirements for High - Quality Current Measurement

Materials of Equipment: The equipment, especially the casing of the ADCP, must be made from reliable materials to ensure high-quality measurement of the Nyabarongo River currents. A durable, water-resistant material such as a titanium alloy is recommended due to water quality and expected debris in the river. Titanium alloy has high strength, allowing the equipment to stand firm against the pressure and impact of the flowing water.

Adequate Dimension and Weight: These shall be as small and light as possible. This facilitates the installation and operation processes of the ADCP in various places along the river, including access conditions with difficulties in some areas.

Low Power Consumption: This is fundamental because it offers the chance for continuous use for extended periods without needing frequent changes of batteries or connection to a power source.

Cost-Effectiveness: The equipment cost should not be so high to allow for large-scale measurement. Lower cost means more ADCPs can be set up along the Nyabarongo River, giving more comprehensive and detailed data about the river necessary for flood management.

7. Selection of Appropriate Equipment for Current Measurement

Type of Measurement: This depends on the type of measurement it is meant for. When it is meant for horizontal cross-section measurement, then one should select a Horizontal ADCP. In cases of vertical cross-section measurement, then a Vertical ADCP shall be appropriate.

Water Depth Consideration: Different frequencies apply to different water depths. Using a 600 kHz ADCP applies to water depth within 70 m. Therefore, a 600 kHz ADCP can be put into consideration if the water depth of Nyabarongo is within the range in the measurement requirements. A 300 kHz ADCP shall mostly be considered for the deeper waters, especially those exceeding 70 m up to 110 m, a depth it best serves with more accuracies.

The market is presently filled with a few well-known ADCP brands like Teledyne RDI, Nortek, and Sontek. However, for cost-effective purposes, the China Sonar PandaADCP will be a good choice. Made from all-titanium alloy material, this ensures that it is durable enough and can be reliable in the water environment. Besides, it offers an amazing cost-performance ratio. 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.