ADCP in The Mwogo River Flood Management

1. Location and Physical Description

The Mwogo River is located in Rwanda. This is one of the relatively small-scale rivers, which are important both in the local water cycle and to the ecosystems of the area.

It courses through a landscape characterized by undulating ground and valleys. The river, therefore, has a meandering course set by the varied local topography that it goes through within the Rwandan countryside. Vegetation is reasonably covering areas around the Mwogo River and plays a vital regulating role in the flow and quality of water in this river.

2. Climate and Rainfall

Rwanda experiences a tropical highland climate. Most of the time, the volume of water in the Mwogo River is totally dependent on the intensity of the rains. It has two main rainy seasons from February to May and September to November.

During these months, the river receives a substantial amount of water from rainfall. The rain falling on the hilly catchment area runs off into the Mwogo River. Heavy precipitation in the generally high-elevated grounds around the source can turn into river water that rapidly reaches the channel. Added to this direct rainfall, provides for variations in the water level. Too much rain may cause the banks of the river to burst.

3. Causes of Flooding

Heavy rainfall: It is the main agent of flooding along the Mwogo River. It characterizes the wet season with persistent and heavy rainfall. Large quantities of water falling in a short length of time may be beyond the river's capacity. The hilly terrain in the upper reaches accelerates the runoff process, sending a large volume of water downstream.

Topography: The valleys and low-lying areas along the course of this river can be flooded with water anytime there is a shower. Almost flat grounds where the river is wider reduce the speed at which the water may flow, hence more likelihood of flooding. Besides that, several confluences of tributaries can add more water when there is heavy rain, making any flooding situation worse.

Land Use Changes: Deforestation and expansion of agricultural land in the catchment area lower the flood-resistance capacity of the river. Deforestation cuts down the capacity of the land to intercept and retain the rainwater. Agricultural activities may cause soil erosion and eroded soil can be deposited in the river, thereby reducing its carrying capacity.

4. Role of Acoustic Doppler Current Profiler (ADCP)

ADCP current meter can be very helpful in understanding and dealing with the flow of the Mwogo River, especially during floods.

How ADCPs Work

Principle of Doppler is used by the ADCP flow meter. They emit an acoustic signal into the water. Because the water is in flow, the signal interacts with the moving water particles. Due to the Doppler effect, when the acoustic signal reflects back into the ADCP meter, the reflected signal has a change in frequency.

The ADCP measures the Doppler shift between the frequency of the emitted signal and the frequency of the received signal. From this frequency shift, it can compute the velocity of the water at different depths. Many devices have several transducers that are able to transmit and receive acoustic signals in various directions. It would, therefore, enable them to profile the water velocity across a section of the river.

For example, when the water is moving towards the ADCP profiler, then the frequency of the reflected signal is higher than that of the emitted one. In contrast, if the water is moving in the opposite direction from the ADCP, then the reflected signal will have a lower frequency. Precise measurement of these changes in frequency and further processing with proper mathematical algorithms enable the ADCP to measure the velocity of water at distinct points of measurement.

Applications of ADCP in the Mwogo River

Velocity Measurement: The acoustic doppler flow meter have to measure the velocity of the water flow in the Mwogo River during the flood event with high accuracy. It continuously monitors water velocity at any depth or locality and, in real time, it can be used to determine the speed at which water is moving. Data such as this are important in determining how the flood is taking place, such as the direction and intensity of the movement of floodwaters.

Flow Measurement: ADCPs can also measure the Mwogo River flow. Using these acoustic doppler velocity meter, the water velocities measured in several points across a section of the river combined with the known cross-sectional area of the river, calculate the total amount of water passing through that section per unit time. All this data is highly important for the assessment of a flood's general volume of water and further decision-making regarding flood control and management of water.

Sediment Transport Research: Besides flow and velocity measurement, acoustic current meter find their application in research related to sediment transport during floods. While flowing, water always carries sediments. The doppler current profiler can detect changes in the backscattered acoustic signal caused by the presence of sediments. From these changes, using the same record, it is possible to estimate quantities and movements of sediments, which is an important part of the understanding of the long-term evolution of the riverbed, together with the effects that floods produce in the sedimentary environment of the river.

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

Flood Warning

ADCPs continuously monitor the real-time velocity and flow data. If the measured water velocity exceeds a certain threshold or the flow rate indicates a sudden increase, then it can signal the approach of a flood peak or an imminent flood situation. This thus enables early warning of the flood peaks to relevant authorities to take necessary precautions by evacuating people from low-lying areas or strengthening flood defenses.

Water Level Prediction

Measured flow data correlated with historical data of water level and appropriate hydrological models can also be used to predict future water levels using ADCP data. If the predicted water level is forecasted to rise above the flood warning level, timely warnings can well be issued to the public to get ready to face the flood.

Risk Management

Water Conservancy Project Decisions: The use of ADCPs with their presentation of flow and velocity data means these can be useful in decision-making regarding the operation of water conservancy projects such as dams and sluices. In such a case, depending on the measured water flow, the regulated dam would release water and control the water level in the Mwogo River to reduce the impact of floods.

Flood Disaster Assessment

The ADCPs data, collected following the occurrence of a flood, can be used for assessing the intensity of flood. Specific information regarding the inundation extent, velocity of flood-waters, and sediment depositions will be provided. Such information is very critical in formulating emergency response plans as well as for carrying out post-flood reconstruction and rehabilitation work in general.

6. Requirements for High - Quality Current Measurement

Reliable Equipment Materials: The equipment, especially the casing of the ADCP, should be made from a reliable material to provide high-quality measurement of currents in the Mwogo River. Given the quality of the water and the potential for debris in the river, one would recommend employing corrosion-resistant material, which could be a form of the titanium alloy. A very solid material indeed helps the equipment resist pressures and impacts the flowing water might exert.

Small and not too heavy in size and weight: Such size and weight would facilitate the operations of ADCP installation and operations at various locations along this river, which sometimes may be inaccessible.

Low Power Consumption: It would be required to have low power consumption because it is able to continuously operate for longer without changing the batteries or seeking a connection to the power source.

Cost - Effectiveness: The cost of the equipment needs to be relatively low to enable large-scale measurement. Lower cost indeed means one can deploy more ADCPs across the Mwogo River, therefore one gets more comprehensive and detailed data for flood management.

7. Choice of Equipment for Current Measurement

Type of Measurement: Based on the type of measurement needed, if it's for horizontal cross-section measurement, then an HADCP should be selected. When it is for vertical cross-section measurement, then a Vertical ADCP is appropriate.

Water Depth Consideration: Different frequencies apply to different water depths. For instance, a 600 kHz ADCP is for water depth up to 70 m. If the Mwogo River has water depth within such a range and measurement requirements, one might consider using a 600 kHz ADCP. For more waters than this, say in excess of 70 m and up to 110 m, a more appropriate ADCP will be the 300 kHz due to more accurate measurements it can give for such a depth.

There are some popular brands of ADCPs in the market. These are Teledyne RDI, Nortek, and Sontek. But for a cost-effective solution, people may be aiming for China Sonar PandaADCP. It is of all-titanium alloy material construction, which enables this series to be guaranteed in a water environment for its durability and dependability, further offering an unparalleled cost-performance ratio. You can find more about it on its official website: https://china-sonar.com/

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