ADCP's Application in Flood Management of the Shebelle River

1. Where is the Shebelle River?

The Shebelle River originates in the Ethiopian Highlands, in the Horn of Africa, and flows through Somalia. The river is an important part of the arid and semi - arid regions it passes through.

It passes through several cities and towns such as Beledweyne in Somalia. Regarding rainfall, the sources of the Shebelle River in Ethiopia are experiencing more rain due to the influence of the highlands. Precipitation is highly seasonal, and most falls during March to May and October to December. The river is a key source of water for irrigation agriculture; it ensures water supply and is thus important for domestic and livestock use along the areas that it traverses.

2. Why does the flood occur in the Shebelle River?

Floods along the course of the Shebelle River are normally induced by heavy rainfall over its upper catchment. The increased rain in excess of the river's normal water-carrying capacity results in a rapid surge in the water level.

Another contributing factor to the flooding includes the topography, as the river floodplain is largely flat. This makes the water spread easily over the low-lying areas. Further, land-use changes, like deforestation and overgrazing in the upper reaches, can reduce the infiltration capacity of the land and enhance surface runoff. The Shebelle River is thus a flood-prone river, and the correct measurement of its flow parameters becomes important. Acoustic Doppler Current Profiler (ADCP) provides a more sophisticated and effective measurement technique to understand and manage flood situations.

3. How do ADCPs using the Doppler principle work?

The ADCPs work on the principle of the Doppler effect. The device sends acoustic signals into the water. These signals interfere with the moving particles of water. When the acoustic waves return after striking the moving particles, the frequency of the reflected waves shifts due to the Doppler effect.

The ADCP current meter has several transducers that can transmit and receive acoustic signals in multiple directions. By accurately measuring the frequency shift of the reflected signals, the velocity of the water flow at different depths and across different sections of the river cross-section can be determined. For example, in the Shebelle River, it can measure the flow velocity from the riverbed to the water surface and across different widths of the river.

4. What are the applications of an ADCP in floods of the Shebelle River?

Flow Velocity Measurement: ADCP current profiler can measure with good accuracy the flow velocity of the Shebelle River in case of floods. This will help in ascertaining the speed at which water is moving along different parts of the river. The monitoring of flow velocity will indicate where the water is flowing rapidly and may give an indication of a surge in flood or a change in the flood behavior.

Flow-rate Measurement Application: In this respect, the determined flow velocity is combined with the cross-sectional area of the river to find the flow-rate. This, in turn, would mean a great volume of water released per unit time across any section in the Shebelle River flood. This assists in forecasting floods and assessing the overall magnitude of the flood.

Sediment-transport Research: During floods, a great amount of sediment is transported along the Shebelle River. ADCP flow meter can detect the movement of sediment particles through the analysis of backscattered acoustic signals. This enables researchers to study the sediment - transport patterns during floods, which is important for understanding the long - term evolution of the riverbed and the impact on the river's hydrological characteristics.

5. How can the data measured by ADCP be utilized for flood warning and risk management of the Shebelle River?

Flood Warning Aspect

• Flow-velocity and Flow-rate Data Monitoring: Continuously monitoring the flow velocity and flow rate using ADCP meter helps in the detection of abnormal changes. If the flow velocity is higher than the threshold value or the flow rate increases suddenly, it can trigger an early flood warning. This information may be forwarded to the local communities and relevant authorities along the river.
• Water-level Prediction and Warning: The flow data obtained from ADCP profiler can be used in conjunction with other hydrological models to predict water - level changes. By analyzing the relationship between flow - rate and water - level, more accurate flood warnings can be issued, especially in the flood - prone areas of the Shebelle River.

Risk Management Aspect

• Water-Conervancy Project Dispatching Decision Support: Precise flow data from ADCP is a very useful input in the operation of water-conservancy projects such as small dams or flood-control structures along the Shebelle River. Decisions on water release or storage can be done in a more efficient way to prevent flood risks downstream.
• Flood-disaster Assessment and Emergency Response: After a flood event, the ADCP-measured data can be used to assess the extent and impact of the flood. This helps in formulating appropriate emergency-response strategies, including rescue operations and damage-assessment surveys.

6. What's needed for high-quality measurement of Shebelle River currents?

The equipment to be used for high-quality current measurement of the Shebelle River should have materials that are reliable. The equipment should be able to handle the harsh aquatic environment with regard to the corrosive nature of the water and other impacts, such as from floating debris and sediment. A small size and light weight are beneficial, as they facilitate the installation and deployment of the equipment in different locations along the river.

Consumed power should also be small so that the ADCPs operate continuously, in a majority of areas where the supply may also not be continuous. Then it has to be economic so as to make it work practically to deploy them in such quantities throughout one river or on different river currents. Casings preferably can be made with the metal-titanium alloy. Because titanium alloys have very good resistance to corrosion, they can resist corrosion by water from the Shebelle River for long periods. Good strength-to-weight ratio implies its strength in handling equipment due to good mechanical properties. This makes equipment stronger and highly manageable.

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

The measurement purpose should be considered in choosing the equipment for current measurement in the Shebelle River. For horizontal cross-section measurement, HADCP is suitable, as it gives detailed flow-velocity information across the width of the river. For vertical cross-section measurement, a Vertical ADCP is more appropriate for the measurement of the flow-velocity distribution from the riverbed to the water surface.

Frequency of ADCP For relatively shallower parts (e.g., water depth less than 70m) of Shebelle River, it is sufficient to apply a 600kHz ADCP; for deeper areas (around 110m), a 300kHz ADCP is required. Several brands are found in the market, like well-known Teledyne RDI, Nortek, and Sontek, among others. There is also a recommended Chinese ADCP brand: China Sonar PandaADCP. It is made of all - titanium alloy material and offers excellent performance and cost-effectiveness. You can visit its website (https://china-sonar.com/) for more information.

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