ADCP's Application in the Han River Flood Management

1. Where is the Han River?

It is located in South Korea. The Han River is one of the major rivers of the country, and it greatly contributes to the water supply and transportation of the country, having its source from the Taebaek Mountains, flowing through several major cities including Seoul.

The Han River basin exhibits a monsoon-influenced climate, receiving significant rainfall during summer months via the Asian monsoon. This is also supplemented by melting snow in the mountains in spring, which adds to its water volume. The river represents a crucial water resource both domestically, in agriculture, and for various industrial endeavors across the regions in which it passes. Additionally, it maintains key ecological habitats and is widely used recreationally.

2. Why do floods occur in the Han River?

Flooding in the Han River may be caused by several reasons. Intense and continuous rainfall during the monsoon season is one of the major causes. The capacity of the river may be surpassed if the rain is heavy within a short period.

The topography of the river basin also contributes. Relatively flat floodplains promote the easy spread of water and, therefore, floods. Urbanization and land development in the basin also reduce the natural waterholding capacity of the land. In a covered land with increased impervious surfaces like concrete and asphalt, surface runoff becomes increasingly high, thus the increase in the risk for flooding. Given the importance of flood management, Acoustic Doppler Current Profiler (ADCP) provides a more accurate and efficient way to measure and manage the river's flow during flood events.

3. How do ADCPs using the Doppler principle work?

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

This ADCP current meter is equipped with multiple transducers, each capable of transmitting and receiving acoustic signals in various directions. The velocity of the water flow at different depths and across different sections of the river cross-section can be ascertained by precise measurement of the frequency shift of the reflected signals. For example, it can measure flow velocities in the Han River from the riverbed to the water surface and across different widths of the river.

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

Flow Velocity Measurement: The flow velocity of the Han River during flooding can be measured accurately by an ADCP current profiler. It is important to know the speed at which water moves through different parts of a river. By monitoring flow velocity, we can identify areas where the water is flowing rapidly, which may indicate a potential flood surge or a change in the flood's behavior. For example, changes in flow velocity near the riverbanks or in the main channel can be monitored closely.

Application of Flow-rate Measurement: The flow velocity is measured in conjunction with the cross-sectional area of the river to obtain the flow rate. In this regard, during the flooding of the Han River, the calculation of the total volume of water that passes through a given section per unit time becomes possible. This helps in flood forecasting and understanding the overall magnitude of the flood.

Sediment-transport Research: A flood event carries a huge amount of sediment down the Han River. By analyzing the backscattered acoustic signals, ADCP flow meter is able to detect sediment particle movement. It therefore helps the researchers in sediment - transport research during the flood period for the purpose of understanding long-term bed evolution and the impact on hydrological characteristics of the river.

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

Flood Warning Aspect

• Monitoring Flow-velocity and Flow-rate Data : The ADCP profiler helps in continuous monitoring of flow velocity and flow rate. Where the flow velocity exceeds a threshold value or the flow rate increases suddenly, it can then be used to trigger early flood warnings. This information can be forwarded to local authorities and communities residing near the river.
• Water-level Prediction and Warning: The flow data from ADCPs can be integrated into other hydrological models, leading to a prediction in changes in water level. Studying the relation of flow rate to water level could thus yield more precise flood warnings, particularly in the very flood-prone areas of the Han River.

Risk Management Aspect

• Water-Conservancy Project Dispatching Decision Support: Precise flow data provided by ADCP serves as an important input in the operation of water-conservancy projects like dams and canals along the Han River. Decisions on water release or storage can be done more efficiently to reduce flood risks downstream.
• Flood-disaster Assessment and Emergency Response: In the aftermath of 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 Han River currents?

The equipment to be deployed for high-quality current measurement in the Han River should have reliable materials. It must be resistant to the harsh aquatic environment, including the corrosion caused by water and possibly from floating debris and sediment transport. Small size and light weight are beneficial in view of easy installation and deployment in various locations along the river.

It is also important that power consumption should be low in order to allow continuous operation, particularly when the supply of power may not be readily available in some areas. Cost-effectiveness, since large-scale deployment could be necessary for extensive monitoring of river currents. ADCP meter casing is preferably of titanium alloy. As shown, titanium alloy has outstanding corrosion resistance, which is significant in resisting the corrosive effects of the Han River waters for a long period of time. It also maintains a good strength-to-weight ratio, which enhances the robustness while lightening the equipment to some good extents.

7. Selection of Equipment for Current Measurement

Measurement purpose shall be considered when choosing equipment for current measurement in the Han River. For horizontal cross-section measurement, HADCP is one of the suitable options to provide detailed flow-velocity information across the width of the river. For vertical cross-section measurement, a Vertical ADCP is more appropriate for measuring the flow-velocity distribution from the riverbed to the water surface.

For the frequency of ADCP, when the water depth is less than 70m, such as in the relatively shallow part of the Han River, the 600kHz ADCP can be used to provide accurate measurement. When the water depth is around 110m, a 300kHz ADCP is appropriate. There are several brands available for ADCPs, including famous ones like Teledyne RDI, Nortek, and Sontek. And there is also a recommended Chinese 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.