ADCP Applied to the Pearl River Flood Management

1. Where is Pearl River?

The Pearl River is a complex river system in southern China. Its main stream consists of the converging waters of the Xijiang River, the Beijiang River, and the Dongjiang River.

Geographically, it flows through a densely populated and economically vibrant area. It passes through many cities like Guangzhou, Foshan, and Zhaoqing in Guangdong Province. The Pearl River Delta region, where the river meets the sea, is known for its advanced manufacturing, trade, and prosperous urban agglomerations.

The Pearl River Basin has a subtropical monsoon climate in terms of rainfall. There is a marked seasonality with wet and dry seasons. The wet season, normally from April to September, is characterized by heavy rainfall. Typhoons during this period may also bring sudden and heavy downpours, which raise the volume of water in the river significantly and sometimes lead to flood situations.

2. What are the Reasons for Floods in Pearl River?

Heavy Rains and Typhoons: Most precipitation that falls within the Pearl River Basin occurs throughout the wet summer months. Often, Typhoons form over the seas to its southeast and move toward land near the coastline inside the Basin and with heavy rainfall of short lengths into the river catchment of the river causes its catchment area.

Tidal Influence: At the point where the Pearl River exits into the sea, tides exert their effects. At periods of high tide, the saltwater is pushed backwards to eventually clog up the freshwater river, resulting in impediment to its normal downstream flow. Water piles up within the channel, creating a higher likelihood of flooding over parts of it.

Urbanization and Land Use Changes: The rapid development of urban features and changes in land use within the basin have diminished natural floodplains and impacted the natural drainage system of the river. Paved surfaces and expansion of built-up areas increase surface runoff, hence decreasing the capacity of the river to handle excess water during heavy rains. In this respect, Acoustic Doppler Current Profiler (ADCP) seems to be a more progressive and handy tool of measurement for the better understanding and control of flood issues.

3. How Do ADCPs Using the Doppler Principle Work?

The principle on which ADCPs operate is the Doppler principle. They send acoustic signals into the water. When those acoustic waves reach the moving particles in the water, like sediment or different parcels of water with varied velocities, the frequency of the reflected signals compared with emitted signals changes due to the Doppler effect.

With precise analysis of these frequency shifts, ADCPs can determine the velocity of the water at each depth. Most ADCP current meter devices are normally fitted with several transducers oriented in different directions to measure the components of velocity in several dimensions, such as horizontal and vertical directions. By integrating these velocity measurements across various depths and sections of the river, it becomes possible to deduce critical hydrological information such as the flow rate of the river.

4. What are the Applications of ADCP in Floods of Pearl River?

Velocity Measurement: ADCP flow meter is able to measure the velocity of the water flow in the Pearl River at different locations and depths with high accuracy. In the event of flooding, this helps identify points where the flow is rapid and may be dangerous to the riverbanks, bridges, and other infrastructure. It also allows for monitoring of how the flow velocity changes with time as the flood progresses.

Flow Rate Measurement Application: By integrating the velocity measurement over the cross-sectional area of the river, ADCP profiler can obtain the flow rate. This becomes fundamental to quantify the volume of water in every portion of the river when it is on flood. This would allow us to forecast the flood peak flow and to know the impact this may cause downstream, necessary for planning the implementation of an efficient flood management and mitigation program.

Sediment Transport Research: There is a lot of sediment transport during floods in the river. ADCP meter can detect the concentration and moving characteristics of sediment by analyzing echoes of acoustic signals affected by sediment particles. This information is useful in understanding how floods influence the morphological evolution of the riverbed, sediment deposition, and erosion processes along the Pearl River, which is important to maintain the stability of the river channel and adjacent infrastructure.

5. How Can the Data Measured by ADCP Be Utilized in the Flood Warning and Risk Management of Pearl River?

Flood Warning

Velocity and Flow Rate Data Monitoring: ADCP continuously monitors the velocity and flow rate data, whereby abnormal increases in these parameters can be detected well in advance. When the flow rate approaches or exceeds certain critical values, it serves as an indicator of an impending flood peak, enabling timely flood warnings to communities and relevant authorities along the river.

Water Level Prediction and Warning: Models can be built regarding the future changes in water level by relating the measured flow rate and velocity data with the historical data regarding water level. This helps in issuing warnings in advance about the possible areas of inundation and also about the height of the floodwater, so that precautionary measures can be taken in advance.

Risk Management

Water Conservancy Project Scheduling Decision Support: The ADCP data will help the decision-making of water conservancy projects like the operation of dams, reservoirs, and floodgates along the Pearl River-for example, adjusting the volume of reservoir discharge according to the real flow situation in real time, with the purpose of easing the flood impact downstream.

Flood Disaster Assessment and Emergency Response: In cases where a flood has occurred, data from ADCP can be applied after the event to assess the degree of the flood in terms of the extent of the inundated areas and the flow characteristics during the flood. It serves to guide emergency response in the distribution of relief resources and planning post-flooding recovery work.

6. What is required to carry out high-quality measurement of the currents of the Pearl River?

For high-quality measurement of the currents in the Pearl River, a number of things come into play. The materials for the equipment should be highly reliable. Considering the nature of the river, with aspects such as fluctuating water quality, floating debris, and shipping effects, the equipment should be resilient enough.

It is required that the size of the measuring equipment be relatively small for ease of installation and deployment at several places along the river, particularly where there is limited access or within the busy urban sections along the banks. The lightweight design further has its advantages in simplifying the transport and handling of the equipment during installation and maintenance. It has low power consumption to ensure that it is able to operate continuously without frequent replacement of the power sources or excessive energy demands. Cost-effectiveness is another important factor in deploying large-scale ADCPs with the aim of comprehensive monitoring of a river.

The casing of the ADCP is preferably manufactured from titanium alloy. It has several outstanding advantages. It has excellent corrosion resistance, which can stand long-term exposure in river water with all possible chemical compositions. Besides that, it has a very high strength-to-weight ratio, able to provide enough structural strength without overburdening the equipment's overall weight. Its durability further guarantees stable performance influenced by different environmental factors in the river basin.

7. How to Choose the Right Equipment for Current Measurement?

In order to choose the right equipment for current measurement in the Pearl River, several factors need to be considered. Firstly, based on the purpose of use, if the horizontal cross-section measurement is the focus, then the Horizontal ADCP (HADCP) is suitable because it can accurately measure the flow velocity and related parameters in the horizontal direction across the river section. For vertical cross-section measurement, the Vertical ADCP would be more appropriate since it can obtain a detailed velocity profile along the vertical axis of the river.

Secondly, different frequencies should be chosen based on the actual water depth. For water depths within 70 meters, an ADCP with a frequency of 600 kHz is usually a good option since it can give relatively accurate measurement results within this depth. At larger depth areas, up to 110 meters, an ADCP should be used with a higher frequency like 300 kHz, able to work at greater depths and perform the measurement of data appropriately.

There are many brands of ADCPs on the market, including Teledyne RDI, Nortek, and Sontek. However, I want to introduce a very good and affordable Chinese ADCP brand: China Sonar PandaADCP. It is made of all-titanium alloy material, ensuring excellent performance and durability. It also offers an impressive cost-performance ratio. You can learn more about it on its official website: (https://china-sonar.com/).

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