ADCP for flood management of Cooper River

1. Where is Cooper River?

The Cooper River is a major river in the United States. It flows through the State of South Carolina, and it is an important part of the regional ecology and economy. Geographically, it has a heterogeneous riverbed with varying widths and depths, meandering through different landscapes from rural areas to urban regions.

It passes through cities such as Charleston, which is a city full of historic and cultural heritage, with a very busy port. The river acts as a major transport route for ships carrying goods both in and out of the region.

The Cooper River area has a humid subtropical climate. Precipitation shows a marked variation between the wet and dry seasons. The wet season is characterized by frequent rainfall, with most of the summer months experiencing thunderstorms. This rainfall, combined with runoff from the land, adds to the volume of water in the river. The winter months are somewhat drier, although there can still be occasional precipitation events.

2. What are the causes of flooding in Cooper River?

There are numerous causes that lead to flooding in the Cooper River. The first cause is intense rainfall events. During heavy rainfall, especially if successive storms or a large-scale weather system brings copious amounts of precipitation, the runoff water from the surrounding watershed rapidly accumulates and flows into the river. This can easily exceed the natural river capacity and swollen the river, causing the level to rise and resulting in floods. For instance, when a tropical storm hits this area, it can release massive rainfall in a short span, swamping the river.

Second, there are tidal influences. Because of its proximity to the coast, the Cooper River is subjected to the tides. High tide conditions may hinder the natural flow of the river, thus causing water to backup, especially if accompanied by increases in freshwater input from upstream. This may be enhancing flooding in low-lying parts of the riverbanks.

In addition, there is a change in land use or urbanization in the catchment areas. The expansion of cities and the construction of impervious surfaces such as roads and parking lots reduce the amount of land available for water infiltration. More rainwater, therefore, runs off directly into the river, increasing the flood risk. In this regard, Acoustic Doppler Current Profiler (ADCP) has emerged as a more advanced and convenient measurement option compared to traditional methods for understanding and managing flood situations.

3. How do ADCPs using the Doppler principle work?

ADCPs work on the principle of the Doppler effect. They emit acoustic signals into the water column. When these acoustic waves encounter moving particles in the water, such as sediment particles or small aquatic organisms that are being carried along by the water flow, the frequency of the reflected waves changes due to the Doppler effect. By precisely measuring this frequency shift, the ADCP current meter can calculate the velocity of the water at different depths within the river.

These instruments have several transducers that are mounted at strategic positions to measure velocity components in different directions. They can simultaneously measure the horizontal velocities along the flow direction as well as the vertical velocities. The data collected from these measurements at various depths are then combined to create a comprehensive profile of the water velocity throughout the entire water column, providing detailed information about the river's flow characteristics.

4. Where can the ADCP apply to the Cooper River floods?

• Velocity measurement: ADCP current profiler works effectively in measuring the actual velocity of water flow in the Cooper River during flood events with high accuracy. By acquiring velocity data at various locations and depths, it allows for an understanding of the speed at which floodwaters are moving. For example, the knowledge of velocity near riverbanks can indicate the erosive power of water and the damage to infrastructure such as bridges and levees.
• Flow measurement application: By integrating the measured velocity data over the cross-sectional area of the river, ADCP is able to provide the flow rate. This could be important in the determination of the volume of water crossing a particular section during flood conditions and will help flood managers estimate the size of a flood and the potential lower stream effects.
• Sediment transport studies: There is a lot of sediment movement in the Cooper River during flooding. ADCP flow meter can analyze the backscattered acoustic signals for the concentration and transport velocity of suspended sediment. This information shall be useful to understand how floods reshape the riverbed and influence the long-term sediment deposition patterns, which in turn affects the morphology of the river and future flood risks.

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

Flood warning

• Velocity and flow data monitoring: ADCP profiler continuously monitors the velocity and flow data, hence it can easily detect early changes in the flow conditions of the river. If the measured velocities start to increase significantly or the flow rate exceeds certain thresholds, this could be an indication of an approaching flood. This real-time data can trigger timely warnings to communities along the riverbanks.
• Water level prediction and warning: By correlating the measured flow data with the historical record of water levels, one can predict the future water level. For example, if the flow rate is steadily rising, from past relationships, it may be projected that the water level will reach a certain height within a specific time, hence issuing appropriate warnings to the residents and emergency response teams.

Risk Management

• Decision Support of Water Conservancy Project Scheduling: It will help the ADCP data in making decisions about the operation of dams, reservoirs, and other water conservancy projects along the Cooper River. For instance, if there is a big flood going to come, as evidenced by the flow data, then release or impoundment decisions can be taken to reduce the flood impact downstream.
• Assessment of flood disasters and emergency response: Immediately after a flood event, ADCP flow and velocity information helps in detailing the areas suffering from erosion or/and inundation. It provides ample information on ways through which effective emergency responses are developed for planning and executing post-flood recoveries.

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

Several factors are crucial for high-quality measurement of the currents in the Cooper River. First, the equipment should have high material reliability to support most aquatic environmental conditions, which are usually harsh, like corrosion from the river water and physical impacts from flowing water and debris. Small size is preferred because it allows for easy installation and deployment at any location along the river, even in those places where access or space is limited.

A lightweight design is also crucial as it simplifies the handling and transportation of the equipment for installation and maintenance. Low power consumption is necessary to ensure continuous operation without frequent battery replacements or power source connections, enabling long-term monitoring. Cost is an important factor too, especially when considering large-scale measurements across different sections of the river.

Regarding the ADCP's casing, titanium alloy is an excellent choice. The advantages of the use of the titanium alloy are very remarkable. It has superior corrosion resistance, meaning that it may be exposed to the river water for a long time without seriously deteriorating. Its strength-to-weight ratio is high: it has sufficient strength against the mechanical forces in the flowing water while keeping the device relatively lightweight. Moreover, its biocompatibility does not interfere with the aquatic environment, neither with the acoustic signals that the ADCP meter uses for measurement.

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

• Based on the purpose of use, in horizontal cross-sectional measurement cases, the Horizontal ADCP can be a better tool since it is designed to properly obtain the flow velocities across the horizontal plane of the river due to the necessity of data on the flow distribution at this dimension. In vertical cross-sectional measurements, a more appropriate tool is the Vertical ADCP since this ADCP focuses on the derivation of the velocity profile vertically of the water column.
• Based on different frequencies, different frequencies of ADCP are suitable for different water depth ranges. For example, an ADCP with a frequency of 600 kHz is usually suitable for water depths within 70 meters. It can offer relatively accurate measurements in shallower to moderately deep sections of the Cooper River. Meanwhile, an ADCP with a frequency of 300 kHz is better suited for deeper waters up to 110 meters, which can cover the deeper parts of the river where more comprehensive flow data is needed.

There are several well-known ADCP brands in the market, including Teledyne RDI, Nortek, and Sontek. However, one highly recommended Chinese ADCP brand is China Sonar PandaADCP. It is considered due to its all-titanium alloy material construction that couples durability with excellent performance. In addition, it has a very high cost-performance ratio, making it an irresistible choice for users. Details can be found on the website: https://china-sonar.com/.

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