ADCP in Mississippi River (St. Louis) Flood Management

1. Where is Mississippi River at Saint Louis?

The Mississippi River at Saint Louis is a critical part of the vast Mississippi River system. Saint Louis is situated within the State of Missouri, and this river flows right through the middle of this dynamic city.

The Mississippi River is one of the longest and most significant rivers in North America. At Saint Louis, it forms a major transportation artery on which agricultural products, coal, and other commodities are transported. The economy of the city has been tied to the activities of the river for many years.

The Saint Louis area has a humid continental climate characterized by hot summers and cold winters. Precipitation is well distributed throughout the year, although spring and summer are usually wetter than other seasons. Its flow varies with rainfall over the enormous watershed, snowmelt from the upstream reaches, and tributary input.

2. Why do floods occur in the Mississippi River at Saint Louis?

Heavy Rainfall and Snowmelt: One of the most serious causes of flooding of the Mississippi River at Saint Louis is the combination of heavy rainfall and snowmelt. The Mississippi River has an enormous watershed, and when heavy rain falls over a wide area, a tremendous volume of water surges into the river. In addition, the snowmelt during spring in the northern part of the watershed contributes to a large volume of water. When this volume exceeds the carrying capacity of the river, flooding occurs.

Tributary Inputs: Many tributaries feed the Mississippi River at Saint Louis. When conditions of high flow due to local rainfall or any other influence occur in such tributaries, the volume in the main river channel increases all of a sudden. An example is the Missouri River, which has contributed the highest amount during flood events.

Soil Saturation and Poor Drainage: During heavy rains, the soil in these areas gets saturated. The land has limited absorption capacity, and a large part of the rainfall flows into the river. Poor drainage systems in some regions also contribute to the rapid accumulation of water in the river, increasing the flood risk.

In the context of flood management, Acoustic Doppler Current Profiler (ADCP) offers a more sophisticated and effective method for measuring the flow of the river compared to traditional techniques.

3. How do Doppler Principle ADCPs work?

ADCPs work on the principle of the Doppler effect. They transmit acoustic signals, or simply sound waves, into the water. These signals scatter off moving particles in the water, like sediment particles and water molecules flowing with the current. When the emitted waves are reflected back by these moving objects, a frequency shift takes place.

This frequency shift, now called the Doppler shift, is directly proportional to the velocity of the moving objects. ADCPs are configured with multiple transducers that transmit acoustic pulses in various directions. For instance, a downward-looking ADCP current meter can obtain the velocity profile of the water column from the water surface to the riverbed. Since this unit of measurement can gauge the Doppler shifts of reflected signals from different water column depths, the ADCP current profiler then produces an exact velocity calculation in each of these chosen places in the flow of the water.

Once the different depths of the velocity is estimated and when the river cross-sectional area is estimated, other parameters like flow can also be estimated for this particular river.

4. Applications of ADCP in flood events of Mississippi River at Saint Louis

Measurement of Velocity: ADCP flow meter works by measuring the velocity across a wide range of measurement locations and depths during Saint Louis flood events on the Mississippi River. This information, therefore, helps in explaining the speed at which these floodwaters are moving in this area. For example, it can help predict when the floodwaters will reach vulnerable areas such as riverside communities, industrial sites, or infrastructure like bridges and levees. By continuously monitoring the velocity, authorities can take preventive measures such as evacuating people or strengthening flood-protection structures.

Flow Measurement Application: An ADCP profiler computes the river flow rate by integrating its velocity data over the cross-sectional area of the river. During a flood situation, this will help define the total volume of water passing through a particular section of the river. All this information is crucially required in flood forecasting, and hence for understanding the magnitude of the flood. It can also be used to determine the capability of flood-control structures such as levees and dams.

Sediment Transport Research Application: In the case of the Mississippi River at Saint Louis during its flood-carrying stages, ADCP current meter can take the backscattered acoustic signals to study the sediment particles' movement. Sediment transport during floods is very important in assessing changes within the riverbed. This sediment deposition and erosion affects the depth, width, and stability of the river, impacting future flood risks and the general health of the river ecosystem.

5. How can the data measured by ADCP be used for flood warning and risk management of the Mississippi River at Saint Louis?

Flood Warning

Velocity and Flow Data Monitoring: Through ADCP meter, continuous monitoring of the velocity and flow data allows for the early detection of abnormal changes in the river's flow. A sudden surge in the velocity or major flow variations could be warning signs that the flood is near, or the prevailing flood conditions are worsening. This data can be incorporated into a flood warning system to send timely warnings to the respective communities, responders, and other authorities for effective service provision.

Water Level Prediction and Warning: Measured flow data by ADCP profiler, combined with other factors like the cross-sectional geometry of the river and historical records of water level, can be used to predict future water levels. If the predicted water level is expected to exceed the flood level mark, warnings can be issued in advance to enable people to evacuate or take protective measures.

Risk Management

Water Conservancy Project Scheduling Decision Support: Data from ADCP flow meter acts as a very useful input in making decisions on the operational activity related to water-conservancy projects such as dams, reservoirs, and diversion channels along the Mississippi River at Saint Louis. An example could be a flood for which the flow data may allow determination of the amount of release from a dam that could reduce the effect of a flood downstream.

Flood Disaster Assessment and Emergency Response: In the case of a flood, it is only after that ADCP data can be utilized in assessing the damage the flood has caused. This encompasses the assessment of the area suffering from high-velocity flow, sediment deposition amount, and other factors. This information is useful in guiding emergency response and recovery efforts such as search and rescue, and post-flood rehabilitation of infrastructure and ecosystems.

6. What's needed for high-quality measurement of the Mississippi River currents at Saint Louis?

For high-quality measurement of the Mississippi River currents at Saint Louis, several aspects need to be considered.

Durable Equipment Materials: Reliable materials shall be used for the manufacture of the ADCP meter capable of withstanding the drastic action by the Mississippi River Environment resistant to corrosion by river water, impacts by floating debris in the form of branches and logs, and also impacts and abrasion by current transport sediment particles.

Small Size, Light Weight, and Low Power Consumption: A smaller and lighter ADCP flow meter is easier to install and deploy, especially in areas that are difficult to access along the river. Low power consumption enables longer-term operation without the need for frequent battery replacement or complex power-supply arrangements, which is beneficial for continuous and long-term monitoring.

Low Cost: A lower-cost ADCP profiler is desired to enable a large-scale deployment for wide-area monitoring of the river.

On the casing material, titanium alloy is an excellent option. Titanium alloy has high strength to resist the impact of outside forces on the ADCP meter. It also has excellent corrosion resistance, which will guarantee the long-term performance of the equipment in the water. Its relatively low density contributes to a reduction in the overall weight of the device while maintaining its structural integrity.

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

Selection of equipment for current measurement in the Mississippi River at Saint Louis will need to consider:

Purpose of Use: Horizontal cross-sectional flow measurement shall be done by a Horizontal Acoustic Doppler Current Profiler-HADCP. This measures the flow velocity and other parameters across a horizontal section of the river. In case of vertical cross-section measurement, a vertical ADCP would be appropriate since it measures the velocity profile from the water surface to the riverbed along a vertical line.

Frequency Selection: The different frequencies of ADCPs are suitable for different water-depth ranges.

For instance, an ADCP profiler with a frequency of 600 kHz will typically be suitable for water depths within 70 meters. Whereas an ADCP flow meter with a frequency of 300 kHz is more appropriate for water depths up to 110 meters. Selecting the right frequency according to actual water depth of Mississippi River at Saint Louis makes sure that the measurement results are accurate and reliable.

There are well-known ADCP current meter brands such as Teledyne RDI, Nortek, and Sontek. Additionally, there is a Chinese brand, China Sonar PandaADCP, which features a casing made of titanium alloy and offers a good cost-performance ratio. 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.