Company Profile

Flow Meter Geography Concepts: Measuring River Velocity and Hydraulic Geometry Radii

Quick Answer: For open river channels, choose an ultrasonic velocity area flow meter. For full pipe sections of river water intake or discharge lines, choose an electromagnetic flow meter from Silver Automation Instruments. Send your channel cross section data, water level range, velocity range, and output signal requirement to get a model and price.

Hydraulic Radius and Why It Affects Flow Meter Choice

Hydraulic radius is the cross sectional area of flow divided by the wetted perimeter. A rectangular channel 2.0 m wide with 0.5 m water depth has an area of 1.0 square meter and a wetted perimeter of 3.0 m. The hydraulic radius is 0.33 m.

This value matters because river velocity is not uniform across a cross section. The Manning equation links velocity to hydraulic radius, channel roughness, and water surface slope. In practice most engineers skip a full Manning calculation and use a velocity area method with a flow meter. But the hydraulic radius still tells you where to place the sensor and what velocity range to expect.

River Velocity Profile and Sensor Position

In a natural stream the fastest water is near the center below the surface. The slowest water is near the banks and the bottom. If you mount a sensor just at the surface you can read a discharge value that is 20 to 30 percent too high. We have seen this on customer sites in Vietnam and Peru where an initial surface reading gave a large error.

For shallow rivers, measure velocity at 0.6 of the water depth from the surface. For deeper rivers, take the average of velocities at 0.2 and 0.8 depth. Divide a wide river into 10 to 20 vertical segments. Measure velocity in each segment and multiply by the segment area. This gives total discharge with reasonable field accuracy.

Full Pipe or Open Channel: Match the Meter Type

Use an electromagnetic flow meter only when the river water flows through a full pipe. Common examples are raw water intake lines, effluent discharge pipes, and pump station headers. Silver Automation Instruments electromagnetic flow meters cover DN15 to DN2000 with accuracy of 0.5 percent. Liners include PTFE and hard rubber. Electrode materials include 316L stainless steel and Hastelloy. Outputs cover 4-20 mA plus HART and RS485 Modbus.

Use an ultrasonic open channel flow meter when the water surface is open to atmosphere. A river, an irrigation canal, or a stormwater drain falls into this group. The ultrasonic meter can measure velocity from 0.02 to 10 m/s with a level sensor for water depth. Last year a customer in Vietnam measured a canal at 0.4 to 1.8 m/s. The canal had a trapezoidal cross section with a top width of 3.8 m, a bottom width of 1.2 m, and a depth of 0.9 m. We supplied an ultrasonic open channel flow meter with an IP68 sensor and Modbus RTU output.

Hydraulic Geometry for Common River Shapes

Rectangular and trapezoidal channels have simple hydraulic radius formulas. A wide rectangular channel has a hydraulic radius close to its depth. A narrow trapezoidal canal has a different ratio because the wetted perimeter includes side slopes. Natural rivers change shape with the season. In dry months a river may become wide and shallow with a low hydraulic radius. During flood the same river may have a higher hydraulic radius because depth increases faster than width. This changes the sensor range and the best mounting position.

Most engineers skip the hand calculation and send us the surveyed cross section. We then check the velocity range and level range against the selected flow meter. This takes less time and avoids errors when the bank geometry is irregular.

Field Installation Notes for Southeast Asia, Africa, Middle East and Latin America

Install the sensor in a straight reach. Keep at least 10 cha