Water flow measurement builds on level measurement to provide not only water levels, but total volume in moving natural waterways, namely rivers and streams. Flow monitoring tracks the movement of water in a watershed and is a critical component of water resource management, hydropower generation, environmental conservation, and other hydrological studies.
Flow rates in water bodies are typically expressed in cubic feet per second (cfs) or cubic meters per second (m³/s), which represent the total volume of water moving through a specific cross-section of a river or stream per unit time. Various methods exist for calculation of flow, often based on other measured parameters such as water level and velocity.
Why Measure Water Flow?
Understanding the dynamics of water flow is fundamental to the field of hydrology and is applicable to many of the same use cases as water level measurement, including flood prediction and management, water resource management, environmental monitoring, and infrastructure planning.
Some other applications where flow measurement is particularly important include:
- Sediment Transport Studies: Sediment deposits have a significant impact on water quality and ecosystem health in some aquatic environments, such as rivers and estuaries, where rates of sedimentation are directly impacted by flow rates.
- Dam Operation: Whether used for flood control, navigation, or energy production, obtaining accurate flow data can help dam operators to effectively manage releases through dams.
- Energy Production: Flow measurements help hydroelectric power plants to estimate potential energy production and expected refill rates of reservoirs, thereby enabling them to optimize operations.
- Wastewater Effluents: Obtaining accurate flow data of incoming raw wastewater and outgoing treated effluents helps wastewater treatment plants to ensure regulatory compliance and efficiently manage treatment processes.
How is Water Flow Measured?
Unlike many other physical and chemical water parameters, flow measurement is not directly measured with a single instrument alone. The geometric component of flow requires an understanding of both the water’s velocity and the cross-sectional area through which it moves. This necessitates a combination of techniques and/or devices to accurately calculate flow rates.
While some manual methods such as dye tracer techniques exist, for the purposes of remote, real-time monitoring, some of the most common flow measurement methods include the stage-discharge relationship, the area-velocity method, and weirs or flumes. Each method has its advantages and limitations.
The stage-discharge relationship method involves developing a rating curve that correlates water level (stage) to the flow rate (discharge) at a particular cross-section of a river or stream. Developing the rating curve requires surveying of the site periodically over time at various stages.
A rating curve developed by taking manual discharge measurements at various water levels allows for estimation of flow rate based on water level measurements alone.
A site survey typically involves dividing the river into subsections and measuring both the geometry and velocity of each section. This can be done manually using a wading rod with a mounted velocimeter, or with more advanced systems that incorporate an Acoustic Doppler Current Profiler (ADCP) instrument that is towed across the water surface and records the depth and velocity profile continuously.
A site survey to develop a rating curve is performed by recording the velocity and depth along the cross-section of the river or stream.
The primary advantage of the stage-discharge relationship is that, once the rating curve is developed, it requires only a simple water level measurement to determine the flow. This significantly reduces the instrumentation needs and costs for maintaining a measurement system.
However, there are several limitations of the stage-discharge relationship to consider. Sites should be periodically resurveyed due to changes of the stream or river cross-section due to erosion, sedimentation, or other obstructions. A rating curve may additionally need to account for seasonal changes resulting from effects such as vegetation during the warm season.
Another limitation is that flow volume can vary at the same stage in some cases. For example, consider a river with a rapidly increasing stage following heavy rainfall and snowmelt. While the river is rising, the flow rate at any given stage may be higher than it is at the same stage while the water level is decreasing. This effect is known as hysteresis, and it produces a loop pattern when plotted.
Hysteresis effects can result in a river or stream having a variable flow rate at the same stage.
To account for hysteresis, more complex rating curves including correction factors for different hydrological conditions may need to be developed.
The area-velocity method leverages the same principle as the process used to develop a stage-discharge rating curve since the cross-sectional area is also determined through a site survey. However, this method utilizes continuous average velocity measurement in addition to water level to improve accuracy. Total discharge (D) is then calculated by area (A) times velocity (V):
D = A x V
The area-velocity method is similar to the stage-discharge method but utilizes a continuous velocity measurement to improve accuracy.
Like water level sensors, velocity meters (or velocimeters) may be either non-contact or contact technologies. Non-contact sensors typically use radar signals that are directed towards the water at an angle. Part of the signal is reflected back to the sensor by surface turbulence or particles in the water.
Due to the Doppler effect, the movement of the water causes a frequency shift of the returned signal. The magnitude of the frequency shift is proportional to the velocity, and the sensor uses internal algorithms to calculate the velocity at the water surface.
A non-contact velocimeter uses a radar sensor and the Doppler effect to measure surface velocity.
Radar velocity sensors are relatively easy to install, low maintenance, and achieve good accuracy under many conditions. However, they are limited to measurement of surface velocity only at one specific point. Single-point measurement may not be representative of the entire cross-section because velocity often varies spatially due to factors such as riverbed/streambed roughness, bends and meanders, obstructions, stratified flow conditions, wind, or tidal influences.
Continuous in-water velocity profiling using an ADCP instrument is an alternative to surface measurement with a radar device in waterways with sufficient minimum water depth. This can be a bottom-mount instrument that focuses on a certain area of the cross-section, or a side-looking instrument that measures across the entire cross-section.
Bottom-mount (left) or side-looking (right) ADCP instruments can provide detailed velocity profiles in a river or stream cross-section.
Such setups offer a more thorough velocity profile and typically include an acoustic or pressure-based water level sensor. The more comprehensive data set delivered can, in some cases, provide better accuracy than a surface measurement with assumptions made about the subsurface velocity characteristics of the river or stream. However, subsurface installation and maintenance is often more complicated than that of a non-contact sensor.
Weirs and flumes are another option for flow calculation in some environments with relatively low flows. These are structures built across streams or channels that have a predictable relationship between water level, velocity, and flow rate. This allows the flow to be calculated solely from the observed water level.
Weirs and flumes have predictable flow rates based on water level.
Weirs and flumes simplify the flow calculation but require installation of a structure in the waterway. This may have environmental impacts since the structure physically alters the movement of water and changes the streambed composition where it is installed.
Weirs and flumes are relatively low maintenance once installed, but periodic removal of debris buildup may be required.
How to Select a Water Flow Measurement System?
Selecting a water flow measurement system depends primarily on the site conditions since not every flow measurement technique is suitable for every environment.
For environments where the water levels fluctuate significantly, the stage-discharge relationship method might be favored due to its simplicity once the initial rating curve is established. This method is particularly beneficial in settings where the installation of complex instrumentation is impractical. However, it requires that the site remains relatively stable over time, or that the rating curve is updated regularly to reflect changes in the stream or river channel.
In contrast, the area-velocity method is preferable in scenarios where water velocity can offer a more accurate picture of flow rates, especially in rivers or streams with complex flow patterns. This method typically utilizes more sophisticated equipment with higher start-up costs but can provide more detailed data.
For sites with relatively stable and low flow rates, weirs and flumes present a straightforward, relatively low-tech option. They are best suited for smaller water bodies where the installation of a physical structure does not pose significant environmental or logistical challenges.
In addition to the feasibility of a particular method for a given location, also consider the maintenance requirements and lifetime costs associated with system operation. Areas prone to high debris loads, significant biofouling, or frequent channel geometry changes may necessitate more robust and easily maintained systems.
What to Consider When Preparing a Water Flow Measurement System?
Having selected a water flow measurement system based on the site characteristics, consider the technical requirements and configuration of the sensors to be used. If using the stage-discharge relationship method or a weir or flume, the only required measurement is water level. In these cases, refer to the chapter on selection, preparation, and deployment of water level sensors.
The area-velocity method typically uses a non-contact radar sensor or submerged ADCP instrument for the velocity component of the calculation.
Radar sensors require a secure, stable mounting structure with an unobstructed view to a representative area of the water surface in the upstream or downstream direction, depending on sensor settings. A slightly turbulent surface normally improves the radar signal quality, but excessive turbulence should be avoided if possible.
ADCP instruments similarly require internal configuration to match the channel geometry and expected flow characteristics. They divide the river’s cross-section into cells of a chosen width. A larger number of cells increases the data resolution but requires more data processing and may increase the system power demand.
How to Deploy a Water Flow Monitoring System?
In the case of a non-contact radar flow meter, the system setup is nearly identical to that of a non-contact radar level sensor.
A non-contact flow measurement system integrates both water level and velocity measurements.
The main difference is that the flow system will include a radar velocimeter directed at an angle in addition to the downward-facing water level radar. This requires a larger unobstructed surface area for the radar signals.
For larger rivers, multiple velocity sensors may be needed at different points in the river cross-section to improve accuracy.
The mounting structure should be free of vibration that can cause measurement interference. The mounting height should be sufficient to avoid submersion during high-flow or flood conditions, but the water surface must remain within the sensor’s measurement range for accurate measurement.
For systems with submersible sensors like an ADCP, many of the same principles apply as with submersible pressure transducers for water level measurement. Most important is a robust structure that protects the instrument and data cable from forces generated by high flow and debris loads yet doesn’t alter flow conditions and compromise measurement accuracy.
A minimum water level is required for the instrument to measure accurately. The site should also provide accessibility for periodic maintenance. Biofouling or other obstructions such as rocks or vegetation can interfere with measurement beams, so regular cleaning is often necessary.
Like water level monitoring systems, remote flow measurement systems will utilize a data logger with wireless telemetry capabilities to transmit logged data. System power can be supplied from an AC (fixed) power supply or solar-charged battery pack.
Conclusion
Water flow is a useful hydrological measurement that can be carried out using a variety of methods, several of which rely on remote sensing systems for water level and/or velocity measurements. Analysis of the flow conditions can help to select an appropriate method.
Site selection also plays a critical role in establishing a continuous measurement system that will provide representative measurements. With careful installation and proper maintenance, systems can provide reliable flow data in long-term monitoring applications.
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