Open channel flow monitoring systems demonstrate multifaceted value in all aspects of water conservancy. Therefore, accurate monitoring of open channel flow is crucial. The commonly used method for measuring open channel flow is to use a standard Parshall flume and an open channel flow meter together. These are used to measure the water level, flow rate, and total volume flowing through weirs, flumes, channels, and partially filled pipes. These flow meters perform best in environments where sediment accumulation is minimal.
This article will detail the concept of open channel flow and related experience in measuring it. We hope you find it helpful.
What is an Open Channel?
An open channel, also known as a canal, man-made river, riverbed, or canal. It is a structural water system used to guide water flow. It is typically an open channel used to transport water from one place to another. While the concept of an open channel is simple, it plays a vital role in agricultural irrigation, urban drainage and river pipelines.
Based on its formation, it can be divided into natural open channels and artificial open channels. Natural open channels include natural rivers. Artificial open channels include man-made water conveyance channels, canals, and pipes that are not yet full of water. During the rainy season, open channels can effectively divert flash floods. In industrial areas, open channels guide wastewater to the ocean or wastewater treatment plants.
What is Open Channel Flow?
Open channel flow refers to free surface water flow in rivers, irrigation canals, etc., as opposed to pressurized flow in pipes. Open channel flow is a key element in agriculture, flood control, and ecosystems. Water flows in natural channels, driven by gravity. Open channel flow plays an important role in irrigation systems. Open channel flow also assists in urban flood discharge. Natural open channels also provide habitats for plants and animals, contributing to ecological restoration.
Flow Monitoring for Open Channel Applications
Open channel flow monitoring plays a crucial role in many fields. Here are some specific applications:
In smart water management systems, monitoring river hydrological stations and reservoir inflow provides decision support for flood control scheduling. Accurate open channel flow monitoring also helps in the allocation and metering of water in the main and branch canals of large irrigation districts, which promotes water-saving irrigation.
In urban construction, monitoring the flow rate of urban flood-prone areas and drainage outlets contributes to the construction of “sponge cities.” Open channel flow monitoring also plays a key role in environmental protection.
Monitoring the open channel flow rate of many industrial enterprises and sewage treatment plants helps optimize resources and protect the environment.
Fundamental Equations of Open Channel Flow
We have summarized four equations for calculating open channel flow rate.
Velocity-Area Method
The velocity-Area Method is the easiest way to understand. The core idea is to calculate the flow rate by measuring the cross-sectional area and water velocity of the open channel. Specifically, we measure the water level by using a level meter. Then, the cross-sectional area is calculated based on the open channel’s geometry. Then, the cross-sectional velocity is measured using a flow meter. Finally, the flow rate is obtained by multiplying the cross-sectional area by the velocity. It is suitable for regular cross-sections and stable flow.
The calculation formula is: Q = S × V.
Where Q is the flow rate, S is the cross-sectional area, and V is the average cross-sectional velocity.
Accurately measuring the average velocity across the cross-section is inaccurate and challenging. We also need to average the velocity measurements from different points on the cross-section to get a more accurate flow.
Water Level Method
This method requires constructing measuring structures in the open channel, such as Parshall flumes or triangular weirs etc.
Flow rate is calculated by measuring the water level upstream and downstream of the structure and correlating these levels with empirical formulas or experimental curves.
For instance, in a Parshall flume, water flow creates specific water level changes within the flume. It reflects the flow rate. The flow rate calculation formula is Q = C × H^n.
Where Q is the flow rate, H is the water level at a specific location within the flume, and C and n are coefficients related to the flume’s size and shape. This method does not require direct velocity measurement. But the construction and maintenance costs of the measuring structures are high. And the presence of downstream water-blocking structures may reduce measurement accuracy.
Fluid Method
This method involves narrowing the cross-section of a section of the channel under test, creating a throat. The water velocity increases at the throat. By measuring the water level and velocity at the throat and combining this with the geometric parameters of the open channel, the flow rate can be accurately calculated. This method applies to all kinds of open channels. However, it requires that the channel’s geometry and dimensions be regular. And the upstream water flow is uniform and stable.
Weir Method
This method involves placing a baffle at an appropriate location in the open channel to block the water flow. The water level rises and flows out through the weir. Flow rate is calculated by measuring the stabilized water level. This method applies to various types of open channels. However, this requires that the geometry and water level of the weir remain stable. Common weirs include triangular weirs and rectangular weirs. Each with its own flow calculation formula. For example, the flow formula for a triangular weir is Q = C × H^(5/2).
Where C is a coefficient, H is the water level upstream of the weir.
Open Channel Flow Measurement Solutions
Open channel flow measurement primarily involves using a Parshall flume and an open channel flow meter. With advancements in technology, Doppler ultrasonic flow meters can now measure flow directly without a Parshall flume. Open channel flow measurement is affected by various factors, including channel size and flow rate etc. If the channel is regular in shape, a flow meter can be used directly. The following are common flow meter measurement principles.
Open Channel Flow Parshall Flume
It is an auxiliary device for measuring flow in open channels. The higher the flow rate in an open channel, the higher the liquid level; the lower the flow rate, the lower the liquid level. For general channels, there is no definite correlation between liquid level and flow rate. This is because, for the same water depth, the flow rate also depends on the channel’s cross-sectional area, slope, and roughness.
When installing weirs and flumes within a channel, the weir’s notch or the flume’s constriction is smaller than the channel’s cross-sectional area. So, the correlation between upstream water level and flow rate mainly depends on the weir’s geometry. The same weir or flume placed in different channels will correspond to the same flow rate for the same liquid level. The weir or flume converts the flow rate into the liquid level. By measuring the liquid level in the weir or flume and then using the corresponding weir-flow rate relationship, we can calculate the open channel flow..
Radar Flow Open Channel Meter
A radar flow open channel meter is a system. It mainly uses radar technology to measure flow rate. It is primarily used to monitor water flow in open waterways or rivers. Open channel flow measurement systems usually include radar sensors, a data processing unit, and display or recording equipment.
The radar sensor is installed above the water surface. The radar sensor transmits signals and receives reflected radar signals. By analyzing the difference between the transmitted and reflected signals, the system can calculate the water surface velocity. Then, the system estimates the right flow rate based on the water surface velocity and the cross-sectional area of the water body.
This system does not directly contact the water. So it makes installation and maintenance relatively simple. And it causes less interference to the fluid. Radar flow monitoring systems can provide accurate flow monitoring data. It also has a significant application value in fields. such as flood warning, water resource management, hydropower generation, and environmental monitoring. By monitoring changes in water flow in real time, some relevant management departments can take fast measures to effectively respond to natural disasters and optimize water resource utilization.
Read More about: An Overview of Radar Level Transmitter
Ultrasonic Open Channel Flow Meters
The core operation principle of ultrasonic open channel flow meters is based on the physical properties of sound waves. They utilize the propagation characteristics of ultrasonic pulses in water flow to measure water velocity and water level.
The equipment is usually mounted above or to the side of the open channel and includes one or more sensor probes. These probes are responsible for transmitting and receiving ultrasonic signals. When an ultrasonic pulse is emitted from the probe, it travels through the air or water, interacts with the water flow, and returns to the probe.
By calculating the propagation time difference of the ultrasonic pulse, the device can deduce the water flow velocity. Simultaneously, combined with data from a water level meter, the system can accurately calculate the flow rate. This principle can ensure the high efficiency and reliability of the measurement process without direct contact with the water body. It reduces equipment wear and the risk of contamination.
In hydraulic engineering, ultrasonic open channel flow meters are usually used in various open waterway scenarios, such as rivers, irrigation canals, or drainage ditches. They featured strong adaptability, capable of handling different water flow conditions, including turbulent or steady flows.
Read More about: Ultrasonic Flow Meter Technical Guide
Doppler Ultrasonic Flow Meters
The key technology of Doppler ultrasonic flow meters is based on the Doppler effect. The device integrates a flow velocity sensor, a pressure sensor, and a temperature sensor. The flow velocity sensor emits ultrasonic pulses. When the sound waves encounter suspended particles in the fluid, they are reflected. The frequency difference between the emitted and received sound waves is positively correlated with the fluid flow velocity. The temperature sensor provides real-time compensation for the sound velocity. This multi-sensor collaborative working mode ensures measurement stability even in water bodies with high sediment content and abundant floating debris.
Read More about: Portable Ultrasonic Flow Meters | Clamp-on Ultrasonic Flow Measurement
Sino-Inst Featured Open Channel Flow Meter
Case Study: Open Channel Flow Monitoring System
This measurement case is located in a small city in China. Our open channel flow meter detects the flow rate in the irrigation area. The monitoring system realizes various functions. such as real-time monitoring of irrigation water level and flow rate, and image monitoring. This achieves the goal of saving irrigation water and managing the irrigation scientifically and efficiently. Our open channel flow meter has the practical functions:
- Accurately monitoring irrigation water consumption. It enables rational scheduling and optimized allocation of irrigation water or fertilizer, which ensures a sustainable water supply.
- With increasingly scarce water resources, this system enables real-time online monitoring of the entire water lifecycle, from intake and supply to use and discharge.
- Providing warning, real-time alarms, and timely reminders to users for abnormal water levels and flow.
- Achieving unattended operation of facilities and equipment, saving personnel costs.
- Real-time alarms for abnormal values improve emergency response capabilities and prevent major losses.
How to Choose the Right Open Channel Flow Meter?
When purchasing an open channel flow meter, the following factors should be considered:
- Water Level Range: The actual water level range of the flow meter should be higher than the actual operating conditions.
- Flow velocity range: Flow velocity also affects flow rate.
- Measurement medium: Water cleanliness, suspended solids content, corrosiveness, etc.
- Installation conditions: Channel size, water flow state (free flow or pressure flow).
- Accuracy requirements: Is high-precision data required (e.g., environmental compliance monitoring)?
- Maintenance ease: Is it easy to clean and calibrate?
- Environmental factors: Temperature, humidity, explosion-proof requirements.
What is the difference between open and closed channel flow?
The core difference between open channel flow and closed channel flow lies in the flow boundary and driving mechanism. Open channel flow has a free surface. And it is driven by gravity. The closed channel flow fills the pipe and is driven by pressure or gravity.
What causes hydraulic jumps in open channel flow?
A hydraulic jump is a phenomenon in fluid mechanics, commonly occurring in open channels such as rivers or spillways. When a high-velocity supercritical flow enters a low-velocity subcritical flow, the fluid velocity suddenly decreases. As a result, some of the fluid's kinetic energy is dissipated by turbulence. While some is converted into potential energy, causing a significant rise in the liquid level. This phenomenon is known as a hydraulic jump.
How to avoid hydraulic jumps?
When water flow transitions from a rapid to a slow flow, a localized hydraulic jump occurs, causing the water level to rise suddenly. In engineering, various energy dissipation measures are designed to eliminate the scouring energy of the water flow within a short area. Common measures include artificial roughening (increasing the roughness of the channel bottom) and stilling basins (using hydraulic jump energy dissipation methods).
In short, open channel flow monitoring plays a crucial role in many industries. Sino-Inst offers a variety of flow meters to help you achieve accurate flow monitoring. Please contact us anytime for a quote. Our professional engineers can provide you with various solutions free of charge!
