Name: weir flow measurement and control
Uploaded: 2025-03-26T15:05:11.000Z
Duration: 1 min 24 s
Channel: JoVE

A weir is a hydraulic structure constructed across open channels to control and measure water flow.It partially obstructs the channel and directs water to flow over or through it, allowing for precise flow measurement.Weirs are widely used in water management. They help regulate flow in rivers, irrigation systems, and flood control channels.There are two main types of weirs: sharp-crested and broad-crested. Sharp-crested weirs feature a thin, vertical plate with a sharp edge over which water flows, forming a distinct drop known as a nappe.These are available in rectangular, triangular or V-notch, and trapezoidal shapes, each designed for specific flow conditions.The flow rate depends on the weir coefficient, width, gravity, and head. In V-notch weirs, the notch angle theta is also included.Broad-crested weirs with broad, flat crests encourage near-uniform critical flow, making them ideal for larger channels. Their flow rate differs from sharp-crested weirs due to geometric differences.&nbsp;Weirs are vital for accurately measuring and controlling flow in natural and constructed waterways.

weir

A weir is a hydraulic structure designed to partially obstruct an open channel, enabling precise control and measurement of water flow. By forcing water to flow over or through it, a weir allows for accurate determination of discharge rates, making it an essential tool in water resource management. These structures are extensively used in regulating river flows, irrigation systems, and flood control channels.<h3 style="margin-left:0cm;">Types of Weirs and Their Features</h3>Weirs are categorized primarily into sharp-crested and broad-crested types, each suited to specific applications. Sharp-crested weirs consist of a thin, vertical plate with a sharp edge that causes water to flow over it, forming a free-falling sheet of water known as the nappe. Common shapes for sharp-crested weirs include rectangular, triangular (V-notch), and trapezoidal configurations, chosen based on the expected flow conditions. The discharge rate for these weirs depends on factors such as the weir coefficient, head (water height above the weir crest), channel width, and gravitational effects. In V-notch weirs, the notch angle is also critical in flow calculation.Broad-crested weirs, on the other hand, feature a wide, flat crest that promotes near-uniform critical flow conditions. These weirs are particularly effective for larger channels, where their stable flow characteristics support reliable flow measurement and regulation.&nbsp;Due to their different geometries, the flow equations for broad-crested weirs and sharp-crested weirs differ significantly.<h3 style="margin-left:0cm;">Applications in Flow Management</h3>Weirs are indispensable in the management of natural and engineered waterways. By enabling controlled water discharge, they help prevent flooding, distribute irrigation water efficiently, and monitor river flows. Their adaptability to different flow conditions and precise measurement capabilities make them integral components in hydrological and engineering systems.

Weir

A weir is a hydraulic structure designed to partially obstruct an open channel, enabling precise control and measurement of water flow. By forcing water ...

Education

JoVE Core

Civil Engineering

Unit 1

Open Channel Flow

KEY TERMS AND CONCEPTS

weir flow measurement and control

A weir is a hydraulic structure that partly blocks an open channel and helps control water flow. It also makes it possible to measure discharge, which is the amount of water moving past a point in a given time. Because of this, weirs are important in water resource management.

Weirs are used in river flow regulation, irrigation systems, and flood control channels. They guide water to flow over or through the structure, which lets engineers estimate flow rates more accurately. This makes them useful in both natural waterways and engineered channels.

Weirs are usually grouped into sharp-crested and broad-crested types. A sharp-crested weir has a thin vertical plate with a sharp edge. Water flows over it as a free-falling sheet called a nappe. Common sharp-crested shapes include rectangular, triangular, also called a V-notch, and trapezoidal forms. The discharge rate depends on the weir coefficient, the head, which is the water height above the crest, the channel width, and gravity. For V-notch weirs, the notch angle also affects the flow calculation.

A broad-crested weir has a wide, flat crest that supports near-uniform critical flow. These weirs work well in larger channels because their flow is more stable for measurement and regulation. Since the shapes are different, the equations used for broad-crested and sharp-crested weirs are also different. In water management, these structures help prevent flooding, distribute irrigation water efficiently, and monitor river flows.

Weir Flow Measurement and Control

energy considerations in open channel flow

energy-considerations-in-open-channel-flow

Flow Depth and Energy in Open Channels

Open channel flow involves water moving with a free surface exposed to the air. Rivers, canals, and artificial channels are common examples. In these systems, gravity and surface effects shape the flow more than the behavior seen in closed pipes. Energy analysis helps explain how depth, velocity, and slope work together. Specific energy combines the potential energy from water depth and the kinetic energy from flow speed. For a given discharge, or flow rate, there are often two possible depths...

uniform depth channel flow

Channel Slope and Flow Balance

Uniform depth channel flow keeps the water depth steady along an open channel. This idea is often used for irrigation canals and is also a useful approximation for rivers. The channel bottom slope must match the energy slope. That balance offsets the gravity energy lost to shear stress at the boundary, so the depth does not change along the channel length. In a constant cross-section channel, uniform flow means the depth and velocity stay consistent. Two key measures are the flow area and the...

uniform depth channel flow: problem solving

uniform-depth-channel-flow-problem-solving

Calculating Trapezoidal Channel Flow Rate

Calculating trapezoidal channel flow rate starts with the channel shape and the depth of flow. For a trapezoidal channel, identify the bottom width, side slope, and flow depth first. These values define the cross-section used in the calculation. Next, find the cross-sectional area for the flowing water. Use the depth of flow, the bottom width, and the side slope angle to determine area. Then calculate the wetted perimeter, which includes the bottom width and the sloped side lengths that touch...

gradually varying flow

Open Channel Depth Changes in GVF

Gradually varying flow in open channels describes water depth that changes slowly along a channel. The depth adjusts over distance as gravity, shear forces, and energy needs stay in balance. This creates a low rate of change in the water surface. Gradually varying flow is common in natural streams, rivers, and canals. In these settings, the depth shifts little by little over long distances. The slope of the channel bed, compared with the energy slope, helps determine how the flow behaves. A...

 rapidly varying flow

Hydraulic Jumps in Open Channels

Rapidly varying flow in open channels describes a sudden change in water depth over a short distance. The depth can change so quickly that the rate of change with distance is close to one. These flows are difficult to analyze exactly because they are transient and multi-dimensional. Approximate solutions with simplified models still give useful insight into how they behave. A common example is the hydraulic jump. In a hydraulic jump, fast and shallow water changes quickly into slower, deeper...

hydraulic jump

Flow Regimes and Energy in Hydraulic Jumps

Hydraulic jumps happen in open channels when fast, shallow water changes into slower, deeper water. This sudden rise in depth depends on the upstream Froude number, which compares flow speed to wave speed. A hydraulic jump can form only when the upstream flow is supercritical, with Fr greater than 1. If Fr is less than 1, the flow is already subcritical, and a jump cannot form. That is because the jump would require a negative head loss, which is not possible under thermodynamic principles.

hydraulic jump: problem solving

Hydraulic Jump in a Rectangular Channel

Hydraulic jump in a rectangular channel shows how flow speed and water depth change when a gate closes downstream. The jump travels upstream at 2 meters per second, so that wave speed is part of the calculation. The initial channel flow speed is 6 meters per second. To find the water depths on each side of the jump, first calculate the effective upstream velocity. This combines the flow moving through the channel with the upstream-moving wave. Next, identify the flow velocities before and...

A weir is a hydraulic structure that partly blocks an open channel and helps control water flow. It also makes it possible to measure discharge, which is the amount of water moving past a point in a given time. Because of this, weirs are important in water resource management. Weirs are used in river flow regulation, irrigation systems, and flood control channels. They guide water to flow over or through the structure, which lets engineers estimate flow rates more accurately. This makes them...

weir: problem solving

Weir Flow Rates and Channel Depth

Weirs are hydraulic structures used to measure water flow in open channels. In a rectangular channel, they help calculate discharge, or the volume of water passing a point each second. The weir head is held at a fixed height above the channel bottom, which makes the math easier and lets students compare depth with flow rate. Three weir types are used in this setting: rectangular sharp-crested, triangular sharp-crested, and broad-crested. The rectangular sharp-crested weir depends on the weir...

underflow gates

Irrigation Canal Gate Flow Control

Underflow gates control water flow in irrigation canals. They help manage discharge so water moves where and when it is needed. The main underflow gate types are vertical gates, radial gates, and drum gates. Vertical gates move up and down. They create a free-flowing water jet. Radial gates pivot to regulate flow, and drum gates rotate for fine adjustments. Each design supports effective flow management in different canal settings. Downstream conditions change how water passes through an...

design example: design of an irrigation channel

design-example-design-of-an-irrigation-channel

Designing Trapezoidal Irrigation Channels

Trapezoidal irrigation channels are a common choice for moving water in farms and other water systems. Their flat bottom and sloping sides make them stable and easier to build than many other channel shapes. They are also cost-effective and efficient for carrying water. The channel size depends on the needed flow capacity and the site conditions. Designers choose the bottom width and the side slope ratio to match those demands. For unlined channels, the side slope is kept gentle to help...

key terms and concepts

Learn how weirs control and measure water flow, including sharp-crested and broad-crested types, discharge, head, and real-world water management uses.

Weir Flow Measurement and Control | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

civil engineering

Weir Flow Measurement and Control

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