A weir is a small dam that regulates the flow of water in an open channel, lake, reservoir, etc. Weirs have been used to power to mills, raise or maintain water levels, create impoundments and to measure flow. Perhaps the most common use of weirs today is as outlet control structures on detention ponds.

A measuring weir is an overflow structure built perpendicular to an open channel axis to measure the rate of flow of water. A properly built and operated weir of a given shape has a unique depth of water at the measuring station in the upstream pool for each discharge. The crest overflow shape governs how the discharge varies with head measurement.

What is weir contraction?

The overflow section shape, usually cut with a sharp upstream corner into a thin plate, is the weir notch, sometimes called the overflow section. When the distances from the sides of the weir notch to the sides of the weir pool are greater than two measurement heads, the water will flow relatively slowly along the bulkhead face toward the overflow opening. As the water from the sides of the channel nears the notch, it accelerates and has to turn to pass through the opening. This turning cannot occur instantaneously, so a curved flow path or side contraction results in which the water springs free to form a jet narrower than the overflow opening width. This effect is also known as an end contraction.

The term vertical contraction includes both crest contraction and drawdown at the weir plate. When approach conditions allow full contractions at the ends and at the bottom, the weir is a contracted weir. For full contraction, the ends of the weir should not be closer to the sides and bottom of the approach channel than a specified distance. If the specified distances are not met, then the weir is partially contracted.

What is a suppressed weir?

When the sides of the flow channel act as the ends of a rectangular weir, no side contraction exists, and the nappe does not contract from the width of the channel. This type of weir is a suppressed weir. The falling sheet of water springing from the weir plate is the nappe.

What is velocity of approach?

Velocity of approach is equal to the discharge divided by the flow section area at the head measuring station. Velocity of approach is important because it can change weir calibrations by effectively reducing the crest length and/or measuring head. In addition, a variable discharge coefficient results as increasing velocity changes the curvature of flow springing from the weir plates.

What are the common types of weirs?

The types of traditional fully contracted weirs commonly used and considered standard for measuring water are:

• Rectangular weirs (including sharp-crested)
• V-notch weirs from 25 degrees to 100 degrees
• Cipoletti (trapezoidal) weirs

Common partially contracted weirs are:

• Partially contracted rectangular weirs (including sharp-crested)
• Partially contracted 90-degree V-notch weirs

What is a sharp-crested weir?

If the notch plate is mounted on the supporting bulkhead such that the water does not contact or cling to the downstream weir plate or supporting bulkhead, but springs clear, the weir is a sharp-crested or thin-plate weir. The top thickness of the crest and side plates should be between 0.03 and 0.08 inches.

For a rectangular sharp-crested weir that takes end contractions and the velocity of approach into account the discharge is determined by:

where

Q Cd g L n H vo

discharge (cfs) coefficient of discharge gravity (ft/s2) crest length (ft) number of end contractions head (ft) velocity of approach (ft/s)

By neglecting the velocity of approach the rectangular sharp-crested weir equation reduces to:

If we assume that the head is not greater than one-third the weir length, the value of the discharge coefficient is between 0.6 to 0.62. If end contractions are also neglected the rectangular sharp-crested weir equation further reduces to a form known as the Francis formula.

What is a V-notch (triangular) weir?

As the name suggests, a V-notch or triangular weir has a triangular opening through which discharge occurs. Regarded as highly accurate, these types of weirs are suitable for low discharges because the head increases more rapidly on a triangular section. V-notch weirs are not noticeably affected by the velocity of approach.

The discharge from a V-notch weir can be determined using:

For fully contracted weirs with a notch angle between 20 and 100 degrees the coefficient of discharge ranges from 0.57 to 0.59.

What is a trapezoidal weir?

A trapezoidal weir has an opening comprised of a rectangle bounded by two triangles. The discharge through this type of weir is determined by summing the discharge over the rectangular section with end contractions with the discharge from the triangular or V-notch section.

What is a Cipoletti weir?

A Cipoletti weir is a trapezoidal weir with a side slope of 1 horizontal to 4 vertical.

The discharge through a Cipoletti weir can be determined using the following formula:

What is a broad-crested weir?

A weir in the form of a relatively long raised channel control crest section is a broad-crested weir. The flow control section can have different shapes, such as triangular or circular. True broad-crested weir flow occurs when the upstream head above the crest is between the limits of about 1/20 and 1/2 the crest length in the direction of flow.

Discharge over a broad-crested weir is determined as follows:

This equation can be modified to account for end contractions and the velocity of approach following the form of the rectangular sharp-crested weir equation. The coefficient of discharge is generally between 0.85 and 1.1. Because of the wide variety in broad crested weir shapes, it is recommended that the coefficient of discharge be calibrated in the field.

What is a long-crested weir?

Long-crested weirs provide more weir crest length by installing the weir at some configuration other than perpendicular to the channel. They are used in open channel irrigation systems to minimize water surface elevation fluctuations. The thickness of the water layer over the crest is directly related to the length of the weir crest. A longer crest length produces a thinner water layer which in turn creates smaller variations in the upstream water levels. It is important to note that long-crested weirs are not intended as hydraulic measurement devices.

What is a duck bill weir?

A duck bill weir is a type of long-crested weir that is designed to control water levels. As shown below the design of a duck bill weir involves a staggered weir crest that has the appearance of teeth or duck bills. This staggering effect increases the weir crest length while minimizing the footprint of the weir. The result is a smaller weir that is effective at controlling the upstream water depth.

What is a compound weir?

A compound weir is a combination of multiple weir types. For example, a V-notch weir might easily handle the normal range of discharges at a structure; but occasionally, much larger flows would require a rectangular weir. A compound weir, consisting of a rectangular notch with a V-notch cut into the center of the crest, might be used in this situation.

What is a side-flow weir?

A side-flow weir is a structure installed along the side of a main channel or pipe. They are used to divert flow during high flow conditions. These types of weirs are commonly seen in irrigation and sewer systems.

What is a transverse weir?

A transverse weir is commonly used in sewer design to divert flow during high flow conditions.

What is an ogee weir?

An ogee weir is a weir whose profile is S-shaped. Flow over the crest adheres to the face of the profile which prevents air access to the underside of the sheet.

How does weir submergence impact discharge measurements?

Downstream water rising above the weir crest elevation produces a submerged weir condition. When the downstream water surface is near or above the crest elevation of a sharp-crested weir, accuracy of measurement should not be expected. Submerged flow correction methods or submerged calibrations only produce estimates of discharge. The use of a submerged weir as a water measurement device is not good practice and should only be done as a temporary, emergency procedure. Because of the large loss of accuracy, designing thin-plate weirs for submergence should be deliberately avoided.

What are some additional sources of information about weirs?

The following documents provide greater detail on weirs and weir design:

USBR Water Measurement Manual (Chapter 7 – Weirs)
Long Crested Weir Design

Can I model weirs in StormNET?

Yes, StormNET supports a variety of common weir types for both open channel and sewer design. With the ability account for backwater effects, StormNET can model complex situations with accuracy while allowing your projects to be completed faster than ever.