What Is a Rectangular Weir?

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  • Written By: Christian Petersen
  • Edited By: Susan Barwick
  • Last Modified Date: 13 September 2019
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The term weir has several meanings. The term rectangular weir refers to a specific type of weir with a rectangular notch cut in the top edge. This notch can be used by engineers to calculate the rate of flow of the body of water, providing valuable information that can be used for environmental management programs, flood management, further dam construction and civil engineering projects.

Generally the term refers either to a structure consisting of pens or fences in a body of water used to trap or hold fish, or a low dam placed across a stream or river for the purpose of managing water depth and flow. A weir is different from other types of dam in that water generally flows over the top rather than through spillways or hydroelectric plants like in other dams.


The equations and calculations used by engineers to calculate the rate of flow of a stream or river over a rectangular weir are somewhat complex. The basic principle, however, revolves around the idea that the depth of the water behind the weir can be used to determine total volume. By using the dimensions of the weir and applying variables that take into account the shape of the top of the weir, or the rectangular notch cut in a rectangular weir, and the depth of the water as it flows over the crest, or over the bottom of the notch, it is possible to calculate the total flow of the body of water over time as a rate, usually expressed in gallons per minute. Other variables are assigned based on the geometry or shape of the crest of the weir or the edges of the notch, such as whether they are square or beveled to make the results as precise as possible.

The data gleaned from flow rate calculations over a rectangular weir can be used in a number of ways. Flood control and general water management policies and practices are often designed around such data. The flow data can be used to determine if a hydroelectric project would be possible or profitable. Water flow data can also be useful for environmental impact studies, specifically in determining how the weir or other structures would affect the ecosystem of a stream or river. Irrigation and other water use needs programs also benefit from this kind of data.


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