Trapezoidal Weir
As per the question, the popular name for a trapezoidal weir is the Cipolletti weir. Here is a detailed explanation of this term:

Definition
A trapezoidal weir, also known as a Cipolletti weir, is a flow measurement structure commonly used in open channel flow applications. It consists of a trapezoidal-shaped notch or opening in a channel through which water flows, allowing for the measurement of flow rate based on the height of the water surface above the weir crest.

Reason for Name
The name "Cipolletti weir" comes from the Italian engineer who first introduced this specific shape of weir, Raffaele Cipolletti. He developed this design to improve the accuracy and efficiency of flow measurement in open channels, leading to its widespread adoption in hydraulic engineering.

Features
- The trapezoidal shape of the weir allows for a more accurate measurement of flow compared to other weir shapes.
- It is particularly useful for measuring flow rates in irrigation canals, stormwater drains, and other open channel systems.
- The design of the weir facilitates smooth flow over the crest, reducing turbulence and ensuring accurate flow readings.

Applications
- Cipolletti weirs are commonly used in water resource management, hydrology, and civil engineering projects for monitoring and controlling water flow.
- They can be found in various settings such as agricultural fields, urban drainage systems, and experimental hydraulic laboratories.
In conclusion, the term "Cipolletti weir" is synonymous with a trapezoidal weir due to its association with the engineer who popularized this particular design for flow measurement in open channels.