The upper surface of the weir over which water flows, is known as

ANS:C - sill

A sill in the context of hydraulic engineering typically refers to a horizontal structure or ledge constructed at the base of a dam, weir, or other hydraulic structure. Here's an explanation of a sill in hydraulic engineering:

1. Function: The primary function of a sill is to control the flow of water and regulate the water level upstream of the structure. It helps maintain a desired water elevation and provides stability to the structure by resisting the forces exerted by the flowing water.
2. Location: Sills are usually located at the base of the structure, either embedded within the foundation or constructed as a part of the structure itself.
3. Design: The design of a sill depends on various factors such as the hydraulic conditions, the desired water level upstream of the structure, and the structural requirements of the hydraulic structure. Sills are typically designed to withstand the hydraulic forces exerted by the flowing water and ensure the stability of the structure.
4. Types: There are different types of sills used in hydraulic engineering, including overflow sills, bottom sills, and training sills. Each type serves a specific purpose in controlling the flow of water and regulating the water level upstream of the structure.

In summary, a sill in hydraulic engineering is a horizontal structure or ledge constructed at the base of a dam, weir, or other hydraulic structure to control the flow of water, regulate water levels, and provide stability to the structure. It plays a crucial role in ensuring the safe and efficient operation of hydraulic structures in various water management applications.