What does the term 'head' refer to in relation to a weir?

The term 'head' in relation to a weir specifically refers to the depth of the water flow over the weir. This measurement reflects the hydraulic pressure exerted by the water, which is crucial for determining how water flows over the structure. The head is an important factor in hydraulic calculations, including flow rate and energy losses, as it impacts how much energy the water possesses as it flows.

Understanding 'head' allows engineers and surveyors to design weirs effectively and manage water flow in rivers, reservoirs, and other water bodies. The other options, such as the angle, width, or structural height, do not pertain to the measurement of hydraulic depth, and while they are relevant to the overall design and function of a weir, they do not define the term 'head.'