What type of weir configuration is likely in use if the crest is 4.5 ft and the water is 2.5 ft high?

The configuration of a rectangular weir is particularly suited for scenarios where the water level is at a specific height, such as the 2.5 ft indicated in the question. A rectangular weir has a horizontal top (the crest) that allows for a consistent flow measurement based on the height of the water above the crest.

When water flows over a rectangular weir, the discharge can be calculated using the height of the water (head) above the weir. In this case, the height of the water, at 2.5 ft, is well below the height of the crest, which is 4.5 ft. This relationship indicates that a rectangular weir can effectively accommodate the flow conditions presented.

Other types of weir configurations, such as a V-notch weir, would not be appropriate for a water height that is significantly less than the crest because they are designed for more precise measurements and typically function better with smaller heads. Similarly, triangular and circular weirs each have specific flow characteristics and formulas that do not align well with the scenario described, especially when considering the relative heights of the water and the crest in this context. Therefore, the rectangular weir is the most suitable configuration given the specified dimensions.