Where should head measurements be taken using a v-notched weir?

Taking head measurements using a v-notched weir is essential for accurate flow measurement. The correct approach is to measure at a distance of at least four times the height of water above the weir crest. This distance ensures that the flow is fully developed and stable by allowing for the proper establishment of the hydraulic conditions downstream.

When measuring too close to the weir, disturbances or turbulence from the weir itself can affect the accuracy of the head measurement. By positioning the measurement point several times the head above the crest, the influence of the weir's presence is minimized, providing a more reliable reading that reflects the true flow conditions. This principle is grounded in fluid dynamics, where flow within certain distances from a flow obstruction can behave unpredictably.

In contrast, measuring at the weir crest would capture head measurement at a point where water flow is transitioning, resulting in less stability. Similarly, measuring directly downstream may still be affected by vortices or eddies created by the weir structure. Finally, measuring at the maximum flow point does not account for the importance of the distance necessary for stable flow conditions. Thus, the recommendation to measure at a specific distance ensures the capturing of accurate and stable hydraulic conditions necessary for flow calculation.