Discharge Relations for Rectangular Broad-Crested Weirs

Compound broad-crested weirs have a small inner rectangular section for measuring low flows and then, they broaden to a wide rectangular section at higher flow depths. It can be used as a water measurement device in irrigation canals. This paper presents data that will be of use in the design of hydraulic structures for flow control and measurement. A series of laboratory experiments were performed in order to investigate the effects of width of the lower weir crest and step height of broad-crested weirs with rectangular compound cross section on the values of the discharge coefficient (Cd) and the approach velocity coefficient. For this purpose, 15 different broad-crested weir models with rectangular compound cross sections for a wide range of discharges were tested. The sillreferenced heads at the approach channel and at the tail water channel were measured in each experiment. The dependence of the discharge coefficient and approach velocity coefficient on model parameters was investigated. Results show that a discontinuity occurs in head-discharge ratings because the section width suddenly changes shape, experiencing a break in slope when the flow enters the outer section. Values of Cd obtained from the experiments on compound broad-crested weirs are lower than those of a broad crested weir with a rectangular cross section because of it’s contraction effects.