Habibi Naser


Hydraulic characteristics of broad-crested weirs with optimum and stable cross-sections


Student Name

Naser Habibi



Mohammad Karim Beirami
(Assist. Prof._Civil Engineering Department_Isfahan University of Technology)

Mohammad Reza Chamani
(Assist. Prof._Civil Engineering Department_Isfahan University of Technology)









Because of economic and simplicity in construction, trapezoidal broad-crested weirs are widely used in small dams and as flow measuring devices. Flow characteristics of these weirs have been extensively studied since the nineteenth century, but no works have been done to investigate the optimum size of cross section of trapezoidal broad-crested weirs. The present investigation is devoted to the study of the optimization the trapezoidal broad-crested weir cross-section. The optimization is based on the stability of the weir against sliding and overturning. In addition, stresses in the weir structure are also checked to avoid negative stresses. Different friction factors, based on the type of soil foundation, are also considered in the analysis.

The present investigation shows that the sliding stability criteria is the main factor for soil foundations, but overturning stability criteria plays a more important role for rock foundations. For soil foundations, it is found that by increasing the friction factor, the area of the longitudinal section and the downstream slope decreases while the upstream slope Increases.

To investigate the effects of cross section on the discharge coefficient, model studies have been carried out for four different cross sections. It is shown that the discharge coefficient is not very sensitive to the upstream or the downstream slope and the variation of the discharge coefficients is the same for all four models. It is found that the effect of the downstream depth on the weir discharge coefficient is the same for all models.

Scale effect is also investigated in a large model of the trapezoidal broad crested weir with the double size of one the previous models. It is shown that the results of the large model don't considerably differ from those of the small size model. A method is also presented to predict the discharge coefficient based on the momentum equation. It is found that the differences between predictions and experimental results for the discharge coefficient are negligible.