Experimental Study on Determination of Flow Coefficient of the Linear and Quadratic Proportional Weirs in Rectangular Channels

Accurate measurement of flow in open channels، water transmission and sewage networks is accounted as an important issue in operational management of hydraulic structures. So far، various types of structures such as weirs، orifices، flumes and slide gates have been used to measure the discharge in conveyance structures. Meanwhile، proportional weirs، a type of sharp-crested weirs، have high accuracy، because of their low sensitivity to upstream depth. In the linear type of proportional weirs، the relationship between discharge and head is linear، while in quadratic and logarithmic types، linear relationships are established between discharge and square head، and between discharge and logarithm of head، respectively. In this research، based on theoretical criteria for proportional weirs and also dimensionless design method، three types of linear proportional weir; i. e. chimney cross section، inverted triangle، and inverted two triangles، and two types of quadratic proportional weir; i. e. one-piece and two pieces linear edge، were designed، constructed and installed at the end of a rectangular channel. In this research، about 600 experiments were carried out with discharge range of 2 to 10 l/s to find experimentally the relationships between head and discharge in the above mentioned proportional weirs. Results of experiments showed that discharge coefficient is a function of dimensionless ratio of the head to the weir height، and the dimensionless ratio of the weir crest length to the weir height. Using some of the experimental results، a relation between discharge and flow coefficient obtained for each weir. Comparison of the results obtained from the relationships with the rest of the results from experiments، showed that the average error of the proposed equation is equal to 1. 5 percent، indicating high accuracy of the proposed equations.