RBC broad-crested weirs for circular sewers and pipes

Abstract

Abstract In recent years, significant advancements have been made in the practical application of long-throated flumes and broad-crested weirs for flow measurements in irrigation canals. The modified RBC (Replogle-Bos-Clemmens) broad-crested weir has many advantages over related open-channel flow devices. These include high accuracy and reliability for a wide variety of shapes, low head-loss requirements which are predictable, and relatively simple inexpensive construction. In this paper we have extended the application of these weirs to circular pipes flowing partially full. The theoretical equations are presented for ideal flow from which approximate ratings can be obtained to within a reasonable accuracy with an empirical discharge coefficient, However, a mathematical model is available which accurately predicts these ratings by directly accounting for the effects of friction. The ratings for a wide variety of shapes and sizes of these weirs were computed with the model and fit to an empirical equation. The constants for this equation are plotted graphically for easy use. The resulting ratings should be well within ±3%. Design examples are given which show how to select the flume dimensions for maintaining free-flowing conditions (modular flow) and for minimizing sediment deposition. Once constructed, the rating for a given flume can be determined even when not constructed as planned.