The Coefficient of Broad-Crested Weir in Natural Channels

Abstract

Weirs have been designed and used extensively in hydraulic structures to control the flow depth and discharge. To estimate the discharge over the weir, a weir coefficient is required. The discharge coefficient can be obtained experimentally as a function of the dimensionless total head of the approaching flow or as a function of various parameters. However, a universally acceptable discharge coefficient does not exist. Beside the man-made weirs that have been constructed by hydraulic engineers, some natural channel characteristics and slopes can be simulated as broad-crested weirs that are clearly noticeable in examining water surface profiles. The study area is Centralia watershed in Central Florida, which contains several cascades of step like waterfalls that can be selected for transecting. Cross-section data from 5 transects and detailed hydraulic data was obtained using the Digital terrain model (DTM) and the Triangular Irregular Network (TIN). The Hydrologic Engineering Center-River Analysis System (HEC-RAS) stepbackwater technique was used to calculate water surface profiles for natural channels that are likely used for storm water drainage. The results show that these channels have chains of steps that dissipate the momentum of falling water in steep areas and maintain a steady rate of flow. The weir coefficients were determined by comparing the steady state discharges to the equivalent weir discharges. The results indicate that the weir coefficient can be expressed as a function of weir height and channel cross-section area. The relationship between this function and the weir coefficient showed a high correlation with R2 = 0.991. The results of this study can be used to estimate the discharge in similar reaches in any water surface profile.