The lowest elevation (h = 0) of the overflow opening of the sharp-crested weirs or the control channel of broad-crested weirs is the head measurement zero reference elevation. For standard weirs - c d - is well defined or constant for measuring within specified head ranges. The flow rate measurement in a rectangular weir is based on the Bernoulli Equation principles and can be expressed as:Ĭ d = discharge constant for the weir - must be determinedĬ d must be determined by analysis and calibration tests. By combining V-notch weirs with broad chested weirs - larger range of flow can be measured with accuracy. Other available weirs are the trapezoidal (Cipolletti) weir, the Sutro (proportional) weir and the compound weirs (combination of the previously mentioned weir shapes). Flow measurement installations with broad-crested weirs will meet accuracy requirements only if they are calibrated. They consist of a sharp edged plate with a rectangular, triangular or v-notch profile for the water flow.īroad-crested weirs can be observed in dam spillways where the broad edge is beneath the water surface across the entire stream. Rectangular weirs and triangular or v-notch weirs are often used in water supply, wastewater and sewage systems. Common Standards and Specifications for Weir Flow Measurements.Water level-discharge relationships can be applied and meet accuracy requirements for sharp-crested weirs if the installation is designed and installed consistent with established ASTM and ISO standards. If the notch plate is mounted on the supporting bulkhead such that the water does not contact or cling to the downstream weir plate or supporting bulkhead, but springs clear, the weir is a sharp-crested or thin-plate weir. Weirs are called sharp-crested if their crests are constructed of thin metal plates, and broad-crested if they are made of wide timber or concrete. The flow rate over a weir is a function of the head on the weir.Ĭommon weir constructions are the rectangular weir, the triangular or v-notch weir, and the broad-crested weir. Weirs are structures consisting of an obstruction such as a dam or bulkhead placed across the open channel with a specially shaped opening or notch.
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